1 kW peak power, 110 ns single-frequency thulium doped fiber amplifier at 2050 nm Erik Lucas,1,* Laurent Lombard,2 Yves Jaouën,3 Sylvain Bordais,1 and Guillaume Canat2 1 2 3
Keopsys, 2 rue Paul Sabatier, 22300 Lannion, France
ONERA, The French Aerospace Lab, F-91123 Palaiseau, Cedex, France
Institut Telecom/Telecom ParisTech, 46 Rue Barrault, 75634 Paris, France *Corresponding author: [email protected]
Received 19 February 2014; revised 2 May 2014; accepted 20 May 2014; posted 29 May 2014 (Doc. ID 206730); published 3 July 2014
We report a high power, single frequency, linearly polarized master oscillator power amplifier emitting 110 ns, 1 kW peak power pulses at 2050 nm. A 20% slope efficiency and a beam quality of M 2 1.21 are achieved with three-stage double-clad Tm3 -doped fiber architecture. Various pump schemes are compared leading to the conclusion that 793 nm pump wavelength is the most efficient for amplification at 2050 nm. Based on numerical simulations, the Brillouin gain coefficient around 2 μm in Tm3 highly doped silica fiber is estimated to 1.2 × 10−11 m∕W. Output peak power is limited by stimulated Brillouin scattering to 535 W without mitigation and to 1 kW with application of a strain distribution along the doped fiber. © 2014 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.2420) Fibers, polarizationmaintaining; (140.3538) Lasers, pulsed; (060.2390) Fiber optics, infrared. http://dx.doi.org/10.1364/AO.53.004413
1. Introduction
High-power single-frequency (SF), linearly polarized fiber lasers operating in the atmospheric transparency window and eyesafe wavelength range of 1.9–2.1 μm are highly desirable for many applications such as LIDAR, pump lasers of optical parametric oscillator crystal emitting in the 6–12 μm band (ZnGeP2 ) [1,2], super-continuum generation [3], spectroscopy, and material processing. Impressive results have been obtained using thulium-doped fiber-laser technologies in the 1900– 2000 nm band, both of which are in the continuous wave regime (CW) [4] and in the pulsed regime. For instance SF laser sources have achieved peak power of the order of 70 kW at 1920 nm, for pulse duration shorter than the phonon lifetime 1559-128X/14/204413-07$15.00/0 © 2014 Optical Society of America
(about 10 ns) [5–8]. For pulse duration longer than the phonon lifetime, the limiting nonlinear effect is stimulated Brillouin scattering (SBS). In this regime, Shi et al. demonstrated at 1920 nm SF Q-switched pulses amplified up to 2.75 kW∕80 ns in Tmgermanate fiber [9]. There have been fewer developments in the pulsed regime for the 2000–2100 nm band especially. Indeed, over this band, the gain per meter of Tm3 -doped fibers is lower. The situation has some similarities with the L band for erbium-doped fiber. Required fiber lengths are large, thus lowering the nonlinearity threshold. For SF fiber amplifiers the SBS has the lowest power threshold among the nonlinear effects. Various techniques exist to mitigate the SBS while preserving the SF characteristics of the laser emission. For example, the transverse acoustic velocity profile can be engineered to reduce the overlap between the optical mode and the acoustic modes involved in the SBS [10,11]. A longitudinal acoustic 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
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velocity gradient can also be used to reduce the effective length determining the SBS threshold. This gradient can be based on strain [12] or temperature variations [13,14]. Goodno et al. reported a 608 W CW SF Tm3 -doped fiber amplifier (TDFA) at 2040 nm [4]. The pump power generated a thermal gradient induced by the nonradiative transitions that allegedly increased the SBS threshold by a factor of 2.9. Unfortunately, the pump-induced thermal gradient is not applicable for pulsed sources due to their low average power. Furthermore, we have previously demonstrated that TDFA efficiency is highly dependent on temperature [15]. We have measured a 40% decrease of the efficiency for a 40°C increase of the fiber temperature. Generating a thermal gradient in TDFA is not desirable as it would decrease the efficiency of the source. In the following experiment we use strain gradient. In this paper we report on a 1 kW peak power of linearly polarized 110 ns pulses, SF at 2050 nm from a master oscillator fiber amplifier (MOPFA). We rely on large mode area (LMA) highly Tm3 -doped silica fibers. Three pump schemes are studied to select the most efficient for 2050 nm amplification. The SBS threshold is enhanced by the application of a strain distribution on fiber used in the last amplification stage. The MOPFA design is described, especially the estimation of Brillouin gain, the calculation of the SBS threshold, and its mitigation. 2. Pump Scheme Selection
Thulium ion Tm3 offers a broad transition (3 F 4 → 3 H 6 ) extending from 1700 to 2100 nm. Tm3 -doped silica fibers have been developed for a decade. Currently, many highly doped fibers are available including polarization-maintaining (PM) and double-clad (DC) fiber types. Tm3 -doped silica fibers accept three pump bands: one centered at 793 nm, one centered at 1210 nm, and one broad absorption band extending from 1550 to 1950 nm. These pump bands correspond to the excitation of an electron from the 3 H 6 energy level to the 3 H 4 , 3 H 5 , and 3 F 4 energy levels, respectively, as shown on Fig. 1. There are several efficient laser sources for these pump bands: multi-mode laser diodes at 793 nm, single-mode (SM) Er3 -doped fiber laser around 1550 nm, and SM Tm3 -doped fiber lasers in the 1800–1900 nm band. These laser sources enable various pump schemes. At 793 nm the peak core absorption is strong and wide (typical FHWM of 17 nm). These characteristics are compatible with highpower broadband multimode laser diodes in clad pumping configuration. This is the most widely used pump source thanks to the 2-for-1 cross-relaxation effects achieved when pumping highly doped Tm3 -doped fibers at 793 nm allows us to reach up to 60% efficiency in CW lasers [16]. Core pumping is possible at 793 nm (but single transversemode pump diodes lack power) at 1210 nm using Raman fiber lasers [17] at 1550–1600 nm using 4414
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Fig. 1. Energy level diagram for the Tm3 -doped silica fiber showing the pump transitions, the cross relaxation process (between the Tm3 ions labeled a and b), and the laser transition. Routes for possible multiphonon decay after optical excitation to the 3 H 4 level are also shown.
