The generation of tunable narrowband terahertz (THz) radiation has shown much interest in recent years. THz systems are used for rotational-vibrational spectroscopy, nondestructive inspection, security screening and others. Monochromatic THz emission has been generated by means of THz parametric oscillation, nonlinear difference frequency generation, and quantum cascade lasers. Intracavity difference frequency generation (DFG) in the nonlinear crystal gallium arsenide (GaAs) is known as an efficient way to generate a continuous wave THz radiation. A novel high power solid state resonator is presented with the use of volume Bragg grating (VBG) technology to create a dual channel system by spectral beam combination. The system consists of two separate Tm:YLF crystals and two VBGs for narrowband wavelength selection. At the end of the resonator both channels share common spherical mirrors, which provide feedback and focuses the beam for nonlinear purposes. This allows each channel to be independent in power and wavelength, eliminating gain competition and allowing individual wavelength tunability. The VBGs are recorded in photo-thermo-refractive glass, which has a high laser induced damage threshold and can withstand the high intracavity power present in the resonator. Tunability of the system has shown spectral spacing from 5 to 20 nm, 0.4 - 1.7 THz, and intracavity continuous wave power levels from 80 to 100 W. By placing the GaAs crystal near the waist, THz radiation can be extracted from the cavity.
Evan Hale, Aleksandr Ryasnyanskiy, George Venus, Ivan Divliansky, Konstantin L. Vodopyanov, and Leonid Glebov, "Wavelength tunable dual channel solid state laser for terahertz difference frequency generation," Proc. SPIE 10082, Solid State Lasers XXVI: Technology and Devices, 1008210 (Presented at SPIE LASE: January 31, 2017; Published: 17 February 2017); https://doi.org/10.1117/12.2252410.
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