Natural fiber span reflectometer providing a virtual differential signal sensing array capability
First Claim
1. A time-domain reflectometer for sensing at a desired set of n spaced sensing positions along an optical fiber span, said sensing positions being for sensing a type of external physical signal having the property of inducing light path changes within the optical fiber span at regions there along where the signal is coupled to the span, comprising:
- an optical fiber span having a first end which concurrently serves as both the interrogation signal input end and the back propagating signal output end for purposes of reflectometry, and having a second remote end;
a first light source for producing a coherent carrier lightwave signal of a first predetermined wavelength;
a binary pseudonoise code sequence modulator modulating said carrier signal for producing a pseudonoise code sequence modulated interrogation lightwave signal which continuously reiterates the binary pseudonoise code sequence, the reiterated sequences being executed in a fixed relationship to a predetermined timing base;
a lightwave heterodyner having first and second inputs for receiving a primary signal and a local oscillator signal, respectively, and operative to produce the beat frequencies of their respective frequencies;
a lightwave directional coupler having a first port which receives said binary pseudonoise code sequence modulated interrogation lightwave, a second port coupled to said first end of said optical fiber span, and a third port coupled to said primary signal input of the heterodyner;
said directional coupler coupling said binary pseudonoise code sequence modulated interrogation lightwave to said second port where it is launched in a forwardly propagating direction along said optical fiber span causing the return to said second port of a composite back-propagating lightwave which is a summation of lightwave back-propagations from a continuum of locations along the length of the span, said composite back-propagating lightwave signal comprising a summation of multiple components including a first signal component comprising the summation of portions of the said pseudonoise code sequence modulated interrogation lightwave signal which the innate properties of the optical fiber cause to backpropagate at a continuum of locations along the span, and a second signal component comprising the modulation of said first signal component caused by longitudinal components of optical path changes induced into said span at a continuum of locations along said span by external physical signals, said second signal component further including a corresponding set of subcomponents comprising the modulation of said first signal component by optical path changes caused by said external signals at the respective sensing positions;
said directional coupler coupling said composite back-propagating lightwave to said third port where it is applied to said first input of the heterodyner;
a second light source coupled to said second input of the lightwave heterodyner, said second light source producing a coherent local oscillator lightwave signal in phase locked relation to said carrier lightwave signal, said local oscillator signal being of a second predetermined wavelength which differs from the first predetermined wavelength by an amount of difference small enough to produce at the output of the heterodyner a radio frequency (r.f.) composite difference beat signal, but by an amount large enough to cause said r.f. composite difference beat signal to have sufficient bandwidth to cause it to include r.f. counterparts of signal components and subcomponents of said composite back propagating lightwave signal;
said r.f. composite difference beat frequency signal being coupled to an n-way splitter providing a corresponding set of n output channels, each transmitting said r.f. composite difference beat signal;
a corresponding set of n correlation-type binary pseudonoise code sequence demodulators having their respective inputs connected to the corresponding output channels of said n-way splitter through a corresponding set of time delay circuits which respectively provide a corresponding set of predetermined time delays in relation to said predetermined timing base of the binary pseudonoise code sequence modulator, to establish said n desired sensing positions along said optical fiber span;
said set of correlation-type binary pseudonoise code sequence demodulators serving to conjunctively temporally and spatially de-multiplex said r.f. composite difference beat signal to provide at their respective outputs r.f. counterparts of the subcomponents of said second signal component of said composite back-propagating lightwave signal caused by changes in the optical path within said optical fiber span induced by external physical signals respectively coupled to the corresponding sensing positions;
a corresponding set of n phase demodulators for transforming each respective r.f. counterpart of said second component into a substantially linear signal representative of radian phase of the corresponding lightwave subcomponent signal; and
at least one subtracter circuit having a first and a second input for respectively receiving outputs from a selected first and a selected second of said set of n phase demodulators, to produce a differential phase signal which is representative of the difference between the radian phases of the r.f. counterparts of two subcomponents of said second component of the composite back-propagating lightwave signal, which two subcomponents are caused by external signals at sensing positions along said optical fiber span established by the time delay circuits respectively associated with said selected first and said selected second phase demodulators.
