5432
Research Article
Vol. 54, No. 17 / June 10 2015 / Applied Optics
Double-pulse laser based on a semiconductor optical amplifier H. G. PAN, A. L. ZHANG,* Y. M. SHI,
AND
Y. M. XUE
School of Electronic & Information Engineering, Tianjin University of Technology, Tianjin 300384, China *Corresponding author: [email protected] Received 24 March 2015; revised 16 May 2015; accepted 16 May 2015; posted 18 May 2015 (Doc. ID 234592); published 5 June 2015
In this paper, a double-pulse laser with a semiconductor optical amplifier (SOA) is proposed. By adjusting the polarization controller, we observe double pulses with repetition frequencies of 10.05 and 12.70 MHz and pulse widths of 33.40 and 30.13 ns, respectively. The laser consists of a SOA asymmetrically placed in a short fiber loop. Its switching time is determined by the off-center position of the SOA within the loop. In the loop, the two pulses, which have the same widths, transmit in the clockwise direction and the counterclockwise direction separately. © 2015 Optical Society of America OCIS codes: (170.0170) Medical optics and biotechnology; (170.1470) Blood or tissue constituent monitoring; (170.3340) Laser Doppler velocimetry; (170.3890) Medical optics instrumentation. http://dx.doi.org/10.1364/AO.54.005432
1. INTRODUCTION The double-pulse (DP) configuration, in which two laser pulses mutually delayed by the proper time are employed, is often used in laser induced breakdown spectroscopy (LIBS) [1–6] and drilling [7–9]. The advantage of DP with respect to the more traditional single-pulse configuration is an enhancement of the signal acquired in the measurements, resulting in a corresponding improvement of the detection limits of the technique [6]. When contrasted to a single-pulse laser of the same power, laser drilling speed can be increased several times with a DP laser, and the drilling quality can be improved at the same time [7]. The DP scheme has been realized by a two-laser configuration or a single laser. Compared with the two-laser configuration, the single-laser configuration simplifies the problems of alignment and cost-effectiveness [10,11]. Many rare-earth-doped fibers have been used as the active media of fiber lasers. Among these active media, erbium-doped fibers and ytterbium-doped fibers have attracted much attention in the construction of either continuous-wave or pulsed fiber lasers [12–14]. Currently, semiconductor optical amplifiers (SOAs) provide interesting alternatives as the gain medium in a fiber-optical laser because of their dominant inhomogeneous broadening properties and the ease of integration with other devices. As compared to the rare-earth-doped fibers, SOAs can generate multiple stable lasing wavelengths at room temperature, and they have many of nonlinear effects, such as cross-phase modulation (XPM) and crossgain modulation (XGM), that make them attractive for applications in telecommunication systems and photonic technologies [15,16]. 1559-128X/15/175432-04$15/0$15.00 © 2015 Optical Society of America
In this paper, a scheme for DPs is obtained using the SOA as the gain and switching medium. The SOA is placed asymmetrically in the nonlinear loop, and XPM and XGM will generate in the nonlinear optical loop mirror (NALM). The phase change of signal pulses via XPM is used for interferometric switching. When we change the fundamental frequency of the laser and the position of the SOA in the nonlinear loop, DPs with repetition frequencies of 10.05 and 12.70 MHz and pulse widths of 33.4 and 30.10 ns are generated. 2. EXPERIMENTAL CONFIGURATION AND PRINCIPLE Figure 1 is the schematic diagram of the experimental setup. The laser consists of a NALM and a linear loop connected by a coupler (C1). The nonlinear loop consists of C1, a polarization controller (PC2), and a SOA. The SOA works as the gain and switching medium with central wavelength 1550 nm, 3 dB optical bandwidth of 60 nm, and polarization-dependent gain of 1 dB. PC2 adjusts the light power distributed to the transverse electric (TE) and transverse magnetic (TM) components of the SOA. The linear loop consists of erbium-doped fiber amplifier 1 (EDFA1), PC1, and a 70:30 output coupler (C2). EDFA1 is the gain medium and it ensures unidirectional operation, and PC1 in the linear loop compensates for the stress birefringence of the fiber. The output coupling is 30% (Port1) for the laser. We use a 50:50 coupler (C3), EDFA2, PC3, and a polarization-dependent isolator (PDI) to test the output pulse polarization states. Port2 is an output port of the PDI, which is recorded by a radio frequency spectrum analyzer (HP 8563E, frequency range:
Research Article
Vol. 54, No. 17 / June 10 2015 / Applied Optics
⇀
⇀
5433
2
jE out j2 ρE sin θ cosΔϕTE E sin θ cosΔϕTM
ρ2 E 2 cos2 θ E 2 sin2 θ 2ρE 2 cosΔϕTE − ΔϕTM : (5)
Fig. 1. Schematic setup of DP optical fiber laser.
