Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1993-10-12
1996-02-20
McGraw, Vincent P.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356334, G01J 320, G01J 336
Patent
active
054933935
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to spectrographs and, in particular, to a compact spectrograph that uses a planar waveguide. The spectrograph of the invention has particular utility in wavelength division multiplexed optical sensing systems.
BACKGROUND OF THE INVENTION
A number of systems have been developed for multiplexing optical fibercoupled transducers. Such systems include optical time division multiplexing, coherence multiplexing, and wavelength division multiplexing (WDM). Although all three systems have been demonstrated, WDM appears to offer the most promise for near term implementation in aircraft and other complex systems. In a WDM system, discrete spectral wavelength ranges or bands propagating along a fiber bus are modulated by one or,more of the sensors or transducers. A crucial component of a WDM system is a demultiplexer/detector, capable of receiving a broadband optical signal and of detecting the optical energy in different wavelength bands.
SUMMARY OF THE INVENTION
The present invention provides a spectrograph that is well suited to serve as a demultiplexer/detector in a WDM system. The spectrograph receives an optical input signal, and detects the optical energy of the input signal in a plurality of different wavelength ranges.
In a preferred embodiment the spectrograph of the present invention comprises a planar waveguide and a detector array. The planar waveguide has a plurality of side edges extending between upper and lower faces. The side edges include a dispersive edge having an inwardly concave shape, an input edge, and a straight output edge. The dispersive edge has a reflective diffraction grating formed on it, the grating comprising a plurality of lines with a variable line spacing. The line spacing and the positions of the input and output edges are selected such that when the optical input signal is introduced into the waveguide at the input edge, the input signal travels through the waveguide and strikes the grating. The grating focuses the optical energy in each of the wavelength ranges of the input signal at a focal spot at the output edge, with the position of each focal spot being a function of wavelength.
The detector array comprises a plurality of photodetectors positioned along a straight line. The detector array is positioned such that the photodetectors are positioned at the respective focal spots, so that each photodetector (or group of adjacent photodetectors) detects the optical energy in a corresponding one of the wavelength ranges. In a preferred embodiment, the waveguide comprises a unitary sheet of material, and the grating comprises grooves formed on the dispersive edge. The grooves may be formed mechanically, by mechanical replication, or by selectively etching the dispersive edge using radiation from a pair of coherent illumination points.
In a second preferred embodiment, a plurality of waveguides of the type described above are stacked one upon the other to form a multi-channel spectrograph. The multi-channel spectrograph receives a plurality of optical input signals, and detects the optical energy of each input signal in a plurality of different wavelength ranges. The output edges are preferably positioned in a plane, so that the stack of waveguides can be directly interfaced to a two-dimensional detector array such as a CCD array. Adjacent waveguides may be isolated from one another by plates, or by thin films deposited on the upper and lower waveguide surfaces, to maintain total internal reflection. Total internal reflection may also be maintained by lowering the refractive index of the waveguide near its surfaces, for example, by ion-diffusion.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B schematically illustrate the operation of Rowland spectrometers;
FIG. 2 illustrates a prior art demultiplexer using a slab waveguide;
FIG. 3 is a second schematic view of the demultiplexer of FIG. 2;
FIG. 4 is a perspective drawing of the spectrograph of the present invention;
FIG. 5 is a graph illustrating the geometry of the s
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Beranek Mark W.
Capron Barbara A.
Griffith David M.
Huggins Raymond W.
Livezy Darrell L.
McGraw Vincent P.
The Boeing Company
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