Stable dual-wavelength Q-switched Nd:YAG laser using a two-step energy extraction technique Atsushi Sato,1,* Shimpei Okubo,1 Kazuhiro Asai,1 Shoken Ishii,2 Kohei Mizutani,2 and Nobuo Sugimoto3 1 2
Tohoku Institute of Technology, 35-1 Yagiyama-Kasumi-cho, Taihaku-ku, Sendai 982-8577, Japan
National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei-shi, Tokyo 184-8795, Japan 3
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan *Corresponding author: [email protected] Received 22 December 2014; revised 2 March 2015; accepted 7 March 2015; posted 9 March 2015 (Doc. ID 231266); published 31 March 2015
The generation of stable dual-wavelength pulses from an actively Q-switched Nd:YAG laser operating at 1064 and 1319 nm was demonstrated. Pulse energies of the dual-wavelength laser were extracted by two temporally separated stimulated-emission processes for a single pumping process, thereby avoiding line competition between the two laser transitions. Total energy of the order of 20 mJ was achieved for the two pulses, and the ratio of the pulse energies of the two lasers could be selected by adjusting the output couplings. The pulse-to-pulse fluctuations for the lasers operating at 1319 and 1064 nm were 4.7%– 4.8% and 1.5%–2.6%, respectively, which were almost equivalent to those for a single emission line in our system. The experimentally observed laser performance agreed reasonably well with theoretical predictions. © 2015 Optical Society of America OCIS codes: (140.3425) Laser stabilization; (140.3540) Lasers, Q-switched; (140.3410) Laser resonators. http://dx.doi.org/10.1364/AO.54.003032
1. Introduction
Almost every aspect of our lives depends on plants, trees, grasses, and other forms of vegetation because they absorb carbon dioxide and provide us with oxygen. Therefore, it is essential to monitor the spatial distribution of vegetation biomass and its variations over time, which represents invaluable information to improve present assessments and future projections of the terrestrial carbon cycle. The normalized difference vegetation index (NDVI) [1] is used to provide a measure of green vegetation, monitor changes in vegetation levels, and understand how these quantities affect the environment, using mostly data taken by satellite-based imaging spectroradiometers, e.g., AVHRR/NOAA [2], MODIS/ 1559-128X/15/103032-11$15.00/0 © 2015 Optical Society of America 3032
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
Terra, Aqua [3], and AVNIR/ALOS [4]. The NDVI can be calculated from spectral reflectance data of vegetation as follows: NDVI
RNIR − RRED ; RNIR RRED
(1)
where RNIR and RRED are spectral reflectances in the near-infrared and red bands, respectively. In the abovementioned passive remote sensing, the spectral regions from 580–690 and 725–1000 nm are used as the red and near-infrared bands, respectively [2–4]. The second-harmonic generation of the 1319 nm transition in Nd:YAG lasers provides red light at 660 nm, which coincides with the absorption band of chlorophyll. Conversely, there is no absorption of chlorophyll and water at 1064 nm, which is the typical lasing wavelength of Nd:YAG. Since these wavelengths correspond to the red and near-infrared
bands, a combination of 660 nm second-harmonic and 1064 nm fundamental outputs from Nd:YAG lasers permits the active remote sensing of the NDVI. Although a lasing wavelength of 1064 nm is slightly longer than 1000 nm, the reflectance of vegetation is expected to be equivalent to that for the near-infrared bands in the passive remote sensing because these wavelengths are far enough from the absorption peak of water around 1.4 μm. Even in the abovementioned passive remote sensing, the nearinfrared wavelength ranges are different [2–4]. If a Q-switched laser is used as a dual-wavelength laser transmitter, we can measure not only range-resolved NDVI values, but also the canopy height. In this case, the 1064 nm laser pulse is suitable for measuring the canopy height because of its high reflectance compared with the 660 nm pulse. Consequently, a novel vegetation lidar for the simultaneous observation of vegetation height and NDVI can be constructed by using both a dual-wavelength laser transmitter and an imaging detection system with a 2D array detector for collecting information regarding vegetation biomass [5]. Air- and spaceborne lidar systems are suitable for this application because they can collect information regarding vegetation over an extensive area by moving the footprint of the transmitted laser pulse over the ground surface. In such a lidar system, a high-energy laser transmitter is needed. In earlier studies, we proposed a spaceborne system with pulse energy of 100 mJ and a pulse width shorter than 30 ns at each wavelength for the observation of both the NDVI and the canopy height from space [5]. In addition to pulse energy requirements, a good spatial overlap between the two footprints of the dual-wavelength laser pulses is required to determine an accurate NDVI value; therefore, simultaneous lasing of the two laser pulses is essential. While dual-wavelength pulses with perfect temporal overlap are ideal, temporally separated dual-wavelength pulses can also be regarded as simultaneously irradiated laser pulses when the time interval between two pulses is negligible. For example, if the lidar platform is a satellite with a velocity of approximately 7 km s−1 , a time interval of 1 μs between two pulses corresponds to an approximate 7 mm displacement in the footprint. In this case, the difference in the position between the two footprints is negligible because it is four orders of magnitude smaller than the footprint size of spaceborne lidars, which is typically several tens of meters [6]. Temporally separated dual-wavelength pulses with a time interval shorter than 1 μs, rather than simultaneously generated dual-wavelength pulses, are well suited to our objective because the reflected lidar signals at 660 and 1064 nm can be successively acquired in a singlechannel detector, facilitating a simple signal-processing system. Dual-wavelength lasers that use a single laser crystal are more cost-effective and compact than a system comprising two lasers operating at different
wavelengths. Dual-wavelength Q-switched pulses are conventionally produced by adjusting the output coupling and cavity length of two lasers so as to compensate for the difference in the stimulatedemission cross section between the two laser transitions [7–9]. Although dual-wavelength lasing can be simultaneously achieved by conventional methods, line competition occurs between the two laser transitions, which can result in considerably unstable pulse energies [10–12]. To solve this problem, the two laser pulses can be either spatially or temporally separated. Spatially independent lasing at two wavelengths can be achieved by using different regions located side by side within the laser crystal as the pumped regions for each wavelength [13]. However, it is difficult to obtain an appropriate overlap between the pumped region and the cavity mode by this method because the cross sections of the two cavity modes are not circular. Conversely, intermittent oscillation of dual-wavelength pulses permits temporally separated lasing, which can avoid line competition and provide an appropriate overlap between the pump and cavity mode volumes. In an earlier study that considered this lasing method, the time interval between the first and second pulses was of the order of tens of microseconds due to the pumping time for the second pulse after the first pulse lasing [14]. In further investigation, Huang et al. pointed out the contribution made to the second pulse by the residual population inversion after extracting the first pulse and demonstrated the generation of dual-wavelength pulses for a single pumping process in a passively Q-switched Nd:YVO4 laser under high-repetition-rate operation at a relatively low output energy level [15]. However, highenergy operations have not been demonstrated, and the validity of two-step energy extraction for the stabilization of pulse energies in dual-wavelength lasers has not been evaluated quantitatively in previous studies. In this paper, we demonstrate an actively Q-switched Nd:YAG laser producing stable highenergy dual-wavelength pulses at 1064 and 1319 nm using a two-step energy extraction technique. The method for designing a dual-wavelength laser with two-step energy extraction was developed using a rate equation model. Furthermore, the pulse-to-pulse stability of the output energies under this lasing method was compared with the stability observed during simultaneous energy extraction in the presence of line competition. 2. Laser Design
In conventional dual-wavelength lasers, key parameters of laser design are the reflectivities of the output mirrors and the cavity lengths for the two lasers, because the same threshold cannot be achieved for the two lasers if a difference in the stimulatedemission cross section between the two laser transitions is not canceled by a specific cavity design [7,16]. On the other hand, for a two-step energy extraction technique, the pulse energies do not directly depend 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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on the two cavity lengths, while the output mirror reflectivities set the pump threshold levels, and thereby, provide a ratio of the pulse energies of the two lasers. In addition, detailed calculations with respect to the population inversion density are required for the design of a laser using this lasing technique, because the population inversion density after the first pulse lasing is related to the initial population density of the second pulse, and they are not the same when the population densities of the two lower laser levels are different. Therefore, we developed a detailed rate equation model [17], compared with conventional models [4,16,18,19], to investigate the operation condition needed to achieve dual-wavelength lasing under two-step energy extraction. Figure 1 shows the energy-level diagram of Nd in YAG. The lasing wavelengths of interest in this study are 1064 and 1319 nm, which correspond to the 4 F 3∕2 → 4 I 11∕2 and 4 F 3∕2 → 4 I 13∕2 transitions, respectively. The upper laser manifold 4 F 3∕2 consists of two Stark levels; only the R2 level is used as a common upper laser level for both transitions. The 1064 and 1319 nm transitions terminated on the Y 3 and X 1 levels, respectively. In the two-step energy extraction technique, a first Q-switched pulse is generated at either wavelength, and a second Q-switched pulse can be subsequently obtained at the other wavelength by using the residual stored energy in the upper laser level after energy extraction by the first Q-switched pulse. The residual population density of the upper laser level is determined by the initial and final population inversion densities of the first pulse lasing, which is given by the transcendental function [20]. Numerical solutions to this equation show that a small difference between the initial and threshold population inversion densities leads to low energy extraction efficiency, i.e., the residual population in the upper laser level increases with higher threshold population inversion density. When the Q-switched pulse width is comparable to or shorter than the lowerlaser-level lifetime, a population of the lower laser level exists during the pulse evolution, thereby decreasing the population inversion. In the previous 4 4
F5/2 F3/2
Pump 805 nm 4
I13/2
4
I11/2
4
I9/2
R2 R1 Laser 1064 nm (R2→Y3) φ1
Nu
1319 nm (R2→X1) φ2 X7 N2 X1 Y6
