Optical Fiber

Abstract

There is provided an optical fiber for providing increased sensitivity in sensing applications by increasing the Rayleigh backscatter coefficient of the fiber while maintaining tolerable levels of signal attenuation (e.g., less than 20% over 10 km). Such an optical fiber comprises a core, a first cladding layer and a second cladding layer. The core comprises at least one core dopant selected from the range of: germanium, phosphorus, aluminium, boron, fluorine. The at least one core dopant is used to increase the core refractive index and enhance the core Rayleigh backscatter coefficient. The first cladding layer comprises at least one dopant selected from: germanium, phosphorus, aluminium, boron, fluorine; wherein at least one first cladding layer dopant is used to reduce the first cladding layer refractive index. The signal attenuation generated in the fiber is less than 20% over 1 km.

Claims

1. An optical fiber for enhancing the sensitivity of a sensing system, the optical fiber comprising: at least one core having a core diameter, a core numerical aperture, a core refractive index and a core Rayleigh backscatter coefficient; a first cladding layer having a first cladding layer thickness, and a first cladding layer refractive index; wherein the core comprises at least one core dopant selected from the range of: germanium, phosphorus, aluminium, boron, fluorine; wherein at least one core dopant is used to increase the core refractive index and enhance the core Rayleigh backscatter coefficient; the first cladding layer comprising at least one first cladding layer dopant selected from the range of: germanium, phosphorus, aluminium, boron, fluorine; wherein at least one first cladding layer dopant is used to reduce the first cladding layer refractive index; wherein the signal attenuation generated in the fiber is less than 20% over 1 km; and wherein the core numerical aperture is greater than 0.13.

2. An optical fiber according to claim 1, wherein the core dopant used comprises one selected from the range: germanium at a concentration of up to 28 mol %; boron at a concentration of up to 24 mol %; fluorine at a concentration of up to 10 mol %.

3. An optical fiber according to claim 1, wherein the core numerical aperture is greater than 0.17.

4. An optical fiber according to claim 1, wherein the core numerical aperture is greater than 0.22.

5. An optical fiber according to claim 1, wherein the core numerical aperture is greater than 0.25.

6. An optical fiber according to claim 1, wherein the optical fiber diameter is in the range of 30 m to 250 m.

7. An optical fiber according to claim 1, wherein the optical fiber diameter is in the range of 50 m to 125 m.

8. An optical fiber according to claim 1, wherein the optical fiber diameter is in the range of 50 m to 80 m.

9. An optical fiber with enhanced Rayleigh backscatter sensitivity according to claim 1.

10. A sensing system comprising: at least one optical fiber according to claim 1 as a sensing element, the optical fiber being arranged to detect at least one predetermined parameter linked to a change in the back-scatter, the sensing system further comprising: at least one input portion arranged to provide an optical signal and accept an optical signal; and at least one detector portion arranged to accept an output optical signal.

11. A method of manufacture of an optical fiber according to claim 1, the method comprising: a) fabricating an optical fiber preform according to the desired specifications according to claim 1; b) drawing an optical fiber from a drawing tower; and further comprising c) coating said glass fiber with a protective coating layer.

12. A method of manufacture of a sensing system as described in claim 10, the method comprising: a) fabricating an optical fiber preform according to the desired specifications according to claim 1; b) drawing an optical fiber from a drawing tower; and further comprising c) coating said optical fiber with a protective coating layer.

13. An apparatus for the manufacture of an optical fiber as described in claim 1, the apparatus comprising: a fiber drawing tower, including tensioning apparatus and furnace apparatus; further comprising a coating station, a curing station and product spool storage.

14. An apparatus for the manufacture of a sensing system as described in claim 10, the apparatus comprising: a fiber drawing tower, including tensioning apparatus and furnace apparatus; further comprising a coating station, a curing station and product spool storage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Specific embodiments will now be described by way of example only, and with reference to the accompanying drawings, in which:

[0041] FIG. 1 shows a cross-sectional diagram of an optical fiber according to a first aspect of the present invention;

[0042] FIG. 2 shows a graph displaying properties of an optical fiber according to a first aspect of the present invention; and

[0043] FIG. 3 shows a sectional diagram of a sensing system according to a second aspect of the present invention incorporating an optical fiber according to a first aspect of the present invention, manufactured using a method of manufacture according to a third aspect of the present invention, wherein the method of manufacture utilises in-part apparatus according to a fifth aspect of the present invention.

[0044] FIG. 4 is a flow diagram of a method of manufacturing an optical fiber.

