Multicore Fiber Laser And Solid-State Laser At Novel Wavelengths

The research in this dissertation includes two parts. One is the theoretical research on multicore fiber laser, the other is the experimental research on Nd:GGG crystal laser operating at novel wavelengths.As the application of the high power fiber laser extends from optical communication to mechanical processing, material processing, laser medicine, thermonuclear fusion, aviation, military and other fields, high power fiber laser with hundreds or even thousands of watt output power has become one the hottest topics in the present optoelectronic technology area.At the same time, the output power of traditional single core fiber laser is limited by lots of factors, such as nonlinear effect, amplified spontaneous emission, and thermal damage threshold, and so on. Therefore scientists have applied methods like double-cladding pump technique, photonics crystal fiber, coherent beam combination of fiber lasers to scale the output power of fiber laser. And muticore fiber, which can guarantee high power laser output, stable phase-locking and good beam beam quality simultaneously, is lately raised for coherent beam comibination and is the best method to accomplish high power fiber laser.It is very important to develop new wavelengths to meet various application requirements. Compared to other Nd-doped crystal, Nd:GGG crystal has good lasing properties, as well as physical and chemical properties. It has been widely used for high power heat capacity lasers and LD pumped lasers. So far the reported Nd:GGG crystal lasers generally operate at 1062 run,1331 nm and 938 nm.Up to now 1110 nm laser has been obtained from Ytterbium-doped fiber lasers, and applied for communication, mechanical working and other fields. As a light source for second harmonic generation,1110 nm can be frequency-doubled to generate yellow-green laser at 555 nm, which is most sensitive for human eyes and has important application for medicine, biological test and so on.There are two general research topics in this thesis:one is theoretical research on supermode propagation properties of multicore fiber, supermode selection and muticore fiber laser; the other is research on lasing performance of Nd:GGG crystal, experimentally verifying and studying its operation at around 1110 nm.The content studied in this paper mainly includes the following several respects: 1. The coupled-mode equation in multicore fiber is derived, and the supermode characteristics in three-core, four-core, six-core and seven-core fibers are calculated and analyzed, to obtain the near-field and far-field intensity distributions of each supermode. The M2 factor and power-in-bucket(PIB) are calulated to weight their beam quality during diffraction and propagation. The in-phase supermode of any mutlicore fiber always has the maximum propagation constant and best beam quality, and it has a Gaussian-like far-field distribution. The power-in-bucket(PIB) has better application for non-continuous mode fields. The analytical coupled-mode theory can be used for any multicore fiber with weakly-coupling, and is more intuitive than numerical methods.2. Take three-core, four-core, six-core and seven-core fibers as examples to discuss the coupling efficiency between supermodes of ring-type and concentric-type multicore fiber and fundamental mode of single-mode (SM) fiber. The coupling efficiencys are analyzed and optimized with core radius of SM fiber and the gap distance between the two fibers as variables. The theoretical results show their couplings with fundamental mode of SM fiber are different, which can be attributed to their different supermode profiles. The study provides good theoretical foundation for the coupling and connection between the two types of fibers.3. A new mode selection method by using single-mode fiber is proposed based on the analysis of coupling efficiencys of three-core, four-core, six-core and seven-core fibers. A parameter of coupling efficiency difference is put forward to measure the in-phase supermode selection efficiency in different mode selection schemes. The coupling efficiency difference is optimized and then compared to that in traditional Talbot cavity. The simulation results show the SM fiber mode selection scheme has superiority and flexiblility for in-phase supermode selection for both ring-type and concentric-type multicore fibers, and the scheme has great potential for pratical applications.4. Take ring-type six-core fiber and concentric-type seven-core fiber as examples, rate equations are established for multicore fiber laser with SM fiber mode selection scheme, to obtain the signal power properties. The SM fiber mode selection provides boundary condition for solving rate equations. So the signal power is affected by the coupling efficiency. Simulation results reveal the output power is propotional to the coupling efficiency, and the ratio of in-phase supermode output to total output is almost propotional to the coupling efficiency difference. It is proved to be reasonable using coupling efficiency difference to weight in-phase supermode selection. Simulation results show that single in-phase supermode output with high optical conversion efficiency can be achieved after optimization in six-core and seven-core fiber lasers based on SM fiber mode slection scheme. This study provides us theoretical foundation for high power, high brightness multicore fiber laser.5. Investigate the lasing behavior of Nd:GGG crystal and obtain 1110nm laser. In experiment, dual wavelength operation at 1110nm and 1105nm is obtained by diode-pumped Nd:GGG crystal laser, with competition existing between them. 1110 nm can be selected successfully by inserting a third plane mirror, which has high transmission at 1110nm,1105nm and 808nm. Adjusting its orientation slightly will change the intracavity losses of different wavelengths. Consequently 1105nm is suppressed, and single wavelength operation at 1110nm is achieved. The innovations in the thesis include:1. For the first time, systematic analysis and comparison has been done for three-core, six-core ring-type and four-core, seven-core concentric-type fibers based on coupled-mode theory and Kirchhoff diffraction theory. The Beam quality of supermodes is calculated with M2 factor and power-in-bucket (PIB). In-phase supermode is proved to have much better beam quality than other supermodes. The theoretical model established is suitable for any multicore fiber if weakly-coupling is satisfied.2. For the first time, the coupling efficiency between SM fiber and multicore fiber is systematically analyzed and optimized. The coupling efficiency and coupling efficiency difference are optimized by varing SM fiber core radius and the gap distance between two fibers. Different coupling characteristics of two types of multicore fiber with SM fiber have been demonstrated. Simulation results provide detailed and reliable basis for coupling between SM fibers and multicore fibers in practical operation.3. The mode selection scheme based on SM fiber is proposed for the first time. And the proposed scheme for two different types of multicore fiber is optimized and compared. In ring-type multicore fiber, the SM fiber can suppress all the high-order supermodes. In concentric-type multicore fiber, the SM fiber can suppress all the high-order supermodes except in-phase supermode and anti-phase supermode. The coupling efficiency difference is put forward to weight in-phase supermode selection. Compared to Talbot cavity, SM fiber can improve in-phase mode selection more than 100%, proving its effectiveness and superiority.4. For the first time, single mode output all-fiber multicore fiber laser based on SM mode selection is obtained theoretically. Based on rate equations, simulations on six-core ring-type and seven-core concentric-type fiber lasers have verified that, output power is propotional to coupling efficiency, and the ratio of in-phase supermode output to total output is almost propotional to coupling efficiency difference. Therefore coupling efficiency difference is proved to be quite reasonable to weight in-phase supermode selection. Single in-phase supermode output can be achieved after optimization. This study provides us theoretical foundation for high power, high brightness all-fiber multicore fiber laser.5. For the first time, continous wave operation at 1110 nm is obtained in diode-pumped Nd:GGG crystal laser in experiment. And continous wave dual wavelength operation at 1110 nm and 1105 nm are obtained for the first time. By inserting a third high transmission mirror to change the intracavity loss, single wavelength operation at 1110 nm is finally achieved.