dN u N c W p − u − σ 1 ϕ1 f u N u − f 1 N 1 dt τ f nr c − σ 2 ϕ2 f u N u − f 2 N 2 ; nr
(2)
dN 2 β2 N u c N σ 2 ϕ2 f u N u − f 2 N 2 − 2 ; dt τf τ2 nr
(3)
dN 1 β1 N u c N σ 1 ϕ1 f u N u − f 1 N 1 − 1 ; dt τf τ1 nr
(4)
dϕ2 cl ϕ γ β N σ 2 ϕ2 f u N u − f 2 N 2 − 2 2 2 u ; dt τc2 τf nr lc2
(5)
dϕ1 cl ϕ γ β N σ ϕ f N − f 1 N 1 − 1 1 1 u ; dt τc1 τf nr lc1 1 1 u u
(6)
where N u is the population density in the upper laser manifold; N 1 and N 2 are the population densities in the lower laser manifolds; ϕ1 and ϕ2 are the laser photon densities in the laser cavities; W p is the pumping rate; τf is the lifetime of the upper laser manifold; τ1 and τ2 are the lifetimes of the lower laser manifolds; σ 1 and σ 2 are the stimulated-emission cross sections for the two transitions; c is the speed of light in vacuum; nr is the refractive index of the laser medium; f u is the Boltzmann thermal occupation factor of the upper level; f 1 and f 2 are the occupation factors of the lower laser levels; l is the length of the laser rod; lc1 and lc2 are the cavity lengths; γ 1 and γ 2 are the rates at which spontaneous emission contributes to the lasers; and β1 and β2 are the branching ratios. The subscripts 1 and 2 in the equations correspond to the laser transitions at 1064 nm and 1319 nm, respectively. The photon decay times τc1 and τc2 in the laser cavities are given by Eqs. (7) and (8):
N1 Y1
Fig. 1. Energy-level diagram of Nd:YAG. 3034
rate equation models for a dual-wavelength Qswitched laser using a common upper laser level [4,16,18,19], it was assumed that the population inversion density is the same as the population density in the upper laser level. However, the initial and threshold population inversion densities for the first Q-switched pulse significantly affect the residual population for the second Q-switched pulse so that the population densities and lifetimes of the two lower laser levels have to be considered in the model in the case of two-step energy extraction. Equations (2)–(6) represent the coupled rate equations for this model [17]:
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
τc1 −
2lc1 ; c ln1 − Lc1 1 − LQ1 R1
(7)
2lc2 ; c ln1 − Lc2 1 − LQ2 R2
(8)
where Lc1 and Lc2 are the round-trip losses in the laser cavities; R1 and R2 are the reflectivities of the output mirrors; and LQ1 and LQ2 represent the time-dependent losses introduced by the Q-switches, which can be approximated by step functions. For convenience, the time interval between the two Q-switch trigger pulses is defined as ΔtQ tQ1 − tQ2 ;
Q-switched pulse energy (mJ)
τc2 −
30 1319 nm 1064 nm
25 20 15 10 5
(a) 0
(9)
0
20
40
60
80
100
Output mirror reflectivity, R1, R2 (%) 200
Q-switched pulse width (ns)
where tQ1 and tQ2 are the delay times of the Qswitches with respect to the beginning of the pump pulse at which the LQ1 and LQ2 values are switched from a maximum to 0. The positive sign of ΔtQ suggests that the Q-switch for the 1319 nm laser opens before that for the 1064 nm laser. Using this model, we calculated the output mirror reflectivities to achieve dual-wavelength lasing under two-step energy extraction. We assumed that the energies of the first and second pulses are extracted from the 1319 nm and 1064 nm lasers, respectively. The parameters used in the simulation are as follows [21,22]: σ 1 45.8 × 10−20 cm2 , σ 2 8.7 × 10−20 cm2 , τϕ 230 μs, f u 0.40, f 1 0.19, f 2 0.32, l 10 mm, lc1 530 mm, lc2 650 mm, nr 1.82, W p 7 × 1022 cm−3 s−1 , β1 0.135, β2 0.018, Lc1 0.03, and Lc2 0.03. The pump pulse length and pump wavelength were set to 140 μs and 805 nm, respectively. The reason for this shorter pump pulse length, compared with the conventional value of 230 μs, is the limitation due to amplified spontaneous emission (ASE) described in a later section. The diameter of the laser rod was set to 3 mm. The values of γ 1 and γ 2 are given by considering the spontaneous emission within the solid angle at the cavity mirrors. A rough estimation of these values is acceptable, because the last terms in Eqs. (5) and (6), which include these parameters, affect only the Q-switched pulse position in time. The lifetime of the lower laser manifold 4 I 11∕2 , τ1 , has been reported in the literature [23–25], and it was assigned a value of 11 ns in our simulation. However, an accurate value for the lifetime of the lower laser manifold 4 I 13∕2 , τ2 , is unknown. This value was assumed to be 80 ns in our simulation as a result of an observed satellite pulse of the main Q-switched pulse caused by the finite lower-laser-level lifetime [25,26]. Figure 2 shows calculated pulse energies and pulse widths in a single-emission-line operation under the same pumping level as a function of output mirror reflectivity. Because of the smaller stimulated-emission cross section for the 1319 nm transition, higher reflectivity is needed to achieve 1319 nm lasing. By contrast, the 1064 nm laser can produce a higher output energy compared with the 1319 nm laser; for example, output energies higher than 20 mJ can be obtained for output mirror reflectivities ranging from 27% to 92%. Accordingly, the 1064 nm laser is suitable
1319 nm 1064 nm 150
100
50
(b) 0
0
20
40
60
80
100
Output mirror reflectivity, R1, R2 (%) Fig. 2. Calculated (a) pulse energies and (b) pulse widths for single-emission-line operations as a function of output mirror reflectivity.
for the second pulse laser, since higher energy extraction efficiency for the second pulse contributes to higher total efficiency for two-step energy extraction. In addition, the higher threshold for the 1319 nm laser is preferable for the first pulse laser, because moderately low energy extraction efficiency leads to a large residual population inversion density, which provides the initial population inversion density for the second pulse. In our system, therefore, the 1319 and 1064 nm transitions are selected for the first- and second pulse lasings, respectively. From a practical viewpoint, the reflectivity of the output mirror for the 1064 nm laser, R1 , used in our experiments was set to 30%, which provides a reasonable balance between a low threshold and damage-free operation. Figure 3 shows calculated output energies of the two lasers as a function of the output mirror reflectivity of the 1319 nm laser, R2 . Although the interval between two Q-switch trigger pulses ΔtQ was roughly chosen so as to avoid a temporal overlap between the two pulses, it is not important because this parameter does not affect the extracted pulse energies. The solid and dashed curves in Fig. 3 represent the results of the numerical simulations using the models with and without the effects associated with the lower-laserlevel lifetimes, respectively. The output energy of the 1319 nm laser calculated by the model that considers the finite lower-laser-level lifetime is the same as the results shown in Fig. 2(a). Because the negligible 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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8
2.5 φ1(1064 nm)
2
25
τ1 = τ2 = 0 ns
4 φ2(1319 nm)
1
Nu
2
0.5
15
0 10 5
50
60
70
80
90
100
Fig. 3. Pulse energies of the dual-wavelength laser calculated using the model with (solid curves) and without (dashed curves) considering the effects of the lower-laser-level lifetimes τ1 and τ2 as a function of the output mirror reflectivity of the first pulse laser operating at 1319 nm. The output mirror reflectivity of the second pulse laser operating at 1064 nm is fixed at 30%.
1.5
(b)
Nu
2
τ1 = 11 ns, τ2 = 80 ns
1319 nm
Output mirror reflectivity, R2 (%)
population of the lower laser level brings about a reduced residual population of the upper laser level after Q-switched pulse lasing, the output energy calculated with the assumption of a negligible lowerlaser-level lifetime is higher than that which accounts for the population in the lower laser level, thereby increasing the energy extraction efficiency in the former case. As a result, the stored energy available for lasing at 1064 nm is reduced, and the second pulse at 1064 nm can be obtained only with a lower reflectivity of the output mirror for the 1319 nm laser, which increases the residual population of the upper laser level due to a higher threshold. A remarkable difference in the output energy between the two models appears when the pulse width, as shown in Fig. 2(b), is comparable to or shorter than the lower-laser-level lifetime. These results indicate that the rate equation model proposed here is required for an accurate analysis of the dual-wavelength Nd:YAG laser with two-step energy extraction having pulse widths of the order of less than or equal to several tens of nanoseconds. The other important parameter in this lasing technique is the ΔtQ value. Figure 4 shows the numerical simulation results with respect to the laser photon densities ϕ1 and ϕ2 in the cavities and the population density in the upper laser manifold N u for different ΔtQ values as a function of time. Note that N u is not equal to the population inversion density. The horizontal axis represents the time after which the pump pulse ends. The output mirror reflectivities of the 1064 and 1319 nm lasers are set to 30% and 70%, respectively, which allow first- and second-pulse generations far above the threshold, as shown in Fig. 3. In Figs. 4(a)–4(c), the two Q-switched pulses were simultaneously generated with a partial overlap, and the photon density for the 1064 nm laser decreased as the value of ΔtQ increased. Under this operation condition, the ratio of the photon densities of the two pulses can be changed by adjusting the ΔtQ APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
0 6
φ1(1064 nm) φ2(1319nm)
4
1 2
0.5 0
(c)
Nu
2
4
φ1(1064 nm)
1
2
0.5
(d)
Nu
2
0 6
φ2(1319 nm)
1.5
4 φ1(1064 nm)
1
2
0.5 200
0 6
φ2(1319 nm)
1.5
0
3036
6
1.5
20
0 40
(a)
Nu (× 1018 cm−3)
R1 = 30%
1064 nm
φ1, φ2 (× 1016 cm−3)
Q-switched pulse energy (mJ)
30
400
600
0 800
Time (ns) Fig. 4. Simulation results of the population density of the upper laser level, N u , and the photon densities ϕ1 and ϕ2 for ΔtQ (a) 40 ns, (b) 60 ns, (c) 80 ns, and (d) 160 ns as a function of time. The output mirror reflectivities of the 1064 and 1319 nm lasers are 30% and 70%, respectively.
value; however, a small change in ΔtQ gives rise to a significant change in the ratio of the two photon densities. Q-switched lasers generally demonstrate pulse timing jitters of the order of several nanoseconds, so that this property of the simultaneous lasing regime leads to instability in the pulse energies, which is the primary cause of line competition between the two laser transitions in dual-wavelength lasers. On the other hand, the stored energy can be extracted with the two-step process by increasing ΔtQ so as to avoid a temporal overlap between the two pulses in the presence of a residual population inversion, as shown in Fig. 4(d). The first pulse is generated in single-emission-line operation and the second pulse is obtained using the residual stored energy. The pulse energies of the two lasers are almost independent of the ΔtQ value under the condition of two-step lasing. The reason for this behavior is that the residual population in the upper laser level remains almost constant until the buildup of the second pulse, because the pulse time interval is significantly shorter than the lifetime of the upper laser level. It is also shown that the pulse energies in this lasing technique are insensitive to the pulse timing jitter. Consequently, the pulse-to-pulse stability of output energies in the two-step energy extraction
technique can be expected to be significantly better than that of dual-wavelength lasers operating under conditions of line competition.
