Update on Laser Development for Lunar Laser Ranging in South Africa

A new fundamental space geodetic observatory for South Africa has been proposed. Lunar Laser Ranging (LLR) is one of the space geodetic techniques to be hosted on-site. LLR requires a pulsed laser that can operate at 100 to 400 mJ energy per ~ 20 pico-second pulses, at a pulse repetition frequency in the kHz region. It must also have excellent beam quality at M2 < 1.1. Since no commercial laser matching the exact specifications could be obtained, it was decided to develop such a laser.<br>The most feasible candidate for such laser parameters was found to be a crystal-based diode pumped system. Several measures must be adhered to in order to obtain the required results: end-pumping will reduce losses and therefore minimise excess heat, a very good overlap between the pump and laser beams will further reduce losses, thermal lens effects must be kept to a minimum and corrections for astigmatism must be incorporated, to name a few.<br>Nd:YLF was identified as a suitable candidate for the gain medium crystal. 4 such crystals were used within a single resonator to multiply the output power. The setup was also built in such a way that the astigmatism of 2 crystals overlapped at right angles with the other 2 crystals’. This minimised the effect of the overall astigmatism. The total output power of this initial continuous wave laser was 87 W at 1053 nm, less than the anticipated 100 W. A revised setup with smaller pump beam diameter is underway.<br>A 4 crystal Nd:YLF laser still seems like the best candidate to achieve the required output for LLR. Future work includes a redesign of the resonator, pulsing the laser and frequency doubling it to green.