We describe a robust system for laser-driven narrowband terahertz generation with high conversion efficiency in periodically poled Lithium Niobate (PPLN). In the multi-stage terahertz generation system, the pump pulse is recycled after each PPLN stage for further terahertz generation. By out-coupling the terahertz radiation generated in each stage, extra absorption is circumvented and effective interaction length is increased. The separation of the terahertz and optical pulses at each stage is accomplished by an appropriately designed out-coupler. To evaluate the proposed architecture, the governing 2-D coupled wave equations in a cylindrically symmetric geometry are numerically solved using the finite difference method. Compared to the 1-D calculation which cannot capture the self-focusing and diffraction effects, our 2-D numerical method captures the effects of difference frequency generation, self-phase modulation, self-focusing, beam diffraction, dispersion and terahertz absorption. We found that the terahertz generation efficiency can be greatly enhanced by compensating the dispersion of the pump pulse after each stage. With a two-stage system, we predict the generation of a 17.6 mJ terahertz pulse with total conversion efficiency η total = 1.6% at 0.3 THz using a 1.1 J pump laser with a two-line spectrum centered at 1 µm. The generation efficiency of each stage is above 0.8% with the out-coupling efficiencies above 93.0%.
A high repetition rate optical parametric oscillator (OPO) generating an idler laser with a wavelength as long as 4.0 μm at 200 and 400 kHz was demonstrated in this paper. The OPO was pumped by a master oscillator power amplifier structure fiber laser with excellent characteristics. The pump pulse from the fiber laser had a steep leading edge, which was theoretically proved to improve the OPO's performance, compared with the Gaussian pump pulse. A homemade periodically poled magnesium-oxide-doped lithium niobate crystal with a grating period of 29 μm was employed in our experiment. By optimizing the resonator, 2.75 and 1.67 W idler lasers were finally achieved at repetition rates as high as 200 and 400 kHz, respectively, with a wavelength as long as 4.0 μm. The conversion efficiencies were 12.03% and 7.31%, respectively.
The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single-to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especially when pushing for high optical-to-terahertz conversion efficiency. A 3D+1 (x,y,z,t) numerical model is necessary in order to fully investigate the terahertz generation problem in the tilted-pulse-front scheme. We compare in detail the differences between 1D+1, 2D+1 and 3D+1 models. The simulations show that the size of the optical beam in the pulse-front-tilt plane sensitively affects the spatio-temporal properties of the terahertz electric field. The terahertz electric field is found to have a strong spatial dependence such that a few-cycle pulse is only generated near the apex of the prism. The part of the beam farther from the apex contains a large fraction of the energy but has a waveform that deviates from a few-cycle. This strong spatial dependence must be accounted for when using the terahertz pulses for strong-field physics and carrier-envelope-phase sensitive experiments such as terahertz acceleration, coherent control of antiferromagnetic spin waves and terahertz high-harmonic generation. arXiv:1908.09581v1 [physics.optics]
Based on the study of laser diode self-mixing interference effects, a simple damping microvibration measuring method that can accurately obtain the damping factor is presented. The damping factor is solved by recording the period and counting the fringe number of the self-mixing signal. The damping factor of 0.0483 s À1 with a standard deviation of 0.0013 and the coefficient of variation of 2.69% was experimentally achieved. Theoretical simulation of the feedback strength on the measuring accuracy shows that the measuring method of the damping factor has high accuracy in the case of weak feedback.
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