通道调制型偏振成像系统的波段宽度限制判据

通道调制型偏振成像技术是一种体积紧凑、空间分辨率高且能够实时获取全偏振信息的新型偏振成像探测技术. 但该技术目前只能实现准单色光的全偏振探测, 严重制约了其实用化. 本文首先对宽带光通道调制型偏振成像出现混叠现象的原因进行了分析, 得出载波频率是限制波段宽度的主要因素. 据此在空间频谱域上分析并推导了通道调制型偏振成像系统的光谱宽度限制判据公式, 同时通过模型仿真得到了系统的极限有效光谱范围, 与理论推导公式结果进行了对比分析, 验证了判据的准确性. 基于该判据可预测给定通道调制型偏振成像系统的有效工作波段, 同时还可为扩展系统波段宽度提供理论支撑.

Imaging spectral bandwidth criterion equation of channeled modulated polarization imaging system

Channeled modulated polarimetry imaging (CMPI) is a novel detection technology which can acquire full-Stokes parameters of each pixel of the sensor. Compared with the other imaging polarimetric technologies, CMPI has advantages in compact, high spatial resolution and acquiring full-Stokes information simultaneously. It has been widely used in remote sensing, military reconnaissance and biomedical diagnosis. However CMPI can only be used for quasi-monochromatic light during full-Stokes imaging, which leads to low signal-to-noise ratio in many cases especially under the condition of low light. Expanding the imaging spectral bandwidth of the CMPI is of great urgency. In order to expand the bandwidth, the limitation factors and conditions of the imaging bandwidth should be clearly understood first. So an imaging bandwidth criterion is deduced in this paper for the researchers to estimate the limitation bandwidth of the CMPI. We analyze the factors which might affect the fringe visibility based on a Savart plate (SP) CMPI and obtain the conclusion that carry frequency (CF) is the main factor which restricts the bandwidth. Then, according to the definition of CF, Ω = Δ/(λf), in which Δ is the shearing distance of SP, λ is the imaging wavelength, and f the focal length of imaging lens, we investigate how these factors influence the CF. It turns out that λ is the main factor which causes the fringe to arise in a certain CPI system while Δ would add an error to CF within 5% in visible light domain. To investigate how the wavelength influences the imaging spectral bandwidth, we deduce the total irradiance on the image plane under broadband light and use Fourier transform for it to obtain the distribution of the spatial frequency of the image plane. And the conclusion is obtained that the CF bandwidth be expressed as (2Ω₀-1/(2L), 2Ω₀ + 1/(2L)) referred to as the Rayleigh criterion, in which Ω₀ is the central CF and L is the range of the imaging plane. After substituting the relevant parameters into the CF bandwidth, we can obtain the imaging spectral bandwidth criterion equation as δ = 2Dλ₀²/(4D²-λ₀²) , in which δ is the maximum imaging bandwidth, D is the maximum optical path difference, and λ₀ is the central wavelength of the CMPI system. To validate the accuracy of the spectral bandwidth criterion, some simulations are conducted to generate a maximum imaging spectral bandwidth while the visibility of the fringes decreases to 0.5 for the fringes which cannot be distinguished when the visibility is less than 0.5. The results show that the error between the simulated spectral bandwidth and the calculated spectral bandwidth is less than 1 nm. This criterion value fits the test well for the SP CMPI system. In addition, it can also be used for estimating the maximum imaging bandwidth of the other CMPI system whose shearing distance is independent or quasi-independent of wavelength.