With the development of underwater optical communication, imaging and sensing, studies of the effect of oceanic turbulence on optical receiving systems have received increasing attention and become a research focus. Until now, the effect of atmosphere turbulence on optical receiving systems have studied widely and in depth, but those in oceanic turbulence have been researched seldom. It is known that the atmosphere turbulence is induced by the temperature fluctuation, while the oceanic turbulence is induced by both the temperature fluctuation and the salinity fluctuation. Thus, the behavior of laser propagation through oceanic turbulence is very different from that in atmosphere turbulence.The paper focuses on the influence of phase disturbance of optical signal caused by oceanic turbulence on optical receiving, including the theoretical modeling of couple efficiency of space optical signal into single-mode fiber received by annular aperture and the variance of angle-of-arrival (AOA) fluctuation received by finite aperture. The research shows that the couple efficiency of single-mode fiber and the variance of angle-of-arrival fluctuation in oceanic turbulence depend on propagation distance, receiving aperture and oceanic turbulence parameter. The results obtained in this paper will be useful for optical receiving systems involving oceanic turbulence channels.
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°.
With the development of optical technology, optical instruments become smaller and more integrated. Because of the high diffraction efficiency and light weight, binary optical elements become more and more popular. Binary optical elements can only blaze at one wavelength, it has high diffraction efficiency at design wavelength. But the diffraction efficiency of binary optical elements will decrease quickly with the change of wavelength. And this situation will have a big impact on image quality. Since double-layer BOE can blaze at two wavelengths, it has high diffraction efficiency at wide spectral bandwidth. There are kinds of fabrication errors. Based on scalar diffraction theory, this paper analyzes the diffraction efficiency of double-layer BOE with fabrication errors and simulates it in MATLAB. Simulation shows the diffraction efficiency decreases quickly if the depth errors of two layers are opposite, and this situation should be avoided. As for periodic errors, tilt errors and angular errors, these fabrication errors have different impact of double-layer BOE on diffraction efficiency.
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