Laser Interferometer Phase Modulation for Ultrasonic Motion Detection
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Solution Overview
Problem
Existing laser interferometric methods for detecting small transient surface motion on optically rough surfaces face limitations in signal processing, particularly near noise levels and at high ultrasonic frequencies, where rectification-based demodulation is inefficient and requires continuous ramp excitation with piezo-mirrors, limiting sensitivity and continuity of detection.
Innovation Solution
A method and apparatus using small-amplitude phase modulation in the optical path difference between reference and scattered beams, synchronized with detection, to generate electrical interference signals that allow for linear demodulation and continuous detection of surface motion, utilizing a piezo translator or phase modulator to introduce a sinusoidal displacement, and processing circuits to correct the sign of the interferometer transfer function.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If rectification-based demodulation is used for detecting small transient surface motion, then the detection can be performed on optically rough surfaces, but the sensitivity deteriorates near noise levels and at high ultrasonic frequencies
Solution Approach 1:
The patent changes the demodulation parameter from rectification-based to synchronous detection with phase modulation. By introducing a small sinusoidal phase modulation and using synchronous detection referenced to this modulation frequency, the system achieves linear demodulation that maintains sensitivity near noise levels and at high ultrasonic frequencies, while still being applicable to optically rough surfaces.
2Device complexity
If signal rectification is used for demodulation, then the processing can be performed with simple circuits, but the efficiency deteriorates for ultrasonic frequencies above a few tens of MHz
Solution Approach 1:
The patent changes the detection parameter from direct rectification to synchronous detection at a modulation frequency. By modulating the optical path difference at a known frequency and detecting signals synchronously at this frequency, the system achieves efficient demodulation at high ultrasonic frequencies (above tens of MHz) while maintaining reasonable circuit complexity through the use of phase-locked loops and synchronous detectors.
3Measurement precision
If piezo-mirror with ramp excitation is used for frequency shifting, then linear demodulation can be achieved, but the detection continuity deteriorates due to required reset excitation
Solution Approach 1:
The patent replaces the continuous ramp excitation with periodic sinusoidal excitation of the piezo-mirror. The mirror is oscillated at a sinusoidal frequency, creating a periodic modulation of the optical path difference. Synchronous detection referenced to this periodic modulation enables continuous detection without requiring reset excitation, maintaining both linear demodulation accuracy and detection continuity.
Solution Approach 2:
The patent introduces a sinusoidal phase modulation as an intermediary signal between the piezo-mirror and the detection system. This intermediary modulation frequency serves as a reference for synchronous detection, enabling continuous linear demodulation without the need for continuous ramp excitation and reset operations.
4Measurement precision
If Doppler shift with frequency shift is used for linear demodulation, then high sensitivity is achieved, but the system requires the Doppler frequency to be well below the ultrasonic signal bandwidth
Solution Approach 1:
The patent changes the frequency shift mechanism from Doppler shift to direct sinusoidal phase modulation of the optical path difference. By modulating the reference beam or object beam at a known sinusoidal frequency, the system achieves linear demodulation with high sensitivity without constraining the ultrasonic frequency range, as the modulation frequency is independently controllable and does not depend on the ultrasonic signal frequency.
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 enables near-ideal sensitivity for detecting very small signals, provides direction information on surface displacement, and supports a wide range of ultrasonic frequencies with continuous detection, reducing noise and harmonic distortion.
Implementation Method 1
Interferometry is a well known technique for measuring the phase difference between two or more optical beams
Implementation Method 2
utilizing a piezo translator or phase modulator to introduce a sinusoidal displacement
Implementation Method 3
Each detector element of the array is optimized for single-speckle detection
Data Source
AI summary
An apparatus and a method for detecting surface motion of an object subject to ultrasound are disclosed. The method comprises generating a laser beam, dividing the laser beam into a reference beam and an object beam to be directed onto the surface, thereby producing a scattered object beam, introducing a small-amplitude modulation in the optical path difference between the reference beam and the scattered object beam, detecting the interference between the scattered object beam and the phase modulated reference beam using a plurality of detecting elements to generate a plurality of electrical interference signals, wherein the electrical interference signals each comprise a wanted signal component indicative of the surface motion and a noise signal component, and processing the electrical interference signals to determine the surface motion of the object.