Optical Coatings for Remote Sensing on FY-1 Meteorological

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1 Invited Paper Optical oatings for Remote Sensing on FY-1 Meteorological Satellite and Airborne Remote Sensing Instrumentations Yixun Yan and Keqi Zhang Shanghai Institute of Technical Physics hinese Academy of Sciences 42 Zhong Shan Bei Yi Road, Shanghai, hina ABSTRAT Optical interference coatings which have been successfully used in hinese FY l Meteorological Satellite and airborne remote sensing instrument are described. These include system requirement for these coatings, design approach, fabrication technique, and the actual results achieved. 1. INTRODUTION Optical interference coatings are always the essential elements in today's modern optical system. They are usually the key elements if compact and sensitive optical systems are needed, this is particularly the case in remote sensing instruments. In this paper, we will describe the use of optical interference coatings in hinese space program and in the airborne remote sensing instrument for mineral search. The FY-l Meteorological Satellite was launched in 1988 and 199 respectively in hina. In which a Very High Resolution Scanning Radiometer is used as principle remote sensing instrument in order to obtain visible and infrared cloud pictures and other remote sensing pictures. To realize this, five channels are arranged in the radiometer including three visible channel, one near infrared channel, and one far infrared channel. These five channels are separated by using two dichroic beamsplliters, one combined microsized bandpass filter, one wide bandpass filter, and the corresponding detectors. The airborne remote sensing instrument for mineral search is a twelve channel narrow band infrared scanner23, the wavelength range is from 1.6 microns to 2.5 microns. These channels are separated and received by twelve microsized bandpass filters combined with a 12 pixels PbS detector array. In the following section, the detailed description of these coatings will be presented. 2.OATINGS FOR FY-1 MTOROLOGIAL SATLLIT 2.1 System requirement for these coatings Figure 1 is a schematic diagram of the optical system of Very High Resolution Scanning Radiometer used in FY-l Meteorological Satellite. It can be seen that the far infrared channel, hannel 5, is selected by a visible-infrared dichroic beamsplitter, one longwave pass filter, and one wide bandpass filter; hannel 2 is selected by a longwave pass filter and the silicon detector itself; 222 / SPI Vol Thin Films for Optical Systems (1992) /93/$4. Downloaded From: on 2/18/216 Terms of Use:

2 the other three channels are determined by a combined microsized bandpass filter in order to simplify the optical system and make it more compact. Table 1 is the design requirement for these five channels. To meet these requirements, all the optical components must be antireflected or highly reflected using optical coating technology depending on their function. And the optical filters, i.e., two dichroic beamsplliters, one microsized bandpass filter, one longwave pass filter, and one wide bandpass filter, must have very high efficiency at the desired spectral ranges, accurate wavelength position, as well as durable coatings to meet the requirements of environmental test. Figure 1 Schematic diagram or the optical system of Very High Resolution Scanning Radiometer for FY l Meteorological Satellite. Table I Scanning Requirements for optical system of Very High Resolution Radiometer. Bands(um) Optical efficiency hl O.58+ O.Ol- O.68+-.Ol.3 h2 O.725+ O.Ol l.l+ O.Ol.3 h3 O.48+ O.5 -O.53+ O.5.16 h4 O.53+ O.5-- O.58+--O.5.18 h5 lo.5+ O.l O.l.18 SPI Vol Thin Films for Optical Systems (1992) / 223 Downloaded From: on 2/18/216 Terms of Use:

3 2.2 Dichroic beamsplliters In two dichroic beamsplliters, the one which transrrtits visible light and reflects infrared light is designed using dielectric-metal-dielectric structure. Based on the thin film theory, if the film thickness of metal is properly chosen, it will behave as high transmitted in a limited wavelength region and behave as high reflected at the other bands3. Thus this structure can be used as a dichroic beamsplitter. In the design, we use zinc sulfide as dielectric material and silver as metal material. The final design is: Substrate! 2 5nmZnS/ l6nmag/ 3 5nmZnS/Air where we use fused quartz as substrate. Using this design, good result has been obtained. Figure 2 is the measured spectral responds of this dichroic beamsplitter which transmits in visible and near infrared, reflects in longer wavelength. The other dichroic beamsplitter should transmit in near infrared and reflect in the whole visible region. So a longwave pass filter is designed: Substrate/O. 77 (. 5HLO. 511) (. 5HLO. 5H) 1(. 5HLO. 5H) 11/Air This is a 65 layers design, where we use fused quartz as substrate, Zr2 as high refractive index material, and 5i2 as low index material. This filter is fabricated using electron beam deposition technique to make the coating durable and stable. Figure 3 is measured spectral transmittance of the actual filter ) o 6. (1) 4 H Wavelength(nm) Wav&ength(nm) Figure 2 Measured spectrum of dichroic beamsplitter which transmits in visible and near infrared and reflects in longer wavelength. Figture 3 Measured spectrum of dichroic beamsplitter which transmits in near infrared and reflects in whole visible. 224 / SPI Vol Thin Films for Optical Systems (1992) Downloaded From: on 2/18/216 Terms of Use:

