Suppresion of Self-Phase Modulation in a Laser Transfer System using Optical Fiber on the Subaru Telescope

摘要

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