Fiber optic sensors for harsh high temperature high pressure environments

Abstract

The present invention relates to a fiber optic sensor comprising a bare silica optical fiber and a connector for mounting on the wall delimiting the area of interest. Characterized in that said optical fiber is stripped on the part extending between the distal end of the silica core (1) and said connector and in that said fiber is bonded to said connector by a ceramic seal (23) between the metal coating and the inner surface of a passage channel in said connector. Abstract Figure: Figure 1

Description

Title of the invention: Fiber optic sensors for severe high temperature high pressure environments Technical field of the invention

[0001] The present invention relates to the field of fiber optic sensors for detecting position, object or UV-VIS-IR radiation in severe environments at high temperatures or high pressures, such as aircraft engines, jet engines, heat engines, boilers, furnaces and thermal test benches. The fiber optic sensors can be used to measure temperature variations in severe environments and can be designed to withstand high temperatures and high pressures.

[0002] The invention specifically relates to a sensor intended to operate in extreme environments of high temperature 1000°C and / or high pressure 500 bars (50 Mpa).

[0003] This type of sensor uses a fiber whose distal end (the one on the side of the area of interest) receives light information, for example an optical reflection. These sensors typically implement a transmission fiber and a reflection fiber as well as a means for reflecting the signal at the point of movement.

[0004] These sensors detect movements or a position, they have the advantage of being insensitive to electromagnetic interference and are both mechanically and thermally robust.

[0005] Such sensors use different types of fibers in the prior art: - Heat-resistant silica-based optical fibers that can be used in high-temperature environments. - High-temperature optical fibers specially designed to withstand extremely high temperatures. They are often made from materials such as boron-doped silica or germanium-doped silica. - Protective glass optical fibers: Protective glass optical fibers can be used to protect optical fibers from extreme conditions such as heat, pressure and UV radiation.

[0006] Some fiber optic sensors use angled optical fibers to direct light to hard-to-reach angles. They are often used in applications such as position and radiation sensing. State of the art

[0007] On the optical detection market, there are a multitude of fiber optic sensors or detectors operating in low-constraint environments (ambient), but very few for severe or extreme environments.

[0008] In order to overcome this difficulty, the use of metal tubes with sapphire at the end of the optical fibers as a protective window is necessary and expensive. Added to this is the degradation of the contrast of the measurements (Input / Output) caused by the mixing of the two waves (outgoing / return) in the sapphire.

[0009] Known in the state of the art is patent US9810557B2 describing fiber optic sensor solutions based on multi-conductor optical fibers intended to be used for detection in harsh environments. This multi-core fiber comprises an arrangement of cores optically coupled in a silica background. The sensors are manufactured by splicing a section of multi-core fiber between two single-mode fibers. This patent proposes fibers comprising a solid silica cladding in which each of the at least seven optical cores of the MCF are Ge-doped cores embedded in a solid silica cladding.

[0010] Patent US6612752 describes a protection system for optical components, comprising a container, at least one optical component secured within said container, a length of optical fiber which has a plastic coating and which is connected to said at least one optical component, and, an optical feedthrough for said length of fiber, placed in a through-hole in a wall of said container and hermetically secured therein, said optical feedthrough comprising an elongate body which has a longitudinal through-hole in which said optical fiber can be placed, and, a portion of said length of fiber being stripped of said coating, wherein said portion of fiber which is stripped of the coating is welded to one end of said elongate body by means of a metal weld,such a weld in a surface portion around said fiber portion being coated with a layer of a polymeric sealant to provide contact between said weld and said polymeric sealant.

[0011] US Patent 5,177,806, which relates to an optical feedthrough in which the fiber, stripped of the protective acrylate layer, is welded in a tube by means of glass powder ("glass solder"). In particular, this patent describes an optical feedthrough in which a fiber is held in a fixed position in a metal sleeve, while a glass powder is brought to a high temperature and then cooled to form a weld inside this sleeve on the bare fiber.

[0012] Patent application EPI 145062A2 relates to a protection system for optical components, which comprises a container at least one optical component fixed inside said container and a length of optical fiber provided with a plastic coating and connected to said optical component. An optical feedthrough for said fiber section is placed in a through-hole of a wall of said container and is hermetically secured therein, said optical feedthrough comprising an elongate body having a longitudinal through-hole in which said optical fiber and a portion of said fiber length stripped of said coating can be placed. This portion of fiber stripped of coating is soldered to one end of said elongate body using a metal solder and this solder is covered, at the surface portion surrounding said fiber portion, with a layer of polymeric sealant.

[0013] Patent US5126558A describes a displacement sensor using a metal-coated optical fiber (copper or aluminum). This sensor is formed by an optical fiber pressure means which comprises:

[0014] (a) at least one optical fiber capable of providing optical attenuation in response to a micro-bending of said optical fiber, said optical fiber having an input portion, an output portion and a loop portion coupled on one side to said pressure pad and on the opposite side to a support structure;

[0015] (b) pressure pad means for measuring pressure applied to a pad pressure wherein said buffer is located adjacent to said element and is laterally movable in response to pressure;

[0016] (c) signal source means coupled to the input portion of the optical fiber of the “metal-coated glass” type to introduce an optical signal;

[0017] (d) detection means coupled to the output end of the optical fiber for measuring the optical signal at the output end of said optical fiber; and

[0018] (e) means for determining the modulation of said optical signal between the part input and output portion of said optical fiber to provide a measurement of the lateral movement of said pressure pad in response to said pressure.

