Theoretical And Experimental Investigations Of Diode-Pumped Solid-State Laser At1.5μm

Since the diode pumped solid state lasers (DPSSLs) at1.5μm based on the4113/2â†'4115/2transition of erbium ions have the advantages of compact, good beam quality, eye-safety, and excellent transparency in both atmosphere and fused-silica fiber, they are of great interest for their applications in optical fiber communications, eye-safe laser range finding, laser radar, remote sensing, laser medicine, laser micromachining, environment detection and so on. In particular, when the DPSSLs at1.5μm are operated in single longitudinal mode, the lasers will possess more advantages, including good power stability, narrow linewidth, long coherent length and low noise, which benifit the generation of an entanglement source with high quality.To date, people have devoted vast efforts in the power scaling of the DPSSLs at1.5μm, and more than twenty kinds of erbium and yetterbium codoped laser materials have been used in the investigations. Recently, lots of progresses were made by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (FJIRSM,CAS) and Moscow State University. The Er3+,Yb3+:YAl3(BO3)4(Er,Yb:YAB) crystal, which can exhibit excellent laser performance, were fabricated and made the watt-level continuous-wave (cw) DPSSL at1.5μm become reality.For the sake of miniaturization and productization of the entanglement source at1.5μm, we carried out the investigations of high-power-level DPSSL at1.5μm by using the high quality Er,Yb:YAB crystal provided by FJIRSM,CAS. The first step of the research is to realize a high power quasi-cw1.5μm laser and a single transverse mode laser at1.5μm with high output power and good beam quality via reasonable cavity design and the optimizations on the pump source and the cooling scheme. The second step that based on the former one is to obtain a single frequency laser by means of intra-cavity etalon or unidirectional traveling wave cavity. The works completed are as follows:1. Based on the detailed analyses of the temperature distribution and the thermal effect in the laser crystal, a theoretical model taking account of temperature-dependent thermal population distribution, temperature-dependent emission cross-section, themal induced variation of cavity waist and thermal induced diffraction loss, was developed for cw Er,Yb:YAB lasers. From the model, the influences of many parameters, including the thermal loading, the radius of aperture in the temperature control oven, the pump duty cycle, the pump waist, and the thickness of the crystal, on the laser output were analyzed. Finally, the guidelines for optimizing the laser behaviors were proposed.2. Based on the simulation results, the Er,Yb:YAB laser was experimentally optimized under quasi-continuous pumping. As a product,2.6W quasi-cw laser output was obtained at the pump duty cycle of10%. Then we let the lasers operate in quasi-cw mode at different pump duty cycles that increased step by step; when the duty cycle was set as100%, a cw1.5μm laser was demonstrated.3. The optimization of the cw laser performance was concentrated on the thermal management of the laser crystal, which was realized by a cooling scheme based on a sapphire heat-spreader. To investigate the effect of the new scheme, the temperature distributions in the crystals cooled by different schemes were simulated by the finite-element-analysis, and the thermal focal lengthes were measured by the knife method. Then the comparative studies of the laser behaviours were made with a three-mirror folded cavity, which was designed to be thermal insensitive. From the experimental results, both the laser output and the beam quality were improved after the optimization.4. On the basis of the cw single transverse mode laser operation, single longitudinal mode was selected by intra-cavity etalon and unidirectional traveling wave cavity, respectively.430mW cw single frequency laser operation was obtained with good stability and excellent noise property. The measured intensity noise and the phase noise achieved the shot noise limit when the analysis frequency was4MHz and5MHz, respectively. The noise level reached the requirement of the generation of a high quality continuous variable quantum entanglement source.The creative works are as follows:1.A model suitable to the diode-pumped Er,Yb:YAB laser was established. The differences between the Er,Yb:YAB crystal and the Er,Yb co-doped phosphate glass, the effect of crystal temperature on the thermal population distribution, emission cross-section, and many other parameters, the variation of the cavity waist and thermal induced diffraction loss, were taken into account. The guidelines for optimizing the laser performance were provided.2. A sapphire-based end-cooling configuration was designed. Both the thermal effect and the temperature of the crystal were strongly reduced. As a consequence, the single-transverse-mode laser output was scaled up to680mW.3. A cw single frequency laser at1.5μm with430mW output was obtained. The laser possesses both compact structure and low noise, which benefit the miniaturization and productization of the entanglement source at1.5μm.