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The research of active solid-state electronic terahertz (THz) imaging systems have been devoted enormous efforts, which play an important role in nondestructive testing. In this paper, a scanning imaging system based on solid-state electronic modulated THz source at 330GHz is described. To investigate the penetrability of the terahertz waves in this band, we use four kinds of shelters in the experiment. A pyroelectric detector is connected to an oscilloscope to achieve the voltage data which processed by a self-developed imaging data collection and processing system. The imaging results are compared and analyzed to prove the potential of THz waves in the application of target recognition with shelters.

In this paper, we demonstrated a passive Q-switched 2μm dual-wavelength intra-cavity PPLN optical parameter oscillator (OPO) with the advantages of compactness, large tuning range, and narrow linewidth. By using c-cut Nd:YLF crystal as the laser gain medium, the output power of the fundamental laser at 1053nm can reach up to 0.916W, corresponding to the incident pump power of 9.79W. When the temperature of PPLN crystal was varied between 40°C and 160°C, the orthogonal polarized dual-wavelength lasers can be tuned from 2026.4nm to 2193.9nm. As to the a-cut situation, the output power at the fundamental wavelength of 1047nm is about 1.192W. In the same temperature range, two orthogonal polarized wavelengths can be tuned from 1991.8nm to 2206.8nm. Such type-Ⅱ OPOs have potential abilities to work for sensing, spectroscopy, imaging, nonlinear frequency conversion and so on.

Compact efficient high-repetition-rate tunable optical parametric oscillators (OPOs) in the 2- μ m region were investigated, based on nonlinear crystals of KTP and periodically polarized LiNbO3 (PPLN) intracavity pumped by a diode-end-pumped acousto-optical (AO) Q-switched Nd:YVO4 laser. The cavity was designed and optimized from the theoretically analysis on thermal stability and mode matching. Using walk-off compensated twin KTP-crystal pairs as the nonlinear media, the maximum output power of 4.15 W was obtained at 25 kHz and the tuning range spanned from 1923.22 nm to 2378.63 nm, pumped at the diode power of 18.41 W at 808 nm. The optical–optical conversion efficiency of 22.5% and the photon conversion efficiency of around 60% are both among the highest in intracavity pumped OPOs. To overcome the power degradation during angle tuning, a type-II quasi-phase matched (QPM) PPLN crystal with temperature tuning was also studied. With a 55-mm-long 14.1- μ m-grating period PPLN crystal, the maximum output power was 3.41 W corresponding to the optical–optical conversion efficiency of 18.5%. Both the KTP and PPLN OPOs demonstrated good efficiency as well as temporal, spectrum and beam characteristics, providing excellent 2- μ m laser source for applications of lidar, medical diagnosis, nonlinear frequency conversion to the long-wave infrared and terahertz range, etc.

Although high-repetition-rate dual-wavelength Nd:YAG lasers at 1319 and 1338 nm have been realized for quite a long time, we have employed it in generating monochromatic terahertz (THz) wave in this paper for the first time. The dual-wavelength laser was LD-end-pumped and acousto-optically (AO) Q-switched with the output power of watt level operating at different repetition rates from 5.5 to 30 kHz. Using a 0.6-mm-thick organic nonlinear crystal DAST for difference frequency generation (DFG), a compact terahertz source was achieved at 3.28 THz. The maximum average output power was about 0.58 μW obtained at a repetition rate of 5.5 kHz, corresponding to the conversion efficiency of about 6.4 × 10−7. The output power scaling is still feasible with higher pump power and a longer nonlinear DFG crystal. Owing to the compactness of the dual-wavelength laser and the nonlinear crystal, a palm-top terahertz source is expected for portable applications such as imaging and so on.

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.

High-order Stokes wave was observed in an x-cut KTP crystal based on stimulated Raman scattering (SRS) pumped by a passively Q-switched Nd:YLF laser with a Cr 4+ :YAG saturable absorber. Output spectra including the fundamental wave at 1047 nm and six Stokes wavelengths at 1077 nm, 1110 nm, 1130 nm, 1143 nm, 1164 nm, 1180 nm based on two Raman frequency shift at 267.4 cm −1 and 693.0 cm −1 were obtained simultaneously. We also detected green light generation with output power of 12 mW from self frequency mixing in the KTP crystal. The maximum total output power reached 452 mW at the repetition frequency of 8.1 kHz, corresponding to the optical-to-optical conversion efficiency of 4.61% and pump-to-Raman conversion efficiency of 3.6%.

A non-critically phase matched eye-safe KTA optical parametric oscillator intracavity pumped by a dual-wavelength acousto-optically Q-switched Nd:GYSGG laser is demonstrated. Simultaneous dual signal wavelength at 1525.1 nm/1531.2 nm can be realized using only one laser crystal and one nonlinear crystal. When the absorbed diode pump power at 808 nm is 7.48 W, the maximum output power, single pulse energy and peak power are 296 mW, 2.96 μJ and 6.4 kW, respectively. As the signal wavelengths exactly locates at the absorption band of C2H2, such an Nd:GYSGG/KTA eye-safe laser has good application prospects in differential absorption lidar (DIAL) for C2H2 detection and difference frequency generation for terahertz waves at 0.77 THz.

