Compact dual-wavelength Nd:GdVO4 laser working ... - OSA Publishing

Compact dual-wavelength Nd:GdVO4 laser working at 1063 and 1065 nm Bo Wu, Peipei Jiang, Dingzhong Yang, Tao Chen, Jian Kong, and Yonghang Shen* State Key Laboratory of Modern Optical Instrumentation, Zhejiang University,Hangzhou 310027, China *Correspondance: e-mail: [email protected], tel: +86 571 87952392

Abstract: We report a compact diode-laser pumped Nd:GdVO4 laser with stable dual-wavelength output at 1063 nm and 1065 nm simultaneously. Two types of resonant cavity configurations were presented to support the stable dual-wavelength operation of the laser. Using a polarization beam splitter(PBS) included T-shaped cavity, we obtained a total power output over 5W in two orthogonal polarized beam directions with 4 W in σ polarization (1065.5 nm) and 1W in π polarization (1063.1 nm). By combining a half-wave-plate with the PBS in the laser cavity, a new configuration favoring one beam direction dual-wavelength output with same polarization direction was realized. A phenomenon of further line splitting was observed in both 1065 nm and 1063 nm. ©2009 Optical Society of America OCIS codes: 140.3410; 140.3580

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C. G. Bethea, “Megawatt power at 1.318µ in Nd3+:YAG and simultaneous oscillation at both 1.06 and 1.318µ,” IEEE. J. Quantum Electron 9, 254-254 (1973). H. Y. Shen, R. R. Zeng, Y. P. Zhou, G. F. Yu, C. H. Huang, Z. D. Zeng, W. J. Zhang and Q. J. Ye, “Simultaneous multiple wavelength laser action in various neodymium host crystals,” IEEE. J. Quantum Electron 27, 2315-2318 (1991). M. B. Danailov and I. Y. Milev, “Simultaneous multiwavelength operation of Nd:YAG laser,” Appl. Phys. Lett. 61, 746-748 (1992). W. Vollmer, M. G. Knight, G. A. Rines, J. C. MeCarthy, E. P. Chicklis, “Five-color Nd:YLF laser,” in: Digest of Conference on Lasers and Electro-Optics, Paper THM 2, Optical Society of America, Washington, DC, 188 (1983). H. Y. Zhu, G. Zhang, C. H. Huang, Y. Wei, L. X. Huang, A. H. Li, Z. Q. Chen, “1318.8nm/1338.2nm simultaneous dual-wavelength Q-switched Nd:YAG laser,” Appl. Phys. B 90, 451-454 (2008). Y. F. Chen, “cw dual-wavelength operation of a diode-end-pumped Nd:YVO4 laser,” Appl. Phys. B 70, 475478 (2000). R. Zhou, B. G. Zhang, X. Ding, Z. Q. Cai, W. Q. Wen, P. Wang and J. Q. Yao, “Continuous-wave operation at 1386nm in a diode-end-pumped Nd:YVO4 laser,” Opt. Express 13, 5818-5824 (2005). 8. Y. Y. Lin, S. Y. Chen, A. C. Chiang, R. Y. Tu, and Y. C. Huang, “Single-longitudinal-mode, tunable dualwavelength, CW Nd:YVO4 laser,” Opt. Express 14, 5329-5334 (2006). R. Zhou, E. B. Li, B. G. Zhang, X. Ding, Z. Q. Cai, W. Q. Wen, P. Wang, and J. Q. Yao, “Simultaneous dual-wavelength CW operation using 4F3/2-4I13/2 transitions in Nd:YVO4 crystal,” Opt. Commun. 260, 641644 (2006). H. Y. Shen, R. R. Zeng, Y. P. Zhou, G. F. Yu, C. H. Huang, Z. D. Zeng, W. J. Zhang and Q. J. Ye, “Comparison of simultaneous multiple wavelength lasing in various neodymium host crystals at transitions from 4F3/2-4I11/2 and 4F/3/2-4I13/2,” Appl. Phys. Lett. 56, 1937-1938 (1990). C. H. Huang, G. Zhang, Y. Wei, L. X. Huang, H. Y. Zhu, “A Q-switched Nd:YAlO3 laser emitting 1080 and 1342nm,” Opt. Commun. 281, 3820-3823 (2008). H. H. Yu, H. J. Zhang, Z. P. Wang, J. Y. Wang, Y. G. Yu, Z. B. Shi, X. Y. Zhang, and M. H. Jiang, “Highpower dual-wavelength laser with disordered Nd:CNGG crystals,” Opt. Lett. 34, 151-153 (2009). J. L. He, J. Du, J. Sun, S. Liu, Y. X. Fan, H. T. Wang, L. H. Zhang, and Y. Hang, “High efficiency singleand dual- wavelength Nd:GdVO4 lasers pumped by a fiber-couple diode,” Appl. Phys. B 79, 301-304 (2004).