[0019] Patent US6978074B2 describes a sensor for detecting flashback occurrences in a premix combustor system having at least one fuel nozzle includes at least one photodetector and at least one fiber optic element coupled between the at least one photodetector and a test region of the combustor system in which a respective flame of the fuel nozzle is not present under normal operating conditions. A signal processor monitors a signal from the photodetector. The fiber optic element may include at least one optical fiber positioned within a protective tube. The fiber optic element may include two fiber optic elements coupled to the test region. The optical fiber and the protective tube may have lengths sufficient to locate the photodetector outside an engine compartment.A plurality of fuel injectors and a plurality of fiber optic elements may be used, the fiber optic elements being coupled to the respective fuel injectors and either the photodetector or, in . in which a plurality of photodetectors are used, to the respective ones of the plurality of photodetectors. This patent proposes in particular the use of optical fibers made of metal-coated silica which can withstand temperatures of the order of 700°C.

[0020] Disadvantages of the prior art

[0021] The solutions of the prior art are not satisfactory because they do not allow uses with severe constraints of high temperature and / or pressure, typically high temperature up to 1000 °C, high pressure bent optical fiber up to 500 bars (50 Mpa). Bare silica optical fibers (without polymer or metallic coating) are very sensitive to mechanical stresses.

[0022] Solutions using metal-coated fibers are also not satisfactory because the expansion rate of the metal (aluminum or copper) is much higher than that of silica, which causes thermal expansion elongation differentials between the silica core and the metal sheath such that the large fiber end damages the distal portion of the fiber.

[0023] Furthermore, the copper sheaths degrade by an oxidation phenomenon upon contact with the oxygen in the air when the distal end of the probe is subjected to high temperatures. Statement of the invention

[0024] The present invention aims to address these drawbacks. To this end, the invention relates, in its most general scope, to a fiber optic sensor capable of operating in extreme environments with high temperature greater than or equal to 1000°C and high pressure greater than or equal to 500 bars (50 Mpa) having the following characteristics.

[0025] The sensor comprises a bare silica optical fiber with a polymer or metallic coating or alternatively inserted in a capillary, and a connector for mounting on the wall delimiting the area of interest. This optical fiber is stripped on the part extending between the distal end of the silica core and said connector and in that said fiber is bonded to said connector by a ceramic seal between the metallic coating and the interior surface of a passage channel in said connector.

[0026] The subject matter of the present invention may also have one or a compatible combination of the following characteristics: • said seal extends over a length of between 10 and 100 millimeters • said fiber has a folding zone • said folding zone (5) forms an angle of between 30° and 170° with a radius of curvature of less than 2.5 millimeters for a fiber having a core with a diameter greater than 0.6 millimeters • the sensor includes a bundle of optical fibers • it comprises a bare optical fiber with a Silica core in a Silica capillary.

[0027] Description of non-limiting examples of embodiment

[0028] The present invention will be better understood on reading the following description, concerning a non-limiting example of embodiment illustrated by the appended drawings where:

[0029] [Fig.l] represents a perspective view of the end of a Silica core optical fiber with its coating

[0030] [Fig.2] represents a schematic view of a first variant of a single-fiber optical sensor or detector

[0031] [Fig. 3] represents a schematic perspective view of a second variant of a single-fiber optical sensor or detector of the elbow type

[0032] [Fig.4] represents a mechanical sectional view of a second variant of a single-fiber optical sensor or detector of the elbow type.

[0033] [Fig. 5] shows a schematic view of a third variant of an elbow-type fiber optic beam sensor or detector

[0034] [Fig.6] represents a schematic view of a fourth variant of an elbow-type fiber optic beam sensor or detector.

[0035] [Fig.7] represents a schematic view of a fifth variant of a single-fiber optical sensor or detector in a silica capillary.

[0036] General principle of the invention

[0037] The invention relates to a solution for detecting position or object or radiation, based on silica optical fibers - alone or in a capillary - straight or bent, in a harsh environment under high temperature up to 1000°C and / or high pressure up to 50 Mpa, environments incompatible with users or instrumentation.

[0038] The principle of the invention consists in producing a sensor with an optical fiber with a silica core (1) and a silica optical sheath (2), the distal end of which is stripped of its polymer or metallic coating, to retain only the silica core (1) over a length extending from the front end of the optical fiber to the connector for mounting on the envelope of the area of interest. The optical fiber is bonded to the connector by a seal (23) using a ceramic powder or paste, in particular Zirconia, which makes it possible to create a bond between the optical fiber and the connector with a low coefficient of expansion, and resistance up to a temperature of the order of 1400 °C.

[0039] This connection between the optical fiber (10) and the connector by a ceramic seal (23) extends over a length of 10 to 100 mm.