Figure 5 shows the cavity configuration of the dualwavelength Q-switched Nd:YAG laser used in this study. While the two laser cavities for both wavelengths shared the pump head, a Q-switch, a polarizer, a rear mirror (M1 or M2), and an output mirror (M6 or M7) for each wavelength were placed in their respective cavity arms. The optical axes of the two lasers were separated using three dichroic mirrors (M3, M4, and M5); the mirrors had a high-transmission coating (>99.8%) for 1064 nm and a high-reflection coating (>99.9%) for 1319 nm. Although it is preferable to use a shared dichroic output mirror with optimum reflectivities for each wavelength, two individual output mirrors were utilized in our experiment, since the preferred dichroic mirror was not available to us. The reflectivity of the output mirror for the 1064 nm laser (M6) was fixed at 30% and that for the 1319 nm laser (M7) was fixed at 55% or 70%. The cavity lengths for the 1064 and 1319 nm lasers were 530 mm and 650 mm, respectively. Although the cavity configuration for the low-gain laser should be simple to minimize cavity loss, the cavity for the 1064 nm laser is simpler than that for the 1319 nm laser in our system. This is because the second pulse laser in the two-step energy extraction should be designed to minimize cavity loss, resulting in maximum total extractableenergy efficiency. If the cavity configurations for two wavelengths are exchanged, the pulse energy at 1319 nm may be slightly increased, whereas the total extracted energy may be decreased, compared with this setup. Nd:YAG has two closely spaced laser transitions at 1319 and 1338 nm, which have almost the same stimulated-emission cross sections, 8.7 × 10−20 cm2 and 9.2 × 10−20 cm2 , respectively [21]. In order to prevent line competition between the two transitions, the rear mirror for the 1319 nm laser (M2) had a high-transmission coating for 1338 nm in addition to the high-reflection coating for 1319 nm. This configuration is essentially equivalent to a conventional Y-shaped cavity. The pump head consists of a
Output 1319 nm
Nd:YAG rod EO Q-switch
Polarizer
M7 Output 1064 nm
M3
M5 M4 Polarizer M2
EO Q-switch
t 140 μs Population inversion
3. Experimental Setup
M1
Pumping
M6
Light guide Laser diode
Fig. 5. Cavity configuration of the dual-wavelength Q-switched Nd:YAG laser. M1 and M2, high-reflection mirrors for the 1064 nm and 1319 nm lasers, respectively; M3, M4, and M5, dichroic mirrors; M6 and M7, output mirrors for the 1064 nm and 1319 nm lasers, respectively.
ΔtQ
Q-switch trigger (1319 nm) Q-switch trigger (1064 nm)
t
t tQ2 = 148 150 μs t tQ1 = 150 μs
Q-switch loss (1319 nm)
t
Q-switch loss (1064 nm)
t
Laser (1319 nm)
t
Laser (1064 nm) t Time
Fig. 6. Timing chart for pump pulses: Q-switch trigger pulses, Q-switch losses, and Q-switched laser pulses.
laser rod, copper heat sinks, three light guides, and six quasi-continuous-wave laser diodes (LDs). Three sets of two LDs were arranged 120° apart around the laser rod. Each LD was capable of operating with 1 kW peak power at a center wavelength of 805 nm. The pulse repetition frequency was 10 Hz. The pump light emitted from the LDs was concentrated by a 50 mm long light guide and was delivered to the lateral surface of the laser rod. The Nd:YAG rod (diameter, 3 mm; length, 15 mm) was doped with 0.6% Nd. The end faces of the laser rod were antireflectioncoated for 1064 and 1319 nm. Potassium dideuterium phosphate and beta-barium borate electro-optic Qswitches were used as the Q-switch elements for the 1064 nm and 1319 nm lasers, respectively. Figure 6 shows a timing chart for the dualwavelength laser experiments in Q-switched mode. The pump pulse and two Q-switch trigger pulses were generated using a digital delay generator (Stanford Research Systems Inc., DG535). The nominal timing jitter of this pulse generator was less than 100 ps. The pump pulse length was set to 140 μs and the Q-switch for the 1064 nm laser was opened at 150 μs after the initiation of the pump pulse. The Q-switch trigger time for the 1319 nm laser was adjusted to between 148 and 150 μs, which correspond to ΔtQ values ranging from 0 to 2 μs. The two Q-switched pulses for the 1064 and 1319 nm lasers were simultaneously monitored on a 350 MHz digital oscilloscope (Instek GDS-3354) with Si and InGaAs photodiodes, respectively. 4. Results and Discussion
The performance of the dual-wavelength laser using two-step energy extraction is characterized by the 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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1064 nm (R = 30%) 1319 nm (R = 70%) 1319 nm (R = 55%)
20 15 10
Q-switched pulse energy (mJ)
1319 nm
15 10 5 0
0
300
600
900
1200
1500
1800
Dual-wavelength laser experiments were performed using the output mirror sets that were used for single-emission-line operation. The Q-switched pulse energies and pulse widths as a function of ΔtQ, for dual-wavelength operation using an output mirror with 30% reflectivity at 1064 nm and 55% reflectivity at 1319 nm, are shown in Fig. 9. Under these conditions, the threshold pump energy for the 1319 nm laser was approximately 2.5 times higher than that for the 1064 nm laser, as shown in Fig. 7. When ΔtQ was greater than 1100 ns, a 4.2 mJ first pulse at 1319 nm, which is the same energy as in single-emission-line operation, and a 17.7 mJ second pulse at 1064 nm were observed. The sum of both pulse energies was 21.9 mJ, which is slightly lower than the expected extractable energy, namely, the maximum pulse energy of the 1064 nm laser shown in Fig. 7. The reason for this discrepancy can be explained by the difference in quantum defects between the two lasing wavelengths. The quantum defect is 24% when the laser
25
Simultaneous
250
Two steps
20
200
15
Energy (1064 nm, exp.) Energy (1319 nm, exp.) Energy (1064 nm, model) Energy (1319 nm, model) Pulse width (1064 nm, exp.) Pulse width (1319 nm, exp.)
10
5
150
100
50
1000
1200
0 1400
ΔtQ (ns) 0
0
200
400
600
800
Pump energy (mJ) Fig. 7. Q-switched pulse energies of the Nd:YAG laser operating at each single emission line as a function of pump energy. 3038
20
Fig. 8. Pulse-to-pulse fluctuations of Q-switched pulse energies under single-emission-line operations. The reflectivities of the output mirrors for the 1064 and 1319 nm lasers are 30% and 70%, respectively.
0 800
5
1064 nm
Pulse width (ns)
Q-switched pulse energy (mJ)
25
25
Shots
Q-switched pulse energy (mJ)
pump threshold and the maximum output energies for operations at a single emission line. The Q-switched pulse energies of the two lasers operating at a single emission line as a function of the pump energy, for different output mirrors, are indicated in Fig. 7. The lines represent least-square fits to the experimental data. Although the pump LDs could produce total pump energy of 1.2 J, the pump energy was set to 603 mJ, with pump power of 4307 W and a pulse length of 140 μs; this is because ASE occurs at higher pumping levels. The roll-off in output energy observed in Fig. 7 can be explained by ASE. The 1064 nm laser pulse, i.e., the second pulse, has maximum energy of 22.8 mJ, which is expected to be the total extractable energy in dual-wavelength operation under two-step energy extraction. The pulse width at this maximum energy was 16 ns. On the other hand, output energies of the 1319 nm laser determine the first pulse energy. Furthermore, compared with the 1064 nm laser, a higher threshold is required for the 1319 nm laser to leave a residual population in the upper laser level to generate the second pulse. Maximum output energies of 4.2 and 12.8 mJ were obtained with output mirror reflectivities of 55% and 70%, respectively. The pulse widths for the 1319 nm laser were longer than 70 ns, even at the highest pumping level, due to the small stimulated-emission cross section. The threshold pump energies for the 1319 nm laser were more than 1.4 times higher than those of the 1064 nm laser. Accordingly, we expected to achieve two-step energy extraction using both the output mirrors that had been used for the 1319 nm laser. The pulse-topulse fluctuations of pulse energies for the 1064 nm laser with a 30% reflectivity output mirror and the 1319 nm laser with a 70% reflectivity output mirror are shown in Fig. 8. The data represent the energy stability in Q-switched operation without line competition between the two wavelengths. The root-meansquare (rms) fluctuations of pulse energies for the 1064 and 1319 nm lasers were 1.5% and 4.7%, respectively; moreover, these values correspond to the minimum achievable fluctuations in dual-wavelength operation.
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Fig. 9. Q-switched pulse energies in dual-wavelength operations under two-step energy extraction as a function of the interval between the two Q-switch trigger pulses, ΔtQ . The reflectivities of the output mirrors for the 1064 and 1319 nm lasers are 30% and 55%, respectively.
operates only at 1064 nm, whereas it increased to 27% in this case because 19% of the total pulse energy was extracted from the 1319 nm laser with a quantum defect of 39%. When the increment of the quantum defect is taken into account, the maximum extractable energy from the dual-wavelength laser decreases from 22.8 to 21.9 mJ, which agrees with the observed total energy. This indicates that the residual energy in the upper laser manifold, after the first pulse, can be efficiently extracted for the second pulse. The solid and dashed curves in Fig. 9 represent the numerical calculation results using the model described in Section 2. The calculated results are in reasonable agreement with the experimental results. In the two-step energy extraction regime, the pulse widths of the 1319 and 1064 nm lasers were 197 ns and 21 ns, respectively, and both the pulse energy and the pulse width were independent of the ΔtQ value. Conversely, the ratio of the two pulse energies was changed by varying ΔtQ in the simultaneous energy extraction regime, because a decrease in ΔtQ increased the fraction of the energy extracted by the 1064 nm laser pulse. Although the pulse width lengthens with decreasing pulse energy in a typical Q-switched laser, both the pulse width and the pulse energy of the 1319 nm laser decreased simultaneously as ΔtQ decreased from 1000 to 850 ns. The reason for this behavior is that the dominant 1064 nm laser pulse, which has a pulse width of approximately 20 ns, depletes the population in the upper laser level during the relatively slow stimulated emission process at 1319 nm. The pulse-to-pulse fluctuations for ΔtQ 1400 ns are shown in Fig. 10. When the laser was operated without a temporal overlap between the two pulses 25 20
1064 nm + 1319 nm
(ΔtQ > 1000 ns), the ΔtQ value was arbitrarily chosen to minimize the fluctuations while maintaining a constant ratio of the 1319–1064 nm pulse energies (0.24 in this case). Furthermore, high stability, which is comparable to that of single-emission-line operation, was achieved by avoiding line competition. As a result, rms fluctuations of 1.5% and 4.8% were achieved with the 1064 nm and 1319 nm lasers, respectively, whereas rms fluctuations of 9%–31% for the 1319 nm laser were observed in the simultaneous lasing region (ΔtQ 800–1000 ns). As shown in Fig. 3, the ratio of the pulse energies for the two wavelengths can be selected by changing the output mirror reflectivity for the 1319 nm laser even under the fixed output mirror reflectivity for the 1064 nm laser. Therefore, we demonstrated dual-wavelength lasing using the two-step energy extraction technique with a different output mirror reflectivity for the 1319 nm laser. In Fig. 11, the Q-switched pulse energies and pulse widths as a function of ΔtQ for the dual-wavelength laser, using a 30% reflectivity output mirror for the 1064 nm laser and a 70% reflectivity output mirror for the 1319 nm laser, are shown. The other conditions were the same as those used for the results shown in Fig. 9. In this study, the reflectivity of the output mirror for the 1064 nm laser was fixed at 30%. However, the optimum reflectivity for operations at the single emission line was calculated to be 70%, as shown in Fig. 2(a). This indicates that the use of the output mirror with a reflectivity of 70% makes the secondpulse energy in the two-step extraction regime to be higher, resulting in the higher extractable total energy. Nevertheless, we had to use a 30% reflectivity mirror because optical damage often occurred on the dichroic mirrors (M3, M4, and M5), which would have a low damage threshold, even at this reflectivity. When the output mirror reflectivity for the 1319 nm laser is 70%, the calculated output energies
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Fig. 11. Q-switched pulse energies in dual-wavelength operations under two-step energy extraction as a function of the interval between the two Q-switch trigger pulses, ΔtQ . The reflectivities of the output mirrors for the 1064 and 1319 nm lasers are 30% and 70%, respectively. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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of the 1064 nm laser with 30%, 50%, and 70% reflectivity output mirrors in the two-step extraction regime are 6.5 mJ, 10.1 mJ, and 11.5 mJ, respectively. Therefore, further improvements in extractable total energy are possible if no damage occurs. Typical pulse shapes of the two lasers for ΔtQ 60, 90, and 140 ns are shown in Figs. 12(a)–12(c), respectively. For ΔtQ < 110 ns, the two Q-switched pulses were generated simultaneously, with a partial temporal overlap [see Figs. 12(a), 12(b)]. In this operational regime, the output energy for the 1064 nm laser decreased with increasing ΔtQ, whereas that for the 1319 nm laser increased. This is because the ratio of the two pulse energies is determined by the temporal positions of both pulses. For example, when ΔtQ is small, the beginning of the 1064 nm laser pulse occurs earlier, resulting in an increased fraction of the energy delivered to that pulse. Conversely, two-step energy extraction occurs when ΔtQ is greater than 110 ns [see Fig. 12(c)]. Within the two-step energy extraction regime, the laser produced output energy
Intensity (a. u.)
