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Intra-cavity frequency doubling with 589 nm emission from a compact c-cut Nd:YVO4 crystal self-Raman laser was investigated. A 15-cm-length LBO with non-critical phase-matching cut (θ = 90°, ϕ = 0°) was used for efficient second-harmonic generation. At a pump power of 16.2 W and a pulse repetition frequency of 40 kHz, output power up to 2.15 W was achieved with a pulse width of 16 ns and a conversion efficiency of 13.3% with respect to the diode pump power. The center wavelength was measured to be 589.17 nm with a Half-Maximum-Full-Width of 0.2 nm, which was well in accordance with the sodium D2 resonance radiation.
©2011 Optical Society of America
1. Introduction
Yellow light sources have attracted attentions for their applications in biomedical, laser projection display, remote sensing, Bose–Einstein condensation, etc [1–3]. There are mainly three ways being adopted for realizing yellow solid-state lasers, which are sum-frequency generation (SFG) of two infrared Nd-based laser lines at 1.06 μm and 1.3 μm [4], second harmonic generation (SHG) of weaker transitions in Nd3+ doped crystal [5], SHG or SFG of Nd-doped solid Raman laser [6,7]. Multi-Watt yellow coherent sources emitting at the sodium D2 line resonance (589.159 nm) are important for coherent interaction in degenerating quantum gases, extending the observation area in laser guide star adaptive optics (LGS AO), as well as replacing dye laser in spectroscopy [8–10]. Recent years, SFG of two laser lines at 1064 nm and 1319 nm from Nd:YAG crystal has been widely investigated [8–10]. In order to reduce the mode competition, those two lines should be generated in two independent solid state lasers following with complex free-space optics systems.
More recently, a 589 nm radiation from the frequency doubling of the novel self-Raman crystal Nd:LuVO4 with double-end diffusion bonding was reported [11]. Maximum output power of 3.5 W was realized with diode laser direct pumping at 880 nm corresponding with a conversion efficiency of 13.3%. In 2001, Kaminskii et al. predicted that Nd:YVO4 and Nd:GdVO4 would be promising self-Raman crystals [12], which were first proved by Chen in 2004 [13]. Then, efficient frequency doubling of a-cut Nd:YVO4 and Nd:GdVO4 self-Raman lasers have been widely investigated [14–17]. 5.7 W (23.5W pump) and 7.9 W (26.5W pump) yellow emission at 588 nm were achieved based on second harmonic generation (SHG) of Nd:YVO4 self-Raman laser with 20-mm-length and 30-mm-length diffusion-bonded crystal [14,16]. CW yellow laser up to 2.51 W was achieved based on Nd:GdVO4 self-Raman laser with 880 nm diode laser direct pumping [17]. For a c-cut Nd:YVO4 crystal, its fundamental wavelength locates at 1066 nm [18], and the SHG of its self-Raman laser emits at 589 nm which is close to the sodium D2 line at 589.159 nm comparing to its a-cut counterpart. In this paper, we demonstrated 589 nm coherent emission generated by frequency doubling in a compact self-Raman laser with c-cut Nd:YVO4 crystal for the first time. At a pump power of 16.2 W and a pulse repetition frequency of 40 kHz, average output power of 2.15 W was obtained with a pulse width of 16 ns and diode-to-yellow conversion efficiency of 13.3%.
2. Experimental setup design
Nd:YVO4 crystal belongs in the crystallographic D4h tetragonal space group of the zircon type, and its unique optical axis is along the four-fold symmetric axis. The stimulated emission cross section of Nd:YVO4 crystal strongly depends on the polarization characteristics of transition spectral line. The stimulated-emission cross section for polarization parallel to the c axis (σ = 25 × 10−19 cm2, corresponding to the fundamental wavelength of 1064 nm), is four times higher than that for polarization orthogonal to the c axis (σ = 6.5 × 10−19 cm2, corresponding to the fundamental wavelength of 1066 nm) [19]. The larger stimulated-emission cross section results in a lower pump threshold for CW laser operation, while a smaller cross section has advantages in Q-switch operation. For c-cut Nd:YVO4 with fundamental wavelength of 1066 nm, its first-Stokes emission is calculated to be around 1178 nm with a stimulated Raman scattering (SRS) active vibration mode of 890 cm−1 [12].
