Open Access
Add to BibSonomyAbstract
An efficient intracavity KTA optical parametric oscillator (OPO) driven by diode-end-pumped acousto-optical Q-switched Nd:YVO4 laser is demonstrated. The mode mismatch between fundamental cavity and OPO cavity caused by the thermal lens effect in Nd:YVO4 crystal as the pump power increased was studied. To lessen mode mismatch, a thermal lens-like cavity mirror made of plane BK7 glass induced by idler absorption was introduced into the OPO cavity. Under a diode pump power of 20 W, a maximum 1535nm light output power of 3.6 W was achieved at the pulse repetition rate of 60 kHz, corresponding to a diode-to-signal conversion efficiency of 18%. This is the highest efficiency reported for eye-safe laser based on intracavity OPO.
©2009 Optical Society of America
1. Introduction
Lasers operating around 1.5 μm, which have the characteristics of eye safety and the transparency in the atmosphere and silica-based optical waveguides and fibers, are vital for applications involving remote sensing of aerosols, laser range finding, coherent laser radar, optical communications, etc. Stimulated Raman scattering [1,2], Er3+ doped solid-state laser [3], and optical parametric oscillators (OPO) [4,5] have been developed to obtain eye-safe 1.5 μm solid-state lasers. To our knowledge, the optical efficiencies of all the above methods at present are lower than 15%. Non-critical phase-matching (NCPM) OPO pumped by 1 μm laser based on KTP and KTA has been convinced to be one promising approach for producing an efficient and reliable pulsed eye-safe laser for the applications mentioned above [6]. In comparison to its analog KTP, KTA not only possesses a larger nonlinear coefficient, higher figure of merit, and lower ionic conductivity, but also owns wider transparency range (0.35–5.3 μm), higher optical damage threshold, which makes it be widely used in the high power mid-infrared optical parameter amplification and oscillation (OPA and OPO) [6–8]
OPO based on KTA was first introduced by P. E. Powers, et al. [9], which was pumped by 780-nm Ti:sapphire laser. Recently, KTA-OPOs driven by both diode-side-pumped [8,10] and diode-end-pumped [11–13] solid state lasers have been explored. Up to now, the highest average power (1.1W) and the most efficiency(diode-to-signal conversion efficiency of 12.5%) for the LD end-pumped KTA-OPO output at 1.5 μm were reported by Huang et al. [12] and Liu et al. [13], respectively. In this paper, based on diode-end-pumped acousto-optical Q-switched KTA intracavity optical parametric oscillator, an efficient and compact 1.5 μm eye-safe laser with high power and repetition rate is demonstrated. Nd:YVO4 crystal was adopted as the laser crystal for its large stimulated emission cross section comparing with Nd:YAG, Nd:YLF and Nd:YAP [14]. The mode mismatch between fundamental cavity and OPO cavity was studied, and the thermal lens-like cavity mirror was introduced into OPO cavity to compensate mode mismatch for efficient OPO conversion.
2. Experiments design
The experimental arrangement of the Q-switched intracavity OPO laser is shown in Fig. 1 . The pump source was a 27 W fibre-coupled laser diode array at 808 nm with a core diameter of 200 μm and a numerical aperture of 0.22. The output beam spot from the fibre was imaged with 1:2 magnification into a 0.3 at.% doped a-cut Nd:YVO4 crystal by a pair of plane-convex lenses and the transmission through the pair lenses was about 96%. The Nd:YVO4 crystal (3mm × 3mm × 10 mm, from CASTECH Inc) was wrapped with indium foil and mounted in a thermoelectric cooled copper block and its surface temperature was kept at about 20 °C during the experiments. Film with high-transmission(HT, T>95%) at 808 nm and high-reflection(HR,R>99.9%) at 1064 nm was coated on the incident facet for the pump light, which acted as one of fundamental cavity mirror. A 30-mm-long acousto-optic Q-switcher (AOS, from Gooch & Housego Co.) is antireflection (AR) coated at 1064 nm on both faces and driven at a 40MHz center frequency with 20 W of Radio-Frequency power.
Fig. 1 Experimental configuration for the Q-switched intracavity Nd:YVO4\KTA optical parametric oscillator
A 5 mm × 5 mm × 20 mm KTA crystal (from CRYSTECH Inc) with type II NCPM (θ = 90°, φ = 0°) cut for the pump wavelength of 1.06 μm and signal wavelength of 1.53 μm was adopted as the nonlinear optical crystal, which was AR coated at 1.06 and 1.53 μm wavelengths on both end faces and mounted in a thermoelectric cooled copper block with its surface temperature kept at about 25 °C. The OPO cavity, 35-mm-long, was formed by a pair of plane-parallel mirrors, M1 and M2. The output coupler M1 with one facet was HR coated at 1.06 μm and partial-reflection(PR) with 13% transmission at 1.53 μm, and other facet was AR coated at 1.53 μm. Whereas one facet of M2 was AR coated at 1.06 μm(R<1%)and HR coated at 1.53μm (R > 99%), and other facet was AR coated at both 1.06 μm and 1.53 μm. All mirrors M0, M1 and M2 are made of BK7 glass. A compact resonator with a total length of 11 cm was designed for 1.53 μm light generation.
