We report a high-power diode-double-end-pumped Q-switched Nd:YVO4 red laser through intracavity frequency-doubling with a type-I critical phase-matched LBO crystal. At a repetition frequency of 21.72 kHz, a maximum average output power of 10.2 W at 671 nm was measured to while the incident pump power was 78.4 W, the corresponding optical conversion efficiency was 13.0%, with a pulse width of about 94 ns and a pulse energy of 469.6 μJ, the peak power was 5.0 kW. At an average output power around 9.6 W a power stability better than 2.3% was maintained for half an hour.
©2007 Optical Society of America
High-power red lasers have a wide application in the fields such as biomedical technology, medical treatment and laser color display, and they can also be applied to femtosecond Kerrlens mode-locked lasers based on Cr3+:LiSrAlF6 (Cr:LiSAF), Cr3+:LiSrGaF6 (Cr:LiSGAF) and Cr3+:LiSrCaAlF6 (Cr:LiSCAF) crystals . Compared with the red laser diodes, diode -pumped solid-state lasers have advantages such as excellent beam quality, high efficiency, high reliability, which makes them more attractive in many fields of application.
The intracavity frequency doubling of infrared radiation with nonlinear optical crystals is an effective method to generate high-power and high-beam-quality red light [2–8]. In recent years, the red light more than 10 W has already been generated by intracavity frequency doubling, but with a very low optical conversion efficiency and poor beam quality for its side-pumping style . The end-pumping technique, which is a more effective way to match the size of pump beam and cavity mode, can improve the beam quality and increase the optical conversion efficiency. By Chenlin Du and his group, quasi-continuous-wave (QCW) coherent red light up to 6 W was generated by intracavity frequency doubling of an end-pumped Nd:GdVO4 laser in a 15-mm LBO crystal . The optical conversion efficiency was up to 12.8%, and the beam quality of M 2 factor was less than 2.47. But for single-end-pumped solid-state red laser, it is hard to obtain higher laser output power due to the strong thermal lens effect in the active medium generated by high power pumping.
In this paper, we report a Q-switched red laser at 671 nm by intracavity frequency doubling of a double-end-pumped 1342 nm Nd:YVO4 laser, based on a type-I critical phase-matched LiB3O5 (LBO) crystal. The average output power of 10.2 W at 671 nm can be obtained at the incident pump power of 78.4 W, with the corresponding optical conversion efficiency of 13.0%.
2. Experimental setup
The experimental setup of intracavity second-harmonic generation (SHG), which had a four-mirror-folded (Z-configuration) resonator, was shown schematically in Fig. 1. The pump sources, fixed on the two ends of the laser crystal, were two commercially available high-power fiber-coupled diode-laser-arrays. The core diameter and numerical aperture (N.A.) of the fiber were 0.4 mm and 0.22, respectively. The pump beams from the fiber at the wavelength of 808 nm were focused into the laser crystal by two optical imaging systems with the imaging ratio of 1:1. The pump mirrors M1 and M2 were flat mirrors, antireflection (AR) coated at 808 nm on their outside surfaces, high-reflectance (HR) coated at 1342 nm and high-transmittance (HT) coated at 808 nm on their inside surfaces. The a-cut Nd:YVO4 crystal with Nd3+ concentration of 0.3 at.% and dimensions of 3×3×10 mm3 was AR coated at 808 nm and 1342 nm on both of its faces, placed in the middle of M1 and M2. To remove the heat generated at high-pump-power levels from the crystal, it was wrapped with indium foil and held in a water-cooled copper block. A temperature sensor was mounted in the copper block near the laser crystal to monitor its surface temperature. The surface temperature of Nd:YVO4 crystal was kept at about 20 °C during the experiments. The output coupler M3 was a meniscus mirror with a radius of curvature of inside surface of 150 mm, HR coated at 1342 nm and AR coated at 671 nm on the inside surface, and HT coated at 671 nm on the outside surface. An acousto-optical (A-O) Q-switch with high diffraction loss at 1342 nm was placed between M2 and M3, and its repetition rate could be tuned continuously from 1 kHz to 100 kHz. A 20-mm-long, (θ, φ)=(86.1°, 0°)-cut LBO crystal for type-I critical phase-matching at 1342 nm was used as the frequency doubler. To minimize the internal losses caused by Fresnel reflection, it was AR coated at 1342 nm and 671 nm on both end faces. It was cooled in the same way as the Nd:YVO4 crystal. The end mirror M4 was a concave mirror with a radius of curvature of 50 mm and with a dual-wavelength HR coating at 1342 nm and 671 nm on its curved surface. In order to adjust the resonator conveniently, the LBO crystal and M4 were mounted on translation stages. To suppress the oscillation of the 4 F 3/2→4 I 11/2 transition (1064 nm) of Nd3+, all the four mirrors had sufficient transmission (>90%) at 1064 nm.
