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In this paper, a cavity dumped Tm:YAG laser was obtained for the first time, whose output pulse width was constant at 54 ns. Maximum repetition rate was 200 kHz, and wavelength was 2013 nm. Its average output power was 595 mW. The laser cavity length was 208 mm with an inserted acousto-optic modulator. Pulses were coupled out of the lateral surface of the cavity when Radio Frequency was added into the modulator. And lens was used to compensate the thermal focal length of Tm:YAG crystal. Numerical calculation of pulse was done.
© 2014 Optical Society of America
1. Introduction
2 μm lasers are in the eye-safe and water-absorption spectrum range, and are widely used in Coherent Doppler LIDAR, Differential Absorption LIDAR and so on [1]. High-repetition-rate and short–pulse-width lasers can be applied in the photoelectric countermeasures [2] and remote-sensing [3].
Tm:YAG is quasi-three-level crystal, which has wide emission range from 1.6 μm to 2.3 μm [4]. Tm:YAG is widely used to obtain 2 μm laser while its absorption peak at 785 nm is the wavelength of commercial AlGaAs laser diodes. Q-switching methods were used to obtain 2 μm pulse lasers for years [5–10]. But the pulse width of Q-switching laser depends on the transmission of the output coupler, the gain of laser medium and the operating repetition rate. The maximum pulse repetition rate is limited by the finite build-up time. As we know, the maximum repetition rate is 27.9 kHz with pulse width of about 2.5 μs [7], and the minimum pulse width is 57 ns with repetition rate of 20 Hz [11]. Q-switching methods are hard to obtain high-repetition-rate and short–pulse-width 2 μm lasers by Tm:YAG. At the same time, the pulse width of cavity dumping is constant, which is determined by cavity length and dumping time. Cavity dumping method was used to get nanosecond pulse width lasers at high repetition rate [12–19]. But cavity dumping methods had not been used for Tm:YAG lasers.
In this paper, cavity dumped Tm:YAG laser was obtained at 2013 nm. The maximum repetition rate was 200 KHz. Pulse width was 54 ns, and average output power was 595 mW. Numerical calculation of pulse was done. The length of cavity, deflection efficiency of AOM, rising time of AOM and width of the light beam on the AOM were considered to simulate output pulse.
2. Experimental setup
The experimental setup was illustrated in Fig. 1. The laser resonator was composed of the cavity mirror M1-M4. M1, M2, M4 and M5 mirrors were high-reflection coated at ~2 μm. M1 and M2 mirrors were plane mirrors. M2 mirror was high-transmission coated at ~790 nm. M4 mirror was a plane-concave mirror that has 200 mm radius of curvature. M3 mirror was high-transmission coated at ~2 μm and was used to compensate the thermal lens of Tm: YAG crystal. M5 was a right angle prism which was used to couple pulses out of the cavity. F-P etalon was inserted at Brewster’s angle to produce polarization with a thickness of 0.2 mm. The distances among M1-M4 were 25 mm, 78 mm, 105 mm with a cavity length of 208 mm.
The laser crystal doped with 3.5% Tm ions was 7 × 7 × 1.5 mm3. Water circulation was used to insure that the temperature of crystal was 15°C all the time. The center wavelength of Laser Diode was 785 nm with fiber diameter of 400 μm and numerical aperture of 0.22. The focused pump beam in the laser medium had a diameter of ~1.2 mm. The acousto-optic modulator (AOM) inserted into the cavity was QSG41-2 produced by Sichuan Institute of Piezoelectric and Acousto-optic Technology (SIPAT). The AOM was made of optical grade quartz crystal that has a damage threshold of 500MW/cm2. The frequency of ultrasonic wave was 40.68 MHZ with a speed of 5750 m/s in the AOM. Its length was 35 mm and rising time was 115 ns/mm. The deflection efficiency of the AOM was 70%.
