WO1987007014A2

WO1987007014A2 - Greatly enhanced spatial detection of optical backscatter for sensor applications

Info

Publication number: WO1987007014A2
Application number: PCT/GB1987/000301
Authority: WO
Inventors: Jeremy Kenneth Arthur Everard
Original assignee: Jeremy Kenneth Arthur Everard
Priority date: 1986-05-09
Filing date: 1987-05-07
Publication date: 1987-11-19
Also published as: GB2190186B; WO1987007014A3; GB8611405D0; GB2190186A

Abstract

A pseudo random bit sequence is amplitude modulated onto a light source (2) (other types of modulation are discussed in the specification) and this modulated beam is transmitted down an optical fibre (3) or any material and the detected backscatted signal is multiplied (9) with a digitally delayed reference version of the transmitted sequence. By varying the delay between the transmitted and the reference pseudo random sequence spatial information can be recovered with improved signal to noise ratios compared to conventional Optical Time Domain Reflectometry. This enables improvements in the detected signal to noise ratio of the backscatter allowing reduced signal averaging times or reduced peak transmitted power. This technique can be used to produce enhanced optical sensors, sensing any external parameter on which the backscatter is dependent using video, and coherent detection. Enhanced fibre loss measurement techniques and enhanced fibre discontinuity measurement techniques are described. Distributed temperature sensors are described using Billouin and Raman backscatter. Because of enhanced detection the spatial resolution for a given signal averaging time can be reduced. The measurement of the amplitude, spectra, phase and polarisation of the scatter can also be used to characterise the properties of the material and the influence of any external parameters which influence the properties of the materials producing the backscatter. The measurement of backscatter can therefore be used to measure any external measurand which influences the amplitude, frequency, phase and polarisation of the backscatter.

Description

GREATLY ENHANCED SPATIAL DETECTION OF OPTICAL BACKSCATTER FOR SENSOR APPLICATIONS

The measurement of the spatially distributed backscatter of light from an optical fibre is often a problem due to the fact that direct detection and signal averaging techniques are often required. The use of high power lasers can reduce the amount of signal averaging, however such lasers are usually bulky and very expensive. The measurement of such backscatter can be used to characterise the losses in the fibre (due to the Rayleigh losses) or in the measurement of distributed temperature sensors (Raman and Brillouin interactions).

The measurement of the amplitude, spectra, phase and polarisation of the backscatter can also be used to characterise the properties of the material and the influence of any external parameters which influence the properties of the materials producing the backscatter.

The measurement of backscatter can therefore be used to measure any external measurand which influences the amplitude, phase, polarisation and frequency of the backscatter.

At present to measure the spatial properties of the backscatter, Optical Time domain reflectometry techniques, herafter defined as OTDR, are used in which a pulse is launched into the fibre and a photodetector, amplifier and sampling gate combination are used to measure the backscatter.

The time delay between the transmitted pulse and the sampling gate being fired, defines the slot in the fibre over which the backscatter is measured.

The pulse width and sampling aperture define the spatial resolution. The signal is then averaged to improve the signal to noise ratio. However the maximum sampling rate is fixed by the length of the fibre to ensure results free from ambiguity. In other words the backscatter from only one pulse should be sampled. The maximum repetition rate is therefore:

repmax = 1/2cL

where c is the speed of light in the fibre and L is the length of the fibre.

NB this assumes that multiple reflections from the fibre ends are negligible.

Signal averaging will improve the s/n ratio by a ratio of the square root of the repetition rate. This is because the noise current in the photodetector or the ensuing load amplifier combination is usually proportional to the root of the bandwidth whereas the signal current is proportional to the optical signal power. Every time the signal is sampled and averaged the rms value of the noise current reduces by the square root of 2 and the signal stays the same. If the signal is integrated the signal component doubles and the noise increases by the square root of 2.

The only ways to improve the signal to noise ratio for a given length of fibre and given resolution using OTDR is to increase the peak power in each pulse or to use a bank of narrow band harmonic filters tailored to pass the signal and reduce the noise. However a very large number of filters would be required. This would also limit the speed of response, however this technique should not be ignored as a satisfactory implementaton may be possible in future.

FMCW can be used, however spectral analysis is required in the receiver after the detector. Also the light source needs to have a narrow spectral width for good spatial resolution.