Er3 -doped fiber lasers or at 1800–1950 nm using Tm3 -doped fiber lasers. Core pumping benefits from very large absorption but the large Tm3 inversion can result in parasitic effects degrading the efficiency [15]. To design high-power Tm3-MOPFAs and compare pumping schemes, we developed a numerical model for CW and pulsed regimes based on standard fiberamplifier models [18,19]. The numerical model takes into account the transitions shown in Fig. 1, radiative emissions from 3 H 4 level to 3 H 6 and 3 F 4 levels, and the energy transfer upconversion from 3 F 4 level to 3 H 6 and 3 H 4 . The numerical model also simulates amplified spontaneous emission (ASE). This numerical model requires accurate cross sections in the 2000–2200 nm range, which have been adapted from Peterka et al. and Moulton et al. [20,21]. Experiments were also performed to compare the efficiency of the various pump schemes. The gain at 2050 nm is a function of the average population of Tm3 -ions in the 3 F 4 energy level and does not depend on the population variations along the fiber. The theoretical gain g in dB per unit length at a wavelength λs for the averaged normalized excited population n¯ 2 on the 3 F 4 level is given by Eq. (1) [22], g 4.34N 0 Γn¯ 2 σ a λs σ e λs − σ a λs ;
(1)
with σ a and σ e the absorption and emission crosssections, respectively, N 0 the Tm3 ion concentration, and Γ the overlap of the optical power distribution and the core profile. This equation was used to draw the wavelength of maximum gain as a function of the averaged normalized excited population n¯ 2 [23], shown Fig. 2. The theoretical n¯ 2 required to optimize signal amplification at 2050 nm is 0.05 for the selected cross sections. For comparison, optimal amplification at 1940 nm, the wavelength used to pump Ho3 -doped lasers,
Fig. 2. Gain peak wavelength versus averaged normalized excited population n¯ 2 for Tm:silica cross-sections from [20,21].
requires n¯ 2 0.14, which requires brighter pump lasers. Actually, n¯ 2 is determined by the pump and signal absorption/emission rate in the fiber. To understand the effect of the pump wavelength choice on the population inversion profile along the fiber, we have performed simulations and experiments for three different pumping schemes: a core-pumped amplifier using 1560 nm Er/Yb-fiber laser, a corepumped amplifier using 1940 nm Tm3 -fiber, and a cladding-pumped amplifier using 793 nm multimode laser diodes. For the sake of comparison, the three amplifiers were simulated in the CW regime, pumped in a forward direction, and based on a 3.6 m 6/130 μm Tm3 -doped fiber with a 2.4 dB∕m cladding absorption at 793 nm. The pump powers were chosen so as to present equal normalized average population inversion n¯ 2 of 0.05. This resulted in a signal gain at 2050 nm of 500 (27 dB), the amplifiers delivering 1 W for 2 mW input power. The pump powers of the 793, 1560, and 1940 nm pumping schemes were respectively 4, 2.5, and 2 W. The three amplifiers would have larger efficiencies if they were operated at a shorter wavelength (typically 1920 nm) closer to the gain maximum of Tm3 -doped fibers. In that case more than 50% efficiency could be expected using 1560 nm core pumping. The three simulated amplifiers were then experimentally tested against the simulations with one difference; the 1940 nm fiber laser pump was limited to 1.2 W. We measured the ratio of ASE forward power to total output power, the ratio of output signal power to input pump power in the fiber, referred to as the optical-optical (o-o) efficiency, and the ratio of output signal power to input diode pump power, referred to as total efficiency. Table 1 Table 1.
summarizes the different measurements and simulation results. Small signal absorption in the fiber core of 1560 nm pump power is very high, 130 dB∕m. For a 1560 nm core-pumped amplifier in forward direction most of the pump power is absorbed in the first 0.5 m of the fiber. The population inversion fraction is very high at the beginning of the fiber with a maximum of 0.9 (cf. Fig. 3). We used Eq. (1) to draw the gain per unit length corresponding to the n2 maximum of each pumping scheme shown in Fig. 4. The n2 maxima are the n2 of the beginning of the fiber in the three pumping scheme. It shows that the amplifier gain is centered to 1860 nm for the inversion population maximum. So the first 0.5 m of the fiber generates a large amount of ASE around 1860 nm in the forward and backward direction. The next 3 m of fiber are pumped by the ASE in the forward direction. But the ASE in the backward direction is lost thus reducing the amplifier efficiency. Experimentally we measured 13% pump efficiency, which is only one third of the 40% simulated pump efficiency. We would expect a larger efficiency as the quantum defect is only 20%. The difference seems to be caused by different cooperative effects, which are not considered by the model. For example, we have observed fluorescence and high photodarkening in the visible and UV regions [15,24] that were not simulated. The ASE power in the forward direction was as high as 14% of total output power at the output of the amplifier. The 6/130 μm fiber used for comparison is a highly Tm3 -doped fiber, and therefore, it is not the most suitable fiber for 1560 nm core pumping. We have also tested 1560 nm core pumping with an eight times lower Tm3 concentration fiber, which is a 9/130 μm fiber with a 20 dB∕m core small signal absorption at 1560 nm. With 3 W input power, 2.7 W pump power, and 3.3 m long 9/130 μm fiber, the o-o efficiency reaches 18%, and the total efficiency reaches 8%, that is still not as high as for clad pumping at 793 nm (23%). Considering these results, corepumping of highly doped fibers at 1560 nm appears to be an unsuitable solution for 2050 nm amplification. The fibers show low absorption of around 2.4 dB∕m at 793 nm in cladding-pump configuration, so the population inversion is better spread out than core pumping at 1560 nm. The maximum population inversion is 0.13. The corresponding gain per unit length is centered on 1950 nm but is only 3 dB higher than at 2050 nm. The quantum defect is 39%, the experimental efficiency for 2050 nm amplification is
Comparison of Various Pump Schemes for 2050 nm Amplification Forward Pumped Single Stage CW MOPFAs with 3.6 m 6/130 μm Tm3 -Doped Fiber
Pumping Scheme Clad (4 W) at 793 nm Core (2.5 W) at 1560 nm Core (1.2 W) at 1940 nm
Sim. o-o Eff. (%)
Meas. o-o Eff. (%)
Sim. ASE (%)
Meas. ASE (%)
Meas. Total Eff. (%)
25 40 35
23 13 32
1 10 0.5
2 14 1
23 7 16
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Fig. 3. Simulation of population inversion fraction for three different pumping schemes.
