US 5,317,383 A · Array retroreflector apparatus for remote seismic sensing

Abstract

A remote sensing system is provided which operates as a bipolarized, differential mode, LDI (laser differential interferometry) system to detect an electrical signal produced at a remote location. The system is adapted to reduce the effects of turbulence induced frequency fluctuations by using an array retroreflector apparatus which converts a single sensing beam into a single return beam having two, overlapping, transversely polarized sensing signals; thus, turbulence induced noise will be common mode to both return signals and cancel at the receiver. In preferred embodiments, the system is used to remotely detect an amplified array signal from a plurality of seismic detectors (e.g., geophones or hydrophones) connected to form an array at the remote location. The target is coupled to common motions at the selected location and the amplified array signal is coupled to an optical component within the target. The target converts the sensing beam into a return beam by separating from the sensing beam two transversely polarized sensing signals, by Doppler shifting the sensing signals to have frequency components that represent common motions coupled to the array retroreflector and to contain between the signals a difference signal which represents the array signal. The target also combines the two sensing signals into a single return beam having two transversely polarized return sensing signals. The return beam is detected. The return sensing signals are separated from the return beam and combined by electronic and/or by optical heterodyning techniques to cancel any common mode signals on the sensing and return beam, thereby leaving a frequency modulated difference signal which represents the frequency modulated array signal. The frequency modulated difference signal may then be demodulated to produce a time varying signal which represents the seismic signal of the seismic motions detected by the seismic detectors.

Claims85

Dependent claims

A remote detector apparatus adapted to provide a single return beam having two transversely polarized overlapping sensing signals, the signals having between them a difference signal which represents an electrical signal produced at a selected location, said signals enabling a bipolarized, differential mode, laser differential interferometry, remote sensing system to detect the electrical signal , to cancel common mode signals on the return sensing signals and to reduce turbulent noise on the difference signal, the turbulent noise being caused by atmospheric effects between a receiver of the system and said remote detector apparatus, the remote detector apparatus comprising:

a casing, the casing being functional to couple,, motions at the selected location, the motions having directions of movements in any of three orthogonal directions at the selected location;
and
a first means mounted on the casing which is functional for converting a single sensing beam into a single return beam (a) by splitting and (b) polarizing the sensing beam to form two polarized sensing signals, (c) by frequency modulating both of the polarized sensing signals to contain Doppler shifted frequency components that represent common motions coupled to the casing, (d) by frequency modulating one of the sensing signals with the electrical signal, the electrical signal being coupled to said first means by a second means for coupling the electrical signal to said first means, whereby, the two polarized sensing signals contain a difference signal between them which represents the electrical signal , (e) by combining the two polarized sensing signals to form the return beam, and (f) by directing the return beam to a return propagation path which takes the return beam to the receiver, the return beam having two transversely polarized return sensing signals which overlap each other on the return propagation path such that the two return sensing signals occupy substantially the same air space, whereby the atmospheric effects on the sensing beam and the overlapping return sensing signals result in turbulent noise on the single sensing beam and the single return beam which are common mode signals to both said return sensing signals thus reducing the effects of turbulent noise on the difference signal.

A method for producing a single return beam having two transversely polarized, overlapping, return sensing signals, said signals having a difference signal between them which represents an electrical signal produced at a selected location, said return beam enabling a bipolarized, differential mode, laser differential interferometry, remote sensing system, to detect the electrical signal, said overlapping return sensing signals also enabling the remote sensing system to reduce turbulent noise on the difference signal, the turbulent noise being caused by atmospheric effects between a receiver of the remote sensing system and the selected location, the method comprising the steps of:

(a) transmitting a single sensing beam to the selected location;
(b) coupling motions at the remote location to a casing, the motions being in any of three orthogonal directions;
(c) separating two polarized sensing signals from the sensing beam, the polarities of the polarized sensing signals being orthogonal to each other, the polarized sensing signals having separate paths;
(d) Doppler shifting both of the polarized sensing signals with common motions coupled to the casing such that each of the two polarized sensing signals contains frequency components which represent the same common motions;
(e) transforming the electrical signal into movements which are inline with the path of one of the two polarized sensing signals, the movements representing the electrical signal;
(f) Doppler shifting a polarized sensing signal of the two polarized sensing signals, the signal having the path inline with the movements representing the electrical motions such the signal has Doppler shifted frequency components which represent the electrical signal;
(g) combining the two Doppler shifted polarized sensing signals to form a return beam having a return propagation path to the receiver of the sensing system, the return beam having two transversely polarized return sensing signals which overlap each other on the return propagation path such that the two return sensing signals occupy substantially the same airspace, the two return sensing signals having a difference signal between them that represents the electrical signal.

