High- power Single-frequency Continuous 589 nm Diamond Sodium Guide Laser(Invited)

Single-frequency 589 nm laser can resonate with sodium atoms in the sodium layer(altitude of similar to 100 km)to generate intense return fluorescence which is used as a bright beacon for adaptive optics and is essential for large aperture telescopes to observe the universe clearly. This Sodium Guide Star Laser (SGSL) technology is of intense interest for applications in astronomical observation, space debris tracking,ground to space communication and mesospheric magnetometry. Due to the lack of efficient direct inversion gain medium at 589 nm and the strict requirements for laser output power and spectral linewidth, it is challenging to generate suitable SGSL. In this paper,we demonstrated a high-power single- frequency SGSL based on diamond Raman laser with intracavity Second Harmonic Generation (SHG). The laser operated in single longitudinal mode is due to the combination spatial-hole- burning-free Raman gain mechanism and harmonic self-suppression of axial- longitudinal modes. The pump was a linearly- polarized Yb-doped fiber laser at 1 018 nm with 3 dB spectral linewidth of 15.7 GHz and maximum output power of 82 W. The dimension of the diamond crystal was 7 mmx4 mmx1.2 mm and the Type I SHG crystal was LiB3O5(LBO)cut at.= 90 degrees,.= 0 degrees with the dimension of 10 mmx4 mmx4 mm. The directions of 1018 nm pump polarization,diamond < 111> axis and LBO slow axis were aligned parallel to each other,providing the highest Raman gain and SHG angle match. The output coupling of the resonator at Stokes wavelength was 0.03%. At the LBO temperature of 37.8.,the SHG power increased up to a maximum of 16.5 W with a beam quality of M2= 1.05 at the pump power of 82 W. The optical- to- optical conversion efficiency from 1 018 nm to 589 nm was 20%. The SHG output spectrum was measured using a scanning F- P interferometer and had a 3 dB linewidth of 16 MHz. Thus,the power spectral density of the SHG was 197 times higher than that of the pump. An analytical model about the external cavity Raman laser with intracavity SHG is adapted and used to predict the SHG and Stokes powers. The calculation results guide that a small output coupling at Stokes is critical to generate high SHG power. However,the influence of output coupling on the Stokes output power is more complicated due to the intracavity SHG. When the pump power is set to a fixed value of 82 W, Stokes output power shows a parabolic trend with the output coupling and reach the maximum at 2%. By tuning the LBO temperature and its phase- matching, the SHG output power and longitudinal-mode characteristics are analyzed theoretically and experimentally at the maximum pump power of 82 W. The temperature of the LBO crystal is tuned from 33. to 51. at a step of 0.5.. The experimental temperature acceptable bandwidth of the LBO crystal is 13. which agrees well with the theoretical temperature acceptable bandwidth of 15.. The maximum SHG power is achieved when the LBO temperature is tuned away from the optimum of 40.5. and the SHG power is only 10.7 W. The reason is that the intracavity Stokes intensity is getting weak due to the strong nonlinear loss when the LBO phasematching is close to the optimum. The laser is maintaining single longitudinal mode operation for the LBO temperature from 33. to 45.,and when the temperature exceeds this temperature,the laser is operating in multi- longitudinal modes. In summary,1 018 nm Yb- fiber laser pumped external diamond Raman resonator with intracavity SHG has been demonstrated as an efficient technology to generate single- frequency 589 nm laser for SGSL applications. Due to the high Raman gain (10 cm/ GW at 1 mu m) and incomparable thermal conductivity (2 000 W/m center dot K)of diamond crystal,the diamond SGSL is well suited to achieve much higher power.