Multi-Channel Laser Interferometry for Ultrasonic Motion Detection
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Solution Overview
Problem
Conventional laser interferometric techniques for detecting ultrasonic motion on optically rough surfaces have low sensitivity due to the requirement for accurate object positioning and are limited by the random phase distribution of speckles, which reduces interference efficiency and depth-of-field.
Innovation Solution
A multi-channel laser interferometric method and apparatus that generates a laser beam with a large collecting aperture, using a beam splitter to create a reference and probe beam, focusing the probe beam onto the surface, expanding the reference beam, and combining it with the scattered probe beam to generate optical fringe signals, which are then processed using various signal processing techniques to extract motion information, including filtering, squaring, rectifying, and heterodyne demodulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional laser interferometric techniques are used with plane wave reference, then interference efficiency is improved, but sensitivity on rough surfaces deteriorates due to single speckle selection requirement
Solution Approach 1:
The detector array divides the speckle field into multiple independent detection channels, each detecting a portion of the optical fringe signal. This segmentation allows simultaneous collection of multiple speckles while maintaining interference efficiency, resolving the contradiction between single-speckle interference quality and multi-speckle light gathering capability.
2Use of energy by moving object
If collecting aperture is increased to gather more light, then light gathering efficiency is improved, but sensitivity deteriorates due to random phase distribution of speckles
Solution Approach 1:
The large collecting aperture is combined with a segmented detector array where each element processes a portion of the speckle field independently. The signals are then coherently summed to reconstruct the interference pattern, maintaining sensitivity while utilizing all collected light energy effectively.
Solution Approach 2:
The invention transitions from single-point detection to spatially distributed detection across a two-dimensional detector array. This dimensional expansion allows simultaneous collection of multiple speckles across the speckle field while maintaining phase information through coherent summation, resolving the sensitivity-light gathering tradeoff.
3Ease of operation
If system is detuned for uniform performance over larger range of sample positions, then ease of operation is improved, but maximum sensitivity deteriorates
Solution Approach 1:
The detector array segments the optical fringe signal spatially, allowing the system to maintain high sensitivity at the focal point while accepting reduced performance at other positions. This segmentation enables the system to operate effectively without requiring precise positioning, as the array collectively captures the interference pattern across a range of positions.
Solution Approach 2:
The multi-channel detector array automatically adapts to variations in sample position by collecting interference signals across multiple spatial channels. The system self-adjusts to maintain measurement capability without requiring external positioning control, achieving both ease of operation and maintained sensitivity.
4Ease of operation
If depth-of-field is increased to reduce positioning requirements, then ease of operation is improved, but sensitivity deteriorates due to reduction by factor two from maximum sensitivity
Solution Approach 1:
The detector array segments the focal spot into multiple detection elements, allowing the system to maintain high sensitivity at the focal point while accepting reduced performance at defocused positions. This segmentation enables the system to achieve adequate depth-of-field without sacrificing maximum sensitivity, as the array collectively captures interference information across the depth range.
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 enhances sensitivity and depth-of-field, allowing for high-sensitivity detection of ultrasonic motion on rough surfaces without the need for precise object positioning, effectively integrating over large speckle fields and improving light gathering efficiency.
Implementation Method 1
dividing the laser beam into a reference beam and a probe beam
Implementation Method 2
passing the probe beam through an optical lens to focalize the probe beam onto the surface of the material
Implementation Method 3
combining the scattered probe beam collected by said optical lens with said expanded reference beam to obtain an optical fringe signal
Implementation Method 4
receiving said optical fringe signal on at least one array of photodetectors and converting said optical fringe signal into an electrical signal
Data Source
AI summary
A multi-channel laser interferometric method and apparatus are provided for optically measuring transient motion from a surface (17). A laser beam (11) is generated and then divided into first and second beams having respective intensities representing minor and major fraction of the predetermined laser intensity. The reference beam (18) illuminates the surface (17) at which deformation is expected. The light back-scattered by the surface is collected by a single aperture lens (15) and then made to interfere with the probe beam (67) which has been expanded (32), onto a two-dimensional array of detectors (71). Each signal (83) corresponding to each detector of the array is converted individually to an electrical signal, each electrical signal is amplified and processed (84), and the plurality of processed signals (85) is then averaged in an electrical summing means (45).