Study On Single Frequency Fiber Raman Amplifier

Fiber Raman amplifiers (FRAs) are of great importance in optical communication, supercontinuum generation and special spectrum lasing benefiting from their essential characters such as broad gain spectrum, wavelength versatility and low amplified spontaneous emission noise. In many special application domains, such as coherent beam combination, spectral beam combination and nonlinear frequency conversion, the linewidth of laser sources are restricted, and single frequency amplifications are required. Single frequency fiber Raman amplifier (SF-FRA) shows great prospect in many applications, such as laser guide star, remote sensing, and high power density visible laser sources obtaining for medicine and laser display. The output characteristics of SF-FRA can be affected by factors such as power and linewidth of seed source, length of Raman gain fiber and pump scheme. Meanwhile, the maximal output power of SF-FRA is limited by stimulated Brilliouin scattering (SBS) effect. The output properties and SBS effect of SF-FRA are investigated theoretically and experimentally in this dissertation. The main contents are as follows:1. The output characteristics of SF-FRA are analyzed systematically. The impacts of factors, such as power and linewidth of seed laser, length of gain fiber and pump scheme, on the output properties are analyzed employing the theoretical model of the intensity equations combining stimulated Raman scattering (SRS) and SBS. The numerical simulation results indicate that: higher efficiency can be obtained accompanying with higher seed power, but the pump threshold will be lower. The pump threshold and maximal output power can be enhanced obviously when the linewidth of seed laser is broader than 1MHz. Higher efficiency can be achieved with longer gain fiber, but the maximal output power will be lower. For forward pumped SF-FRA, higher efficiency can be acquired,but the pump threshold will be low. With reference to the maximal output power, higher power can be obtained in forward pumped SF-FRA while the gain fiber is long, in addition, when the gain fiber is short, backward pump scheme should be employed to obtain higher maximal output power. The numerical simulation results can provide reliable theoretical laws for the optimal design of high power SF-FRA system.2. The impacts of strain distribution on SBS effect in SF-FRA are studied in detail, and optimal solution is proposed. The simulation results indicate that: the signal power and pump power are variant at different location, and the Brilliouin gain and Raman gain of SBS light are dissimilar, so the SBS effect in gain fiber can be affected by the parameters of strain distribution, such as location, stage number, intensity and non-uniform strain distribution. When the SBS power is separated to discrete frequency homogeneously, the best SBS suppression effect can be achieved. The maximal output power can be increased about 100% utilizing the optimum condition. The simulation results present an operable technical approach to improve the maximal output power of single channel SF-FRA.3. A 1080nm single frequency fiber oscillator is constructed as seed source, and a 1031nm master oscillator power amplifier (MOPA) is constructed as pump source, then SF-FRA system is demonstrated. The optical spectrum, frequency spectrum, power characteristics, optical to optical conversion efficiency and coherence property of amplified laser are investigated experimentally. Firstly, a ring cavity single frequency 1080nm fiber oscillator is demonstrated, single frequency oscillation can be realized by the adoption of optimal saturable absorber and polarization controller, the stability of seed oscillator is proved by the 100W level high power amplification of 1083nm single frequency oscillator which has the same structure with 1080nm single frequency oscillator. Afterwards, 791mW Raman amplified laser is obtained with the construction of 1031nm MOPA laser and SF-FRA system with different pump scheme and gain fiber. The analysis of optical spectrum and frequency spectrum indicate that no SBS effect can be observed under maximal output power. Meanwhile, the linewidth of output laser broadens slightly. The impacts of factors, such as power of seed laser, length of gain fiber and pump scheme, on the output power and efficiency are explicitly studied, and the experiment results are compatible with the simulation. At last, the experimental investigation on coherence properties of Raman amplified laser indicates that coherent beam combination of SF-FRAs to obtain high power output laser is feasible. 4. Experimental realization of dual longitudinal modes SF-FRA. Duallongitudinal modes SF-FRA shows great prospect in laser guide star, and there is no experimental report about the demonstration previously. By adjusting home-made single frequency fiber oscillator, dual longitudinal modes single frequency seed laser with 1.4GHz frequency interval and a power proportion of 3:1 is obtained. The linewidth of each longitudinal mode is about 10MHz. 1.07W dual longitudinal modes Raman amplified laser is obtained from forward pumped SF-FRA. The measurements of output laser indicate that frequency interval and power proportion of the two laser modes are preserved in the Raman amplification process, and the linewidth broadens slightly.