1山西大学光电研究所量子光学与光量子器件国家重点实验室, 山西 太原 030006
2山西大学极端光学协同创新中心, 山西 太原 030006
Significance High-power low-noise continuous-wave (cw) single-frequency all-solid-state lasers (ASSL) have various advantages, including high conversion efficiency, high beam quality, low noise, and long coherence length, and have been widely used in many application fields, e.g. quantum science and technology, cold atom physics, high-precision measurement, high-efficiency frequency conversion, coherent communication, lidar, and optical sensing. This type of laser promotes the development of essential quantum physics research and the practicality of quantum technology. For example, such lasers can be used to prepare a high entanglement, multi-component quantum entanglement source, which is an important technical element in quantum secure communications and quantum computing. In addition, such lasers are basic light sources for high-precision measurement based on laser interferometers. With the observation that gravitational waves from a binary black hole merge, high-precision measurement has attracted significant attention recently. The sensitivity of the gravitational wave device is inversely proportional to the root mean square of the injected laser power, and thus if the injection laser power of the advanced LIGO device is expected to reach 125 W, its detection sensitivity will be 10 times higher than that of the LIGO device.
Progress To satisfy the application requirements of ASSLs in quantum science and technology, cold atom physics, and high-precision measurement, high-power low-noise cw single-frequency mode-hop-free ASSLs with different central wavelengths were investigated and fabricated. First, based on the analysis of the thermal effect of laser crystal in laser-diode end-pumped high-power ASSLs and the improvement measures for the thermal effect of laser crystals, a universal model of sufficient conditions for a stable single-longitudinal-mode operation for high-power cw ASSLs was established by introducing the nonlinear losses of the fundamental wave when a nonlinear crystal was inserted into the cavity, and a high-power cw single-frequency mode-hop-free ASSL was fabricated. Second, the output power of the cw single-frequency ASSL was scaled up under intense pump. A 125W cw single-frequency 1.064μm laser was achieved by a homemade 50.3W cw single-frequency laser, and a two-stage dual end-pumped master-oscillator power amplifier acted as the seed source and amplifier. The measured power stability of the 1.064μm laser over 8h was better than ±0.73%. In addition, a 25.3W cw single-frequency 532nm laser was obtained with an optical-optical conversion efficiency of 32.2%, and the power stability was greater than 0.4% over 8h. Third, the noise characteristics of the cw single-frequency ASSLs were studied and suppressed. The intensity and phase noise of the cw single-frequency ASSLs reached the shot noise level (SNL) for analysis frequencies greater than 5MHz. The intensity noises of the ASSLs were much greater than the SNL in the low analysis frequency range and less than several megahertz. The intensity noises could be manipulated by intra-cavity nonlinear loss, and the resonant relaxation oscillation noise peak of a 1.064μm laser was reduced and shifted toward low frequencies when the nonlinear loss was increased. The intensity noises of the ASSLs were suppressed by controlling the stimulated emission rate in the laser cavity and by a control system based on a Mach-Zehnder interferometer outside the laser cavity. Fourth, the central wavelengths of cw single-frequency ASSLs were extended. 1) A high-power stable low-noise cw single-frequency 540nm and 1.08μm dual-wavelength Nd∶YAP/LBO laser was fabricated. Maximum output powers of 4.5W at 540nm and 1.5W at 1.08μm were achieved simultaneously. 2) Stable low-noise cw single-frequency 473nm and 946nm Nd∶YAG/PPKTP lasers were fabricated. Maximum output powers of 1.01W at 473nm and 1.5W at 946nm were achieved, respectively. 3) A high-power stable low-noise cw single-frequency 671nm and 1.34μm dual-wavelength Nd∶YVO4/LBO laser was fabricated. Maximum output powers of 3.17W at 671nm and 2.15W at 1.34μm were achieved simultaneously. 4) A stable low-noise cw single-frequency 1.55μm Er,Yb∶YAB laser was fabricated. Maximum output power of 400mW at 1.55μm was achieved. 5) An all-solid-state cw single-frequency Ti:sapphire laser with continuous frequency-tuning was achieved via an intra-cavity locked LiNbO3 electro-optic etalon. A maximal tuning range of 110nm (760--870nm) was obtained by rotating the BRFs, and a continuous frequency-tuning range of 20GHz was realized after the electro-optic etalon was locked to the oscillating mode of the Ti:sapphire laser. Maximal output power of 2.88W at 795nm was obtained under a pump power of 16.53W. 6) A continuously tunable single-frequency 461nm Ti:sapphire laser was obtained by PPKTP intra-cavity doubling. Output power at 460.86nm was 1.05W under a pump power of 12W, and the continuous frequency-tuning range of the 461nm blue laser was 15.756GHz. 7) A continuously tunable single-frequency 455nm blue laser was implemented by an intra-cavity frequency doubled Ti:sapphire laser with an LBO crystal. The output power at 455nm was 1.0W under a pump power of 13.5W, and the continuous frequency-tuning range of the 455nm blue laser was up to 32GHz.
Conclusion and Prospect Based on the theoretical study and experimental design of lasers, a series of high-power, low-noise cw single-frequency mode-hop-free ASSLs with different central wavelengths were fabricated. Relative to the future development of high-power, low-noise cw single-frequency ASSLs, scaling up the output power and output energy of the ASSLs remains an important topic. In addition, the central wavelengths and linewidths of these lasers should be extended and narrowed, respectively, and the beam quality and directional stability of the laser beams should be improved further. Finally, to improve the sensitivity of high-precision optical measurements, the extra intensity noise of cw single-frequency ASSLs should be suppressed further, and it is the best that intensity noise can reach the SNL in the whole analysis frequency range.