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Accused Products
Abstract
A CW lightwave modulated by a continuously reiterated pseudorandom (PN) code sequence is launched into an end of a span of ordinary optical fiber cable. Portions of the launched lightwave back propagate to the launch end from a continuum of locations along the span because of innate fiber properties including Rayleigh scattering. This is picked off the launch end and heterodyned producing a r.f. beat signal. The r.f. beat signal is processed by a plurality (which can be thousands) of correlator pseudonoise code sequence demodulation and phase demodulator units operated in different delay time relationships to the timing base of the reiterated modulation sequences. Pairs of outputs of the units are connected to respective substractor circuits, each providing a signal representative of a differential signal between acoustic, or other forms of, signals incident the bounds of virtual increments of the span.
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Citations
24 Claims
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1. A time-domain reflectometer for sensing at a desired set of n spaced sensing positions along an optical fiber span, said sensing positions being for sensing a type of external physical signal having the property of inducing light path changes within the optical fiber span at regions there along where the signal is coupled to the span, comprising:
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an optical fiber span having a first end which concurrently serves as both the interrogation signal input end and the back propagating signal output end for purposes of reflectometry, and having a second remote end;
a first light source for producing a coherent carrier lightwave signal of a first predetermined wavelength;
a binary pseudonoise code sequence modulator modulating said carrier signal for producing a pseudonoise code sequence modulated interrogation lightwave signal which continuously reiterates the binary pseudonoise code sequence, the reiterated sequences being executed in a fixed relationship to a predetermined timing base;
a lightwave heterodyner having first and second inputs for receiving a primary signal and a local oscillator signal, respectively, and operative to produce the beat frequencies of their respective frequencies;
a lightwave directional coupler having a first port which receives said binary pseudonoise code sequence modulated interrogation lightwave, a second port coupled to said first end of said optical fiber span, and a third port coupled to said primary signal input of the heterodyner;
said directional coupler coupling said binary pseudonoise code sequence modulated interrogation lightwave to said second port where it is launched in a forwardly propagating direction along said optical fiber span causing the return to said second port of a composite back-propagating lightwave which is a summation of lightwave back-propagations from a continuum of locations along the length of the span, said composite back-propagating lightwave signal comprising a summation of multiple components including a first signal component comprising the summation of portions of the said pseudonoise code sequence modulated interrogation lightwave signal which the innate properties of the optical fiber cause to backpropagate at a continuum of locations along the span, and a second signal component comprising the modulation of said first signal component caused by longitudinal components of optical path changes induced into said span at a continuum of locations along said span by external physical signals, said second signal component further including a corresponding set of subcomponents comprising the modulation of said first signal component by optical path changes caused by said external signals at the respective sensing positions;
said directional coupler coupling said composite back-propagating lightwave to said third port where it is applied to said first input of the heterodyner;
a second light source coupled to said second input of the lightwave heterodyner, said second light source producing a coherent local oscillator lightwave signal in phase locked relation to said carrier lightwave signal, said local oscillator signal being of a second predetermined wavelength which differs from the first predetermined wavelength by an amount of difference small enough to produce at the output of the heterodyner a radio frequency (r.f.) composite difference beat signal, but by an amount large enough to cause said r.f. composite difference beat signal to have sufficient bandwidth to cause it to include r.f. counterparts of signal components and subcomponents of said composite back propagating lightwave signal;
said r.f. composite difference beat frequency signal being coupled to an n-way splitter providing a corresponding set of n output channels, each transmitting said r.f. composite difference beat signal;
a corresponding set of n correlation-type binary pseudonoise code sequence demodulators having their respective inputs connected to the corresponding output channels of said n-way splitter through a corresponding set of time delay circuits which respectively provide a corresponding set of predetermined time delays in relation to said predetermined timing base of the binary pseudonoise code sequence modulator, to establish said n desired sensing positions along said optical fiber span;
said set of correlation-type binary pseudonoise code sequence demodulators serving to conjunctively temporally and spatially de-multiplex said r.f. composite difference beat signal to provide at their respective outputs r.f. counterparts of the subcomponents of said second signal component of said composite back-propagating lightwave signal caused by changes in the optical path within said optical fiber span induced by external physical signals respectively coupled to the corresponding sensing positions;
