Comparison of CW Pumping and Quasi-CW Pumping for a Passively Q-switched Nd:YAG Laser

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1 Journal of the Korean Physical Society, Vol. 57, No. 2, August 2010, pp Comparison of CW Pumping and Quasi-CW Pumping for a Passively Q-switched Nd:YAG Laser Kangin Lee, Youngjung Kim, Jin Seog Gwag, Jin Hyuk Kwon and Jonghoon Yi Department of Physics, Yeungnam University, Gyeongsan , Korea (Received 31 December 2009, in final form 5 July 2010) A passively Q-switched 1064-nm laser pulse was generated when a quasi-cw laser diode beam with a pulsewidth of several hundreds of microseconds pumped a Nd:YAG crystal. A Cr:YAG crystal was used as a saturable absorber. The temporal delay between the onset of the laser diode pump and the Q-switched output pulse was studied as a function of the initial transmission of Cr:YAG for several output couplers with different reflectivities. By pumping the Nd:YAG laser with a quasi-cw laser diode, we could actively control the pulse repetition rate of the passively Q-switched Nd:YAG laser. Also, the temporal jitter of the Q-switched pulse for the cw-laser-diode-pumped case could be reduced by using quasi-cw pumping. The temporal width and the peak power of the Q-switched pulse for the quasi-cw-pumped case were compared with those for the cw-laser-diode-pumped case. PACS numbers: v, Gd, Xi Keywords: Nd:YAG laser, Passive Q-switching, Diode-pumped laser, Quasi-cw pumping DOI: /jkps I. INTRODUCTION Diode-pumped lasers have reduced the volume of a laser considerably compared with that of flashlamppumped lasers [1,2]. For a pulsed laser, an acousto-optic or an electro-optic Q-switch is still usually employed inside the cavity. The passive Q-switching method can be used to replace such bulky devices, thus reducing the volume of Q-switched lasers [3 5]. Owing to their merits of very simple structure, robust mounting, and cheap price, passively Q-switched solid state lasers have been extensively studied for several decades in an attempt to develop small and compact pulsed laser. However, passively Q-switched lasers are still lacking in stability and efficiency compared with the lasers Q-switched by using active methods. In a passively Q-switched laser, the pulse repetition rate still cannot be controlled precisely. Thus, active methods are commonly employed for industrial lasers, despite the expensive optics and the bulky control electronics [1 6]. However, there are some applications in which the laser volume should be small, so in these cases, active methods cannot be employed. While solid organic dyes were used for passive Q- switching elements during the early stage of the development, solid state materials, such as semiconductor saturable absorber mirror (SESAM) or Cr:YAG crystals, have been widely used recently due to their high stability in lasing and long lifetime compared to dye [7 9]. jhyi@yu.ac.kr Especially, Cr:YAG has attracted considerable attention for Q-switching Nd-based lasers, but in spite of intensive study, applications of Cr:YAG in industrial lasers have been very limited due to pulse fluctuations. The pulses from passively Q-switched lasers show considerable fluctuations not only in amplitude but also in temporal behavior [10 12]. The temporal interval between pulses shows large jitter, which limits applications for industrial purposes. The lower energy conversion efficiency is another serious problem. In this work, we tried a quasi-cw-pumping method to reduce the fluctuation of the temporal interval between pulses. This quasi-cw-pumping method was applied to generate pulses from a passively Q-switched microchip laser for laser ignition of a combustion engine [13]. Compared to the work in Ref. 13, we report various lasing characteristics, such as pulse buildup time, peak power, and pulse width, as functions of the output coupler reflectivity, the initial transmission of Cr:YAG and the pump power. Based on the results, we will discuss the differences in output characteristics between quasi-cw pumping and cw pumping. II. EXPERIMENTS AND RESULTS A schematic of the experimental setup is given in Fig. 1. The laser crystal used in the experiment was Nd:YAG with a doping level of 1 at.%, and dimension of mm 3. The exit and the entrance faces of the Nd:YAG -359-

