Off-axis holography spatial demodulation for interferometric sensing
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Solution Overview
Problem
Existing optical sensing systems using digital cameras for phase measurement face a trade-off between noise reduction and bandwidth, with phase and frequency modulation methods limiting detection bandwidth and being sensitive to Doppler noise, whereas spatial demodulation using off-axis holography offers reduced incoherent noise and increased bandwidth without the need for carrier frequencies.
Innovation Solution
The use of off-axis holography in optical sensors, where a camera captures interference patterns between a reference and interrogation beam at different angles, enabling spatial demodulation to extract optical phase information, thereby maximizing camera bandwidth and reducing noise through pixel averaging, while being less sensitive to Doppler noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase or frequency modulation is used to extract optical phase with a camera, then the detection bandwidth is reduced, but the incoherent noise reduction is achieved through pixel averaging
Solution Approach 1:
The patent transitions from temporal modulation (phase/frequency modulation in time domain) to spatial demodulation (interference fringes in space domain). By tilting the reference beam to create spatial interference fringes across the camera sensor, the system extracts phase information from the spatial domain rather than requiring temporal modulation, thereby achieving both noise reduction through pixel averaging and maximum bandwidth utilization.
2Measurement precision
If a carrier frequency is added for phase or frequency modulation, then the optical phase can be extracted, but the highest measurable signal bandwidth is limited to 1/16th of the original detector bandwidth
Solution Approach 1:
The patent extracts the carrier frequency requirement entirely from the system by using spatial demodulation. Instead of adding a temporal carrier frequency through phase or frequency modulation, the system uses the spatial interference pattern created by tilted beams to encode phase information directly in the spatial domain, eliminating the need for carrier frequencies and their associated bandwidth limitations.
3Measurement precision
If phase or frequency modulation is used, then optical phase measurement is enabled, but the system becomes sensitive to Doppler noise
Solution Approach 1:
By moving from temporal modulation to spatial demodulation, the patent changes the domain in which phase measurement occurs. The spatial interference fringes are created by the geometric arrangement of tilted beams rather than temporal modulation, making the measurement insensitive to Doppler shifts that affect temporal frequency-based methods.
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 achieves significant noise reduction (up to 60 dB) and maintains high-frequency signal detection capabilities, outperforming traditional phase and frequency modulation methods by utilizing the full camera bandwidth and minimizing signal degradation from low-frequency motion.
Implementation Method 1
a beam splitter configured to split the laser beam into a reference beam and an interrogation beam
Implementation Method 2
the optical sensing element is configured to convert a desired signal to a change in the optical path of the interrogation beam
Implementation Method 3
the interrogation beam is incident upon the sensing surface of the camera at a first angle and the reference beam is incident upon the sensing surface of the camera at a second angle different from the first angle, thereby creating an interference pattern at the sensing surface
Implementation Method 4
Exemplary embodiments use off-axis holography which enables single-shot phase measurements by measuring the interference fringes formed between a reference beam and the interrogation beam
Data Source
AI summary
A method of interferometric optical sensing via spatial demodulation includes emitting a laser beam; splitting the laser beam into a reference beam and an interrogation beam; converting a desired signal into a change in the optical path of the interrogation beam via an optical sensor; and capturing the reference beam and the interrogation beam via a camera, wherein the interrogation beam is incident to the camera at a first angle and the reference beam is incident to the camera at a second angle different from the first angle, thereby causing an interference pattern at the camera.