Laser Intensity Noise Rejection in Interferometric Displacement Measurement

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Solution Overview

Problem

Multi-speckle interferometers face limitations in sensitivity due to laser intensity noise, which can become a dominant factor at ultrasound frequencies below a few MHz, making it difficult to achieve theoretical sensitivity limits without using expensive low-intensity lasers.

Innovation Solution

A laser interferometric apparatus and method that utilize balanced detection by generating two electrical interference signals from a reference and object beam, with one being much stronger than the other, and processing these signals to subtract intensity noise, thereby generating a displacement signal that is substantially free of intensity noise.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If balanced detection is used to reject intensity noise, then measurement precision is improved, but device complexity increases

Engineering Contradiction:

Improveinterferometer sensitivityVSAvoidoptical set-up complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The detection system is segmented into two separate detection channels, each detecting one of the two interference beams. This allows independent processing of each beam's signal, enabling noise rejection through differential detection while maintaining modular system architecture.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The signals from both detection channels are merged through electronic subtraction in the signal processing unit. This combines the information from both beams while canceling out the common intensity noise component, achieving noise rejection without requiring complex optical alignment.

Inventive Principle:

Principle #5Merging (Combining)

2Measurement precision

If specific lasers with low intensity noise are used, then measurement precision is improved, but cost increases

Engineering Contradiction:

Improveinterferometer sensitivityVSAvoidsystem cost

Core Design Contradiction:

Measurement precisionVSEase of manufacture

Solution Approach 1:

The invention converts the harmful intensity noise from standard lasers into a rejectable common-mode signal. By using the intensity noise present in both interference beams simultaneously, the system transforms what was a limiting factor into a cancelable artifact, enabling the use of cost-effective standard lasers.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The signal processing unit acts as an intermediary that mediates between the two detection channels. It performs electronic subtraction of the detected signals, serving as the mechanism that eliminates intensity noise and allows standard lasers to be used without compromising measurement precision.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Measurement precision

If a comparison beam is used in all-electronic noise rejection, then intensity noise is rejected, but laser light available for measurement is reduced

Engineering Contradiction:

Improveintensity noise rejectionVSAvoidlaser light utilization

Core Design Contradiction:

Measurement precisionVSUse of energy by moving object

Solution Approach 1:

Both interference beams are generated from the same laser source and are used simultaneously for measurement. The system merges the utilization of both beams in the differential detection scheme, ensuring that all laser light contributes to the measurement while the common intensity noise is rejected through signal processing.

Inventive Principle:

Principle #5Merging (Combining)

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 effectively rejects laser intensity noise, allowing shot noise to become the dominant source, thereby achieving the theoretical sensitivity limit of the interferometric system while maximizing the use of available laser light.

Implementation Method 1

two photo-detectors... The two electrical signals corresponding to the interference beams

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

Interferometry is a well known technique for measuring the phase difference between two or more optical beams

Methodology Applied

Scientific EffectOptical interference: Interference

Data Source

PatentUS7729881B2Laser intensity noise rejection for interferometric apparatus

Publication Date: 2010.06.01 SOUND & BRIGHT

AI summary

A laser interferometric apparatus for measuring a displacement of an object is disclosed, the apparatus comprising a laser source for producing a laser beam having a given intensity, a beam splitter for dividing the laser beam into a reference beam and an object beam to be directed to the object, thereby producing a scattered object beam being modulated according to the displacement of the object, interference means adapted to provide at least two electrical interference signals from the scattered object beam and the reference beam, the at least two electrical interference signals each comprising a wanted signal component indicative of the object displacement and an intensity noise component, and processing means for subtracting the at least two electrical interference signals, thereby generating an output signal, the output signal comprising substantially the wanted signal component alone, wherein the intensity noise is substantially rejected.