We have demonstrated a laser-diode pumped continuous-wave (CW) and passively Q-switched laser with a Nd:Sc0.2Y0.8SiO5 (Nd:SYSO) crystal for the first time. In the CW operation, the laser was found to oscillate in tri-wavelength regime at 1074.8 nm, 1076.6 nm and 1078.2 nm, respectively. The maximum CW output power of 1.96 W was obtained, giving an optical-to-optical conversion efficiency of 35% and a slope efficiency of 39%. Using either Cr4+:YAG or V3+:YAG crystal as saturable absorber, stable passively Q-switched laser was obtained at dual-wavelength of 1074.8 nm and 1078.2 nm with orthogonal-polarization. The maximum average output power, pulse repetition rate, and shortest pulse width were 1.03 W, 50 kHz, and 24 ns, respectively. The passively Q-switched dual-wavelength laser could be potentially used as a source for generation of terahertz radiation.
©2012 Optical Society of America
With the development of high-energy physics and medical imaging, the need for the scintillating crystal has lately become pressing . Binary and ternary inorganic silicate crystal doped with rare-earth Cerium (Ce) can be used as scintillating crystal and have attracted much attention, due to the good properties of high density, high output and short decay time. In this regime, Ce-doped Lu2SiO5 (LSO), Y2SiO5 (YSO) and LuYSiO5 (LYSO) are the representative crystals [2,3]. As the multifunctional crystal, the silicate crystals have been used as not only the scintillating crystal but also laser gain medium doped with different rare ions including Nd3+, Yb3+, Tm3+ and Ho3+ etc [4–8]. Zhuang et al have experimentally demonstrated continuous-wave and actively Q-switched Nd:LSO crystal, with the dual-wavelength output power of 1 W . Comaskey et al have realized 1.8 J signal pulse energy output at 1.074 nm with Nd:YSO crystal . By substituting part of yttrium ions with scandium ions, a novel laser crystal Nd:Sc0.2Y0.8SiO5 (Nd:SYSO) was grown with Czochralski method along the crystallographic b axis by Shanghai Institute of Ceramics. The mixed Nd:SYSO crystal has the same structure as Nd:YSO, which is a monoclinical biaxial positive crystal (class 2/m, space group C2/c). As a consequence, the Nd:SYSO crystal is birefringent. The strong natural birefringence of monoclinical biaxial crystal overwhelms the thermally induced stress birefringence, which can induce the thermal depolarization observed in isotropic media such as YAG . Multiple types of the substitutional sites in this crystal provide a strong inhomogeneous lattice field for rare earth dopants, which results in large ground-state splitting and broad emission spectra. Therefore, it is possible to realize the multi-wavelength and ultra-fast laser operation.
In this paper, we have demonstrated a novel continuous-wave (CW) and passively Q-switched laser with a Nd:SYSO crystal. The CW laser was operating in tri-wavelength regime at 1074.8 nm, 1076.6 nm and 1078.2 nm, respectively. Under an absorbed pump power of 5.6 W, a maximum CW output power of 1.96 W was obtained, corresponding to an optical-to-optical conversion efficiency of 35% and a slope efficiency of 39%. In the passive Q-switching regime, the laser was stably operating at dual-wavelength of 1074.8 nm and 1078.2 nm. The maximum average output power, pulse repetition rate, and shortest pulse width were 1.03 W, 50 kHz, and 24 ns, respectively.
2. Experimental setup
A compact concave-plano cavity was employed, as shown in Fig. 1 . The pump source was a fiber-coupled laser diode emitting at 811 nm with a radius of 200 µm and a numerical aperture (N.A.) of 0.22. The pump beam was coupled into the laser crystal with a coupling system and the waist radius was about 200 µm. The laser cavity consists an input mirror M1, a Nd:SYSO crystal, a saturable absorber and an end mirror M2. M1 was a plano-concave mirror with a curvature radius of 500 mm. The plane face was anti-reflection at 811 nm (R<0.2%); the concave face was coated for high transmission at 811 nm (R<3%) and high reflection at 1000-1100 nm (R>99.8%). The output mirror M2 was coated with partial transmission (Toc = 5%, 10% and 27% were available) at 1075 nm. The Nd:SYSO laser crystal was 0.8 at-% Nd3+-doped and cut perpendicular to <010> direction with dimensions of 3 × 3 × 3 mm3. Both sides of the Nd:SYSO crystal were uncoated. A Cr4+:YAG with the initial transmission of 97% and a V3+:YAG crystal with that of 95% were used as saturable absorber. In order to reduce the influence of thermal lens effects, both the laser crystal and saturable absorber were wrapped with indium and mounted in a copper block cooled by water at 20 °C. The laser pulse signal was recorded by a Tektronix DPL7104 digital oscilloscope (1 GHz bandwidth, 5 Gs/s sampling rate) and a photo-detector (EOT, model ET5000A). The average output power was measured by a laser power meter (Fieldmax-II, coherent). The laser spectra were measured by an optical analyzer (0.5 nm spectral resolutions, Avantes, AcaSpec-3648-NIR256-2.2).
3. The results and discussions
The absorption and emission spectrum of Nd:SYSO crystal at room temperature are shown in Fig. 2 . As can be seen from Fig. 2(a), a maximum absorption coefficient of 3.45 is located at 811 nm with an FWHM about 7 nm. The asymmetric broadening of the emission spectrum around 1075 nm shown in Fig. 2(b) indicates the presence of non-resolved lines.
