Wavelength switched passive interferometric sensor system

Claim 2

A sensor system for sensing changes in a physical parameter, comprising: an interferometer, the interferometer including a reference optical fiber for propagating a reference signal, the reference optical fiber being isolated from the physical parameter, the interferometer including a sensing optical fiber for propagating a sensing signal, the sensing optical fiber being exposed to the physical parameter, the sensing signal having an initial phase relation to the reference signal, the sensing optical fiber producing a phase change in the sensing signal relative to the reference signal in response to changes in the physical parameter; a light source that supplies a sequence of optical pulses for input to the reference optical fiber and the sensing optical fiber of the interferometer; a frequency shifter for shifting the frequency of the optical pulses such that each the 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 the sensing signal and the reference signal, the first and second phase differences being in quadrature with one another; and signal processing apparatus for determining the change in phase between the sensing signal and the reference signal to measure changes in the physical parameter.

Claim 3

The sensor system of claim 1 wherein the frequency shifter forms each optical pulse to have a pair of portions having equal magnitudes.

Claim 4

The sensor system according to claim 2 wherein the signal processing apparatus comprises: a first signal storage device for storing a first signal [1] indicative of the reference signal intensity; a second signal storage device for storing a second signal [2] dependent upon the reference signal intensity, the sensing signal intensity, and the cosine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for an optical signal having the first optical frequency; a third signal storage device for storing a third signal [31 indicative of interference between a portion of the reference signal having the second optical frequency and a portion of the sensing signal having the first optical frequency; a fourth signal storage device for storing a fourth signal [4] dependent upon the reference signal intensity, the sensing signal intensity, and the sine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for an optical signal having the second optical frequency; a fifth signal storage device for storing a fifth signal [5] indicative of the sensing signal intensity; and computing apparatus for processing the signals [1]-[5] to determine the phase angle between the sensing and reference signals by computing the tangent of the phase angle according to the equation tan θ = {[4] - [5] - [1]{[2] - [5] - [1]} -1 . where [1], [2], [4] and [5] are the first, second, fourth and fifth signals, respectively.

Claim 5

A sensing system according to claim 1 including apparatus for separating the first and second pulse portions by a predetermined time interval and wherein the means for determining the change in phase between the sensing signal and the reference signal to measure changes in the physical parameter comprises: a sixth signal storage device for storing a sixth signal [6] indicative of the reference signal intensity for the first pulse portion; a seventh signal storage device for storing a seventh signal [7] dependent upon the reference signal intensity, the sensing signal intensity, and the cosine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for the first pulse portion; an eighth signal storage device for storing an eighth signal [8] indicative of the sensing signal intensity for the first pulse portion; a ninth signal storage devicefor storing a ninth signal indicative of the reference signal intensity for the second pulse portion; a tenth signal storage device for storing a tenth signal [10] dependent upon the reference signal intensity, the sensing signal intensity, and the sine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for an optical signal having the second optical frequency; an eleventh signal storage devicefor storing an eleventh signal [11] indicative of the sensing signal intensity for the second pulse portion; and computing apparatus for processing the signals to determine the phase angle between the sensing and reference signals [6]-[11] by computing the tangent of the phase angle according to the equation tan 0 = {[10]- [11] - [9]}{[8] [6]} 1/2 {[7] - [8] - [6]} -1 {[11] [9]} -1/2 , where [6], [7], [8], [9], [10] and [11] are the sixth through eleventh signals, respectively.

Claim 6

The sensor system of claim 12 further including: a first fiber optic evanescent field coupler for supplying the sensing and reference signals to the sensing optical fiber and reference optical fibers, respectively, one of the sensing and reference optical fibers forming an input port and a first output port of the first fiber optic evanescent field coupler, the other of the sensing and reference optical fibers forming a second output port of the first fiber optic evanescent field coupler, the first fiber optic evanescent field coupler being formed to divide an optical signal input to the input port between the first and second output ports; and a second fiber optic evanescent field coupler connected to the reference and sensing optical fibers for recombing the sensing and reference signals to produce an interference signal.

Claim 9

The method of claim 6 further including the step of forming the first and second pulse portions to have equal amplitudes and wherein the step of determining the change in phase between the sensing signal and the reference signal to measure changes in the physical parameter comprises the steps of: storing a first signal [1] indicative of the reference signal intensity; storing a second signal [2] dependent upon the reference signal intensity, the sensing signal intensity, and the cosine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for an optical signal having the first optical frequency; determining the duration of a third signal [3] indicative of interference between a portion of the reference signal having the second optical frequency and a portion of the sensing signal having the first optical frequency; storing a fourth signal [4] dependent upon the reference signal intensity, the sensing signal intensity, and the sine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for an optical signal having the second optical frequency; storing a fifth signal [5] indicative of the sensing signal intensity; and processing the signals to determine the phase angle between the sensing and reference signals by computing the tangent of the phase angle according to the equation. tan 0 = {14] - [5] - [1]}{[2] - [5] - [1]} -1 , where [1], [2], [4] and [5] are the first, second, fourth and fifth signals, respectively.

Claim 10

The method of claim 23, including the step of forming the first and second pulse portions to be sepaated by a predetermined time interval and wherein the step of determining the change in phase between the sensing signal and the reference signal to measure changes in the physical parameter comprises the steps of: storing a sixth signal [6] indicative of the reference signal intensity for the first pulse portion; storing a seventh [7] signal dependent upon the reference signal intensity, the sensing signal intensity, and the cosine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for the first pulse portion; storing an eighth [8] signal indicative of the sensing signal intensity for the first pulse portion; storing a ninth [9] signal indicative of the reference signal intensity for the second pulse portion; storing a tenth [10] signal dependent upon the reference signal intensity, the sensing signal intensity, and the sine of the phase angle between the sensing and reference signals for interference between the reference signal and the sensing signal for an optical signal having the second optical frequency; storing an eleventh signal [11] indicative of the sensing signal intensity for the second pulse portion; and processing the signals to determine the phase angle between the sensing and reference signals by computing the tangent of the phase angle according to the equation tan 0 = {[10] - [11 ] - [9]}{[8] [6]} 1/2 {[7] - [8] - [6]} -1 {[11 ] [9]} -1/2 , where [6], [7]. [8], [9], [10] and [11] are the sixth through eleventh signals, respectively.

Claim 11

The method of claim 8 including the steps of: forming the reference arm of a reference optical fiber; and forming the sensing arm of a sensing optical fiber.

Claim 12

The method of claim 9, further including the steps of: supplying the sensing and reference signals to the sensing optical fiber and the reference optical fiber with a first fiber optic evanescent field coupler; and combining the reference and sensing signals in a second fiber optic evanescent field coupler to produce an interference signal.

Claim 13

The method of claim 10 further comprising the steps of: transmitting optical signals to a plurality of interferometers with a transmit fiber; coupling signals output from the plurality of interferometers to a receive fiber; and detecting the signals output from the plurality of interferometers.