This unit would feature coarse and fine resolution along two orthogonal axes.
The figure is a simplified depiction of a proposed spectrometer optical unit that would be suitable for incorporation into a remote-sensing instrumentation system. Relative to prior spectrometer optical assemblies, this unit would be compact and simple, largely by virtue of its predominantly two-dimensional character.
The proposed unit would be a combination of two optical components. One component would be an arrayed-waveguide grating (AWG) — an integratedoptics device, developed for use in wavelength multiplexing in telecommunications. The other component would be a diffraction grating superimposed on part of the AWG.
Notwithstanding the conceptual simplicity as described thus far, the function is complicated by the fact that the response of each output waveguide is characterized by a spectral periodicity with multiple frequency components spaced at multiples of the free spectral range appearing in each frequency bin. Hence, in the absence of a corrective measure, each output waveguide would carry multiple wavelength components, resulting in an ambiguous output.
In the proposed device, the degeneracy would be broken by means of the diffraction grating, which would be lithographically formed on the surface in the output waveguide region. The grating lines would cross the output waveguides, establishing orthogonal coordinate axes. One axis would represent coarse spectral resolution; the other, fine spectral resolution. The net result of superimposing the grating on the output waveguides would be to divert some of the light from waveguide modes to free-space-propagating modes. Because the output diffraction angle of each mode would depend on its wavelength, the output waves propagating in free space would be sorted with coarse spectral resolution along one coordinate axis and fine spectral resolution along the other axis. The proposed unit could be designed, in conjunction with a planar photodetector array, to obtain an optimal match between the array pixel pattern and the wavelength-dispersion pattern.
This work was done by John Hong of Caltech for NASA’s Jet Propulsion Laboratory.
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