Maorong Wang

High-Repetition-Rate Terahertz Generation in QPM GaAs With a Compact Efficient 2-

An efficient tunable high-repetition-rate KTiOPO4 (KTP) optical parametric oscillator (OPO) was deployed as a compact terahertz (THz) source for the first time. The KTP OPO was intracavity pumped by an acousto-optical Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy. The OPO tuning range extended continuously from 2.068 to 2.191 μm with a maximum output power of 3.29 W at 24 kHz, corresponding to an optical-optical conversion efficiency of 20.69%, believed to be among the highest reported. A periodically inverted quasi-phase-matched GaAs crystal was used to generate the THz wave by the difference frequency generation (DFG) method based on a dual-wavelength laser. The maximum output THz power was 0.6 μW at 1.244 THz, and the DFG energy conversion efficiency was 4.32 × 10-7. This provides a potential practical palm-top tunable THz sources for portable applications.

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A compact efficient high-repetition-rate doubly-resonant dual-wavelength KTP optical parametric oscillator (OPO), with output power up to 3.65 W and tuning ranges of 2.088-2.133 μm/2.171-2.122 μm for signal/idler waves, was deployed for terahertz (THz) generation in a GaSe crystal. Based on difference frequency generation (DFG), the THz wave was continuously tunable from 730.9 μm (0.41 THz) to 80.8 μm (3.71 THz), believed to be the first report of a compact high-repetition-rate widely-tunable THz source. The maximum THz average power reached 1.2 μW at 1.54 THz and the corresponding DFG efficiency was 7.8 × 10-7, entirely suitable for portable applications. The utility of the THz source was also demonstrated through spectroscopy and imaging experiments.

KTP OPO

A simultaneous dual-wavelength intracavity pumped non-critical eye-safe optical parametric oscillator (OPO) is realized using a Nd:GYSGG laser crystal and a KTP nonlinear crystal. A folded cavity is used for thermal stability and mode matching, which greatly improves the output characteristics versus a linear cavity. The maximum output power of the 1562.1 nm/1567.4 nm dual-wavelength eye-safe laser is 750 mW at 10 kHz, corresponding to the optical-to-optical conversion efficiency, single-pulse-energy and peak power of 5.8%, 75 μJ and 22.7 kW. Such a dual-wavelength OPO provides a good laser source for remote sensing for CO and CO2 gases or difference frequency generation for terahertz wave at the important 0.65 THz band.

Widely-tunable high-repetition-rate terahertz generation in GaSe with a compact dual-wavelength KTP OPO around 2 μm

Abstract. We demonstrated a data-processing method based on multiple beam interference and Fresnel equations that simultaneously gave the refraction index and absorption coefficient from the raw data of terahertz (THz) time-domain spectroscopy (TDS), which laid the foundation for obtaining the dielectric parameters. This method was independent of phase processing, and complete material information was reserved without having to cut the time-domain signal. The optical coefficients including refractive indices and absorption coefficients of white polyethylene and quartz samples at different thicknesses were obtained. The applicability and accuracy of this method were discussed and verified by comparison with the traditional data-processing method of THz TDS.

Simultaneous dual-wavelength eye-safe KTP OPO intracavity pumped by a Nd:GYSGG laser

Broadband terahertz (THz) atmospheric transmission characteristics from 0 to 8 THz are theoretically simulated based on a standard Van Vleck–Weisskopf line shape, considering 1696 water absorption lines and 298 oxygen absorption lines. The influences of humidity, temperature, and pressure on the THz atmospheric absorption are analyzed and experimentally verified with a Fourier transform infrared spectrometer (FTIR) system, showing good consistency. The investigation and evaluation on high-frequency atmospheric windows are good supplements to existing data in the low-frequency range and lay the foundation for aircraft-based high-altitude applications of THz communication and radar.