The focal length was measured by the method in [20]. Its value changed from 652 mm to 163 mm when pump power changed from 10 W to 20 W. Considering the well-known ABCD matrix, |A + D|/2 value of the resonator changed from 0.84 to 0.50, which indicated the resonator always kept stable.
3. Experimental results and discussion
The cavity dumped Tm:YAG laser could be divided into two phases, circulation phase and dumping phase. In the circulation phase, the AOM didn’t work, so the loss of cavity was low, and laser circulated in the cavity between the three high-reflection mirrors M1, M2, and M4 without output. In the dumping phase, AOM worked to deflect the laser, and the laser was reflected from the flat sound waves into the first diffraction order quickly. Then, the laser was reflected out of the cavity by M5.
The pulses were observed from 60 kHz to 200 kHz when RF width was 140ns. Average output power increased when the repetition rate rose as shown in Fig. 2(a). In that process, the width of output pulse remained unchanged. The circulation period was 5μs when repetition rate was 200 kHz. In the circulation phase, the number of photons would reach its upper limit in less than 5μs. So the number of photons would be almost the same in the beginning of dumping phase when repetition rate ranged from 60 kHz to 200 kHz. There were more output pulses in the same observation time while the repetition rate increased, so the average output power increased too. However, cavity dumping has the minimum repetition rate that was analyzed by [21]. In this paper, the minimum repetition rates were 60 kHz, 90 kHz and 110 kHz when pump powers were 10 W, 15 W and 20 W. Average output power was approximate to linear when the repetition rate was bigger than the minimum repetition rate. If the repetition rate was smaller than the minimum repetition rate, the pulse height would be unstable, so the average output power would be smaller than expected. That was the reason why it was sub-linear in Fig. 2(a).
Fig. 2 (a) Average output power vs. repetition rate when RF width was 140 ns; (b) Average output power vs. RF width when repetition rate was 200 kHz.
RF width was changed to observe its effect on output pulses when the repetition rate was 200 kHz. Average output power remained nearly unchanged while RF width changed from 140 ns to 2030 ns as shown in Fig. 2(b). Because there was no output in the circulation process, photons number in cavity would reach an upper limit. However, the output pulse was unstable when RF width was more than 140 ns. Light field established from a noise field at the beginning of circulation phase when RF width was too wide to dump all photons in the end of dumping phase.
The acousto-optically cavity dumped Tm:YAG laser was stable at a repetition rate of 200 kHz and RF width of 140ns. Average output power of pulses was 595 mW when pump power was 20 W. The average output power as a function of pump power was shown in Fig. 3(a). The wavelength of laser was 2013 nm as shown in Fig. 3(b).The pulse remained stable when pump power ranged from 10 W to 20 W. Power was detected by PM30 power meter produced by COHERENT Company. The wavelength was detected by 721 Spectrum Analyzer produced by Bristol Company.
Constant pulse width of 54 ns was obtained under the cavity dumped operation. One pulse was shown in Fig. 4(a). As a comparison, output pulse of Q-switching operation in the same cavity with only a different M4 mirror, which has the same curvature radius and transmission efficiency of 2%, was shown in Fig. 4(b), and its width was ~200 ns at repetition rate of 5 kHz. Pulse was detected by DS4034 digital oscilloscope produced by RIGOL Company.
Fig. 4 Comparison of cavity dumping and Q-switching. (a) One output pulse of cavity dumping;(b) one output pulse of Q-switching mode as a comparison in the same cavity structure and repetition rate of 5 kHz.
In the cavity dumping, the width of output pulse was mostly determined by the length of cavity, deflection efficiency of AOM, rising time of AOM and width of the light beam on the AOM. The output pulse could be approximately simulated by Eq. (1).
In Eq. (1), S was the acousto-optical interaction region, P(t) was the output photons number, I(t) was the all photons number in acousto-optical interaction region, g(x,y) was the normalized Gaussian distribution, η(x,y) was the distribution of deflection efficiency. Light field was divided to many segments equally. Every segment passed through the AOM one by one. g(x,y) was given by Eq. (2). We considered the change of deflection efficiency in the rising time of ultrasonic wave was linear, and the ultrasonic wave passed through the AOM along the x-axis negative direction. η(x,y) was given by Eq. (3). In Eq. (3), ηmax was the maximum deflection efficiency, x1 was the front edge of rising time, and x2 was the back edge of rising time.
