We demonstrated efficient high-power continuous-wave (cw) and passively Q-switched disordered Nd:CLNGG laser performance. In cw operations, the output power was obtained to be 3.81 W with a slope efficiency of 30.3%. To our knowledge, they are the highest cw power and efficiency with Nd:CLNGG as the gain medium and a laser-diode (LD) as the pump source. Recorded with a spectrum analyzer, no splitting was found in the Nd:CLNGG laser, which is different with that of its isomorph Nd:CNGG. The LD pumped passively Q-switched Nd:CLNGG laser was obtained for the first time to our knowledge. The shortest pulse width, largest pulse energy and highest peak power were achieved to be 12.3 ns, 199.1 μJ and 16 kW, respectively, with Cr4+:YAG crystals as the saturable absorbers.
©2009 Optical Society of America
Nowadays, the disordered crystals have been of current interests because they posses both advantages of Nd-doped glass and ordered crystals in spectral and thermal properties [1–4]. In this regime, Nd:CNGG is one of the representative crystals. In this crystal, the Nb5+, Ga3+ and cationic vacancies distributed randomly, which generated its disordered structure and large inhomogeneous broadening in its spectra. Recently, its laser-diode (LD) pumped continuous-wave, passively Q-switched and mode-locked Nd:CNGG lasers has been reported and their potential applications in the generation of terahertz (THz) radiation has been proposed [4,5]. With Li+ ions introduced in this disordered crystal, parts of cationic vacancies were filled and another disordered crystal Nd:CLNGG was generated. The previous studies on these two crystals have revealed their considerable differences, besides the spectral inhomogeneous broadening. For instance, the power of thermal lens and divergence of the Nd:CLNGG laser were measured to be only 55% and 75% of those of Nd:CNGG at about 1.06 μm, respectively . These excellent properties make Nd:CLNGG more favorable to be used in the laser operations than that of Nd:CNGG. With this crystal, subpicosecond pulse has been achieved recently . Just as the disordered Nd:CNGG crystal, this Nd:CLNGG should also have large energy storage capacity and excellent Q-switching performance. Unfortunately, maybe lack of excellent crystals in the past, no LD pumped Q-switched Nd:CLNGG laser has been reported to our knowledge and the power of cw Nd:CLNGG laser is low (170 mW) with a efficiency of only 10% . With the Czochralski method, we have gotten the high-quality Nd:CLNGG crystals recently. With this crystal, the output power over than 1 W was ever achieved but with a low efficiency (12.4%) . In this letter, we demonstrated high-power efficient cw and passively Q-switched Nd:CLNGG laser performance. The cw output power was obtained to be 3.81 W with a slope efficiency of 30.3%. Its beam quality parameter M2 was also measured to be 2.06 smaller than that of Nd:CNGG (2.89) under the same incident pump power. With Cr:YAG as saturable absorbers, its passive Q-switching performance was achieved with the shortest pulse width, largest pulse energy and highest peak power of 12.3 ns, 199.1 μJ and 16 kW, respectively. No split was found in its laser spectrum, which is different with that of its isomorph Nd:CNGG.
The experimental configuration shown in Fig. 1 is based on a plano-concave resonator. The pump source used in all the operations was a fiber-coupled LD with a central wavelength around 808 nm. Through focusing optics (N. A. = 0.22), the output of the source was focused into the laser crystal with a spot radius of 0.256 mm. The input mirror M1 was a concave one with a curvature radius of 200 mm, AR coated at 808 nm on the flat face, high-reflection coated at 1.06μm and high-transmission coated at 808 nm on the concave face. The output coupler (OC) M2 was two flat mirrors with different transmissions at 1.06 μm of 1.6% and 8%. A Nd:CLNGG crystal with Nd concentration of 0.5at.% was used as a gain material. It was cut along its <111> direction with the dimensions of 3 mm × 3 mm × 6 mm. Its end-faces were polished and anti-reflection (AR) coated at 808 nm and 1.06 μm. Two Cr4+:YAG samples were used as saturable absorbers, their initial transmissions T0 were 97.5% and 93.7%, respectively, with AR coatings at 1.06 μm on their end-faces. To remove the heat generated from laser crystals and saturable absorbers under high pump power levels, laser crystals were wrapped with indium foil and mounted in a water-cooled copper block, and Cr4+:YAG were held in a aluminum block without cooling water. The temperature of cooling water was controlled to be 10 °C. The laser output power was measured by a power meter (EPM 2000. Melectron Inc.) and temporal behaviors of the Q-switched laser were recorded by a TED620B digital oscilloscope (500-MHZ bandwidth and 2.5-Gs/s sampling rate, Tektronix Inc.). With a optical spectrum analyzer (0.1 nm spectral resolution ANRITSU. MS9710C), the laser spectra were measured.
