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We report efficient multi-Stokes Raman output of a KTiOAsO4 (KTA) crystal driven by a laser diode end-pumped acousto-optic Q-switched Nd:YAG laser. The Raman outputs of two x-cut KTA crystals, one with a length of 20 mm and the other one of 25-mm, were experimentally compared. Under an incident pump power of 10.9 W, a maximum output power of 1.12 W with a pulse width of 7.6 ns and a pulse repetition frequency of 15 kHz were obtained. The conversion efficiency and slope efficiency with respect to the incident diode pump power were 10.3% and 15.2%, respectively. The laser output contains multiple Raman Stokes lines with different spectral strengths that varied with the pump power.
© 2014 Optical Society of America
1. Introduction
Stimulated Raman scattering (SRS) has attracted a great deal of attention for its capability of generating new wavelength coherent light. In particular, it has become an important approach to generate coherent light in the 1.1-1.2 μm where the direct generation of laser emission from the rare earth ion doped materials is difficult [1,2]. Recently, a variety of crystals such as vanadates [3–6], tungstates [7–9], and molybdates [10,11] were discovered to have big Raman gain coefficient, and were adopted for efficient Raman lasers. Compared with the Raman crystals listed above, the KTA and its isomorph KTiOPO4 (KTP) own small Raman shift and strong Raman gain coefficient. They permit generating multi-order Stokes radiation through the cascade stimulated Raman scattering.
The Raman spectrum of KTA crystal was investigated in 1990’s by Watson [12] and Tu et al. [13]. The strongest Raman scattering is from the A1 (ZZ) geometry and the Raman peak is at 234 cm−1. Liu et al reported the first Stokes at 1091 nm [14] and second Stokes at 1120 nm [15]. Maximum output power of 4.55 W for first Stokes at 1091 nm was achieved by diode-side-pumped Nd:YAG/KTA Raman configuration, with the diode-to-Stokes conversion efficiency of 7.5%. Using diode-end-pumped Nd:YAG/KTA Raman configuration, 0.63 W second Stokes at 1120 nm with conversion efficiency of 9.4% was obtained. Lan et al. also reported a diode end-pumped passively Q-switched Nd:YAG/KTA Raman laser at 1091nm [16]. The first Stokes power of 250 mW at 1091 nm was obtained with conversion efficiency of 3.3%. Chang et al reported simultaneous multi-order Stokes radiation with high peak powers at room temperature. It could be a potential pump source for the terahertz (THz) generation based on the nonlinear difference frequency method [17]. In [17], the first, second, third and fourth Stokes for the KTP crystal with Raman shift of 270 cm−1 was reported.
In this paper, multi-order Raman Stokes emission in KTA crystal was obtained based on intracavity cascade Raman conversion. The KTA was pumped by a laser diode (LD) end-pumped acousto-optic Q-switched Nd:YAG laser. At an incident pump power of 10.9 W and a pulse repetition frequency of 15 kHz, an average output power of 1.12 W and pulse width of 7.6 ns were obtained, a diode to Raman output conversion efficiency of 10.3% and slope efficiency of 15.2% was obtained.