cladding pumped at 793 nm thus reducing the total o-o efficiency of core-pumping at 1940 nm to 16%. It is 30% less efficient than clad pumping at 793 nm to amplify the 2050 nm signal. This 1940 nm laser was operating close to threshold because of the lack of power of the 793 nm diodes. More powerful lasers could be used and the total o-o efficiency of corepumping at 1940 nm would certainly reach 20%. So even if clad pumping at 793 nm generates a larger ASE ratio than core pumping at 1940 nm, it appears to be the best compromise among the three pump schemes when considering total efficiency and simplicity. These results were obtained with a CW amplifier but can be extrapolated to pulsed amplifiers of the same average power. 3. Brillouin Gain Estimation at 2 μm
23%, which is close to the 25% of simulated pump efficiency. Our measurements showed that the output ASE was only 2% of the total output power. The last configuration used a pump wavelength of 1940 nm, which minimizes the quantum defect with the signal wavelength. This wavelength is at the limit of the absorption band of Tm3 -doped fiber, but the small signal absorption of the 1940 nm pump in the fiber core still reaches 6 dB∕m. The saturation power at pump wavelength is only 81 mW. Therefore the pump in the core at 1940 nm saturates the population inversion over the first 2 m, which is then homogeneously distributed, see Fig. 3. The population inversion fraction maximum is 0.07. The gain is almost perfectly centered at 2050 nm all along the fiber. Furthermore, the quantum defect for 1940 nm (7.5%) is the smallest of the three pumping schemes. These facts explain the larger efficiency of the 1940 nm core pumping compared to the two other pumping methods. The total efficiency of this pumping scheme however, is lower than direct-clad pumping with diodes at 793 nm, which was confirmed by experiments. We measured a 32% pump efficiency and a low output ASE ratio of 1%. However, 1940 nm pump sources are Tm3 -doped fiber lasers
In order to design the various stages, we need an estimate of the Brillouin gain value in Tm- doped fibers at 2 μm. A threshold measurement was performed on a Tm-doped SM fiber amplifier. The Brillouin reflectivity is given by the ratio of the backscattered Stokes power Ps 0 to the output power of the fiber-amplifier Pp L. When this reflectivity is large enough, the signal is depleted and a dip appears on the amplified pulse. This threshold condition is typically reached for a reflectivity of 10%. The SBS process in a passive fiber can be described by the scattering of the optical wave on the thermal phonon. The reflectivity of the passive fiber is then in the limit of low reflectivity and large gain [25]: exp GSBS RSBS ηβL p 3∕2 ; π GSBS
(2)
with GSBS the single-pass Brillouin gain GSBS
gB P LL; Aeff p
(3)
and gB the Brillouin gain, Aeff the effective area, η the capture fraction of the fiber, and β a phenomenological Stokes backscatter coefficient per unit length. In a long passive fiber with fiber of length L, the threshold of the SBS is given by the classical Smith relation [26], gB P LL 21: Aeff p
(4)
A fiber amplifier can be approximated as a lumped amplifier formed by a gain segment producing a gain GA without the SBS followed by a passive fiber segment [11]. The passive fiber length is the effective fiber length Leff of the amplifier defined as Fig. 4. Lineic gains of the simulated fraction maxima of excited Tm3 -ions shown Fig. 3. 4416
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Leff
1 Pp L
Z
L 0
Pp zdz:
(5)
Fig. 5. Three-stage amplifier layout. AOM, acousto-optic modulator; HR, high reflexion; FBG, fiber Bragg grating.
The reflectivity of the fictive passive segment is then exp GSBS Rg SBS ηβLeff p 3∕2 : π GSBS
6
Taking into account that the reflectivity should be defined at the amplifier input exp GSBS RSBS GA ηβLeff p 3∕2 : π GSBS
(7)
For ηβ ∼ 10−9 m−1, Leff 1.4 m, RSBS at threshold ∼10%, and GA 25 dB, we would get ln
p RSBS π 3 18.7 ln GA GSBS − ln GSBS : (8) ηβLeff 2
Equation (8) can now be solved to get GSBS 17.2. An approximation of GSBS for the fiber amplifier is GSBS 21 − ln GA . To estimate gB , we built a SM fiber amplifier using L 3.7 m of Tm3 -doped PM fiber with 10 μm core diameter. It was cladding pumped in the forward direction to amplify 1 μs, 300 mW peak power impulsions, and 1997 nm pulses. We measured the SBS threshold Pth 95 W corresponding to an amplification gain GA 25 dB. Using the power distribution Pp z provided by the amplifier model when the SBS threshold is reached, we computed Leff 1.4 m. Then with Aeff 89 μm2 from the Marcuse formula [27] we obtain gB 1.2 × 10−11 m∕W. This estimated value is somewhat lower than the value measured in passive SM germanium doped fibers at 1.5 μm of gB 2 × 10−11 m∕W [28]. This low value is consistent with the presence of aluminum in the core. The Tm3 -doped fibers are co-doped with strong concentration of aluminum. It is known that aluminum doping increases the acoustic velocity and helps reduce the effective Brillouin gain as it reduces the acoustic-phonon core guidance [29].
is built using a 9 m long PM DC Tm3 -doped fiber with 10 μm core diameter and 130 μm cladding diameter. Output pulse peak power is 70 W, i.e., 23 dB gain, limited by pump power. The SBS threshold is estimated to be 90 W. Our amplifier model predicts an SBS threshold for 6/130 and 10/130 μm forward-pumped fiber amplifiers of 20 and 50 W, respectively. Backward pumping further increases the SBS threshold by changing signal-power repartition in the fiber. The effective length of the backward pumped amplifier is 1.8 times lower than the forward pumped amplifier, thus increasing the SBS threshold to 90 W. ASE at the second-stage output represents 60% of the total output power. This large value is due both to backward pumping configuration and low saturation. Indeed saturation power of the 10/130 μm fiber (345 mW at signal wavelength) is large compared to the saturation power that would have a 6/130 μm doped fiber (141 mW at signal wavelength) and to the 0.8 mW input average power. A bandpass filter is added to prevent saturation of the third amplifier stage with ASE of the second stage. The effect of the filter on the ASE spectrum is shown in the inset of Fig. 6. A second circulator is inserted after the filter to monitor the counterpropagating beams (ASE and SBS) of the third amplifier stage. Signal losses in the filter amount to 3 dB, resulting in 35 W peak-power pulses at the third-stage input. The third amplifier is built using a 4.5 m long, PM, LMA, double clad, Tm3 highly doped fiber with 25 μm diameter, 0.09 NA core, spliced to match passive fibers on both ends. The third-stage fiber is slightly multi-mode with a V number of 3.44. It is
4. Experiment
The MOPFA layout is shown in Fig. 5. A linearly polarized DFB laser emitting a power of 4 mW with 40 dB). The amplified output is modulated into 110 ns, 320 mW peak power pulses at 20 kHz repetition rate using a fiber acousto-optic modulator (AOM) from AA opto-electronic. Pulses are then injected into the second amplification stage, which
Fig. 6. Normalized output spectrum of the MOPFA. Inset: output spectra of the second stage, unfiltered and filtered, respectively. 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
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unstrained fiber and Cs 4.6 in silica. Δf B0 is close to 9 GHz at 2 μm and BB is about 50 MHz in an unstrained fiber. Consequently, the Brillouin gain for an input power with Δf bandwidth could be written as a function of the tensile strain, cf. Eq. (9): gB ε; Δf
Fig. 7. Third-stage output peak power, simulated and experimental. Inset: far field beam profile measured at 1 kW peak power.