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of 12.6 mJ in the first pulse at 1319 nm and output energy of 6.7 mJ in the second pulse at 1064 nm, which corresponds to an energy ratio of 1.9. The observed pulse energies in this region were somewhat higher than those predicted by the model (solid curve in Fig. 11), because the observed pulse width of 96 ns is longer than the calculated value of 64 ns, which reduces the effect of the lower-laser-level lifetime on the pulse energy. In Figs. 13(a)–13(c), the pulse-to-pulse fluctuations of pulse energies for ΔtQ 60 ns, 90 ns, and 140 ns, respectively, are shown. Significantly large fluctuations were observed under simultaneous energy extraction due to line competition between the two laser transitions, whereas the total energy of the two lasers was relatively stable [see Figs. 13(a), 13(b)]. In our previous study, it was found that the pulse-topulse fluctuations due to line competition in simultaneous energy extraction are strongly related to the timing jitters of the two Q-switched pulses [27]. Typical timing jitters for single-emission-line operations were measured to be 5–10 ns for both lasers. Although the corresponding increase and decrease in output energies for the 1319 and 1064 nm lasers were almost equal in magnitude for ΔtQ 60 ns, the observed changes appear to be asymmetrical for ΔtQ 90 ns, as shown in the inset in Fig. 13(b). This results from the difference of temporal positions between the partially overlapped pulses. When ΔtQ 90 ns, a large fraction of the output energy of the 1319 nm laser is extracted before the beginning of the 1064 nm laser pulse. Therefore, timing jitters that further decrease the temporal overlap between the two pulses have a smaller influence on the fluctuations compared with jitters that increase the overlap. Conversely, the magnitude of the fluctuations was independent of the direction of the jitter when ΔtQ 60 ns; this is because the temporal positions of the two pulses were nearly equivalent. In contrast, stable operations were achieved for two-step energy extraction, and rms fluctuations of 4.7% and 2.6% were observed for the 1319 nm and 1064 nm lasers, respectively. However, the rms value for the 1064 nm laser was 1.7 times larger than that for single-emission-line operation. This results from the relatively unstable pulse energy of the 1319 nm laser; that is, the initial population density of the upper laser level for the 1064 nm laser is affected by fluctuations of the 1319 nm laser, which are dominant in this case. The rms fluctuation of the 1319 nm laser energy is the same as that for singleemission-line operation, because the first pulse is not affected by the second pulse in the absence of a temporal overlap. In Fig. 14, the rms fluctuations of the two lasers as a function of ΔtQ are shown. When both pulse energies are comparable, almost the same rms fluctuation is obtained at both wavelengths, with both exceeding 21% for ΔtQ 90 ns. As ΔtQ decreases from 90 to 40 ns, the pulse-to-pulse stability of the 1064 nm laser improves to 5.3%, whereas the stability of the 1319 nm laser deteriorates significantly. This is because a slight change in pulse energy at the dominant wavelength causes a significant change for the other
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Fig. 13. Pulse-to-pulse fluctuations of Q-switched pulse energies for different values of ΔtQ : (a) 60 ns, (b) 90 ns, and (c) 140 ns. The conditions are the same as those under which the results illustrated in Fig. 11 were obtained. The inset in (b) shows Q-switched pulse energies from 900 to 1000 shots.
50 Simultaneous Two steps
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laser. As a result, the minimum fluctuations for both lasers could be obtained only for two-step energy extraction. In this experiment, the interval between the two Q-switch trigger pulses should be more than 120 ns for stable operation. Although the interval between the peaks of the two laser pulses was then more than a few hundreds of nanoseconds, an interval less than 1 μs is negligible for the observation of the NDVI. 5. Conclusions
We have demonstrated stable dual-wavelength pulse generation by using a two-step energy extraction technique from an actively Q-switched Nd:YAG laser operating with high energies at wavelengths of 1064 and 1319 nm. Numerical simulations of the temporal behavior of the dual-wavelength Q-switched pulses were performed using a rate equation model considering the finite lower-laser-level lifetime. The
simulation results show that the first and second pulses can be successively extracted by introducing a higher threshold in the first pulse laser compared with the second pulse. In laser experiments, the first and second pulses were extracted at 1319 nm and 1064 nm, respectively, using the two-step energy extraction technique, as described in Section 2. Total energies of the order of 20 mJ of the two pulses were achieved for two combinations of output mirrors. While a ratio of 1.9 was demonstrated for the 1319–1064 nm pulse energies, a higher ratio might be required for the observation of the NDVI since the 660 nm laser pulse is strongly absorbed by chlorophyll. This can be achieved by increasing the reflectivity of the output mirror at 1319 nm. The pulse-to-pulse fluctuations of output energies for the 1319 and 1064 nm lasers were 4.7%–4.8% and 1.5%–2.6%, respectively, which were significantly smaller than those obtained with simultaneous energy extraction, and were nearly equivalent to those obtained for a single emission line in our system. The observed behavior of the dual-wavelength Qswitched pulses was in reasonably good agreement with the theoretical predictions. In the experiments described here, the Q-switched pulse widths of the 1319 nm laser were 2–10 times longer than those of the 1064 nm laser, depending on the combination of the output couplers used. Since the cavity lengths can be arbitrarily selected in this method, dual-wavelength Q-switched operation with the same pulse width and with shorter pulse widths can be achieved by adjusting the length of each cavity. References 1. P. J. Sellers, “Canopy reflectance, photosynthesis, and transpiration,” Int. J. Remote Sens. 6, 1335–1372 (1985). 2. National Oceanic, and Atmospheric Administration, “NOAA KLM User’s Guide (April 2007 revision),” http://www.ncdc .noaa.gov/oa/pod‑guide/ncdc/docs/klm/index.htm. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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3. Land Processes Distributed Active Archive Center, “MODIS Overview,” https://lpdaac.usgs.gov/products/modis_products_ table/modis_overview. 4. “AVNIR/ALOS Overview,” http://www.jaxa.jp/projects/sat/ alos/. 5. K. Asai, A. Sato, T. Terasaki, and N. Sugimoto, “Lidar for observation of vegetation environment from International Space Station (ISS)/Japanese Experiment Module (JEM)Exposed Facility (EF),” Proc. SPIE 7860, 78600G (2010). 6. B. E. Schutz, H. J. Zwally, C. A. Shuman, D. Hancock, and J. P. DiMarzio, “Overview of the ICESat mission,” Geophys. Res. Lett. 32, L21S01 (2005). 7. W. Lin and H. Shen, “A configuration of the laser cavity for simultaneous dual wavelength Q-switch pulsed Nd:YAlO3 laser,” J. Appl. Phys. 86, 2979–2983 (1999). 8. Y. Chen and S. W. Tsai, “Diode-pumped Q-switched Nd:YVO4 yellow laser with intracavity sum-frequency mixing,” Opt. Lett. 27, 397–399 (2002). 9. B. M. Walsh and N. P. Barnes, “Line tunable ultraviolet laser,” in Advanced Solid-State Photonics, Technical Digest (Optical Society of America, 2005), paper TuB16. 10. H. Y. Shen, W. X. Lin, R. R. Zeng, Y. P. Zhou, G. F. Yu, C. H. Huang, Z. D. Zeng, W. J. Zhang, R. F. Wu, and Q. J. Ye, “1079.5and 1341.4-nm: larger energy from a dual-wavelength Nd:YAlO3 pulsed laser,” Appl. Opt. 32, 5952–5957 (1993). 11. J. L. He, J. Du, J. Sun, S. Liu, Y. X. Fan, H. T. Wang, L. H. Zhang, and Y. Hang, “High efficiency single- and dualwavelength Nd:GdVO4 lasers pumped by a fiber-coupled diode,” Appl. Phys. B 79, 301–304 (2004). 12. L. Zhang, Z. Wei, B. Feng, D. Li, and Z. Zhang, “Simultaneous dual-wavelength Q-switched Nd:YAG laser operating at 1.06 μm and 946 nm,” Opt. Commun. 264, 51–54 (2006). 13. H. Kisov, M. Deneva, and M. Nenchev, “Development of a tunable, competition less flash-lamp pumped Nd:YAG laser generated of a chosen pair of two lines,” Proc. SPIE 8770, 87701Q (2013). 14. X. Fan, J. He, H. Huang, and L. Xue, “An intermittent oscillation dual-wavelength diode-pumped Nd:YAG laser,” IEEE J. Quantum Electron. 43, 884–888 (2007). 15. H. T. Huang, J. L. He, B. T. Zhang, K. J. Yang, C. H. Zuo, J. L. Xu, X. L. Dong, and S. Zhao, “Intermittent oscillation of
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16. 17. 18.
19. 20. 21.
22. 23. 24. 25. 26.
27.
