A passively Q-switched dual-wavelength laser with tunable power ratio and pulse interval is demonstrated based on the coaxial diode end-pumping configuration by varying the pump wavelength. A theoretical model was built by a set of time-domain rate equations illustrating the dynamic process of both the pulse generation of two wavelengths from a shared pump source and a saturable absorber. The simulation showed that both the power ratio and the time interval between the pulses at two resonant wavelengths in the Q-switched mode could be tuned by balancing the gains in two laser crystals by varying the pump wavelength, realized by controlling the working temperature of the pump laser diode. The experiment was performed with Nd:YAG/Nd:YAP composite laser crystals and a Cr ⁴⁺ :YAG slice working as the saturable absorber for pulse generation. The continuous-wave and Q-switched total output power reached 5.62 W and 2.12 W, corresponding to overall optical-optical conversion efficiencies of 37.5% and 14.1%, respectively. The manipulation of power ratio and pulse interval between 1064 nm and 1080 nm agreed well with the simulation results.

A novel scheme to realize widely tunable optical parametric oscillator (OPO) based on an x -cut KTP crystal is demonstrated. The tuning range from 1572.5 nm to 1697 nm was enabled by a 1:1 imaging optical system with a variable deflection mirror to couple the 1064-nm laser pulses into the OPO cavity in the noncollinear phase-matching (PM) configuration. Benefitting from varying the pump direction rather than the cavity axis, the OPO cavity was unaffected so that the stability of the output signal energy and beam direction could be maintained, which greatly facilitates subsequent applications. The change of pump threshold, output energy, linewidth and temporal pulse shapes during wavelength tuning were also discussed.

The 4H-SiC crystal is found to have great potential in terahertz generation via nonlinear optical frequency conversion due to its extremely high optical damage threshold, wide transparent range, etc. In this paper, optical rectification (OR) with tilted-pulse-front (TPF) setting based on the 4H-SiC crystal is proposed. The theory accounts for the optimization of incident pulse pre-chirping in the TPF OR process under high-intensity femtosecond laser pumping. Compared with the currently recognized LiNbO₃-based TPF OR, which generates a single-cycle terahertz pulse within 3 THz, 4H-SiC demonstrates a significant advantage in producing ultra-widely tunable (up to over 14 THz, TPF angle 31°–38°) terahertz waves with high efficiency (~10^–2) and strong field (~MV/cm). Besides, the spectrum characteristics, as well as the evolution from single- to multi-cycle terahertz pulses can be modulated flexibly by pre-chirping. The simulation results show that 4H-SiC enables terahertz frequency extending to an unprecedent range by OR, which has extremely important potential in strong-field terahertz applications.

In order to achieve collinear phase-matched nonlinear optical frequency conversion in cubic crystals, a novel method to induce and modulate the birefringence based on the linear electro-optic effect was studied. Taking terahertz generation with ZnTe and CdTe crystals of the $\overline{4}3\mathrm{m}$ point group as an example, an external electric field provided the freedom to realize perfect phase matching (PM) in a wide bandwidth up to 2 THz for difference frequency generation, with monochromatic optical waves at around 800 nm and 1010 nm, respectively. Theoretical simulations showed that such a method helps to extend the terahertz frequency, enhance the conversion efficiency, and alleviate the limitation on the pump wavelength, which is highly favorable for nonlinear optical uses of cubic crystals. Simultaneous wideband terahertz generation pumped by ultrashort laser pulses via optical rectification or difference frequency mixing was also discussed, indicating that an electric field can be used to modulate the PM characteristics without changing the group velocity-matching condition. Tuning the nonlinear interaction by an applied voltage is fast, robust, and convenient compared to other approaches. In addition, the linear electro-optic effect can make the nonlinear crystal a compact and high-speed terahertz amplitude modulator, which has great potential in radar, communication, imaging, etc.

A widely tunable eye-safe noncollinear phase-matched (PM) KTP optical parametric oscillator (OPO) with fixed output direction was proposed. Based on a novel confocal optics system, the input pump beam from a pulsed 1064 nm Nd:YAG laser could be deflected into the OPO with a tunable and agile noncollinear angle while maintaining the resonator unaffected. As a result, stable OPO operation with a wide tuning range of 124 nm was achieved easily with a beam scanner. The pump threshold, output energy, linewidth and temporal pulse shapes during wavelength tuning were also measured and discussed.