2. Experimental setup
A schematic diagram of the LD end-pumped acousto-optically Q-switched Nd:YAG/KTA intra-cavity cascade Raman laser is shown in Fig. 1. A fiber coupled LD array operating at 808 nm was used as the pumping source. The coupling fiber has a core diameter of 200 μm and a numerical aperture of 0.22. The pumping light was re-imaged into an Nd:YAG crystal with a spot size of about 320 μm in diameter using a pair of achromatic lenses. The Nd:YAG crystal has Nd doping concentration of 0.8 at.% and a dimension of 3 mm × 3 mm × 10 mm in size. It was wrapped with indium foil and mounted in a water-cooled copper block whose temperature was kept at 20 °C. KTA crystals along X axis (θ = 90°, ϕ = 0°) cut were adopted as the Raman crystal to meet the X(Z,Z)X Raman configuration, under which the Raman shift at 234 cm−1 has the strongest Raman scattering gain. KTA crystal with two different lengths, one with a size of 5 mm × 5 mm × 25 mm and the other 5 mm × 5 mm × 20 mm, were used for comparison. Both end-faces of the KTA and Nd:YAG crystals were antireflection (AR) coated from 1050 to 1200 nm. In order to achieve the Q-switching operation, a 30-mm-long acousto-optic Q-switching module (AOM, Gooch & Housego Co.) was placed between the Nd:YAG and KTA crystals. The fundamental and the Raman laser oscillation shared the same flat–flat resonator of a total length of 75 mm, composed of an input mirror (IM) and an output coupler (OC). The mirror IM was high-transmission (HT, T>95%) coated at 808 nm and high reflection (HR, R > 99.9%) coated at 1000-1200 nm. The mirror OC was HR coated at 1064 nm. Its transmission is slowly increased for the wavelength from 1064 to 1200 nm. At a fundamental pump wavelength of 1064 nm, the wavelength of the first to fourth-order Raman Stokes lines of the KTA can be calculated. They are at 1091, 1120, 1150 and 1182 nm. Table 1 listed the reflectivity of the mirror OC at the Stokes wavelengths. The laser output power was detected by a thermal sensor power meter (Model: PM310D, Thorlabs Inc).
Fig. 1 Experimental arrangement of the LD end-pumped acousto-optically Q-switched Nd:YAG/KTA intra-cavity Raman laser.
Table 1. Reflectivity of mirror OC for different Stokes wavelengths
3. Experimental results and discussion
The output characteristics of the intra-cavity Raman laser with the two different KTA crystal lengths were experimentally compared. Through optimizing the Q-switch pulse repetition frequency and duty cycle, a maximum output power was realized at the pulse repetition frequency of 15 kHz and the duty cycle of 3%. The average output power as a function of the incident pump power for both lengths of KTA at the optimized pulse repetition frequency are shown in Fig. 2.
Fig. 2 The average output power versus incident pump power of the Raman lasers with the two different KTA crystal lengths.
Both average output powers increased with the incident pump powers above the Raman thresholds which were around 3 and 3.5 W, respectively. The Raman slope efficiency with respect to incident pump power for the 25 mm length KTA was about 15.2%, which was higher than that for the 20 mm length KTA. Under the incident pump power of 10.9 W, the maximum output power obtained was 1.12 W. A conversion efficiency of 10.3% was achieved for the 25 mm length KTA crystal. Figure 3 shows maximum average output power versus pulse repetition frequencies at the incident pump power of 10.9 W.
Fig. 3 Maximum average output power versus pulse repetition frequency at the incident pump power of 10.9 W.
The output spectra were measured by a grating monochromater (ZOLIX, model Omni-λ500). Figure 4 shows the spectra of the Raman output using 25 mm length KTA with the optimized pulse repetition frequency of 15 kHz. Multiple Raman Stokes lines were detected in the wavelength range from 1.06 to 1.2 μm. Based on the measured spectra, when the incident pump power was below 4.8 W, only the first to third order Stokes light at the vibration mode frequency of 234 cm−1 are visible. The fourth order Stokes light became visible at the incident pump power of about 5 W. Figure 4(b) and Fig. 4(c) shows the spectra of the laser output under an incident pump power of 6.2 W and 10.9 W, respectively. Increasing pump power, the ratios of the spectral strength for the third and fourth order Stokes light to the total output increased. Beside the first to fourth order Stokes light at the vibration mode frequency of 234 cm−1, Raman light at 1146 nm was also detected, which is the first Stokes line at the vibration mode frequency of 671 cm−1. Under the incident pump power of 10.9 W, a maximum average output power of 1.12 W was obtained. The output spectrum contains six spectral lines, two of them have strong spectral strength and four have low spectral strength, as shown in Fig. 4(b) and Fig. 4(c). The two lines with high spectral strength are the third and fourth Stokes at 1150 and 1182 nm with the Raman shift of 234 cm−1. The four lines with low spectral strength are the fundamental at 1064 nm, the first and the second Stokes at 1091 and 1120 nm with the Stokes shift of 234 cm−1, and the first Stokes at 1146 nm with the Stokes shift of 671 cm−1. The third and fourth Stokes lines with similar intensity were about seven times that of fundamental and other Stokes light.