1064 nm and 1342 nm obtained in a diode-pumped doubly passively Q-switched Nd:YVO4 laser,” Appl. Phys. B 96, 815–820 (2009). H. Su, H. Y. Shen, W. X. Lin, R. R. Zeng, C. H. Huang, and G. Zhang, “Computational model of Q-switch Nd:YAlO3 dual-wavelength laser,” J. Appl. Phys. 84, 6519–6522 (1998). A. Sato, S. Okubo, K. Asai, S. Ishii, and K. Mizutani, “Energy extraction in dual-wavelength Q-switched laser with a common upper laser level,” Appl. Phys. B 117, 621–631 (2014). W. Lin, S. Lin, and J. Huang, “Cavity configuration for a Q-switched simultaneous dual-wavelength Nd:YAlO3 laser with the characteristic of polarization-dependent,” J. Opt. Soc. Am. B 20, 479–483 (2003). G. A. Henderson, “A computational model of a dualwavelength solid-state laser,” J. Appl. Phys. 68, 5451–5455 (1990). W. Koechner, Solid-State Laser Engineering, 6th ed. (SpringerVerlag, 1999). S. Singh, R. G. Smith, and L. G. Van Uitert, “Stimulatedemission cross section and fluorescent quantum efficiency of Nd3+ in yttrium aluminum garnet at room temperature,” Phys. Rev. B 10, 2566–2572 (1974). D. Hua, Z. Song, S. Wang, and Z. Rong, “Study on YAG:Nd3+ spectra. I. Absorption spectra,” J. Chem. Phys. 89, 5398–5403 (1988). T. Y. Fan, “Effect of finite lower level lifetime on Q-switched lasers,” IEEE J. Quantum Electron. 24, 2345–2349 (1988). F. E. Hovis, M. Stuff, C. J. Kennedy, and B. Vivian, “Lower level relaxation of Nd:YAG,” IEEE J. Quantum Electron. 28, 39–42 (1992). J. J. Degnan, D. B. Coyle, and R. B. Kay, “Effects of thermalization on Q-switched laser properties,” IEEE J. Quantum Electron. 34, 887–899 (1998). S. P. Ng, D. Y. Tang, L. J. Qian, and L. J. Qin, “Satellite pulse generation in diode-pumped passively Q-switched Nd:GdVO4 lasers,” IEEE J. Quantum Electron. 42, 625–632 (2006). A. Sato, T. Abe, S. Okubo, K. Asai, N. Sugimoto, S. Ishii, and K. Mizutani, “Development of a simultaneous dual-wavelength Q-switched Nd:YAG laser at 1064 and 1319 nm,” Proc. SPIE 8526, 852606 (2012).
Tohoku Institute of Technology, 35-1 Yagiyama-Kasumi-cho, Taihaku-ku, Sendai 982-8577, Japan
National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei-shi, Tokyo 184-8795, Japan 3
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan *Corresponding author: [email protected] Received 22 December 2014; revised 2 March 2015; accepted 7 March 2015; posted 9 March 2015 (Doc. ID 231266); published 31 March 2015
The generation of stable dual-wavelength pulses from an actively Q-switched Nd:YAG laser operating at 1064 and 1319 nm was demonstrated. Pulse energies of the dual-wavelength laser were extracted by two temporally separated stimulated-emission processes for a single pumping process, thereby avoiding line competition between the two laser transitions. Total energy of the order of 20 mJ was achieved for the two pulses, and the ratio of the pulse energies of the two lasers could be selected by adjusting the output couplings. The pulse-to-pulse fluctuations for the lasers operating at 1319 and 1064 nm were 4.7%– 4.8% and 1.5%–2.6%, respectively, which were almost equivalent to those for a single emission line in our system. The experimentally observed laser performance agreed reasonably well with theoretical predictions. © 2015 Optical Society of America OCIS codes: (140.3425) Laser stabilization; (140.3540) Lasers, Q-switched; (140.3410) Laser resonators. http://dx.doi.org/10.1364/AO.54.003032
1. Introduction
Almost every aspect of our lives depends on plants, trees, grasses, and other forms of vegetation because they absorb carbon dioxide and provide us with oxygen. Therefore, it is essential to monitor the spatial distribution of vegetation biomass and its variations over time, which represents invaluable information to improve present assessments and future projections of the terrestrial carbon cycle. The normalized difference vegetation index (NDVI) [1] is used to provide a measure of green vegetation, monitor changes in vegetation levels, and understand how these quantities affect the environment, using mostly data taken by satellite-based imaging spectroradiometers, e.g., AVHRR/NOAA [2], MODIS/ 1559-128X/15/103032-11$15.00/0 © 2015 Optical Society of America 3032
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Terra, Aqua [3], and AVNIR/ALOS [4]. The NDVI can be calculated from spectral reflectance data of vegetation as follows: NDVI
RNIR − RRED ; RNIR RRED
(1)
where RNIR and RRED are spectral reflectances in the near-infrared and red bands, respectively. In the abovementioned passive remote sensing, the spectral regions from 580–690 and 725–1000 nm are used as the red and near-infrared bands, respectively [2–4]. The second-harmonic generation of the 1319 nm transition in Nd:YAG lasers provides red light at 660 nm, which coincides with the absorption band of chlorophyll. Conversely, there is no absorption of chlorophyll and water at 1064 nm, which is the typical lasing wavelength of Nd:YAG. Since these wavelengths correspond to the red and near-infrared
bands, a combination of 660 nm second-harmonic and 1064 nm fundamental outputs from Nd:YAG lasers permits the active remote sensing of the NDVI. Although a lasing wavelength of 1064 nm is slightly longer than 1000 nm, the reflectance of vegetation is expected to be equivalent to that for the near-infrared bands in the passive remote sensing because these wavelengths are far enough from the absorption peak of water around 1.4 μm. Even in the abovementioned passive remote sensing, the nearinfrared wavelength ranges are different [2–4]. If a Q-switched laser is used as a dual-wavelength laser transmitter, we can measure not only range-resolved NDVI values, but also the canopy height. In this case, the 1064 nm laser pulse is suitable for measuring the canopy height because of its high reflectance compared with the 660 nm pulse. Consequently, a novel vegetation lidar for the simultaneous observation of vegetation height and NDVI can be constructed by using both a dual-wavelength laser transmitter and an imaging detection system with a 2D array detector for collecting information regarding vegetation biomass [5]. Air- and spaceborne lidar systems are suitable for this application because they can collect information regarding vegetation over an extensive area by moving the footprint of the transmitted laser pulse over the ground surface. In such a lidar system, a high-energy laser transmitter is needed. In earlier studies, we proposed a spaceborne system with pulse energy of 100 mJ and a pulse width shorter than 30 ns at each wavelength for the observation of both the NDVI and the canopy height from space [5]. In addition to pulse energy requirements, a good spatial overlap between the two footprints of the dual-wavelength laser pulses is required to determine an accurate NDVI value; therefore, simultaneous lasing of the two laser pulses is essential. While dual-wavelength pulses with perfect temporal overlap are ideal, temporally separated dual-wavelength pulses can also be regarded as simultaneously irradiated laser pulses when the time interval between two pulses is negligible. For example, if the lidar platform is a satellite with a velocity of approximately 7 km s−1 , a time interval of 1 μs between two pulses corresponds to an approximate 7 mm displacement in the footprint. In this case, the difference in the position between the two footprints is negligible because it is four orders of magnitude smaller than the footprint size of spaceborne lidars, which is typically several tens of meters [6]. Temporally separated dual-wavelength pulses with a time interval shorter than 1 μs, rather than simultaneously generated dual-wavelength pulses, are well suited to our objective because the reflected lidar signals at 660 and 1064 nm can be successively acquired in a singlechannel detector, facilitating a simple signal-processing system. Dual-wavelength lasers that use a single laser crystal are more cost-effective and compact than a system comprising two lasers operating at different
wavelengths. Dual-wavelength Q-switched pulses are conventionally produced by adjusting the output coupling and cavity length of two lasers so as to compensate for the difference in the stimulatedemission cross section between the two laser transitions [7–9]. Although dual-wavelength lasing can be simultaneously achieved by conventional methods, line competition occurs between the two laser transitions, which can result in considerably unstable pulse energies [10–12]. To solve this problem, the two laser pulses can be either spatially or temporally separated. Spatially independent lasing at two wavelengths can be achieved by using different regions located side by side within the laser crystal as the pumped regions for each wavelength [13]. However, it is difficult to obtain an appropriate overlap between the pumped region and the cavity mode by this method because the cross sections of the two cavity modes are not circular. Conversely, intermittent oscillation of dual-wavelength pulses permits temporally separated lasing, which can avoid line competition and provide an appropriate overlap between the pump and cavity mode volumes. In an earlier study that considered this lasing method, the time interval between the first and second pulses was of the order of tens of microseconds due to the pumping time for the second pulse after the first pulse lasing [14]. In further investigation, Huang et al. pointed out the contribution made to the second pulse by the residual population inversion after extracting the first pulse and demonstrated the generation of dual-wavelength pulses for a single pumping process in a passively Q-switched Nd:YVO4 laser under high-repetition-rate operation at a relatively low output energy level [15]. However, highenergy operations have not been demonstrated, and the validity of two-step energy extraction for the stabilization of pulse energies in dual-wavelength lasers has not been evaluated quantitatively in previous studies. In this paper, we demonstrate an actively Q-switched Nd:YAG laser producing stable highenergy dual-wavelength pulses at 1064 and 1319 nm using a two-step energy extraction technique. The method for designing a dual-wavelength laser with two-step energy extraction was developed using a rate equation model. Furthermore, the pulse-to-pulse stability of the output energies under this lasing method was compared with the stability observed during simultaneous energy extraction in the presence of line competition. 2. Laser Design
In conventional dual-wavelength lasers, key parameters of laser design are the reflectivities of the output mirrors and the cavity lengths for the two lasers, because the same threshold cannot be achieved for the two lasers if a difference in the stimulatedemission cross section between the two laser transitions is not canceled by a specific cavity design [7,16]. On the other hand, for a two-step energy extraction technique, the pulse energies do not directly depend 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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on the two cavity lengths, while the output mirror reflectivities set the pump threshold levels, and thereby, provide a ratio of the pulse energies of the two lasers. In addition, detailed calculations with respect to the population inversion density are required for the design of a laser using this lasing technique, because the population inversion density after the first pulse lasing is related to the initial population density of the second pulse, and they are not the same when the population densities of the two lower laser levels are different. Therefore, we developed a detailed rate equation model [17], compared with conventional models [4,16,18,19], to investigate the operation condition needed to achieve dual-wavelength lasing under two-step energy extraction. Figure 1 shows the energy-level diagram of Nd in YAG. The lasing wavelengths of interest in this study are 1064 and 1319 nm, which correspond to the 4 F 3∕2 → 4 I 11∕2 and 4 F 3∕2 → 4 I 13∕2 transitions, respectively. The upper laser manifold 4 F 3∕2 consists of two Stark levels; only the R2 level is used as a common upper laser level for both transitions. The 1064 and 1319 nm transitions terminated on the Y 3 and X 1 levels, respectively. In the two-step energy extraction technique, a first Q-switched pulse is generated at either wavelength, and a second Q-switched pulse can be subsequently obtained at the other wavelength by using the residual stored energy in the upper laser level after energy extraction by the first Q-switched pulse. The residual population density of the upper laser level is determined by the initial and final population inversion densities of the first pulse lasing, which is given by the transcendental function [20]. Numerical solutions to this equation show that a small difference between the initial and threshold population inversion densities leads to low energy extraction efficiency, i.e., the residual population in the upper laser level increases with higher threshold population inversion density. When the Q-switched pulse width is comparable to or shorter than the lowerlaser-level lifetime, a population of the lower laser level exists during the pulse evolution, thereby decreasing the population inversion. In the previous 4 4
