Apparatus and method for metal coating optical fibres

Abstract

Apparatus and method for metal coating optical fibres. The apparatus comprises: an electroplating tank (2) containing an electroplating solution (3) and a first electrode (4) for metal coating an optical fibre (5) having an outer electrically conductive layer (8); a driving system comprising a motor (M) and pulleys for driving the optical fibre (5) through the electroplating solution (3) such that the outer electrically conductive layer (8) is in contact with a second electrode (9) outside the electroplating solution (3) and is coated in the electroplating tank (2) with a metallic layer (14), obtaining a metal-coated optical fibre (10); a power supply unit (11) for applying a configurable DC voltage and / or current to the electrodes (4,9); a thickness detector unit (12) for measuring a thickness (TH) of the metal-coated optical fibre (10); and a control unit (13) that adjusts the voltage and / or current of the power supply unit (11) based on the thickness (TH) of the metal-coated optical fibre (10) and a target thickness.

Description

[0001] APPARATUS AND METHOD FOR METAL COATING OPTICAL FIBRES

[0002] DESCRIPTION

[0003] FIELD

[0004] The present invention is comprised within the field of methods and apparatuses for metal coating optical fibres.

[0005] BACKGROUND

[0006] An optical fibre is a waveguide transmission mean widely used in telecommunications. It is a thin glass or plastic flexible wire that transmits the light from one end to another. A fibre optic sensor (FOS) is a device that uses an optical fibre as a sensing element (intrinsic sensor) or as a medium for transmitting the information (extrinsic sensor). Intrinsic sensors can be used for measuring a physical parameter (e.g. strain, temperature, pressure) using the modifications that the physical parameter produces on the light that travels through the optical fibre. Intrinsic sensors include, among others, punctual FOS sensors based on Fiber Bragg Grating (FBG) and distributed sensors based on Rayleigh and Brillouin Scattering. Extrinsic sensors only use the fibre as a transmitting medium between the detector and the sensing element (which can be either a non-fibre optic sensor or an electronic sensor and a detector). Fibre optic sensors can be used in different applications, such as physics, chemistry, biomedicine, environmental applications, etc.

[0007] For harsh environment applications, mainly very high temperatures (350°C-1000°C) and / or corrosive environments, it is necessary to protect the fibre optic sensors to increase their robustness. This protection can be done with metallic tubes or with polymeric protective layers. The latter suffer from long term degradation and do not withstand temperatures above 400°C, and the metallic tubes prevent strain measurement. For high temperature environments, between 350 and 1000°C, the protective layers should isolate the sensor from humidity, because water and, at high temperatures, hydrogen would enter the core of the fibre, damaging it. The best option to avoid this negative effect is to coat the optical fibre sensor with a hermetic layer made of carbon or metal. The standard optical fibre coating is a polymeric material (acrylate or polyimide). This material acts like a shock absorber to minimize attenuation caused by possible microflections, and to protect the fibre from mechanical damage and / or moisture ingress (although not totally hermetically sealed). This type of coating does not withstand temperatures above 350°C, therefore in high temperature environment the coating is required to be resistant to the operation temperatures as well as hermetic to assure the seal of the optical fibre from hydrogen penetration. The hydrogen molecules are able to entry in the silica of the optical fibre causing surface degradation at room temperature, which causes the strength degradation of the fibre because it generates cracks on the surface. This effect is minimized by the typical polymeric coating. Also, at high temperatures, the hydrogen is diffused into the core of the fibre which causes faster core degradation, increasing the optical losses and ultimately causing the optical fibre fault. For these environments, other resistant coatings are applied, like carbon, metallic or ceramic coating.

[0008] Carbon coated optical fibres are made by depositing a thin carbon layer onto the surface of the silica during the fibre draw manufacturing process. The final carbon layer is around 20 - 50 nm, sufficient for against both water and hydrogen diffusion, without causing micro-bend losses. However, it is thin and brittle, which cannot protect the fibre from mechanical damage such as scratches. For this reason, carbon-coated fibres are always protected by polymer coatings. But the polymeric coating makes it non-resistant to temperatures over 350°C.

[0009] In contrast with the carbon-coated optical fibres, another way to obtain a sealed optical fibre to avoid their degradation due to hydrogen exposure and to obtain a high resistance at high temperatures (>350°C), is to use a metal-coated optical fibre. Metal coating is hermetic and robust enough for protecting mechanically and thermally the optical fibres. They are widely used for high temperature environments due to their thermal properties. They have a high thermal conductivity and high heat capacity which favors an effective transfer of thermal energy from the coating to the sensor, thus increasing the thermal sensitivity and thermal response.

[0010] Optical fibres coated with Cu, Al and Au are commercially available. Some fibres include a carbon layer sandwiched between the fibre cladding and the metallic coating to improve their bonding. All of them can be used as distributed FOS, with a maximum coating thickness of 25 gm (for a single-mode fibre) and able to operate at a maximum of 400°C for Al coating, 450°C for Cu coating, and 600°C for Au coating, for long-term exposure application.

[0011] The commercial metallic coatings can be applied to the optical fibre by different techniques, such as freezing or extrusion. However, the metallic coatings available in the market are thin (e.g. the achieved metal thickness with the freezing technique is of dozens of microns) and do not provide sufficient protection to the FOS to withstand the welding process used for embedding them in metallic structures, so that they cannot be integrated in many harsh environment applications or be used to monitor strain (when not in direct contact with the material of the component or structure). Therefore, the metal coated FOS must have a higher coating protection to allow their embedding.