Fig. 4 Optical spectra of the Raman laser emission under different pump power level: (a) 4.8W, (b) 6.2W, and (c) 10.9W.
Accompany the output of the multi-Stokes, the yellow light irradiated from the KTP crystal can be seen, which is due to the frequency mixing among the fundamental and the multi-Stokes lights. The spectra were detected by AvaSpec-3648 Fiber Optic Spectrometer, the results is shown in Fig. 5. There are more than ten lines detected in the region from 530 to 600 nm. The temporal pulse profile of the Raman output of the laser was recorded by a PIN photodiode, and displayed on a 500 MHz oscilloscope (Model DPO3052B). Because the different order Stokes light couldn’t be separated, Fig. 6 shows the temporal pulse profile of the total output at the pulse repetition frequency of 15 kHz, and an incident pump power of 10.9 W. The pulse width was about 7.6 ns. The average power stability of the Raman output was investigated with a power meter. We found that the average output power fluctuation was about 8% in an hour at the maximum output power of 1.12 W.
4. Conclusion
In conclusion, efficient multi-order Stokes emission was demonstrated in a LD end-pumped acousto-optic Q-switched Nd:YAG/KTA intra-cavity Raman laser. The Raman outputs under two different length x-cut KTA crystals were experimentally compared. Under an incident pump power of 10.9 W, a maximum output power of 1.12 W with the pulse width of 7.6 ns and a pulse repetition frequency of 15 kHz was obtained. The Raman conversion efficiency and slope efficiency with respect to the incident diode pump power were 10.3% and 15.2%, respectively. A total of six spectral lines were observed on the laser output. The intensity of third Stokes at 1150 nm and fourth Stokes at 1182 nm are about seven times of that of the fundamental and others low order Stokes light. The two intense Stokes lines with interval of 234 cm−1 could be a potential pump source for the THz generation.
Acknowledgments
This work was supported by the National Science Foundation of Zhejiang Province under Grants LQ13F050004, high-level talent innovation technology project fund of Wenzhou, national natural science foundation of China under Grant 10904143, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
References and links
1. Y. T. Chang, H. L. Chang, K. W. Su, and Y. F. Chen, “High-efficiency Q-switched dual-wavelength emission at 1176 and 559 nm with intracavity Raman and sum-frequency generation,” Opt. Express 17(14), 11892–11897 (2009). [CrossRef] [PubMed]
2. H. Y. Zhu, Y. M. Duan, G. Zhang, C. H. Huang, Y. Wei, W. D. Chen, L. X. Huang, and Y. D. Huang, “Efficient continuous-wave YVO4/Nd:YVO4 Raman laser at 1176 nm,” Appl. Phys. B-Lasers Opt. 103(3), 559–562 (2011). [CrossRef]
3. A. A. Kaminskii, K. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4 – new efficient χ(3)-materials for Raman lasers,” Opt. Commun. 194(1-3), 201–206 (2001). [CrossRef]