F5/2 F3/2
Pump 805 nm 4
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R2 R1 Laser 1064 nm (R2→Y3) φ1
Nu
1319 nm (R2→X1) φ2 X7 N2 X1 Y6
dN u N c W p − u − σ 1 ϕ1 f u N u − f 1 N 1 dt τ f nr c − σ 2 ϕ2 f u N u − f 2 N 2 ; nr
(2)
dN 2 β2 N u c N σ 2 ϕ2 f u N u − f 2 N 2 − 2 ; dt τf τ2 nr
(3)
dN 1 β1 N u c N σ 1 ϕ1 f u N u − f 1 N 1 − 1 ; dt τf τ1 nr
(4)
dϕ2 cl ϕ γ β N σ 2 ϕ2 f u N u − f 2 N 2 − 2 2 2 u ; dt τc2 τf nr lc2
(5)
dϕ1 cl ϕ γ β N σ ϕ f N − f 1 N 1 − 1 1 1 u ; dt τc1 τf nr lc1 1 1 u u
(6)
where N u is the population density in the upper laser manifold; N 1 and N 2 are the population densities in the lower laser manifolds; ϕ1 and ϕ2 are the laser photon densities in the laser cavities; W p is the pumping rate; τf is the lifetime of the upper laser manifold; τ1 and τ2 are the lifetimes of the lower laser manifolds; σ 1 and σ 2 are the stimulated-emission cross sections for the two transitions; c is the speed of light in vacuum; nr is the refractive index of the laser medium; f u is the Boltzmann thermal occupation factor of the upper level; f 1 and f 2 are the occupation factors of the lower laser levels; l is the length of the laser rod; lc1 and lc2 are the cavity lengths; γ 1 and γ 2 are the rates at which spontaneous emission contributes to the lasers; and β1 and β2 are the branching ratios. The subscripts 1 and 2 in the equations correspond to the laser transitions at 1064 nm and 1319 nm, respectively. The photon decay times τc1 and τc2 in the laser cavities are given by Eqs. (7) and (8):
N1 Y1
Fig. 1. Energy-level diagram of Nd:YAG. 3034
rate equation models for a dual-wavelength Qswitched laser using a common upper laser level [4,16,18,19], it was assumed that the population inversion density is the same as the population density in the upper laser level. However, the initial and threshold population inversion densities for the first Q-switched pulse significantly affect the residual population for the second Q-switched pulse so that the population densities and lifetimes of the two lower laser levels have to be considered in the model in the case of two-step energy extraction. Equations (2)–(6) represent the coupled rate equations for this model [17]:
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
τc1 −
2lc1 ; c ln1 − Lc1 1 − LQ1 R1
(7)
2lc2 ; c ln1 − Lc2 1 − LQ2 R2
(8)
where Lc1 and Lc2 are the round-trip losses in the laser cavities; R1 and R2 are the reflectivities of the output mirrors; and LQ1 and LQ2 represent the time-dependent losses introduced by the Q-switches, which can be approximated by step functions. For convenience, the time interval between the two Q-switch trigger pulses is defined as ΔtQ tQ1 − tQ2 ;
Q-switched pulse energy (mJ)
τc2 −
30 1319 nm 1064 nm
25 20 15 10 5
(a) 0
(9)
0
20
40
60
80
100
Output mirror reflectivity, R1, R2 (%) 200
Q-switched pulse width (ns)
where tQ1 and tQ2 are the delay times of the Qswitches with respect to the beginning of the pump pulse at which the LQ1 and LQ2 values are switched from a maximum to 0. The positive sign of ΔtQ suggests that the Q-switch for the 1319 nm laser opens before that for the 1064 nm laser. Using this model, we calculated the output mirror reflectivities to achieve dual-wavelength lasing under two-step energy extraction. We assumed that the energies of the first and second pulses are extracted from the 1319 nm and 1064 nm lasers, respectively. The parameters used in the simulation are as follows [21,22]: σ 1 45.8 × 10−20 cm2 , σ 2 8.7 × 10−20 cm2 , τϕ 230 μs, f u 0.40, f 1 0.19, f 2 0.32, l 10 mm, lc1 530 mm, lc2 650 mm, nr 1.82, W p 7 × 1022 cm−3 s−1 , β1 0.135, β2 0.018, Lc1 0.03, and Lc2 0.03. The pump pulse length and pump wavelength were set to 140 μs and 805 nm, respectively. The reason for this shorter pump pulse length, compared with the conventional value of 230 μs, is the limitation due to amplified spontaneous emission (ASE) described in a later section. The diameter of the laser rod was set to 3 mm. The values of γ 1 and γ 2 are given by considering the spontaneous emission within the solid angle at the cavity mirrors. A rough estimation of these values is acceptable, because the last terms in Eqs. (5) and (6), which include these parameters, affect only the Q-switched pulse position in time. The lifetime of the lower laser manifold 4 I 11∕2 , τ1 , has been reported in the literature [23–25], and it was assigned a value of 11 ns in our simulation. However, an accurate value for the lifetime of the lower laser manifold 4 I 13∕2 , τ2 , is unknown. This value was assumed to be 80 ns in our simulation as a result of an observed satellite pulse of the main Q-switched pulse caused by the finite lower-laser-level lifetime [25,26]. Figure 2 shows calculated pulse energies and pulse widths in a single-emission-line operation under the same pumping level as a function of output mirror reflectivity. Because of the smaller stimulated-emission cross section for the 1319 nm transition, higher reflectivity is needed to achieve 1319 nm lasing. By contrast, the 1064 nm laser can produce a higher output energy compared with the 1319 nm laser; for example, output energies higher than 20 mJ can be obtained for output mirror reflectivities ranging from 27% to 92%. Accordingly, the 1064 nm laser is suitable
1319 nm 1064 nm 150
100
50
(b) 0
0
20
40
60
80
100
Output mirror reflectivity, R1, R2 (%) Fig. 2. Calculated (a) pulse energies and (b) pulse widths for single-emission-line operations as a function of output mirror reflectivity.
for the second pulse laser, since higher energy extraction efficiency for the second pulse contributes to higher total efficiency for two-step energy extraction. In addition, the higher threshold for the 1319 nm laser is preferable for the first pulse laser, because moderately low energy extraction efficiency leads to a large residual population inversion density, which provides the initial population inversion density for the second pulse. In our system, therefore, the 1319 and 1064 nm transitions are selected for the first- and second pulse lasings, respectively. From a practical viewpoint, the reflectivity of the output mirror for the 1064 nm laser, R1 , used in our experiments was set to 30%, which provides a reasonable balance between a low threshold and damage-free operation. Figure 3 shows calculated output energies of the two lasers as a function of the output mirror reflectivity of the 1319 nm laser, R2 . Although the interval between two Q-switch trigger pulses ΔtQ was roughly chosen so as to avoid a temporal overlap between the two pulses, it is not important because this parameter does not affect the extracted pulse energies. The solid and dashed curves in Fig. 3 represent the results of the numerical simulations using the models with and without the effects associated with the lower-laserlevel lifetimes, respectively. The output energy of the 1319 nm laser calculated by the model that considers the finite lower-laser-level lifetime is the same as the results shown in Fig. 2(a). Because the negligible 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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8
2.5 φ1(1064 nm)
2
25
τ1 = τ2 = 0 ns
4 φ2(1319 nm)
1
Nu
2
0.5
15
0 10 5
50
60
70
80
90
100
Fig. 3. Pulse energies of the dual-wavelength laser calculated using the model with (solid curves) and without (dashed curves) considering the effects of the lower-laser-level lifetimes τ1 and τ2 as a function of the output mirror reflectivity of the first pulse laser operating at 1319 nm. The output mirror reflectivity of the second pulse laser operating at 1064 nm is fixed at 30%.
1.5
(b)
Nu
2
τ1 = 11 ns, τ2 = 80 ns
1319 nm
Output mirror reflectivity, R2 (%)
population of the lower laser level brings about a reduced residual population of the upper laser level after Q-switched pulse lasing, the output energy calculated with the assumption of a negligible lowerlaser-level lifetime is higher than that which accounts for the population in the lower laser level, thereby increasing the energy extraction efficiency in the former case. As a result, the stored energy available for lasing at 1064 nm is reduced, and the second pulse at 1064 nm can be obtained only with a lower reflectivity of the output mirror for the 1319 nm laser, which increases the residual population of the upper laser level due to a higher threshold. A remarkable difference in the output energy between the two models appears when the pulse width, as shown in Fig. 2(b), is comparable to or shorter than the lower-laser-level lifetime. These results indicate that the rate equation model proposed here is required for an accurate analysis of the dual-wavelength Nd:YAG laser with two-step energy extraction having pulse widths of the order of less than or equal to several tens of nanoseconds. The other important parameter in this lasing technique is the ΔtQ value. Figure 4 shows the numerical simulation results with respect to the laser photon densities ϕ1 and ϕ2 in the cavities and the population density in the upper laser manifold N u for different ΔtQ values as a function of time. Note that N u is not equal to the population inversion density. The horizontal axis represents the time after which the pump pulse ends. The output mirror reflectivities of the 1064 and 1319 nm lasers are set to 30% and 70%, respectively, which allow first- and second-pulse generations far above the threshold, as shown in Fig. 3. In Figs. 4(a)–4(c), the two Q-switched pulses were simultaneously generated with a partial overlap, and the photon density for the 1064 nm laser decreased as the value of ΔtQ increased. Under this operation condition, the ratio of the photon densities of the two pulses can be changed by adjusting the ΔtQ APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
0 6
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4
1 2
0.5 0
(c)
Nu
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4
φ1(1064 nm)
1
2
0.5
(d)
Nu
2
0 6
φ2(1319 nm)
1.5
4 φ1(1064 nm)
1
2
0.5 200
0 6
φ2(1319 nm)
1.5
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3036
6
1.5
20
0 40
(a)
Nu (× 1018 cm−3)
R1 = 30%
1064 nm
φ1, φ2 (× 1016 cm−3)
Q-switched pulse energy (mJ)
30
400
600
0 800
Time (ns) Fig. 4. Simulation results of the population density of the upper laser level, N u , and the photon densities ϕ1 and ϕ2 for ΔtQ (a) 40 ns, (b) 60 ns, (c) 80 ns, and (d) 160 ns as a function of time. The output mirror reflectivities of the 1064 and 1319 nm lasers are 30% and 70%, respectively.
value; however, a small change in ΔtQ gives rise to a significant change in the ratio of the two photon densities. Q-switched lasers generally demonstrate pulse timing jitters of the order of several nanoseconds, so that this property of the simultaneous lasing regime leads to instability in the pulse energies, which is the primary cause of line competition between the two laser transitions in dual-wavelength lasers. On the other hand, the stored energy can be extracted with the two-step process by increasing ΔtQ so as to avoid a temporal overlap between the two pulses in the presence of a residual population inversion, as shown in Fig. 4(d). The first pulse is generated in single-emission-line operation and the second pulse is obtained using the residual stored energy. The pulse energies of the two lasers are almost independent of the ΔtQ value under the condition of two-step lasing. The reason for this behavior is that the residual population in the upper laser level remains almost constant until the buildup of the second pulse, because the pulse time interval is significantly shorter than the lifetime of the upper laser level. It is also shown that the pulse energies in this lasing technique are insensitive to the pulse timing jitter. Consequently, the pulse-to-pulse stability of output energies in the two-step energy extraction
technique can be expected to be significantly better than that of dual-wavelength lasers operating under conditions of line competition.