Optical coefficients extraction from terahertz time-domain transmission spectra based on multibeam interference principle

We investigated the optical properties of zinc germanium phosphide (ZnGeP2 or ZGP) crystals in a wide terahertz (THz) range from 0.2 THz to 6 THz, and made comparisons between crystals grown by both horizontal gradient freezing (HGF) and vertical gradient freezing (VGF) techniques. THz time-domain spectroscopy (TDS) and Fourier transform infrared spectroscopy (FTIR) systems were used to measure and analyze the transmittance, refractive indices and absorption coefficients. It was found that the HGF grown crystals have different birefringence and absorption in the THz range compared with the VGF grown crystals. The anisotropic absorption in the THz range was observed and the polar phonon modes at 3.6 THz and 4.26 THz were also discussed. The dispersion and absorption data of ZGP given in this report enabled us to know it better in the THz range and optimize its THz applications.

Theoretical and experimental study on broadband terahertz atmospheric transmission characteristics

A novel coaxial diode-end-pumping configuration with combined two laser crystals was proposed for simultaneous, compact, and flexible dual-wavelength laser generation. Theoretical simulations showed that by balancing the gain in both laser crystals by varying the pump focusing depth or pump wavelength, the power ratio for each wavelength could be tuned for either continuous wave or Q-switched operation, and the time interval between two pulses at different wavelengths in Q-switched mode was also tunable. Experimental verifications were performed, demonstrating coincident conclusions. As there was no gain competition between two wavelengths, the output characteristics were much more stable than dual-wavelength generation in a single-laser crystal. It is believed that this is a feasible and promising method for generating dual-wavelength laser for applications of spectroscopy, precision measurement, nonlinear optical frequency conversion to terahertz wave, and so on.

Linear optical properties of ZnGeP 2 in the terahertz range

A compact optical terahertz (THz) source was demonstrated based on an efficient high-repetition-rate doubly resonant optical parametric oscillator (OPO) around 2 μm with two type-II phase-matched KTP crystals in the walk-off compensated configuration. The KTP OPO was intracavity pumped by an acousto-optical (AO) Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy. The tuning range extended continuously from 2.068 μm to 2.191 μm with a maximum output power of 3.29 W at 24 kHz, corresponding to an optical-optical conversion efficiency (from 808 nm to 2 μm) of 20.69%. The stable pulsed dual-wavelength operation provided an ideal pump source for generating terahertz wave of micro-watt level by the difference frequency generation (DFG) method. A 7.84-mm-long periodically inverted quasi-phase-matched (QPM) GaAs crystal with 6 periods was used to generate a terahertz wave, the maximum voltage of 180 mV at 1.244 THz was acquired by a 4.2-K Si bolometer, corresponding to average output power of 0.6 μW and DFG conversion efficiency of 4.32×10-7. The acceptance bandwidth was found to be larger than 0.35 THz (FWHM). As to the 15-mm-long GaSe crystal used in the type-II collinear DFG, a tunable THz source ranging from 0.503 THz to 3.63 THz with the maximum output voltage of 268 mV at 1.65 THz had been achieved, and the corresponding average output power and DFG conversion efficiency were 0.9 μW and 5.86×10-7 respectively. This provides a potential practical palm-top tunable THz sources for portable applications.

Compact and Flexible Dual-Wavelength Laser Generation in Coaxial Diode-End-Pumped Configuration

We demonstrated a compact, widely tunable, and monochromatic high-repetition-rate terahertz (THz) source based on difference frequency generation in a GaSe crystal. The dual-wavelength pump laser was a doubly resonant near-degenerate type-II periodically poled LiNbO3 (PPLN) optical parametric oscillator (OPO), intracavity pumped by a 1.064-μm Q-switched Nd:YVO4 laser operating at 25 kHz. The PPLN OPO covered the wavelength range of 2072-2186 nm through temperature tuning with a 55-mm-long nonlinear crystal, and the maximum output power was 3.27 W. The generated THz wave was continuously tunable from 0.24 (1250 μm) to 3.78 THz (79 μm) with the maximum average output power of 1.8 μW at 1.57 THz. Such a compact and portable THz source has great potential for spectroscopy and imaging applications.