NO343066B1 - Process for the preparation of an optical fiber, an optical fiber and its use - Google Patents

Abstract

Method of coating an optical fiber with a fiber Bragg grating (FBG) with a hermetic (airtight) coating. The coating is in particular a coating of carbon, to avoid the ingress of gases, vapors or liquids from the environment, which can be water or hydrogen, and which can diffuse into the fiberglass and cause drift in the measured Bragg wavelengths. Fiber Bragg gratings are used which maintain the grating strength at temperatures up to and above 1000°C, which are heated to over 1000°C in a chamber with a reactive gas that forms carbon deposits. An optical fiber, a fiber optic sensor and a method for using a fiber optic Bragg grating for a measurement, where the grating is formed in a fiber according to the method are also described.

Description

The invention relates to a method for coating an optical fiber as expressed in the introductory part of claim 1, as well as an optical fiber produced by this method, and a use of such an optical fiber.

A fiber optic Bragg grating (FBG) is a permanent periodic variation of the refractive index in the core of a single-mode optical quartz glass fiber along a typical length of 1-100 mm. It can be formed in a photosensitive fiber by illuminating the fiber from the side with a periodic interference pattern formed by ultraviolet (UV) laser light. It is assumed that variation of the refractive index in a standard FBG is formed by UV-induced breaking of electron bonds in Ge-based defects, which release electrons which are again bound at other places in the glass matrix. This results in a change in the absorption spectrum and in the density, and thus a change in the glass's refractive index. An FBG reflects light within a narrow bandwidth (typically 0.1-0.3 nm), centered on the Bragg wavelength, λB= 2neffL, where neff is the effective refractive index seen by the light propagating through the fiber, and L is the physical period of the refractive index variation . It is known that the reflected Bragg wavelength from an FBG will change with any external influence, such as temperature and mechanical stress (stretch), which changes the effective refractive index seen by the propagating light, and/or the physical grating period ( the fiber length). By measuring the reflected Bragg wavelength, for example using a broadband light source and a spectrometer, an FBG can be used as a sensor to measure such external influences. A standard UV-induced FBG can be made thermally stable up to approximately 300-400 °C, at higher temperatures the UV-induced refractive index variation will weaken rapidly and the grating will be erased.

It is possible to make so-called chemical FBGs that can withstand temperatures up to 1100-1200 °C [Fokine, M., Sahlgren, B. E., and Stubbe, R., "High temperature resistant Bragg gratings fabricated in silica optical fibres", ACOFT- 96, postdeadline paper, 1996, Sidney, Australia and PCT patent application WO 98/12586 to Fokine]. A chemical lattice is typically formed by first writing a regular lattice into hydrogen-containing, fluorine (F) co-doped, Ge-doped quartz glass fibers. UV exposure of such fibers forms OH in the illuminated areas of the fibres, which on heating reacts with F to form HF. Post-heating at temperatures >1000 °C causes HF to diffuse out of the fiber core, leaving UV-exposed areas that are more depleted of F than non-exposed areas, forming a spatially varying F concentration, and thus a refractive index variation (lattice). It is also possible to make other types of special FBGs, which can withstand high temperatures, such as type II FBGs [W.X. Xie et al., Opt. Commun. 1993, Vol. 104, pp. 185-195]. It is known that type II FBGs in germanium-free, nitrogen-doped fibers with quartz glass cores are much more stable at elevated temperatures than conventional type I FBGs [E.M. Dianov et al., Electron. Lett., Vol. 33, pp. 236-237, 1997].

Several FBGs can be wavelength-multiplexed along one fiber, which makes them very attractive for distributed measurements of mechanical impact (stretch) and temperature.