3. Results and discussions
Cw operations were first carried out by removing Cr:YAG from the cavity and optimizing the cavity length to be 1.5 cm. The change of cw output powers with absorbed pump power was shown in Fig. 2 . The absorption efficiency of this crystal at the pump wavelength was measured to be 78%. By using OC = 1.6%, the maximum output power Pmax was obtained to be 3.81 W under the absorbed pump power of 13.4 W, with a slope efficiency (η) of 30.3%. Increasing the pump power, the output power was saturated. It can be found that this results are a bit lower than that obtained with Nd:CNGG (Pmax = 4.03 W, η = 31%) , which is caused by its higher thermal induced loss under the high pump power due to its lower thermal conductivity (2.98 Wm−1K−1)  than that of Nd:CNGG (4.7 Wm−1K−1) . Up to now, this is the highest-power and most efficient Nd:CLNGG laser to our knowledge. The threshold was measured to be 0.63 W. Replacing OC to be that with a transmission of 8%, the threshold became 2.1 W and slope efficiency was 28.9%. We also measured the threshold to be 2.56 W with OC = 10%. Based on the cw model developed by N. Mermilliod et.al , the threshold (Pth) can be given by:10]. The smaller emission cross-section of Nd:CLNGG than that of Nd:CNGG and its lower slope efficiency with OC = 8% than that with OC = 1.6% indicated that the inhomogeneous broadening in the spectra of Nd:CLNGG is larger than that of Nd:CNGG with the same Nd-doped concentration . Using the knife edge method, the beam quality parameter M2 of the cw laser beam under the incident pump power of 15 W was measured to be 2.06 much smaller than that with Nd:CNGG (2.89) in the same incident pump power . We believed that this better beam quality was induced by its lower thermal focal power. This phenomenon is similar with that reported with flashlamps as pump source . With a spectrum analyzer, the laser spectrum was recorded which was shown in Fig. 3 . From this figure (b), it can be found that the laser band is centered at 1060 nm, which is different from that obtained with Nd:CNGG (Fig. 3 (a)) with centers at 1059 nm and 1061 nm . For comparison, the fluorescence spectrum centered at 1.06 µm with the width of 16.5 nm was also shown in this figure (c).
Extending the cavity to be 2.5 cm and inserting Cr:YAG into the cavity close to M2, the passively Q-switched Nd:CLNGG laser was achieved. During all the passive Q-switching, the OC with a transmission of 8% was used. The variation of average output powers with increasing absorbed pump power is also shown in Fig. 2. With T0 = 97.5%, the maximum average output power of 1.64 W was obtained with optical conversion efficiency of 13.6%. The threshold increased and average output power decreased obviously with T0 = 93.7%. The maximum output power was achieved to be 0.95 W at the absorbed pump power of 12.1 W with optical conversion efficiency of 7.9%.
Same with that of Nd:CNGG, the pulse repetition rate increased and pulse width decreased with the increasing of pump power with the same saturable absorber. The highest pulse repetition rate were measured to be 15.5 and 4.78 kHz by using T0 = 97.5% and 93.7% Cr:YAG, respectively, in the Q-switching operations. The shortest pulse with width of 12.3 ns was obtained with T0 = 93.7%. The pulse profile with width of 12.3 ns is shown in Fig. 4 . From this figure, it can be observed that the falling time is obviously longer than the rising, which means that the transmission of OC used in the experiment was a bit low. But if OC with a high transmission is used in the Q-switching, the average output power would be much lower.
With the pulse repetition rate and average output power, the pulse energy could be calculated. We found that the pulse energy varied in the range of 84 μJ to 106 μJ with T0 = 97.5%, and 110 μJ to 199.1 μJ with T0 = 93.7%. The pulse energy is larger than that Nd:CNGG (173.16 μJ) . By comparing the emission cross-sections of Nd:CNGG and Nd:CLNGG, it can be believed that the energy storage capacity of Nd:CLNGG is larger than that of Nd:CNGG, which induces the Q-switched Nd:CLNGG laser with larger pulse energy. By using the pulse energy and pulse width, the peak power can be calculated. The highest peak power was achieved to be 16 kW with T0 = 93.7% under the absorbed pump power of 12.1 W. The peak power was also much higher than that of the Nd:CNGG laser (12.3 kW) .
In conclusion, the high-power laser performance was demonstrated with Nd:CLNGG crystal as the gain material. The cw output power was gotten to be 3.81 W with a slope efficiency of 30.3%. By measuring the M2 value, the Nd:CLNGG laser has much better quality than that of Nd:CNGG under the same incident pump power. With a spectrum analyzer, no split was found in the Nd:CLNGG laser spectrum. In the passive Q-switching, due to the small emission cross-section of Nd:CLNGG, the pulse energy as high as 199.1 μJ was achieved with the peak power of 16 kW.
This work is supported by the National Natural Science Foundation of China (No. 50672050, 50721002 and 50911120083) and the Grant for State Key Program of China (2004CB619002).
References and links
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