[0012] The present invention allows coating the optical fibre with a metal layer (e.g. Ni, Cu) with a desired thickness according to the application, from a few microns to hundreds of microns. In addition, the invention provides greater homogeneity and precision in the desired thickness of the metallic layer applied to the fiber optic cable.

[0013] SUMMARY

[0014] The invention relates to an apparatus and method for coating optical fibres with metals based on electroplating deposition that solves the aforementioned problems, providing automatization and control of the plating coating process that achieves precise thickness and homogeneous coating all along the coated length. The optical fibres are preferably coated with Ni or Cu, but they may me coated with other metals by changing the anode and the bath solution of the electroplating.

[0015] The apparatus for metal coating optical fibres comprises an electroplating tank containing an electroplating solution and a first electrode for metal coating an optical fibre having an outer electrically conductive layer; a driving system comprising a motor and a plurality of pulleys configured to drive the optical fibre through the electroplating solution such that the outer electrically conductive layer is in contact with at least one second electrode arranged outside the electroplating solution and is coated in the electroplating tank with a metallic layer to obtain a metal-coated optical fibre; a power supply unit configured to apply a configurable DC voltage and / or current to the electrodes; a thickness detector unit configured to measure a thickness of the metal-coated optical fibre; and a control unit configured to adjust the voltage and / or current of the power supply unit based at least on the thickness of the metal-coated optical fibre and a target thickness.

[0016] The method of metal coating optical fibres comprises driving an optical fibre having an outer electrically conductive layer through an electroplating tank containing an electroplating solution and a first electrode for metal coating the optical fibre, such that the outer electrically conductive layer is in contact with at least one second electrode arranged outside the electroplating solution and is coated in the electroplating tank with a metallic layer, obtaining a metal-coated optical fibre; measuring a thickness of the metal-coated optical fibre; and applying a configurable DC voltage and / or current to the electrodes based at least on the thickness of the metal-coated optical fibre and a target thickness.

[0017] The present invention can be applied to any type of optical fibers (single mode optical fibres, multimode optical fibres, distributed optical fibres, Fibre Bragg Grating sensors, etc.), as long as they have an outer electrically conductive layer which is required for applying electroplating.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A series of drawings which aid in better understanding the invention and which are expressly related with an embodiment of said invention, presented as a non-limiting example thereof, are very briefly described below.

[0020] Figure 1 shows an apparatus for metal coating optical fibres according to an embodiment of the present invention.

[0021] Figure 2 depicts an embodiment in which the apparatus further adjusts the speed of the motor.

[0022] Figure 3 depicts another embodiment of the apparatus in which the temperature of the electroplating solution is adjusted.

[0023] Figure 4 shows an embodiment of the apparatus in which the pH of the electroplating solution is adjusted. Figure 5 shows another embodiment in which the apparatus adjusts the tension of the optical fibre in the direction of movement.

[0024] Figures 6A and 6B depict an exemplary embodiment of the apparatus.

[0025] Figures 7A and 7B show different roller diameters used for the feed pulley.

[0026] Figures 8A and 8B show an embodiment of the thickness detector unit.

[0027] Figure 9 represents an embodiment of the control unit.

[0028] Figure 10 shows a control process performed by the control unit to calculate the thickness of the optical fibre and control the apparatus based on the thickness value.

[0029] Figure 1 1 is a diagram showing the functioning of the pH monitoring system according to an embodiment.

[0030] Figure 12 shows an embodiment of a tensor unit installed in the apparatus.

[0031] Figures 13A-13B depict a cross-section of Ni-Cu coated fibre without chemical attack (Fig. 13A) and with chemical attack (Fig. 13B).

[0032] Figures 14A-14B show the Rayleigh frequency variation for a Ni coated distributed sensor heated in a tubular furnace of 300 mm of length.

[0033] Figures 15A-15B represent the response to the temperature test for a freely nickel coated FBG (Fig. 15A) and a nickel coated FBG pasted on a nickel plate (Fig. 15B).

[0034] Figure 16 shows the response of a Ni coated and polymeric coated FBG sensors during a mechanical tensile, compression and fatigue tests applied.

[0035] Figure 17 depicts a Ni coated FBG sensor embedded by laser cladding into antifriction material. Figure 18 shows the response of two Ni coated FBGs manufactured on two different optical fibre composition. The two Ni coated FBGs were thermally characterized from 20- 850°C, applying continuous and thermal cycles.

[0036] Figures 19A-19B represents the temperature distribution in the concrete layers of an energy storage concrete module tank through the interpolation of the measured data from the embedded Ni coated FOS.

[0037] Figures 20A and 20B show the response of the Ni coated embedded FOS for different mechanical tests. Figure 20A is a comparison of measurements for Ni coated FBG sensor, embedded by automatic TIG welding, and strain gauge, glued on the specimen surface, during a tensile test. Figure 20B shows the response of the Ni coated FBG embedded in an antifriction bearing during some of the tests carried out in a test-bench to test the sensorized antifriction material bearing.

[0038] Figure 21 is a flow diagram of a method of metal coating optical fibres according to an embodiment.

[0039] DETAILED DESCRIPTION

[0040] Figure 1 schematically depicts the components of an apparatus 1 for metal coating optical fibres according to an embodiment of the present invention.

[0041] The apparatus 1 comprises:

[0042] - An electroplating tank 2 containing an electroplating solution 3 and a first electrode 4 (acting as an anode) for metal coating an optical fibre 5 that has an outer electrically conductive layer 8. The zoom-in image shows the layers of an optical fiber 5, including the core 6, the cladding 7 and the outer electrically conductive layer 8 (e.g. a metal coating).