[0040] The transmission range extends from 220 to 2400 nm depending on the choice of UV-VIS or VIS-NIR silica fiber core. The working temperature range is from -273°C to 1000°C; humidity - up to 100%.

[0041] The invention proposes a sensor comprising a bare silica optical fiber.

[0042] The variant illustrated by [Fig.2] corresponds to a single-fiber variant where the light is transmitted by the fiber (1) to illuminate the area of interest (4) and the reflected or scattered light is returned to a photometer by the same optical fiber (10).

[0043] The variant illustrated by [Fig. 3] corresponds to a bent variant, comprising a longitudinal tube (6) opening into a cylindrical sleeve (7) whose axis is inclined, in the example described by 90°, relative to the axis of the longitudinal tube (6). The optical fiber (10) has a bending zone (5).

[0044] The variant illustrated by [Fig.4] corresponds to a variant based on a bundle of fibers (11 to 14) making it possible to assign the central fiber to the emission of light radiation in the direction of the area of interest, and the peripheral fibers to the transmission of the radiation reflected or diffused by the object (4).

[0045] The variant illustrated by [Fig.5] corresponds to a variant based on a bundle of fibers (11 to 14) making it possible to assign one or more of the fibers to the emission of light radiation in the direction of the area of interest, and one or more fibers to the transmission of the radiation reflected or diffused by the object (4), in a bent configuration with a folding zone (5).

[0046] According to another variant, the sensor consists of a multimode optical fiber with a core having a diameter of more than 0.4 mm, introduced into a capillary in the form of a thick tube (internal diameter < 1 mm / external diameter < 2 mm). The two tubes are made of silica (SiO2). The assembly can be straight or bent. The external surface of the optical fiber (10) is held to the internal wall of the capillary (3) by ceramic bonding or fusion between fiber and silica tube.

[0047] In this configuration, the light is emitted by the fiber core and the reflected light is captured by the capillary. The refractive index of the capillary is close to that of the fiber core and higher than that of the cladding. The latter is the buffer layer between the core and the capillary and acts as an optical insulator. As a result, the light waves are forced, according to the Snell-Descartes law, to propagate either in the core or in the capillary, without interfering. This will greatly improve the contrast of the measurements.

[0048] The possible curvature of these optical fibers is achieved by thermal effect with a very low RC curvature radius of 3 mm (for a fiber version with a 0.6 mm core), radius much smaller than the bending radius recommended by the fiber manufacturer and therefore much more interesting for very confined areas.

[0049] Straight or angled optical / capillary fiber sensors or detectors are positioned and maintained in a high temperature or high pressure environment. They allow reaching narrow and difficult to access places depending on the fiber lengths used. These fibers can pass through the walls confining the environments through specific sealed feedthroughs to be connected to transmission and measurement equipment, outside in user or instrumentation compatible environments.

[0050] Optical assembly: optical fiber / capillary Silica

[0051] The multimode optical fiber (core > 0.4 mm plus cladding) introduced into a capillary (internal diameter < 1 mm / external diameter < 2 mm), both are made of silica. The optical fiber is held inside the capillary by ceramic bonding or fusion with the capillary. The assembly has very high temperature compatible materials which can be shaped, sized and packaged according to the size.

[0052] Method of “folding” the fiber:

[0053] The optical fiber or bundle of silica optical fibers (multimode) are bent by thermal effect. The mechanical coating of the fiber is removed in the fusion zone (approximately 1700°C) on which the thermal bending is carried out.

[0054] We can thus obtain a 90° bend (or an angle curvature of 30° to 170°) on a fiber > 600 pm in diameter with a very small radius of curvature RC < 3 mm, the size of which is compatible with a cube of approximately 5 mm.

[0055] At no time is there any break in the optical guidance. The device obtained is therefore perfectly stable, insensitive to vibrations and compatible with very high temperatures (up to 1000°C).

[0056] The optical transmission loss generated by this action is of the order of 20% for a 90° bend.

Claims

Claims

1. Optical fiber sensor capable of operating in extreme environments with high temperature greater than or equal to 1000°C and / or high pressure greater than or equal to 50 Megapascals comprising a silica optical fiber without polymer or metallic coating and a connector for mounting on the wall delimiting the area of interest characterized in that said optical fiber is stripped on the part extending between the distal end of the silica core (1) and said connector and in that said fiber (1) is bonded to said connector by a ceramic seal (23) between a metallic coating and the inner surface of a passage channel in said connector.

2. Fiber optic sensor according to claim 1 characterized in that said seal (23) extends over a length of between 10 and 100 millimeters.

3. Optical fiber sensor according to claim 1 characterized in that said fiber has a folding zone (5).

4. Fiber optic sensor according to the preceding claim characterized in that said folding zone (5) forms an angle between 30° and 170° with a radius of curvature less than 2.5 millimeters for a fiber having a core (1) with a diameter greater than 0.6 millimeters.

5. Fiber optic sensor according to claim 1 characterized in that it comprises a bundle of optical fibers.

6. Optical fiber sensor according to claim 1 characterized in that it comprises a bare optical fiber with a Silica core in a Silica capillary.

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