Fibre optic sensors for severe high-temperature and high-pressure environments

Abstract

The present invention relates to a fibre optic sensor comprising an exposed silica optical fibre and a connector for mounting on the wall delimiting the area of interest, characterized in that said optical fibre is stripped on the part extending between the distal end of the silica core (1) and said connector, and in that said fibre is connected to said connector by a ceramic seal (23) between the metal coating and the inner surface of a passage channel in said connector.

Description

[0001] DESCRIPTION

[0002] Title: Fiber optic sensors for harsh high temperature high pressure environments

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The present invention relates to the field of fiber optic sensors for position, object or UV-VIS-IR radiation detection in harsh 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 harsh environments and can be designed to withstand high temperatures and high pressures.

[0005] The invention specifically relates to a sensor intended to operate in extreme environments of high temperature 1000°C and / or high pressure 500 bars.

[0006] 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 of reflecting the signal at the point of movement.

[0007] These sensors detect movement or position, they have the advantage of being insensitive to electromagnetic interference and are both mechanically and thermally robust.

[0008] Such sensors use different types of fibers in the prior art:

[0009] Heat-resistant silica-based optical fibers that can be used in high-temperature environments.

[0010] High-temperature optical fibers are specially designed to withstand extremely high temperatures. They are often made from materials such as boron-doped silica or germanium-doped silica.

[0011] 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. Some fiber optic sensors use angled optical fibers to direct light at hard-to-reach angles. They are often used in applications such as position and radiation sensing.

[0012] STATE OF THE ART

[0013] 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 harsh or extreme environments.

[0014] 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 measurement contrast (Input / Output) caused by the mixing of the two waves (forward / return) in the sapphire.

[0015] Known in the prior art is patent US2002 / 106179 describing an optical path extending between a first and a second zone sealed with respect to each other by means of a seal, said path comprising at least one optical fiber. This optical fiber is provided with a metallized coating and passes through the seal such that said metallized coating is in direct contact with said seal.

[0016] Patent CN209068453 discloses a high-temperature fiber optic sensor comprising a hollow-type temperature sensing metal shell, an optical fiber, a fiber optic sensing element, and a metal-armored optical cable at both open ends. The optical fiber and a fiber optic sensing element are placed in the temperature sensing metal shell, and around the optical fiber and the fiber optic sensing element are filled with nitrogen or inert gas. The two-port metal shell is sealed by the tail plug and the preceding terminal adapter.

[0017] US2008232745 relates to a method of assembling a Fabry-Perot interferometer comprising depositing a first metal layer on an end portion of a ferrule, depositing a second metal layer on a back portion of a die, placing the first metal layer and the second metal layer in contact with each other with the respective first and second orifices aligned with each other, and bonding the ferrule to the die by thermocompression. The resulting interferometer comprises a glass die with a cavity, a silicon diaphragm disposed over the opening of the cavity and bonded to the glass die, a ferrule bonded to the glass die by thermocompression with the first and second orifices aligned with each other, and an optical fiber inserted through the other end of the ferrule in direct contact with a back portion of the die and aligned with the first orifice.

[0018] Patent US2008273852 relates to remote sensing in an environment with temperatures above 300 gC, using an optical fiber having a core (10), a cladding (20), and a protective metal coating (30) on the cladding to protect a surface of the cladding, the cladding having a diameter greater than 150 μm, and a thickness of at least 50 μm. The larger diameter cladding means that stress from the protective metal layer can be reduced, resulting in lower optical loss and better hydrogen protection. A metal conduit encapsulates the sensing fiber, and a pump evacuates the conduit to reduce hydrogen loss. Ceramic splice protectors are used. Optical time domain reflectometry is used to determine the differential loss at various locations along the fiber. A reflective element at the end of the fiber facilitates calibration.

[0019] Patent application EP3546087 relates to a temperature sensor intended for measuring the temperature of a section of a component of a metallurgical installation, which has:

[0020] - at least one optical waveguide which comprises fiber Bragg gratings made by being arranged against the optical waveguide, spaced from each other in the longitudinal direction of the optical waveguide

[0021] - at least one protective tube made of a metal or a metal alloy which peripherally surrounds the optical waveguide with radial clearance, which tube can be introduced together with the optical waveguide incorporated therein, into a sensor bore against the component section and

[0022] - at least one connector intended to connect the optical waveguide to an electronic device acting as an optoelectric type sensor.