Fig. 8. Normalized pulses at the SBS threshold and at 1 kW peak power with SBS mitigation. The 30 ns gaps between the dips correspond approximately to the round trip of the pulses reflected by the SBS.
cladding pumped in a forward direction by 793 nm diodes. The end of the fiber is collimated and launched in a free space isolator providing 28 dB isolation. The far field beam shape is shown in Fig. 7, displaying a very good beam quality thanks to careful splice optimization. Beam quality was measured using a ModeScan and a 25 μm slit PYROCAM, leading to M 2 1.2 in both cases. No dependance of M 2 on pump power was observed. The SBS threshold is reached in this configuration at 535 W output peak power for 18 W of pump power (cf. Fig. 7). The average output power is 1.2 W. When the SBS threshold is reached, instabilities in pulse shape can be observed as shown in Fig. 8. The next step is dedicated to further increase the SBS threshold. 5. SBS Mitigation
In order to increase the maximum peak power, a strain-based SBS mitigation was implemented. The Brillouin gain gB Δf has a Lorentzian lineshape centered at the Brillouin frequency shift Δf B with a bandwidth BB FWHM. The Brillouin shift of a silica fiber depends on the tensile strain ε by Δf B Δf B0 1 Cs ε with Δf B0 the Brillouin shift for 4418
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BB ∕22 : Δf − Δf B0 1 Cs ε2 BB ∕22
(9)
If the local Brillouin frequency shift Δf B z changes by more than one Brillouin bandwidth BB over the fiber length, the Stokes amplification is no longer efficient over the whole fiber length. Spontaneously backscattered light is only amplified locally, and there is no accumulated SBS over large distances anymore [30]. This idea was recently used in an Er/Yb amplifier using a staircase strain distribution [31]. In this work we applied a triangular shape strain distribution on the doped fiber of the third stage. The strain linearly increased from the beginning to the middle of the active fiber, with the maximum tensible strain εmax , and linearly decreases from this point to the end of the active fiber so the tensile strain εz εmax 2z∕L for z ∈ 0; L∕2, and εz εL − z beyond. εmax is set to 0.5%. It is chosen reasonably low to preserve the fiber lifetime. In these conditions the maximum output peak power is 1.05 kW for a pump power of 23 W (cf. Fig. 7). Corresponding pulse characteristics are 110 ns FWHM duration, 20 kHz repetition frequency, energy of 115 μJ, and an average power of 2.2 W. The pulse shape is close to Gaussian with no SBS instabilities, cf. Fig. 8. Maximum peak power is thus increased by 3 dB compared to the unstrained case and is limited by the SBS in the 50 cm transport passive fiber used at the amplifier output. Without the transport fiber, the maximum output peak power is estimated to 1.3 kW for εmax 0.5%. Its length is determined by handling convenience and could be reduced below 10 cm without affecting functionality, thus increasing the SBS threshold. The third-amplifier gain is 15 dB with 20% average power-slope efficiency. The overall three-stage MOPFA gain is then 55 dB. The output spectrum measured using a 0.05 nm resolution optical spectral analyzer is shown Fig. 6. We measured a linewidth smaller than 2 pm limited by our OSA resolution. ASE is 20 dB. 6. Conclusion
In summary we have presented a three-stage MOPFA based on Tm3 -doped fibers with 1 kW peak power, 110 ns duration, SF linearly polarized pulses at 2050 nm. To the best of our knowledge this is the highest peak power generated for single frequency 100 ns class pulses beyond 2,000 nm in Tm3 -silica fiber. Work to improve the optical efficiency is on going. We also estimated the Brillouin gain coefficient to be 1.2 × 10−11 m∕W around 2 μm in Tm3 -silica
fiber. This MOPFA is well suited for an optical parametric oscillator ZnGeP2 crystal. The authors want to acknowledge the Region Ile de France, which partially supported this work. References 1. D. Creeden, P. A. Budni, and P. A. Ketteridge, “Pulsed Tm-doped fiber lasers for mid-IR frequency conversion,” Proc. SPIE 7195, 71950X (2009). 2. A. Godard, “Infrared (2–12 μm) solid-state laser sources: a review,” C.R. Physique 8, 1100–1128 (2007). 3. M. Duhant, W. Renard, G. Canat, T. N. Nguyen, F. Smektala, J. Troles, Q. Coulombier, P. Toupin, L. Brilland, P. Bourdon, and G. Renversez, “Fourth-order cascaded Raman shift in AsSe chalcogenide suspended-core fiber pumped at 2 μm,” Opt. Lett. 36, 2859–2861 (2011). 4. G. D. Goodno, L. D. Book, and J. E. Rothenberg, “Singlefrequency, single-mode emission at 2040 nm from a 600-W thulium-doped fiber amplifier chain,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2009), paper MF2. 5. J. Geng, Q. Wang, Z. Jiang, T. Luo, S. Jiang, and G. Czarnecki, “Kilowatt-peak-power, single-frequency, pulsed fiber laser near 2 μm,” Opt. Lett. 36, 2293–2295 (2011). 6. Q. Wang, J. Geng, T. Luo, and S. Jiang, “2 μm mode-locked fiber lasers,” Proc. SPIE 8237, 82371N (2012). 7. A. M. Heidt, Z. Li, J. Sahu, P. C. Shardlow, M. Becker, M. Rothhardt, M. Ibsen, R. Phelan, B. Kelly, S. U. Alam, and D. J. Richardson, “100 kW peak power picosecond thuliumdoped fiber amplifier system seeded by a gain-switched diode laser at 2 μm,” Opt. Lett. 38, 1615–1617 (2013). 8. Q. Fang, W. Shi, K. Kieu, E. Petersen, A. Chavez-Pirson, and N. Peyghambarian, “High power and high energy monolithic single frequency 2 μm nanosecond pulsed fiber laser by using large core Tm-doped germanate fibers: experiment and modeling,” Opt. Express 20, 16410–16420 (2012). 9. W. Shi, E. B. Petersen, D. T. Nguyen, Z. Yao, A. Chavez-Pirson, N. Peyghambarian, and J. Yu, “220 μJ monolithic singlefrequency Q-switched fiber laser at 2 μm by using highly Tm-doped germanate fibers,” Opt. Lett. 36, 3575–3577 (2011). 10. M. J. Li, X. Chen, J. Wang, A. Ruffin, D. Walton, S. Li, D. Nolan, S. Gray, and L. Zenteno, “Fiber designs for reducing stimulated Brillouin scattering,” in Optics Fiber Communication Conference (2006), pp. 1–3. 11. M. D. Mermelstein, M. J. Andrejco, J. Fini, A. Yablon, C. Headley III, D. J. DiGiovanni, and A. H. McCurdy, “11.2 dB SBS gain suppression in a large mode area Yb-doped optical fiber,” Proc. SPIE 6873, 68730N (2008). 12. N. Yoshizawa and T. Imai, “Stimulated Brillouin scattering suppression by means of applying strain distribution to fiber with cabling,” J. Lightwave Technol. 11, 1518–1522 (1993). 13. J. Hansryd, F. Dross, M. Westlund, P. A. Andrekson, and S. N. Knudsen, “Increase of the SBS threshold in a short highly nonlinear fiber by applying a temperature distribution,” J. Lightwave Technol. 19, 1691–1697 (2001). 14. M. D. Mermelstein, A. D. Yablon, and C. Headley, “Suppression of stimulated Brillouin scattering in an Er-Yb fiber amplifier utilizing temperature-segmentation,” in Optical Amplifiers and Their Applications, Technical Digest (CD) (Optical Society of America, 2005), paper TuD3.
