We are commissioning the Laser Guide Star Adaptive Optics (LGS/AO188) system for Subaru Telescope at
Hawaii, Mauna Kea. This system utilizes a combination of an
all-solid-state mode-locked sum-frequency generation
(SFG) laser (1.7GHz-bandwidth, 0.7ns-pulse width) as a light source and single-mode optical fiber for beam
transference. 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 μ m, and a step index fiber (SIF) cable whose MFD was 4.2 μ m to evaluate the threshold levels for
non-linear effects. We observed SRS in the 200-m-long SIF when we input 1.3W. The material losses of them
were 10db/km and 6.4dB/km, respectively. However, SRS and SBS were not induced in the 200-m-long PCF,
even for an input power of 5.3W. As a result, we estimated the threshold of SRS to be 33W for the 35-m-long
PCF designed for the Subaru LGSAO system.
Other than SRS and SBS, we found self phase modulation (SPM) in our PCF. SPM makes the spectrum
of the laser beam broaden and it causes less efficiency of generating bright LGS. We measured width of the
spectrum by spectrum analyzer. As the result, we found it was 9.1GHz of full width half maximum (FWHM)
in comparison with the original FWHM of our laser spectrum, 2.1GHz. This shows 70% of the laser energy for
brightening the LGS was lost.
We also measured the brightness of the LGS and evaluated its relationship with wavelength of the laser.
The LGS's brightness showed a peculiar tendency that did not be extinguish even though the wavelength has
varied about 2pm. The tendency was not shown with the experiment using sodium gas cell. Therefore, it may
be concerned the environment of the sodium layer in the mesosphere.