Abstract-By discussing the basic schemes of the terahertz generation methods based on the 1550-nm ultrafast lasers briefly, GaAs and ZnGeP 2 are likely to be promising nonlinear optical crystals for terahertz waves generation by using optical rectification process. However, the mismatches of velocities between the terahertz waves and optical pulses are so large that the phase-matching coherent lengths are quite short, for example, the coherent length of 0.7 mm for GaAs and 0.5 mm for ZnGeP 2 at 2 THz around, respectively. That limited extremely the applications of these bulk excellent nonlinear optical crystals in terahertz regime. In this paper, we demonstrated theoretically that the dielectric planar waveguide could be used to enhance the coherent length of optical rectification process in THz regime. And for the first time, a dielectric planar THz waveguide that has potential applicable value in THz generation by optical rectification method was proposed. We predicted that the effective coherent length could be extended to 4 mm at 2 THz in a GaAs/ZnGeP 2 dielectric planar waveguide during optical rectification process pumped by ultrafast optical pulses at wavelength of 1550 nm.

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The damage growth characteristic determines the lifetime of optical components and the operating costs of high power laser systems. Knowledge of the lateral size and the depth of damage sites are crucial to evaluate the subsequent growth. An on-line detection method based on optimal modified lateral shearing interferometry is proposed to simultaneously measure the lateral size and the depth of damage sites. Thus, the damage growth characteristics can be estimated more comprehensively and efficiently. In the presented method, critical parameters of the common-path configuration are analyzed and optimized to meet the measurement requirements. Experimental results are presented to confirm the feasibility of the proposed method for on-line measurement. The damage growth characteristics of an optical film under different fluences are also analyzed and discussed simply.

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