1. Introduction

Diode-pumped solid-state green lasers (DPSSGL) with high average power and high beam quality are important for various demands in both scientific and industrial applications, such as pumping tunable lasers and optical parametric oscillators or amplifiers [1, 2], laser display [3], medical treatment [4], material precise processing [5] and so on, especially for the green lasers with high beam quality. There are two ways to achieve high power DPSSGL: intracavity frequency doubling [6] and extra- cavity frequency doubling [7]. The intracavity frequency doubling can afford very high average power green lasers with high efficiency [8, 9, 10, 11, 12, 13], because of the high infrared (IR) power density in the resonator. The intracavity frequency doubling also has the advantage of compact structure. However, the beam quality of the high power intracavity frequency doubling green lasers is always poor for the unstable oscillator cavity caused by the thermal perturbation. Meanwhile, the CW intracavity frequency doubling causes a chaotic fluctuation of output power in the scale range from microsecond to millisecond, which is known as the “green problem.” [14]

B. Yong et al. reported a green laser with the beam quality of M ²=6.2 at an average output power of 120 W by intracavity frequency doubling of a diode-side-pumped, Q-switched Nd:YAG rod laser with a repetition rate of 10 kHz and an optical-to-optical conversion efficiency of 15.2%, and the type II phase-matched LiB₃O₅ (LBO) crystal was used as the nonlinear crystal [9]. They also reported a 218 W green laser with the similar structure, and the M ² factor was 20.2 [10]. S. Konno reported a 138 W green laser at an estimated beam quality of M ²=11 by intracavity frequency doubling of a diode-pumped Nd:YAG laser [11]. A type II phase-matched LBO crystal was placed in the resonator. This group achieved a further result of over 400W 532 nm output power in 2004.

The main method to produce high average power green output with high beam quality is to use the external frequency doubling. H. Kiriyama et al. presented a Nd:YAG master oscillator power amplifier (MOPA) system, and with an external KTiOPO₄ (KTP) doubler this system generated 132 W of green average power at 1 kHz with a conversion efficiency of 60% when pumped at a IR power level of 222W, but the beam quality was not mentioned [15]. Y. Hirano et al. reported an external two-stage KTP crystal architecture, and the system produced a 131 W green average power with a frequency conversion efficiency as high as 65.2% with the M ² factor of 5.2 [16]. P. Dupriez et al. presented an 80Wgreen laser based on a frequency-doubled picoseconds, single-mode, linearly-polarized fiber laser with a LBO crystal, and the frequency conversion efficiency was 45.4% with the M ² factor of 1.15 [17].

In this paper, we reported a high efficiency and high beam quality green laser with an extracavity second harmonic generation (SHG). A high power Q-switched Nd:YVO₄ MOPA laser was used as the fundamental frequency IR source while the nonlinear crystal was a type I phase-matching LBO. The green laser pulses with the peak power of 1.5 MW were obtained at 103.5 Waverage power output of 60 kHz, and the pulse duration was 11.8 ns. The diode-to-green and IR-to-green optical efficiency were up to 24% and 67% respectively. The beam quality factors was M ²<1.45 in both orthogonal directions. To the best of our knowledge, this is the highest efficiency, highest beam quality green laser that has ever been reported through an extra- cavity SHG with more than 100 W average power. We also investigated the output characteristics of the SHG varying with the pulse repetition frequency (PRF) of the IR laser.

2. Experimental setup

The experimental setup of the extra-cavity second harmonic generation is shown in Fig.1. In the continuous wave operation at 1064nm, the 32 W Nd:YVO₄ oscillator was scaled up to 178 W through the 4-stages Nd:YVO4 amplifiers at 500 kHz, corresponding to the measured beam quality factors as M ² _x=1.21 and M ² _y=1.27 in the orthogonal directions respectively. At the acousto-optic Q-switching operation of 100 kHz, the output power of the MOPA IR laser was 161 W with the pulse duration of 18 ns.

 

Fig. 1. Experimental setup of the extra- cavity SHG.

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The LBO crystal was chosen as the frequency doubling crystal, because of its relatively large acceptance angle (acceptance angle×length~52 mrad·cm), good mechanical characteristics, especially for its high damage threshold. The damage threshold of the LBO crystal is as high as 2.5 GW/cm² at τ=1.3 ns @1064nm [18], which is about 10 times as large as that of KTP crystal. Therefore, LBO is suitable for high power second harmonic generation, especially for high peak power pulse operation, even though its nonlinear coefficient is relatively small (~1.16×10^-12m/V). The IR laser was focused into the extra-cavity harmonic generation module by a lens with focal length of f=150mm. The LBO crystal with dimension of 5mm×5mm×25 mm, was type I phase-matching and was cut at θ=90°, φ=0°. Both the entrance and the exit surfaces of the LBO were antireflection coated at 1064 nm (T>99.8%) and at 532 nm (T>97%).

