IMAGING SPECTROMETRY
Flash spectrometer has holographic grating
Kristin Lewotskyo
A computer-generated hologram (CGH) is the dispersing element in a `flash`, or instantaneous, imaging spectrometer for studying dynamic events. Conventional imaging spectrometers capture a scene over a large number of narrowband spectral channels and must scan the region of interest to generate the spatially resolved images. While this approach is adequate for static scenes, any motion within the region being imaged generates spurious spatial artifacts that further complicate the already formidable task of image cube data analysis.
Flash imaging spectrometry surmounts this difficulty by acquiring spectral and spatial data on a given scene in minimal elapsed time. Michael Descour and colleagues at the University of Arizona Optical Sciences Center (Tucson, AZ), in collaboration with engineers at the Jet Propulsion Laboratory (JPL; Pasadena, CA) have used a CGH grating disperser and computer tomographic techniques to build a nonscanning imaging spectrometer capable of capturing data from a scene within the response time of the detector.
Precisely controlled diffraction
The optical system consists of imaging foreoptics followed by a CGH disperser sandwiched between a collimating system and a reimaging lens (see figure). The scene is imaged, then spectrally dispersed by the CGH to form a rectangular array of two-dimensional (2-D) images focused onto a large-area focal-plane array. Each of these images constitutes a representation of the scene that can converted to a three-dimensional image cube by computer tomography.
The JPL team developed the CGH dispersing element, a 2-D grating consisting of multiple 8 ¥ 8 arrays of 2.5 ¥ 2.5-µm pixels; the grating period is 20 µm. The component diffracts an incident broadband signal into a 5 ¥ 5 array of diffractive orders, each with equal irradiance. The design minimizes energy diffracted into higher orders falling outside of the focal-plane array; it is also optimized to produce diffraction patterns with approximately equal irradiance across the spectral range of interest. The JPL team used electron beam lithography to fabricate the element in polymethyl methacrylate (PMMA) on a quart¥substrate.
Angular deflection of nonzero diffracted orders varies as a function of wavelength. While images at all wavelengths in the zeroth order are super imposed, the outer diffractive orders in the array consist of a spatially distributed collection of spectrally dispersed images. Once these data are captured by the focal-plane array and sent to a computer, the images can be assembled into a data cube using tomographic algorithms.
In an initial demonstration of the flash imaging spectrometer, the Arizona team captured spectral data from a color image on a computer monitor. Com parison of the directly measured and reconstructed spectral signatures of the display showed reasonably good agreement, with some variation at the high and low bounds of the visible spectral region. Future work will include development of CGH dispersers that generate higher diffractive orders than the present version, increasing spectral resolution of the system.