FBGs can also be used as a pressure sensor by measuring the shift in Bragg wavelength caused by hydrostatic pressure-induced compression of the quartz glass fiber [Xu, M. G., Reekie, L., Chow, Y. T., and Dakin, J. P., "Optical infibre grating high pressure sensor ”, Electron. Lett., Vol, 29, pp. 398-399, 1993]. This provides a very simple sensor design with small dimensions and good reproducibility and long-term stability given that the entire sensor construction is made of quartz glass. An all-fiber FBG sensor with improved pressure sensitivity and inherent temperature compensation can be made by using a passive or an active (fiber laser) FBG written into a birefringent side-hole fiber, which has two open channels symmetrically located on either side of the fiber core, [Udd, E., US patents 5,828,059 and 5,841,131, Kringlebotn, J. T., Norwegian patent application 19976012 (passive FBG sensors) and Kringlebotn, J. T., US patent 5,844,927 (active FBG sensor)]. It is also possible to make FBG pressure sensors with improved pressure sensitivity by using a glass transducer element surrounding the optical fiber, either to convert pressure into mechanical impact (stretch)/compression in the fiber, or to convert pressure into birefringence in the fiber [ Udd, E., US patent 5,841,131].

When fiber optic sensors are used under conditions of high temperature, such as in oil wells, significant drift effects can occur both in the FBG and birefringent interferometric sensors, as shown by J. R. Clowes et al. in "Effects of high temperature and pressure on silica optical fiber sensors", IEEE Photon. Technol. Lett., Vol. 10, pp. 403-405, 1998. The drift effect occurs when the fiber is surrounded by a liquid, such as water or oil, and increases with increasing temperature. It is assumed that the effect is due to the penetration of liquid molecules into the outer layers of the fiber sheath, which leads to the formation of a strongly mechanically stressed layer and consequently a mechanical tensile stress in the fiber core. This increases the optical path length of the fiber and changes the Bragg wavelength of an FBG. This effect will also change the birefringence of a strongly birefringent fiber. Clowes et al. demonstrated that the increase in optical path length in a fiber was reduced by one order of magnitude by using a hermetic (tight), carbon-coated fiber.

In addition, diffusion of gases, such as hydrogen, into the fiber will cause the refractive index to change proportionally with the hydrogen concentration, and thus lead to operation in the Bragg wavelength of an FBG written into the fiber core, as described by Malo et al. in "Effective index drift from molecular hydrogen diffusion in hydrogen-loaded optical fibers and its effect on Bragg grating fabrication", Electronics Letters, Vol. 30, pp. 442-444, 1994. Hydrogen will also cause an increase in the loss in an optical fiber, which can be destructive to FBG-based fiber lasers doped with rare earths. Finally, diffusion of gases into the holes in a side hole fiber will change the pressure inside the holes, and thus the pressure difference, which affects the measurement of the external hydrostatic pressure.

As shown by Kringlebotn in Norwegian patent application 19976012, a practical whole fiber FBG pressure sensor without operation when used under high temperature can be realized by recoating an FBG in a side hole fiber with a hermetic coating to prevent the ingress of gases, vapors or liquids from the surrounding environment. However, nothing is mentioned about how such a coating can be applied to an FBG.

A. Hay [US patent 5,925,879] shows the use of a carbon coating, or other hermetic coating on an FBG sensor to protect the optical fiber and sensors from harsh environments.

Carbon has been shown to provide a good hermetic coating for optical fibers, making them largely impermeable to both water and hydrogen, while maintaining the mechanical strength and low loss in the fiber. A carbon coating can be applied to an optical fiber during the drawing process, before the fiberglass is cooled, by a pyrolytic process [see, for example, US patent 5,000,541, to DiMarcello et al.]. Carbon coating by a similar technique can also be applied to joints between hermetic fibers to maintain air tightness after the splicing of carbon coated fibers [US patent 4,727,237, to Schantz, C. A., et al.]. In this latest patent, a pyrolytic technique is used based on heating the fiber joint area with a CO2 laser inside a chamber containing a reactant gas, causing a carbon coating to form on the glass surface by pyrolysis of the reactant gas. During such a process, the temperature in the fiber will typically exceed 1000 °C. This high temperature pyrolytic process has been shown to produce highly airtight coatings and appears to be the technique of choice for carbon coating optical fibers. However, a standard FBG, i.e. a so-called type I FBG in a germanium doped quartz glass fiber, cannot be carbon coated using this process, as it will be erased at the high temperature involved.