2 -360- Journal of the Korean Physical Society, Vol. 57, No. 2, August 2010 Fig. 1. Schematic diagram of the experimental setup for the passively Q-switched Nd:YAG laser. crystal were anti-reflection (AR) coated at 1064 nm (reflectance <1%). The Nd:YAG crystal was wrapped with thin indium foil, and then inserted in the water-cooled copper mount. The cooling water temperature was set at 24.7 C. The Nd:YAG crystal was longitudinally pumped by using a fiber-coupled cw laser diode (JDSU, 808 nm). The diode laser had a maximum power of 4 W. The core diameter of the fiber was 100 µm, and the numerical aperture of the fiber was The pump beam emitted from the fiber was collimated and then focused by using a set of lenses with 50-mm focal lengths. The diameter of the pump beam spot focused on the entrance surface of the Nd:YAG was 160 µm. The radius was measured by using a scanning knife-edge method. The laser diode was mounted on a copper block. The water flowing inside the block was directed to the inlet of the crystal cooling mount. A flat dichroic mirror, AR coated at 808 nm and high reflection (HR) coated at 1064 nm, was used as the input coupler. Also, several flat mirrors with reflectivities (R) of 77%, 81%, 88%, and 94% at 1064 nm were used as the output coupler to test the dependence of the lasing characteristics on the reflectivity. For the cooling mount of crystal occupied space, the shortest cavity length was 30 mm. The output power of the cw laser was studied. Figure 2 shows the cw Nd:YAG laser output powers (1064 nm) versus the incident pump power (808 nm) for several output couplers with different reflectivities. The maximum cw output power was achieved when the output coupler reflectivity was 77%. When the incident pump power was 4 W, a maximum power of 1.12 W was obtained (optical conversion efficiency = 28%). For reflectivities in the range from 77% to 88%, the dependences of the output power on the pump power are very similar. When the reflectivity of the output coupler was 94%, the output power of the cw Nd:YAG laser dropped to 0.82 W. After inserting the saturable absorber inside the laser cavity, we observed the pulsed output from the cavity. The Cr:YAG crystals with initial transmissions (T 0 ) of 50%, 60%, 70%, 80%, and 90% were used to determine Fig. 2. Output power of the Nd:YAG laser as a function of the incident power of the cw laser-diode pump for output couplers with R = 77%, 81%, 88%, and 94%. the dependence of the output power characteristics on the initial transmission. The dimensions of each Cr:YAG crystals were mm 3, and both surfaces were AR coated at 1064 nm. The Cr:YAG crystal was wrapped with thin indium foil and then tightly enclosed in the copper housing. The housing temperature was stabilized by using flowing water. For each Cr:YAG crystal, passively Q-switched pulse s characteristics as functions of the output coupler reflectivity and the incident pump power were measured. In Fig. 3, the average output power of the passively Q- switched Nd:YAG laser as a function of the pump power and the initial transmission of the Cr:YAG is shown. A maximum Q-switched power of 710 mw was obtained when the output coupler reflectivity was 77% and the T 0 was 90% (optical conversion efficiency = 17.8%). When Cr:YAG with a lower T 0 was used, the average output power decreased. For a Cr:YAG with a T 0 of 50%, the average output became 235 mw (optical conversion efficiency = 11.9%). The pulse repetition rates and the pulsewidth, which are important characteristics of a passively Q-switched laser, were measured. The dependences of the pulse repetition rate and the pulsewidth on the T 0 and the output coupler reflectivity are shown in Fig. 4(a), and Fig. 4(b), respectively. Figure 4(a) shows that the pulse repetition rate increase from 5 khz to 55 khz when the T 0 is increased from 50% to 90%. At 5-kHz repetition rate (T 0 = 50%), the Q-switched pulse showed a temporal jitter of 40 µs. Figure 4(b) shows that the pulsewidth also increases as the T 0 is increased. When the T 0 was 90%, the average pulsewidth was 22.3 ns. When a T 0 of 50% and an output coupler reflectivity of 81% were used, a minimum pulsewidth of 2.64 ns was obtained. Compared with the case of Cr:YAG with a 90% T 0, the pulsewidth was almost reduced by a factor of 1/8. These results demonstrate that the pulse repetition rate and the pulsewidth depend slightly on the output coupler reflec-