4 more elements

A method for producing a single return beam having two transversely polarized, overlapping, return sensing signals, said signals having a difference signal between them which represents an electrical signal produced at a selected location, said return beam enabling a bipolarized, differential mode, laser differential interferometry, remote sensing system, to detect the electrical signal, said overlapping return sensing signals also enabling the remote sensing system to reduce turbulent noise on the difference signal , the turbulent noise being caused by atmospheric effects between a receiver of the remote sensing system and the selected location, the method comprising the steps of:

(a) transmitting a single sensing beam to the selected location;
(b) coupling motions at the remote location to a casing, the motions being in any of three orthogonal directions;
(c) separating two sensing signals from the sensing beam such that the two sensing signals have separate paths and polarizing the two sensing signals into two polarized sensing signals, the polarities of the polarized sensing signals being orthogonal to each other;
(d) Doppler shifting both of the sensing signals and both of the polarized sensing signals with common motions coupled to the casing such that each of the two polarized sensing signals contains frequency components which represent the same common motions;
(e) transforming the electrical signal into movements which are inline with the path of one of the two sensing signals, the movements representing the electrical signal;
(f) Doppler shifting a polarized sensing signal of the two polarized sensing signals, the signal having the path inline with the movements representing the electrical motions such that the signal has Doppler shifted frequency components which represent the electrical signal;
(g) combining the Doppler shifted polarized sensing signals to form a return beam having a return propagation path to the receiver of the sensing system, the return beam having two transversely polarized return sensing signals which overlap each other on the return propagation path such that the two return sensing signals occupy substantially the same airspace, the two return sensing signals having a difference signal between them that represents the electrical signal.

4 more elements

A method for remotely obtaining a difference signal which is representative of an electrical signal produced at a selected location, comprising the steps of:

(a) producing a laser beam of substantially monochromatic frequency;
(b) splitting the laser beam into a sensing beam and a reference beam;
(c) changing the frequency of the reference beam to produce a frequency offset between the frequency of the sensing beam and the frequency of the reference beam;
(d) splitting the reference beam into two polarized reference signals, a first reference signal and a second reference signal, the polarity of the first reference signal being orthogonal to the polarity of the second reference signal;
(e) directing the sensing beam to the selected location;
(f) coupling motions at the selected location to a casing;
(g) converting the sensing beam at the casing into a return beam having two overlapping polarized return sensing signals, both of the return sensing signals having been frequency modulated to have common mode frequency components which represent common motions on the casing and to have a difference signal between the two signals which represents the electrical signal produced at the selected location, the polarities of the polarized return sensing signals being transversely aligned with respect to each other and the two return signals being aligned on the return signal such that the two signals occupy substantially the same air space on a return propagation path from the selected location;
(h) detecting the return beam;
(i) separating the two polarized return signals from the return beam into two separated polarized signals, a first return signal and a second return signal, the polarity of the first return signal being orthogonal to the polarity of the second return signal and having the same polarity as the first reference signal;
(j) combining in an optical heterodyning process the first reference signal with the first return signal and the second reference signal with the second return signal to produce a first channel signal and a second channel signal, each channel signal having a center frequency approximately at the offset frequency;
(k) reversing the polarity of either channel signal;
(l) canceling the common mode signals that were on the sensing beam and the return beam by combining the first channel signal and the second channel signal in a heterodyning process to produce a frequency modulated difference signal;
and
(m) demodulating the deference signal to produce a time varying signal which represents the electrical signal.

11 more elements

An apparatus for remotely obtaining a difference signal which is representative of an electrical signal produced at a selected location, comprising:

