Coupling Kinetics Of Non - Cavity Cavity High Energy Solid - State Lasers

Recently, solid state laser(SSL) is becoming one of the hot topics in the laser field. Especially, the output power and the beam quality of the high energy solid-state laser (HESSL) has been improved rapidly. Continuous SSL output power with good beam quality has reached the 100-kW level. Unfortunately, because the electro-optic efficiencies of the HESSLs are still low, it generates large amount of waste heat, which not only requires higher demanding of the power and thermal management, but also induces thermal stress and thermal induced optical aberrations. All of these effects may restrict the scaling of laser power and beam quality. In this thesis, the coupled mechanism between the laser dynamics (LD), the cavity dynamics (CD) and the thermal-optic dynamics (TOD) in HESSL with unstable resonator (UR) is studied. The amplified spontaneous emission (ASE), the eigen-mode (EM) analysis of resonator with intra-cavity phase aberration (ICPA), intra-cavity adaptive optics (ICAO) phase correction, and transverse modal instability (TMI) are all investigated in theory, numerical simulation and experiment.The ASE in the HESSL with UR has been regarded as the most important reason which limits the power scaling beside the thermal effects. The distinct physical property caused by the coupling between ASE and laser oscillations (LO) is investigated theoretically and experimentally in this thesis. In theory, two types of ASE numerical simulation method, i.e. the Monte Carlo type ray-trace technique, and the spatial volume integral technique based on mirror imaging and diffused illumination are proposed. A coupled model involving the ASE, laser dynamics (LD) and cavity dynamics (CD) is developed. Then an experiment is carefully designed to verify the theoretical model and to investigate the competition between ASE and LO. It shows that ASE decreases the output laser power by affecting the threshold pumping power, while the slope efficiency is not changed by ASE. This observation provides us a simple way to estimate the decrease of the optical efficiency by ASE. Varies of ASE suppressing techniques are compared theoretically, and the criterion to ignore the ASE in TDL is given. A (3+1)-dimensional ray tracing simulation, which integrates the finite-difference in time domain and the Fox-Li type iteration, is carriedout to achieve the time evolution of ASE and LO in TDL. It is shown that therelaxation oscillations of LO during its establishment process is suppressed by ASEand the relaxation time is also shortened. On the other hand, the effective suppression and correction of ICPA in HESSLwith UR is important to beam quality scaling. However, we should understand themechanism of modal evolution in cavity with ICPA before the compensation of ICPA.In this paper, modal properties in optical unstable resonator with ICPA areinvestigated theoretically. An eigen-mode expansion (EME) method is proposed tosolve the laser EMs of optical resonator semi-analytically with ICPA. By using a setof appropriate basis functions, the eigen-equation of optical resonator is translatedinto a linear eigen-value problem. Then the EMs of the optical resonator are obtainedby solving the eigen-value problem numerically. The EME method is applied to theUR with ICPA. It is shown that the EMs of UR are non-orthogonal with each other,and then the necessarity of the ICAO in the UR with ICPA is proved theoretically.Beam qualities of the unstable resonators utilizing graded reflectivity mirror as theoutput coupler (GRM-UR) is fully investigated by comparing with traditional unstableresonator (TUR). It is shown that the GRM-URs possess poorer beam quality thanTURs in the presence of the ICPA, although the threshold gain of GRM-URdetermined by the geometric magnification is much lower than that of the TUR.Physically, it is because the lifetime of photons in the GRM-UR is longer than that inthe TUR so that photons pass through the phase screen much more times in GRM-UR.It is noticed that a stable-like oscillation, so called localized mode, is formed locallyin the presence of larger ICPA which may raise the risk of local damage oncomponents in optical cavity. According to the investigation on the modal properties in UR with ICPA, thearithmetic of the ICAO is studied in theory and experiment. Firstly, the limitation ofthe ICPA active phase correction method based on the geometric-optic approximationis analyzed. Secondly, a practical ICAO arithmetic, which integrates theidentical-optical-path detection, conjugate correction and relaxation iteration, isdeveloped. It is called "a round-trip detection method (RTD)". Both EME and Fox-Liiteration algorithms are employed to validate the feasibility of the RTD theoretically.The experiments of UR with or without gain medium are proposed later. Theexperimental results show that the detection system of RTD is simple and the arithmetic has good convergence and stability. It can not only compensate the ICPA effectively, but also improve the laser beam quality greatly. These results may suggest a new approach to the ICAO phase correction in HESSL with UR.The aforementioned conclusion on modal characteristics and compensation of phase distortion in UR are all based on stable-state theory. However, the time evolution of ICPA and laser mode may become significant in UR with strong thermo-optic coupling. And then the phenomenon of TMI is investigated theoretically in some HESSLs with UR. The TMI is caused by a thermo-optic-thermo feedback coupled with an asymmetrical laser oscillation inside some unstable cavities. The physical mechanisms of thermo-optic coupling in three laser schemes, the Nd:YAG Thin Disk Laser (Nd:YAG TDL), the Yb:YAG Thin Disk Laser (Yb:YAG TDL) and the Directed-Liquid-Cooled Thin Slab Laser (DLC-TSL), are different. Numerical simulations indicate that the TMI can appear in all these three lasers in the condition of high pumping power, which is similar with the high power fiber lasers. It is also shown that the TMI can appear within a special power range only in the Nd-or Yb-doped TDL due to certain saturation phenomenon, but exhibits a power threshold in the direct-liquid-cooled slab lasers. In other words, the TMI in Nd:YAG TDL and Yb:YAG TDL can be mitigated by avoiding those instability parameter ranges in the design of lasers, but in DLC-TSL we have to decrease the laser absorption of the coolant to avoid the instability. An experiment of Nd:YAG TDL with eight disks has been performed and the presence of TMI is verified qualitatively.In conclusion, the theoretical and experimental investigations of coupled dynamics in the HESSL with UR have been proposed. Some physical properties behind these processes are obtained and the available solutions are discussed. These results in this dissertation may provide some suggestions for the scaling on power and beam quality in the HESSL with UR.