4 2.3 ombined microsized bandpass filter hannels 1, 3, 4 are in visible region, using a three element array silicon detector, they are combined into one optical path, so the optical system can be more compact and have less energy loss. From figure 1 we can see that at the front of the detector is a combined microsized bandpass filter to separate three independent channels on the elements of detector array respectively. The structure of combined microsized bandpass filter is shown in figure 4. It can be seen that they are combined in three parts, each part corresponds to one optical channel. Filters for channels 3 and 4 are bandpass filters, they are using triple halfwave design: Substrate/HLH (HLHLHLH) 2HLH/Air. Figure 4 The structure of combined microsized hannel 1 is designed by the filter. combination of a longwave pass filter and a shortwave pass filter. All these bandpass filters are blocked from UV to 1.5 micrometers. Figure 5 is measured spectral transmittance of these bandpass filters for channels 1, 3, and ci-) I. o 6. (1) 4. F- 2.. Figure 5 Measured spectrum of combined microsized bandpass filter. SPI Vol Thin Films for Optical Systems (1992)! 225 Downloaded From: on 2/18/216 Terms of Use:

5 2.4 oatings for far infrared channel The far infrared channel is set from 1.5 to 12.5 micrometers. This channel is also realized by optical coatings, which include a 1.5 micrometers longwave pass filter and a broad bandpass filter. Using PbTe and ZnS coating materials, these two filters are designed and produced on germanium substrates. Figures 6 and 7 are their measured spectrum ) o 6. (1) 4. 2 ) 8. o WaveIength(um Wavelength urn Figure 6 Measured spectrum of longwave pass filter. Figure 7 Measured spectrum of wide bandpass filter. 2.5 nvironmental test It is clear that all optical components must have high reliability for use in space program. So these coatings must pass following strict environmental test before put into practical use. They are: (a) tape test. (b) abrasion test. (c) hot water emerging test. merged in 45 hot water for 8 hours; (d) humidity test. oatings are in the environment of >95% humidity and 4 for 24 hours. (e) low temperature storing test. Stored in 45 environment for 12 hours. After the environmental test, all coatings should not have any visible damage and optical degradation. 3. OATINGS FOR 12 HANNL IR NARROW BAND SANNR This infrared narrow band scanner is used for searching gold mineral and oil resources. It contains 12 near infrared channels from 1.6 micrometers to 2.5 micrometers. One requirement for this instrument is that it should be suitable for installation on smallsized remote sensing aircraft. So a compact structure of the optical system is needed. The simplest way to implement this is to use a 12 pixels detector array but it can not be realized unless microsized bandpass filter is used to separate each channel to different pixels. Figure 8 is the schematic diagram of the optical system of this scanner. 226 / SPI Vol Thin Films for Optical Systems (1992) Downloaded From: on 2/18/216 Terms of Use:

6 De t ec t or ar r ay with fi1trs Target raditoin Figure 8 Schematic diagram of the channel infrared narrow band scanner. optical system of 12 A 12 pixels PbS detector array is used in the scanner, the size of pixel is l.4mm*l.4mm. ach pixel is linearly arranged by 1.4mm and pasted by a selected microsized bandpass filter. Thus the maximum size of the filter is 2.8mm*2.8mm. Table 2 is the system requirement for these microsized filters. To meet these requirement, we choose triple halfwave design combined with special deposition technique to produce these very small size filters. The design approach are common to the others. Figure 9 is one of the measured spectral performance of these bandpass filters. 4. AKNOWLDGMNTS The authors would like to acknowledge Mr Wu hangyong, Mr Xu Buyun, Ms. Zhu uiyuan, And Mr Zhu Bingsheng for providing useful data. 5. RFRNS [1] Gong Huixing, "Visible-Infrared Scanning Radiometer of FY-1 Meteorological Satellite and Its Technical Advances,"hinese Journal of Infrared Research, Vol.9, pp.81-9, 199. [2] Wu hangyong, "Near Infrared Twelve hannel Airborne Spectral SPI Vol Thin Films for Optical Systems (1992) / 227 Downloaded From: on 2/18/216 Terms of Use:

7 Scanner," Shanghai Institute of Technical Physics Annual Technical Report, 199. [3] Baumeister, P.W., "Radiant Power Flow and Absorptance in Thin Films," Applied Optics, Vol.8, pp (1969). Table II Requirements for 12 channel bandpass filters. hannel entral wavelengh (urn) Bandwidth (urn) T (%) U o 6. (1) 4. ol. F Wavelength(um) Figure 9 A bandpass filter centered on 2.33 microns used for 12 channel infrared narrow band scanner. 228/ SPI Vol Thin Films for Optical Systems (1992) Downloaded From: on 2/18/216 Terms of Use:

Filters for Dual Band Infrared Imagers

Filters for Dual Band Infrared Imagers Thomas D. Rahmlow, Jr.* a, Jeanne E. Lazo-Wasem a, Scott Wilkinson b, and Flemming Tinker c a Rugate Technologies, Inc., 353 Christian Street, Oxford, CT 6478; b