9 kHz–26.5 GHz) and an oscilloscope (TOKOWGA, sample rate: 500 MHz) through a photodetector (speed: 150 MHz), while the output spectrum is measured by an optical spectrum analyzer (Anritus MS9710C, resolution: 0.05 nm). The SOA gain saturation and the refractive index saturation are polarization dependent. When the pulse passed through the SOA, the polarization states of the light changed because the gain and optical path length for the TE and TM components are different. To develop a more quantitative understanding of the change of polarization states, we have written a model based ⇀
on a propagating electric field E , having two components aligned along the modes of the waveguide [17]: ⇀
E e
iωt−kz
⇀
⇀
E 0TE e
iϕTE
E 0TM e
iϕTM
;
(1)
⇀
where E 0TE0TM is the E component along the TE(TM) direction and its phase is ϕTETM . E TE and E TM are expressed as n ⇀ ⇀ 2 E 0TE ρE cosθuTE E 0TM E sinθuTM ; where ρ is the ratio of the single-pass gain in the TE mode to the single-pass gain in the TM mode, θ is the angle between the input polarization and TE polarization, and uTETM is the unit ⇀
vector along the horizontal (vertical) axis. The amplitude E of the input field is taken to be unity in our model. PC2 adjusts the polarization of input light in the counterclockwise (CCW) direction only, so after pulse propagation from the SOA, the angle θ is different between the light in the clockwise (CW) direction and that in the CCW direction. The changed phases ΔϕTE and ΔϕTM are ΔϕTE
2πLnTE ; λ
ΔϕTM
2πLnTM : λ
From Eq. (5) we can conclude that, after passing through the SOA, the power and polarization states are different between the pulses in the CW direction and the CCW direction. When a pulse injects into the nonlinear loop through C1, it splits into two parts with equal power and the same phase. Then they counterpropagate around the loop and recombine at tC1. Assuming that after the cycle. the gain and phase shifts of the CW and CCW are G cw t, Δϕcw t, G ccw t, and Δϕccw t, the power at the reflection port and the transmission port are [15] 1 P R P in t G cw t G ccw t 4 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 G cw tG ccw t cosΔϕcw t − Δϕccw t ; (6) 1 P T P in t G cw t G ccw t 4 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi − 2 G cw tG ccw t cosΔϕcw t − Δϕccw t : (7) The pulse evolution process is shown in Fig. 2. When the pulse propagates from the linear loop into the nonlinear loop through C1, it splits into two pulses, and then they transmit in the CW direction and the CCW direction. The pulse transmit in the CW direction is composed of pulse 1 and pulse 2, and the pulse transmit in the CCW direction is composed of pulse 3 and pulse 4. Supposing that the single-mode fiber on the left of the SOA is shorter than that on the right and the D-value is Δx, after pulse 1 passes the SOA, pulse 2 and pulse 3 begin to pass the SOA, and encounter in the SOA, then XPM and XGM [17] are generated. Thus, a large phase difference is generated between the two pulses. In this case, the two pulses will destructively combine at the reflection port of C1, which results in the majority of the energy emerging from the transmission port, i.e., P T ≠ 0. Then the switching window is opened and DPs with the same widths are formed.