DETAILED DESCRIPTION

[0045] Referring to FIG. 1, a cross-sectional diagram of an optical fiber 10 according to a first aspect of the present invention is shown. The optical fiber 10 comprises an optical fiber core 12 arranged about a linear plane L and comprising a plurality of optical gratings 15, whereby about the optical fiber core 12 there is comprised a first cladding layer 14. Arranged about the first cladding layer 14 is comprised a second cladding layer 16. In the embodiment shown, the optical fiber 10 further comprises a coating 20, arranged to cover the circumferential area of the second cladding layer 16.

[0046] Referring to FIG. 2, a graphical representation of the iterative dimensions of the optical fiber layers emerging from the core can be seen on the horizontal axis 42, along with the corresponding refractive indices on the vertical axis 40. Optical fiber core 12 comprises up-doped silica, and therefore the optical fiber core refractive index 24 has been increased compared to the refractive index of un-doped silica. The optical fiber core refractive index 24 has been increased through the use of germanium-containing compounds.

[0047] The first cladding layer 12 comprises down-doped silica, and as such the first cladding layer refractive index 28 has been reduced from the refractive index of un-doped silica. The first cladding layer refractive index 28 has been reduced through the use of fluorine-containing compounds.

[0048] The corresponding increased optical fiber core refractive index 24 and reduced first cladding layer refractive index 28 provides a reduced critical angle for light entering the core, thus providing for an increased acceptance angle 18. As such the optical fiber core diameter 22 can be suitably reduced, thereby providing for an increased optical fiber core numerical aperture.

[0049] The second cladding layer 26, possessing a second cladding layer refractive index 32, comprises un-doped silica.

[0050] The doping regime for the optical fiber according to a first aspect of the present invention provides for a reduced optical fiber core diameter 22 and thus an increased optical fiber core numerical aperture. This increases the optical power density of an optical transmission and has the effect of increasing Rayleigh backscatter coefficient of the fiber thus increasing the resultant Rayleigh backscatter. This backscatter increase enhances the sensitivity of the fiber in applications where the fiber is used as a sensing element.

[0051] Referring to FIG. 3, a sectional diagram of a sensing system is shown, the sensing system incorporating an optical fiber 10 according to a first aspect of the present invention. Additional to the optical fiber 10, there is provided distributed acoustic sensor (DAS), used to interrogate the optical fiber 10 by providing an optical signal and detecting the level of backscatter provided. The use of an optical fiber 10 in the sensing system provides for increased sensitivity of the system, while maintain the range of the system due to signal losses remaining within a tolerable range. In the embodiment shown, the distributed acoustic sensor 34 is one of many deployed along a pipe-system 36 and is used for monitoring the properties of the pipe-system, such as fluid flow and temperature. The enhanced sensitivity of the system by virtue of the incorporation of optical fiber 10 allows the sensing system to detect smaller changes than were previously possible. The improved levels of signal attenuation allow for fewer interrogators 34 due to the ability to use optical fibers 10 that cover a much greater distance. As depth of oil wells continue to increase, together with more stringent global legislation on pipeline monitoring, the low-attenuation aspects of this invention are particularly relevant and important. In the embodiment shown, sensor 34 is further arranged to provide details from the interrogation of optical fiber 10 in order to inform a control system (not shown) of changes to the properties of pipe-system 36 by way of communication line 38. In this way, the sensing system can be used to inform a controller to enact changes to fluid provision to the pipe-system 36 as a result of real-time remote-monitoring of subtle changes within the pipe-system.

[0052] In the embodiments shown, germanium-comprising compounds were used to increase the optical fiber core refractive index, and fluorine-comprising compounds were used to reduce the first cladding layer refractive index. Alternative embodiments will be conceivable wherein other elements or compounds are used as doping agents to increase or decrease the refractive index in the present invention. In alternative embodiments, more than one doping agent may be used for each element, wherein the doping agents provide an increased or reduced refractive index in a cooperative manner, or wherein doping agents are used to counteract extreme refractive index changes caused by other doping agents.

[0053] Further alternative embodiment will be conceivable wherein the optical power within the core together with dopant-dependent backscatter sites are sufficient to cause backscatter without the use of optical gratings within the optical fiber core.

[0054] In the embodiments described, the second cladding layer 26 comprises no refractive index modifying doping agents. Additional embodiments will be envisioned wherein the second cladding layer comprises a second cladding layer dopant.

[0055] There are a number of applications envisaged for the sensing system according to the second aspect of the present invention. There are however, additional embodiments wherein these are not the only applications available for use of a sensing system according to this invention.

[0056] Referring to FIG. 4, a schematic of a method for manufacture of an optical fiber is shown. The method includes fabricating an optical fiber preform according to the desired specifications (44); drawing an optical fiber from the optical fiber preform using a drawing tower (46); and protecting the optical fiber with a protective coating layer (48).

[0057] It will be appreciated that the above described embodiments are given by way of example only and that various modifications thereto may be made without departing from the scope of the invention as defined in the appended claims.