a laser, the laser being functional for producing a laser beam of substantially monochromatic frequency;
a beamsplitter, the beamsplitter being functional for splitting the laser beam into a sensing beam and a reference beam;
a first Bragg cell, the Bragg cell being functional for changing the frequency of the reference beam to produce a frequency offset between the frequency of the sensing beam and the frequency of the reference beam;
a first polarizing beamsplitter, the first polarizing beamsplitter being functional for splitting the reference beam into two polarized reference signals, a first reference signal and a second reference signal, the polarity of the first reference signal being orthogonal to the polarity of the second reference signal;
a casing, the casing being functional to couple to motions at the selected location;
a means mounted on the casing which is functional for converting the sensing beam into a return beam having two overlapping transversely polarized return sensing signals, the two return sensing signals having been frequency modulated to contain common mode frequency components that represent common motions coupled to the casing and to have between the two signals a difference signal which represents the electrical signal produced at the selected location, the two return sensing signals being aligned on the return beam to overlap each other such that the two signals occupy substantially the same air space on a return path to a receiver;
a return beam detector, the return beam detector being functional for detecting the return beam;
a second polarizing beamsplitter, the second polarizing beamsplitter being functional for separating the two polarized return signals from the return beam into two separated polarized signals, a first return signal and a second return signal, the polarity of the first return signal being orthogonal to the polarity of the second return signal and having the same polarity as the first reference signal;
a first photodetector, the first photodetector being functional for combining in an optical heterodyning process the first reference signal with the first return signal to produce a first channel signal having a center frequency approximately at the frequency offset;
a second photodetector, the second photodetector being functional for combining in an optical heterodyning process the second reference signal with the second return signal to produce a second channel signal having a center frequency approximately at the frequency offset, the first channel signal and the second channel signal being channel signals;
an oscillator, the oscillator being functional to produce a signal having a frequency which can be heterodyned with one of the channel signals to produce a frequency shifted channel signal which has a polarity reversal with respect to an initial channel signal;
a means for combining the oscillator signal with said one of the channel signals to produce a polarity reversed channel signal;
a means for combining the polarity reversed channel signal with a channel signal of the channel signals which was not used to form the polarity reversed channel signal in a heterodyning process to cancel any common mode signals that were on the sensing beam and the return beam and to produce a frequency modulated difference signal which represents the electrical signal.

10 more elements

A method for remotely obtaining a difference signal which is representative of an electrical signal produced at a selected location, comprising the steps of:

(a) producing a laser beam of substantially monochromatic frequency;
(b) directing the sensing beam to the selected location;
(c) coupling motions at the selected location to a casing;
(d) converting the sensing beam at the casing into a return beam having two overlapping polarized return sensing signals, both of the return sensing signals having been frequency modulated to have common mode frequency components which represent common motions on the casing and to have a difference signal between the two signals which represents the electrical signal produced at the selected location, the polarities of the polarized return sensing signals being transversely aligned with respect to each other and the two return signals being aligned on the return signal such that the two signals occupy substantially the same air space on a return propagation path from the selected location;
(e) detecting the return beam;
(f) separating the two polarized return signals from the return beam into two separated polarized signals, a first return signal and a second return signal, the polarity of the first return signal being orthogonal to the polarity of the second return signal and having the same polarity as the first reference signal
(g) changing the frequency of the first return signal to produce a frequency offset between the frequency of the first return signal and the frequency of the second return signal;
(h) reversing the polarity of the second return signal;
(i) canceling any common mode signals on the sensing beam and the return beam by combining in an optical heterodyning process the first return signal with the second return signal to produce a channel signal, the optical heterodyning producing a frequency modulated difference signal which represents the electrical signal;
(j) demodulating the deference signal to produce a time varying signal which represents the electrical signal.

7 more elements

An apparatus for remotely obtaining a difference signal which is representative of an electrical signal produced at a selected location, comprising:

a laser, the laser being functional for producing a laser beam of substantially monochromatic frequency;
a casing, the casing being functional to couple to motions at the selected location;
a means mounted on the casing which is functional for converting the sensing beam into a return beam having two overlapping transversely polarized return sensing signals, the two return sensing signals having been frequency modulated to contain common mode frequency components that represent common motions coupled to the casing and to have between the two signals a difference signal which represents the electrical signal produced at the selected location, the two return sensing signals being aligned on the return beam to overlap each other such that the two signals occupy substantially the same air space on a return path to a receiver;
a return beam detector, the return beam detector being functional for detecting the return beam;
a polarizing beamsplitter, the polarizing beamsplitter being functional for separating the two polarized return signals from the return beam into two separated polarized signals, a first return signal and a second return signal, the polarity of the first return signal being orthogonal to the polarity of the second return signal and having the same polarity as the first reference signal;
a means for changing the frequency of the first return signal to produce a frequency offset between the frequency of the first return signal and the second return signal;
a means for rotating the polarity of the second return signal such that it has the same polarity as the first return signal;
and
a photodetector, the photodetector being functional for combining in an optical heterodyning process the first return signal with the second return signal to cancel any common mode signals which were on the sensing beam and the return beam and to produce a channel signal having a center frequency approximately at the frequency offset, the channel signal being the frequency modulated difference signal which represents the electrical signal.

6 more elements

Dependencies are inferred from the claim text.

Array retroreflector apparatus for remote seismic sensing

Life of this patent

Array retroreflector apparatus for remote seismic sensing

Life of this patent

Abstract

Claims85