(3)
Because nTE is different from nTM , when the power of the pulse is changed, the refractive difference between the TE and TM changes nonlinearly. The phase difference Δϕ is Δϕ ΔϕTE − ΔϕTM :
(4)
E out is the complex amplitude of SOA output optical field, so
Fig. 2. Schematic diagram of pulses evolution in the NALM.
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Research Article
Fig. 3. Optical waveforms, optical spectrum, and radio frequency spectrum of the laser when the pulse width is 30.10 ns: (a) double- pulses in the laser cavity, (b) (c) optical staircase waveform out of the laser cavity, (d) RF spectrum, and (e) optical spectrum.
3. EXPERIMENTAL RESULTS AND DISCUSSION When the SOA current exceeds 220 mA, double pulses successfully generate with a carefully adjusted polarization state of PC2. The pulse width is about 30.10 ns, as shown in Fig. 3(a). The length of the laser cavity is about 16 m (the repetition frequency is about 12.70 MHz), and the difference between the two delay arm lengths of the SOA is 6 m. In Fig. 3(d), we can see that the laser fundamental frequency is 12.66 MHz. When the current of the SOA is changed, the output power of the laser changes accordingly, but the pulse widths do not change. We can obtain the corresponding optical spectrum, as is shown in Fig. 3(e), where the central wavelength of the optical spectrum is 1583 nm and its 3 dB bandwidth is about 10 nm. By changing the length of the cavity of the laser and the position of the SOA in the NALM, we can obtain double pulses with different repetition rates and different pulse widths. When the length of the cavity is about 20.0 m, the fundamental frequency of the cavity is 10 MHz. The difference between the two delay arm lengths of the SOA is 3.4 m and the pulse width is 33.4 ns (Port1), as shown in Fig. 4(a). Figure 3(d) shows the fundamental frequency of the laser of 10.05 MHz. The corresponding optical spectrum is shown in Fig. 4(e), where the central wavelength of the optical spectrum is 1600 nm and its 10 dB bandwidth is 40 nm. To test the polarization states and composition of the DP, we monitored the output of the laser by using an optical spectrometer,
Fig. 4. Optical waveforms, optical spectrum, and RF spectrum of the laser when the pulse width is 30.10 ns: (a) double pulse in the laser cavity, (b) and (c) optical staircase waveforms out of the laser cavity, (d) RF spectrum, and (e) optical spectrum.
Research Article a RF spectrometer, and an optical oscilloscope at Port2. By adjusting PC3, we can obtain a staircase waveform. The electric level with the highest power is named code 2 and the electric level with the higher power is named code 1, which are shown in Fig. 3(b). Figures 3(b) and 3(c), and Figs. 4(b) and 4(c) are some of the staircase waveform pulses recorded when we adjusted PC3. These demonstrate that the DP is composed of two pulses and they have different polarization states but the same pulse widths. Code 1 or code 2 cannot disappear when we adjust PC3, which shows that the polarization states of code 1 and code 2 are not orthogonal. 4. CONCLUSION In this paper, a double-pulse (DP) laser with a semiconductor optical amplifier (SOA) was proposed. The laser consists of a SOA asymmetrically placed in a short fiber loop. Its switching time is determined by the off-center position of the SOA within the loop. When the current of the SOA exceeds 220 mA, by adjusting the PC, DPs with repetition frequencies 10.05 and 12.70 MHz and pulse widths 33.40 and 30.13 ns are generated. When we change the cavity length of the laser and the SOA position in the NALM, we obtain DPs with the same pulse widths. Through experimental demonstration, we find that pulses transmitted in the CW direction and the CCW direction have the same widths but different polarization states. The single-laser configuration simplifies the problems of alignment and cost-effectiveness. It is highly practical for a number of applications, including LIBS, drilling, and communications. Natural Science Foundation of China (NSFC) (61377075). REFERENCES 1. A. Guarnaccio, G. P. Parisi, D. Mollica, A. DeBonis, R. Teghil, and A. Santagat, “Fs-ns double-pulse laser induced breakdown spectroscopy of copper-based-alloys: Generation and elemental analysis of nanoparticles,” Spectrochim. Acta, Part B 101, 261–268 (2014). 2. Z. Haider, Y. B. Munajat, R. Kamarulzaman, and N. Shahami, “Comparison of single pulse and double simultaneous pulse laser induced breakdown spectroscopy,” Anal. Lett. 48, 308–317 (2014). 3. R. Ahmed and M. A. Baig, “A comparative study of enhanced emission in double pulse laser induced breakdown spectroscopy,” Opt. Laser Technol. 65, 113–118 (2015).
