Natural fiber span reflectometer providing a spread spectrum virtual 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 spectrum spreading signal modulator for temporally structuring said carrier lightwave signal into a spread spectrum modulated interrogation lightwave signal which continuously reiterates sequences of an autocorrelatable spectrum spreading signal, the reiterated sequences being executed in a fixed relationship to a predetermined timing base;
a light wave 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 spread spectrum modulated interrogation lightwave signal, 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 hetrodyner;
said directional coupler coupling said spread spectrum modulated interrogation lightwave signal 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 signal which is a summation of the 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 spread spectrum modulated interrogation lightwave signal which the innate properties of the optical fiber cause to back propagate 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 n 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 signal 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 and 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. 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 de-spreaders and de-multiplexers 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 spectrum spreading signal modulator, to establish said n desired sensing positions along said optical fiber span; and
said set of r.f. de-spreaders and de-multiplexers concurrently serving multiple functions including;
a first function of performing a coherent signal correlation process upon said r.f. composite difference beat signal to de-spread the r.f. counterparts of the interrogation lightwave signal; and
a second function of conjunctively temporally and spatially demultiplexing 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 optical fiber span at corresponding sensing positions.
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Abstract
A CW lightwave modulated by a continuously reiterated autocorrelated spectrum-spreading signal is launched into an end of a span of ordinary optical fiber cable. Portions of this lightwave back propagate to the launch end from a continuum of span locations because of innate fiber properties including Rayleigh effects. This is picked off the launch end and heterodyned producing an r.f. beat signal. The beat signal is processed by a plurality (can be thousands) of multifunction despreader, autocorrelator and de-multiplexer units respectively operated in different time delayed relationships to the timing base of launch signal reiteration. This provides r.f. time-domain reflectometry outputs representative of acoustic, or other signals incident upon virtual sensors at positions along the fiber corresponding to the various delay relationships. Material attenuation of undesired noises (e.g., reflections due to presence of couplers in the fiber cable line) is effected by the spectrum spreading and de-spreading.
22 Citations
32 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 spectrum spreading signal modulator for temporally structuring said carrier lightwave signal into a spread spectrum modulated interrogation lightwave signal which continuously reiterates sequences of an autocorrelatable spectrum spreading signal, the reiterated sequences being executed in a fixed relationship to a predetermined timing base; a light wave 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 spread spectrum modulated interrogation lightwave signal, 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 hetrodyner; said directional coupler coupling said spread spectrum modulated interrogation lightwave signal 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 signal which is a summation of the 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 spread spectrum modulated interrogation lightwave signal which the innate properties of the optical fiber cause to back propagate 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 n 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 signal 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 and 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. 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 de-spreaders and de-multiplexers 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 spectrum spreading signal modulator, to establish said n desired sensing positions along said optical fiber span; and said set of r.f. de-spreaders and de-multiplexers concurrently serving multiple functions including; a first function of performing a coherent signal correlation process upon said r.f. composite difference beat signal to de-spread the r.f. counterparts of the interrogation lightwave signal; and a second function of conjunctively temporally and spatially demultiplexing 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 optical fiber span at corresponding sensing positions. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30)
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31. 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 input signals, comprising:
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means for illuminating an optical fiber span with a CW optical signal; means 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 with a reiterative binary psuedonoise code sequence in a manner which further temporally structures the modulated CW optical signal into a spread spectrum reiterative binary psuedonoise code sequence signal; means for picking off a radio frequency (r.f.) counterpart of the retrieved signal, wherein the r.f. counterpart is in phase locked synchronism with the CW optical signal; means for performing a corresponding plurality of coherent autocorrelation detection processes upon said r.f. counterpart of the retrieved optical signal to conjunctively perform correlation detection and dispreading of the r.f. counterparts in the respective autocorrelation detections of the plurality of autocorrelation detection processes in a corresponding plurality of different timed relationships with respect to the reiterative autocorrelatable form of modulation of the CW optical signal.
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32. 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 regions influenced by such input 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 and temporally structure the CW optical signal into a CW optical signal with a reiterative spread spectrum form of binary psuedonoise code sequence form of modulated signal; a heterodyner which, in phase locked synchronism with said transmitter laser, receives said retrieved back-propagated portions of illumination and derives therefrom a radio frequency (r.f.) counterpart; and a corresponding plurality of autocorrelation detectors operative to respectively perform coherent correlation processes upon said r.f. counterpart of the retrieved optical signal to conjunctively perform correlation detection and dispreading functions therewith, in respective timed relationships of a corresponding plurality of different timed relationships with respect to said reiterative autocorrelatable form of modulation code.
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Specification