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Keopsys, 2 rue Paul Sabatier, 22300 Lannion, France
ONERA, The French Aerospace Lab, F-91123 Palaiseau, Cedex, France
Institut Telecom/Telecom ParisTech, 46 Rue Barrault, 75634 Paris, France *Corresponding author: [email protected]
Received 19 February 2014; revised 2 May 2014; accepted 20 May 2014; posted 29 May 2014 (Doc. ID 206730); published 3 July 2014
We report a high power, single frequency, linearly polarized master oscillator power amplifier emitting 110 ns, 1 kW peak power pulses at 2050 nm. A 20% slope efficiency and a beam quality of M 2 1.21 are achieved with three-stage double-clad Tm3 -doped fiber architecture. Various pump schemes are compared leading to the conclusion that 793 nm pump wavelength is the most efficient for amplification at 2050 nm. Based on numerical simulations, the Brillouin gain coefficient around 2 μm in Tm3 highly doped silica fiber is estimated to 1.2 × 10−11 m∕W. Output peak power is limited by stimulated Brillouin scattering to 535 W without mitigation and to 1 kW with application of a strain distribution along the doped fiber. © 2014 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.2420) Fibers, polarizationmaintaining; (140.3538) Lasers, pulsed; (060.2390) Fiber optics, infrared. http://dx.doi.org/10.1364/AO.53.004413
1. Introduction
High-power single-frequency (SF), linearly polarized fiber lasers operating in the atmospheric transparency window and eyesafe wavelength range of 1.9–2.1 μm are highly desirable for many applications such as LIDAR, pump lasers of optical parametric oscillator crystal emitting in the 6–12 μm band (ZnGeP2 ) [1,2], super-continuum generation [3], spectroscopy, and material processing. Impressive results have been obtained using thulium-doped fiber-laser technologies in the 1900– 2000 nm band, both of which are in the continuous wave regime (CW) [4] and in the pulsed regime. For instance SF laser sources have achieved peak power of the order of 70 kW at 1920 nm, for pulse duration shorter than the phonon lifetime 1559-128X/14/204413-07$15.00/0 © 2014 Optical Society of America
(about 10 ns) [5–8]. For pulse duration longer than the phonon lifetime, the limiting nonlinear effect is stimulated Brillouin scattering (SBS). In this regime, Shi et al. demonstrated at 1920 nm SF Q-switched pulses amplified up to 2.75 kW∕80 ns in Tmgermanate fiber [9]. There have been fewer developments in the pulsed regime for the 2000–2100 nm band especially. Indeed, over this band, the gain per meter of Tm3 -doped fibers is lower. The situation has some similarities with the L band for erbium-doped fiber. Required fiber lengths are large, thus lowering the nonlinearity threshold. For SF fiber amplifiers the SBS has the lowest power threshold among the nonlinear effects. Various techniques exist to mitigate the SBS while preserving the SF characteristics of the laser emission. For example, the transverse acoustic velocity profile can be engineered to reduce the overlap between the optical mode and the acoustic modes involved in the SBS [10,11]. A longitudinal acoustic 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
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velocity gradient can also be used to reduce the effective length determining the SBS threshold. This gradient can be based on strain [12] or temperature variations [13,14]. Goodno et al. reported a 608 W CW SF Tm3 -doped fiber amplifier (TDFA) at 2040 nm [4]. The pump power generated a thermal gradient induced by the nonradiative transitions that allegedly increased the SBS threshold by a factor of 2.9. Unfortunately, the pump-induced thermal gradient is not applicable for pulsed sources due to their low average power. Furthermore, we have previously demonstrated that TDFA efficiency is highly dependent on temperature [15]. We have measured a 40% decrease of the efficiency for a 40°C increase of the fiber temperature. Generating a thermal gradient in TDFA is not desirable as it would decrease the efficiency of the source. In the following experiment we use strain gradient. In this paper we report on a 1 kW peak power of linearly polarized 110 ns pulses, SF at 2050 nm from a master oscillator fiber amplifier (MOPFA). We rely on large mode area (LMA) highly Tm3 -doped silica fibers. Three pump schemes are studied to select the most efficient for 2050 nm amplification. The SBS threshold is enhanced by the application of a strain distribution on fiber used in the last amplification stage. The MOPFA design is described, especially the estimation of Brillouin gain, the calculation of the SBS threshold, and its mitigation. 2. Pump Scheme Selection
Thulium ion Tm3 offers a broad transition (3 F 4 → 3 H 6 ) extending from 1700 to 2100 nm. Tm3 -doped silica fibers have been developed for a decade. Currently, many highly doped fibers are available including polarization-maintaining (PM) and double-clad (DC) fiber types. Tm3 -doped silica fibers accept three pump bands: one centered at 793 nm, one centered at 1210 nm, and one broad absorption band extending from 1550 to 1950 nm. These pump bands correspond to the excitation of an electron from the 3 H 6 energy level to the 3 H 4 , 3 H 5 , and 3 F 4 energy levels, respectively, as shown on Fig. 1. There are several efficient laser sources for these pump bands: multi-mode laser diodes at 793 nm, single-mode (SM) Er3 -doped fiber laser around 1550 nm, and SM Tm3 -doped fiber lasers in the 1800–1900 nm band. These laser sources enable various pump schemes. At 793 nm the peak core absorption is strong and wide (typical FHWM of 17 nm). These characteristics are compatible with highpower broadband multimode laser diodes in clad pumping configuration. This is the most widely used pump source thanks to the 2-for-1 cross-relaxation effects achieved when pumping highly doped Tm3 -doped fibers at 793 nm allows us to reach up to 60% efficiency in CW lasers [16]. Core pumping is possible at 793 nm (but single transversemode pump diodes lack power) at 1210 nm using Raman fiber lasers [17] at 1550–1600 nm using 4414
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Fig. 1. Energy level diagram for the Tm3 -doped silica fiber showing the pump transitions, the cross relaxation process (between the Tm3 ions labeled a and b), and the laser transition. Routes for possible multiphonon decay after optical excitation to the 3 H 4 level are also shown.
