US 4,789,240 A · Wavelength switched passive interferometric sensor system

Abstract

This invention includes a fiber optic interferometer having a reference arm isolated from a physical parameter being monitored and a sensing arm exposed to the physical parameter being monitored. The sensing arm produces changes in the relative phase of the optical signals propagating in the sensing and reference arms in response to changes in the physical parameter. The sensing and reference signals are combined to produce an interference pattern after the sensing signal has been exposed to the physical parameter being monitored. A frequency shifter changes the frequency of the optical signal input to the interferometer, which produces interference signals at two frequencies. The optical path difference of the sensing and reference arms for the two frequencies produces phase changes that are in quadrature, which permits the interference signal for one frequency to be expressed as a function of the cosine of the phase change and the interference signal for the other frequency to be expressed as a function of the sine of the phase change. A demodulation algorithm uses data from the two interference signals to calculate either the tangent or cotangent of the phase angle to determine the phase change in the sensing signal caused by changes in the parameter.

Claims29

Dependent claims

A sensor system for sensing changes in a physical parameter, comprising:

an interferometer, said interferometer including a reference arm for propagating a reference signal, said reference arm being isolated from the physical parameter, said interferometer including a sensing arm for propagating a sensing signal, said sensing arm being exposed to the physical parameter, said sensing signal having an initial phase relation to said reference signal, said sensing arm producing a phase change in said sensing signal relative to said reference signal in response to changes in the physical parameter;
means for supplying a sequence of optical pulses for input to said reference arm and said sensing arm of said interferometer, said means including an input optical fiber;
means for shifting the frequency of said optical pulses such that each said sequence includes a first pulse portion having a first optical frequency and a second pulse portion having a second optical frequency, said first and second pulse portions producing first and second phase differences between said sensing signal and said reference signal, said first and second phase differences being in quadrature with one another;
and
means for processing said first and second pulse portions to determine the change in phase between the sensing signal and the reference signal to measure changes in the physical parameter.

2 more elements

A method for sensing changes in a physical parameter, comprising the steps of:

coupling a reference signal into a reference arm of an interferometer;
isolating the reference arm from the physical parameter;
coupling a sensing signal into a sensing arm of the interferometer such that the sensing signal has an initial phase relation to said reference signal;
exposing the sensing arm to the physical parameter to produce a phase change in the sensing signal relative to the reference signal in response to changes in the physical parameter;
supplying a sequence of optical pulses for input to the reference arm and the sensing arm of said interferometer;
shifting the frequency of the optical pulses such that each sequence includes a first pulse portion having a first optical frequency and a second pulse portion having a second optical frequency, the first and second pulse portions producing first and second phase differences between said sensing signal and said reference signal so that the first and second phase differences are in quadrature with one another;
and
processing said first and second pulse portions to determine the change in phase between the sensing signal and the reference signal to measure changes in the physical parameter.

5 more elements

Dependencies are inferred from the claim text.

Wavelength switched passive interferometric sensor system

Life of this patent

Wavelength switched passive interferometric sensor system

Life of this patent

Abstract

Claims29