3 Comparison of CW Pumping and Quasi-CW Pumping for a Passively Q-switched Nd:YAG Laser Kangin Lee et al Fig. 3. Average output power of passively Q-switched Nd:YAG as a function of the incident power of the cw laserdiode pump for Cr:YAG with different initial transmissions (T 0). Fig. 5. Peak power of pulses from a cw-pumped, passively Q-switched Nd:YAG laser as a function of the initial transmission of Cr:YAG for output couplers with different reflectivities. Fig. 4. (a) Pulse repetition rate and (b) Pulsewidth of output pulses from the cw-pumped, passively Q-switched Nd:YAG laser as functions of the initial transmission of Cr:YAG for output couplers with different reflectivities. tivity while they markedly depend on the T 0 of Cr:YAG. For higher pulse energy and high peak power, Cr:YAG with a lower T 0 is preferred. Figure 5 shows the pulse peak power as a function of the output coupler reflectance and the T 0 of Cr:YAG. A maximum pulse energy of 48 µj and a peak power of about 18 kw were obtained when Cr:YAG with a T 0 of 50% and an output coupler with a reflectivity of 77% were used. In view of Q-switching theory, the pulse energy increases when the stored energy is large. Cr:YAG with a higher T 0 is bleached more quickly than Cr:YAG with a lower T 0. Thus, a lower T 0 leads to a long storage time. As the T 0 of Cr:YAG was decreased, the pulse fluctuation also decreased, and the optical conversion efficiency decreased significantly due to the increased spontaneous decay of the excited population [14 16]. Figure 5 shows that not only the T 0 of Cr:YAG but also the reflectivity of the output coupler affects the peak power considerably. For a cw laser-diode-pumped, passively Q-switched laser, the pulse repetition rate is vulnerable to external perturbations. The instability of the output pulse energy and the temporal jitter can be reduced by sending the pulsed diode beam to a cw diode laser by using a pulse current driver (Lightwave, LDX-3600). Figures 6(a) and 6(b) show the temporal profile of the output pulse from a quasi-cw laser-diode-pumped, passively Q-switched laser. Figure 6(b) is temporally expanded graph of the passively Q-switched pulse shown in Fig. 6(a). In the experiment, the pulse repetition of the passively Q-switched laser was decided by the modulation frequency of driving current. Careful control of the peak power and the temporal width of the current pulse generated a single Q-switched pulse for a single QCW pumping laser pulse. The temporal width of the Q-switched pulse shown in Fig. 6(b) was 2.45 ns when the T 0 of Cr:YAG was 50% and the output coupler reflectivity was 81%. The temporal width of the Q-switched pulse from the quasi-cw laser-diode-pumped laser was very close to that of the pulse from the cw laser-diodepumped laser (2.76 ns) when the peak pump powers in

4 -362- Journal of the Korean Physical Society, Vol. 57, No. 2, August 2010 Fig. 6. (a) Photo of the quasi-cw laser-diode output and passively Q-switched laser, and (b) Graph of passively Q- switched output pulse for a quasi-cw-pumped Nd:YAG laser. The output coupler reflectivity is 77%, and the Cr:YAG initial transmission is 60%. Fig. 8. (a) Pulsewidth and (b) Peak power of quasi-cwpumped, passively Q-switched Nd:YAG laser output pulses as functions of the initial transmission for output couplers with different reflectivities. Fig. 7. Variations of the buildup time of pulses from a quasi-cw-pumped Nd:YAG laser as a function of the initial transmission of Cr:YAG for output couplers with different reflectivities. both cases were the same. In Fig. 6, a Q-switched output pulse was generated 340 µs after the pulsed laser diode had been turned on. The time required for pulse generation (pulse buildup time) was affected by the reflectance of the output coupler and the initial transmission of the saturable absorber as shown in Fig. 7. Finding the optimal pulse diode duration was important for stable operation of the passively Q-switched laser. The shortest pulse buildup time was 68 ± 3 µs for Cr:YAG with a 90% initial transmission and an output coupler with a 94% reflectivity. The fluctuation in the pulse buildup time corresponds to the temporal jitter. The jitter was reduced by a factor of almost 1/3 compared with the cw pumped case. The more significant difference in jitter between the cw pumped and quasi-cw pumped cases is in the accuracy of the pulse timing. For the cw pumped case, the jitter can accumulate in time because the pulse-to-pulse interval depends on the excited Nd-ion population remaining after the Q- switched pulse. However, in the quasi-cw pumped case, the pulse start time is controlled by the pulse current supply, and the temporal fluctuation is the fluctuation in only the pulse buildup time. Remaining excited Nd ions decay to the ground state before they can be affected by the energy from the next pump pulse. Figures 8(a) and 8(b) show the pulsewidth and