The main purpose of the invention is to produce a method for re-coating an FBG in an optical fiber of non-crystalline quartz glass, or an FBG inside an element of non-crystalline quartz glass, with a hermetic (air-tight) carbon coating to prevent in-diffusion of molecules from the surrounding liquid or gas into the glass at high temperatures, thereby removing or reducing drift in the Bragg wavelength of the FBG. This is particularly important for FBG-based sensors for temperature, mechanical stress (stretch) and pressure, and which are used at high temperatures, for example in oil wells, in refineries or applications in industrial processing. Furthermore, it is an object to provide a method which prevents losses in FBGs resulting from in-diffusion of hydrogen. Finally, it is a purpose that the method maintains the mechanical strength of the fiber or the glass element and maintains the lattice strength (lattice reflectivity).

The object of the invention is achieved by a method which has the characterizing features in claim 1. Further features are expressed in the dependent claims. The method involves applying a high-temperature carbon coating technique to a special fiber with Bragg gratings that can withstand and maintain its grating strengths (reflectivity) at elevated temperatures, typically above 1000°C. These grids can be so-called chemical grids, i.e. grids that include a variation of the chemical composition along the grid. The chemical grating can be formed in a suitable hydrogen-containing optical fiber, usually a germanium, fluorine co-doped quartz glass fiber, where the fiber can be a weakly birefringent fiber or a strongly birefringent fiber, such as a side hole fiber. The grating can also be other types of special high-temperature gratings, such as type II gratings, for example in germanium-free, nitrogen-doped quartz glass fibres. The high temperature carbon coating technique will usually be a pyrolytic technique based on heating the fiber in a chamber containing a reactant gas which causes a carbon coating to form on the glass surface by pyrolysis of the reactant gas.

The carbon-coated FBG should be coated with a second protective coating that protects the carbon mechanically and prevents the carbon from burning off at high temperatures.

This additional coating can be a polyimide, silicone or acrylate coating, or a thin metal coating such as gold.

The carbon coating process can be combined with the high-temperature heating process needed to make the chemical lattice, so that heating the fiber inside a chamber containing a reactant gas both forms the chemical lattice and deposits a carbon coating on the glass surface. This reduces the number of processing steps and reduces the handling of the fiber to a minimum and gives the grid a high mechanical strength. The second recoating process can also be incorporated inside the chamber by placing the carbon coated fiber in a mold filled with the coating material, and heating this with an internal heating device to harden the coating material and form a suitable coating.

A particular method of providing a hermetically sealed fiber optic Bragg grating (FBG) to limit penetration by diffusion into the optical fiberglass from ambient fluids is set forth in claim 21.

A special optical sensor provides a hermetically protected fiber optic Bragg grating (FBG) to limit penetration by diffusion into the optical fiberglass from ambient fluids is expressed in claim 22.

EXAMPLES

In the following, the invention will be described with reference to the illustrations, there

Fig. 1 shows the section of a chemical FBG written into a side hole fiber with a carbon coating and an additional coating of polyimide.

Fig. 2 is a schematic illustration of a chamber for re-coating a high temperature FBG with carbon.

Fig. 3 shows the long-term drift of the Bragg wavelength for a chemical FBG exposed to silicone oil at 200 °C, both with and without carbon re-coating.

Fig. 1 shows the cross-section of a chemical FBG in the core 1 of a side-hole fiber 2 with a carbon coating 3 formed by a pyrolytic process and an additional coating 4 of polyimide which protects the carbon-coated FBG.

