Research Progress of Infrared Thin Film Coatings Applied in Space Remote Sensing Systems(Invited)

As one of the great achievements of modern science and technology, remote sensing technology is an effective means for human beings to understand the objective world. The demands in the fields of weather forecast, resource exploration, environmental monitoring, disaster assessment and deep space exploration have promoted the rapid development of space infrared remote sensing technology. In the space infrared optical system, optical thin film coatings undertake the functions of optical energy transfer, optical spectrum division and distribution, working band limitation and out of band suppression, system polarization sensitivity regulation and so on. Space optical remote sensing instruments are widely used in weather forecasting,earth mapping, astronomical observation and other fields. Reflective optical systems are mostly used due to long detection distance, large aperture and limited volume and weight;In order to obtain as much target information as possible, its spectral coverage is generally wide,and the working spectral band from near ultraviolet to long wave infrared is also common; It is necessary to accurately limit the spectrum of the working band to obtain the detailed characteristics of the target; In addition, in order to improve the signal-to-noise ratio of the system, it is necessary to suppress the spectrum of specific wavelength efficiently. The coatings mentioned above are used frequently in space optical remote sensing systems. The detection range of space optical instruments can be extended from 100 km to light years. In order to improve the system's ability to collect target beam energy and distinguish target details, it is necessary to improve the aperture of the optical system. It is difficult for the transmission optical elements to achieve large size because of the limitations of material uniformity and defects; The reflective optical system has no chromatic aberration and no secondary spectrum, so it is easier to realize wide-band imaging; The reflection system can fold the optical path by using the folding mirror to reduce the volume of the system; The large aperture mirror is easy to be lightweight,which can effectively reduce the weight of the system. Based on the above reasons, the reflective structure plays a leading role in the design of space remote sensing optical instruments, and the mirrors have also become an indispensable optical element in the system. The design of mirror film can be roughly divided into three technical routes:metal reflective film,all dielectric multilayer reflective film, and metal dielectric composite multilayer reflective film. For space optical instruments with a wide spectrum, all dielectric multilayers are rarely used due to the technical characteristics of narrow reflection band, film thickness and stress. Metal film and metal dielectric composite film are common technical schemes. In spaceborne optical instruments, the materials commonly used to prepare metal reflective films are aluminum(Al), silver (Ag) , gold (Au) , etc. Aluminum is the only metal material with stable reflectivity from ultraviolet to infrared band. It has been successfully used in the preparation of main mirror reflection film of many large aperture space telescopes. Aluminum film mirrors for space applications usually need to prepare protective films on the outer layer of metal aluminum. The materials and processes of protective films are different according to the application requirements. For example, the working band of the Hubble telescope is from ultraviolet to infrared. MgF2 is used as the protective film in order to prevent the reflectivity of ultraviolet band from decreasing due to the oxidation of aluminum film in the air; SiOx is selected as the protective film of Al for the Herschel telescope working in the infrared band. With the increasing requirements of space remote sensing system for the optical performance of large aperture mirror, the problem of low absolute reflectivity of aluminum mirror restricts its wider application. Silver material has high and stable reflectivity from visible light to long wave infrared band,and also has the smallest polarization effect in all metal thin film materials. Therefore,it has become the choice for the preparation of main mirror reflection film of large aperture space remote sensing instrument in recent years. Gold material has obvious absorption at < 600 nm,which limits its application in space remote sensing instruments with visible light in the working spectrum. However,for space remote sensing instruments working in infrared band, gold is a good choice for preparing mirror film. Firstly,the gold film has high reflectivity in the whole infrared band; Secondly, the chemical properties of the gold film are stable, and it hardly reacts with other substances, resulting in performance degradation,or even no need to prepare a protective film. Au was used to coat the primary mirrors of the James Webb Space Telescope, which was launched at the end of 2021. Though the main optical telescope of space infrared optical instruments is usually designed with reflective structure, lens or optical windows are indispensable optical thin film elements in the subsequent optical path. Antireflection coatings are used to improve the optical transmission efficiency and eliminate the ghosting artifacts caused by the residual reflectance. The refractive index span of optical materials used as infrared lenses or windows is large. The refractive index of low refractive index materials such as BaF2 and MgF2 is less than 1.4, while the refractive index of high refractive index materials represented by Si and Ge is more than 3.4. According to Fresnel formula,the