[0043] - A driving system configured to drive the optical fibre 5 through the electroplating solution 3 such that the outer electrically conductive layer 8 is in contact with at least one second electrode 9 (acting as a cathode) arranged outside the electroplating solution 3 and is coated in the electroplating tank 2 with a metallic layer 14 (disposed on top of the outer electrically conductive layer 8), thereby obtaining a metal-coated optical fibre 10. The driving system comprises at least one motor (M) and a plurality of pulleys (P1 ,P2,P3,P4,PT). The apparatus 1 preferably comprises a set of pulleys (P1 ,P2,P3,P4) that feed the metal-coated optical fibre 10 to a take-up reel (PT), wherein the takeup reel, which is connected to a motor (M), receives the finished metal-coated optical fibre 10.

[0044] - A power supply unit 11 configured to apply a configurable DC voltage and / or current to the first 4 and second 9 electrodes. Figure 1 shows the electrons (e-) running from the anode (first electrode 4) to the cathode (second electrode 9).

[0045] - A thickness detector unit 12 configured to repeatedly measure a thickness (TH) of the metal-coated optical fibre 10.

[0046] - A control unit 13 configured to adjust the voltage and / or current of the power supply unit 11 based at least on the thickness (TH) of the metal-coated optical fibre 10 and a target thickness.

[0047] In an embodiment, the voltage and / or current provided by the power supply unit 1 1 is determined based on the difference (i.e. an error e) between the thickness measured and the target thickness: e=TH-target. This error e may feed, for instance, a PI or PID controller, whose output is applied to the power supply unit 11 to obtain a certain voltage and / or current output that allows adjusting the thickness of the optical fibre to the desired value. Other different types of controllers may be used instead. Basically, if the coating thickness or diameter of the optical fiber falls below a lower threshold, an order will be given by the control unit 13 for the current or voltage to increase and if the thickness / diameter of the optical fiber is greater than an upper threshold (which may have the same value as the lower threshold), the current or voltage will be decreased. Note that while the measured thickness (TH) corresponds to the total thickness of the optical fibre, the thickness of the metal-coated layer applied in the electroplating tank 2 may be obtained by subtracting the diameter of the optical fibre 5 (which is a known value) from the measured thickness (TH).

[0048] To improve homogeneity on the metal coating of the optical fibre, the apparatus 1 preferably incorporates two or more anodes that partially surround the optical fibre 5, in the form of at least two metal sheets vertically disposed and facing each other such that the optical fibre 5 is arranged between them as it passes through the electrolytic bath. As shown in Figure 1 , the apparatus uses electroplating for applying a metal coating on the optical fibre 5. Electroplating is an electrolytic process for depositing a layer of metal upon a substrate to enhance its properties. An electric current flows through two electrodes that are submerged in an electrolyte (electroplating solution 3). The positive charged electrode is known as the anode (first electrode 4) while the negative charged electrode is the cathode (second electrode 9).

[0049] In the embodiment of Figure 1 , two second electrodes 9 are employed, each one corresponding to a different pulley (P1 and P4) of the driving system; however, in other embodiments a single second electrode (a single pulley) may be used. These two pulleys (P1 , P4) are made of a metallic material, such as brass, and act as the cathodes since they are used for transmitting the electrical power needed for the electroplating process through the optical fibre 5. Pulleys P2 and P3 are made of a non-conductive material, such as Teflon®, that withstands the acid properties of the bath solution, and they are used for enabling the optical fibre movement and for keeping the optical fiber 5 submerged in the electroplating solution 3.

[0050] In this embodiment, the second electrodes 9 are not submerged in the electroplating solution 3, but in electrical contact with the electroplating solution 3 through the outer electrically conductive layer 8 of the optical fiber 5. The electroplating solution 3 contains electrically charged particles. When an electrical potential is applied between the electrodes, the charged ions migrate towards the opposite charged electrode. For instance, if the first electrode 4 is made of copper or nickel, charged ions (Cu2+or Ni+2) in the electroplating solution 3 migrate towards the second electrode 9, thus being deposited on the optical fiber 5. This way, a metal coating of the material of the anode (e.g Cu or Ni) is applied on the optical fiber 5. The electron (e-) transfer completes the electrical circuit. This technique has to be applied over an electrically conductive material (outer electrically conductive layer 8), so it is not possible to apply it directly on the glass optical fibre.

[0051] Figure 2 schematically represents an embodiment of the apparatus 1 in which the control unit 13 is further configured to adjust the speed of the motor (M) based at least on the thickness (TH) of the metal-coated optical fibre 10 and the target thickness. In this case there is only one second electrode 9 (pulley P1 ). As explained before, in an embodiment the voltage and / or current provided by the power supply unit 11 may be determined based on the difference or error e between the thickness measured (TH) and the target thickness (e=TH-target), this error e feeding a controller (e.g. PI, PID). Furthermore, the speed of the motor (M) may also be adjusted based on said error e. The motor speed may be increased (if the thickness is larger than an upper threshold) or decreased (if the diameter is smaller than a lower threshold), e.g. for a certain time X, as long as the inertia of the process does not manage to readjust the thickness. Basically, at higher speeds for the same l / V values, the thickness of the optical fibre will be lower and, for lower speeds, the thickness will be greater.

[0052] Figure 3 schematically depicts another embodiment of the apparatus 1. In this embodiment, the apparatus 1 comprises a temperature sensor 15 configured to obtain a temperature (T) of the electroplating solution 3, and a heating unit 16 configured to heat the electroplating solution 3. The control unit 13 receives the temperature (T) from the temperature sensor 15 and adjusts the temperature (T) of the electroplating solution 3 to a target temperature by controlling the heating unit 16. The temperature is normally set to a reference value, preferably around 55°C, because at this temperature the electroplating solution has better conductivity and there are fewer risks of defects (burns) appearing in the optical fiber.