In the numerical calculation, the acousto-optical interaction region was set as a square area whose side length was 4ω, and ω was the radius of light beam which was expected to 0.23 mm. The pulse width in theory was 32ns. The contrast calculation was shown in Fig. 5. The contrast proved that the pulse width was proportional to the length of cavity, light beam radius, and rising time of AOM. It was inversely proportional to deflection efficiency.
Fig. 5 Numerical calculation of pulse. (a) the contrast of light beam radius; (b) the contrast of deflection efficiency; (c) the contrast of cavity length; (d) the contrast of rising time.
There was an inaccuracy in that numerical calculation. That was because the AOM was considered as a plane without three-dimensional volume and the light beam radius was inaccurate as a result of inaccurate measurement of focal length. And Light beam radius in the AOM was variable, but in the calculation, it was considered as constant.
4. Conclusion
In conclusion, cavity dumped Tm:YAG laser using an acousto-optic modulator was obtained, and pulse width was 54 ns. The maximum repetition rate was 200 kHz. Average output power was linear to the pump power, proportionate to the repetition rate, and remained unchanged while RF width changed. In the same cavity structure, the pulse width of Q-switching operation was 200ns at a repetition rate of 5 kHz. The pulse width was proportional to the length of cavity, light beam radius, and rising time of AOM. And it was inversely proportional to deflection efficiency.
Acknowledgment
This work was supported by National Natural Science Foundation of China (No. 61308009), Science Fund for Outstanding Youths of Heilongjiang Province (JQ201310) and Fundamental Research funds for the Central Universities (Grant No. HIT.NSRIF.2014044).
References and links
1. C. Wu, Y. L. Ju, Q. Wang, Z. G. Wang, B. Q. Yao, and Y. Z. Wang, “Injection-seeded Tm:YAG laser at room temperature,” Opt. Commun. 284(4), 994–998 (2011). [CrossRef]
2. F. Huang, Y. F. Wang, J. Y. Wang, and Y. X. Niu, “Study on application of high2repetition2rate solid state lasers in photoelectric countermeasure,” Infrared and Laser Engineering. 32(5), 465–467 (2003).
3. M. C. Abrams and D. M. Tratt, “Progress in Laser Sources for Lidar Applications:Laser Sources for 3D Imaging Remote Sensing,” SPIE 5653, 241–248 (2005). [CrossRef]
4. P. X. Song, Z. W. Zhao, X. D. Xu, B. X. Jiang, P. Z. Deng, and J. Xu, “Research progress in TM:YAG crystal,” J. Synthetic Crystals. 34(1), 131–135 (2005).
5. D. Cao, S.-F. Du, Q.-J. Peng, Y. Bo, J.-L. Xu, Y.-D. Guo, J.-Y. Zhang, D.-F. Cui, and Z.-Y. Xu, “A 171.4 W diode-side-pumped Q-switched 2 μm Tm:YAG laser with a 10 kHz repetition rate,” Chin. Phys. Lett. 29(4), 044210 (2012). [CrossRef]
6. I. Rohde, J. M. Masch, D. T. Kunde, M. Marczynski-Bühlow, G. Lutter, and R. Brinkmann, ” Cardiovascular Damage after cw and Q-switched 2μm Laser irradiation,” Proc of OSA Biomedical Optics-SPIE. 8803,880301(2013).
