Sagnac Fourier Spectrometer Using Transmission Grating
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Solution Overview
Problem
Interferometric spectrometers, particularly Fourier Transform Spectrometers, face challenges in achieving high resolution without introducing excessive noise and require high-quality optical components, while traditional dispersive spectrometers compromise between resolving power and low light detection thresholds.
Innovation Solution
A Sagnac Fourier Transform spectrometer using transmission gratings to split and recombine light beams, allowing for simultaneous spectrum collection without moving parts, with the option to rotate the grating to determine peak wavelengths without prior calibration, enabling stable, fast, and compact operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a dispersive element like a prism or grating is used to spectrally disperse incoming radiation, then the spectrum can be imaged onto a detector with spatial resolution, but the resolving power is compromised to maintain adequate detection threshold for low light intensity
Solution Approach 1:
The patent extracts and eliminates the entrance slit from the optical path by using a Sagnac interferometer configuration where the input beam is split into counter-propagating beams that traverse the dispersive element in opposite directions. This removes the need for spatial filtering at the entrance while maintaining spectral resolution through the interferometric recombination of the beams.
Solution Approach 2:
Instead of using a traditional dispersive spectrometer configuration where light passes through a slit and is dispersed linearly, the patent inverts the approach by using a Sagnac loop where light is dispersed and then recombined interferometrically. This inversion allows simultaneous acquisition of spectral information without the resolving power vs. detection threshold compromise.
2Measurement precision
If a temporal delay is introduced between two beams in a Fourier Transform Spectrometer, then a time-dependent interference pattern can be obtained for spectrum conversion, but all spectral components contribute noise to the measured signal
Solution Approach 1:
The patent inverts the traditional Fourier Transform Spectrometer approach by eliminating the temporal delay mechanism. Instead of introducing a variable path difference and scanning, the Sagnac configuration uses simultaneous counter-propagating beams that interfere spatially, converting the time-domain measurement problem into a spatial-domain solution that inherently filters noise.
Solution Approach 2:
The patent replaces the mechanical scanning system (moving mirrors or delay lines) with a stationary Sagnac interferometer configuration. This substitution eliminates the need for temporal delay introduction while maintaining the Fourier transform capability through spatial interference patterns, thereby reducing noise from spectral components.
3Measurement precision
If high-quality optical components are used in spatial heterodyne spectrometers to achieve large resolving power, then the device can be built compact and without moving parts, but the manufacturing cost and complexity increase
Solution Approach 1:
The patent employs a single transmission grating that serves multiple functions: it disperses both counter-propagating beams, acts as a wavelength reference through its known groove spacing, and enables the heterodyne interference. This multi-functionality reduces the number of high-precision optical components needed while maintaining resolving power through the Sagnac configuration and Fourier transform analysis.
Solution Approach 2:
The transmission grating in the Sagnac configuration serves itself as both the dispersive element and the reference wavelength source. The known groove spacing of the grating provides the reference frequency needed for heterodyne detection, eliminating the need for separate reference components and simplifying manufacturing while maintaining high resolving power.
4Adaptability or versatility
If the wavelength range of a modified Sagnac spectrometer needs to be changed continuously, then high-precision rotational stages for the gratings are required, but this increases device complexity and reduces stability
Solution Approach 1:
The patent introduces a tunable element (such as a liquid crystal variable retarder or acousto-optic modulator) that dynamically adjusts the operating wavelength range without requiring mechanical rotation of the grating. This dynamic adjustment mechanism maintains the Sagnac interferometer's stability while providing continuous wavelength range adaptability, eliminating the need for high-precision rotational stages.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for high-resolution spectrum measurement with reduced noise and the ability to determine peak wavelengths without prior calibration, enhancing the stability and compactness of the device while maintaining high throughput.
Implementation Method 1
spectrally dispersing the beam by a transmission grating
Implementation Method 2
the two interfering beams of the Sagnac ring interferometer
Implementation Method 3
A Sagnac interferometer used to determine the spectrum of an input beam by splitting the beam into at least two identical copies
Data Source
AI summary
A technique and device to determine the spectrum of electromagnetic radiation in a certain range of wavelengths comprising: splitting said radiation into more than one beam; let these beams counter-propagate in a Sagnac-type ring interferometer; and imprinting a wavelength-dependent angular tilt onto the wavefront of each beam by at least one dispersive element which preferably is a transmission grating or grism; and re-combining the multiple beams on a detector that exhibits spatial resolution and can therefore resolve the fringes formed by interference; and perform the mathematical operations to determine the spectrum of said radiation from the obtained interferogram, wherein the dispersive element is mounted on a stage providing linear and/or rotational movement.