Er3 -doped fiber lasers or at 1800–1950 nm using Tm3 -doped fiber lasers. Core pumping benefits from very large absorption but the large Tm3 inversion can result in parasitic effects degrading the efficiency [15]. To design high-power Tm3-MOPFAs and compare pumping schemes, we developed a numerical model for CW and pulsed regimes based on standard fiberamplifier models [18,19]. The numerical model takes into account the transitions shown in Fig. 1, radiative emissions from 3 H 4 level to 3 H 6 and 3 F 4 levels, and the energy transfer upconversion from 3 F 4 level to 3 H 6 and 3 H 4 . The numerical model also simulates amplified spontaneous emission (ASE). This numerical model requires accurate cross sections in the 2000–2200 nm range, which have been adapted from Peterka et al. and Moulton et al. [20,21]. Experiments were also performed to compare the efficiency of the various pump schemes. The gain at 2050 nm is a function of the average population of Tm3 -ions in the 3 F 4 energy level and does not depend on the population variations along the fiber. The theoretical gain g in dB per unit length at a wavelength λs for the averaged normalized excited population n¯ 2 on the 3 F 4 level is given by Eq. (1) [22], g 4.34N 0 Γn¯ 2 σ a λs σ e λs − σ a λs ;
(1)
with σ a and σ e the absorption and emission crosssections, respectively, N 0 the Tm3 ion concentration, and Γ the overlap of the optical power distribution and the core profile. This equation was used to draw the wavelength of maximum gain as a function of the averaged normalized excited population n¯ 2 [23], shown Fig. 2. The theoretical n¯ 2 required to optimize signal amplification at 2050 nm is 0.05 for the selected cross sections. For comparison, optimal amplification at 1940 nm, the wavelength used to pump Ho3 -doped lasers,
Fig. 2. Gain peak wavelength versus averaged normalized excited population n¯ 2 for Tm:silica cross-sections from [20,21].
requires n¯ 2 0.14, which requires brighter pump lasers. Actually, n¯ 2 is determined by the pump and signal absorption/emission rate in the fiber. To understand the effect of the pump wavelength choice on the population inversion profile along the fiber, we have performed simulations and experiments for three different pumping schemes: a core-pumped amplifier using 1560 nm Er/Yb-fiber laser, a corepumped amplifier using 1940 nm Tm3 -fiber, and a cladding-pumped amplifier using 793 nm multimode laser diodes. For the sake of comparison, the three amplifiers were simulated in the CW regime, pumped in a forward direction, and based on a 3.6 m 6/130 μm Tm3 -doped fiber with a 2.4 dB∕m cladding absorption at 793 nm. The pump powers were chosen so as to present equal normalized average population inversion n¯ 2 of 0.05. This resulted in a signal gain at 2050 nm of 500 (27 dB), the amplifiers delivering 1 W for 2 mW input power. The pump powers of the 793, 1560, and 1940 nm pumping schemes were respectively 4, 2.5, and 2 W. The three amplifiers would have larger efficiencies if they were operated at a shorter wavelength (typically 1920 nm) closer to the gain maximum of Tm3 -doped fibers. In that case more than 50% efficiency could be expected using 1560 nm core pumping. The three simulated amplifiers were then experimentally tested against the simulations with one difference; the 1940 nm fiber laser pump was limited to 1.2 W. We measured the ratio of ASE forward power to total output power, the ratio of output signal power to input pump power in the fiber, referred to as the optical-optical (o-o) efficiency, and the ratio of output signal power to input diode pump power, referred to as total efficiency. Table 1 Table 1.
summarizes the different measurements and simulation results. Small signal absorption in the fiber core of 1560 nm pump power is very high, 130 dB∕m. For a 1560 nm core-pumped amplifier in forward direction most of the pump power is absorbed in the first 0.5 m of the fiber. The population inversion fraction is very high at the beginning of the fiber with a maximum of 0.9 (cf. Fig. 3). We used Eq. (1) to draw the gain per unit length corresponding to the n2 maximum of each pumping scheme shown in Fig. 4. The n2 maxima are the n2 of the beginning of the fiber in the three pumping scheme. It shows that the amplifier gain is centered to 1860 nm for the inversion population maximum. So the first 0.5 m of the fiber generates a large amount of ASE around 1860 nm in the forward and backward direction. The next 3 m of fiber are pumped by the ASE in the forward direction. But the ASE in the backward direction is lost thus reducing the amplifier efficiency. Experimentally we measured 13% pump efficiency, which is only one third of the 40% simulated pump efficiency. We would expect a larger efficiency as the quantum defect is only 20%. The difference seems to be caused by different cooperative effects, which are not considered by the model. For example, we have observed fluorescence and high photodarkening in the visible and UV regions [15,24] that were not simulated. The ASE power in the forward direction was as high as 14% of total output power at the output of the amplifier. The 6/130 μm fiber used for comparison is a highly Tm3 -doped fiber, and therefore, it is not the most suitable fiber for 1560 nm core pumping. We have also tested 1560 nm core pumping with an eight times lower Tm3 concentration fiber, which is a 9/130 μm fiber with a 20 dB∕m core small signal absorption at 1560 nm. With 3 W input power, 2.7 W pump power, and 3.3 m long 9/130 μm fiber, the o-o efficiency reaches 18%, and the total efficiency reaches 8%, that is still not as high as for clad pumping at 793 nm (23%). Considering these results, corepumping of highly doped fibers at 1560 nm appears to be an unsuitable solution for 2050 nm amplification. The fibers show low absorption of around 2.4 dB∕m at 793 nm in cladding-pump configuration, so the population inversion is better spread out than core pumping at 1560 nm. The maximum population inversion is 0.13. The corresponding gain per unit length is centered on 1950 nm but is only 3 dB higher than at 2050 nm. The quantum defect is 39%, the experimental efficiency for 2050 nm amplification is
Comparison of Various Pump Schemes for 2050 nm Amplification Forward Pumped Single Stage CW MOPFAs with 3.6 m 6/130 μm Tm3 -Doped Fiber
Pumping Scheme Clad (4 W) at 793 nm Core (2.5 W) at 1560 nm Core (1.2 W) at 1940 nm
Sim. o-o Eff. (%)
Meas. o-o Eff. (%)
Sim. ASE (%)
Meas. ASE (%)
Meas. Total Eff. (%)
25 40 35
23 13 32
1 10 0.5
2 14 1
23 7 16
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Fig. 3. Simulation of population inversion fraction for three different pumping schemes.
