Investigation On High Power High Brightness Solid-state Lasers And Their Harmonic Generation

This dissertation is composed of two parts. As a Ph. D candidate of Zhejiang University and Technische Universitat Berlin (TU Berlin), the author had studied in Institut für Optik und Atomare Physik, TU Berlin for two years for some research work. The first five chapters are based on the work finished in TU Berlin. The topic is high power high brightness solid state lasers and their harmonic generation. The 6th chapter is based on the work finished in Zhejiang University, the topic of which is the theoretical study on external cavity feedback semiconductor lasers.A lamp-pumped high power high brightness Q-switched laser system is investigated theoretically and experimentally in this dissertation. The thermal induced birefringence is a key factor of the improvement of output power of the laser system in fundamental mode. Birefringence compensation is mandatory for high power lasers with good beam quality.The principle of birefringence compensation using two Nd: YAG rods are investigated in Chapter 2. Considering the thickness of the quartz polarization rotator, a revised model is presented for the condition of birefringence compensation. The depolarization ratio of less than 2.5% is achieved experimentally.By means of matrix optics, a cavity containing a two-rod system is simplified as a thin lens, which allows one to analyze the properties of the system such as stability of the resonator and beam radius in the cavity. Inserting a positive lens into the cavity to enlarge the fundamental mode volume, 61W linearly polarized output is obtained in a lamp cw-pumped Nd:YAG laser. It is shown that the birefringence compensation is essential for a dynamic stable resonator with lager fundamental mode volume.The spherical aberration of the thermal lens of the active media is obvious when the solid state lasers are strongly pumped. It can change the mode profile, the single-pass loss, the output power and so on, especially in a large-volume TEM₀₀-mode resonator. The self-consistent mode is calculated using Fox-Li diffraction iterative algorithm. There are side lobes with the fundamental mode when the spherical aberration is introduced, hence the beam quality is decreased. The calculation results agree well with the experimental results. The beam filling factor plays an important role in the design of a laser system. The optimized value of the beam filling factor should be decided by the experimental results according to the spherical aberration effect.Second harmonic generation (SHG) and fourth harmonic generation (FHG) are investigated with KTP and BBO crystals respectively. Due to the high repetition rate and long pulse width, the grey tracking effect is obvious in the experiment of SHG. By enlarge the beam size of the green light on the output surface of the KTP crystal, the grey tracking effect is successfully eliminated. 18W green laser with 10kHz repetition rate is obtained with converge efficiency of 44%. Ultraviolet light generation by FHG with BBO crystal is investigated. 1.9W ultraviolet is achieved experimentally.The influence of the external cavity feedback on the output beam quality, thereshold curruent and output power of a semiconductor laser is discussed in the 6th chapter. In according the theory of self-consistency, the wave propagation of the output laser of external cavity BAL is calculated. Through the introduction of effective reflectivity, the influence of external optical feedback on the oscillating characteristics of a semiconductor laser is discussed such as threshold gain, laser frequency and output power, according to the self-consistency condition and the photon and carrier density ratio equation.