4. H. Y. Zhu, Y. M. Duan, G. Zhang, C. H. Huang, Y. Wei, W. D. Chen, Y. D. Huang, and N. Ye, “Yellow-light generation of 5.7 W by intracavity doubling self-Raman laser of YVO4/Nd:YVO4 composite,” Opt. Lett. 34(18), 2763–2765 (2009). [CrossRef] [PubMed]
5. A. A. Kaminskii, M. Bettinelli, J. Dong, D. Jaque, and K. Ueda, “Nanosecond Nd3+:LuVO4 self-Raman laser,” Laser Phys. Lett. 6(5), 374–379 (2009). [CrossRef]
6. J. Lin and H. M. Pask, “Nd:GdVO4 self-Raman laser using double-end polarised pumping at 880 nm for high power infrared and visible output,” Appl. Phys. B-Lasers Opt. 108(1), 17–24 (2012). [CrossRef]
7. A. J. Lee, H. M. Pask, J. A. Piper, H. J. Zhang, and J. Y. Wang, “An intracavity, frequency-doubled BaWO4 Raman laser generating multi-watt continuous-wave, yellow emission,” Opt. Express 18(6), 5984–5992 (2010). [CrossRef] [PubMed]
8. Y. M. Duan, H. Y. Zhu, G. Zhang, C. H. Huang, Y. Wei, C. Y. Tu, Z. J. Zhu, F. G. Yang, and Z. Y. You, “Efficient 559.6 nm light produced by sum-frequency generation of diode-end-pumped Nd:YAG/SrWO4 Raman laser,” Laser Phys. Lett. 7(7), 491–494 (2010). [CrossRef]
9. M. T. Chang, W. Z. Zhuang, K. W. Su, Y. T. Yu, and Y. F. Chen, “Efficient continuous-wave self-Raman Yb:KGW laser with a shift of 89 cm⁻¹,” Opt. Express 21(21), 24590–24598 (2013). [CrossRef] [PubMed]
10. H. H. Yu, Z. Li, A. J. Lee, J. Li, H. J. Zhang, J. Y. Wang, H. M. Pask, J. A. Piper, and M. H. Jiang, “A continuous wave SrMoO4 Raman laser,” Opt. Lett. 36(4), 579–581 (2011). [CrossRef] [PubMed]
11. J. A. Piper and H. M. Pask, “Crystalline Raman Lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 692–704 (2007). [CrossRef]
12. G. H. Watson, “Polarized Raman spectra of KTiOAsO4 and isomorphic nonlinear-optical crystals,” J. Raman Spectrosc. 22(11), 705–713 (1991). [CrossRef]
13. C. S. Tu, A. R. Guo, R. W. Tao, R. S. Katiyar, R. Y. Guo, and A. S. Bhalla, “Temperature dependent Raman scattering in KTiOPO4 and KTiOAsO4 single crystals,” J. Appl. Phys. 79(6), 3235–3240 (1996). [CrossRef]
14. Z. J. Liu, Q. P. Wang, X. Y. Zhang, S. S. Zhang, J. Chang, Z. H. Cong, W. J. Sun, G. F. Jin, X. T. Tao, Y. X. Sun, and S. J. Zhang, “A diode side-pumped KTiOAsO4 Raman laser,” Opt. Express 17(9), 6968–6974 (2009). [CrossRef] [PubMed]
15. Z. J. Liu, Q. P. Wang, X. Y. Zhang, S. S. Zhang, J. Chang, H. Wang, S. Z. Fan, W. J. Sun, X. T. Tao, S. J. Zhang, and H. J. Zhang, “1120 nm second-Stokes generation in KTiOAsO4,” Laser Phys. Lett. 6(2), 121–124 (2009). [CrossRef]
16. W. X. Lan, Q. P. Wang, Z. J. Liu, X. Y. Zhang, F. Bai, H. B. Shen, and L. Gao, “A diode end-pumped passively Q-switched Nd:YAG/KTA Raman laser,” Optik (Stuttg.) 124(24), 6866–6868 (2013). [CrossRef]
17. Y. T. Chang, Y. P. Huang, K. W. Su, and Y. F. Chen, “Diode-pumped multi-frequency Q-switched laser with intracavity cascade Raman emission,” Opt. Express 16(11), 8286–8291 (2008). [CrossRef] [PubMed]