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4. M. Baudelet and B. W. Smith, “The first years of laser-induced breakdown spectroscopy,” J. Anal. At. Spectrom. 28, 624–629 (2013). 5. D. A. Cremers and R. C. Chinni, “Laser-induced breakdown spectroscopy—Capabilities and limitations,” Appl. Spectrosc. Rev. 44, 457– 506 (2009). 6. L. Guo, Z. Hao, M. Shen, W. Xiong, X. He, Z. Xie, M. Gao, X. Li, X. Zeng, and Y. Lu, “Accuracy improvement of quantitative analysis by spatial confinement in laser-induced breakdown spectroscopy,” Opt. Express 21, 18188–18195 (2013). 7. X. J. Wang, F. Y. Meng, M. Li, J. B. Wang, and G. B. Gao, “Nanosecond dual pulse laser controlling method,” Chinese Journal of Lasers (Chinese Edition) (1983-) / Zhongguo Jiguang / Chin J Lasers 38, 0203004 (2011). 8. X. J. Wang and Z. J. Li, “Thermally induced birefringence compensation in double-pulse Q-switched laser,” Proc. SPIE 8192, 81923A (2011). 9. A. C. Forsman, P. S. Banks, M. D. Perry, E. M. Campbell, A. L. Dodell, and M. S. Armas, “Double-pulse machining as a technique for the enhancement of material removal rates in laser machining of metals,” J. Appl. Phys. 98, 033302 (2005). 10. C. Gautier, P. Fichet, D. Menut, J. L. Lacour, D. L’Hermite, and J. Dubessy, “Study of the double-pulse setup with an orthogonal beam geometry for laser-induced breakdown spectroscopy,” Spectrochim. Acta Part B 59, 975–986 (2004). 11. G. Galbacs, V. Budavari, and Z. Geretovszky, “Multi-pulse laser-induced plasma spectroscopy using a single laser source and a compact spectrometer,” J. Anal. At. Spectrom. 20, 974–980 (2005). 12. I. N. Duling, C. J. Chen, P. K. A. Wai, and C. R. Menyuk, “Operation of a nonlinear loop mirror in a laser cavity,” IEEE J. Quantum Electron. 30, 194–199 (1994). 13. M. J. Guy, D. U. Noske, and J. R. Taylor, “Generation of femtosecond soliton pulses by passive mode locking of an ytterbium-erbium figureof-eight fiber laser,” Opt. Lett. 18, 1447–1449 (1993). 14. T. O. Tsun, M. K. Islam, and P. L. Chu, “High-energy femtosecond figure-eight fiber laser,” Opt. Commun. 141, 65–68 (1997). 15. J. M. Tang, P. S. Spencer, P. Rees, and K. A. Shore, “Enhanced TOAD performance by negative frequency-detuned signal and control picosecond optical pulses,” IEEE J. Quantum Electron. 36, 574–582 (2000). 16. S. Min, Y. Zhao, and S. Fleming, “Semiconductor optical amplifier based high duty-cycle, self-starting figure-eight 1.7 GHz laser source,” Opt. Express 17, 6187–6193 (2009). 17. M. Usami, R. Inohara, K. Nishimura, and M. Tsurusawa, “Experimental analysis of cross gain modulation and cross phase modulation in SOA with assist light injection,” in Lasers and Electro-Optics Society Summer Topic (IEEE, 2002), paper TuK1.