A more satisfactory way of improving the s/n ratio is to increase the average transmitted and hence received power in the time interval without causing ambiguity thus allowing more effective time integration.

In this invention a pseudo random bit sequence (hereafter defined as PRBS) is amplitude modulated onto a light source (other types oi modulation are discussed in other parts of this specification) and this modulated beam is transmitted down an optical fibre or any material and the detected backscatted signal is multiplied with a digitally delayed version of the transmitted sequence, herafter the transmitted sequence will be called the the reference PRBS.

The Pseudo random bit sequence referred to in this patent means a pseudo random sequence of bits which appear to have a noise like spectra where the bit sequence is repeated after a specific number of bits and hence time interval. The sequences may consist of binary (on and off pulses) Fig.1. or multiple level pulses (for example -1,0,+1 Fig.2.).The pseudo random sequence can also be multilevel with levels from two to infinity. The pseudo random sequence is also designed to have specific autocorrelation properties. The number of bits in the pseudo random sequence before the sequence repeats and the time taken before the sequence starts to repeat (hereafter called the sequence repeat time) can be varied according to the specifications of the sensor system.

Spatial information is obtained by multiplying the detected backscattered signal with a delayed version of the pseudo random bit sequence, the delay being implemented digitally. By varying the delay the backscatter from different points can be measured. The delay can also be varied using analogue techniques (for example a delay line).

In the Pseudo random bit sequences the sequence repeat time is equivalent to the repmax of the OTDR case and the bit length is equivalent to the sampling aperture. The bit length also defines the spatial resolution.

This allows the average power transmitted and received to be increased by having many bits in a sequence allowing more effective use of signal averaging.

The* extra signal power occupies the full noise bandwidth. This technique can be thought of in terms of sampling in that the ambiguity due to the increase in sampling rate is removed by arranging for the unwanted sampled terms to average out to be approximately zero by designing the pseudo random bit sequences to have specific autocorrelation properties.

Pseudo random sequences can be designed with a varierty of properties.

By using this technique with a maximally flat pseudo random bit sequence a specific autocorrelation function can be produced where the peak occurs when the delay between the transmitted and received sequence is zero. If this peak is normalised to 1 then the correlation between the two pseudo random sequences when, the delay is not zero is constant at -1/n where n is the number of bits before the sequence repeats. This assumes that the integration is performed over one complete sequence and that the peak of the autocorrelation function was normalised to V.. Fig.3.

Pseudo random sequences with different autocorrelation functions can also be used.

The sampling rate is now therefore increased to the bit rate which is now ./ bit width in time). For a given bandwidth photodiode (hence resolution) the bit rate can be increased until inter symbol interference becomes a problem which allows a s/n improvement over conventional OTDR of approximately between:

SQUARE ROOT 0F((Bit width in time)/(sampling rate of conventional OTDR)).

and

(The bit width in time)/(sampling rate of conventional OTDR)

The exact improvement in the signal to noise ratio is dependent on the correlation between the bandwidth limited photo-detector noise and the bandwidth limited pseudo random bit sequence as well as the detector integration and averaging times.

The integration time for optimum performance should be one or N times (where N is a positive integer) the sequence repeat time where the sequence repeat time is defined as the time after which the bit sequence starts to repeat. There is usually no advantage in integrating fro more than one sequence repeat time.

For the same peak transmitted power, the average power transmitted and received using pseudo random bit sequences can now be increased to approximately half the peak power transmitted in the OTDR case. In conventional OTDR the average power transmitted was:

(Peak power).(repetition rate/bit length)

The invention can be used in a number of ways: SECTION A

Video detection (In this case video detection means that the backscatterd signal is directly incident on the Photodiode) where the signal out of the photo-diode is multiplied by an identical suitably delayed pseudo random bit sequence, the delay being produced using digital techniques. The system is shown in Fig. .