The experimental arrangement of intra-cavity frequency-doubled acousto-optic Q-switched c-cut Nd:YVO4 crystal self-Raman laser is shown in Fig. 1 . The pump source was a fibre-coupled laser diode array at 808 nm with a numerical aperture of 0.22 and a core diameter of 200 μm. The pump light was coupled into the Nd:YVO4 crystal with a focus spot diameter of about 400 μm using two different plano-convex lenses with a focal length of 60 mm and 30 mm, respectively. Both coupling lenses were anti-reflection (AR) coated at 808 nm and the coupling efficiency through the pair lenses was about 96%. The c-cut, 0.3-at.% Nd3+ doped Nd:YVO4 (3 × 3 × 20 mm3) functioning as a self-Raman laser crystal was wrapped in an indium foil and mounted on a thermoelectric cooled copper block. The temperature on its surface was kept at about 20 °C during the experiments. To make the cavity compact, custom coating design was used on both faces of the laser crystal. The film with high transmission (HT, T>98%) at 808nm, high reflectivity (HR, R>99.95%) at 1066 nm and first-Stokes light of 1178 nm (R>99.99%) was coated on the incident facet (S1 in Fig. 1) which formed the fundamental and Raman cavities pairing with the output coupler(OC). The opposite side (S2) was HR coated at 589 nm (R>99%, guaranteed at maximum unidirectional output) and AR coated (R<0.2%) at both the fundamental and the first-Stokes wavelengths. In order to reduce the diffraction loss and misalignment sensitivity of cavity, a concave output coupler with 100 mm radius of curvature was adopted, which was HR(R>99.95%) coated at both the fundamental and first-Stokes wavelengths, and HT coated at 589 nm wavelength as well.
A 3 × 3 × 15 mm3 LBO crystal (from CRYSTECH Inc.) with type-II NCPM (θ = 90°, ϕ = 0°) cut was used as the frequency doubling crystal for the first-Stokes light of 1178 nm. The crystal was AR coated at 1066 nm and 1178 nm on both end faces and mounted on a thermoelectric cooled copper block with its surface temperature kept at about 40 °C. Between the Nd:YVO4 crystal and nonlinear optical crystal, a 20-mm-long acousto-optic Q-switcher (AOS, Model:QS041-10G-GH28, from Gooch & Housego Co.) was insert to realize Q-switch mode operation. The AOS driven at a 40 MHz center frequency with 10 W of radio-frequency power is antireflection (AR) coated at 1064 and 1176 nm on both faces and HT(T>90%) at 589 nm. The duty cycle (the ratio of the cavity opening time to the cavity modulation period) of Q-switch drive was set at 5%. All the optics components have been set close to each other to shorten the cavity length and reduce the cavity loss, corresponding to the overall cavity length of about 75 mm.
3. Experimental results and discussion
To realize SHG of the first-Stokes wavelength, both LBO and KTP crystals can be used as nonlinear optical crystal. Table 1 shows the calculated phase matching (PM) parameters. The KTP crystal has the advantage of larger deff. However, its walk-off angle is much bigger and the acceptance angle is rather small. For the LBO crystal, PM direction is very close to the non-critical phase-matching (NCPM) angle which can be realized by controlling the crystal temperature at about 313 K. Though the deff is smaller than KTP crystal, the conversion efficiency of NCPM LBO can be raised by using longer crystal for large acceptance angle and no walk-off angle. Therefore, both KTP and LBO have their advantages and disadvantages. In the experiment, both KTP (θ = 68.8°, ϕ = 0°, 4 × 4 × 10 mm3) and LBO (θ = 90°, ϕ = 0°, 3 × 3 × 15 mm3) have been adopted for comparison. The yellow laser thresholds for both crystals were almost the same, but the conversion efficiency for KTP was lower than that for NCPM LBO. The output power of 589 nm laser was measured by a LPM-100 laser power meter. At the pump power of about 11 W and with the similar setup, output power at 589 nm was only about 560 mW for KTP, while for the NCPM LBO, output power up to 1 W was obtained. The experiment results demonstrate the superiority of NCPM LBO in terms of SHG output for no Walk-off angle and large acceptance angle.
