Terahertz (THz) sources, ranging from 0.1 THz to 10 THz, between microwaves and infrared waves, have important applications in spectral detection, medical imaging, communication, etc. Difference frequency generation (DFG) is an effective method for generating terahertz with the characteristics of low cost, simple structure, widely tunable range, no threshold, and room-temperature operation. This paper reviews various optical terahertz sources of difference frequency generation based on nonlinear crystals, including DFG with inorganic crystals, DFG with organic crystals, DFG with quasi-phase-matching (QPM) crystals, DFG in waveguides, cavity-enhanced DFG, and cascaded DFG. Their recent advances, as well as their advantages and disadvantages, are fully present and discussed. This review is expected to provide a comprehensive reference for researchers in this field and a quick understanding of optical THz sources of difference frequency generation with nonlinear crystals.
A flexible method of generating stable dual-wavelength laser pulses with tunable power ratio and pulse interval is proposed, through integrating a coaxially end-pumped laser with compound gain media and an intracavity pumped optical parametric oscillator (IOPO). A theoretical model was built by a set of time-domain coupling wave equations containing both the generation of two fundamental waves from a shared pump source and the conversion to signal waves through the parametric process. Simulations showed that by simply varying the pump focal position or pump wavelength, the gains in two laser crystals could be changed, leading to simultaneous change in average power ratio and time interval between two wavelengths. Experimental verifications were performed with combined laser crystals (Nd:YAG and a-cut Nd:YLF) and a nonlinear crystal (KTA), which enabled dual-wavelength signal output in the 1.5-1.6 μm eye-safe region and demonstrated coincident conclusions with theoretical results. As there was no gain competition between two fundamental waves, stable signal output was obtained. Moreover, various wavelength pairs in any wavelength ranges are possible by using different laser crystals and nonlinear crystals. It is believed that this is a promising method for generating simultaneous dual-wavelength laser pulses for applications in lidar, remote sensing, nonlinear frequency conversion, etc.
A high-power dual-wavelength laser based on coaxial end-pumping scheme with composite Nd:YAG and Nd:YAP crystals was demonstrated. A theoretical model was built to analyze the thermal lens and accurately calculate the thermal focal length of laser gain medium pumped by a divergent beam. The experiment was carried out with a plane-convex cavity and a V-shape cavity for high-power output. The Q-switched laser produced 9.1 W pulsed dual-wavelength output (4.7 W at 1064 nm and 4.4 W at 1079 nm) at 25-kHz pulse repetition rate with synchronized pulse duration of 35 ns, corresponding to the optical-to-optical efficiency of 30.3%. The continuous-wave (CW) output power reached 15.5 W at laser diode (LD) pump power of 40 W, corresponding to a slope efficiency of 40.9% and an optical-to-optical conversion efficiency of 38.9%.The powers at two wavelengths were almost equivalent and the fluctuations were both less than 2.3%. To our best knowledge, it was the highest output power for stable CW dual-wavelength lasers based on diode-end-pumped configuration.
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