[0053] The embodiment schematically shown in Figure 4 refers to an apparatus 1 comprising a pH sensor 18 configured to obtain a pH measurement (pH) of the electroplating solution 3, and a pH control system 19 comprising one or more valves (V1 ,V2) configured to inject into the electroplating tank 2 a first liquid (L1 ) that increases the pH of the electroplating solution 3 or a second liquid (L2) that reduces the pH of the electroplating solution 3. The control unit 13 repeatedly receives the pH measurement (pH) from the pH sensor 18 and adjusts the pH of the electroplating solution 3, by actuating the pH control system 19, within a predetermined range. The pH regulation may be performed, for instance, when the pH measurement (pH) is out of said range.

[0054] Figure 5 schematically shows another embodiment in which the apparatus 1 comprises a tensor unit 20. The tensor unit 20 is a tensioning device configured to apply a configurable force to the optical fibre 5 to adjust the tension of the optical fibre 5 in the direction of movement. In an embodiment, the tensor unit 20 comprises a tensor pulley 21 and a pneumatic brake 22. The pneumatic brake 22 acts on a reel of optical fibre arranged at the feed pulley (P0), and the tensor unit 20 adjusts the position of the tension pulley 21 by automatically changing the braking pressure to keep the tension pulley 21 in a certain position. This ensures that the fiber tension is constant throughout the process. The modification of the tension provides benefits in the metal-coated optical fibre 10 manufactured by the apparatus 1 , since a metal-coated optical fibre 10 manufactured with higher tension has less losses (optical power) than a metal-coated optical fibre 10 that is manufactured with lower tension.

[0055] In the embodiments of Figures 1 to 5 a single control unit 13 is depicted. However, the control unit 13 may be implemented as an individual control unit or a set of control units arranged at the same location (adjacent each other) or different locations (separated by a distance), independent or in communication with each other.

[0056] Figure 6A depicts an elevation view of an exemplary embodiment of an apparatus 1 for metal coating optical fibres in a running configuration where the apparatus is coating the optical fibre. The apparatus comprises two polypropylene tanks mounted in series. The first tank is the electroplating tank 2 where the electrodeposition process is carried out. The electroplating tank 2 contains a nickel bath (electroplating solution 3), a nickel anode (first electrode 4), a resistance (heating unit 16) for increasing the temperature and a level meter (not shown in the figure). The second tank is a rinse tank 30. The rinse tank 30 contains water 31 for rinsing the metal-coated optical fibre 10 (in this case, nickel- coated). The anodes (first electrode 4), which are nickel plates, are mounted in baskets and covered by bags. The baskets are used to hold the nickel plates at a specific location while the bags are used to avoid the contamination of the bath due to impurities that can be released from the nickel plates while the electroplating process is carried out. Also, the machine (apparatus 1 ) has a recirculating pump, to ensure that the electroplating bath is homogeneous and a suction pump 33 to aspirate and filter the gases that are emitted from the electroplating bath.

[0057] Figure 6B depicts an elevation view of the apparatus 1 of Figure 6A in a set-up position. In this set-up position a reel 35 of optical fibre 5 is first arranged at a feed pulley (P0). Then, the optical fibre 5 is placed through a tensor unit 20 and through different pulleys (P1 , P2, P3, P4, P5, P6). Finally, the optical fiber 5 is attached at the end to a take-up pulley (PT). The pulleys are from different materials: two of the pulleys (P1 , P4), which act as cathodes (second electrodes 9), are preferably made of brass and are used for transmitting the electrical power needed for the electroplating process through the optical fibre; the other pulleys (P2, P3, P5, P6) are preferably made of Teflon™, which is a non- conductive material that withstands the acid properties of the bath solution, and they are used for enabling the optical fibre movement and for keeping it submerged in the nickel bath and in the water. The pulleys have a few millimeters groove to ensure that the optical fibre is correctly arranged along the set-up process. The take-up pulley (PT) is connected to a motor (M), that allows the rolling of the optical fibre.

[0058] Also, at least one additional motor allows the lifting of a structure 32 (including the pulleys and supports thereof) where the optical fibre has to be allocated. This equipment is connected to an electric control panel (which includes the control unit 13), and it is controlled through a touch screen 34 which has an interface that allows the switching on / off the pumps, motors, and heating resistance. The screen also shows the speed of the motor, the temperature and the alarms related to the optical fibre breakage (using a sensor in the tensor unit 20), tank level of the electroplating solution (using the level meter), the current and voltage applied, the pressure applied to the brake and the position of the tensor pulley. For applying the voltage and current needed for the electroplating process, a voltage source is used (power supply unit 11 ).

[0059] In an embodiment relating Figures 6A and 6B, the electric control panel is first switched on and then the recirculating and the suction pumps are also switched on. Next, the temperature set point is selected and the heating starts, which only happens if the bath level is under a predetermined range. While the temperature is increasing, the structure 32 is lifted (Fig. 6B) so that the optical fibre can be arranged along the machine. The first meters of fibre will not be coated by nickel, but they are needed to fix the fibre at the take-up pulley (PT). Once the optical fibre is allocated and the bath reaches the set point temperature, the structure 32 is descended (Fig. 6A), submerging the optical fibre on both the electroplating solution and the rinse bath. Then, the motor speed is set and the desired current or voltage are set in the voltage source and the electroplating process starts. It is possible to fix the current or the voltage according to the operator’s knowledge. If the current is fixed, depending on the conductivity and resistance of the bath and optical fibre, the voltage is applied automatically by the voltage source; and vice-versa in the case of setting the voltage. The temperature (ideal range 45-55°C) and speed (0.02-2.09 m / min) are defined through the tactile screen 34, while the current (0.2 - 1 A) or voltage (1 - 3 V) are defined in the voltage source (which is not included in the electric control panel). These parameters are the ones that would define the final thickness of the coated optical fibre.