5 Comparison of CW Pumping and Quasi-CW Pumping for a Passively Q-switched Nd:YAG Laser Kangin Lee et al the peak power of the quasi-cw-pumped, passively Q- switched Nd:YAG laser. Comparing the results with those for the cw laser-diode-pumped case, we find that the effects of the reflectance on the pulse characteristics are quite different. First, the range of variation of temporal width of the pulse (pulsewidth) when the T 0 is changed from 50% to 90% is larger for the quasi-cwpumped case. Second, the optimal output coupler reflectance is shifted to higher reflectance for the quasi-cwpumped case. When the pulsewidth was 2.45 ns and the pulse energy was 25 µj, the energy conversion efficiency was 1.56%. The energy conversion efficiency dropped to about 1/10 that for the cw laser-diode-pumped case. After the Q-switched laser pulse is generated, a significant number of excited Nd ions remain. For the cw laserdiode-pumped case, the energy stored in excited ions contributes to the generation of the next pulse. However, for the quasi-cw laser-diode-pumped case, the energy stored in excited ions becomes an energy loss. Due to the low efficiency, the reflectivity of the output coupler needs to be increased, as can be seen in Fig. 8(b). If the pulse buildup time is considered, the maximum possible pulse repetition rate can reach 15 khz for a 65-µs pump pulsewidth. III. CONCLUSIONS The output characteristics of a passively Q-switched Nd:YAG laser pumped by a quasi-cw laser were investigated. The pulsewidth, pulse buildup time, and pulse energy were compared with the results from the cwlaser-diode-pumped case. When quasi-cw pumping was used, the temporal jitter observed in the cw-laser-diodepumped case was much reduced. The temporal width of the output pulses from the quasi-cw-laser-diode-pumped case showed a larger variation when the initial transmission of the Cr:YAG was changed, the optimal reflectivity of the output coupler shifted to the higher reflectivity side, and the energy conversion efficiency became very low. However, active control of pulse repetition rate has advantages in some applications, such as laser ignition of combustion engines. ACKNOWLEDGMENTS This research was supported by Yeungnam University Research grants in REFERENCES [1] R. Scheps, Introduction to laser diode-pumped solid state laser (SPIE Optical Engineering Press, Bellingham, 2002), pp [2] Koechner, Solid State Engineering (Springer, New York, 1999), Chap. 7. [3] P. Yan, X. Tian, M.-l. Gong and T. Xie, Optical Engineering 44, (2005). [4] H.-H. Wu, S.-F. Chen and C.-H. Cheng, J. Opt. A: Pure Appl. Opt. 9, 376 (2007). [5] J. J. Zayhowski, J. Alloys Comp. 303, 393 (2000). [6] F. Q. Liu, H. R. Xia, S. D. Pan, W. L. Gao, D. G. Ran, S. Q. Sun, Z. C. Ling, H. J. Zhang, S. R. Zhao, J. Y. Wang, Opt. Laser Technol. 39, 1449 (2007). [7] J. Dong, K.-I. Ueda, H. Yagi and A. A. Kaminskii, Optical Rev. 15, 57 (2008). [8] M. Tsunekne, T. Inohara, A. Ando, K. Kanehara and T. Taira, Proceedings of Advanced Solid-State Photonics (Nara, Japan, Jan. 2008), MB4. [9] M. Tsunekne, T. Inohara, A. Ando, K. Kanehara and T. Taira, Proceedings of Conference on Lasers and Electro- Optics/Quantum Electronics and Laser Science (San Jose, Califonia, USA, May 2008), CFJ4. [10] B. Cole, L. Goldberg, C. Ward Trussell, A. Hays, B. W. Schilling and C. McIntosh, Opt. Express 17, 1766 (2009). [11] Y. Kalisky, Prog. Quantum Electron. 28, 249 (2004). [12] N. Pavel, J. Saikawa, S. Kurimura and T. Taira, Jpn. J. Appl. Phys. 40, 1253 (2001). [13] H. Sakai, H. Kan and T. Taira, Opt. Express 16, (2008). [14] J. J. Degnan, IEEE J. Quantum Electron. 25, 214 (1989). [15] J. J. Degnan, IEEE J. Quantum Electron. 31, 1890 (1995). [16] H. Sakai, H. Kan and T. Taira, Opt. Express 16, (2008).