According to our numerical calculation based on the transmission matrix theory and our previous studies [11–13], it was found that the stable parameter and fundamental mode size in the laser crystal are sensitive to both the lengths of M2M3 arm (L3) and M3M4 arm (L4), furthermore, the size and location of the beam waist between M3 and M4 are mainly dependent on the length of L4. To obtain high efficient second-harmonic generation, two important points should be noticed in the process of resonator design. Firstly, the size of oscillating beam in the laser crystal should be matched with the pump beam to optimize the laser mode and improve the conversion efficiency . Secondly, since the frequency-doubling efficiency is proportional to the power density of fundamental waves, it is necessary to reduce the beam size in the nonlinear optical crystal to provide a high power density of fundamental waves. Therefore, the arms L3 and L4 should be adjusted to enable stability within the range of high pump power and the LBO crystal should be placed close to the end mirror M4 where a beam waist existed to gain high optical conversion efficiency. In our experiment, a group of cavity parameters were chosen as follow: L1=L2=10 mm, L3=235 mm and L4=135 mm. The folding angle between the M2M3 arm and the M3M4 arm was kept at 8° to minimize astigmatism.
3. Results and discussion
The pump wavelength was set equal to the absorption peak wavelength of Nd:YVO4 crystal, by adjusting temperature. The average output power of the red light as a function of the incident pump power was experimentally measured. As shown in Fig. 2, in the high pump power region, the optical conversion efficiency was better than low pump power region. It was mainly due to reasonable design of cavity which made the high power region close to the center of stable region of resonator. At the incident pump power of 78.4 W, the maximum average output power at 671 nm was measured to be 10.2 W with a corresponding optical conversion efficiency of 13.0%, while the pulse width was 94 ns (Full Width Half Maximum, FWHM) and the repetition frequency was 21.72 kHz, respectively. As the pump power was increased to more than 80 W, the 671 nm output power was saturated and not increased along with the pump power any more. This was due primarily to the too serious thermal effect making the resonator away from the center of stable region.
The red laser pulse signal was detected at different pump powers and modulation frequency by using a fast photodiode detector (Newport 818-BB-20), and it was observed and measured with a 300 MHz oscilloscope (Tektronix TDS 3032B). Figure 3 shows the Q-switched pulse sequence with equal spaces in the screen of the oscilloscope. From this figure, it could be seen that the pulse sequence was very stable when the laser operated at a high output power level. Figure 4 shows a single pulse profile with the repetition frequency of 21.72 kHz at the incident pump power of 78.4 W.
The output power stability of the red laser was monitored at a average output power around 9.4 W. As shown in Fig. 5, for half an hour, the output power was measured every two minutes. The fluctuation (rms) of the average output for half an hour was calculated to be about 2.3%. It was mainly due to the pump wavelength and linewidth variations of the laser diodes. Moreover, these variations were impossible to be avoided because it was hard to set the wavelengths of two laser diodes the same at all and no attempt was made to actively control their temperature.
As shown in Fig. 6, setting the incident pump power to about 73 W, we also measured the average output power and pulse width of 671 nm light at different repetition frequencies. The maximum 671 nm light average output power of 9.6 W was obtained at the repetition frequency of 25 kHz; meanwhile, the corresponding pulse width was 102 ns. When the repetition frequency was not less than 15 kHz and not more than 40 kHz, the pulse width was shortened along with the decrease of repetition frequency. It can be presumed that if keep on decreasing the repetition below 15 kHz, the pulse width would keep shortening, but meanwhile, the output power would also be decreased badly.
In conclusion, a high-power double-end-pumped Q-switched intracavity-frequency-doubling Nd:YVO4/LBO red laser has been demonstrated. The maximum average 671 nm output power as much as 10.2 W was obtained with a corresponding repetition frequency of 21.72 kHz, while the pulse width was 94 ns and the optical conversion efficiency was 13.0%. The perfect performance indicated that the double-end-pumping technique could effectively diminish the thermal lens effect to allow more pump power being injected into the laser crystal so that a higher output power could be obtained.
This work was supported by the Science and Technology Project of Guangdong Province of China (2004B16001210), the Natural Science Foundation of Guangdong Province of China (No. 04300858), the Science and Technology Project of Shenzhen (No.200429), and the Key Natural Science Research Project of Guangdong Higher Education Institutions (No.05Z019).
References and links
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