7. Q. Wang, H. Teng, Y. W. Zou, Z. G. Zhang, D. H. Li, R. Wang, C. Q. Gao, J. J. Lin, L. W. Guo, and Z. Y. Wei, “Graphene on SiC as a Q-switcher for a 2 μm laser,” Opt. Lett. 37(3), 395–397 (2012). [CrossRef] [PubMed]
8. C. Q. Gao, Z. F. Lin, M. W. Gao, Y. S. Zhang, L. N. Zhu, R. Wang, and Y. Zheng, “Single-frequency operation of diode-pumped 2 microm Q-switched Tm:YAG laser injection seeded by monolithic nonplanar ring laser,” Appl. Opt. 49(15), 2841–2844 (2010). [CrossRef] [PubMed]
9. C. Li, J. Song, D. Y. Shen, N. S. Kim, and K. Ueda, “Diode-pumped Q-switched Tm:YAG laser in active mirror configuration,” Proc. SPIE 3889, 604–609 (2000). [CrossRef]
10. G. Stoppler, C. Kieleck, and M. Eichhorn, “High-Pulse Energy Q-switched Tm3+:YAG Laser for Nonlinear Frequency Conversion to the Mid-IR,” Proc. SPIE 7836, 783609 (2010). [CrossRef]
11. S. Goldring, E. Lebiush, and R. Lavi, “RTP Q-Switched 2-Micron Tm:YAG laser,” Proc. SPIE 4630, 13–16 (2002). [CrossRef]
12. Y. D. Lv, J. Q. Liu, and L. J. Hao, “Electro-optical cavity-dumped Ce:Nd:YAG laser for aesthetic medicine,” Opt. Laser Technol. 44(8), 2432–2435 (2012). [CrossRef]
13. S. Kaspar, M. Rattunde, T. Töpper, U. Schwarz, C. Manz, K. Köhler, and J. Wagner, “Electro-optically cavity dumped 2m semiconductor disk laser emitting 3ns pulses of 30 W peak power,” Appl. Phys. Lett. 101(14), 141121 (2012). [CrossRef]
14. P. He, H. L. Wang, L. L. Zhang, J. L. Wang, C. T. Mo, C. Wang, X. D. Li, and D. Y. Chen, “Cavity-dumped electro-optical Q-switched Nd:GdVO4 laser with high repetition rate,” Opt. Laser Technol. 44(3), 631–634 (2012). [CrossRef]
15. H. A. Kruegle and L. Klein, “High peak power output, high PRF by cavity dumping a Nd:YAG laser,” Appl. Opt. 15(2), 466–471 (1976). [CrossRef] [PubMed]
16. X. X. Tang, Z. W. Fan, J. S. Qiu, F. Q. Lian, and X. L. Zhang, “High efficiency and good beam quality of electro-optic, cavity-dumped and double-end pumped Nd:YLF laser,” Laser Phys. 22(6), 1015–1020 (2012). [CrossRef]
17. X. Yu, C. Wang, Y. F. Ma, F. Chen, R. P. Yan, and X. D. Li, “Performance improvement of high repetition rate electro-optical cavity-dumped Nd:GdVO4 laser,” Appl. Phys. B 106(2), 309–313 (2012). [CrossRef]
18. Y. F. Ma, J. W. Zhang, H. Li, and X. Yu, “High stable electro-optical cavity-dumped Nd:YAG laser,” Laser Phys. Lett. 9(8), 561–563 (2012). [CrossRef]
19. V. G. Savitski, J. E. Hastie, S. Calvez, and M. D. Dawson, “Cavity-dumping of a semiconductor disk laser for the generation of wavelength-tunable micro-Joule nanosecond pulses,” Opt. Express 18(11), 11933–11941 (2010). [CrossRef] [PubMed]
20. Y. T. Chang, Y. P. Huang, K. W. Su, and Y. F. Chen, “Comparison of thermal lensing effects between single-end and double-end diffusion-bonded Nd:YVO4 crystals for 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions,” Opt. Express 16(25), 21155–21160 (2008). [CrossRef] [PubMed]
21. J. M. Moran, “calculation of the minimum repetition rate of a cavity-dumped four-level laser,” IEEE J. Quantum Electron. 12(10), 639–644 (1976). [CrossRef]
