The 900 nm laser based on a KTiOPO₄ (KTP) optical parametric oscillator (OPO) was demonstrated for the first time. The 532 nm laser was produced from a LiB₃O₅(LBO) pumped by 1064 nm laser. The wavelength tuning range of 898-911 nm, tuning resolution better than 1 nm, output energy of 1.85 mJ, and repetition rate of 1-10 Hz was obtained, which can be applied to optoelectronic countermeasures for its solidification, miniaturization, wide tuning, high tuning resolution, and high beam quality.

  Objective   The spectral response of a 900 nm wavelength laser on a silicon detector is better than that of 1064 nm, and it is located at the transition absorption line of atoms, ions, etc. It can also generate blue light laser with frequency doubling. Therefore, it can be widely applied in fields such as optoelectronic guidance, atmospheric detection, and deep-sea communication. Currently, semiconductor lasers commonly used to generate 900 nm wavelength bands have a low tuning range and are difficult to output high-energy pulses. The optical system of titanium sapphire laser is relatively complex, and it is difficult to achieve miniaturization and lightweight, and the tuning speed is slow. Nd³⁺ doped solid-state lasers are difficult to achieve wavelength tuning output and are prone to energy level competition. We need a suitable crystal and technology to solve the above problems and achieve better optical performance of 900 nm laser output. Optical Parametric Oscillator(OPO) can convert mature 1064 nm laser into ultraviolet, visible, near-infrared, mid to far infrared laser bands, and has advantages such as solid-state design, miniaturization, high efficiency, high beam quality, and wide tunable output wavelength. The use of high damage threshold KTiOPO₄(KTP) and OPO can achieve high conversion efficiency, wide tuning range, and high tuning resolution of 900 nm laser output.

  Method   A tunable 900 nm wavelength laser based on KTP OPO has been built (Fig.2). We chose LBO frequency doubling crystal to double the 1064 nm laser output from Nd: YAG laser to 532 nm and use it as the pump light for KTP OPO. The angle of the KTP crystal changes with the adjustment knob above, and the output wavelength of the laser can be tuned. The light at the output end is collected by the optical fiber and guided into the spectrometer for spectral analysis. By turning the knob on the upper adjustment bracket, a 900 nm laser within a certain tuning range is successfully captured.

  Results and Discussions   The tuning range of the laser is measured, and a tuning range from 898 nm to 911 nm is achieved (Fig.4). Angle tuning resolution \Delta \lambda _i/\Delta \theta =9.56 nm/(°), slightly different from the theoretical result of 10.26 nm/(°). Possible reason is the cutting angle φ of KTP crystals not in the 0° position. The repetition rates of Nd: YAG lasers are set to 1, 2, 5, and 10 Hz, respectively, and the energy of the frequency doubling laser at 532 nmand the energy of the output idler light at 900 nm are measured (Fig.5). When the repetition rates are 1, 2, 5, and 10 Hz, the slope efficiencies are 43.62%, 38.14%, 36.35%, and 31.78%, respectively. The maximum measured output energy is 1845 μJ, 1656 μJ, 1571 μJ, 1397 μJ, respectively. The corresponding light to light conversion efficiencies are 37.81%, 33.93%, 32.19%, and 28.63%. The energy conversion efficiency decreases with the increase of repetition frequency. The possible reason is that the thermal effect caused by the high pulse repetition rate consumes some energy. The pump threshold is maintained at 402-570 μJ under four different repetition rates, remaining basically unchanged.

  Conclusion   A wide tunable and high-precision 900 nm laser output based on KTP OPO has been successfully achieved. The tuning resolution reaches 9.56 nm/(°). The energy output of idler light reaches 1.85 mJ, when the pump energy is 4.88 mJ. The energy conversion efficiency under different repetition rates is also comprehensively measured. The tuning range of the laser system was analyzed by a spectral analyzer, and the measured tuning range spanned from 898 nm to 911 nm. The tunable output of 900 nm laser based on KTP OPO has been proven to be a simple, wide tunable range, high tuning accuracy, and fast tuning speed method.