Research Article
Vol. 54, No. 17 / June 10 2015 / Applied Optics
Double-pulse laser based on a semiconductor optical amplifier H. G. PAN, A. L. ZHANG,* Y. M. SHI,
AND
Y. M. XUE
School of Electronic & Information Engineering, Tianjin University of Technology, Tianjin 300384, China *Corresponding author: [email protected] Received 24 March 2015; revised 16 May 2015; accepted 16 May 2015; posted 18 May 2015 (Doc. ID 234592); published 5 June 2015
In this paper, a double-pulse laser with a semiconductor optical amplifier (SOA) is proposed. By adjusting the polarization controller, we observe double pulses with repetition frequencies of 10.05 and 12.70 MHz and pulse widths of 33.40 and 30.13 ns, respectively. The laser consists of a SOA asymmetrically placed in a short fiber loop. Its switching time is determined by the off-center position of the SOA within the loop. In the loop, the two pulses, which have the same widths, transmit in the clockwise direction and the counterclockwise direction separately. © 2015 Optical Society of America OCIS codes: (170.0170) Medical optics and biotechnology; (170.1470) Blood or tissue constituent monitoring; (170.3340) Laser Doppler velocimetry; (170.3890) Medical optics instrumentation. http://dx.doi.org/10.1364/AO.54.005432
1. INTRODUCTION The double-pulse (DP) configuration, in which two laser pulses mutually delayed by the proper time are employed, is often used in laser induced breakdown spectroscopy (LIBS) [1–6] and drilling [7–9]. The advantage of DP with respect to the more traditional single-pulse configuration is an enhancement of the signal acquired in the measurements, resulting in a corresponding improvement of the detection limits of the technique [6]. When contrasted to a single-pulse laser of the same power, laser drilling speed can be increased several times with a DP laser, and the drilling quality can be improved at the same time [7]. The DP scheme has been realized by a two-laser configuration or a single laser. Compared with the two-laser configuration, the single-laser configuration simplifies the problems of alignment and cost-effectiveness [10,11]. Many rare-earth-doped fibers have been used as the active media of fiber lasers. Among these active media, erbium-doped fibers and ytterbium-doped fibers have attracted much attention in the construction of either continuous-wave or pulsed fiber lasers [12–14]. Currently, semiconductor optical amplifiers (SOAs) provide interesting alternatives as the gain medium in a fiber-optical laser because of their dominant inhomogeneous broadening properties and the ease of integration with other devices. As compared to the rare-earth-doped fibers, SOAs can generate multiple stable lasing wavelengths at room temperature, and they have many of nonlinear effects, such as cross-phase modulation (XPM) and crossgain modulation (XGM), that make them attractive for applications in telecommunication systems and photonic technologies [15,16]. 1559-128X/15/175432-04$15/0$15.00 © 2015 Optical Society of America
In this paper, a scheme for DPs is obtained using the SOA as the gain and switching medium. The SOA is placed asymmetrically in the nonlinear loop, and XPM and XGM will generate in the nonlinear optical loop mirror (NALM). The phase change of signal pulses via XPM is used for interferometric switching. When we change the fundamental frequency of the laser and the position of the SOA in the nonlinear loop, DPs with repetition frequencies of 10.05 and 12.70 MHz and pulse widths of 33.4 and 30.10 ns are generated. 2. EXPERIMENTAL CONFIGURATION AND PRINCIPLE Figure 1 is the schematic diagram of the experimental setup. The laser consists of a NALM and a linear loop connected by a coupler (C1). The nonlinear loop consists of C1, a polarization controller (PC2), and a SOA. The SOA works as the gain and switching medium with central wavelength 1550 nm, 3 dB optical bandwidth of 60 nm, and polarization-dependent gain of 1 dB. PC2 adjusts the light power distributed to the transverse electric (TE) and transverse magnetic (TM) components of the SOA. The linear loop consists of erbium-doped fiber amplifier 1 (EDFA1), PC1, and a 70:30 output coupler (C2). EDFA1 is the gain medium and it ensures unidirectional operation, and PC1 in the linear loop compensates for the stress birefringence of the fiber. The output coupling is 30% (Port1) for the laser. We use a 50:50 coupler (C3), EDFA2, PC3, and a polarization-dependent isolator (PDI) to test the output pulse polarization states. Port2 is an output port of the PDI, which is recorded by a radio frequency spectrum analyzer (HP 8563E, frequency range:
Research Article
Vol. 54, No. 17 / June 10 2015 / Applied Optics
⇀
⇀
5433
2
jE out j2 ρE sin θ cosΔϕTE E sin θ cosΔϕTM
ρ2 E 2 cos2 θ E 2 sin2 θ 2ρE 2 cosΔϕTE − ΔϕTM : (5)
Fig. 1. Schematic setup of DP optical fiber laser.