cladding pumped at 793 nm thus reducing the total o-o efficiency of core-pumping at 1940 nm to 16%. It is 30% less efficient than clad pumping at 793 nm to amplify the 2050 nm signal. This 1940 nm laser was operating close to threshold because of the lack of power of the 793 nm diodes. More powerful lasers could be used and the total o-o efficiency of corepumping at 1940 nm would certainly reach 20%. So even if clad pumping at 793 nm generates a larger ASE ratio than core pumping at 1940 nm, it appears to be the best compromise among the three pump schemes when considering total efficiency and simplicity. These results were obtained with a CW amplifier but can be extrapolated to pulsed amplifiers of the same average power. 3. Brillouin Gain Estimation at 2 μm
23%, which is close to the 25% of simulated pump efficiency. Our measurements showed that the output ASE was only 2% of the total output power. The last configuration used a pump wavelength of 1940 nm, which minimizes the quantum defect with the signal wavelength. This wavelength is at the limit of the absorption band of Tm3 -doped fiber, but the small signal absorption of the 1940 nm pump in the fiber core still reaches 6 dB∕m. The saturation power at pump wavelength is only 81 mW. Therefore the pump in the core at 1940 nm saturates the population inversion over the first 2 m, which is then homogeneously distributed, see Fig. 3. The population inversion fraction maximum is 0.07. The gain is almost perfectly centered at 2050 nm all along the fiber. Furthermore, the quantum defect for 1940 nm (7.5%) is the smallest of the three pumping schemes. These facts explain the larger efficiency of the 1940 nm core pumping compared to the two other pumping methods. The total efficiency of this pumping scheme however, is lower than direct-clad pumping with diodes at 793 nm, which was confirmed by experiments. We measured a 32% pump efficiency and a low output ASE ratio of 1%. However, 1940 nm pump sources are Tm3 -doped fiber lasers
In order to design the various stages, we need an estimate of the Brillouin gain value in Tm- doped fibers at 2 μm. A threshold measurement was performed on a Tm-doped SM fiber amplifier. The Brillouin reflectivity is given by the ratio of the backscattered Stokes power Ps 0 to the output power of the fiber-amplifier Pp L. When this reflectivity is large enough, the signal is depleted and a dip appears on the amplified pulse. This threshold condition is typically reached for a reflectivity of 10%. The SBS process in a passive fiber can be described by the scattering of the optical wave on the thermal phonon. The reflectivity of the passive fiber is then in the limit of low reflectivity and large gain [25]: exp GSBS RSBS ηβL p 3∕2 ; π GSBS
(2)
with GSBS the single-pass Brillouin gain GSBS
gB P LL; Aeff p
(3)
and gB the Brillouin gain, Aeff the effective area, η the capture fraction of the fiber, and β a phenomenological Stokes backscatter coefficient per unit length. In a long passive fiber with fiber of length L, the threshold of the SBS is given by the classical Smith relation [26], gB P LL 21: Aeff p
(4)
A fiber amplifier can be approximated as a lumped amplifier formed by a gain segment producing a gain GA without the SBS followed by a passive fiber segment [11]. The passive fiber length is the effective fiber length Leff of the amplifier defined as Fig. 4. Lineic gains of the simulated fraction maxima of excited Tm3 -ions shown Fig. 3. 4416
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Leff
1 Pp L
Z
L 0
Pp zdz:
(5)
Fig. 5. Three-stage amplifier layout. AOM, acousto-optic modulator; HR, high reflexion; FBG, fiber Bragg grating.
The reflectivity of the fictive passive segment is then exp GSBS Rg SBS ηβLeff p 3∕2 : π GSBS
6
Taking into account that the reflectivity should be defined at the amplifier input exp GSBS RSBS GA ηβLeff p 3∕2 : π GSBS
(7)
For ηβ ∼ 10−9 m−1, Leff 1.4 m, RSBS at threshold ∼10%, and GA 25 dB, we would get ln
p RSBS π 3 18.7 ln GA GSBS − ln GSBS : (8) ηβLeff 2
Equation (8) can now be solved to get GSBS 17.2. An approximation of GSBS for the fiber amplifier is GSBS 21 − ln GA . To estimate gB , we built a SM fiber amplifier using L 3.7 m of Tm3 -doped PM fiber with 10 μm core diameter. It was cladding pumped in the forward direction to amplify 1 μs, 300 mW peak power impulsions, and 1997 nm pulses. We measured the SBS threshold Pth 95 W corresponding to an amplification gain GA 25 dB. Using the power distribution Pp z provided by the amplifier model when the SBS threshold is reached, we computed Leff 1.4 m. Then with Aeff 89 μm2 from the Marcuse formula [27] we obtain gB 1.2 × 10−11 m∕W. This estimated value is somewhat lower than the value measured in passive SM germanium doped fibers at 1.5 μm of gB 2 × 10−11 m∕W [28]. This low value is consistent with the presence of aluminum in the core. The Tm3 -doped fibers are co-doped with strong concentration of aluminum. It is known that aluminum doping increases the acoustic velocity and helps reduce the effective Brillouin gain as it reduces the acoustic-phonon core guidance [29].
is built using a 9 m long PM DC Tm3 -doped fiber with 10 μm core diameter and 130 μm cladding diameter. Output pulse peak power is 70 W, i.e., 23 dB gain, limited by pump power. The SBS threshold is estimated to be 90 W. Our amplifier model predicts an SBS threshold for 6/130 and 10/130 μm forward-pumped fiber amplifiers of 20 and 50 W, respectively. Backward pumping further increases the SBS threshold by changing signal-power repartition in the fiber. The effective length of the backward pumped amplifier is 1.8 times lower than the forward pumped amplifier, thus increasing the SBS threshold to 90 W. ASE at the second-stage output represents 60% of the total output power. This large value is due both to backward pumping configuration and low saturation. Indeed saturation power of the 10/130 μm fiber (345 mW at signal wavelength) is large compared to the saturation power that would have a 6/130 μm doped fiber (141 mW at signal wavelength) and to the 0.8 mW input average power. A bandpass filter is added to prevent saturation of the third amplifier stage with ASE of the second stage. The effect of the filter on the ASE spectrum is shown in the inset of Fig. 6. A second circulator is inserted after the filter to monitor the counterpropagating beams (ASE and SBS) of the third amplifier stage. Signal losses in the filter amount to 3 dB, resulting in 35 W peak-power pulses at the third-stage input. The third amplifier is built using a 4.5 m long, PM, LMA, double clad, Tm3 highly doped fiber with 25 μm diameter, 0.09 NA core, spliced to match passive fibers on both ends. The third-stage fiber is slightly multi-mode with a V number of 3.44. It is
4. Experiment
The MOPFA layout is shown in Fig. 5. A linearly polarized DFB laser emitting a power of 4 mW with 40 dB). The amplified output is modulated into 110 ns, 320 mW peak power pulses at 20 kHz repetition rate using a fiber acousto-optic modulator (AOM) from AA opto-electronic. Pulses are then injected into the second amplification stage, which
Fig. 6. Normalized output spectrum of the MOPFA. Inset: output spectra of the second stage, unfiltered and filtered, respectively. 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
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unstrained fiber and Cs 4.6 in silica. Δf B0 is close to 9 GHz at 2 μm and BB is about 50 MHz in an unstrained fiber. Consequently, the Brillouin gain for an input power with Δf bandwidth could be written as a function of the tensile strain, cf. Eq. (9): gB ε; Δf
Fig. 7. Third-stage output peak power, simulated and experimental. Inset: far field beam profile measured at 1 kW peak power.