An effective method for wavelength tuning in an optical parametric oscillator (OPO) was proposed using noncollinear phase-matching (PM) in a ring cavity. This method was particularly useful for noncritically phase-matched (NCPM) KTP/KTA OPOs where changing the crystal orientation or working temperature is ineffective. A wide tuning range in the eye-safe band from 1572.9 nm to 1684.2 nm was realized pumped by an Nd:YAG laser at 1.06 μm in an NCPM KTP OPO while the internal noncollinear angle was tuned from 0 to 3.1° or the external angle from 0° to 5.8°, with slight variation of the deflection angle of one cavity mirror. The good beam quality and high spectrum intensity of the narrow-linewidth Nd:YAG laser resulted in 33.3% conversion efficiency for the collinear case and above 11% throughout the tuning range. Such OPOs have many potential applications where tunable eye-safe lasers are required, and the proposed wavelength tuning method can be extended to all kinds of OPOs.

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 × 10sup-7/sup, entirely suitable for portable applications. The utility of the THz source was also demonstrated through spectroscopy and imaging experiments.

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.

High-repetition-rate, monochromatic and tunable terahertz (THz) source is demonstrated. We use an orthogonally polarized dual-wavelength intracavity OPO to complete the type-II phase-matched collinear difference-frequency generation in GaSe. A high average-power 2 μm laser with 12 W output power and good beam quality based on an intracavity KTP OPO is experimentally designed. The KTP OPO is intracavity pumped by an acousto-optical Q-switched side-pumped Nd:YAG with the repetition rate of 10 kHz. Two identical KTP crystals were 7 × 8 × 15 mm3 in size, cut at θ = 51.2°, φ = 0°, which were tuned in the x-z plane to achieve type-II phase-matching. The KTP OPO consists of two identical KTP crystals to reduce the walk-off effect and improve the beam overlap area of the output signal and idler waves. The pulse-width of the 2-μm KTP OPO laser is about 11 ns with the linewidth about 0.8 nm. The focused OPO beam is injected into the uncoated GaSe with the length of 8 mm, and the generated THz wave is detected with a 4.2-K Si-bolometer after focusing with a polyethylene lens. The tunable and coherent radiation from 0.2 to 3 THz has been achieved based on the type-II phase-matching DFG when the two pump waves are in the range of 2.1064 - 2.1272 μm and 2.1516 - 2.1304 μm while symmetrically tuning the phase-matching angle of the KTPs. The maximum output THz average power can reach μW-level around 1.48 THz.

An efficient tunable high-repetition-rate KTP optical parametric oscillator (OPO) was deployed as a compact terahertz source. The KTP OPO was intracavity pumped by an acousto-optical (AO) Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy around 2 μm. A periodically inverted quasi-phase-matched (QPM) GaAs crystal and a GaSe crystal were used to generate terahertz (THz) wave of micro-watt level by the difference frequency generation (DFG) method. This provides a potential practical palm-top tunable THz sources for portable applications.

We demonstrate a method for obtaining optical coefficients over a broad terahertz spectral range from 1.5 THz to 16 THz at room temperature. Based on the interferograms directly acquired by a Fourier transform infrared spectrometer (FTIR), multi-beam interference principle combining Fresnel’s formula is employed to extract the refraction index and the extinction coefficient, giving the basis for calculating dielectric coefficients. It avoids the uncertainty and phase instability while using Kramers–Kronig (KK) relations and overcomes the limited frequency range of terahertz time-domain spectroscopy (TDS). Moreover, this method has better stability and is needless of cutting useful information between neighboring interference peaks for thin samples compared with TDS, making it a general processing method for interferograms and a good alternative for terahertz dielectric measurement.

A compact and stable terahertz (THz) source is demonstrated based on difference frequency generation (DFG) pumped by an efficient dual-wavelength acousto-optic (AO) Q-switched solid-state Nd:YLF laser with composite gain media (a-cut and c-cut) in the coaxial pumping configuration. Optimal power ratio and pulse synchronization of the orthogonal polarized 1047/1053 nm dual-wavelength laser could be realized by varying the pump focusing depth and/or pump wavelength. The total power of 2.92 W was obtained at 5 kHz pumped by 10-W laser-diode power at 803 nm. Such an efficient dual-wavelength laser demonstrated good stability and inconspicuous timing jitter benefiting from the suppressed gain competition between two resonating wavelengths. An 8-mm-long GaSe crystal was employed to generate THz waves at 1.64 THz by DFG and the maximum THz average output power was about 0.93 μW. This compact coaxial pumping method can be extended to all kinds of neodymium (Nd) doped laser crystals to produce different dual-wavelength lasers for various THz wavelength generation, which have good prospects for portable and costless applications like imaging, non-destructive inspection, etc.

We presented a high power, widely tunable narrowband 2 μm dual-wavelength source employing intracavity optical parametric oscillator with potassium titanium oxide phosphate (KTP) crystal. Two identical KTP crystals were oriented oppositely in the OPO cavity to compensate the walk-off effect. The output average power of dual-wavelength 2 μm laser was up to 18.18 W at 10 kHz with the peak power of 165 kW. The two wavelengths can be tuned in the range of 2070.7 nm to 2191.1 nm for ordinary light while in the range of 2190.7 nm to 2065.9 nm for extraordinary light with the full width at half maximum (FWHM) about 0.8 nm. The pulse width of the tunable laser was as narrow as 11 ns. The beam quality factor M 2 was less than 4 during wavelength tuning.

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.

The characteristics of terahertz (THz) generation based on cascaded difference frequency generation (DFG) process in periodically inverted gallium arsenide ( GaAs ) and gallium phosphide ( GaP ) is calculated from coupled wave equations, in which the output enhancement factors are 5.4 and 3.9 in the two crystals, respectively, compared with DFG without cascading. The optimal interaction length, influence of crystal absorption, wave vector mismatch and pump intensity are analyzed. A short discussion on wavelength tuning is also given. The calculated optimal operating parameters and conclusions can provide good directions for the experimental design.