Research On High Pulse Repetition Frequency Optically Pumped Terahertz Laser And Stability Control

THz radiation has many unique characteristics which has great value and broad application prospects in THz imaging, laser radar, biomedical, and other basic research areas. THz technolgy is the leading edge in the field of scientific study. It required THz laser pulse with high repetition frequency and stable power output in the applications of high-speed THz imaging, THz radar, communications and other areas. The optically pumped THz laser pulse is one of the important means currently to obtain THz laser pulse. At present, TEA CO2laser is mainly used as the pumping source for optically pumped THz laser system which can produce THz laser pulse with higher peak power. However, the repetition frequency is generally less than1kHz. In order to obtain THz laser with high stability, the pump laser frequency should be offset and stabilized at the resonance absorption frequency of the laser medium. The THz laser stability is directly influnced by the pumping laser frequency stability. An offset frequency to the line center of the pump laser is requried to consist with resonance absorption of the laser medium. And the pump laser frequency stability directly affects the stability of the THz laser. How to obtain pulsed THz laser with high repetition rate and high stability is the focus and also the purpose of this paper.The main work of this paper is the research on optically pumped THz laser with high pusle repetation frequency and stabilization, which includs theoretical analysis of optically pumped THz laser kinetics, high-repetition-rate pulsed pump source, high repetition rate pulsed THz laser system and stability control system and THz laser wavelength measurements using rotating F-P etalon. The main contents are as follows:(1) Using the three-level system rate equation theory to analyze the kinetics of pulsed optically pumped THz laser. In theory, THz laser wavelength and polarization measurements were discussed by using rotary F-P mirror.(2) Four kinds of high-repetition-rate optical pump THz laser pump sources were researched, which includs dual-channel RF-excited Z folded waveguide CO2laser with line selected, electro-optical Q-switched Z folding dual-channel RF waveguide CO2laser, RF-excited and DC discharged mechanical chopper Q CO2lasers. The maximum pulse repetition is20kHz. The laser wavelength can be selected in9~11μm range by adjusting the angle of the grating. In this paper, the mechnical Q-switched CO2laser DC-discharged is used as THz laser pumping source which meets the requriements of high repetition rate (20kHz), high peak power demands.(3) To obtain the desired pumping source, active frequency stabilization using pulse build-up time method for stabilizing the pump laser source frequency is proposed. On the basis of active frequency stabilization, the offset frequency locking technique is used for the pump laser frequency offset and frequency stabilization. In experiments, the active frequency control and offset frequency locking were applied on the Z-fold dual channel RF-excited CO2laser and two pulsed laser respectively. The long term frequency stability is up to10-8and the offset frequency fluctuation was less than±2MHz when the frequency control system is in closed loop operation. It verifies the feasibility of the method.(4) The THz laser oscillator was designed in this paper, which was pumped by the high-repetition-rate mechanical Q-switched CO2laser. By adjusting the grating angle, CO2laser lines can be selected for pumping CH3OH and CH2F2gases. In the experiment, a total of7lines THz laser were obatained. The waveforms of pulses THz laser with pulse repetation rate20kHz were detected by pyroelectric detector respectively. The THz laser power was measured, and the maximum pulse peak power is about3.5W. The wavelength and the polarization state of four THz laser lines obtained by optically pumped CH3OH, CH2OH were measured by rotating F-P mirror method. The measurement accuracy is0.1~0.5μm. The experimental results are greatly consistent with the theoretical caculation.(5) The extreme value method was used to stabilize THz laser power by adjusting the pump laser resonator cavity length. In closed loop operation,158.5μm THz laser power is stabilized at10mW with±0.5mW fluctuation in50minutes. The power stability is improved to be0.09compared with operation in free mode which stability is0.24.The power drift speed reduced about10times. The THz laser transmittances for different lenses are measured in the experiments. One-dimensional transmission imaging for objects wrapped was conducted by118.8μm laser. The result shows that different widths objects and different materials can be distinguished.