greater the difference in refractive index between the two optical interfaces, the higher the residual reflection generated on the two optical surfaces. The residual reflection on one side of Ge window without antireflection reaches 36%. The more optical surfaces,the more serious the optical energy loss. Therefore, infrared antireflection coatings are of great significance for infrared lenses and windows. For the design and fabrication of infrared AR coatings, the choice of film materials is relatively limited, and the materials have more or less certain absorption. Both high optical efficiency and the adaptability of space environment are very important to the infrared AR coatings for space application. The working wavelength of space optical instruments often covers a wide spectral range. No detector can respond the full spectral range from visible to far infrared. The common technical scheme uses multi-stage dichroic beam splitters to separate the optical signals of different spectral range into different optical paths. In order to improve the performance of detectors, infrared detectors for space applications generally work in low temperature environment,while detectors in ultraviolet, visible and near-infrared bands work near room temperature. Moreover, some instruments even put the whole infrared light path in the cold box to reduce the background noise. So the broadband dichroic beam splitter, which covers the whole spectral range of the instrument, is designed to separate the spectra of visible and near-infrared from infrared. There are two methods to design the broadband dichroic beam splitter. One is induced-transmission dielectric-metal-dielectric structure, the other is multiple long- wave pass stacks. The first scheme is adopted by GOES-R ABI and FY-4A/B AGRI, and the second is used by FY-4C AGRI and MTG FCI. The infrared hyper-spectral vertical sounder can take measurements of three dimensional atmospheric structures from interference of split light beams, via a Michelson interferometer working over different infrared bands for large-area,continuous, fast, and accurate vertical air soundings of temperature and humidity. Infrared neutral beam splitter is the key thin film component of the infrared Fourier-transform spectrometer. A device which divides a beam of light into two parts is known as a beam splitter. The functional part of a beam splitter generally consists of a plane surface coated to have a specified transmittance and reflectance over a certain wavelength range. Meteorological satellites represented by NOAA in the United States and METOP in Europe are equipped with interferometric infrared vertical detection instruments. The development and application of interferometric space infrared hyperspectral instrument in China is relatively late due to the constraints of some materials and key technologies. However,with the breakthrough of broadband infrared neutral beam splitter, of which the wavelength band is from 4.44 mu m to 14.29 mu m and the surface shape is about 1/ 50 lambda, China has risen rapidly in this field. A filter which possesses a region of transmission bounded on either side by regions of rejection is known as a bandpass filter. In the traditional multi-band space remote sensing instrument, the filter is usually matched with the detector chip to realize the accurate positioning of the required working band. With the maturity and development of grating splitting, prism splitting and interference splitting, there are more ways to subdivide and select the working band, but the filter still plays an important role. In recent years, new progress has been made in infrared integrated filters. Integrated devices such as multi-channel integrated filters,dual-band-pass filters,and linearly variable filters have been successfully used in space optical instruments. Polarization describes the vibration mode of electric vector in the process of light propagation. There are some differences in the polarization information of radiated or reflected light on different target surfaces, which provides a new degree of freedom for space remote sensing to obtain information. Polarization has been successfully applied in space remote sensing instruments. The polarization of light is affected by the shape and material of the object surface. Infrared imaging with additional polarization information can greatly improve the contrast between different objects, which is of great value for target recognition. However, compared with the visible light and near-infrared wavelengths, polarization in the infrared band is more difficult to control, which brings a series of challenges to the design and preparation of infrared polarization regulated films. The reliability of optical thin film elements refers to the probability of completing the specified functions within the specified time under the working environment conditions. The working life, which is related to the orbit environment,character of the task, budget and so on, may vary from one application to another. The working life of space remote sensing instruments is an important assessment index. The reliability and space environment simulation test on the ground is an important guarantee for the optical thin film elements to meet the on orbit operation life. Since the birth of space remote sensing instrument, human beings have never stopped pursuing higher performance indicators. The demand of the system has promoted the continuous progress of optical thin film technology. Some new optical thin film elements have been widely used in space remote sensing systems. Larger aperture, wider spectral range, higher optical transmission efficiency, finer light splitting and higher integration are still the unremitting goals of optical thin film development.