a corresponding set of n phase demodulators for transforming each respective r.f. counterpart of said second component into a substantially linear signal representative of radian phase of the corresponding lightwave subcomponent signal; and
at least one subtracter circuit having a first and a second input for respectively receiving outputs from a selected first and a selected second of said set of n phase demodulators, to produce a differential phase signal which is representative of the difference between the radian phases of the r.f. counterparts of two subcomponents of said second component of the composite back-propagating lightwave signal, which two subcomponents are caused by external signals at sensing positions along said optical fiber span established by the time delay circuits respectively associated with said selected first and said selected second phase demodulators. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22)
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23. A system wherein, at respective span increment sensing stations of a plurality of span increment sensing stations along a span of optical fiber, the system senses the phase differential between input signals respectively incident upon the bounds of each station, for signals of a type having a property of inducing light path changes at regions of the span influenced by such signals, said system comprising:
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means for illuminating the span with a CW optical signal and for retrieving back-propagating portions of the illumination back propagating from a continuum of locations along the span;
means for modulating said CW optical signal in accordance with a reiterative autocorrelatable form of modulation with respect to a reference timing base;
means for picking off from said retrieved back-propagating portions of the illumination a radio frequency (r.f.) counterpart of the retrieved modulated CW optical signal;
means for performing a second plurality of autocorrelation detection processes upon said r.f. counterpart of the retrieved back-propagated modulated CW optical signals, said second plurality corresponding to the number of bounds of sensing stations, said second plurality of autocorrelation detection processes being performed in different timed relationships relative the reference timing base determined by propagation distances which the modulated CW optical signal travels to the respective bound;
means for detecting the radian phase of each input signal at each respective timed relationship by which said autocorrelation detection means performs processing for a respective bound, said detection means performing the radian phase detection of each input signal in phase locked relationship to said CW optical signal; and
means for determining the difference between radian phases of the signals at the respective timed relationships associated with each pair of bounds of each span increment sensing station.
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24. Signal sensing array apparatus, wherein at respective span increment sensing stations of a first plurality of span increment sensing stations along a span of optical fiber the apparatus senses phase differentials between input signals respectively incident the bounds of each span increment sensing station, the input signals being of a type having a property of inducing light path changes at regions of the span influenced by such signals, said apparatus comprising:
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an optical wave network comprising a transmitter laser and a lightwave directional coupler, said network being operative to illuminate an optical fiber span with a CW optical signal and to retrieve portions of the illumination back-propagating from a continuum of locations along the fiber span;
a modulator operative to modulate the CW optical signal in accordance with a reiterative autocorrelable form of modulation code;
a heterodyner which receives said retrieved, back-propagated, portions of illumination and derives therefrom a radio frequency (r.f.) counterpart there of; and
a phases-of-input-signals detector for determining the phase of input signal at each bound of each span increment sensing station of said first plurality of span increment sensing stations, said phases-of-input-signals detector being operative in respective timed relationships of a second plurality of timed relationships corresponding to the bounds of the sensing stations, said timed relationships being with respect to said reiterative autocorrelable form of modulation code, said phases-of-input-signals detector including a corresponding second plurality of autocorrelation detectors which respectively detect temporal portions of the r.f. counterparts of the retrieved, back-propagated, portions of said illumination that occur in the timed relationships corresponding to the bounds of each span increment sensing station and detect electronic representations of the optical input signals incident the respective bounds, and a corresponding second plurality of phase demodulators which are operative in phase locked relationships with said CW optical signal and which respectively receive said detected electronic representations of the optical input signals; and
a subtracter network coupled to receive the outputs of said second plurality of phase demodulators and which is operative to determine the differences between the phase demodulator outputs of each pair of phase demodulator outputs which correspond to a pair of bounds of each span increment sensing station of the first plurality of said stations.
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Specification