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Highly efficient diode-pumped actively Q-switched Nd: YAG– SrWO4 intracavity Raman laser Xiaohan Chen,1 Xingyu Zhang,1,3 Qingpu Wang,1 Ping Li,1 Shutao Li,1 Zhenhua Cong,1 Guohua Jia,2 and Chaoyang Tu2,* 1
School of Information Science and Engineering, Shandong University, Jinan, 250100, China Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China 3 E-mail:
[email protected] *Corresponding author:
[email protected]
2
Received October 31, 2007; revised February 2, 2008; accepted February 14, 2008; posted February 28, 2008 (Doc. ID 89154); published March 28, 2008 A highly efficient diode-pumped actively Q-switched intracavity Raman laser with SrWO4 as the Ramanactive medium is presented. As high as 23.8% diode-to-Stokes optical conversion efficiency is obtained with an incident pump power of 7.17 W and a pulse repetition rate of 15 kHz. © 2008 Optical Society of America OCIS codes: 140.3550, 140.3580, 140.3540, 140.3480.
Stimulated Raman scattering (SRS) is an attractive method of frequency conversion based on a thirdorder nonlinear optical process [1–3]. With the development of high-quality Raman crystals in recent years, there has been a rapid increase of interest in solid-state Raman lasers [4,5]. The crystalline Raman media have advantages in their favorable thermal and mechanical properties, a narrow linewidth of the vibrational modes, and their compatibility with the compact, all-solid-state laser technology. In light of the fact that the diode-pumping technology has become mature, it is practical to develop the diodepumped solid-state SRS lasers. These devices have shown efficient and reliable performance in various experimental configurations, such as intracavity Raman resonators [6–11], external Raman resonators [12,13], etc. The intracavity Raman laser configuration is very attractive because it takes advantage of the high intensity inside the laser cavity and uses multiple round trips of the fundamental laser inside the Raman cavity to increase the effective interaction length, leading to low-threshold operation and high overall conversion efficiencies. The diode-to-Stokes conversion efficiency based on the LD pumped intracavity Raman laser is ⬃10% in general [7–9], and the obtained highest conversion efficiency was 16.9%, achieved by Chen et al. in 2005 with a Nd:YAG crystal as the gain medium and a BaWO4 crystal as the Raman-active medium [10]. As a new promising Raman-active crystal, SrWO4 has attracted much attention for its good mechanical and optical properties. Its steady-state Raman gain coefficient is 5.0 cm/ GW at 1064 nm. The Raman mode at 921 cm−1 has the strongest intensity, and its linewidth is 2.7 cm−1 at room temperature [14]. The spontaneous Raman spectroscopy of SrWO4 crystal was investigated by Basiev et al. in 2000 [15]. Shortly thereafter, the intracavity Raman generation employing SrWO4 as the Raman-active material was studied using an alexandrite free-running multimode laser as the pump source [14,16,17]. In 2006, Ding et al. developed an external cavity Raman converter with SrWO4 Raman crystal [18]. However, to our 0146-9592/08/070705-3/$15.00
knowledge, no research on the LD end-pumped intracavity Raman frequency conversion based on the SrWO4 Raman-active crystal has been carried out. In this Letter, we report our results on a highly efficient intracavity SrWO4 Raman generation in an actively Q-switched diode-pumped Nd:YAG laser. At an incident pump power of 7.17 W, the intracavity Raman laser system delivers 1.71 W average output at 1179.6 nm at a pulse repetition frequency (PRF) of 15 kHz. The corresponding conversion efficiency from diode to Stokes is as high as 23.8%, much higher than the previously obtained diode-to-Stokes conversion efficiencies [6–10,19]. Figure 1 shows the experimental setup of the diode-pumped actively Q-switched Nd: YAG– SrWO4 intracavity Raman laser. The pump source is a 30 W fiber-coupled 808 nm laser diode with a core diameter of 400 m and a numerical aperture of 0.22. A focusing lens system with a focal length of 50 mm and a coupling efficiency of 95% is used to reimage the pump beam into the laser crystal. The resonator has a concave-plane configuration. The rear mirror M1 is a concave mirror with a curvature radius of 1000 mm. It is coated for high reflection (HR) at 1064 and 1180 nm 共R ⬎ 99.8% 兲 and high transmission (HT) at 808 nm 共T ⬎ 95% 兲. The output coupler M2 is a plane mirror. It is coated for HR at 1064 nm 共R ⬎ 99.8兲 and partial reflection (PR) at the first Stokes wavelength 1180 nm 共R = 75% 兲. The laser medium is a 1 at. % Nd3+ : YAG crystal with a length of 5 mm. The Raman active medium is an a-cut SrWO4 crystal with a length of 35 mm. Both sides of the Nd:YAG
Fig. 1. Schematic of the experimental setup. © 2008 Optical Society of America
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Fig. 2. Average output power at 1064 nm with respect to the incident pump power for the PRFs of 10, 15, and 20 kHz.