9 kHz–26.5 GHz) and an oscilloscope (TOKOWGA, sample rate: 500 MHz) through a photodetector (speed: 150 MHz), while the output spectrum is measured by an optical spectrum analyzer (Anritus MS9710C, resolution: 0.05 nm). The SOA gain saturation and the refractive index saturation are polarization dependent. When the pulse passed through the SOA, the polarization states of the light changed because the gain and optical path length for the TE and TM components are different. To develop a more quantitative understanding of the change of polarization states, we have written a model based ⇀
on a propagating electric field E , having two components aligned along the modes of the waveguide [17]: ⇀
E e
iωt−kz
⇀
⇀
E 0TE e
iϕTE
E 0TM e
iϕTM
;
(1)
⇀
where E 0TE0TM is the E component along the TE(TM) direction and its phase is ϕTETM . E TE and E TM are expressed as n ⇀ ⇀ 2 E 0TE ρE cosθuTE E 0TM E sinθuTM ; where ρ is the ratio of the single-pass gain in the TE mode to the single-pass gain in the TM mode, θ is the angle between the input polarization and TE polarization, and uTETM is the unit ⇀
vector along the horizontal (vertical) axis. The amplitude E of the input field is taken to be unity in our model. PC2 adjusts the polarization of input light in the counterclockwise (CCW) direction only, so after pulse propagation from the SOA, the angle θ is different between the light in the clockwise (CW) direction and that in the CCW direction. The changed phases ΔϕTE and ΔϕTM are ΔϕTE
2πLnTE ; λ
ΔϕTM
2πLnTM : λ
From Eq. (5) we can conclude that, after passing through the SOA, the power and polarization states are different between the pulses in the CW direction and the CCW direction. When a pulse injects into the nonlinear loop through C1, it splits into two parts with equal power and the same phase. Then they counterpropagate around the loop and recombine at tC1. Assuming that after the cycle. the gain and phase shifts of the CW and CCW are G cw t, Δϕcw t, G ccw t, and Δϕccw t, the power at the reflection port and the transmission port are [15] 1 P R P in t G cw t G ccw t 4 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 G cw tG ccw t cosΔϕcw t − Δϕccw t ; (6) 1 P T P in t G cw t G ccw t 4 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi − 2 G cw tG ccw t cosΔϕcw t − Δϕccw t : (7) The pulse evolution process is shown in Fig. 2. When the pulse propagates from the linear loop into the nonlinear loop through C1, it splits into two pulses, and then they transmit in the CW direction and the CCW direction. The pulse transmit in the CW direction is composed of pulse 1 and pulse 2, and the pulse transmit in the CCW direction is composed of pulse 3 and pulse 4. Supposing that the single-mode fiber on the left of the SOA is shorter than that on the right and the D-value is Δx, after pulse 1 passes the SOA, pulse 2 and pulse 3 begin to pass the SOA, and encounter in the SOA, then XPM and XGM [17] are generated. Thus, a large phase difference is generated between the two pulses. In this case, the two pulses will destructively combine at the reflection port of C1, which results in the majority of the energy emerging from the transmission port, i.e., P T ≠ 0. Then the switching window is opened and DPs with the same widths are formed.
(3)
Because nTE is different from nTM , when the power of the pulse is changed, the refractive difference between the TE and TM changes nonlinearly. The phase difference Δϕ is Δϕ ΔϕTE − ΔϕTM :
(4)
E out is the complex amplitude of SOA output optical field, so
Fig. 2. Schematic diagram of pulses evolution in the NALM.
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Research Article
Fig. 3. Optical waveforms, optical spectrum, and radio frequency spectrum of the laser when the pulse width is 30.10 ns: (a) double- pulses in the laser cavity, (b) (c) optical staircase waveform out of the laser cavity, (d) RF spectrum, and (e) optical spectrum.