Fig. 8. Normalized pulses at the SBS threshold and at 1 kW peak power with SBS mitigation. The 30 ns gaps between the dips correspond approximately to the round trip of the pulses reflected by the SBS.
cladding pumped in a forward direction by 793 nm diodes. The end of the fiber is collimated and launched in a free space isolator providing 28 dB isolation. The far field beam shape is shown in Fig. 7, displaying a very good beam quality thanks to careful splice optimization. Beam quality was measured using a ModeScan and a 25 μm slit PYROCAM, leading to M 2 1.2 in both cases. No dependance of M 2 on pump power was observed. The SBS threshold is reached in this configuration at 535 W output peak power for 18 W of pump power (cf. Fig. 7). The average output power is 1.2 W. When the SBS threshold is reached, instabilities in pulse shape can be observed as shown in Fig. 8. The next step is dedicated to further increase the SBS threshold. 5. SBS Mitigation
In order to increase the maximum peak power, a strain-based SBS mitigation was implemented. The Brillouin gain gB Δf has a Lorentzian lineshape centered at the Brillouin frequency shift Δf B with a bandwidth BB FWHM. The Brillouin shift of a silica fiber depends on the tensile strain ε by Δf B Δf B0 1 Cs ε with Δf B0 the Brillouin shift for 4418
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BB ∕22 : Δf − Δf B0 1 Cs ε2 BB ∕22
(9)
If the local Brillouin frequency shift Δf B z changes by more than one Brillouin bandwidth BB over the fiber length, the Stokes amplification is no longer efficient over the whole fiber length. Spontaneously backscattered light is only amplified locally, and there is no accumulated SBS over large distances anymore [30]. This idea was recently used in an Er/Yb amplifier using a staircase strain distribution [31]. In this work we applied a triangular shape strain distribution on the doped fiber of the third stage. The strain linearly increased from the beginning to the middle of the active fiber, with the maximum tensible strain εmax , and linearly decreases from this point to the end of the active fiber so the tensile strain εz εmax 2z∕L for z ∈ 0; L∕2, and εz εL − z beyond. εmax is set to 0.5%. It is chosen reasonably low to preserve the fiber lifetime. In these conditions the maximum output peak power is 1.05 kW for a pump power of 23 W (cf. Fig. 7). Corresponding pulse characteristics are 110 ns FWHM duration, 20 kHz repetition frequency, energy of 115 μJ, and an average power of 2.2 W. The pulse shape is close to Gaussian with no SBS instabilities, cf. Fig. 8. Maximum peak power is thus increased by 3 dB compared to the unstrained case and is limited by the SBS in the 50 cm transport passive fiber used at the amplifier output. Without the transport fiber, the maximum output peak power is estimated to 1.3 kW for εmax 0.5%. Its length is determined by handling convenience and could be reduced below 10 cm without affecting functionality, thus increasing the SBS threshold. The third-amplifier gain is 15 dB with 20% average power-slope efficiency. The overall three-stage MOPFA gain is then 55 dB. The output spectrum measured using a 0.05 nm resolution optical spectral analyzer is shown Fig. 6. We measured a linewidth smaller than 2 pm limited by our OSA resolution. ASE is 20 dB. 6. Conclusion
In summary we have presented a three-stage MOPFA based on Tm3 -doped fibers with 1 kW peak power, 110 ns duration, SF linearly polarized pulses at 2050 nm. To the best of our knowledge this is the highest peak power generated for single frequency 100 ns class pulses beyond 2,000 nm in Tm3 -silica fiber. Work to improve the optical efficiency is on going. We also estimated the Brillouin gain coefficient to be 1.2 × 10−11 m∕W around 2 μm in Tm3 -silica
fiber. This MOPFA is well suited for an optical parametric oscillator ZnGeP2 crystal. The authors want to acknowledge the Region Ile de France, which partially supported this work. References 1. D. Creeden, P. A. Budni, and P. A. Ketteridge, “Pulsed Tm-doped fiber lasers for mid-IR frequency conversion,” Proc. SPIE 7195, 71950X (2009). 2. A. Godard, “Infrared (2–12 μm) solid-state laser sources: a review,” C.R. Physique 8, 1100–1128 (2007). 3. M. Duhant, W. Renard, G. Canat, T. N. Nguyen, F. Smektala, J. Troles, Q. Coulombier, P. Toupin, L. Brilland, P. Bourdon, and G. Renversez, “Fourth-order cascaded Raman shift in AsSe chalcogenide suspended-core fiber pumped at 2 μm,” Opt. Lett. 36, 2859–2861 (2011). 4. G. D. Goodno, L. D. Book, and J. E. Rothenberg, “Singlefrequency, single-mode emission at 2040 nm from a 600-W thulium-doped fiber amplifier chain,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2009), paper MF2. 5. J. Geng, Q. Wang, Z. Jiang, T. Luo, S. Jiang, and G. Czarnecki, “Kilowatt-peak-power, single-frequency, pulsed fiber laser near 2 μm,” Opt. Lett. 36, 2293–2295 (2011). 6. Q. Wang, J. Geng, T. Luo, and S. Jiang, “2 μm mode-locked fiber lasers,” Proc. SPIE 8237, 82371N (2012). 7. A. M. Heidt, Z. Li, J. Sahu, P. C. Shardlow, M. Becker, M. Rothhardt, M. Ibsen, R. Phelan, B. Kelly, S. U. Alam, and D. J. Richardson, “100 kW peak power picosecond thuliumdoped fiber amplifier system seeded by a gain-switched diode laser at 2 μm,” Opt. Lett. 38, 1615–1617 (2013). 8. Q. Fang, W. Shi, K. Kieu, E. Petersen, A. Chavez-Pirson, and N. Peyghambarian, “High power and high energy monolithic single frequency 2 μm nanosecond pulsed fiber laser by using large core Tm-doped germanate fibers: experiment and modeling,” Opt. Express 20, 16410–16420 (2012). 9. W. Shi, E. B. Petersen, D. T. Nguyen, Z. Yao, A. Chavez-Pirson, N. Peyghambarian, and J. Yu, “220 μJ monolithic singlefrequency Q-switched fiber laser at 2 μm by using highly Tm-doped germanate fibers,” Opt. Lett. 36, 3575–3577 (2011). 10. M. J. Li, X. Chen, J. Wang, A. Ruffin, D. Walton, S. Li, D. Nolan, S. Gray, and L. Zenteno, “Fiber designs for reducing stimulated Brillouin scattering,” in Optics Fiber Communication Conference (2006), pp. 1–3. 11. M. D. Mermelstein, M. J. Andrejco, J. Fini, A. Yablon, C. Headley III, D. J. DiGiovanni, and A. H. McCurdy, “11.2 dB SBS gain suppression in a large mode area Yb-doped optical fiber,” Proc. SPIE 6873, 68730N (2008). 12. N. Yoshizawa and T. Imai, “Stimulated Brillouin scattering suppression by means of applying strain distribution to fiber with cabling,” J. Lightwave Technol. 11, 1518–1522 (1993). 13. J. Hansryd, F. Dross, M. Westlund, P. A. Andrekson, and S. N. Knudsen, “Increase of the SBS threshold in a short highly nonlinear fiber by applying a temperature distribution,” J. Lightwave Technol. 19, 1691–1697 (2001). 14. M. D. Mermelstein, A. D. Yablon, and C. Headley, “Suppression of stimulated Brillouin scattering in an Er-Yb fiber amplifier utilizing temperature-segmentation,” in Optical Amplifiers and Their Applications, Technical Digest (CD) (Optical Society of America, 2005), paper TuD3.
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