[0060] Figures 7A and 7B show different roller diameters that may be used for the feed pulley (P0). The apparatus 1 may comprise a mounting for supporting a reel 35 of optical fibre. Taking into account that the optical fibre is sold rolled up in reels 35 that, depending on the provider, may have different diameters, different mountings have been designed so that the different reels 35 fit on the machine.

[0061] A voltage and current source (power supply unit 1 1 ) is connected to the control unit 13 so the voltage or current is directly defined by an operator (e.g. through a tactile screen 34). The voltage and current values can be registered along the coating process using a l / V sensor installed. In manual operation, the current is fixed to a desired value and then the voltage is modulated to ensure that the current is constant. In automatic operation, these values will change taking to achieve a desired coating thickness all along the coated length. This is possible by using a thickness detector unit 12, such as an artificial vision sensor, which monitors the coated fibre thickness. The current and voltage monitoring are important because fluctuations in these values affect to the achieved microstructure and thus, to the response of the sensor.

[0062] For monitoring the thickness (TH) and the quality of the coated fibre, an artificial vision system is preferably employed as a thickness detector unit 12, although other types of thickness detectors may be used. The artificial vision system allows the control unit 13 for automatizing the electroplating process because in automatic process when the specified thickness is not achieved it modifies the current and / or voltage until the desired thickness is obtained. Also, the artificial vision system may be used to monitor the quality of the fibre, since pores or other Ni coating layer anomalies or defects can be detected using artificial vision.

[0063] Figures 8A and 8B show an embodiment of the artificial vision system 40 which includes a high-resolution camera 41 that can be equipped with different objectives that provides different magnification and focal distances. Also, a light source (backlight illumination panel 42) is needed. Both parts and their arrangement are shown in Figures 8A (lateral view) and 8B (perspective view). Likewise, the camera and the light source are integrated in a way that is possible its displacement to move closer or farther apart from each other. This artificial vision system 40 is located at the output of the optical fibre from the electroplating tank, prior to the take-up pulley (PT).

[0064] The camera 41 records images of the nickel-coated optical fibre that passes in front of it and send them to the control unit 13 (e.g. a computer) installed at the electric control panel. The recording frequency ranges from 1 to 10 Hz. A calibration of the camera is performed prior to the first measurement and if the camera is moved a chessboard shape pattern is used. Then, an image recognition algorithm processes the images and extracts the thickness and quality of the coated layer. For the thickness, an edge detection algorithm is used together with the contrast comparison. Every image is analysed by columns so that the information of the edges will be localized in two points, the thickness of the fibre being the difference between those points. For each image, a value of thickness will be obtained for each column so that the mean thickness is calculated. Also, the 3o is obtained as an uncertainty assessment. The data are shown in real time and recorded for product traceability.

[0065] In an embodiment, depicted in Figure 9, the control unit 13 is based on a Linux PC running multiple subsystems on Python, which communicate over the messaging passing framework MQTT. The camera 41 is connected to the control unit 13 using the GenICam protocol and to a PLC 45 (which may also be considered as part of the control unit 13) of the machine using the Modbus protocol. The image acquired by the camera 41 is received and processed by an artificial vision subsystem 47, which obtains the optical fibre thickness. Although Figures 1 to 5 depict the thickness detector unit 12 as a device separated from the control unit 13, the thickness detector unit 12 may comprise some elements that are part of the control unit itself, as in the example shown in Figure 9 wherein the artificial vision subsystem 47 is part of the control unit 13.

[0066] In an embodiment, regulation is achieved by implementing a PID controller 46, which receives the fibre thickness and multiple variables from the PLC 45 and calculates an appropriate action to act on the electrical parameters of the process (see Figure 10, which depicts a control process according to an embodiment). The voltage and / or current are changed within a predetermined range. Other different types of controllers may be used instead of a PID controller 46, such as a PI controller, artificial neural networks, etc. The composition of the electroplating solution 3 should be preferably monitored to know the status of the solution, because it affects to the coating quality. It is possible to do this by taking a solution sample from the bath and analyse each component solution concentration. Also, it is possible to employ systems that can analyse the components concentration in-situ, installing this system directly on the bath, but this is too expensive. However, the present invention aims instead to measure only a key parameter of the solution: the pH. If the pH is not in a specific range (in an embodiment, 3.8 - 4.2), the solution is not in the working conditions and some actions must be taken (add more solution, add distilled water, etc). A pH continuous monitoring system is preferably included in the electroplating machine to know the status of the solution.

[0067] For the continuous pH monitoring and control (Fig. 4), a pH controller is mounted in the electroplating tank 2 (pH sensor 18). The pH value measured may be shown at the display of the pH controller and also at the screen 34 because it is connected with the PLC 45 of the machine. When the measured pH is out of range, it is necessary to act over the electrolytic bath. If the measured pH is below to a lower threshold (e.g. 3.8), it can be increased by adding a first liquid (L1 ), such as nickel carbonate or 10% sodium hydroxide solution, although distilled water is preferred; however, if it is above an upper threshold (e.g. 4.2), a second liquid (L2), such as sulfuric acid, should be added to decrease the pH. This last case is more usual because the pH increases with the usage of the system. The pH monitoring system is connected with the PLC 45 as aforementioned, and a control system has been stablished with the pneumatic valves, as depicted in the diagram of Figure 11 . When the pH measurement is below the range, the PLC 45 acts over the pneumatic valve that goes through a first liquid container to add the first liquid (e.g. distilled water) so that the pH is increased. If the pH measurement is above the range, a pneumatic valve that goes through a second liquid container (e.g. containing sulfuric acid) is opened. Then normal operation is reestablished to mix and homogenise the solution. This should be done at least before starting the coating of a new fibre, but it may also be applied during the coating process.

