RAMAN AMPLIFICATION IN DISTRIBUTED OPTICAL FIBER SENSING SYSTEMS

• US 20090263069A1
• Filed: 07/12/2007
• Published: 10/22/2009
• Est. Priority Date: 07/28/2006
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Hartog, Arthur H.
• Granted Patent: US 8,989,526 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: G01D 5/353 influencing the transmission properties of an optical fibre
• CPC Class Codes: G01D 5/35358 using backscattering to detect the measured quantity
• CPC Class Codes: G01D 5/35364 using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01M 11/319 Reflectometers using stimulated back-scatter, e.g. Raman or fibre amplifiers
• CPC Class Codes: G01N 2021/638 Brillouin effect, e.g. stimulated Brillouin effect
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/094076 Pulsed or modulated pumping H01S3/1024 takes precedence
• CPC Class Codes: H01S 3/302 in an optical fibre
• CPC Class Codes: H04B 10/2916 using Raman or Brillouin amplifiers

Abstract

A method of providing Raman amplification in an optical fiber sensing system, comprises generating a probe pulse of light and launching the pulse into a sensing optical fiber, generating pump light at a shorter wavelength and modulating it to produce a time-varying intensity profile, and launching the pump light into the sensing fiber. such that the intensity of the launched pump light during launch of the probe pulse is different from the intensity at other times. Raman amplification of backscattered light produced by the probe pulse as it propagates along the fiber is achieved, as is amplification of the probe pulse if the pump power is non-zero during launch of the probe pulse.

Claims

1. A method of providing Raman amplification in an optical fiber sensing system, comprising:

generating a probe pulse of light at a probe wavelength;

launching the probe pulse into a sensing optical fiber;

generating pump light at a pump wavelength shorter than the probe wavelength;

modulating the pump light to produce a time-varying intensity profile;

launching the pump light into the sensing fiber such that the intensity of the launched pump light during launch of the probe pulse is different from the intensity at one or more other times; and

propagating the pump light and the probe pulse along the sensing fiber to achieve Raman amplification of backscattered light produced by the probe pulse as it propagates.

2. A method according to claim 1, in which the intensity of the pump light during launch of the probe pulse is substantially zero so that the probe pulse experiences substantially no Raman amplification.

3. A method according to claim 1, in which the intensity of the pump light during launch of the probe pulse is greater than zero so that the probe pulse experiences some Raman amplification.

4. A method according to claim 3, in which the intensity of the pump light during the launch of the probe pulse is sufficient to amplify the probe pulse to an intensity below a threshold above which the probe pulse would be distorted by nonlinear effects.

5. A method according to claim 1, and further comprising introducing a time delay into forward propagating light in the sensing fiber at either the probe wavelength or the pump wavelength such that after the delay, the probe pulse overlaps a part of the intensity profile of the pump light having a greater intensity than the part overlapped by the probe pulse before the delay, and experiences Raman amplification.

6. A method according to claim 5, in which the time delay is introduced such that the intensities of the probe pulse and of the pump light with which it overlaps after the delay are such that the probe pulse is amplified by the pump light to an intensity below a threshold above which the probe pulse would be distorted by nonlinear effects.

7. A method according to claim 1, and further comprising passing forward propagating light in the sensing fiber through one or more rare earth-doped optical fiber amplifiers integrated into the sensing fiber to amplify the intensity of the light.

8. A method according to claim 1, and further comprising:

collecting backscattered light returned from the sensing fiber; and

processing the collected backscattered light to determine information about the sensing fiber and/or an environment in which it is deployed.

9. An optical fiber sensing system for providing Raman amplification, comprising:

a sensing optical fiber deployable in a sensing environment;

a first optical source operable to generate probe pulses of light at a probe wavelength;

a second optical source operable to generate pump light at a pump wavelength shorter than the probe wavelength;

means operable to modulate the pump light generated by the second optical source to produce a time-varying intensity profile; and

an optical coupling device arranged to receive the probe pulses from the first optical source and modulated pump light from the modulation means and to launch the probe pulses and the pump light into the sensing fiber for propagation along the sensing fiber to achieve Raman amplification of backscattered light produced by the probe pulses as they propagate, such that the intensity of the launched pump light during launch of the probe pulses is different from the intensity at one or more other times.

10. An optical fiber sensing system according to claim 9, in which the intensity of the pump light during launch of the probe pulse is substantially zero so that the probe pulse experiences substantially no Raman amplification.

11. An optical fiber sensing system according to claim 9, in which the intensity of the pump light during launch of the probe pulse is greater than zero so that the probe pulse experiences some Raman amplification.

12. An optical fiber sensing system according to claim 11, in which the intensity of the pump light during the launch of the probe pulse is sufficient to amplify the probe pulse to an intensity below a threshold above which the probe pulse would be distorted by nonlinear effects.

13. An optical fiber sensing system according to claim 9, and further comprising an optical time delay device integrated into the sensing fiber and operable to introduce a time delay into forward propagating light in the sensing fiber at either the probe wavelength or the pump wavelength such that after the delay, the probe pulse overlaps a part of the intensity profile of the pump light having a greater intensity than the part overlapped by the probe pulse before the delay, and experiences Raman amplification.

14. An optical fiber sensing system according to claim 13, in which the optical time delay device is located such that the intensities of the probe pulse and of the pump light with which it overlaps after the delay are such that the probe pulse is amplified by the pump light to an intensity below a threshold above which the probe pulse would be distorted by nonlinear effects.

15. An optical fiber sensing system according to claim 13, in which the optical time delay device comprises an optical fiber delay line coupled at both ends to the sensing fiber by a wavelength division multiplexer arrangement configured to pass light at one of the pump wavelength and the probe wavelength directly along the sensing fiber and to direct light at the other of the pump wavelength and the probe wavelength along the optical delay line before directing it further along the sensing fiber.

16. An optical fiber sensing system according to claim 9, and further comprising one or more rare-earth optical fiber amplifiers integrated into the sensing fiber and operable to amplify the intensity of forward propagating light in the sensing fiber.

17. An optical fiber sensing system according to claim 9, and further comprising:

an optical detector for collecting backscattered light returned from the sensing fiber; and

a processor operable to process one or more signals output from the optical detector to determine information about the sensing fiber and/or an environment in which it is deployed.