Figure 5 shows the cavity configuration of the dualwavelength Q-switched Nd:YAG laser used in this study. While the two laser cavities for both wavelengths shared the pump head, a Q-switch, a polarizer, a rear mirror (M1 or M2), and an output mirror (M6 or M7) for each wavelength were placed in their respective cavity arms. The optical axes of the two lasers were separated using three dichroic mirrors (M3, M4, and M5); the mirrors had a high-transmission coating (>99.8%) for 1064 nm and a high-reflection coating (>99.9%) for 1319 nm. Although it is preferable to use a shared dichroic output mirror with optimum reflectivities for each wavelength, two individual output mirrors were utilized in our experiment, since the preferred dichroic mirror was not available to us. The reflectivity of the output mirror for the 1064 nm laser (M6) was fixed at 30% and that for the 1319 nm laser (M7) was fixed at 55% or 70%. The cavity lengths for the 1064 and 1319 nm lasers were 530 mm and 650 mm, respectively. Although the cavity configuration for the low-gain laser should be simple to minimize cavity loss, the cavity for the 1064 nm laser is simpler than that for the 1319 nm laser in our system. This is because the second pulse laser in the two-step energy extraction should be designed to minimize cavity loss, resulting in maximum total extractableenergy efficiency. If the cavity configurations for two wavelengths are exchanged, the pulse energy at 1319 nm may be slightly increased, whereas the total extracted energy may be decreased, compared with this setup. Nd:YAG has two closely spaced laser transitions at 1319 and 1338 nm, which have almost the same stimulated-emission cross sections, 8.7 × 10−20 cm2 and 9.2 × 10−20 cm2 , respectively [21]. In order to prevent line competition between the two transitions, the rear mirror for the 1319 nm laser (M2) had a high-transmission coating for 1338 nm in addition to the high-reflection coating for 1319 nm. This configuration is essentially equivalent to a conventional Y-shaped cavity. The pump head consists of a
Output 1319 nm
Nd:YAG rod EO Q-switch
Polarizer
M7 Output 1064 nm
M3
M5 M4 Polarizer M2
EO Q-switch
t 140 μs Population inversion
3. Experimental Setup
M1
Pumping
M6
Light guide Laser diode
Fig. 5. Cavity configuration of the dual-wavelength Q-switched Nd:YAG laser. M1 and M2, high-reflection mirrors for the 1064 nm and 1319 nm lasers, respectively; M3, M4, and M5, dichroic mirrors; M6 and M7, output mirrors for the 1064 nm and 1319 nm lasers, respectively.
ΔtQ
Q-switch trigger (1319 nm) Q-switch trigger (1064 nm)
t
t tQ2 = 148 150 μs t tQ1 = 150 μs
Q-switch loss (1319 nm)
t
Q-switch loss (1064 nm)
t
Laser (1319 nm)
t
Laser (1064 nm) t Time
Fig. 6. Timing chart for pump pulses: Q-switch trigger pulses, Q-switch losses, and Q-switched laser pulses.
laser rod, copper heat sinks, three light guides, and six quasi-continuous-wave laser diodes (LDs). Three sets of two LDs were arranged 120° apart around the laser rod. Each LD was capable of operating with 1 kW peak power at a center wavelength of 805 nm. The pulse repetition frequency was 10 Hz. The pump light emitted from the LDs was concentrated by a 50 mm long light guide and was delivered to the lateral surface of the laser rod. The Nd:YAG rod (diameter, 3 mm; length, 15 mm) was doped with 0.6% Nd. The end faces of the laser rod were antireflectioncoated for 1064 and 1319 nm. Potassium dideuterium phosphate and beta-barium borate electro-optic Qswitches were used as the Q-switch elements for the 1064 nm and 1319 nm lasers, respectively. Figure 6 shows a timing chart for the dualwavelength laser experiments in Q-switched mode. The pump pulse and two Q-switch trigger pulses were generated using a digital delay generator (Stanford Research Systems Inc., DG535). The nominal timing jitter of this pulse generator was less than 100 ps. The pump pulse length was set to 140 μs and the Q-switch for the 1064 nm laser was opened at 150 μs after the initiation of the pump pulse. The Q-switch trigger time for the 1319 nm laser was adjusted to between 148 and 150 μs, which correspond to ΔtQ values ranging from 0 to 2 μs. The two Q-switched pulses for the 1064 and 1319 nm lasers were simultaneously monitored on a 350 MHz digital oscilloscope (Instek GDS-3354) with Si and InGaAs photodiodes, respectively. 4. Results and Discussion
The performance of the dual-wavelength laser using two-step energy extraction is characterized by the 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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1064 nm (R = 30%) 1319 nm (R = 70%) 1319 nm (R = 55%)
20 15 10
Q-switched pulse energy (mJ)
1319 nm
15 10 5 0
0
300
600
900
1200
1500
1800
Dual-wavelength laser experiments were performed using the output mirror sets that were used for single-emission-line operation. The Q-switched pulse energies and pulse widths as a function of ΔtQ, for dual-wavelength operation using an output mirror with 30% reflectivity at 1064 nm and 55% reflectivity at 1319 nm, are shown in Fig. 9. Under these conditions, the threshold pump energy for the 1319 nm laser was approximately 2.5 times higher than that for the 1064 nm laser, as shown in Fig. 7. When ΔtQ was greater than 1100 ns, a 4.2 mJ first pulse at 1319 nm, which is the same energy as in single-emission-line operation, and a 17.7 mJ second pulse at 1064 nm were observed. The sum of both pulse energies was 21.9 mJ, which is slightly lower than the expected extractable energy, namely, the maximum pulse energy of the 1064 nm laser shown in Fig. 7. The reason for this discrepancy can be explained by the difference in quantum defects between the two lasing wavelengths. The quantum defect is 24% when the laser
25
Simultaneous
250
Two steps
20
200
15
Energy (1064 nm, exp.) Energy (1319 nm, exp.) Energy (1064 nm, model) Energy (1319 nm, model) Pulse width (1064 nm, exp.) Pulse width (1319 nm, exp.)
10
5
150
100
50
1000
1200
0 1400
ΔtQ (ns) 0
0
200
400
600
800
Pump energy (mJ) Fig. 7. Q-switched pulse energies of the Nd:YAG laser operating at each single emission line as a function of pump energy. 3038
20
Fig. 8. Pulse-to-pulse fluctuations of Q-switched pulse energies under single-emission-line operations. The reflectivities of the output mirrors for the 1064 and 1319 nm lasers are 30% and 70%, respectively.
0 800
5
1064 nm
Pulse width (ns)
Q-switched pulse energy (mJ)
25
25
Shots
Q-switched pulse energy (mJ)
pump threshold and the maximum output energies for operations at a single emission line. The Q-switched pulse energies of the two lasers operating at a single emission line as a function of the pump energy, for different output mirrors, are indicated in Fig. 7. The lines represent least-square fits to the experimental data. Although the pump LDs could produce total pump energy of 1.2 J, the pump energy was set to 603 mJ, with pump power of 4307 W and a pulse length of 140 μs; this is because ASE occurs at higher pumping levels. The roll-off in output energy observed in Fig. 7 can be explained by ASE. The 1064 nm laser pulse, i.e., the second pulse, has maximum energy of 22.8 mJ, which is expected to be the total extractable energy in dual-wavelength operation under two-step energy extraction. The pulse width at this maximum energy was 16 ns. On the other hand, output energies of the 1319 nm laser determine the first pulse energy. Furthermore, compared with the 1064 nm laser, a higher threshold is required for the 1319 nm laser to leave a residual population in the upper laser level to generate the second pulse. Maximum output energies of 4.2 and 12.8 mJ were obtained with output mirror reflectivities of 55% and 70%, respectively. The pulse widths for the 1319 nm laser were longer than 70 ns, even at the highest pumping level, due to the small stimulated-emission cross section. The threshold pump energies for the 1319 nm laser were more than 1.4 times higher than those of the 1064 nm laser. Accordingly, we expected to achieve two-step energy extraction using both the output mirrors that had been used for the 1319 nm laser. The pulse-topulse fluctuations of pulse energies for the 1064 nm laser with a 30% reflectivity output mirror and the 1319 nm laser with a 70% reflectivity output mirror are shown in Fig. 8. The data represent the energy stability in Q-switched operation without line competition between the two wavelengths. The root-meansquare (rms) fluctuations of pulse energies for the 1064 and 1319 nm lasers were 1.5% and 4.7%, respectively; moreover, these values correspond to the minimum achievable fluctuations in dual-wavelength operation.
