[Analysis and Design of Interference Imaging System in Fourier Transform Imaging Spectrometer Based on Multi-Micro-Mirror].

To realize the static state and high throughput of Fourier transform imaging spectrometer (FTIS), a temporal spatial mixed modulated FTIS based on multi-micro-mirror was put forward in this paper, whose interference system was based on Michelson interferometer with a multi-micro-mirror to replace the plane mirror. The remarkable characteristics of this FTIS were no movable parts and slit existing in this system, and the interferogram and image of object could be gained at the same time. The fore-optics system imaged the object on the plane mirror and multi-micro-mirror of the interference system, due to the structure feature of multi-micro-mirror, the optical path difference (OPD) of two imaging beam could be modulated. Through the reimaging system, the image of object with different interference order could be obtained. By means of the analysis to the spectrum signal-to-noise ratio (SNR) of interference system, the relationship between spectrum SNR and image SNR was definite, and the characteristic parameters of multi-micro-mirror were determined. To ensure the constancy of OPD corresponding to each step plane, by means of the analysis to the imaging process of fore-optics system, the optical path structure of telecentric in image space was determined. According to the calculation of the relationship between field of view and OPD, the design indexes of fore-optics system were determined and the optical design was completed. To ensure no extra OPD was introduced by reimaging system, through the analysis of the imaging feature by reimaging system, the optical path structure of double telecentric was determined. According to the calculation of the relationship between incidence aperture angle and step number, the optical system that satisfied the system requirement was designed. By means of the theory analysis and optical design to each unit system, this research can provide a novel development strategy for static and high throughput FTIS.

Design of spatio-temporally modulated static infrared imaging Fourier transform spectrometer.

A novel static medium wave infrared (MWIR) imaging Fourier transform spectrometer (IFTS) is conceptually proposed and experimentally demonstrated. In this system, the moving mirror in traditional temporally modulated IFTS is replaced by multi-step micro-mirrors to realize the static design. Compared with the traditional spatially modulated IFTS, they have no slit system and are superior with larger luminous flux and higher energy efficiency. The use of the multi-step micro-mirrors can also make the system compact and light.

[Study on transmission efficiency of interference system in spatially modulated Fourier transform spectrometer].

The reflection of the optic system surface and the absorption of the infrared material could reduce the transmission of the incident light in spatially modulated Fourier transform infrared spectrometer. Through the calculation of the transmission function of the interference system and the simulation of the interferogram image and recovered spectrum affected by transmission function, it was indicated that the contrast of the interferogram image declined and the spectral line intensity weakened. The theoretical analysis shows that the contrast of the interferogram image was related to the intensity reflectance of the anti-reflection film, and the attenuation of the spectrum was determined by transmission efficiency concerned with intensity reflectance R1 of the anti-reflection film, intensity reflectance R2 of the beam splitter film, and the absorption coefficient. By means of the analysis and argumentation, the absorption of the material could be ignored in our investigative wave band. So the transmission efficiency was determined only by R1 and R2. Then taking the transmission efficiency as the design target, according to the transmission required by system, the tolerance of the R1 and R2 could be gained.

[Study on spectrum inversion of spatially modulated Fourier transform spectrometer].

The sampling mode of static Fourier transform spectrometer is spatial domain sampling. The interferogram function is sampled by two orthogonal stepped mirrors and the interference irradiance is received by the detector. The interferogram image is a planar spatial array which consists of MXM interferogram units. After image segmentation, the interferogram image is divided into M x M interferogram units according to comparability criterion. By means of addressing location, the sampled interferogram sequence which matches up to the discrete optical path difference sequence could be gained. Using over-zero sampling technique, the big single side interferogram sequence and the small double side interferogram sequence are apodizated by different window functions. For the sake of correcting phase error, the frequency-domain spectrum correction and space-domain interferogram correction are researched and improved. The simulation result shows that the two methods can both gain the perfect spectrum line shape, and the effect of space-domain interferogram correction is better than others with the spectrum standard deviation only 0.012 088.

[Study on uniformity of optical field in static Fourier transform spectrometer].

Since the Fourier transform spectrometer based on multi-micro mirrors samples the interferogram image which corresponds to each OPD in the transverse optical field, the spatial distribution of the irradiance has much effect on the quality of the interferogram. In the present paper, distribution function is introduced in this spectrometer system. By means of simulation, the contrast of the interferogram modulated by distribution function is depressed in the fringe area and the concomitant line appears in the spectrum. According to theory analysis, the decline of the contrast lies on the distribution function which scans the interferogram periodically. And the concomitant line is the effect of the spectrum shift, which belongs to the modulation function. Finally, the difference image and conversed recovery arithmetic are proposed. Via the simulation, the conversed recovery arithmetic can recover the interferogram and the spectrum evidently.

[Analysis of collimation error in Fourier transform infrared spectrometer based on step mirrors].

The collimation error in Fourier transform spectrometer has many effects on the interferogram and the spectrogram. By means of modulation, the authors found the reverse of contrast in the interferogram and the side slope noise in the spectrogram due to the collimation error. The authors analyzed the signal-to-noise ratio and resolution at different factor of beam divergence. When the factor of the beam divergence is 0.15 degrees x mm(-1), the signal-to-noise ratio can reduce to 6 dB and the resolution can go bad to 13.4 cm(-1).

Spectrum Reconstruction of a Spatially Modulated Fourier Transform Spectrometer Based on Stepped Mirrors.

Based on the basic configuration and interference principle of a static step-mirror-based Fourier transform spectrometer, an image segmentation method is proposed to obtain a one-dimensional interferogram. The direct current component of the interferogram is fit using the least squares (LS) method and is subsequently removed. An empirical-mode decomposition-method-based high-pass filter is constructed to denoise the spectrum and enhance the spectral resolution simultaneously. Several experiments were performed and the spectrum is reconstructed based on these methods. The spectrum resolution is 81 cm-1 at 2254 cm-1.