A digital pseudo random generator is built using digital circuits (1). The output of (1) is amplitude modulated onto a laser (2). The light out of the laser (2) is coupled into an optical fibre (3) via a beam splitter (4) and a lens (5). The backscattered signal from the fibre is collimated by the lens (5; and deflected by the beam splitter (4) via a lens (7) onto the photodetector (6). The electrical output of the photo-detector (6) is amplified in an amplifier (8) and then multiplied in a multiplier (9) with a time delayed version of the original pseudo random sequence (1) using digital circuits (10,11) where (10) is a delay circuit and (11) is another PRBS generator. The digital circuits (1,10,11) could be combined in one circuit and the delayed PRBS sequence can be produced in a number of ways. A typical circuit for the pseudo random generators (1,10,11) is shown in Fig.5 where this circuit scans through the delay increasing the delay by one bit every time the pseudo random sequence starts to repeat. This is achieved using two identical PRBS generators fed by clock pulses where the clock pulses applied to one of the PRBS generators drops one pulse every time the pseudo random bit sequence starts to repeat. Another technique would be to detect the end of the sequence in each generator by looking at the states of the shift registers used to generate the pseudo random bit sequences and setting the delay using logic contolled either by a computer or by hardwired counters and logic controlled by switches.

The output from the the multiplier (9) is amplified (12) and then integrated or averaged (13) over N times the sequence repeat time where N is a positive integer. There is usually no advantage in integrating over more than one complete seqquence repeat time. The infomation is then displayed (14) or processed and stored. The signal can be averaged over different times however the dynamic range would be reduced. By varying the delay between the pseudo random generators (1,11) and observing the amplitude of the signal out of the integrator a picture showing the variation of backscatter with distance can be built up. The dynamic range is improved by using longer sequences, in other words a larger number of bits before the sequence repeats..

As the optical backscatter from the fibre is distributed all along the fibre then each bit not on the peak of the correlation will be multiplied by -1/n. All the unwanted bits of backscatter will be summed together causing a large component of signal which could be similar in value to the wanted signal. The number of bits in the pseudo random bit sequence before it repeats in conjunction with the sequence repeat time sets the spatial resolution. The large unwanted component can be reduced by arranging for the pseudo randon sequence repeat time to be considerably longer than the round trip time of the fibre. Another method would be chop the signal and subtract the value when there is no correlation at any point in the fibre from the value when the correlation peak is at the wanted position. The amplitude of the backscatter when there is no correlation in the fibre could be produced by adjusting the delay between the two PRBS generators to be less than the round trip propagation time of the laser beam between the laser and the input of the fibre or by arranging for the delay to be longer than the round trip propagation time of the fibre.

This system can be used as it stands to measure the Rayleigh backscatter and the loss and discontinuities along a fibre.

By looking at the spectral properties of the backscatter using optical bandpass (or lowpass or highpass) filters placed between the beam splitter (4) and the lens (7) or detector (6) of the system shown in fig.4., the Brillouin and Raman lines and other spectra can be measured. The spectra and amplitude of these lines can be used to give direct measurement of temperature along the fibre. It can also be used to measure any external parameter on which the bacscatter is dependent. The system is shown in Fig.6. where the only change from Fig.4. is the filter or filters or polarisation detectors or modifiers (15). The Raman and Brillouin backscatter are caused by acousto-optic interactions between the input optical wave and the pnonon waves in the fibre. These interactions produce sidebands on either side of the optical beam, the amplitude and spectrum of which are frequency dependent.

The spatial temperature distribution can be derived by measuring the amplitude of the. Brillouin or Raman Stokes line Ctbe lower frequency sideband) or by measuring the Brillouin or Raman antistokes line (the upper frequency sideband) or by measuring the ratio of the amplitudes oi the Stokes to the Antistokes lines at the same offset frequency using the techniques described in section A and Fig.6. By measuring the ratio of the stokes and antistokes line by either switching filters or using two optical paths the optical cross section can be removed from the equations. Calibration can be enhanced by putting parts of the fibre in known temperature regions. Enhanced temperature measurement can also be made by launching optical signals into both ends of the fibre and thus obtaining a measurement from either end of the fibre.

A temperature sensor could also be made by measuring either the Stokes line or the antistokes line and taking a ratio of one of these lines to the Rayleigh backscatter to normalise out the losses in the fibre.