According to the above experimental setup, the mode radius at the center of the KTA for both fundamental cavity and OPO cavity have been simulated by the ABCD ray transfer matrix(the thermal effects of Q-switch crystal and KTA are ignored). Figure 2 shows mode radii of both cavities at the center of the KTA versus thermal focal length of laser crystal. The mode radius of fundamental cavity is closely related to thermal focal length of laser crystal, whereas, that of the OPO cavity is independent of the thermal focal length of laser crystal. With the increasing of pump power, the thermal focal length of laser crystal is shortened, which leads to the fundamental cavity mode much smaller than OPO cavity mode. The mismatching between both cavity modes will cause the reduction of OPO conversion and the decrease of output power. To our knowledge, no efficient means has been reported on compensating for mode mismatch of intracavity OPO. Based on theoretical designing of resonant cavity, mode mismatch between the fundamental cavity and OPO cavity could be lessened by inserting a lens inside OPO cavity. Figure 2 also shows mode radii of both cavities at the center of the KTA when the OPO cavity was inserted with different focal lengths lens. It can be seen the mode matching between two cavities could be realized only at a certain thermal focal length of laser crystal when the OPO cavity inserted with a focal length lens. In order to well compensate mode mismatch, the focal length of lens inside OPO cavity should be shorten with the thermal focal length of laser crystal while increasing the pump power. Therefore, the mode mismatch can be lessened by adopting a lens with variable focal length inside OPO cavity.
Fig. 2 Mode radius of the fundamental cavity and OPO cavity at the center of the KTA as a function of thermal focal length of Nd:YVO4 (The parameter f is the lens focus length inside the OPO cavity).
Since generating of signal wave at 1.5μm, the idler wave around 3.5 μm usually be absorbed by cavity mirror(BK7), which will result in gradient distribution of refractive index in the cavity mirror and works as a thermal lens-like mirror with variable focal length. Its focal length varies inversely proportional to the OPO idler power. For the general 1.5 μm OPO cavity, the facets of both cavity mirror M2 with HR coating at 1.53 μm and cavity mirror M1 with PR coating at 1.53 μm always face inside OPO cavity, and the thermal lens-like cavity mirrors are excluded outside OPO cavity. Therefore, if reversing the OPO cavity mirror such that the facet of M1 with PR coating at 1.53 μm or M2 with HR coating at 1.53 μm face outside OPO cavity, the lens-like cavity mirror will be included inside of OPO cavity and compensate mode mismatch of the OPO and fundamental cavities mode. In Fig. 1, the cavity mirror M2 was reversed in this intracavity Nd:YVO4\KTA optical parametric oscillator.
3. Experimental results and discussion
In order to investigate the effect of lessening mode mismatch by employing thermal lens-like cavity mirror M1 or M2, the experiments under four kinds of combination by reversing OPO cavity mirrors were studied. Four kinds of combination are listed as following: both M1 with PR coating at 1.53 μm and M2 with HR coating at 1.53 μm facing inside OPO cavity(General cavity); M1 with PR coating at 1.53 μm facing outside OPO cavity and M2 with HR coating at 1.53 μm facing inside OPO cavity(M1 Reversed); M1 with PR coating at 1.53 μm facing inside OPO cavity and M2 with HR coating at 1.53 μm facing outside OPO cavity(M2 Reversed); both M1 with PR coating at 1.53 μm and M2 with HR coating at 1.53 μm facing outside OPO cavity (M1&M2 Reversed). Table 1 shows the output power and lens-like mirror for four kinds of combination. From the results, it can be concluded that the OPO cavity with M1 reversed and M2 reversed both could lessen mode mismatch and improve the OPO conversion. The difference of reversing M1 and M2 is that the lens-like mirror inside fundamental cavity doesn’t change by reversing M2, but the lens-like mirror M1 is included inside fundamental cavity by reversing M1. In our experiment, the highest power was easily achieved with M2 Reversed as the experiment setup shown in Fig. 1. Therefore, OPO cavity with M2 reversed may well compensate mode mismatch comparing with others.