Fig. 2 is a schematic illustration of a chamber for re-coating a high temperature FBG, such as a grating, with carbon. A high-temperature chemical grating 1 in a stripped area 2 of an optical fiber 3 is placed inside a sealed chamber 4. The fiber enters and exits the chamber through pressure seals/throughs 5. A gas mixture 6 of a reactive gas, for example acetylene , and nitrogen enters the chamber through an inlet opening 7 and exits through an outlet 8, creating a gas flow with a small excess pressure inside the chamber that keeps other gases out. The fiber is heated by a scanning CO2 laser beam 9 that enters the chamber through a transmitting window 10.

Fig. 3 shows the long-term drift of the Bragg wavelength in a chemical FBG exposed to silicone oil at 200 °C, with (1) and without (2) a carbon coating and shows that a carbon coating eliminates the drift of the wavelength in such an environment, which is believed to arising from changes in the fiber's mechanical stresses due to the penetration of molecules from the oil in the environment into the outer layers of the fiber sheath.

Claims (27)

Requirement 1. Method for providing a hermetically protected fiber optic Bragg grating (FBG) which provides minimum penetration by diffusion into the optical fiberglass from gases in the surrounding environment, in the form of vapor or liquid and causing unwanted deviation/drift in the measurement of the grating's Bragg wavelengths, comprehensive to provide a high temperature Bragg grating sustaining optical fiber, comprising an optical fiber with an inscribed grating which maintains the grating strength at temperatures above 1000°C has been inscribed, and providing said fiber with a hermetic coating in a process comprising heating said optical fiber. 2. Method according to claim 1, where the gases in the surrounding environment, in the form of steam or liquid, are water or hydrogen. 3. Method according to claim 1 or 2, wherein the step of providing a high temperature stable optical Bragg grating fiber comprises providing an optical fiber in which a standard Bragg grating is written; and to form a chemical lattice in said optical fiber which has a periodic chemical variation in composition along said fiber with a corresponding periodic variation in refractive index upon heating said fiber, causing components of the optical fiber to outgas/diffuse out of the glass, thereby forming a refractive index variation. 4. Method according to claim 1 or 2, wherein providing an optical fiber in which a standard Bragg grating is written comprises providing an optical fiber in which a standard Bragg grating is written by illumination with ultraviolet light. 5. Method according to claim 3 or 4, where the two processes of developing the chemical lattice and providing the fiber with a hermetic coating are included in the same heating process. 6. Method according to claim 3 or 4, where the formation of the Bragg grating and the hermetic coating are performed in the same step. 7. Method according to claim 3 or 4, where the periodic chemical variation is formed in a hydrogen-containing, fluorine-doped, germanium-doped quartz glass fiber. 8. Method according to one of claims 1 to 7, wherein the step of providing a high temperature stable optical Bragg grating fiber comprises to provide a type II optical Bragg grating fiber. 9. Method according to one of claims 1 to 7, wherein the step of providing a high temperature stable optical Bragg grating fiber comprises to provide a type II optical Bragg grating fiber written into a germanium-free, nitrogen-doped, quartz glass fiber. 10. Method according to one of claims 1 to 9, wherein said hermetic coating is formed as a result of a pyrolytic process, wherein the pyrolytic process comprises pyrolysis of a reactive gas causing the formation of a carbon coating which is deposited on the surface of said optical fiber. 11. Method according to claim 10, where the carbon-coated fiber is coated with an additional coating, to protect the carbon coating from mechanical damage and damage from heating. 12. Method according to claim 10, where the carbon-coated fiber is coated with an additional coating of polyimide, silicone or acrylate. 13. Method according to claim 10, where the carbon-coated fiber is coated with an additional thin metal coating. 14. Method according to claim 13, where additionally thin metal coating comprises a coating of gold. 15. Method according to one of claims 10 to 13, where the reactive gas is acetylene. 