Terahertz Parametric Oscillator THz-OPO crystal

Featured publications:
Energy scaling and extended tunability of terahertz wave parametric oscillator with MgO-doped near-stoichiometric LiNbO3 crystal
April 2017 Optics Express 25(8):8926
Projects: Terahertz wave source based on optical method Laser and THz sources.

Widely tunable, high-energy TPO based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been demonstrated in this paper. With the external tuning angle varied from 0.2° to 5°, the THz frequency was continuously tuned from 1.16 THz to 4.64 THz. The maximum THz wave output energy was 17.49 μJ at 1.88 THz under the pump energy of 165 mJ/pulse, corresponding to the THz wave conversion efficiency of 1.06 × 10−4 and the photon conversion efficiency of 1.59%. Moreover, the THz output energy of MgO:SLN TPO was enhanced compared with MgO:CLN TPO. The THz energy enhancement in the MgO:SLN TPO can be attributed to its larger Raman scattering cross section and smaller absorption coefficient qualitatively. The energy fluctuation at around 12.17 μJ was about 12.5% over 1 hour. It is expected that such high output energy THz wave systems with widely tunability can provide good advantages and enlarge its applicable scope.

A widely tunable, high-energy terahertz wave parametric oscillator based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been demonstrated with 1064 nm nanosecond pulsed laser pumping. The tunable range of 1.16 to 4.64 THz was achieved. The maximum THz wave output energy of 17.49 µJ was obtained at 1.88 THz under the pump energy of 165 mJ/pulse, corresponding to the THz wave conversion efficiency of 1.06 × 10?4 and the photon conversion efficiency of 1.59%, respectively. Moreover, under the same experimental conditions, the THz output energy of TPO with MgO:SLN crystal was about 2.75 times larger than that obtained from the MgO:CLN TPO at 1.60 THz. Based on the theoretical analysis, the THz energy enhancement mechanism in the MgO:SLN TPO was clarified to originate from its larger Raman scattering cross section and smaller absorption coefficient.

Experimental setup
The schematic diagram of MgO:SLN TPO is shown in Fig. 1(a). The pump source was a multimode Q-switched Nd:YAG laser with the repetition rate of 10 Hz and pulse width of 10 ns. The pump beam from the Nd:YAG laser was first collimated by the telescope lens T1, reducing the spot size in order to increase the power density. The attenuator M3 was used to control the incident pump intensity of the TPO under good beam quality for high conversion efficiency [13]. Then, an aperture with adjustable diameter was utilized to reduce the beam diameter to a suitable value, which was 4 mm in our experiment. The resonant cavity with the length of 220 mm was consisted of a pair of plane-parallel mirrors M1 and M2, which were both coated with high transmission (more than 98%) in the 1063 to 1064.7 nm wavelength range and high reflection in the range of 1067 to 1078 nm (R more than 70% at 1067 to 1070 nm, R more than 90% at 1070 to 1078 nm). The nonlinear gain medium was an 1 mol. % MgO-doped near-stoichiometric LiNbO3 (MgO:SLN) crystal with the composition Li:Nb = 49.6:50.4 (mol. %). Figure 2(b) shows the crystal configuration and the cutting angles. The isosceles trapezoid crystal was cut from a rectangle crystal whose dimensions were 40 mm × 20 mm × 10 mm in x, y and z directions, respectively. The angles between the base and the waist of the isosceles trapezoid is 65°, which allows the generated THz wave to be emitted nearly normal to the crystal surface without any coupler and guarantees that the pump wave and the Stokes wave are totally reflected at the crystal surface. The polarization directions of the pump wave and the Stokes wave were both along the z-axis of MgO:SLN crystal. Besides, the resonant cavity mirrors and MgO:SLN crystal were mounted on a rotating stage. Continuous frequency tuning can be obtained by rotating the stage to vary the phase-matching angle between the pump and oscillating Stokes waves inside the crystal. The THz output energy was measured by the calibrated Golay cell detector GC-1P. The calibration of Golay cell detector is 86.95 kV/W at the repetition rates of 10 Hz. In order to block the injection of intense pump and Stokes waves into the detector, transmittance-calibrated black polyethylene sheet (1mm thickness) was used as the THz low-pass filter. The wavelength of the Stokes wave was measured by an optical spectrum analyzer.