[0068] With regard to the tensile forces performed in Figure 5, the metallic coating generates micro-bendings on the silica optical fibre which increases the losses and cause limitations in the use of the coated fibre optic sensors. The micro curvatures modify the light pathway inducing changes in the angle which hits the boundary and thus avoiding its propagation. This is important because the more losses a fibre has, the less data / light length can be transmitted along it. Therefore, it also reduces its potential as a sensor. To minimise the losses generated during the coating procedure, a controlled strength is preferably applied along the fibre during its metallic coating. A tensor unit 20 (or tensile sensor) is included in the machine (Figure 12). The tensor unit 20 modifies the tensile applied to the optical fibre. There are two ways of use:

[0069] Manual mode: where the pressure of the pneumatic brake 22 is fixed forcing the position of the tensor pulley 21 to the maximum position when the force of the fibre compensates the pressure of the pulley chambre.

[0070] - Automatic mode: the position of the tensor pulley 21 is given as a set point and the pressure of the pneumatic brake 22 is modified to maintain the position of the tensor pulley 21 as close as possible to the set point.

[0071] The coating process of the machine is described hereinafter, according to an embodiment. All the different electronic modules of the machine are switched on (the control unit, the PLC, the suction pump to start the aspiration mode, the recirculating pump, the electro valve that goes through the solution tank, etc.). Later, the heaters are switched on to achieve the desired temperature of the solution. Meanwhile, a reel 35 with commercially copper coated optical fibre is placed in the mounting. Then, the optical fibre is extended and fixed to the final roller (take-up pulley). Beginning from the last pulley, the optical fibre is arranged along the different pulleys. The last part of the optical fibre that is accommodated is the one that goes through the tensile sensor.

[0072] Once the optical fibre is placed, the compressed air valve has to be opened in order to feed the pneumatic circuit of the tensile sensor. Then, in the screen 34 the desired specifications should be defined; first of all, taking into account the desired thickness that has to be achieved, the speed of the machine and the current and voltage should be defined. These last parameters should preferably be first defined in manual mode, and then, once the coating starts to be homogeneous, the automatic mode should be selected. Also, the tensile sensor should be set either on manual or automatic mode, defining the tensile that should be achieved all along the fibre optic. From now on, the artificial vision system will act together with the current and voltage source in order to achieve a homogeneous coating all along the length of the optical fibre.

[0073] The copper coated fibre enters the electrolytic bath. Then, it passes through the vision camera that will record the images and the program will analyse them in order to activate the control, ensuring that the recoated optical fibre has the desired thickness. The metal- coated optical fibre 10 will then be submerged into the rinse tank 30 and finally it is rolled into the take-up pulley (PT). Once the whole length of the fibre is coated with nickel, the procedure is finished, and the machine can be switched off.

[0074] The apparatus 1 of the present invention is able to coat the optical fibre with nickel, or other metal such as copper, in a continuous way, at an average speed of 20 m / h (and able to reach 24 m / h or more) depending on the needed thickness which is directly related with the current / voltage parameters and with the rollers speed. As previously commented, the apparatus may have different control and monitoring parameters such as temperature, tank level, pH, coating speed, coating thickness and tensile measurement.

[0075] The nickel-coated optical fibres obtained with the apparatus 1 has the following properties:

[0076] - Base fibre: single mode optical fibre with 9 pm core diameter and 125 - 200 pm cladding diameter. It is commercially coated with a metal (Cu, Au, Al...) and has a C interlayer.

[0077] - Wavelength range: 1300 - 1600 nm.

[0078] - Nickel-coated layer thickness: from 15 ± 1 pm to 500 ± 50 pm.

[0079] - Final diameter: 175 - 1200 pm.

[0080] - Temperature stable up to 700-800°C for long term exposure and it can withstand peaks up to 1000°C. Also withstands short term exposure to cryogenic temperatures.

[0081] - Temperature and strain simultaneous measurement.

[0082] Possible to embed them in different metals (Aluminum, Inconel®, Tin based alloys, Steel, Invar, etc.) and other materials (e.g. high thermal resistant concrete) by casting, laser, TIG, thermal spray coating, etc.

[0083] The achieved coating is homogeneous along all the coating length with a radial growing around the glass fibre as shown in Figures 13A and 13B, depicting a cross-section of Ni-Cu coated fibre without chemical attack (Fig. 13A) and with chemical attack (Fig. 13B), in which the central black circle 50 is the silica fibre. The inner ring 51 is the Cu coated layer from a Cu coated commercial fibre. The outer ring 52 is the Ni coated layer applied by the apparatus 1 .

[0084] Thermal and mechanical tests were performed on nickel coated optical fibres obtained by the apparatus and, when the optical fibre 5 includes metallic-coated FBG sensors, on Fiber Bragg Grating (FBG) sensors obtained by the apparatus. Figures 14A and 14B show the response of a Ni coated optical fibre monitored by a Rayleigh interrogator during some thermal calibration cycles from room temperature to 700°C, heating the distributed fibre optic sensor (FOS) in steps of 100°C in a tubular oven of 300 mm length. Figure 14A presents the distributed FOS response in an image representation. Figure 14B shows the response of some points along the length of the fibre in comparison with the thermocouple’s response. The central part of the fibre is exposed to a higher temperature than the extremes of it because the tubular oven operation.