16. Method according to one of claims 1 to 15, where the heat is supplied by means of a laser beam. 17. Use of an optical fiber with a hermetically protected Bragg grating as provided by a method according to one of claims 1 to 16 for measuring pressure differences in an environment at elevated temperatures and with a liquid environment, where the hermetically protected Bragg grating fibers provide mainly measurements of pressure or pressure differences that are free from operation. 18. Use of an optical fiber according to claim 17, where the fluid environment comprises the heated water or oil in an oil well. 19. Optical fiber produced by the method defined in one of claims 1 to 18, and used as a hydrostatic pressure sensor, with optical fibers having a core and a sheath, where a high-temperature stable fiber optic Bragg grating is written into the core of the optical fiber, there two side holes are included in the optical fiber's jacket, where the side hole optical fibers are spliced between ordinary single mode fibers, so that a change in the pressure difference between the surroundings and the side holes will cause a change in the birefringence of the fiber and thus a change in the wavelength difference between the two the reflection peaks of the grating according to each of the two orthogonally polarized eigenmodes in the fiber, with an increased pressure sensitivity compared to the pressure-induced change of wavelength in a regular fiber grating, and the fiber with two holes has a total birefringence in the operating pressure range, so that the two the peaks do not cross each other during operation, and further that the fiber optic Bragg grating in the optical fiber with side holes is coated with a hermetic coating to avoid the penetration of gases, vapors or liquids in the surrounding environment, which may be water or hydrogen which diffuses into the fiberglass and causes deviations/drift in the measured Brag the g wavelengths. 20. Fiber optic sensor for measuring temperature and hydrostatic pressure, where a fiber Bragg grating is written into a quartz glass optical fiber with at least two side holes in the sheath of the optical fiber, where the fiber with side holes is spliced between two common single mode fibers, and where the grating is of a type which maintains the grating strength at temperatures above 1000°C according to one of claims 1 to 6, and that the grating in the fiber with side holes is coated with a hermetic coating of carbon by a method defined in one of claims 1 to 16. 21. Method of providing a hermetically sealed fiber optic Bragg grating (FBG) to limit penetration by diffusion into the optical fiberglass from ambient fluids, comprising: writing a Bragg grating into an optical fiber, where the Bragg grating maintains its strength at temperatures above 1000°C; and forming a hermetic coating on the optical fiber by a process including heating the optical fiber; wherein writing the Bragg grating comprises: introducing a precursor Bragg grating into the optical fiber by illumination with ultraviolet light; and then heating the optical fiber to form the Bragg grating with a periodic chemical variation in composition along the optical fiber with a corresponding periodic variation in refractive index due to components of the optical fiber gassing/diffusing out of the optical fiber thereby producing a refractive index variation. 22. An optical sensor provides a hermetically sealed fiber optic Bragg grating (FBG) to limit penetration by diffusion into the optical fiberglass from ambient fluids, comprising: an optical fiber having a Bragg grating written therein, the Bragg grating maintaining its strength at temperatures above 1000°C; and a pyrolytic hermetic coating formed on the optical fiber by heating the optical fiber. 23. The optical sensor according to claim 22, wherein the optical fiber is configured to provide measurement of temperatures and hydrostatic pressure. 24. The optical sensor according to claim 23, where the coating is a carbon coating. 25. The optical sensor according to claim 22, wherein the optical fiber is configured to provide measurement of hydrostatic pressure. 26. The optical sensor according to claim 25, where the optical fiber has a core and cladding with the Bragg grating written into the core and with at least two side holes in the cladding. 27. The optical sensor according to claim 26, where the optical fiber is spliced between two ordinary single-mode fibers, so that a change in the pressure difference between the surroundings and the side holes will cause a change in the birefringence of the optical fiber and thus a change in the wavelength difference between the two reflection peaks of the grating according to each of the two orthogonally polarized eigenmodes in the optical fiber.