The CW tri-wavelength operation of the Nd:SYSO crystal was first carried out by removing the saturable absorber from the cavity and optimizing the cavity length to be 15 mm. The CW output power versus the absorbed pump power is shown in Fig. 3 . The absorption efficiency of this crystal at pump wavelength was measured to be only 50%, because this crystal was short and uncoated at pump wavelength. As can be seen from Fig. 3, the threshold pump power were 0.5, 0.9 and 2.2 W with the output coupler of Toc = 5%, 10% and 27%, respectively. Based on the model developed by Findlay Clay et.al , the threshold pump power can be given by:
With a spectrum analyzer, the CW laser spectrum was recorded under the absorbed pump power of 5.6 W and is shown in Fig. 4(a) . As can be seen, the laser operated in tri-wavelength regime, with respective wavelengths of 1074.8 nm, 1076.6 nm and 1078.2 nm. By using a Glan-Taylor polarizer, we found that the laser emissions at the three wavelengths were all linearly polarized. The transmission polarization direction of the Glan-Taylor polarizer was parallel to the Y-axis of the Nd:SYSO crystal. The emission at 1076.6 nm and 1078.2 nm had the same polarization state along Z-axis, while the polarization direction of 1074.8 nm was along Y-axis. The transmission and reflection spectra are shown in Fig. 4(d) and Fig. 4(e).
In order to investigate the influence of saturable absorbers on the multi-wavelength lasers, we employed a Cr4+:YAG crystal and a V3+:YAG crystal as the saturable absorber to realize the passive Q-switching operation. The saturable absorber could be regarded as the intra-cavity loss. We used different saturable absorbers with different initial transmissions and crystal lengths to regulate oscillating modes. However, the multi-wavelength oscillation regime in the passively Q-switched laser was not easily obtained by using the saturable absorber with small initial transmission because of the large intra-cavity absorption loss. Therefore, we chosen a Cr4+:YAG crystal and a V3+:YAG crystal with large initial transmissions (To = 97% and 95%, respectively) to realize the multi-wavelength operation simultaneously. The dimension of the Cr4+:YAG and V3+:YAG crystal were Φ 9.5 × 0.285 mm3 and 4 × 4 × 3 mm3, respectively. The variation of the output power with absorbed pump power is shown in Fig. 5(a) . Under the absorbed pump power of 5.6 W, a maximum average output power of 1.03 W was obtained with the Cr4+:YAG saturable absorber and the output coupler of 5%. The corresponding maximum optical-to-optical efficiency and slope efficiency were determined to be 18.4% and 23.8%. With the V3+:YAG crystal, the maximum average output power of 0.92 W was obtained. With an optical spectrum analyzer, the spectra of the passively Q-switched lasers were recorded. Using either Cr4+:YAG crystal or V3+:YAG crystal, the laser operated in stable dual-wavelength regime at 1074.8 nm and 1078.2 nm. Figure 4(b) and 4(c) showed the stable dual-wavelength spectra obtained under the absorbed pump power of 5.6 W. As can be seen, the emission at 1078.2 nm was somehow suppressed by using V3+:YAG crystal compared with that of Cr4+:YAG crystal. This was because V3+:YAG crystal had a relatively smaller initial transmission and lager inserted loss induced by the crystal thickness of 3 mm than that of Cr4+:YAG crystal. The emission at 1074.8 nm had a polarization direction along Y-axis of the Nd:SYSO crystal, however, the emission at 1078.2 nm had polarized direction along Z-axis of the Nd:SYSO crystal. The simultaneous dual-wavelength pulsed Nd:SYSO laser at 1074.8 nm and 1078.2 nm has potential applications for the generation of THz radiations .
In the passively Q-switched regime, the repetition rate increased and the pulse width decreased with the augment of the absorbed pump power for both different output couplers (Toc = 5% and 10%). The dependences of the repetition rate and pulse width on the absorbed pump power with Toc = 10% are shown in Fig. 5(b). A maximum repetition rate of 50 kHz and a minimum pulse width of 24 ns were obtained with the Cr4+:YAG saturable absorber under the absorbed pump power of 5.6 W, corresponding to single pulse energy and peak power of 16.2 µJ and 0.7 kW, respectively. The single pulse profile with duration of 24 ns is shown in Fig. 6(a) . From this figure, one can observe that there was no satellite pulse before or after the main pulse and the pulses of the dual-wavelength overlap in the time domain. The inset of Fig. 6(a) showed the pulse train with the repetition rate of 50 kHz. It can be seen that the time jitter from pulse to pulse is small, which indicating the passively Q-switched laser was stable. The stable orthogonally polarized dual-wavelength laser with pulse overlapped in the time domain could be used as a source for generation of THz radiation by suitable nonlinear crystals with type-II phase matching scheme. The 2D and 3D output beam spatial distributions of the passively Q-switching laser under the absorbed pump power of 5.6 W are shown in Fig. 6(b). As can be seen, the transverse mode presents a perfectly Gaussian profile.
The performance of CW and passively Q-switching operation with a Nd:SYSO crystal were investigated for the first time. A maximum CW output power of 1.96 W was obtained, giving an optical-to-optical conversion efficiency of 35% and a slope efficiency of 39%. The CW laser operated in tri-wavelength regimes at 1074.8 nm, 1076.6 nm and 1078.2 nm, simultaneously. In the passively Q-switching regime, stable dual-wavelength laser was realized at 1074.8 nm and 1078.2 nm with orthogonal polarizations. The maximum average output power, pulse repetition rate, and shortest pulse width were 1.03 W, 50 kHz, and 24 ns, respectively. We believe that the dual-wavelength Nd:SYSO laser would have potential application in the generation of 5.5 THz radiation by using suitable nonlinear crystals with type-II phase matching scheme.
This work was supported by the National Natural Science Foundation of China (Grant No: 51021062 and 91022003), and the Project (2011YQ030127). L.H. Zheng would like to thank the National Natural Science Foundation of China (Grant No: 60908030 and 60938001) Jing-Liang He's e-mail address is email@example.com.
References and links
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