[0085] A thermal and strain test was performed with nickel coated FBGs. A first FBG was freely located at a furnace and a second FBG was pasted on a nickel plate. The first FBG is used to measure temperature independently while the second FBG is used to measure strain. Two thermocouples have been installed in the furnace to measure the temperature, one on top of the nickel plate and the other is free near the first FBG. The furnace was programmed to perform an up and down temperature ramp, where the temperatures under study are 150, 280, 410, 540, 670 and 800°C in both the increase and decrease sides, the cycle being repeated continuously. The response of the nickel coated FBGs to the thermal tests is shown in Figures 15A and 15B for both sensors under study. Both sensors follow faithfully the temperature cycle of the furnace. The freely located sensor (first FBG) has a very repeatable response, as depicted in Fig. 15A. As shown in Fig. 15B, the pasted sensor (second FBG) gets stable after the first thermal test due to the material movement caused by temperature effect, once the material has relaxed, the sensor response becomes repeatable and reproducible recovering the same wavelength after each thermal test.

[0086] Figure 16 represents the response of a Ni coated FBG sensor and a standard polymeric coated FBG sensor, bonded to a titanium specimen, during a tensile test up to 30 kN, a compression test up to -10 kN and a fatigue test from -10 kN to 20 kN.

[0087] Ni coated FOS offers the advantage to resist embedding process at high temperatures as laser embedding process (cladding or others), TIG welding, casting, etc. Figure 17 shows a Ni coated FBG sensor embedded in a steel specimen coated with an antifriction material based on tin.

[0088] More information about the response of nickel coated optical fibre sensors embedded in different materials is reported hereinafter.

[0089] The nickel coated optical fibres obtained by the apparatus have been validated in different use cases:

[0090] - Long-term high temperature validation (TRL7) monitoring of a superheated biomass boiler and of a superheated combined cycle boiler during one year of normal operation. The Ni coated FOS was exposed to erosion, corrosion and continuous temperatures of 250°C with peaks of 900°C. The Ni coated sensor has showed a stable signal and operated normally during the year tested.

[0091] Figure 18 shows how two Ni coated FBG sensors responds to a thermal fatigue test. The Ni coated FBGs were manufactured on two different base optical fibres. First, they were subjected to an annealing and thermal fatigue cycles at 200, 400, 600 and 800°C. The annealing was carried-out at 50°C over the thermal fatigue maximum temperature. Secondly, FBG sensors were subjected to continuous exposition to 200, 400, 600 and 800°C.

[0092] - Temperature and strain validation (TRL5). The Ni coated fibre also has been validated in an energy storage concrete module tank for existing Concentrated Solar Power (CSP). The Ni coated FOS was directly embedded in the concrete module for monitoring strain (to detect cracks and fatigue) and temperature to monitor the operation and the durability of the module, operating up to 550°C. Figures 19A and 19B show the thermal map for a concrete block heated up to 550°C, on one of its surfaces, obtained though the Ni coated FOS embedded on it. Figure 19A is a representation of the temperature gradient for the left side of the block, whereas Figure 19B represents the gradient temperature along the central part of the concrete block.

[0093] Embedding validation (TRL6): The Ni coated FOS were embedded in the antifriction material of a very difficult-access component in the naval sector, a bearing, to monitor the component condition monitoring and detect the degradation of the antifriction material layer. The Ni coated FOS was successfully embedded by laser cladding and by tungsten inert gas (TIG) welding and the smart component was validated in a test-bench at real conditions. The Ni embedded FOS were able to monitor load, speed, strain, vibrations and temperature. Figures 20A and 20B show the response of the Ni coated embedded FOS for different mechanical tests. Figure 20A shows the response of the Ni coated embedded sensor by automatic Tungsten Inert Gas (TIG) welding in the steel-antifriction material specimen, for a tensile test applying loads up to 30kN and comparing its response with a typical gauge. Figure 20B shows the response of a Ni coated FOS embedded in an antifriction material bearing, by manual TIG welding, subjected to load, speed and temperature variations, simulating the real operation conditions of the bearing. In both cases, the embedded FOS shows a clear and stable response which is sensible to any of the parameters involved in the bearing operation.

[0094] Another aspect of the present invention refers to a method 100 of metal coating optical fibres. As shown in Figure 21 , the method comprising the following steps:

[0095] - Driving 1 10 an optical fibre 5 having an outer electrically conductive layer 8 through an electroplating tank 2 containing an electroplating solution 3 and a first electrode 4 for metal coating the optical fibre 5, such that the outer electrically conductive layer 8 is in contact with at least one second electrode 9 arranged outside the electroplating solution 3 and is coated in the electroplating tank 2 with a metallic layer, obtaining a metal-coated optical fibre 10.

[0096] - Measuring 120 a thickness (TH) of the metal-coated optical fibre (10).

[0097] - Applying 130 a configurable DC voltage and / or current to the electrodes (4,9) based at least on the thickness (TH) of the metal-coated optical fibre 10 and a target thickness.

[0098] The method may further comprise adjusting the speed of the motor (M) based at least on the thickness (TH) of the metal-coated optical fibre and the target thickness. In an embodiment, the method comprises repeatedly measuring the temperature (T) of the electroplating solution and adjusting said temperature (T) to a target temperature by actuating a heating unit.

[0099] The method may comprise measuring the pH of the electroplating solution and adjusting the pH of the electroplating solution within a predetermined range by actuating a pH control system.

