Numerical study on the influence of the linewidth of a QCW pulsed sodium laser on the brightness of a guide star.

In this paper, we investigated the impact of the linewidth of a QCW pulsed sodium laser on the brightness performance of a generating sodium laser guide star by using the numerical simulation tool PRS. We compared the field test results with the simulation results for two TIPC's 30W class sodium guide star lasers and found the results are in good agreement which proves the tool can be used for prediction. Then, we used the tool to study the influence of D2b repumping and different linewidths from 10MHz to 1GHz on the coupling efficiency and the photon return flux. For the TIPC's QCW pulsed solid-state laser, when the on-sky power density is 1 W/m, the coupling efficiency is 79.6 (photons/s/W/(atoms/m)) without D2b repumping, however, the value is up to 213.3 (photons/s/W/(atoms/m)) with 15% D2b enabled and is increased by 168% than the value without D2b; when the power density reaches 10 W/m, the coupling efficiencies without D2b and with 15% D2b are 66.6 and 233.6 (photons/s/W/(atoms/m)), respectively. The results show that for the QCW pulsed laser, D2b repumping is necessary. With D2b enabled, if the spectral linewidth is too wide or too narrow, the photon return flux will be adversely affected. The return flux of 60MHz is 52.5% higher than that of 1GHz, while the return flux of 300MHz is 37.8% higher than that of 10 MHz when the laser power is 100W.