and SrWO4 crystals are antireflection (AR) coated at 1064 and 1180 nm 共R ⬍ 0.2% 兲. The entrance face of the Nd:YAG is also HT coated at 808 nm. Both crystals are wrapped with In foil and mounted in watercooled Cu blocks. The water temperature is maintained at 20° C. A 35 mm long acousto-optic (AO) Q-switch (Gooch & Housego Company) driven at 41 MHz center frequency with 15 W of rf power is placed between the Nd:YAG crystal and the SrWO4 crystal. The overall laser cavity length is approximately 12.5 cm. A dichroic mirror is used to block the 1064 nm fundamental laser. The average output power is measured by a power meter (Molectron PM10) connected to Molectron EPM2000. Before using the mirror M2 to generate the Raman output, we first study the performance of this laser operating at 1064 nm. Four output couplers with different reflectivities at 1064 nm (R = 92%, 84%, 80%, and 70%) are employed instead of the Raman output coupler M2. The optimum reflectivity of the output coupler is approximately 80%. Figure 2 gives the re-
Fig. 3. Average output power at the Stokes wavelength with respect to the incident pump power for the PRFs of 10, 15, and 20 kHz.
lation between the average output power at 1064 nm and the incident pump power with the 80% reflectivity mirror. The threshold for 1064 nm oscillation is 1.55 W. At an incident pump power of 8 W, the average output powers at 1064 nm are 3.16, 3.40, and 3.51 W at the PRFs of 10, 15, and 20 kHz, respectively. When the Raman output coupler M2 is used, we investigate the characteristics of the 1180 nm Raman laser. Figure 3 shows the average output power with respect to the incident 808 nm pump power. The thresholds at the PRFs of 10, 15, and 20 kHz are 2.35, 3.09, and 3.47 W, respectively. For all pumping powers, no second Stokes is observed. Although reducing the PRF leads to a lower threshold for the Raman laser, as can be seen from Fig. 3, the average output power goes saturated at higher pump power under the PRF of 10 kHz. However, no saturation is found under the PRFs of 15 and 20 kHz. The main reason for this problem may be self-focusing, which was discussed by Chen et al. [10]. The highest average output power is 1.78 W, which is obtained at a PRF of 20 kHz and a LD pump power of 8.0 W. The corresponding conversion efficiency from diode pump power to Raman output power is 22.3%. The highest diode-to-Stokes optical conversion efficiency is 23.8%, which is obtained at a PRF of 15 kHz and a pump power of 7.17 W. To the best of our knowledge, this is the highest conversion efficiency obtained by a LD pumped intracavity Raman laser. Based on Fig. 3, the slope efficiencies at the PRFs of 10, 15, and 20 kHz are 34.8%, 41%, and 38.8%, respectively. Figure 4 gives the relationship between the pulse width of the Raman laser with respect to the incident pump power at the PRFs of 10, 15, and 20 kHz, respectively. The pulse’s temporal behavior is recorded by a Tektronix digital phosphor oscilloscope (TDS3052B, 500 MHz bandwidth) with two fast p-i-n photodiodes. One photodiode is placed behind the dichroic mirror to record the 1180 nm laser, while the other is placed
Fig. 4. Pulse width at the Stokes wavelength with respect to the incident pump power for the PRFs of 10, 15, and 20 kHz.
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agreement with the optical vibration modes of the tetrahedral WO42− ionic groups. In summary, the characteristics of a diode-pumped actively Q-switched Nd: YAG– SrWO4 intracavity Raman laser has been investigated for what we believe to be the first time. With an incident pump power of 7.17 W, as much as 1.71 W of average output power at the Stokes wavelength is generated at a PRF of 15 kHz, corresponding to a diode-to-Stokes conversion efficiency of 23.8%. To the best of our knowledge, this is the highest intracavity Raman conversion efficiency to date.