[0100] The method may comprise adjusting the tension of the optical fibre in the direction of movement using a tensor unit.

[0101] The present invention provides the following advantages:

[0102] • Applicable to distributed and / or punctual optical fibre sensors.

[0103] • Cost-effective, with an affordable extra cost for the achieved properties. The optical fibre obtained is 1 .3 € / m more expensive than the commercial Cu coated optical fibre but it can be used for higher temperature ranges, because the Cu- coated fibre oxidizes for temperatures above 450°C and breaks. The optical fiber is also cheaper than the Au-coated fibre and it is not sticked to stainless steel tubes when encapsulated for temperature monitoring at high temperatures.

[0104] • Efficient: the FOS may measure strain and temperature simultaneously.

[0105] • Highly accurate: measurement of strain (±1 p£) or temperature (±0.1 °C) without loss of resolution.

[0106] • Versatile: optimized for nickel coating but it can be used for other metals.

[0107] • Manipulate: nickel coated optical fibres are resistant, they don’t deteriorate or break easily.

[0108] • Flexible: Wide range of nickel coated optical fibre thicknesses can be achieved.

[0109] • Wide application range of the resulting optical fibre.

[0110] • Resulting Ni-coated fibre can be embedded by welding, laser, ultrasound or other bonding processes.

Claims

CLAIMS1. An apparatus for metal coating optical fibres, comprising: an electroplating tank (2) containing an electroplating solution (3) and a first electrode (4) for metal coating an optical fibre (5) having an outer electrically conductive layer (8); a driving system comprising a motor (M) and a plurality of pulleys configured to drive the optical fibre (5) through the electroplating solution (3) such that the outer electrically conductive layer (8) is in contact with at least one second electrode (9) arranged outside the electroplating solution (3) and is coated in the electroplating tank (2) with a metallic layer (14) to obtain a metal-coated optical fibre (10); a power supply unit (1 1 ) configured to apply a configurable DC voltage and / or current to the electrodes (4,9); a thickness detector unit (12) configured to measure a thickness (TH) of the metal-coated optical fibre (10); and a control unit (13) configured to adjust the voltage and / or current of the power supply unit (1 1 ) based at least on the thickness (TH) of the metal-coated optical fibre (10) and a target thickness.

2. The apparatus of claim 1 , wherein the control unit (13) is further configured to adjust the speed of the motor (M) based at least on the thickness (TH) of the metal-coated optical fibre (10) and the target thickness.

3. The apparatus of any preceding claim, further comprising: a temperature sensor (15) configured to obtain a temperature (T) of the electroplating solution (3), and a heating unit (16) configured to heat the electroplating solution (3); wherein the control unit (13) is further configured to adjust the temperature (T) of the electroplating solution (3), by controlling the heating unit (16), to a target temperature.

4. The apparatus of any preceding claim, comprising: a pH sensor (18) configured to obtain a pH measurement (pH) of the electroplating solution (3), and a pH control system (19) comprising at least one valve (V1 ,V2) configured to inject into the electroplating tank (2) a first liquid (L1 ) to increase the pH of theelectroplating solution (3) or a second liquid (L2) to decrease the pH of the electroplating solution (3); and wherein the control unit (13) is further configured to adjust the pH of the electroplating solution (3) within a predetermined range by actuating the pH control system (19).

5. The apparatus of claim 4, wherein the first liquid (L1 ) is distilled water and the second liquid (L2) is sulfuric acid.

6. The apparatus of any preceding claim, comprising a tensor unit (20) configured to apply a configurable force to the optical fibre (5) to adjust the tension of the optical fibre (5) in the direction of movement.

7. The apparatus of claim 6, wherein the tensor unit (20) comprises a tensor pulley (21 ) and a pneumatic brake (22).

8. The apparatus of any preceding claim, comprising at least one motor configured to vertically move a structure (32) that supports the pulleys of the driving system between a first position in which none of the pulleys are in contact with the electroplating solution (3) and a second position in which two of the pulleys (P2,P3) are immersed in the electroplating solution (3).

9. The apparatus of any preceding claim, wherein the thickness detector unit (12) comprises a camera (41 ), a backlight illumination panel (42) and an artificial vision subsystem (47).

10. A method of metal coating optical fibres, comprising: driving (1 10) an optical fibre (5) having an outer electrically conductive layer (8) through an electroplating tank (2) containing an electroplating solution (3) and a first electrode (4) for metal coating the optical fibre (5), such that the outer electrically conductive layer (8) is in contact with at least one second electrode (9) arranged outside the electroplating solution (3) and is coated in the electroplating tank (2) with a metallic layer (14), obtaining a metal-coated optical fibre (10); measuring (120) a thickness (TH) of the metal-coated optical fibre (10); and applying (130) a configurable DC voltage and / or current to the electrodes (4,9) based at least on the thickness (TH) of the metal-coated optical fibre (10) and a targetthickness.

11. The method of claim 10, further comprising adjusting the speed of the motor (M) based at least on the thickness (TH) of the metal-coated optical fibre (10) and the target thickness.

12. The method of any of claims 10 to 11 , the method further comprising repeatedly measuring the temperature (T) of the electroplating solution (3) and adjusting said temperature (T) to a target temperature by actuating a heating unit (16).

13. The method of any of claims 10 to 12, the method comprising measuring the pH of the electroplating solution (3) and adjusting the pH of the electroplating solution (3) within a predetermined range by actuating a pH control system (19).

14. The method of any of claims 10 to 13, comprising adjusting the tension of the optical fibre (5) in the direction of movement using a tensor unit (20).

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