Raman laser oscillator to convert the Nd:YAG laser at the wavelength of $1.06 \mu m$ to eye-safe laser at the wavelength of $1.54 \mu m$ was studied. High pressure methane($CH_4$; Raman shift, $2917 cm^{-1}$) was used as a nonlinear medium in which both the stimulated Raman scattering(SRS) and the stimulated Brillouin scattering(SBS) could occur. By using the backward SBS as a resonator mirror for the pump wave at the wavelength of $1.06 \mu m$, the Raman conversion efficiency was highly increased in comparison with a conventional Raman oscillator. The focusing lens (meniscus type,f=12 cm) was made to have the back focal length exactly equal to the radius of curvature of one of its surfaces, and it was coated for dichroic reflection (99 \% reflectivity at $1.54 \mu m$ and 5 \% reflectivity at $1.06 \mu m$). This surface was used as an output coupler of the initial pump cavity to generate the seed pulse for backward SBS process. The main pump pulse for SRS process was generated in the Brillouin resonator which was formed by the backward SBS process. The forward and the backward Raman wave at the wavelength of $1.54 \mu m$ were generated, and the backward Raman wave were reflected by the dichroic refection coating on the surface of the focusing lens. The optical length of the resonator between the focusing lens and the rear mirror was approximately 40 cm. The dimensions of the Nd:YAG rod were 5.7 mm diameter and 76.2 mm length, and its surfaces were coated for antireflection. Two Brewster angled glass plates were placed inside the cavity to generate the linearly polarized pump wave. A passive Q-switch filter(Kodak 15064, optical density 0.48) was used to make the system compact. The length of the Raman cell was 15 cm. At the pressure of 40 atm of $CH_4$ gas, the maximum Raman output energy was obtained. At this pressure, the Raman output energy was about 38 mJ and the pulse width was 7 ns(FWHM). The Raman conversion efficiency upto 48\% was obtained in this case. When the focusing lens was replaced with the uncoated lens which had the same geometrical shape as the former one, much lower Raman output energy was obtained. For comparison with a conventional method, an output coupler with 20 \% reflectivity was placed. In this case the maximum Raman output energy at 600 psi was about 7 mJ, and the Raman conversion efficiency was about 14\%. This Raman laser oscillator can be used as an eye-safe laser source for a laser range finder or lidar systems.
