Optical Thin Film Technology for Quantitative Remote Sensing on Ocean Color

Objective Precisely fabricated optical film elements of high performance are the basis for quantitative remote sensing instruments. For the Chinese ocean color and temperature scanner onboard HY-1C/D satellites [COCTS ( HY-1C/D)], excellent characteristics such as precisely positioned spectral response, low stray light coefficient, and low polarization sensitivity are needed to reduce the effects of atmospheric absorption and scattering. The realization of these targets ultimately requires the development of specific optical films, such as multichannel integrated filters and optical films with low polarization sensitivity. Methods Based on the optical thin-film characteristic matrix theory, we employ commercial software to calculate the spectral and polarization properties of band-pass filters at different incidence angles of light. Additionally, we also study the spectral and polarization effects caused by the spatial angular frequency distribution of light and simulate the spectral and polarization properties of converging beams in COCTS (HY-1C/D) through an integration method. Dual ion beam sputtering is adopted to prepare multichannel integrated band-pass filters on a single substrate at 90 degrees C to ensure reliability and spectral performance. The Jones matrix method is utilized to analyze the effect of different optical elements on the polarization sensitivity of the system, and film designs with special polarization tuning are finished for the key optical elements. The polarization sensitivity of the system is regulated and reduced by the mutual compensation of polarization characteristics among different optical elements. Results and Discussions The integrated filters are well prepared as shown in Fig. 1, the simulated spectral distribution of the filter in the converging beam matches very well with the measured results as shown in Fig. 3, and after a variety of environmental simulations and reliability tests, the spectra remain consistent as shown in Fig. 4. The polarization characteristic analysis of the filters ensures that the polarization sensitivity of all the filters is less than 0. 31% as shown in Table 2. The sound polarization design and mutual compensation of optical films ensure that the system polarization sensitivity is effectively controlled. Equipped with well-coated optical elements, COCTS (HY-1C/D) exhibits excellent relative spectral response and low polarization sensitivity when compared with the approved space-borne radiance sensors for ocean color detection, such as MODIS and VIIRS. For COCTS (HY-1D) launched in June 2020, the B7 band for atmospheric correction still has a bandwidth of 20 nm, without overlapping with atmospheric water vapor or oxygen absorption bands as shown in Fig. 2. The measured average polarization sensitivity is less than 1% at the scanning angle of 0 degrees (Fig. 8 and Table 3). The chlorophyll-a mass concentrations of global ocean from COCTS (HY-1C/D) exhibit relatively high consistency with MODIS and VIIRS products, which indicates the satisfactory capability of COCTS (HY-1C/D) for quantitative remote sensing as shown in Fig. 9. Conclusions Quantitative space remote sensing requires remote sensing instruments with accurate detection bands and low polarization sensitivity to reduce the effects of atmospheric absorption and scattering, and optical thin films play a key role. The technology of multichannel integrated band-pass filters on the single substrate can effectively reduce stray light and inter-channel crosstalk light, thus becoming the development direction of band-pass filters for quantitative remote sensing. To obtain the precise spectral response of the instrument, the influence of beam angle distribution on the spectral properties of band-pass filters should be fully considered in the design and fabrication processes. To reduce the polarization sensitivity of the instrument based on optical coating, we need to pay attention to the polarization regulation of each optical thin film and the complementary polarization characteristics of different components. The application of advanced optical thin film technology in COCTS (HY-1C/D) guarantees high-quality quantitative ocean color remote sensing and imaging, and the products of COCTS (HY-1C/D) show satisfactory performance.