Measuring brillouin backscatter from an optical fibre using digitisation

Patent metadata

Field	Value
Publication number	US-9429450-B2
Application number	US-37754807-A
Country	US
Kind code	B2
Filing date	Aug 7, 2007
Priority date	Aug 16, 2006
Publication date	Aug 30, 2016
Grant date	Aug 30, 2016

Abstract

A method for measuring Brillouin backscattering from an optical fiber, comprising mixing backscattered light received from the optical fiber and having a Brillouin frequency fβ(t) with coherent light at a frequency f i in an optical detector to produce an electrical signal with a difference frequency ΔF(t)=fβ(t)−f15 and directly digitizing the electrical signal using an analog-to-digital converter to generate a sequence of samples representing the electrical signal, the samples then being processed to determine one or more properties of the Brillouin spectral line. The difference frequency may be further reduced by an additional frequency mixing stage to allow digitization at a lower sampling rate.

First claim

What is claimed is:

1. A method for measuring Brillouin backscattering from an optical fibre, comprising:
launching a probe pulse of coherent light with a frequency f0 into an optical fibre;
receiving backscattered light from the optical fibre that includes at least one Brillouin spectral line at a frequency fB(t) shifted from f0 by a Brillouin shift, the Brillouin spectral line varying with time and distance along the fibre;
providing coherent light at a frequency f1;
mixing the received backscattered light at fB(t) with the coherent light at f1 in an optical detector to generate an electrical signal at a difference frequency ΔF(t)=fB(t)−f1;
digitising the electrical signal using an analog-to-digital converter to sample the electrical signal at a sampling rate and hence generate a sequence of digital samples representing the electrical signal;
processing the digital samples to determine one or more properties of the Brillouin spectral line of the received backscattered light as a function of time and distance along the optical fibre; and
mixing the electrical signal at the difference frequency ΔF(t) with a secondary electrical signal at a constant frequency fC to reduce the frequency of the electrical signal to a frequency ΔF2(t) which is less than the difference frequency ΔF(t);
wherein the probe pulse is generated from a first optical source and the coherent light at the frequency f1 is generated from a second optical source by modulating an output of one of the first optical source and the second optical source to generate modulation sidebands and injection-locking the other of the first optical source and the second optical source to one of the modulation sidebands.

2. A method according to claim 1 , in which the sampling rate is at least twice a highest anticipated value of the difference frequency ΔF(t) of the electrical signal.

3. A method according to any one of claim 1 or 2 , in which f1=f0.

4. A method according to any one of claim 1 or 2 , in which the one or more properties of the Brillouin spectral line determined from the digital samples include at least one of the Brillouin frequency fB(t); the intensity of the Brillouin spectral line; and the linewidth of the Brillouin spectral line.

5. A method according to any one of claim 1 or 2 , and further comprising calculating a value of one or more physical parameters to which the optical fibre is subject from the one or more determined properties of the Brillouin spectral line and converting time into distance along the optical fibre to obtain an indication of a distribution of the one or more physical parameters over a length of the optical fibre.

6. A method according to any one of claim 1 or 2 , and further comprising repeating the method for further probe pulses, and averaging over a plurality of probe pulses to obtain a determination of the one or more properties of the Brillouin spectral line and/or one or more physical parameters.

7. A method according to claim 1 or claim 2 , in which the difference frequency ΔF(t) is less than 100 GHz.

8. A method according to claim 7 , in which the frequency ΔF2(t) is less than 5 GHz.

9. Apparatus for measuring Brillouin backscattering from an optical fibre comprising:
a first optical source operable to generate probe pulses of coherent light at a frequency f0 and launch probe pulses into an optical fibre;
a second optical source operable to generate coherent light at a frequency f1;
an optical detector arranged to receive backscattered light from the optical fibre that includes at least one Brillouin spectral line at a frequency fB(t) shifted from f0 by a Brillouin shift, the Brillouin spectral line varying with time and distance along the fibre, and to receive the coherent light at frequency f1, and operable to generate an electrical signal at a difference frequency ΔF(t)=fB(t)−f1 from frequency mixing of the received backscattered light at fB(t) and the coherent light at frequency f1;
an analog-to-digital converter arranged to receive the electrical signal, and operable to sample the electrical signal at a sampling rate to generate a sequence of digital samples representing the electrical signal; and
a processor operable to process the digital samples to determine one or more properties of the Brillouin spectral line of the received backscattered light as a function of time and distance along the optical fibre, wherein the processor is further operable to calculate a value of one or more physical parameters to which the optical fibre is subject from the one or more determined properties of the Brillouin spectral line and convert time into distance along the optical fibre to obtain an indication of a distribution of the one or more physical parameters over a length of the optical fibre;
wherein an output of one of the first optical source and second optical source is modulated to generate modulation sidebands, and the other of the first optical source and the second optical source is injection-locked to one of the modulation sidebands.

10. Apparatus according to claim 9 , in which the sampling rate is at least twice a highest anticipated value of the frequency ΔF(t) of the electrical signal.

11. Apparatus according to claim 9 , in which the difference frequency ΔF(t) is less than 100 GHz.

12. Apparatus according to claim 9 , in which f1=f0.

13. Apparatus according to claim 9 , in which the one or more properties of the spectral line determined from the digital samples include at least one of: the Brillouin frequency fB(t); the intensity of the Brillouin spectral line; and the linewidth of the Brillouin spectral line.

14. Apparatus according to claim 9 , and further comprising an electrical local oscillator operable to generate a secondary electrical signal at a constant frequency fC, and an electrical signal mixer operable to frequency mix the electrical signal at the difference frequency ΔF(t) with the secondary electrical signal to reduce the frequency of the electrical signal to a frequency ΔF2(t) which is less than ΔF(t) before the analog-to-digital converter receives the electrical signal.

15. Apparatus according to claim 14 , in which the frequency ΔF2(t) is less than 5 GHz.

Assignees

• Hartog Arthur H
• Schlumberger Technology Corp

Inventors

• Hartog Arthur H

Classifications

• G01K2011/322

  Physics · mapped topic

• G01D5/35364 Primary

  using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering · CPC title

• G01M11/3145

  Details of the optoelectronics or data analysis · CPC title

• G01M11/3109

  Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR · CPC title

• G01D5/353

  influencing the transmission properties of an optical fibre · CPC title