High-Power Hybrid-Gain Fiber Lasers

All-fiberized fiber lasers possess the advantages of compacted configuration, high efficiency and convenient heat management, which have found plenty of applications in industry, biology, lidar, and military counterwork. With the rapid development of various applications’ demands, fiber lasers now move forward to the direction of higher power, higher efficiency and wider spectral emission range. There are two categories of optical gain for fiber laser. One comes from the stimulated emission between two real states of ions, such as Yb-doped fiber lasers(YDFL). The other gain results from the nonlinear effect, such as Raman fiber lasers(RFL), which can be regarded as the virtual gain in quantum-mechanical viewpoint. These two kinds of optical gains have both been demonstrated holding the potential to achieve high-power, wide-emission-spectrum lasers output. This thesis focuses on the study of the characters of Yb-ions gain, Raman gain and especially the combination of these two gains in the potential of power enhancement and wavelength transformation.Comparing with the lasers in common emission wavelength range of YDFL, the published study on the trails of long wavelength YDFL is relatively less. However, wavelength in long wavelength range can match well with the emission wavelength of high power RFL and the long wavelength YDFL can be the seed of hybrid gain system, thus this thesis firstly thoroughly studies the emission performance of long wavelength YDFL. The laser emission of long wavelength YDFL is restricted by amplified spontaneous emission(ASE) due to the small gain in Yb-doped fiber(YDF). In order to suppress ASE we theoretically analyze the influence of the core diameter of YDF, the length, small signal absorption coefficient, absorption and emission cross section, fiber loss, reflectivity of output coupler, and pump scheme on long wavelengh laser generation. In experiment, we also study the influence of reflectivity of output coupler, YDF length, pump source wavelength, and fiber loss on ASE suppression. Based on the theoretical and experimental study results, we design and optimize the 1173 nm and 1120 nm fiber oscillators with output power of 16 W and 322 W, respectively, which are both the highest power reported in corresponding wavelength by common double clad YDF. Moreover, we analyse the performance of amplification of long wavelength laser in YDF. The influences of seed power, fiber end reflection, and fiber parameters on laser amplification are carefully studied. Finally, we demonstrate a high power 1120 nm YDF amplifier with output power of 309 W, the slope efficiency is about 80%.Raman gain is as important as Yb-ions gain for the hybrid gain system, the study on Raman gain is composed by two topics: core pumped Raman oscillator and short-cavity random fiber laser based on Raman gain. On the topic of core pumped Raman oscillator, we theoretically derive the Raman threshold formula for oscillator, which can be applied to measure the Raman gain coefficient. The influence of passive fiber length, Raman gain coefficient, fiber loss, reflectivity of output coupler, and core diameter on the efficiency of Raman oscillator have been theoretically studied. The results show that optimized the parameters of fiber length and reflectivity of output couple can ensure the achievement of maximal efficiency. In the experiment, we demonstrate a high power 1173 nm Raman oscillator, the output power is 119 W corresponding to the optical-to-optical efficiency of 82%. The experimental results are also agreed with our numerical study.On the other topic of Raman-gain based short-cavity random fiber laser, much attention has been paid to theoretically analyze the relationship between laser threshold and fiber length. The numerical results point out that the threshold of short-cavity random fiber laser is sensitive to parasitic reflectivity, which can be used to judge the existence of parasitic reflectivity. We can also find that the shorter the cavity length, the higher the output power for this random fiber laser. We successfully realize hundred-watt random fiber laser in experiment. Power of 124 W with optical efficiency of 79% is achieved by open cavity in 1150 nm, and power of 112 W with optical efficiency of 84.8% is also demonstrated in half-open cavity, which are the highest power and efficiency results reported in random fiber laser. Due to the steady output in time domain of random fiber laser, it can be regarded as the seed of high power fiber amplifier system. We also demonstrate that the high power 1150 nm random fiber laser can be used to pump Ho-doped fiber; power of 23 W in 2050 nm is obtained.We carry out the numerical study about hybrid gain fiber laser in detail. Firstly, the definition of hybrid gain fiber amplifier is proposed and a rate equations set is built up, based on which the advantage of hybrid gain system in pump power extraction, wavelength conversion, and backward Raman scattering suppression are carefully discussed. The calculation of thermal distribution shows that hybrid gain fiber system would not introduce additional thermal burden. Another model taking into account of four waves mixing(FWM) effect is built up to evaluate the influence of FWM on the hybrid gain system. The calculation results indicate that the high-order Raman Stokes threshold would be decreased by FWM, but it is possible to restrict the power portion in an acceptable range by optimizing the system parameters for a kilo-watt long wavelength laser system. We also find FWM has less impact on the backward Raman Stokes wave generation, which guarantees the hybrid gain system can still suppress the backward scattering laser.Experimental study on emission range broadening by hybrid gain fiber amplifier in high power level has been conducted. An output power of 732 W at 1120 nm is achieved in YDF by hybrid gain scheme, which is consistent with our theoretical results. By further optimization, power of 1.52 k W at 1120 nm is demonstrated, which is the highest power in this wavelength as we know and the power can be further promoted. We try three-tone seed amplification for longer wavelength achievement. Finally, 1178 nm laser with record power of 536 W is demonstrated. A numerical model is used to describe the characteristic of this amplifier. The results reveal that kilo-watt 1178 nm laser can be realized by increasing the seed power, while the high-order Raman induced by FWM would be less than 5%.Investigation about the hybrid gain amplifier in multi-mode fiber has been carried out. The experimental and theoretical study shows that the output beam quality would be improve when fundmental mode dominates the seed laser. There is a difference between the simulation of Raman effect in single mode and multi-mode fiber because of the insufficient mode overlapping between pump and Stokes waves in multi-mode fiber. We propose a method that modified the Raman gain coefficient by pump and Stokes waves’ power ratio to make the single mode model suitable to the multi-mode case. The calculated results are agreed with the experiments.