The analytical expressions of the M2-factor for laser beams propagating in oceanic turbulence are derived by using the extended Huygens-Fresnel principle and the definition of the second moment of the Wigner distribution function(WDF). Taking the standard Hermite-Gaussian (SHG) beams and elegant Hermite-Gaussian (EHG) beams as typical examples of laser beams, the propagation properties of the SHG beams and EHG beams in oceanic turbulence are analyzed quantitatively. It is shown that in oceanic turbulence the M2-factor of SHG and EHG beams are different except for beam order m=0 and m=1. At a given propagation distance, the relative M2-factor of SHG and EHG beams in turbulence monotonously decreases with increasing beam order. The relative M2-factor of SHG beams are less than those of EHG beams under the same condition, implying that turbulence influences SHG beams less than EHG beams.
A mathematical model of the diffraction efficiency change with the ambient temperature for the double-layer harmonic diffractive elements (HDE) is presented, and its effects are analyzed in this paper. The double-layer HDE structure is investigated and the optimization procedure is based on the equation of diffraction efficiency of the double-layer HDEs. By selecting appropriate design wavelength, the average diffraction efficiency of the system is reaching 99% in working wavelength and working temperature, which improves the image contrast and the image quality significantly. A set of dual-wavelength infrared optical system is designed based on dual-wavelength 320x240 element cooled thermal IR focal plane arrays detector. By introducing double-layer HDEs and aspheric surfaces, the chromatic aberration and the off-axis aberration are well corrected and the system structure is simplified. The system working in the wave band of 3.7~4.8μm and 7.7~9.5μm and with the F number of 2 is consisted of 8 elements and has 100% cold shield efficiency. The image quality evaluating results show that the performance of the dual-wavelength infrared optical system is very well in
temperature from -40C° to +60C°.
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