Table 1. Phase Matching Parameter of KTP and LBO for SHG Calculated
Average output powers with different pulse repetition frequencies (PRF) of 30, 40, and 60 kHz have been studied. Figure 2 shows the average output power at 589 nm versus incident pump power with different PRFs for Q-switch operation. The laser threshold was about 0.9 W. Above the threshold, the output power increased with the pump power. At the pump power of about 14 W, average output powers at 589 nm were 1.25 W, 1.64 W, and 1.4 W for the PRFs of 30, 40, and 60 kHz, respectively. The output power begins to saturate firstly for the PRF of 30 kHz. The maximum average output power up to 2.15 W was achieved at the pump power of 16.2 W with the PRF of 40 kHz. The conversion efficiency was 13.3% for the 2.15 W output power with respect to the diode pump power. The output power was compromised by the thermal effects while the pump power was further increased. Based on ABCD ray transfer matrix and omitting the thermal lens effect of the Q-switch crystal and the LBO crystal, the stability of the resonator affected by thermal effects in Fig. 1 could be calculated easily. The results show that the laser system will become unstable when the focal length of thermal lens is shorter than 52 mm which corresponds to the focal length of thermal lens at the pump power of 16.2 W and the PRF of 40 kHz.
Fig. 2 Average output power at 589 nm versus incident pump power with different pulse repetition frequencies (PRF) of 30, 40 and 60 kHz.
The temporal pulse profile of 589 nm light was recorded by a PIN photodiode and displayed by a digital oscilloscope with bandwidths of 1 GHz (Agilent DSO6102A). At the PRF of 40 kHz, the pulse width was about 16 ns as shown in Fig. 3 and the pulse energy was 54 μJ with the incident pump power of 16.2 W. The pulse-to-pulse stability of the pulse train was better than 7%. The spectrum of yellow light was measured by Fiber Optic Spectrometer (AvaSpec-3648), only the wavelength of 589 nm appeared in the measured wavelength range from 500 nm to 650 nm. The center wavelength was about 589.17 nm as displayed in Fig. 4 with the recorded wavelength data interval of 0.066 nm, and the spectrum width was about 0.2 nm. The spectrum of this frequency doubled c-cut Nd:YVO4 crystal self-Raman laser which is well including the spectrum of sodium D2 radiation (589.159 nm, natural linewidth of about 10 MHz [20]), so the system is a potential sodium D2 resonance radiation source.
4. Conclusion
In conclusion, we have demonstrated an intra-cavity frequency doubled c-cut Nd:YVO4 crystal self-Raman laser at 589 nm. A 15-cm-length LBO with non-critical phase-matching cut (θ = 90°, ϕ = 0°) was used for efficient second-harmonic generation. Resonator cavity structure and coating parameters of crystals were designed to reduce its insert loss and diffraction loss. At the pump power of 16.2 W and the pulse repetition frequency of 40 kHz, maximum output power up to 2.15 W was achieved with a pulse width of 16 ns and a diode-to-yellow emission conversion efficiency of 13.3%. The overall laser efficiency and output power might be further improved using the direct pump at 880 nm as that in Ref. [21] to reduce quantum-defect deficit and the thermal lens within the Nd:YVO4 crystal. The center wavelength was 589.17 nm with the width of 0.2 nm, which was well in accordance with the sodium D2 resonance radiation.
Acknowledgment
This work was supported by the National Natural Science Foundation of China under Grant No. 10904143, and partially supported by Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics and the National Natural Science Foundation of China under Grant No. 90922035.
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