Optical time domain reflectometry

• US 20050117830A1
• Filed: 01/30/2003
• Published: 06/02/2005
• Est. Priority Date: 01/30/2002
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Wait, Peter C., Hartog, Arthur H.
• Granted Patent: US 7,304,725 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: G01M 11/3109 Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR
• CPC Class Codes: G01M 11/319 Reflectometers using stimulated back-scatter, e.g. Raman or fibre amplifiers
• CPC Class Codes: H04B 10/071 using a reflected signal, e.g. using optical time domain reflectometers [OTDR]

Abstract

An optical time domain reflectometry method of sensing a parameter to be measured in a region of interest comprises launching optical radiation (4) at a probe wavelength into an optical fibre (5, 6) and producing electrical output signals in response to optical radiation backscattered from the optical fibre (5, 6). In one aspect of the invention the optical fibre comprises first and second sections (5, 6), the second section (6) having a lower intensity threshold for the onset of non-linear effects than the first section (5) and being deployed in the region of interest. Optical radiation (4) at the probe wavelength is launched into the said first section (5) at an intensity lower than the non-linear effects intensity threshold of the first section (5) but higher than the non-linear effects intensity threshold of the second section (6), the attenuation characteristics of the said first section (5) being chosen so that the intensity of the optical radiation (4) at the probe wavelength reaching the second section (6) is below that of the non-linear effects intensity threshold of the said second section (6). In another aspect of the invention the optical fibre comprises first and second sections connected together by a remote amplifier, the gain of the amplifier being selected so as to compensate for attenuation losses in the probe wavelength in the first section.

Claims

1. Optical time domain reflectometry apparatus, for sensing a parameter to be measured in a region of interest, which apparatus comprises:

an optical fibre (5,6);

source means (1) operable to launch optical radiation (4) into the optical fibre (5,6) at a probe wavelength; and

detection means (2) operable to produce electrical output signals in response to optical radiation backscattered from the optical fibre (5,6);

characterised in that the optical fibre (5,6) comprises a first section (5), into which the said optical radiation (4) at the probe wavelength is launched, and a second section (6) deployed in the region of interest, the said first section (5) having a higher intensity threshold for the onset of non-linear effects than the said second section (6), the said source means (1) being operable to launch optical radiation (4) at the probe wavelength into the said first section (5) at an intensity lower than the non-linear effects intensity threshold of the said first section (5) but higher than the non-linear effects intensity threshold of the second section (6), the attenuation characteristics of the first section (5) being such that the intensity of the optical radiation (4) at the probe wavelength reaching the second section (6) is below the threshold for the onset of non-linear effects of the said second section (6).

19. Apparatus as claimed in claim 1, or in any one of claims 8 to 18, wherein the first section (5;

50) comprises first (5a;

50a) and second fibres (5b;

50b), the first fibre (5a;

50a) being arranged so as to convey optical radiation (4) at the probe wavelength towards the second section (6;

60) and the second fibre (5b;

50b) being arranged so as to convey the backscattered optical radiation returned from the second section (6;

60) to the detecting means (2).

2. An optical time domain reflectometry method of sensing a parameter to be measured in a region of interest, which method comprises launching optical radiation (4) at a probe wavelength into an optical fibre (5,6) and producing electrical output signals in response to optical radiation backscattered from the optical fibre (5,6), characterised in that the optical fibre (5,6) comprises first and second sections (5, 6), the second section (6) having a lower intensity threshold for the onset of non-linear effects than the first section (5), the said second section (6) being deployed in the region of interest and optical radiation (4) at the probe wavelength being launched into the said first section (5) at an intensity lower than the non-linear effects intensity threshold of the first section (5) but higher than the non-linear effects intensity threshold of the second section (6), the attenuation characteristics of the said first section (5) being chosen so that the intensity of the optical radiation (4) at the probe wavelength reaching the second section (6) is below that of the non-linear effects intensity threshold of the said second section (6).

3. A method as claimed in claim 2, wherein attenuation of optical radiation (4) at the probe wavelength in the said first section (5) is proportional to the length of the first section (5).

4. A method as claimed in claim 2 or 3, wherein adjacent to the second section (6) the effective area A_eff of the first section (5) decreases towards the effective area A_eff of the second section (6).

5. A method as claimed in claim 4, wherein the decrease in the effective area A_eff is gradual.

6. A method as claimed in claim 4, wherein the decrease in the effective area A_eff takes place in two or more steps.

7. A method as claimed in claim 6, wherein, for a fibre (5,6) consisting of i sections of fibre having a progressively decreasing effective area A_eff, given a known ratio in the backscatter level in the ith and (i+1)th sections and the attenuation per unit length of the (i+1)th section, the optimum length Lopt_i+1 of the (i+1)th section is given by:

${\mathrm{Lopt}}_{i+1}=\frac{10·<br><br>\log <br><br>\left[\frac{B_{i+1}}{B_i}·<br><br>\frac{{\mathrm{\alpha <br><br>}}_{s_{i+1}}}{{\mathrm{\alpha <br><br>}}_{s_i}}\right]-S_{i,i+1}}{{\mathrm{\alpha <br><br>}}_{i+1}}$

where B is the backscatter capture faction, α

is the scattering loss for the spectral component of interest, S is the splicing loss in dB between section i and section i+1 and α

is the total loss of the section in dB/km, and assuming Lopt_i+1 is less than the length of the region of interest and the intensity of the radiation at the junction between the i and (i+1)th sections is below the non-linear effects intensity threshold of the (i+1)th section.