Fig. 5. (Color online) Typical oscilloscope traces for the fundamental and Raman pulses.
Fig. 6. Optical spectra for the actively Q-switched intracavity Nd: YAG– SrWO4 Raman laser.
ahead the dichroic mirror to record the 1064 nm laser. The typical time shapes for the fundamental and Raman laser pulses are shown in Fig. 5, which are recorded at a pump power of 5.93 W and a PRF of 15 kHz. The pulse durations of the fundamental and the Raman lasers are approximately 20.9 and 9.2 ns, respectively. The spectral information is monitored by a widerange optical spectrum analyzer (Yokogawa AQ 6315A, 350– 1750 nm). To obtain the spectra including both the fundamental and the Raman waves, we replace the dichroic mirror with a a filter that is coated for HT at 1064 nm and PR 共R = 80% 兲 at 1180 nm. Figure 6 depicts the optical spectra of the actively Q-switched Nd: YAG– SrWO4 intracavity Raman laser output, which is recorded at an incident pump power of 7.17 W and a PRF of 15 kHz. It can be seen that the frequency shift between the first-Stokes and the fundamental laser is ⬃921 cm−1, which is in
This research is supported by the Science and Technology Development Program of Shandong Province (grant 2004GG2203098), National Science and Technology Innovative Fund Program of the Chinese Academy of Sciences (CXJJ-182), and the Natural Science Foundation of the Fujian Province (grants 2005HZ1026 and 2007H0037). References 1. G. Eckhardt, D. P. Bortfeld, and M. Geller, Appl. Phys. Lett. 3, 137 (1963). 2. W. Kaiser and M. Maier, in Laser Handbook, F. T. Arecchi and E. O. Schulz-Dubois, eds. (North-Holland, 1972), Vol. II, p. 1077. 3. Y. R. Shen, Principles of Nonlinear Optics (Wiley, 1984). 4. H. M. Pask, Prog. Quantum Electron. 27, 3 (2003). 5. J. T. Murray, W. L. Austin, and R. C. Powell, Opt. Mater. 11, 353 (1999). 6. H. M. Pask and J. A. Piper, IEEE J. Quantum Electron. 36, 949 (2000). 7. Y. F. Chen, Opt. Lett. 29, 1915 (2004). 8. Y. F. Chen, Opt. Lett. 29, 2632 (2004). 9. F. F. Su, X. Y. Zhang, Q. P. Wang, S. H. Ding, P. Jia, S. T. Li, S. Z. Fan, C. Zhang, and B. Liu, J. Phys. D 39, 2090 (2006). 10. Y. F. Chen, K. W. Su, H. J. Zhang, J. Y. Wang, and M. H. Jiang, Opt. Lett. 30, 3335 (2005). 11. S. T. Li, X. Y. Zhang, Q. P. Wang, X. L. Zhang, Z. H. Cong, H. J. Zhang, and J. Y. Wang, Opt. Lett. 32, 2951 (2007). 12. R. P. Mildren, M. Convery, H. M. Pask, J. A. Piper, and T. Mckay, Opt. Express 12, 85 (2004). 13. H. M. Pask, S. Myers, J. A. Piper, J. Richards, and T. Mckay, Opt. Lett. 28, 435 (2003). 14. L. I. Ivleva, T. T. Basiev, I. S. Voronina, P. G. Zverev, V. V. Osiko, and N. M. Polozkov, Opt. Mater. 23, 439 (2003). 15. T. T. Basiev, A. A. Sobol, Yu. K. Voronko, and P. G. Zverev, Opt. Mater. 15, 205 (2000). 16. H. Jelinkova, J. Sulc, T. T. Basiev, P. G. Zverev, and S. V. Kravtsov, Laser Phys. Lett. 2, 4 (2005). 17. J. Sulc, H. Jelinkova, T. T. Basiev, M. E. Doroschenko, L. I. Ivleva, V. V. Osiko, and P. G. Zverev, Opt. Mater. 30, 195 (2007). 18. S. H. Ding, X. Y. Zhang, Q. P. Wang, F. F. Su, S. T. Li, S. Z. Fan, Z. J. Liu, J. Chang, S. S. Zhang, S. M. Wang, and Y. R. Liu, IEEE J. Quantum Electron. 42, 78 (2006). 19. J. A. Piper and H. M. Pask, IEEE J. Sel. Top. Quantum Electron. 13, 692 (2007).