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
Fig. 9. Q-switched pulse energies in dual-wavelength operations under two-step energy extraction as a function of the interval between the two Q-switch trigger pulses, ΔtQ . The reflectivities of the output mirrors for the 1064 and 1319 nm lasers are 30% and 55%, respectively.
operates only at 1064 nm, whereas it increased to 27% in this case because 19% of the total pulse energy was extracted from the 1319 nm laser with a quantum defect of 39%. When the increment of the quantum defect is taken into account, the maximum extractable energy from the dual-wavelength laser decreases from 22.8 to 21.9 mJ, which agrees with the observed total energy. This indicates that the residual energy in the upper laser manifold, after the first pulse, can be efficiently extracted for the second pulse. The solid and dashed curves in Fig. 9 represent the numerical calculation results using the model described in Section 2. The calculated results are in reasonable agreement with the experimental results. In the two-step energy extraction regime, the pulse widths of the 1319 and 1064 nm lasers were 197 ns and 21 ns, respectively, and both the pulse energy and the pulse width were independent of the ΔtQ value. Conversely, the ratio of the two pulse energies was changed by varying ΔtQ in the simultaneous energy extraction regime, because a decrease in ΔtQ increased the fraction of the energy extracted by the 1064 nm laser pulse. Although the pulse width lengthens with decreasing pulse energy in a typical Q-switched laser, both the pulse width and the pulse energy of the 1319 nm laser decreased simultaneously as ΔtQ decreased from 1000 to 850 ns. The reason for this behavior is that the dominant 1064 nm laser pulse, which has a pulse width of approximately 20 ns, depletes the population in the upper laser level during the relatively slow stimulated emission process at 1319 nm. The pulse-to-pulse fluctuations for ΔtQ 1400 ns are shown in Fig. 10. When the laser was operated without a temporal overlap between the two pulses 25 20
1064 nm + 1319 nm
(ΔtQ > 1000 ns), the ΔtQ value was arbitrarily chosen to minimize the fluctuations while maintaining a constant ratio of the 1319–1064 nm pulse energies (0.24 in this case). Furthermore, high stability, which is comparable to that of single-emission-line operation, was achieved by avoiding line competition. As a result, rms fluctuations of 1.5% and 4.8% were achieved with the 1064 nm and 1319 nm lasers, respectively, whereas rms fluctuations of 9%–31% for the 1319 nm laser were observed in the simultaneous lasing region (ΔtQ 800–1000 ns). As shown in Fig. 3, the ratio of the pulse energies for the two wavelengths can be selected by changing the output mirror reflectivity for the 1319 nm laser even under the fixed output mirror reflectivity for the 1064 nm laser. Therefore, we demonstrated dual-wavelength lasing using the two-step energy extraction technique with a different output mirror reflectivity for the 1319 nm laser. In Fig. 11, the Q-switched pulse energies and pulse widths as a function of ΔtQ for the dual-wavelength laser, using a 30% reflectivity output mirror for the 1064 nm laser and a 70% reflectivity output mirror for the 1319 nm laser, are shown. The other conditions were the same as those used for the results shown in Fig. 9. In this study, the reflectivity of the output mirror for the 1064 nm laser was fixed at 30%. However, the optimum reflectivity for operations at the single emission line was calculated to be 70%, as shown in Fig. 2(a). This indicates that the use of the output mirror with a reflectivity of 70% makes the secondpulse energy in the two-step extraction regime to be higher, resulting in the higher extractable total energy. Nevertheless, we had to use a 30% reflectivity mirror because optical damage often occurred on the dichroic mirrors (M3, M4, and M5), which would have a low damage threshold, even at this reflectivity. When the output mirror reflectivity for the 1319 nm laser is 70%, the calculated output energies
0 15 10 1319 nm
5 0 15
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Energy (1064 nm, exp.) Energy (1319 nm, exp.) Energy (1064 nm, model) Energy (1319 nm, model) Pulse width (1064 nm, exp.) Pulse width (1319 nm, exp.)
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Shots Fig. 10. Pulse-to-pulse fluctuations of Q-switched pulse energies for ΔtQ 1400 ns. The reflectivities of the output mirrors for the 1064 and 1319 nm lasers are 30% and 55%, respectively.
Fig. 11. Q-switched pulse energies in dual-wavelength operations under two-step energy extraction as a function of the interval between the two Q-switch trigger pulses, ΔtQ . The reflectivities of the output mirrors for the 1064 and 1319 nm lasers are 30% and 70%, respectively. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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of the 1064 nm laser with 30%, 50%, and 70% reflectivity output mirrors in the two-step extraction regime are 6.5 mJ, 10.1 mJ, and 11.5 mJ, respectively. Therefore, further improvements in extractable total energy are possible if no damage occurs. Typical pulse shapes of the two lasers for ΔtQ 60, 90, and 140 ns are shown in Figs. 12(a)–12(c), respectively. For ΔtQ < 110 ns, the two Q-switched pulses were generated simultaneously, with a partial temporal overlap [see Figs. 12(a), 12(b)]. In this operational regime, the output energy for the 1064 nm laser decreased with increasing ΔtQ, whereas that for the 1319 nm laser increased. This is because the ratio of the two pulse energies is determined by the temporal positions of both pulses. For example, when ΔtQ is small, the beginning of the 1064 nm laser pulse occurs earlier, resulting in an increased fraction of the energy delivered to that pulse. Conversely, two-step energy extraction occurs when ΔtQ is greater than 110 ns [see Fig. 12(c)]. Within the two-step energy extraction regime, the laser produced output energy
Intensity (a. u.)
(a)
1064 nm
1319 nm
0
100 200 300 400 500 600
Time (ns)
(b) Intensity (a. u.)
1064 nm 1319 nm
0
100 200 300 400 500 600
Time (ns)
Intensity (a. u.)
(c)
1064 nm
1319 nm
0
100 200 300 400 500 600
Time (ns) Fig. 12. Typical pulse shapes of the dual-wavelength laser for different values of ΔtQ : (a) 60 ns, (b) 90 ns, and (c) 140 ns. The conditions are the same as those under which the results illustrated in Fig. 11 were obtained. 3040
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
of 12.6 mJ in the first pulse at 1319 nm and output energy of 6.7 mJ in the second pulse at 1064 nm, which corresponds to an energy ratio of 1.9. The observed pulse energies in this region were somewhat higher than those predicted by the model (solid curve in Fig. 11), because the observed pulse width of 96 ns is longer than the calculated value of 64 ns, which reduces the effect of the lower-laser-level lifetime on the pulse energy. In Figs. 13(a)–13(c), the pulse-to-pulse fluctuations of pulse energies for ΔtQ 60 ns, 90 ns, and 140 ns, respectively, are shown. Significantly large fluctuations were observed under simultaneous energy extraction due to line competition between the two laser transitions, whereas the total energy of the two lasers was relatively stable [see Figs. 13(a), 13(b)]. In our previous study, it was found that the pulse-topulse fluctuations due to line competition in simultaneous energy extraction are strongly related to the timing jitters of the two Q-switched pulses [27]. Typical timing jitters for single-emission-line operations were measured to be 5–10 ns for both lasers. Although the corresponding increase and decrease in output energies for the 1319 and 1064 nm lasers were almost equal in magnitude for ΔtQ 60 ns, the observed changes appear to be asymmetrical for ΔtQ 90 ns, as shown in the inset in Fig. 13(b). This results from the difference of temporal positions between the partially overlapped pulses. When ΔtQ 90 ns, a large fraction of the output energy of the 1319 nm laser is extracted before the beginning of the 1064 nm laser pulse. Therefore, timing jitters that further decrease the temporal overlap between the two pulses have a smaller influence on the fluctuations compared with jitters that increase the overlap. Conversely, the magnitude of the fluctuations was independent of the direction of the jitter when ΔtQ 60 ns; this is because the temporal positions of the two pulses were nearly equivalent. In contrast, stable operations were achieved for two-step energy extraction, and rms fluctuations of 4.7% and 2.6% were observed for the 1319 nm and 1064 nm lasers, respectively. However, the rms value for the 1064 nm laser was 1.7 times larger than that for single-emission-line operation. This results from the relatively unstable pulse energy of the 1319 nm laser; that is, the initial population density of the upper laser level for the 1064 nm laser is affected by fluctuations of the 1319 nm laser, which are dominant in this case. The rms fluctuation of the 1319 nm laser energy is the same as that for singleemission-line operation, because the first pulse is not affected by the second pulse in the absence of a temporal overlap. In Fig. 14, the rms fluctuations of the two lasers as a function of ΔtQ are shown. When both pulse energies are comparable, almost the same rms fluctuation is obtained at both wavelengths, with both exceeding 21% for ΔtQ 90 ns. As ΔtQ decreases from 90 to 40 ns, the pulse-to-pulse stability of the 1064 nm laser improves to 5.3%, whereas the stability of the 1319 nm laser deteriorates significantly. This is because a slight change in pulse energy at the dominant wavelength causes a significant change for the other
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Number of shots
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(c)
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Fig. 13. Pulse-to-pulse fluctuations of Q-switched pulse energies for different values of ΔtQ : (a) 60 ns, (b) 90 ns, and (c) 140 ns. The conditions are the same as those under which the results illustrated in Fig. 11 were obtained. The inset in (b) shows Q-switched pulse energies from 900 to 1000 shots.
50 Simultaneous Two steps
Fluctuation (%)
40
30 1064 nm 20
1319 nm
10
0 30
60
90
120
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ΔtQ (ns) Fig. 14. Rms fluctuations of Q-switched pulse energies as a function of the interval between the two Q-switch trigger pulses, ΔtQ . The conditions are the same as those under which the results illustrated in Fig. 11 were obtained.
laser. As a result, the minimum fluctuations for both lasers could be obtained only for two-step energy extraction. In this experiment, the interval between the two Q-switch trigger pulses should be more than 120 ns for stable operation. Although the interval between the peaks of the two laser pulses was then more than a few hundreds of nanoseconds, an interval less than 1 μs is negligible for the observation of the NDVI. 5. Conclusions
We have demonstrated stable dual-wavelength pulse generation by using a two-step energy extraction technique from an actively Q-switched Nd:YAG laser operating with high energies at wavelengths of 1064 and 1319 nm. Numerical simulations of the temporal behavior of the dual-wavelength Q-switched pulses were performed using a rate equation model considering the finite lower-laser-level lifetime. The
simulation results show that the first and second pulses can be successively extracted by introducing a higher threshold in the first pulse laser compared with the second pulse. In laser experiments, the first and second pulses were extracted at 1319 nm and 1064 nm, respectively, using the two-step energy extraction technique, as described in Section 2. Total energies of the order of 20 mJ of the two pulses were achieved for two combinations of output mirrors. While a ratio of 1.9 was demonstrated for the 1319–1064 nm pulse energies, a higher ratio might be required for the observation of the NDVI since the 660 nm laser pulse is strongly absorbed by chlorophyll. This can be achieved by increasing the reflectivity of the output mirror at 1319 nm. The pulse-to-pulse fluctuations of output energies for the 1319 and 1064 nm lasers were 4.7%–4.8% and 1.5%–2.6%, respectively, which were significantly smaller than those obtained with simultaneous energy extraction, and were nearly equivalent to those obtained for a single emission line in our system. The observed behavior of the dual-wavelength Qswitched pulses was in reasonably good agreement with the theoretical predictions. In the experiments described here, the Q-switched pulse widths of the 1319 nm laser were 2–10 times longer than those of the 1064 nm laser, depending on the combination of the output couplers used. Since the cavity lengths can be arbitrarily selected in this method, dual-wavelength Q-switched operation with the same pulse width and with shorter pulse widths can be achieved by adjusting the length of each cavity. References 1. P. J. Sellers, “Canopy reflectance, photosynthesis, and transpiration,” Int. J. Remote Sens. 6, 1335–1372 (1985). 2. National Oceanic, and Atmospheric Administration, “NOAA KLM User’s Guide (April 2007 revision),” http://www.ncdc .noaa.gov/oa/pod‑guide/ncdc/docs/klm/index.htm. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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