8. Optical time domain reflectometry apparatus, for sensing a parameter to be measured in a region of interest, which apparatus comprises:

an optical fibre (50, 60);

source means (1) operable to launch optical radiation (4) into the optical fibre (50, 60) at a probe wavelength having a preselected intensity; and

detection means (2) operable to produce electrical output signals in response to optical radiation backscattered from the optical fibre (50, 60);

characterised in that the optical fibre (50, 60) comprises a first section (50), into which optical radiation (4) at the said probe wavelength is launched, and a second section (60) deployed in the region of interest, there being a remote amplifier (7;

11a) arranged between the first and second sections (50, 60) which is operable to compensate for attenuation losses in the intensity of the probe wavelength in the said first section (50).

9. Apparatus as claimed in claim 8, wherein the amplifier (7;

11a) comprises a section of fibre doped with rare-earth ions, which fibre has the property of exhibiting gain at the probe wavelength when illuminated by optical radiation at a pump wavelength (10).

10. Apparatus as claimed in claim 9, wherein the amplifier (7;

11a) comprises a single mode fibre doped with erbium ions.

11. Apparatus as claimed in claim 9 or 10, wherein optical radiation (10) at the pump wavelength is transmitted along the same fibre as optical radiation (4) at the probe wavelength and backscattered optical radiation.

12. Apparatus as claimed in any one of claims 9 to 11, further comprising a wavelength-selective reflector (9) provided at the remote end of the amplifier (7) for reflecting any unabsorbed optical radiation (10) at the pump wavelength back through the amplifier (7).

13. Apparatus as claimed in claim 12, wherein the reflector (9) preferably comprises a fibre Bragg grating.

14. Apparatus as claimed in any one of claims 8 to 13, wherein the gain of the said remote amplifier (7;

11a) is selected so as to increase the intensity of the optical radiation (4) transmitted into the second section (60) to a value just below the intensity threshold for the onset of non-linear effects of the said second section (60), thereby allowing the intensity of the radiation launched into the first section (50) to be low compared to the intensity threshold for the onset of non-linear effects of the first section (50).

15. Apparatus as claimed in any one of claims 8 to 14, wherein the amplifier (11a) forms part of an optical sub-assembly (71) provided between the first and second sections (50, 60), which sub-assembly (71) is operable to separate the optical radiation (4) at the probe wavelength from the backscattered optical radiation and transmit it, via the said amplifier (11a) and filtering means (12) for filtering out optical radiation at unwanted wavelengths, into the said second section (60).

16. Apparatus as claimed in claim 15, wherein the optical sub-assembly (71) has a further amplifier (11b) for amplifying backscattered optical radiation returned from the said second section (60).

17. Apparatus as claimed in claim 15 or 16, wherein the optical sub-assembly (71) comprises first and second input/output circulators (13a, 13b) optically connected to the said first and second sections (50, 60) respectively.

18. Apparatus as claimed in claim 17, wherein the said first input/output circulator (13a) is connected to the said first section (50) by a directional coupler (81a), which coupler (81a) is arranged to transmit optical radiation (10) at the pump wavelength to the said amplifier (11a).

20. Apparatus as claimed in claim 19, when read as appended to any one of claims 9 to 18, wherein optical radiation (10) at the pump wavelength is also launched into the first fibre (50a) of the first section (50).

21. Apparatus as claimed in claim 19 or 20, when read as appended to claim 16, wherein optical radiation (10) at the pump wavelength of the said further amplifier (11b) is launched into the second fibre (50b) of the first section (50).

22. An optical time domain reflectometry method of sensing a parameter to be measured in the region of interest, which method comprises launching optical radiation (4) at a probe wavelength with a preselected intensity into an optical fibre (50, 60) and producing electrical output signals in response to optical radiation backscattered from the optical fibre (50, 60), characterised in that the optical fibre (50, 60) comprises first and second sections (50, 60) connected together by a remote amplifier (7;

11a), the gain of the amplifier (7;

11a) being selected so as to compensate for attenuation losses in the probe wavelength in the first section (50).

23. A method as claimed in claim 22, wherein optical radiation (4) is launched into the first section (50) at the probe wavelength at an intensity which is low compared with the intensity threshold for the onset of non-linear effects in the first section (50) and the gain of the remote amplifier (7;

11a) is selected so as to increase the intensity of optical radiation (4) at the probe wavelength transmitted into the second section (60) to an intensity just below the intensity threshold for the onset of non-linear effects of the second section (60).

24. Optical time domain reflectometry apparatus, for sensing a parameter to be measured in a region of interest, which apparatus comprises:

an optical fibre;

source means operable to launch optical radiation into the optical fibre at a probe wavelength; and

detection means operable to produce electrical output signals in response to optical radiation backscattered from the optical fibre;

characterised in that the optical fibre comprises a first section, into which the said optical radiation at the probe wavelength is launched, and a second section deployed in the region of interest, the first section being optically connected to the second section at a location remote from the source means and comprising first and second fibres, the first fibre being arranged so as to convey optical radiation at the probe wavelength towards the second section and the second fibre being arranged so as to convey the backscattered optical radiation returned from the second section to the detecting means.

25. Apparatus as claimed in claim 24, wherein a remote amplifier is provided between the first fibre of the first section and the second section, for amplifying optical radiation at the probe wavelength transmitted along said first fibre of the first section into the second section, and optical radiation at the pump wavelength of the amplifier is also launched into the first fibre of the first section.

26. Apparatus as claimed in claim 25, wherein a further remote amplifier is provided between the second section and the second fibre of the first section, for amplifying backscattered optical radiation returned from the second section.

27. Apparatus as claimed in claim 26, wherein optical radiation at the pump wavelength of the said further amplifier is launched into the second fibre of the first section.