Spatial Temperature sensors can also be built using the pseudo random modulated optical beam by using a fibre with absorbtion edges which move in frequency with temperature. The amplitude of the back scattered Rayleigh light would then be dependent on the temperature at specific points along the fibre. The signal can be further improved by launching two pseudo random modulated beams into the fibre each at a different optical frequency. The frequency of one beam should be arranged to be varying with temperature by being on the absorbtion edge. The other beam should be arranged to be away from the absorbion edge to calibrate out varying losses in the fibre and pulse to pulse variation. In this invention absorbtion edge means an optical frequency where the absorbtion coefficient alpha in the fibre varies with small changes in optical frequency.

The absorption edge could be either due to the materials within the fibre or could be absorbing materials placed in line with the fibre at specific intervals along the fibre.

The coherent detection schemes described in Section B and C could also be used to measure the spatial temperature distribution described above.

Having detected the backscatter using the system described in section A the amplitude, spectra, phase and polarisation of the signal can be measured and spatialy resolved at different points along the fibre by using filters or polarisation detectors or modifiers or by using phase modulators in the path between the laser and the beam splitter or the path between the beam splitter and the photo-detector.

Therefore the use of the pseudo random sequence modulated signal can be used to obtain spatial information on the amplitude, spectra, phase and polarisation of the backscatter and hence any external measurand on which the amplitude, spectra, phase and polarisation of the backscatter are dependent. The invention is therefore useful for obtaining spatial information on any external measurand which affects ampl tude, spectrum, phase and polaristion of the backscatter.

SECTION B

Conerent detection (In this case coherent detection means that the optical backscatter is incident on the photo-detector with an optical local oscillator. A constant local oscillator signal similar to the transmitted optical signal without any modulation on it is applied with the optical backscattered light from the fibre to the input of the photodetector.

The system is shown in Fig 7.

A digital pseudo random generator is built using digital circuits (1). The output of (1) is amplitude modulated onto a laser (2;. Otner forms of modulation can be used as described elsewnere in this specification. The light out of the laser (2) is coupled into an optical fibre (3) via a beam splitter (4). The backscattered signal from the fibre is deflected by the beam splitter (4) via a lens (7) onto the photodetector (6). An optical local oscillator beam (15) is derived from the other reflection from the beam splitter (4) and by using a mirror (17),the optical local oscillator beam is arranged to be incident onto the photo-detector (6) via a lens (7) simultaneously with the optical backscatter. The electrical output of the photo-detector (6) is then amplified,if necessary, in an amplifier (8). The output of the amplifier (8) or the pnoto-detector (6) is then power detected (16) or demodulated if other modulation schemes are used.

To obtain spatial information the demodulated signal is then multiplied in a multiplier (9) with a time delayed version of the original pseudo random sequence (1) using digital circuits (10,11) where (10) is a delay circuit and (11) is another PRBS generator. The digital circuits (1,10,11) could be combined in one circuit and the delayed PRBS sequence can be produced in a number of ways. A typical circuit for the pseudo random

generators (1,10,11) is shown in Fig.5 where this circuit scans through the delay increasing the delay by one bit every time the pseudo random sequence starts to repeat. This is achieved using two identical PRBS generators fed by clock pulses where the clock pulses applied to one of the PRBS generators drops one pulse every time the pseudo random sequence starts to repeat. The output from the the multiplier (9) is amplified (12) and then integrated or averaged (13) over N times the sequence repeat time where N is a positive integer. There is usually no advantage in integrating over more than one complete sequence repeat time. The infomation is then displayed (14) or processed and stored. The signal can be averaged over different times however the dynamic range would be reduced. By varying the delay between the pseudo random generators (1,11) and observing the amplitude of the signal out of the integrator a picture showing the variation of backscatter with distance can be built up. The dynamic range is improved by using longer sequences, in other words a larger number of bits before the sequence repeats.

As the optical backscatter from the fibre is distributed all along the fibre then each bit not on the peak of the correlation will be multiplied by -1/n. All the unwanted bits of backscatter will be summed together causing a large component of signal which could be similar in value to the wanted signal. This problem can be overcome by arranging for the pseudo randon sequence repeat time to be considerably longer (for example one hundred times) than the round trip time of the fibre. Another method would be chop the signal and subtract the value when there is no correlation at any point in the fibre from the value when the correlation peak is at the wanted position.