Table 1. The output power and lens-like mirror for four kinds of combination by reversing cavity mirrors
The further study on the operation with experiment setup shown in Fig. 1 is carried out. Figure 3 shows the output power at the signal wavelength 1.53 μm as a function of the diode pump power with the optimized repetition rate at 60 kHz. The idler wave around 3.5 μm was absorbed by both OPO output coupler M1 and filter mirror M0. As a result, the idler wave was not observed through the output mirror M0. The OPO threshold was found to be lower than 3 W. The signal output power reaches a maximum of 3.6 W for a given incident pump power of 20 W, and then rapidly decreases at higher pump powers which caused by the resonator instability. The diode-to-signal conversion efficiency is up to 18%, and the slope efficiency is about 20.2%. To the best of our knowledge, this is the highest average power and efficiency for eye-safe laser produced by diode-end-pumping intracavity OPOs. The power stability of the 1.53 μm laser was investigated by a Model LPM-100 power meter and the fluctuation was lower than 4% at the maximum output power of 3.6 W in half an hour. When the output beam irradiated on the white paper, we also can see the weak green and red light, which were generated by second harmonic generation (SHG) and sum frequency generation (SFG) of fundamental wave and signal wave. The wavelength of the red light is about 628.6 nm, therefore, the calculated wavelength of signal wave is about 1535 nm.
Fig. 3 Output power at 1.53 μm versus diode pump power for both with mode mismatch compensating(MMC) and without MMC
The pulse temporal behavior of signal light was recorded by a fast speed PIN photodiode(sampling rate: 4GHz), and displayed by a Model TDS3052B (500 MHz) dual-line oscilloscope. At the repetition rate of 60 kHz and pump power of 20 W, the temporal pulse profiles of the fundamental and signal waves are shown in Fig. 4 as R2 and R1, respectively. Multi-pulses for signal light were detected and the principal pulse followed with two sub-pulses which has also been observed in Referees [13,15]. The pulse width of principal pulse measured to by 2.2 ns by this 500 MHz oscilloscope, the accurate pulse width maybe shorter than 2.2 ns for the limitation by critical time resolution of oscilloscope. The reason for generating the two sub-pulses maybe that the width of the principal pulse is much shorter than that of fundamental wave, after principal pulse disappeared, fundamental photons in cavity re-accumulated and exceeded the OPO threshold again. Therefore, pulse-series was also observed in the fundamental laser.
In the Fig. 3, the OPO operating with general cavity was also listed for comparison. The OPO threshold was higher, and the maximum signal output power of only about 1.28 W was obtained. It shows a stronger roll-over at higher pump levels, which may be related to the mode mismatching as discuss in Part 2. Therefore, the improvement of output power and efficiency benefits from lessening mode mismatch between the fundamental cavity and OPO cavity. The average output power versus the repetition rate is shown in Fig. 5 , which indicates that the maximum average output power was achieved at the repetition rate of 60 kHz. While the output coupler was replaced by the mirror with the transmission optimized (T = 13%) for 1064 nm output, the output power of 8.9 W was obtained at the repetition rate of 60 kHz and the pump power of 20 W.
4. Conclusion
In conclusion, an efficient intracavity optical parametric oscillator driven by diode-end-pumped acousto-optical Q-switched Nd:YVO4 laser has been demonstrated. A 20-mm-length KTA crystal with non-critical phase matching (θ = 90°, φ = 0°) cut was adopted for efficient OPO conversion. For the purpose of improving OPO conversion, a cavity mirror made of plane BK7 glass working as thermal lens induced by absorbing the idler wave was introduced for lessening mode mismatch between the fundamental and OPO cavities. With the optimization of Q-switch repetition rate at 60 kHz, the maximum average power of 3.6 W eye-safe laser was achieved under the pump power of 20 W, corresponding to a diode-to-signal conversion efficiency of 18%. The output power and conversion efficiency are substantially improved by comparing with the results carried out by Huang et al. [12] and Liu et al. [13] This is the highest efficiency reported for the intracavity OPOs. The instability of the signal power was measured to be less than 4% during half an hour operation.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (under Grant NO.10904143), innovation projects of Fujian Institute of Research on the Structure of Matter (under Grant No. SZD08001-4) and Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2008DP173016).
