Laser Interferometric Motion Detection with Frequency Shift
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Solution Overview
Problem
Existing interferometric systems face challenges in efficiently detecting very small transient surface motions on optically rough surfaces subjected to ultrasound, particularly at high ultrasonic frequencies, due to noise interference and inefficiencies in signal processing, especially when using multi-channel random-quadrature interferometers.
Innovation Solution
The method involves introducing a small frequency shift, such as a Doppler shift, between the reference and object beams in a laser interferometric system, allowing for linear demodulation of signals by synchronizing detection with the applied frequency shift, which enhances signal processing and reduces noise interference, using elements like acousto-optic modulators or piezo translators to achieve this.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If signal rectification-demodulation is used in MRCQ interferometer, then the interferometer response is linear for measurements above noise level, but the detection efficiency deteriorates for ultrasonic frequencies above a few tens of MHz and for very small amplitude signals
Solution Approach 1:
The patent changes the detection parameter from rectification-demodulation to synchronous detection with frequency shift. By introducing a frequency shift (e.g., 40 kHz) and using synchronous detection, the system achieves both high precision for very small amplitudes and high efficiency for ultrasonic frequencies above tens of MHz, resolving the contradiction between measurement precision and productivity.
2Measurement precision
If rectification process is used, then the output signal is proportional to absolute value of displacement, but noise amplitude increases and degrades the rectification process especially for high frequency signals
Solution Approach 1:
The patent introduces a frequency shift as an intermediary mechanism. By shifting the signal frequency and using synchronous detection, the system separates the signal from noise more effectively. The frequency shift acts as a mediator that allows the detection system to distinguish signal from noise without the degradation caused by rectification processes.
3Measurement precision
If DC offset from noise is present, then the rectification process is degraded, but the noise amplitude increases with detection bandwidth
Solution Approach 1:
The patent employs periodic action through synchronous detection at a specific frequency shift (e.g., 40 kHz). By detecting only at this periodic frequency and rejecting other frequencies, the system maintains effective detection even in the presence of DC offset from noise. The periodic detection rhythm filters out non-synchronized noise components.
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 the detection of very small surface motions with improved sensitivity and accuracy, providing signals proportional to the surface motion and determining the direction of displacement, while maintaining a high signal-to-noise ratio across a range of ultrasonic frequencies.
Implementation Method 1
generating a laser beam
Implementation Method 2
the object beam 16 is then directed onto a scattering surface of an object 24 subjected to ultrasound using an optical lens 26. The back-scattered light 28 is collected by the lens 26, thus generating a scattered object beam 30
Implementation Method 3
The reference beam 18 and the scattered object beam 30 are then combined using a second beam splitter 33, thus forming two interference beams 34, 36
Implementation Method 4
The two interference beams 34, 36 are each received by two detector arrays 38, 39, respectively, and converted into electrical interference signals
Implementation Method 5
The processing circuit is used to carry out electronic parallel signal processing based on signal rectification, i.e., on the rectification of the amplitude of a sinusoidal signal
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 frequency shift between the reference beam and the scattered object beam, wherein the frequency shift is smaller than the ultrasonic frequency, detecting the interference between the scattered object beam and the frequency shifted 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.