The amplitude of the backscatter when there is no correlation in the fibre could be produced by adjusting the delay between the two PRBS generators to be less than the round trip propagation time of the laser beam between the laser and the input of the fibre or by arranging for the delay to be longer than the the round trip propagation time of the fibre.

This technique can be used directly as it stands to measure the loss and Rayleigh backscatter and discontinuities along the fibre.

To ensure that the phase of the optical local oscillator and detected signal are not in quadrature (causing zero output) a phase wouulator can be incorporated in one of the optical paths between (2) and (4) or 2 detectors could be used. The phase wobulator consists of an optical device which modulates the phase by an angle around 90 degrees. If two detectors are used each detector has the backscattered signal combined with a local oscillators applied to it, where the phase of each local oscillator is in quadrature.

To remove problems of DC offset in the detector and multiplier circuits the optical or electronic signal should be chopped at say 1 KHz, the chopping frequency preferably being above the flicker noise corner of the ensuing amplifier. The signals should be coupled into the multiplier in such a way that the voltage excursions are equally above and below DC. DC offsets larger than the received signals would produce incorrect correlation functions. The phase wobulator can also be used in place of the chopper.

The technique described in section B can also be used to measure temperature by measuring the amplitude and spectra of the Brillouin backscatter from the fibre using coherent and heterodyne detection and performing the filtering at microwave frequencies Fig.8. The backscatter would be detected coherently by mixing the backscattered signal with a part of the input beam on a photo-detector. The detected microwave signal would then be mixeα with a microwave local oscillator to produce a signal around DC.

The Brillouin backscatter consists of sidebands on either side of the carrier which are temperature dependent. The backscatter obeys the Bose Einstein thermal factor shown below where the Stokes line (lower frequency sideband) and the antistokes line (higher frequency sideband) give different results due to quantum effects.

I₈ = lσ_f (1 + N_fT) I_ag - Iσ_f(N_fτ) Where

(Stokes) (_An_ti-_Stoke_s) ^{∑ he incident} °P^tical in^ten^si^ty w __*n,e-,,r.e ^j - _(hf)J k is Boltzmann's constant

^■ _ζ ~ - T is temperature ( K) h is Planck¹s constant f is the offset frequency σ, is the frequency dependent cross section

For temperatures around room temperature and for an offset of around 30 GHz the Bose Einstein part of the relationship simplifies to KT/hf as hf/KT is considerably lower than one. (hf/KT = .7T). This also means that the Stokes and anti-stokes lines are similar in amplitude at room temperature.

In the system described above the sum of the two lines will be detected as they will overlay on each other in the frequency domain. This is because a single mixer cannot distinguish between positive and negative frequencies. To separate the stokes and antistokes lines to enable temperature measurement using the ratio of the Stokes line to the anti-stokes line or to look at only one of the lines or to measure the ratio of the Stokes or anti-stokes lines to the Rayleigh backscatter lines, the lines need to be separated in the frequency domain using an optical frequency shifter (for example a bragg modulator). The frequency shifter can be used to insert a frequency shift in the reference optical beam to the mixer. This allows a controllable separation between the two lines as well as the Rayleigh line.

The system is snown in Fig.8.

A digital pseudo random generator is built using digital circuits (1). The output of (1) is amplitude modulated onto a laser (2j. Other forms of modulation are described elsewhere in this specification. The light out of the laser (2) is coupled into an optical fibre (3) via beam splitters (4) and (5) and a lens (b). These two beam splitters (4) and (5) are used to derive a frequency shifted optical local oscillator (for example using a Bragg modulator (7) to enable separation of the Brillouin Stokes and Anti-stokes lines. If it is not necessary to separate the Stokes and anti-stokes lines the beam splitter (4) and frequency shifter (7) can be left out and the optical local oscillator can be derived using a mirror as shown in Fig(7). The backscattered signal from the fibre is deflected by the beam splitter (5) via a lens (8) onto the photodetector (9). The frequency shifted optical local oscillator beam (10) is derived from the other reflection from the beam splitter (4) and this is arranged to be incident onto the photo-detector (9) via a lens (8) simultaneously with the optical ba-ckscatter. The electrical output of the photo-detector (9) is then amplified,if necessary or possible, in an amplifier (11). The output of the amplifier (11) or the photo-detector (9) is then applied to an RF mixer (12) with the output of an RF local oscillator (13) which is oscillating at a frequency similar to the frequency of the detected Stokes and Anti-stokes lines causing the Stokes and Antistokes lines to be downconverted to frequencies lower than a few Gigahertz where signal processing is easier. The two lines are then filtered and power detected (14) or demodulated if other modulation schemes are used as described elsewhere in this specification.

