In part one of the thesis, efficient eye-safe Raman laser system which utilize stimulated Brillouin scattering for feedback mechanism are described. A Raman laser at the wavelength of 1543 μm is generated in high pressure $CH_4$ gas from a passively Q-switched Nd:YAG laser at 1.064 μm. The high reflectivity and phase conjugation property of stimulated Brillouin scattering in $CH_4$ gas are used as a dual cavity resonator mirror for fundamental and Raman waves.
Two types of focusing lenses (meniscus and plano-convex types) are designed and compared with each other for their performances. Employing a meniscus lens with an optimized reflectance of 10% at 1.064 μm, the laser system produced a maximum Raman energy of 33 mJ with a residual fundamental energy of 20 mJ, where the pressure of $CH_4$ gas is 800 psi. When the plano-convex lens is used, a Raman energy of 25 mJ with a 26 mJ fundamental energy is obtained. Theoretical and experimental results show that the shift of the focus by dispersion in the case of plano-convex lens is responsible for the lower Raman conversion efficiency since it prevents the formation of the Raman half resonator. A meniscus focusing lens is preperred for the efficient Raman generation with a reflecting surface having radius of curvature similar to the back focal length of the lens.
A Nd:YAP laser at 1.079 μm is also tried instead of a Nd:YAG fundamental oscillator. A $Cr^{4+}$:YAG crystal is used as a passive Q-switching filter and a Raman wavelength of 1.575 μm is generated in $CH_4$ gas. By employing a meniscus lens with a reflectance of 6% at 1.079 μm, a maximum Raman energy of 21.4 mJ with a residual fundamental energy of 17.9 mJ has been obtained where the pressure of $CH_4$ gas is 800 psi.
These Raman laser systems can be used as an eye-safe laser source for a laser range finder or lidar systems.
In part two an efficient cascaded Raman fiber amplifier is described. Signal amplification at 1.3 μm is achieved through the use of stimulated Raman scattering in a silica-based fiber with a moderate $GeO_2$ content (18 mole %) pumped by a Nd:YAG laser at 1.064 μm. Cascaded Raman generation up to third Stokes order of 1.24 μm is achieved with an intracavity ring resonator configuration realized by using wavelength division multiplexing (WDM) fused fiber couplers. It is shown experimentally that optical filters are needed to obtain high Raman gain and good signal quality. Output signal powers of up to 20 dBm (100 mW) with a 28 dB of Raman gain at 1.315 μm have been attained, where the pump power is 3.4 W. This cascaded Raman amplifier can provide an efficient and effective means of expanding the usable bandwidth of existing and future optical transmission systems.
A cascaded Raman fiber laser operating at 1.24 μm is also demonstrated. In this laser system, single-mode fiber with a moderate $GeO_2$ content (18 mole %) is pumped by a Yitterbium fiber laser at 1.060 μm. Efficient cascaded Raman generation of third Stokes order at 1.24 μm is obtained in a simple intracavity resonator configuration. Output powers of 740 mW with 3.3 W pump power is achieved. The Raman laser should provide an amplification of 1.3 μm signal through SRS in the standard telecommunication fiber.
A single longitudinal mode erbium doped fiber laser operating at 1.534 μm is demonstrated. Twisted mode technique with fiber optic components is utilized to achieve single mode operation in a simple linear cavity fiber laser. Using a self-heterodyne technique a detection-limited linewidth of 10 kHz is measured.