#107766 - $15.00 USD Received 20 Feb 2009; revised 15 Mar 2009; accepted 17 Mar 2009; published 30 Mar 2009

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14. K. Lunstedt, N. Pavel, K. Petermann, and G. Huber, “Continuous-wave simultaneous dual-wavelength operation at 912nm and 1063nm in Nd:GdVO4,” Appl. Phys. B 86, 65-70 (2007). 15. A. I. Zagumennyi, V. G. Ostoumov, I. A. Shcherbakov, T. Jensen, J.-P. Meyn, and G. Huber, “The Nd:GdVO4 crystal: a new material for diode-pumped laser,” Sov. J. Quantum Electron 22, 1071-1072 (1992). 16. Y. Sato, N. Pavel, and T. Taira, “Spectroscopic properties and near quantum-limit laser-oscillation in Nd:GdVO4 single crystal,” in OSA TOPS on Advanced Solid-State Photonics, Vol. 94, G. J. Quarles, ed., (Optical Society of America, Washington, DC), 405-409 (2004).

Introduction Lasers emitting simultaneously at multiple wavelengths can find wide applications in many fields such as environmental monitoring, laser radar, spectral analysis and THz research, etc. In traditional laser systems, however, only one wavelength (or line) operation can normally be obtained if no special measure is taken. The emission lines with weak gain are usually depressed by the line with strong gain because of the gain competition between the laser emission lines, and this competition generally results in only the laser line with the strongest gain being generated. To meet the requirement of simultaneously multiple laser line oscillation, special design of the laser oscillation cavity is necessary to control and to reduce the gain competition among the multiple wavelength lines of the gain medium. To our knowledge, the first report about the multiple wavelength laser was presented by Bethea in 1973 by using a Nd:YAG as the gain medium [1]. After that, multiple wavelength lasers based on Nd:YAG[2-4], Nd:YLF[5], Nd:YVO4[6-9], Nd:YAP[10,11] and Nd:CNGG[12] have been reported, by means of designing a typical coating for the output coupler, generating in spatially shifted regions of the gain medium and using two quarter wave plates (QWP) to make the intrinsic frequency split, etc. With Nd:GdVO4 crystal, multi-wavelength laser operation was also realized at 1063 nm and 1342 nm[13], or at 912nm and 1063nm[14]. However, to our knowledge, no work on the simultaneous multi-wavelength laser operation at two close wavelengths of 1063 nm and 1065 nm with Nd:GdVO4 crystal was once reported. Such a dual-wavelength laser would be especially valuable as a compact and strong laser source to generate the THz emission because the frequency difference between 1063 nm and 1065 nm is about 0.53 THz. Since being first introduced by Zaguniennyi et al in 1992[15], the Nd:GdVO4 crystal has been proved very suitable for high power laser system. It has the characteristics of strong polarization dependent absorption spectra and fluorescent emission spectra. The strongest emission line of Nd:GdVO4 is typically 1063.1 nm in π polarization (E//c). Comparing with this line, the emission lines of 1063.1 nm and 1065.5 nm in σ polarization (E⊥ c) are medium strong[16]. However, because the gain of 1063.1 nm in π polarization(emission cross section σ =10.3×10-19 -cm2) is almost five times higher than that of both 1063.1 nm and 1065.5 nm in σ polarization(σ =2.1×10-19 -cm2) [16], the σ polarization emission is normally depressed by the π polarization emission of 1063.1 nm if no specific measure is applied to control the gain competition in the laser cavity. In this paper, we will present our recent results of exploring a dual-wavelength Nd:GdVO4 laser. By separating the orthogonal polarized beams with a polarization beam splitter(PBS) and then controlling their feedback separately, simultaneous dual-wavelength Nd:GdVO4 laser operation was stably realized at two close wavelengths of 1063 nm and 1065 nm. To our knowledge, this is the first work of realizing dual-wavelength laser operation by means of polarization dependent gain control. The method presented in this paper can be extended to other polarization dependent solid state laser, such as those with host materials of YVO4, YAP, YLF or even ruby for dual-wavelength output.