If it is necessary to remove problems of DC offset in the detector and multiplier circuits the optical or electronic signal could be chopped at say 1 KHz, the chopping frequency preferably being above the flicker noise corner of the ensuing amplifier. The signals should be coupled into the multiplier in such a way that the voltage excursions are equally above and beiL.ow DC. DC offsets larger than the received signals would produce incorrect correlation functions. The chopped output cαul.d be detected using a tuned amplifier tuned to the chopping frequency and a power detector or by using a Lock-in amplifier.

To obtain spatial information about the spectral line required the demodulated signal is then multiplied in a multiplier (15) with a time delayed version of the original pseudo random sequence (1) using digital circuits (16,17) where (16) is a delay circuit and (17) is another PRBS generator. The digital circuits (1,16,17) could be combined in one circuit and the delayeα PRBS sequence can be produced in a number of ways. A typical circuit for the pseudo random generators (1,16,17) is shown in Fig.5 where this circuit scans through the delay increasing the delay by one bit every time the sequence starts to. repeat. This is achieved using two identical PRBS generators f£d by clock pulses where the clock pulses applied to one of the PRB_> generators drops one pulse every time the sequence starts to repeat. ( The delayed PRBS signal could also be produced by applying the first PRBS output to a shift register). The output from the the multiplier (15) is amplified (18) and then integrated or averaged (19) over one or N times the sequence repeat time where N is a positive integer. There is usually no advantage in integrating over more than one complete seqquence repeat time. The infomation is then displayed (20) or processed and stored. The signal can be averaged over different times however the dynamic range would be reduced. By varying the delay between the pseudo random generators (1,17) and observing the amplitude of the signal out of the integrator a picture showing the variation of backscatter with distance can be built up. The dynamic range is improved by using longer sequences, in other words a larger number of bits before the sequence repeats.

NB Either of the above two techniques (The techniques described in sections A and B) can be improved upon by using multiple correlators to give simultaneous information from different spatial positions using multiple delays of the original pseudo random sequences Fig.9. Digital techniques (for example shift registers) could be used to produce the delay. A tapped charge coupled delay line could also be used to produce the delay and the correlation. In other words a number of pseudo random bit sequences with different delays between them would be separately multiplied with the received signal to produce simultaneous output of the backscatter from different positions along the fibre Fig.9.

The output or the photo-detectors would be amplified and buffered and applied to one port of the correlators (multipliers). The other port of the multipliers would be driven by multiple delayed versions of the PRBS. Each correlator would simultaneously produce and output which when integrated or averaged would produce spatial information about the amplitude, spectrum, phase and polarisation of the backscatter. The spatial position being probed by each correlator is set by the delay between the transmitted PRBS and the PKBS applied to the correlators via the delay circuitry.

The measurement of the amplitude, spectra, phase and polarisation of the scatter can also be used to characterise the properties of the material and the influence of any external parameters which influence the properties of the materials producing the backscatter.

The measurement of backscatter can therefore be used to measure any external measurand which influences the amplitude, phase and frequency of the backscatter.

SECTION C

Coherent detection where the correlation is done in the ^"photo-detector. In other words the local oscillator now contains a delayed version of the pseudo-random bit sequence amplitude modulated onto it Fig.10. Other forms of modulation can. be used as described elsewhere in this specification. This system requires the laser to have a large coherence length.

The system is shown in Fig.10.

