Exploring the Principle and Application of Time-Air Conditioning Type Interferometric Spectrometer

Among the many types of spectrometers, interferometric spectrometers are distinguished by their...High light throughput, high spectral resolution, high signal-to-noise ratio, and wide band coverageIt possesses significant advantages. Unlike traditional dispersive spectrometers that rely on prisms or gratings, it uses the principle of interference to superimpose light of different wavelengths to form fringes, and then uses Fourier transform to decode the interference information into a spectrum. This allows it to distinguish extremely close wavelengths, maintain a high signal-to-noise ratio under weak light or infrared conditions, and achieve wide-band measurements from visible light to mid- and far-infrared. It is widely used in atmospheric monitoring, environmental remote sensing, and infrared material analysis. The core of an interferometric spectrometer lies in generating and recording changes in optical path difference (OPD) to obtain complete interference fringes and recover the spectral distribution. Based on the method of OPD acquisition, it can be divided into time-modulated, spatially modulated, and time-controlled types. This article will focus on the time-controlled type interferometric spectrometer, using the classic Large Aperture Static Interferometric Imaging Spectrometer (LASIS) as an example.

The LASIS structure mainly consists of a front telescope optical system, a transverse shearing interferometer made of cemented half-pentagonal prisms, a Fourier imaging objective, and an area array detector. The front telescope is used to compress the aperture of the target beam to match the interferometer, thereby effectively reducing the system size (as shown in Figure 1). During operation, the transverse shearing interferometer splits the incoming target radiation into two coherent beams with an optical path difference, forming interference fringes on the back focal plane of the optical system. The magnitude of the optical path difference determines the spectral resolution: the larger the optical path difference, the higher the resolution. The detector then records the interference-modulated "interference image" (as shown in Figure 2), which simultaneously contains both two-dimensional spatial information and one-dimensional spectral information of the target.

Figure 1 Schematic diagram of LASIS optical principle

(Image source: Hu Bingliang. A Review of the Development of Interferometric Spectroscopic Imaging Technology (Invited))

Figure 2. The "interference image" modulated by the interferometer from the LASIS output.

(Image source: Li Xiangbin. Research on image correction technology for large aperture static interferometric imaging spectrometer)

Figure 3 illustrates the process of LASIS acquiring the spectral information of a specific ground feature. Figure 3(a) shows the imaging process, with different symbols (■, ★, ●) representing the locations of different ground features. As the push-broom operation proceeds, the detector gradually acquires interference fringes of the same ground feature under different optical path differences, and then stitches them together in the scanning order to obtain a complete interferogram (Figure 3(b)). The fringe curve of a single pixel is extracted from this interferogram (Figure 3(c)), and after Fourier transform, the spectral curve of that pixel can be decoded (Figure 3(d)). The entire process includes data processing steps such as fringe stitching, dark current correction and noise processing, Fourier decoding, and spectral calibration. These steps ensure the accuracy and reliability of the spectral information of each pixel, providing a solid foundation for further analysis and application.