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Experiment and result analysis The phenomenon of simultaneous dual-wavelength laser oscillation at both 1063 nm and 1065 nm was once observed from an end pumped normal Nd:GdVO4 laser. We noted the laser wavelength switched from 1065 nm (σ polarization) to 1063 nm(π polarization) when the pump power increased gradually. Further analysis showed the phenomenon was resulted from the feedback difference between the π polarization and σ polarization, which was caused by the output coupler. It was then noted there existed a narrow transition pump power range, in which dual-wavelength output with orthogonal polarization direction could be realized. However, as the gain competition between the modes of π polarization and σ polarization still existed in the system, the dual-wavelength output was severely unstable and the mode shifting occurred from time to time between π polarization and σ polarization.

π @ 1063nm M3

LD@808nm

fiber σ @ 1065nm Nd:GdVO4 M2

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M1

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Fig. 1. T-shaped cavity configuration featuring orthogonal polarized two beam output. M1: input mirror, M2 and M3: output couplers for σ polarized 1065 nm emission and π polarized 1063 nm emission respectively.

In order to obtain a practically applicable dual-wavelength laser with stable power output, we designed a T-shaped cavity as shown in Fig. 1. The Nd:GdVO4 laser was end pumped by a fiber-pigtailed laser diode working at 808 nm(LIMO60-F400-DL808M3, LIMO Corp., German) with a maximum power output of 60 W. A piece of Nd:GdVO4 crystal (Nd3+ doping, 0.5at%, from WITCORE company, China) with size of 3×3×8 (mm) was used as the gain medium. Both sides of the crystal were anti-reflection coated at 1064 nm and 808 nm. The crystal was wrapped using an indium foil and placed onto an aluminum stage for temperature stabilization. The input mirror of the cavity was flat with coating of high reflection around 1064 nm and transmittance of 95% at 808 nm. A PBS was placed in the cavity to split the beams polarizing in two orthogonal directions. The output coupler for the mode with horizontal polarization, M2, was a concave mirror with a radius of curvature -1000 mm and was coated with transmittance of 10% around 1064nm. Another concave mirror, M3, with a radius of curvature -500 mm was used as the output coupler for the mode with vertical polarization and was coated with transmittance of 30% around 1064 nm.


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Fig. 2. Power dependence of the π polarized and the σ polarized laser emissions on the pump power in the T-shaped cavity.

By using such a T-shaped cavity configuration, stable dual-wavelength laser emission of 1063 nm (π polarization) and 1065 nm (σ polarization) was simultaneously obtained in two directions with much better power stability over a large adjustable power range as shown in Fig. 2. The laser power output of both the π polarized 1063.1 nm and the σ polarized 1065.5 nm beams increased simultaneously with the pump power until the pump power reached 33W. A total output power over 5W was obtained with 4 W in the σ polarization (1065.5 nm) and 1W in the π polarization (1063.1 nm). It was found in the experiment that the oscillation at 1065.5nm (σ polarized) was much more sensitive to the environmental factors than that at 1063.1nm (π polarized). Slight temperature fluctuation and pump power variation might induce strong mode competition and then large power fluctuation. Such power fluctuations were especially distinct as shown in Fig. 2 when the pump power was 23W and 29W respectively. With pump power higher than 34W, the output power of the σ polarized beam decreased quickly as a result of the net gain increase of π polarized oscillation at 1063.1nm. In the mean time, the power of π polarized beam rapidly increased and finally overrun its counterpart at 1065.5 nm. It was noted the saltation point of the pump power (pump power 39.4W) was strongly affected by the transmittances of the two output couplers, M2 and M3, and could thus be adjusted accordingly for system optimization. The spectrum of the laser emissions, as shown in Fig. 3 was measured by using an optical spectral analyzer (OSA, Ando AQ6317C) with a spectral resolution of 0.01nm when two output beams from the perpendicular directions were directed together onto an optical fiber using mirror reflectors. 1.4

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Fig 3. Spectrum of the output emissions measured when two output beams from the perpendicular directions of the T-shaped cavity were directed together.


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D output M2

M1 L

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Nd:GdVO4 HWP

LD 808nm

fiber

Fig. 4. Cavity configuration featuring one beam π polarized multi-wavelength laser output. The PBS also acted as the output coupler of the vertical polarized laser beam. D-Detector.