[0023] The connector has a connecting socket made of a metal or a metal alloy, through which is guided an end section of the optical waveguide connected directly to the connector which has been incorporated into the connector, and which is welded to the protective tube.

[0024] DISADVANTAGES OF PRIOR ART

[0025] 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. Bare silica optical fibers, i.e. without polymer or metallic coating and without mechanical protection, are very sensitive to mechanical stresses

[0026] Solutions using metal-clad 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 part of the fiber.

[0027] Furthermore, copper sheaths degrade through oxidation when exposed to oxygen in the air when the distal end of the probe is subjected to high temperatures.

[0028] STATEMENT OF THE INVENTION

[0029] 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 temperatures greater than or equal to 1000°C and high pressure greater than or equal to 500 bars, having the characteristics set out in claim 1.

[0030] The sensor comprises a bare silica optical fiber or, alternatively, inserted into a capillary, as well as 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 it is bonded to said connector by a ceramic seal between the silica core and the inner surface of a passage channel in said connector. The assembly is protected by a mechanical tube held by the connector, which makes the fiber floating inside said tube.

[0031] The subject matter of the present invention may also have one or a compatible combination of the following characteristics:

[0032] ■ said seal extends over a length of between 10 and 100 millimeters

[0033] ■ said fiber has a folding zone

[0034] ■ 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

[0035] ■ the sensor includes a bundle of optical fibers

[0036] ■ it comprises a bare optical fiber with a Silica core in a Silica capillary. DESCRIPTION OF NON-LIMITING EXAMPLES OF EMBODIMENT

[0037] 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:

[0038] Figure 1 shows a perspective view of the end of a Silica core optical fiber with its cladding

[0039] Figure 2 represents a schematic view of a first variant of a single-fiber optical sensor or detector.

[0040] Figure 3 shows a schematic perspective view of a second variant of a single-fiber optical sensor or detector of the elbow type.

[0041] Figure 4 shows a mechanical cross-section of a second variant of a single-fiber optical sensor or detector of the elbow type.

[0042] Figure 5 shows a schematic view of a third variant of an elbow-type fiber optic beam sensor or detector

[0043] Figure 6 shows a schematic view of a fourth variant of an elbow-type fiber optic beam sensor or detector.

[0044] Figure 7 represents a schematic view of a fifth variant of a single-fiber optical sensor or detector in a silica capillary.

[0045] GENERAL PRINCIPLE OF THE INVENTION

[0046] 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 500 bars, environments incompatible with users or instrumentation.

[0047] 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 keep 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 linked to the connector by a seal (23) by a ceramic powder or paste, in particular Zirconia, which makes it possible to create a link 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. This connection between the optical fiber (10) and the connector by a ceramic seal (23) extends over a length of 10 to 100 mm.

[0048] 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 -273°C to 1000°C; humidity - up to 100%. The invention provides a sensor comprising a bare silica optical fiber.

[0049] The variant illustrated in Figure 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).

[0050] The variant illustrated in Figure 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).

[0051] The variant illustrated by figure 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).

[0052] The variant illustrated by Figure 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).

[0053] 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 (SiCh). 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.

[0054] In this configuration, 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, 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.

[0055] 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), a radius much lower than the curvature radius recommended by the fiber manufacturer and therefore much more interesting for very confined areas.

[0056] 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, outdoors in user-compatible or instrumentation environments.

[0057] Optical assembly: optical fiber / capillary Silica

[0058] The multimode optical fiber (core > 0.4 mm plus cladding) is inserted into a capillary (inner diameter < 1 mm / outer diameter < 2 mm), both made of silica. The optical fiber is held inside the capillary by ceramic bonding or fusion with the capillary. The assembly features very high temperature compatible materials that can be shaped, sized and packaged according to the size.

[0059] Fiber “folding” method:

[0060] 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.

[0061] We can thus obtain a 90° bend (or an angle bend 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.

[0062] At no time is there any break in the optical guidance. The resulting device is therefore perfectly stable, insensitive to vibrations and compatible with very high temperatures (up to 1000°C). The optical transmission loss generated by this action is around 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 500 bars 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 the metallic coating and the interior 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. Optical fiber sensor according to the preceding claim, characterized in that 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 (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.

Images