Cavity Enhanced THz Generation in Nonlinear Crystals Pumped by Near-IR Fiber Lasers

A coherent optical THz (1.5 THz, 200 μm) source was developed based on pulsed, near IR, fiber lasers, and frequency mixing in nonlinear crystals. The generated THz frequency is determined by the difference frequency of two high peak power pulsed fiber lasers at 1550 nm and 1538 nm. When incident to the crystal, the near IR lasers induce a polarization at their beat frequency which generates the THz radiation. The pulsed fiber lasers are single transverse mode, have high pulse energy and peak powers of 0.38 mJ and 128 kW respectively. They are transform limited at a few ns in duration with very good beam quality of M≈1.2. The pulse seed was created by modulating a constant laser beam with an electro-optic modulator. An arbitrary waveform generator was used to pre-shape these pulses to compensate for pulse distortion caused by pump gain depletion in the subsequent fiber amplifiers. Preamplifiers were constructed using commercial erbium doped silica fiber. Special, highly doped, large core, phosphate fiber was developed in-house to further amplify the pulses, while avoiding nonlinear scattering processes such as stimulated Brillouin scattering and stimulated Raman scattering. THz generation was achieved in both ZnGeP2 and GaP which were chosen based on their low pump and THz absorption, as well as high nonlinear coefficient. Angle tuning was used to phase match all three optical frequencies in ZnGeP2 thanks to its birefringence. Layers of GaP ~500 μm thick were pressed together alternately rotated 180 around the normal to quasi-phase match the pump and THz frequencies.