High-Power Laser Beam Transfer through Optical Relay Fibers for a Laser Guide Adaptive Optics System

We are developing a laser guide star adaptive optics (LGSAO) system for the Subaru Telescope on Mauna Kea, Hawaii. The AO188 system will dramatically increase the observable sky area. This system differs from systems used at other large telescopes in that it utilizes the combination of an all-solid-state mode-locked sumfrequency generation (SFG) laser as a light source and single-mode optical fiber for beam transference. Optical fibers transfer laser light from the source, located at the Nasmyth platform, to the laser launching telescope more flexibly and more easily than do mirror relay optics. However, optical fibers induce nonlinear scattering effects, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), beyond certain threshold levels in high-power lasers. We measured the laser transmission characteristics of a photonic crystal fiber (PCF) whose mode field diameter (MFD) was 11 � m, and a step index fiber (SIF) cable whose MFD was 4.2 � m to evaluate the threshold levels for non-linear effects. We observed SRS in the 200-m-long SIF when we input 1.3 W. However, SRS and SBS were not induced in the 200-m-long PCF, even for an input power of 5.3 W. As a result, we estimated the threshold of SRS to be 33 W for the 35-m-long PCF designed for the Subaru LGSAO system. Given that the fiber will carry a laser beam of about 30 W with a pulse width of less than 1 ns, we conclude that nonlinear scattering will pose no problems for this application.