The Theoretical And Experimental Studies Of Crystalline Raman Amplifier And Anti-Stokes Laser

Stimulated Raman Scattering (SRS) is a third nonlinear effect. It is one of important approaches for expanding the present laser lines. SRS includes Stokes Raman scattering and anti-Stokes Raman scattering. The frequency of the Raman scattering light depends on the pumping laser frequency and the Raman frequency shift of the Raman medium. By changing the pumping laser wavelength and the Raman medium, the laser spectrum with SRS can extend from the ultraviolet to the near infrared. Compared with the gas and liquid Raman media, crystalline Raman media have the advantages of small physical size, high gain coefficient, excellent thermal conductivity, good mechanical properties, high molecule density, and so on. Because the Stokes Raman scattering does not need phase-matching and has high conversion efficiency, the solid-state Raman lasers based on crystal Raman media have been widely investigated in recent years.In many applications, e.g. lidar and range-meter, Raman laser radiations with high energies, high beam qualities, and high spectral purities are needed. In this situation, they are hard to be realized only by Raman lasers. Raman amplifiers become necessary alternatives. First, satisfactory Raman seed pulses are generated in advance by a Raman laser. And then, the Raman seed radiations are amplified by one-or multi-stage Raman amplifiers to generate high-energy Raman outputs. Compared with the traditional laser amplifiers, the realization of the Raman amplifiers are more difficult. First, the pumping radiation must have high intensity to ensure that the Raman seed is fully amplified. Second, the pumping laser pulses and the Raman seed pulses must pass through one Raman gain medium simultaneously, with good spatial and temporal overlaps. So far, Raman amplifiers using optical fibers as the Raman gain media have been widely investigated. Generally speaking, fiber Raman amplifiers are suitable for continuous-wave (CW) lasers and low peak-power pulsed lasers. However, very few researches with regard to the crystalline Raman amplifiers have been reported.To make full use of SRS to obtain more wavelengths, the anti-Stokes generation is necessary. In contrast to Stokes generation, it is a four-wave mixing (FWM) process of two pumping photons, one first Stokes photon, and one first anti-Stokes photon. So, to realize the effective anti-Stokes generation, the three laser fields need to satisfy phase-matching condition. Meanwhile, the anti-Stokes scattering is an up-conversion process with low conversion efficiency. The anti-Stokes generation is harder than the Stokes generation. So far, most of the researches with regard to the anti-Stokes generation are based on gas Raman media. Only a few reports related to the anti-Stokes Raman lasers use crystals as the Raman media.In this dissertation, we mainly investigated two aspects. First, we theoretically and experimentally studied crystalline pulsed Raman amplifier based on BaWO4crystal. Second, we theoretically and experimentally studied the anti-Stokes Raman lasers. The concrete research contents of this thesis are as follows.1. An a-cut BaW04pulsed Raman amplifier at1180nm was realized for the first time. The pumping source was a flash-lamp pumped passively Q-switched single-longitudinal-mode laser at1064nm. Four Raman laser seeds with the pulse energies of8.0mJ,2.0mJ,200μJ and40μJ were used to investigate the properties of the Raman amplifier. In the case of200mJ pumping laser energy, the highest Raman amplification ratio of418was obtained with the Raman seed energy of40μJ. The largest amplified Raman energy of71.5mJ was obtained with the Raman seed energy of8.0mJ. Serious depletion of the pumping laser pulse was detected.2. A theoretical model of the Raman amplifier was deduced by simplifying the radiative transfer equations of the extracavity pumped Raman laser. An analytic solution that depicted the output Raman intensity of the Raman amplifier was obtained with known incident pumping and Raman seed intensities. The theoretical output energy, the amplification ratio and the optical-optical conversion efficiency with regard to the pumping energy were analyzed. The theoretical results were in well accordance with the experimental ones.3. An extracavity pumped a-cut BaW04anti-Stokes Raman laser was demonstrated for the first time. By separating the optical axis of the Raman external cavity from the pumping coupled wave direction at34.1mrad, the phase-matching between the pumping, the first Stokes, and the first anti-Stokes laser beams was achieved. When the pumping energy was128mJ, the output energy of the anti-Stokes radiation at968nm obtained was2.2mJ and the corresponding optical-optical conversion efficiency was1.7%. Meanwhile, three orders of Stokes radiations were obtained with the total energy of42.5mJ.4. Under the plane-wave approximation, by using coupled wave equations, the relational equation between the pumping, the first Stokes, and the first anti-Stokes intensities of the extracavity pumped anti-Stokes Raman lasers were deduced for the first time. Together with this equation, the rate equations of extracavity Raman lasers are were used to simulate the properties of the anti-Stokes laser. The theoretical predictions agreed well with the measured data.5. An extracavity pumped a-cut SrWO4anti-Stokes Raman laser was studied for the first time. The optical axis of the Raman external cavity was deviated from the pumping coupled wave direction at31.3mrad. When the pumping energy was120mJ, the output energy of the anti-Stokes radiation at968nm obtained was0.74mJ and the corresponding optical-optical conversion efficiency was0.6%. Meanwhile, three orders of Stokes radiations were obtained with the total energy of23.9mJ.6. With an Nd:YAG laser medium and an a-cut BaWO4Raman medium, a LD-side-pumped actively Q-switched intracavity anti-Stokes Raman laser at968nm was demonstrated for the first time. A Raman coupled cavity was placed in the fundamental cavity. The optical axis of the Raman coupled cavity was separated from the optical axis of the fundamental oscillating direction at the phase-matching angle. When the pumping energy was170W and the repetition rate was7.5kHz, the obtained maximum anti-Stokes power was0.94mW with the shortest pulse width of3.9ns.The main innovations of this thesis are summarized as follows:1. For the first time, the single-longitunidal-mode1064nm laser pumped highly efficient BaWO4Raman amplifier at1180nm was demonstrated. In the case of200mJ pumping laser energy, the highest Raman amplification ratio of418was obtained with the Raman seed energy of40μJ. The largest amplified Raman energy of71.5mJ was obtained with the Raman seed energy of8.0mJ.2. The extracavity pumped BaWO4anti-Stokes Raman laser was realized for the first time. When the pumping energy was128mJ, the output energy of the anti-Stokes radiation at968nm obtained was2.2mJ and the corresponding optical-optical conversion efficiency was1.7%.3. For the first time, by using coupled wave equations, the relational equation between the pumping, the first Stokes, and the first anti-Stokes intensities of the extracavity pumped anti-Stokes Raman lasers were deduced under the plane-wave approximation.4. The rate equations of extracavity Raman laser were used to simulate the properties of the anti-Stokes lasers for the first time.5. The extracavity pumped anti-Stokes Raman laser at969nm was realized based on SrWO4crystal for the first time. Under the pumping energy of120mJ, the anti-Stokes energy obtained was0.74mJ and the corresponding optical-optical conversion efficiency was0.6%.6. A LD-side-pumped actively Q-switched Nd:YAG/BaWO4intracavity anti-Stokes Raman laser at968nm was demonstrated for the first time, a. When the pumping energy was170W and the repetition rate was7.5kHz, the obtained maximum anti-Stokes power was0.94mW with the shortest pulse width of3.9ns.