Pixel scale Fabry Perot filter array for chemical agent vapor detection
Fabry-Pèrot filters having different cavity thicknesses are formed in arrays on a substrate for spectral sensing. The filter array can be coupled to a detector array to make a compact, low size, weight and power spectral sensor that detects spectral information in a scene viewed by the detector array. The spectral sensor can be used in a compact spectrometer to detect the presence of harmful chemical vapors.
1 . A spectral sensor comprising:
a first substrate;
an array of optical filters formed on the first substrate, each optical filter having:
a first mirror formed on the first substrate;
a second mirror spaced apart from the first mirror and in optical communication with the first mirror; and
an etalon cavity formed between the first mirror and the second mirror;
a second substrate;
an array of long wavelength infrared (LWIR) detectors formed on the second substrate, the array of LWIR detectors in optical communication with the array of optical filters; and
an optical element, disposed between a field-of-view of the spectral sensor and the array of optical filters, to homogenize emissions from the field-of-view across all of the array of LWIR detectors in the spectral sensor and eliminate a spatial dependence of the field-of-view in a spectral domain for the spectral sensor such that the array of LWIR detectors does not image the field-of-view, wherein:
the array of optical filters includes at least 10 optical filters for which the etalon cavities of the 10 optical filters have different thicknesses such that the 10 optical filters have different transmission functions or different reflectivity functions,
the etalon cavities of the 10 optical filters having the different thicknesses are all etched from a same layer of material on the first substrate, and
the spectral sensor is assembled with the array of optical filters, the optical element, and the array of LWIR detectors in a package without Peltier cooling, the package having a size and weight to mount on an unmanned aerial vehicle weighing less than 20 pounds and to be flown by the unmanned aerial vehicle.
2 . The spectral sensor of claim 1 , wherein:
the 10 different optical filters each have a transmission function with a peak transmission wavelength and a full-width half-maximum value of the transmission function;
the peak transmission wavelengths of the 10 different optical filters are different from each other by a value that is at least 10% of the full-width half-maximum value; and
each transmission function of the 10 different optical filters partially overlaps in wavelength with another transmission function of another optical filter of the 10 different optical filters.
3 . The spectral sensor of claim 2 , wherein a partial overlap of two of the transmission functions comprises a crossing of the two transmission functions at a value no less than 10% of peak values of the two transmission functions.
4 . The spectral sensor of claim 2 , wherein the full-width half-maximum value of each transmission function is no larger than 0.5 microns.
5 . The spectral sensor of claim 1 , wherein the first substrate and the second substrate are a same substrate.
6 . The spectral sensor of claim 1 , wherein the 10 different optical filters each have a transmission function with a peak value in a range between 8 microns and 12 microns.
7 . The spectral sensor of claim 1 , wherein at least one optical filter in the array of optical filters is in optical communication with more than one detector in the array of LWIR detectors.
8 . The spectral sensor of claim 1 , wherein the array of optical filters includes at least two optical filters that have a same transmission function or a same reflectivity function and the 10 different optical filters are located randomly in the array of optical filters.
9 . The spectral sensor of claim 1 , wherein the optical element comprises a random phase plate, a diffusor, a fly's-eye lens, micro lenses, prismatic sheets, or some combination thereof.
10 . The spectral sensor of claim 1 , wherein the same layer of material is formed from germanium.
11 . The spectral sensor of claim 1 , wherein the first mirror is a multilayer Bragg mirror.
12 . A method of making a spectral sensor, the method comprising:
illuminating a grayscale photomask to expose a photoresist and pattern an array of etalon cavities for Fabry-Pèrot filters, wherein the photoresist is deposited over an etalon layer on a substrate;
developing the photoresist such that the photoresist has at least 10 different thicknesses over the etalon layer;
etching the photoresist and at least a portion of the etalon layer to pattern the array of etalon cavities, wherein the array of etalon cavities includes at least 10 etalon cavities having different thicknesses;
forming an array of the Fabry-Perot filters from the array of etalons;
disposing the array of Fabry-Perot filters adjacent to an array of LWIR detectors;
disposing an optical element between a field-of-view of the spectral sensor and the array of etalon cavities, wherein the optical element homogenizes emissions from the field-of-view across all of an array of LWIR detectors for the spectral sensor and eliminates a spatial dependence of the field-of-view in a spectral domain for the spectral sensor such that the array of LWIR detectors does not image the field-of-view; and
assembling the optical element, the array of Fabry-Perot filters, and the array of LWIR detectors in a package without Peltier cooling, the package having a size and weight to mount on an unmanned aerial vehicle weighing less than 20 pounds and to be flown by the unmanned aerial vehicle.
13 . The method of claim 12 , wherein the etching is a single processing step that etches both the photoresist and the etalon layer.
14 . The method of claim 12 , further comprising:
depositing a mirror over the array of etalon cavities to form the array of Fabry-Perot filters that comprises 10 different optical filters, wherein
the 10 different optical filters each have a transmission function with a peak transmission wavelength and a full-width half-maximum value of the transmission function;
the peak transmission wavelengths of the 10 different optical filters are different from each other by a value that is at least 10% of the full-width half-maximum value; and
each transmission function of the 10 different optical filters partially overlaps in wavelength with another transmission function of another optical filter of the 10 different optical filters.
15 . The method of claim 14 , further comprising:
optically coupling the array of Fabry-Perot filters to the array of LWIR detectors.