Method and apparatus for multiplexed fabry-perot spectroscopy
Inventors
Yetzbacher, Michael K. • Miller, Christopher W. • Deprenger, Michael J. • Boudreau, Andrew J.
Assignees
Publication Number
US-9304040-B2
Publication Date
2016-04-05
Expiration Date
2034-05-20
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Abstract
A method of optical spectroscopy and a device for use in optical spectroscopy. The device includes a substrate, and a plurality of etalon cavities affixed to or coupled to the substrate. A signal is received from a Fabry-Perot interferometer. The signal is sampled using the device according to a generalized Nyquist-Shannon sampling criterion. The signal is sampled using the device according to a phase differential criterion for wave number resolution. An input spectrum for the signal is reconstructed based on the signal sampled according to the generalized Nyquist-Shannon sampling criterion and the signal sampled according to the phase differential criterion for wave number resolution.
Core Innovation
The invention disclosed involves a method of optical spectroscopy and a device for use in optical spectroscopy, particularly directed to robustly recovering an optical spectrum or spectra associated with a scene using Fabry-Perot transmission data. The device comprises a substrate and a plurality of etalon cavities affixed or coupled to the substrate, where the cavity thicknesses satisfy a generalized Nyquist-Shannon sampling criterion and a phase differential criterion for wavenumber resolution.
The problem addressed is the limitations of prior art spectral measurement methods such as dispersive elements, Michelson interferometers, macroscopic optical filters, mosaic array filters, and micro-optical components for imaging spectroscopy. Prior methods suffer from size, weight, spatial and spectral resolution tradeoffs, limited bandwidth, poor signal-to-noise ratios, manufacturing difficulties, and ambiguity for spectral components separated beyond one free-spectral range (FSR). Specifically, prior Fabry-Perot devices are limited by device size, bandwidth constraints, and resolution tied to cavity finesse or physical step precision.
The core innovation overcomes these challenges by using a multiplexed Fabry-Perot spectroscopy technique employing a plurality of etalon cavities with thicknesses spaced according to a generalized Nyquist-Shannon sampling criterion and covering an overall height range that meets a phase differential criterion for wavenumber resolution. This approach allows accurate reconstruction of input spectra from the Fabry-Perot interferometer signals using matrix inversion or transform methods, providing improvement in spectral resolution independent of device thickness precision. The device design allows for compact, broadband, imaging spectroscopy over the detector bandwidth minimizing noise and enabling disambiguation of spectra beyond one FSR, unlike prior art devices.
Claims Coverage
The patent includes fifteen claims with one independent claim defining the device and additional dependent claims specifying optional features. The main inventive features focus on the device structure, sampling criteria, and associated measurement methods.
Device with plurality of etalon cavities satisfying generalized Nyquist-Shannon sampling criterion and phase differential criterion for wavenumber resolution
A device comprising a substrate with a plurality of etalon cavities affixed or coupled thereto. Each etalon cavity has a respective height value, with height differences between adjacent cavities satisfying a generalized Nyquist-Shannon sampling criterion. The overall height range between maximum and minimum cavity thickness satisfies a phase differential criterion for wavenumber resolution.
Generalized Nyquist-Shannon sampling criterion for cavity step size
The sampling criterion is defined mathematically as δ ≤ 1/(4Fnσ_max), where δ is the step size between cavity thicknesses, F the cavity finesse, n the cavity refractive index, and σ_max the maximum vacuum wavenumber the detector is sensitive to.
Phase differential criterion for wavenumber resolution
The overall cavity height range d satisfies dr ≤ 1/(2FnΔσ), where Δσ is the desired vacuum wavenumber resolution, ensuring spectral resolution requirements are met over the range of cavity thicknesses.
Etalon cavity spacing
The plurality of etalon cavities may be evenly spaced or unevenly spaced in their thickness values, allowing design flexibility while satisfying the sampling criteria.
Substrate including detector or image sensor
The substrate can be a detector or image sensor, including two-dimensional focal plane arrays, point detectors, or linear arrays, enabling varied detection configurations.
Fabry-Perot cavity structure
Each etalon cavity comprises two mirrors sandwiching a dielectric material, with mirrors having flat reflective surfaces and coatings including metal or dielectric layered coatings, or reflecting boundaries due to refractive index differences.
Device cross-sectional profile and additional components
The etalon cavities may form a staircase cross-sectional profile. The device optionally includes imaging optics and scanners with configurations such as patterned illumination, diffractive optical elements, spatial light modulators, various scanner types, and detector arrays.
The claims cover a device with multiplexed Fabry-Perot etalon cavities precisely spaced per sampling and phase criteria for spectral resolution, incorporating specific physical and optical structures and optionally combined with imaging and scanning components to perform robust optical spectroscopy beyond prior art limitations.
Stated Advantages
Improvement in spectral resolution relative to Fourier Transform processing techniques, with resolution gain equal to the cavity finesse.
Increase in fringe contrast and signal-to-noise ratio relative to micro-optic devices with two-beam interference.
Disambiguation of Fabry-Perot transmission signals having spectral components separated by more than one free-spectral range of the maximum cavity retardation, enabling increased usable bandwidth.
Capability to measure optical signals over the entire bandwidth of the detector.
Instrument resolving power depends on the number of steps in the instrument rather than physically resolvable step size, reducing reliance on manufacturing precision.
Ability to tune or design the cavity finesse to optimize recovered spectral resolution and maximum frequency for particular signal-to-noise conditions.
Documented Applications
Recovery of moderate resolution (e.g., 5-500 cm⁻¹) optical spectra using compact staircase spectrometers.
Electromagnetic wave spectroscopy including X-ray, Ultraviolet, infrared, microwave, and radio-wave spectroscopy using resonant reflective cavities.
Compact, broadband imaging spectrometers for measuring spectra associated with scenes in integrating or imaging configurations.
Standalone spectrometers or line-scanning imaging spectrometers in configurations where image lines or scene components are scanned across etalon cavities.
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