  We are developing the Laser Guide Star Adaptive Optics (LGS/AO188) system for
the Subaru Telescope at Mauna Kea, Hawaii. This system utilizes a combination
of an all-solid-state mode-locked sum-frequency generation (SFG) laser (1.7-GHz
bandwidth, 0.7-ns pulse width) as a light source and a single-mode optical
fiber for beam transference. However, optical fibers induce nonlinear effects,
especially self-phase modulation (SPM). We studied SPM in our photonic crystal
fiber (PCF). SPM broadens the spectrum of a laser beam and decrease the
efficiency of bright laser guide star generation. We measured the spectrum
width using a spectrum analyzer. We found a spectrum width of 8.4 GHz at full
width at half maximum (FWHM). The original FWHM of our laser spectrum was 1.4
GHz. This was equivalent to a 70 % loss in laser energy. We also measured the
brightness of the sodium cell and evaluated its performance as a function of
laser wavelength. The cell's brightness showed a peculiar tendency;
specifically, it did not extinguish even though the wavelength varied by more
than 5 pm. To reduce the impact of SPM, we developed an optical system that
divides one laser pulse into four lower-power pulses. The laser peak power
after passing through the new optical system was decreased to one-fourth the
original, reducing the impact of SPM on the sodium cell. An actual laser guide
star created with the new system was 0.41 mag brighter than the laser guide
star created with the original system. We achieved brighter laser guide star
generation by dividing a laser pulse to reduce its peak intensity. This is an
effective method for laser relay using optical fiber.