The LBO was placed with a distance of L=115 mm between the lens and the center of the LBO, i.e., the frequency doubling crystal was placed before the focused spot of the IR laser, which is in order to reduce the power intensity on the entrance of the LBO crystal and to prevent the damage of the coatings (damage threshold ≈100 kW/cm²). The spot size at the waist spot is with the radius of ω ₀≈0.09mm, and the spot size at L=115 mm is with the radius of about ω _L≈ 0.25 mm. The power intensity are 550 kW/cm² and 80kW/cm² for the two locations respectively. The temperature of the LBO was monitored by a precise temperature controlling module. The calculated non-critical type I phase matched temperature (NCPM temperature) of the crystal was at 150°C, but the temperature was kept at 149.5°C in the experiment, due to the self-heating effect in the nonlinear crystal [19]. The temperature distribution in the nonlinear crystal can be presented as

where T ₀ is the boundary temperature (i.e., the controlling temperature being set), P is the pumping power of the IR laser, η=1×10^-4cm^-1 is the absorption coefficient, K=3.5 W/m/K is the thermal conductivity, and d is the diameter of the pumping power. Although the boundary temperature of LBO crystal can be kept constant at T ₀, the temperature in the middle of LBO crystal still get higher than T ₀, which yields the thermal induced phase mismatching for SHG. In order to compensate this phase mismatching, the boundary temperature was set lower than the ideal temperature of 150 °C. Taking our operated power of P=155W as example, the boundary temperature T0 should be set in the interval of 149.5 °C±0.5oC, and the incident angle to the LBO crystal was fine adjusted to satisfy the optimal phase matching condition.

3. Experimental results and discussion

 

Fig. 2. The average output power and optical conversion efficiency Vs. the PRF.

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The output power of the SHG increased nonlinearly while the repetition rate of the fundamental IR laser decreased, as shown in Fig. 2. At the repetition rate of 60 kHz, 103.5Waverage power of green laser at 532 nm was achieved, corresponding to 154.5 W average power of the fundamental IR laser. The conversion efficiency from IR to green laser was about 67% at 60 kHz, corresponding to the diode-to-green optical conversion efficiency of 24%. Enhancement of the repetition rate of the IR laser yields to the widening of the pulse duration and the nonlinear decreasing of the peak power of the IR laser. Since the frequency conversion efficiency of the SHG is proportional to the power density of the fundamental frequency laser, therefore, the conversion efficiency of the SHG went down as the repetition rate increased, even though the average power of the IR laser increased to 170W at 500 kHz. 24W green laser was obtained at 500 kHz. The repetition rate of the IR laser wasn’t decreased lower than 60 kHz to prevent the damage of the dichromatic coating on LBO crystal.

 

Fig. 3. The characters of the green laser pulse varying with the PRF.

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The pulse characters of the green laser were detected using a high speed photoelectric detector and a Aglient Infiniium oscilloscope with the bandwidth of 1.5 GHz. Figure 4 shows the pulse duration and peak power of the green laser varying with the PRF. The pulse duration of the SHG was 11.8 ns (FWHM) at 60 kHz, which corresponds to a peak power as high as 1.5 MW. The peak power reduced to 9.6 kW while the pulse duration was 50 ns. The oscilloscope traces of the pulse series at both 60 kHz and 500 kHz are shown in Fig. 4(a) and (b) respectively. The standard deviation jitters were used to describe the instability of pulse energy. The instability of pulse energy was better than ±4% (rms) at 60 kHz and which degenerated to be better than ±10% at 500 kHz.

The beam quality of the SHG was measured with 90/10 Knife Edge method using a Spiricon M2-200 laser beam analyzer. The beam quality of the 103 W average output was measured as M ² _x=1.44 and M ² _y=1.23 (see Fig. 5(a)), which is a near diffraction limited beam output up to 100 W at 532 nm. Compared with the beam quality of IR source, the beam quality of the SHG was degraded which is due to the thermal effects especially the thermal aberration in the nonlinear crystal. The beam quality of the green laser was deteriorated slightly when the PRF of the IR laser increased, from M ² _x=1.44 and M ² _y=1.23 at 60 kHz to M ² _x=1.52 and M ² _y=1.33 at 500 kHz. Figure 5(b) shows the near-Gaussian-like spatial distribution of laser intensity on far-field.

4. Conclusion

In summary, we reported a high brightness and high beam quality extra-cavity second harmonic generation. 103.5 W average power 532 nm doubling frequency laser was achieved which corresponds to a IR-to-green optical conversion efficiency of 67% and diode-to-green optical conversion efficiency of 24%, and the peak power of the green laser was as high as 1.5 MW with the instability of pulse energy less than ±4%. The beam quality was measured better than M²<1.45 in both orthogonal directions. The output characteristics of the SHG varying with the PRF of the IR laser were also investigated. We consider that with a higher threshold coating of LBO, the SHG with higher average power and more efficient will be achieved in the case that the PRF is kept lower than 60 kHz, the IR power is enhanced, or the focus spot of the IR laser moves towards the center of the LBO.

 

Fig. 4. The oscilloscope traces of the green pulse series at (a) 60 kHz and (b) 500 kHz.

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Fig. 5. The beam characters of the frequency doubling green laser at 103 W output. (a) beam quality measurement; (b) Spatial form.

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