Natural fiber span reflectometer providing a virtual phase signal sensing array capability
DCFirst 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 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; and
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.
1 Assignment
Litigations
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Accused Products
Abstract
A CW lightwave modulated by a continuously reiterated psuedorandom 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 type pseudonoise code sequence demodulation and phase demodulator units, operated in different time delay relationships to the timing base of the reiterated modulation sequences. These units provide outputs representative of phase variations in respective unique spectral components in the r.f. beat signal caused by acoustic, or other forms of, signals incident to virtual sensors at fiber positions corresponding to the various time delay relationships.
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Citations
22 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 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; and 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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
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21. A system wherein at respective sensing stations of a plurality of sensing stations along a span of optical fiber, the system senses input signals of a type having a property of inducing light path changes at regions of the span influenced by such signals, comprising:
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means for illuminating an optical fiber 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 the CW optical signal in accordance with a reiterative autocorrelatable form of modulation code; means for picking off from said retrieved back-propagating portions of the illumination a radio frequency (r.f.) counterpart of the retrieved back-propagated modulated CW optical signal; means for performing a corresponding plurality of autocorrelation detection processes on said r.f. counterpart of the retrieved back-propagated modulated CW optical signal in respective predetermined timed relationships to said reiterated modulation code; and means for performing a corresponding plurality of phase demodulation processes upon said r.f. counterpart of the retrieved back-propagated modulated CW optical signal, said demodulation processes performed in phase locked synchronism with said CW optical signal.
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22. Signal sensing apparatus for sensing input signals at an array of a plurality of sensing stations along an optical fiber span, wherein at respective sensing stations of the array the apparatus senses input signals of a type having the property of inducing light path changes within 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 thereof; and a subsystem which receives said r.f. counterpart of the retrieved back-propagated portions of said illumination and is operative to derive signals representative of the phases of input signals respectively influencing the sensing stations of said plurality of sensing stations, said subsystem including; a corresponding plurality of autocorrelation detectors which respectively detect components of said r.f. counterpart of the retrieved portions of the illumination representative of the optical input signals influencing the corresponding sensing stations; and a corresponding plurality of phase demodulators which receive respective corresponding detected components of the input signals, said demodulators being operative in phase locked synchronism with said CW optical signal.
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Specification