A digital pseudo random generator is built using digital circuits (1). The output of (1) is amplitude modulated onto a laser (2; using a modulator (16). Other forms of modulation are described elsewhere in this specification. The light out of the laser- (2) is coupled into an optical fibre (3) via a beam splitter (4; and (5) and a lens (6). These two beam splitters (4) and (5) are used to derive an optical local oscillator which is amplitude modulated (7) by a second PRBS (8) which has been delayed" (9) using digital circuits. (Other forms of modulation can be used as described elsewhere in this specification). If a frequency shift is required as well as the modulation a frequency shifter (for example a Bragg modulator) can be incorporated in (7). The backscattered signal from the fibre is approximately collimated by lens (6) and deflected by the beam splitter (5) via a lens (10) onto the photodetector (11). The optical local oscillator beam (15) is derived from the other reflection from the beam splitter (4) and this is arranged to be incident onto the photo-detector (11) via a lens (10) simultaneously with the optical backscatter. The electrical output of the photo-detector (11) is then amplified ,if necessary, in an amplifier (12). The output of the amplifier (12) or the photo-detector (11) is then integrated or averaged (13) and displayeα or processed and stored (14). The amplitude of the output is dependent on the backscatter from a specific region in the fibre where the position probed is defined by the delay between PRBS (1) and PRBS (8).

The system described in Section C can be used to measure the temperature using Brillouin backscatter as described in section B and also to measure the temperature using absorption edge techniques as described in section B.

In the system snown in Fig.10. the output from the photodetector could be applied to an RF mixer with the output of an RF local oscillator oscillating at a frequency similar to the frequency of the detected Stokes and Anti-stokes lines causing the Stokes and Antistokes lines to be downconverted to frequencies lower than a few Gigahertz where signal processing is easier. The two lines are then filtered and power detected or demodulated if other modulation schemes are used as described elsewhere in this specification. The temperature can be derived using the stokes or antistokes lines or the ratio of the two lines or the ratio of the higher frequency sideband to the Rayleigh backscatter or by taking backscatter measuerments from either end of the fibre as described in Section B.

NB

THESE DESCRIPTIONS APPLY TO SECTIONS A. B. C.

The beam splitter arrangements described in figures 4,6,7,8 and 10 can be replaced by optical fibre couplers. The local oscillator can be derived from the unused end of the fibre either directly by injecting a signal or by reflection using a mirror to couple power back into the fibre. The Rayleigh backscatter can also be used as the local oscillator. The fibres can be placed directly onto the photo-detector and or lasers reducing the requirements for lenses if necessary.

To ensure that the phase of the optical local oscillator and detected signal are not in quadrature (causing zero output) a phase wobulator can be incorporated in one path or 2 detectors could be used. The phase wobulators modulate the phase by around 90 degrees (other angles can be used) by effectively modulating the optical path length. If two detectors are used each detector has the optical backscattered signal combined with an optical local oscillator applied to it, where the phase of each local oscillator on each detector is in quadrature.

If it is necessary to remove problems of DC offset in the detector and multiplier circuits the optical or electronic signal could be chopped at say 1 KHz, the chopping frequency preferably being above the flicker noise corner of the ensuing amplifier. The signals should be coupled into the multiplier in such a way that the voltage excursions are equally above and below DC. DC offsets larger than the received signals would produce incorrect correlation functions. The phase wobulator can also be used in place of the chopper. The chopped output could be detected in a tuned amplifier tuned to the chopping frequency and a power detector or a lock in amplifier.

The pseudo random bit sequence can be modulated onto the carrier (optical or radio) in a number of different ways. The amplitude can be modulated either partly or completely. The phase or frequency can be modulated (for example phase shift keying or frequency shift keying) or a combination of amplitude and phase modulation can be used for example SSB.

In the AM system the optical carrier would be switched on and off by the pseudo random sequence.

The techniques mentioned above can be used in any form of ranging (spatial sensing) system for example radar utilising optical, electromagnetic, radio or sonic waves.

The signal received can be processed to obtain spatial information about the backscatter which can be used to sense any parameter on which the backscatter is dependent.

Counter propagating sequences can be used to enhance localised electromagnetic intensities at a particular point. The output from the multiplier should be integrated or averaged where maximum dynamic range is achieved by integrating or averaging over one or N times the sequence repeat time where N is a positive integer.lt is usually only necessary to integrate or average over one sequence repeat time.

By attaching ring (loop fibre resonators) at specific points along a main fibre and transmitting a pseudo random sequence modulated signal down the main fibre the ring resonators can be probed to obtain information about the state of the resonators at different positions.