3. EXPERIMENTAL RESULTS AND DISCUSSION When the SOA current exceeds 220 mA, double pulses successfully generate with a carefully adjusted polarization state of PC2. The pulse width is about 30.10 ns, as shown in Fig. 3(a). The length of the laser cavity is about 16 m (the repetition frequency is about 12.70 MHz), and the difference between the two delay arm lengths of the SOA is 6 m. In Fig. 3(d), we can see that the laser fundamental frequency is 12.66 MHz. When the current of the SOA is changed, the output power of the laser changes accordingly, but the pulse widths do not change. We can obtain the corresponding optical spectrum, as is shown in Fig. 3(e), where the central wavelength of the optical spectrum is 1583 nm and its 3 dB bandwidth is about 10 nm. By changing the length of the cavity of the laser and the position of the SOA in the NALM, we can obtain double pulses with different repetition rates and different pulse widths. When the length of the cavity is about 20.0 m, the fundamental frequency of the cavity is 10 MHz. The difference between the two delay arm lengths of the SOA is 3.4 m and the pulse width is 33.4 ns (Port1), as shown in Fig. 4(a). Figure 3(d) shows the fundamental frequency of the laser of 10.05 MHz. The corresponding optical spectrum is shown in Fig. 4(e), where the central wavelength of the optical spectrum is 1600 nm and its 10 dB bandwidth is 40 nm. To test the polarization states and composition of the DP, we monitored the output of the laser by using an optical spectrometer,
Fig. 4. Optical waveforms, optical spectrum, and RF spectrum of the laser when the pulse width is 30.10 ns: (a) double pulse in the laser cavity, (b) and (c) optical staircase waveforms out of the laser cavity, (d) RF spectrum, and (e) optical spectrum.
Research Article a RF spectrometer, and an optical oscilloscope at Port2. By adjusting PC3, we can obtain a staircase waveform. The electric level with the highest power is named code 2 and the electric level with the higher power is named code 1, which are shown in Fig. 3(b). Figures 3(b) and 3(c), and Figs. 4(b) and 4(c) are some of the staircase waveform pulses recorded when we adjusted PC3. These demonstrate that the DP is composed of two pulses and they have different polarization states but the same pulse widths. Code 1 or code 2 cannot disappear when we adjust PC3, which shows that the polarization states of code 1 and code 2 are not orthogonal. 4. CONCLUSION In this paper, a double-pulse (DP) laser with a semiconductor optical amplifier (SOA) was proposed. The laser consists of a SOA asymmetrically placed in a short fiber loop. Its switching time is determined by the off-center position of the SOA within the loop. When the current of the SOA exceeds 220 mA, by adjusting the PC, DPs with repetition frequencies 10.05 and 12.70 MHz and pulse widths 33.40 and 30.13 ns are generated. When we change the cavity length of the laser and the SOA position in the NALM, we obtain DPs with the same pulse widths. Through experimental demonstration, we find that pulses transmitted in the CW direction and the CCW direction have the same widths but different polarization states. The single-laser configuration simplifies the problems of alignment and cost-effectiveness. It is highly practical for a number of applications, including LIBS, drilling, and communications. Natural Science Foundation of China (NSFC) (61377075). REFERENCES 1. A. Guarnaccio, G. P. Parisi, D. Mollica, A. DeBonis, R. Teghil, and A. Santagat, “Fs-ns double-pulse laser induced breakdown spectroscopy of copper-based-alloys: Generation and elemental analysis of nanoparticles,” Spectrochim. Acta, Part B 101, 261–268 (2014). 2. Z. Haider, Y. B. Munajat, R. Kamarulzaman, and N. Shahami, “Comparison of single pulse and double simultaneous pulse laser induced breakdown spectroscopy,” Anal. Lett. 48, 308–317 (2014). 3. R. Ahmed and M. A. Baig, “A comparative study of enhanced emission in double pulse laser induced breakdown spectroscopy,” Opt. Laser Technol. 65, 113–118 (2015).
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