Research On Eye-safe, Mid-infrared Laser And Terahertz Radiation Based On Nonlinear Optical Frequency Conversion

The wavelength regions of 1.5-1.6μm eye-safe band, 3-5μm and 8-12μm mid-infrared bands are three important atmospheric windows in the optical spectrum. Moreover, the terahertz (THz) radiation has been widely exploited in basic and application research areas in recent years with the increase of knowledge for people in this area. Practical radiation sources in these bands are the basic elements for applications. Compared with other methods, nonlinear optical frequency conversion technologies such as optical parametric oscillator (OPO) and difference frequency conversion (DFG) can provide us with all-solid-state, compact, room-temperature operating, low-threshold, high-efficiency, high-power (high-energy), tunable and coherent sources, by converting near-infrared lasers to the required wavelength regions mentioned above. In this dissertation, the eye-safe and 3-5μm mid-infrared KTP/KTA OPOs, as well as the 8-12μm and THz radiation sources based on DFG in GaSe and ZGP crystals, are theoretically and experimentally studied. Some new phenomena observed in the experiments are also further explored.The main contents and key innovative points are as follows:1. The low-repetition-rate, high-pulse-energy eye-safe KTP/KTA OPOs are researched. The optimum operating conditions are derived based on the calculations and comparisons on the phase-matching relations, effective nonlinear coefficients and walk-off angles of KTP OPO and KTA OPO pumped by 1064-nm lasers. In the experiment, high-efficiency, high-pulse-energy eye-safe lasers are achieved from noncritically phase-matched KTP and KTA OPOs pumped by electro-optically Q-switched pulsed Nd:YAG lasers in a compound cavity configuration. The maximum pulse energy of KTP OPO at 1572nm is 66.5mJ, corresponding to the optical-to-optical conversion efficiency of 12.1% and electrical-to-optical conversion efficiency of 4.47%; the maximum pulse energy of KTA OPO at 1536nm is up to 74.9mJ, corresponding to the optical-to-optical conversion efficiency of 12.14% and electrical-to-optical conversion efficiency of 4.74%. The conversion efficiencies are the higher than any other reported results of similar configuration.2. The high-pulse-energy mid-infrared KTA OPOs at 3.47μm intracavity pumped by an electro-optical Q-switched Nd:YAG laser are achieved. A folded cavity is adopted in order to fully extract the generated idler wave of the OPO in two directions. The total output energy reaches 31mJ, corresponding to the optical-to-optical conversion efficiency of 4.76% and electrical-to-optical conversion efficiency of 1.82%. The photon conversion efficiency from the fundamental to idler wave is 87%. This is the highest output energy and conversion efficiency of mid-infrared KTA OPOs that have been reported.3. Eye-safe KTA OPOs intracavity pumped by end-pumped acousto-optically (A-O) Q-switched high-repetition-rate lasers are theoretically and experimentally researched. Based on the theoretical analysis on the cavity parameters of intracavity pumped OPOs (IOPOs), we achieve a 30-kHz, 1.03-W KTA OPO at 1536nm in the experiment, pumped by an A-O Q-switched Nd:YVO₄ laser. The optical-to-optical conversion efficiency is 12.26% and the threshold is as low as 0.75W. The dynamic processes of KTA IOPOs using Nd:YLF and Nd:YVO₄ as the pump lasers are compared, which conclude that laser materials with higher energy storage ability are more suitable in low-threshold, high-peak-power IOPOs.4. A high-repetition-rate 3.47-μm mid-infrared KTA IOPO is achieved, pumped by an LD-end-pumped A-O Q-switched Nd:YVO₄ laser. With a linear cavity in which the signal wave is resonant and the idler is coupled out in one direction, the maximum mid-infrared output power is 435mW when the LD pump power is 9.9W. The optical-to-optical conversion efficiency is 4.4% and the photon conversion efficiency from fundamental to idler wave reaches 64%.5. Stimulated Raman scattering (SRS) in KTA is observed in the experiment of eye-safe IOPO pumped by a Nd:YAG laser and further experiments are performed on this phenomenon. We first report the Stokes wave at 1178nm in KTA pumped by a Nd:YAG laser. Based on cascaded SRS and self-frequency-doubling in KTA, we achieve a multi-wavelength laser with wavelengths from visible to the near-infrared, including spectra of 573nm, 1064nm, 1091nm, 1120nm, 1146nm and 1178nm. The output power for each line is from tens to hundreds of milliwatts. The yellow laser at 573nm reaches 470mW with an optimized scheme.6. The difference frequency generation (DFG) of 8-12μm mid-infrared laser in GaSe and ZGP crystals is theoretically and experimentally researched. Firstly we calculated the phase-matching relations, effective nonlinear coefficients and walk-off angles of the DFG process based on a dual-wavelength KTP OPO working near the degenerate point pumped by a 1064nm laser. In the experiment, the tuning range of 8.28-18.365μm is achieved using a GaSe crystal. The maximum generated mid-infrared pulse energy is 31.7μJ at 8.76μm, corresponding to the peak power of 7kW and the energy conversion efficiency of about 0.9%. With a ZGP crystal for DFG, the maximum mid-infrared pulse energy is 10μJ at 8.76μm, the peak power of which is 2.2kW and the energy conversion efficiency is about 0.45%.7. THz difference frequency generation (DFG) in GaSe based on a dual-wavelength KTP OPO is researched. The DFG process is theoretically analyzed considering the effects of absorption, crystal length and pump intensity, etc. A walk-off compensated dual-wavelength KTP OPO is firstly employed as the pump source for DFG. The generated THz wave is tunable from 0.186 to 3.7THz, with the maximum peak power of 11W at 1.68THz. The stability and threshold of the KTP OPO, as well as the generated THz power is obviously improved compared with that using a common KTP OPO.8. Theoretical analysis is performed on the principles and process of cascaded DFG for THz wave in a ZnTe crystal. We obtain the optimum pump condition and crystal length through solving the coupling wave equations of cascaded DFG, and analyze the influence of absorption, wave-number mismatch and pump intensity on the cascade DFG process. Based on our calculations in which the pump intensity of 50MW/mm2 and the pump frequencies of 369THz and 368THz are taken for example, the generated THz intensity at 1THz reaches 0.815MW/mm2. The output intensity is increased by 7.7 times, corresponding to the energy conversion efficiency of 0.815%. The photon conversion efficiency reaches 600%, which goes far beyond the Manley-Rowe limit.