Because the laser output from the cavity configuration as shown in Fig. 1 featured two beams, it is sometimes inconvenient for applicants though the beams can be easily recombined using reflectors and PBS. To improve this, another cavity configuration featuring one beam output was presented as shown in Fig. 4. A half-wave plate (HWP) was included between the PBS, which also acted as an output coupler here, and the Nd:GdVO4 crystal. It is not difficult to understand the polarization direction of the beams oscillating in the cavity would rotate (for both polarizations) according to the axis direction of the HWP when the beams pass the HWP. After the HWP, the portion of the beams with horizontal polarization would transmit through the PBS and reflect back by mirror M2 and then transmit through the PBS again without further loss, while the other portion of the beams with vertical polarization would be reflect out of the cavity by the PBS as the laser output. The beams feeding back from M2 and passing through the PBS were all polarizing in horizontal direction but would experience further polarization rotation when they reversely pass the HWP. The π polarized and σ polarized beams after the HWP would then be amplified separately depending on their gains in these polarization directions. The remained horizontal polarized beam at 1063.1 nm and vertical polarized beam at 1065.5 nm would not experience amplification in this round trip but would merge to the whole beam after they pass the HWP again. Thus, there would be no extra loss incurred if the transmittance of the HWP was perfect. In this way, the gains for beams with π polarization and σ polarization could be modified continuously by rotating the HWP. As a result, the laser could easily be regulated to work in single wavelength mode or in multi–wavelength mode. Under this configuration, dual wavelength laser emission with power output exceeding 1W was obtained when the pump power was 15 W. As the transmittance of the HWP we used was not good enough in this experiment, no higher pump power was applied for the time being. By measuring the spectra of the laser emission with the OSA(with the same spectral resolution of 0.01nm), we found there existed a HWP position with dualwavelength output at 1063.7 nm and 1065.5 nm as shown in Fig. 5 (It was noted the wavelength values were not exactly the same as that obtained in the T-shaped cavity configuration). Besides, the phenomenon of further laser line splitting was also observed. Both the laser lines at 1065 nm and at 1063 nm might split into two lines when the HWP was rotated. When the HWP was rotated to a specific direction, for example, there were two lines near 1063 nm which were corresponded to the former line of 1063.1 nm. Similar to this, there were also several HWP directions with two laser lines near 1065 nm which were


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corresponded to the former line of 1065.7 nm. In a whole, as many as 10 lines (5 couples of lines) have been observed during the experiment and the spectra of some of these emission lines are illustrated in Fig. 5. 30

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Fig 5. The laser output spectra when the HWP was rotated, showing (a) the emission lines at 1063.8 nm and 1065.5 nm. (b) the emission line at 1065 nm splitting into two lines at 1065.3 nm and 1065.6 nm, or into other two lines at 1065.5 nm and 1065.8 nm; (c)the emission line at 1063 nm splitting into two lines at 1062.7 nm and 1063.0 nm, 1063.2 nm and 1063.5 nm, 1063.6 nm and 1063.8 nm respectively;

It is interesting to note that in most cases the emission lines appeared in pair. Though the wavelength values of these lines were found to vary with the HWP direction, the wavelength difference within the pair lines seems unchanged (a value between 0.2 and 0.3 nm, limited by the resolution of the OSA). We believe this phenomenon might relate to the additional phase difference of two orthogonal polarized beams through the HWP, one experiencing amplification in the Nd:GdVO4 crystal while another not. Deeper understanding of this emission line splitting may require further effort. Conclusion Briefly in summary, we succeeded in obtaining a stable dual-wavelength output at 1063 nm and 1065 nm simultaneously by designing two types of cavity configuration in a Nd:GdVO4 laser. A T-shaped cavity favored stable π and σ polarization emission output in two perpendicular directions. A total power output of over 5 W was realized. A HWP included cavity favored continuously adjustable one beam dual-wavelength and multi-wavelength emission output. The phenomenon of further laser line splitting at both wavelengths of 1063 nm and 1065 nm was observed in the HWP included cavity configuration. Higher power output of dual-wavelength operation can be expected in both cavities by optimizing the parameters of cavity reflectivity and the transmittance of the HWP. This work was partly supported by Natural Science Foundation of China (project No. 60778001), the program for NCET in University and the National Basic Research Program (973) of China (2007CB307003).


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