As the optical backscatter from the fibre is distributed all along the fibre then each bit not on the peak of the correlation will be multiplied by -1/n. All the unwanted bits of backscatter will be summed together causing a large component of signal which could be similar in value to the wanted signal. This problem can be overcome by arranging for the pseudo randon sequence repeat time to be ponsiderably longer than the round trip time of the fibre. The number of bits in the pseudo-random sequence before it repeats in conjunction with the sequence repeat time sets the spatial resolution. These parameters can be varied dependent on the specification of the sensor system.

Another method would be chop the signal and subtract the value when there is no correlation at any point in the fibre from the value when the correlation peak is at the wanted position.

The optical filters used in Fig.6. can also include polarisation filters or polarisation changers or polarisation detectors or polarisation rotators to measure the polarisation states.

The beam splitter arrangements shown in Fig. ,8,10. can be repositioned to reduce the light loss of the backscatter and reduce the loss in the optical local oscillator power.

The delayed pseudo random bit sequence can be produced by applying the reference pseudo random sequence to a shift register.

The diagrams described above are block schematics, the detailed layout can be varied.

The spatial temperature sensor which uses the Brillouin line can be made into a point source sensor by using a length of fibre wrapped into a loop and launching a CW signal not a pulsed signal and analysing the amplitude and spectrum of the CW backscattered signal.

The number of bits in the pseudo random sequences before the sequence repeats, described in Sections A,B,C, can be varied from small integer numbers to very large integer numbers. The number of bits before the sequence repeats and the sequence repeat time define the dynamic range and the resolution of the measurement. The nu_r.ber of bits (n) in a sequence before it repeats can normally be expressed by the equation : n » 2^m - 1

where m is a positive integer.

Claims

A pseudo-random sequence is modulated onto a light source which is coupled into an optical fibre or other material where the backscattered light is detected and correlated with a delayed version of the origional pseudo-random sequence. Variation of the delay between the two pseudo-random sequences produces a spatial picture of the optical backscater along the fibre allowing distributed measurements of the fibre properties through the measurement of the amplitude, spectrum, phase and polaristion of the backscatter and thereby the measurement of any field or parameter external to the fibre which affects the amplitude, spectrum, phase and polarisation properties of the backscatter.

The modulation described in (1) can be amplitude modulation, phase modulation or frequency modulation or any combination of these modulation types.

The technique described in (1) can be used in a distributed sensor system to measure any external field or parameter by measuring the effect of these parameter on the amplitude, phase, spectrum and polarisation of the bacscattered signal.

The system described in (1) can be used to perform optical time domain reflectometry on fibre systems to measure breaks in the fibre or changes in the amplitude or spectrum or phase or polarisation of the backscatter or any combinations of these where the system may include some of the improvements described in any of the other claims. In the system described in (1) optical filters or polarisors can be placed in front of the photo-detector to select a particular part of the spectrum or polarisation of the backscatter.

In the system described in (1) the delayed pseudo random sequence can be chopped between two delays where one delay correlates at the point to be measured and the other delay correlates at a point outside the fibre thereby producing two signals which can be subtracted to obtain calibration and normalisation.

In the system described in (1) the light level can be chopped to produce an AC signal from the correlator enabling amplification and detection via narrow band filters or lockin amplifiers.

The technique described in (1) can be used to measure the Brillouin or Raman optical lines using optical or electrical filtering and thereby to measure the distributed temperture along a fibre where calibration can be obtain by taking the ratio of the stokes to antistokes lines or by taking the ratio of either the stokes or anti-stokes line to the Rayleigh line.

The technique described in (1) can be expanded by using many correlators each driven by a delayed reference pseudo-random sequence at one input where the delay is different for each correlator and a buffered version of the received backscattered signal at the other input thereby producing at the output of each correlator simultaneous information from different positions in the fibre.

A combination of scanning and multiple correlators can be used as in (1) and (9)

The detection of the backscattered signal on a photo-detector as described in (1) can be by direct power detection or coherent detection using an optical local oscillator or coherent detection and demodulation using an optical local oscillator modulated with a delayed version of the pseudo-random sequence.

Two optical beams can be modulated with a pseudo random sequence and counter propagated down a fibre to produce enhanced intensities at a point within the fibre.

The correlation can be performed in the optical detector where the optical local oscillator is modulated with the delayed version of the pseudo-random sequence.