A spatially modulated prism interferometer (SMPI) has been developed that overcomes the complexities of traditional interferometers and the inherent limitations of diffraction gratings, dispersion prisms, and spectral selection filters. Applications include atmospheric sounding, geologic mapping,in-situ mineralogy, oceanography, pollution monitoring, poisonous gas detection, medical spectroscopic imaging, and industrial inspection.
At the heart of the SMPI is the prism triplet shown in Figure 1. Its function is to shear the input beam into two mutually coherent output beams with chief rays that are parallel to the optical axis. A Fourier optical system, shown in Figure 2, collimates the two beams, tilts them, and then recombines them at a pupil plane. The tilted wavefronts generate a spatially modulated interference pattern that is recorded as an interferogram by a detector array. If the Fourier optical system is made anamorphic, then a line image is formed in a direction orthogonal to the series of interferograms. Interferometers with a 25° image field have been designed.
Because the SMPI generates instantaneous interferograms at a pupil plane, it benefits from the following attributes:
Field-Widened: The entrance slit can be widened to any width to increase the signal flux without affecting the spectral resolution. This gives it a significant advantage over grating and prism spectrometers, which must trade throughput for spectral resolution. Image plane interferometers suffer a similar fate because their modulation efficiency degrades in proportion to the slit width and fringe frequency, a phenomenon known as self-apodization.
Broadband Efficiency: As shown in Figure 1, the SMPI efficiency is nearly constant with wavelength. In contradistinction, the efficiency of a grating spectrometer is high only at the blaze wavelength, and then it diminishes rapidly. The SMPI has double the efficiency of the Michelson, Sagnac, and Wollaston prism interferometers because it utilizes all (instead of half) the incident light.
- No Stray Light Induced Spectral Errors: Stray light in the SMPI increases the noise floor but does not necessarily contribute to an erroneous spectral signal. In filter, prism, and grating spectrometers, stray light is indistinguishable from spectral signals and introduces large radiometric errors. Gratings are particularly troublesome because they behave like badly scratched mirrors. The edge of each groove, even when perfectly fabricated, scatters the incident white light directly across the spectrum.
- Radiometric Purity: When the detector array is at a pupil plane the radiance contributions from the various objects in the field are uniformly distributed across the pixels in the array. A pupil plane interferometer has the additional benefit of distributing all the colors of the spectrum uniformly across all the pixels of the array. This simplifies calibration and eliminates the radiometric errors that are routinely generated in image-plane spectrometers and filters when high radiance objects are lost in the dead zones between pixels. Responsivity variations across the active regions of pixels also contribute to radiometric errors in image-plane spectrometers, which is why they should not be used in science applications that require high spectral radiometric accuracy.
- Single Instrument Line Shape Function: There are no diffraction effects at a pupil plane, so the SMPI can be designed to have a single line shape for all colors and field positions. This greatly simplifies calibration and spectral retrievals in comparison to image-plane gratings, dispersive prisms, and filters. The line shape generated by these devices broadens with wavelength-dependent diffraction and changes with the aberration-dependent point-spread function.
- Instantaneous Interferogram: The entire interferogram is recorded instantaneously across the detector array, which eliminates recording errors. Scanning interferometers and filters that require the movement of the observational platform or an optical component are prone to irrecoverable spectral errors when the platform motion is not perfectly rectilinear or the scene changes during the scan period.
- Mechanical Stability: Unlike Michelson interferometers, the SMPI is relatively insensitive to mechanical shock, focal plane jitter, and misalignment because there are no moving parts, and because the two beams converging on the detector array are collimated. The collimation attribute relaxes the focal plane axial position tolerance: a 1-mm axial shift generates less than 0.5-percent change in the spectral line width at 5 cm-1resolution, and no change in the spectral line position. Since a typical axial position tolerance is 10 µm and the typical axial vibration amplitude of an active cooler is 1 µm, the detector array can be mounted directly onto the cold finger without concern for vibration-induced spectral errors. This significantly reduces the cooling power requirements and the complexity of the thermal-mechanical focal-plane mount.
The SMPI incorporates several important optical design characteristics that enable it to achieve high spectral resolution and high efficiency in a compact form. The telescope is designed with a shifted pupil so that the chief ray strikes the edge rather than the middle of the detector array. This shifts the zero path difference point to one side of the array and effectively doubles the maximum possible optical-path difference and spectral resolution without requiring a doubling of the pupil width.
The beam-shearing prism is designed so that the beam splitter (BS) coating on prism A is tilted less than 10° to the input beam. This prevents total internal reflection at the airgap between prisms A and B, and it eliminates the need for an oil or adhesive to fill the gap. Adhesives have strong absorption features in the thermal infrared, so their omission is desirable.
The prism configuration is governed by a requirement to maintain the same optical path length for two light beams whose chief rays must emerge parallel to each other and perpendicular to a flat output surface. When the entrance and exit surfaces are perpendicular to the chief rays, then astigmatism and dispersion are eliminated. Astigmatism reduces the spectral resolution of the interferometer, and dispersion changes the instrument line shape as a function of wavelength.
The prism is designed for minimum volume and maximum beam shear. The beam shear distance, Δ S, is proportional to the spectral resolution. A thumb-sized beam-shearing prism with a 60-mm focal length Fourier lens can achieve a spectral resolution of 1 cm-1. This is a factor of 40 reduction in volume with respect to an equivalent Sagnac interferometer. Likewise, a 0.5 cm-1 prism interferometer can improve by a factor of two the NEΔT of the Atmospheric InfraRed Sounder (AIRS) and reduce its volume by a factor of 25. AIRS is a pupil plane grating spectrometer.
The high-resolution performance of the SMPI is due in no small part to the recent advances in large format, GaAs based Quantum Well Infrared Photoconductor (QWIP) detector arrays. The SMPI requires a large array of pixels with high pixel operability and uniform responsivity, which are two unique characteristics of the QWIP arrays being developed at JPL (see Tech Briefs, Vol. 24, No. 5, p. 26a-30a). The QWIP arrays also have low 1/f noise, which increases the calibration stability of the detector array and of the interferometer.
This work was done by Francis Reininger of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Spatially Modulated Prism Interferometer
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