References and links
1. Y. T. Chang, K. W. Su, H. L. Chang, and Y. F. Chen, “Compact efficient Q-switched eye-safe laser at 1525 nm with a double-end diffusion-bonded Nd:YVO4 crystal as a self-Raman medium,” Opt. Express 17(6), 4330–4335 ( 2009). [CrossRef] [PubMed]
2. Y. X. Fan, Y. Liu, Y. H. Duan, Q. Wang, L. Fan, H. T. Wang, G. H. Jia, and C. Y. Tu, “High-efficiency eye-safe intracavity Raman laser at 1531 nm with SrWO4 crystal,” Appl. Phys. B 93(2-3), 327–330 ( 2008). [CrossRef]
3. J. H. Huang, Y. J. Chen, Y. F. Lin, X. H. Gong, Z. D. Luo, and Y. D. Huang, “Enhanced efficiency of Er:Yb:Ce:NaGd(WO4)2 laser at 1.5-1.6 microm by the introduction of high-doping Ce3+ ions,” Opt. Lett. 33(21), 2548–2550 ( 2008). [CrossRef] [PubMed]
4. R. Dabu, C. Fenic, and A. Stratan, “Intracavity pumped nanosecond optical parametric oscillator emitting in the eye-safe range,” Appl. Opt. 40(24), 4334–4340 ( 2001). [CrossRef] [PubMed]
5. H. Y. Zhu, G. Zhang, C. H. Huang, H. Y. Wang, Y. Wei, Y. F. Lin, L. X. Huang, G. Qiu, and Y. D. Huang, “Electro-optic Q-switched intracavity optical parametric oscillator at 1.53 μm based on KTiOAsO4,” Opt. Commun. 282(4), 601–604 ( 2009). [CrossRef]
6. K. Vodopyanov, “Pulsed Mid-IR Optical Parametric Oscillators, Topics,” Appl. Phys. (Berl.) 89, 141–180 ( 2003).
7. J.-P. Fève, B. Boulanger, O. Pacaud, I. Rousseau, B. Ménaert, G. Marnier, P. Villeval, C. Bonnin, G. M. Loiacono, and D. N. Loiacono, “Phase-matching measurements and Sellmeier equations over the complete transparency range of KTiOAsO4, RbTiOAsO4, and CsTiOAsO4,” J. Opt. Soc. Am. B 17(5), 775–780 ( 2000). [CrossRef]
8. R. F. Wu, K. S. Lai, H. F. Wong, W. J. Xie, Y. L. Lim, and E. Lau, “Multiwatt mid-IR output from a Nd:YALO laser pumped intracavity KTA OPO,” Opt. Express 8(13), 694–698 ( 2001). [CrossRef] [PubMed]
9. P. E. Powers, S. Ramakrishna, C. L. Tang, and L. K. Cheng, “Optical parametric oscillation with KTiOAsO4.,” Opt. Lett. 18(14), 1171–1173 ( 1993). [CrossRef] [PubMed]
10. Z. J. Liu, Q. P. Wang, X. Y. Zhang, Z. J. Liu, J. Chang, H. Wang, S. Z. Fan, S. T. Li, S. S. Huang, W. J. Sun, G. F. Jin, X. T. Tao, S. J. Zhang, and H. J. Zhang, “2.54W 1535nm KTiOAsO4 optical parametric oscillator within a diode-side-pumped acousto-optically Q-switched Nd:YAG laser,” J. Phys. D Appl. Phys. 41(13), 135112 ( 2008). [CrossRef]
11. J. G. Miao, J. Y. Peng, B. S. Wang, and H. M. Tan, “Compact KTA-based intracavity optical parametric oscillator driven by a passively Q-switched Nd:GdVO4 laser,” Appl. Opt. 47(23), 4287–4291 ( 2008). [CrossRef] [PubMed]
12. H. T. Huang, J. L. He, X. L. Dong, C. H. Zuo, B. Y. Zhang, G. Qiu, and Z. K. Liu, “High-repetition-rate eye-safe intracavity KTA OPO driven by a diode-end-pumped Q-switched Nd:YVO4 laser,” Appl. Phys. B 90(1), 43–45 ( 2008). [CrossRef]
13. Z. Liu, Q. Wang, X. Zhang, Z. Liu, J. Chang, H. Wang, S. Fan, W. Sun, G. Jin, X. Tao, S. Zhang, and H. Zhang, “Efficient acousto-optically Q-switched intracavity Nd:YAG/KTiOAsO4 optical parametric oscillator,” Appl. Phys. B 92(1), 37–41 ( 2008). [CrossRef]
14. L. Aihong, Z. Haiyong, Z. Ge, H. Chenghui, W. Yong, H. Lingxiong, and C. Zhenqiang, “Diode side-pumped 1.3414 microm Nd:YAP laser in Q-switched mode,” Appl. Opt. 46(33), 8002–8006 ( 2007). [CrossRef] [PubMed]
15. Y. F. Chen, S. W. Chen, S. W. Tsai, and Y. P. Lan, “Output optimization of a high-repetition-rate diode-pumped Q-switched intracavity optical parametric oscillator at 1.57μm,” Appl. Phys. B 77(5), 505–508 ( 2003). [CrossRef]
