Document Type

Article

Journal/Book Title/Conference

Proceedings of SPIE

Issue

156

Publication Date

1-1-1978

First Page

233

Last Page

240

Abstract

A method of using the SIMS (the selective modulation interferometric spectrometer) to measure the difference between the spectral content of two optical beams is given. The differencing is done optically; that is, the modulated detector signal is directly proportional to the difference between the two spectra being compared. This optical differencing minimizes the dynamic -range requirements of the electronics and requires only a simple modification of the basic cyclic SIMS spectrometer. This technique can be used to suppress background radiation for the enhancement of target detection and tracking. Laboratory measurements demonstrating the application of this technique are reported. Introduction The capability of optically detecting targets in the presence of a radiating background can be increased by using double- beaming techniques to suppress the effects of a radiating background. Background suppression is accomplished by ascertaining the difference in power between an optical beam from the target plus the background and an optical beam from the background only. One method of ascertaining the difference between the optical power transmitted by these two beams is to sequentially measure the power of these two beams and then compute the difference. However, this method has two serious disadvantages. First, errors may result because the difference between two numbers that are nearly equal must be computed. Second, time variation in the instrument's response will cause errors because the measurements of the two beams are made at different times. Vanasse et al.(1) have proposed a double- beaming technique* for optical background suppression that does not have these two disadvantages. This technique does the differencing optically; the differencing is done simultaneously so the measured signal is proportional to the difference between the power in the two beams. This paper describes an analogous double- beaming technique for the spectromètre interférentiel à modulation sélective (SIMS). The SIMS was introduced by Fortunato and Maréchal(3),(4). It has been extensively analyzed by Fortunato(5) and described by Esplin(6). Fortunato(5) has done double- beaming with a birefringent SIMS configuration. This paper describes another method of double - beaming that uses the cyclic SIMS configuration. A cyclic SIMS configuration is shown schematically in Figure 1. Interference fringes analogous to the fringes formed in Young's double -pinhole experiment(7) are formed in the grill plane located in the focal plane of L2. However, in contrast to Young's configuration, the SIMS, being an interferometer, has a very large optical throughput. The spatial distribution of the irradiance in the grill plane is analogous to the detector output of a conventional Fourier spectrometer: the SIMS forms a spatial Fourier transform of the source spectrum in the grill plane whereas a conventional Fourier spectrometer forms a temporal Fourier transform of the source spectrum.(8) The SIMS differs from the conventional Fourier spectrometer in the method used to recover the spectrum from the Fourier transform. An electromechanical method is used in the SIMS; a digital computer is used in the conventional Fourier spectrometer. The electromechanical inversion method used in the cyclic SIMS configuration of Figure 1 is accomplished by translating mirror M1 while vibrating the grill in the focal plane of lens L2. For this SIMS configuration, the central wavelength of the narrow band of wavelengths modulated by the vibrating grill is determined by the location of mirror M1. Since the output of the synchronous demodulator is proportional to the modulated optical power, *First suggested by Fellgett.(2) SPIE Vol. 156 Modern Utilization of Infrared Technology IV (1978) / 233 THE SIMS TECHNIQUE APPLIED TO BACKGROUND SUPPRESSION George A. Vanasse Air Force Geophysics Laboratory and Roy W. Esplin Ronald J. Huppi Stewart Radiance Laboratory Utah State University 1 DeAngelo Drive Bedford, Massachusetts 01730 Abstract A method of using the SIMS (the selective modulation interf erometric spectrometer) to measure the difference between the spectral content of two optical beams is given. The differencing is done optically; that is, the modulated detector signal is directly proportional to the difference between the two spectra being compared. This optical differencing minimizes the dynamic-range requirements of the electronics and requires only a simple modification of the basic cyclic SIMS spectrometer. This technique can be used to suppress background radiation for the enhancement of target detection and tracking. Laboratory measurements demonstrating the application of this technique are reported.

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