NL2037081B1

NL2037081B1 - A Distributed Bragg Reflector based temperature sensor for object monitoring and a method of manufacturing thereof

Info

Publication number: NL2037081B1
Application number: NL2037081A
Authority: NL
Inventors: Theodorus Cornelis Koren Ivar; Verius Velu Ewoud
Original assignee: Photonfirst Ip B V
Priority date: 2024-02-21
Filing date: 2024-02-21
Publication date: 2025-08-28
Also published as: WO2025178493A1

Abstract

A Distributed Bragg Reflector, DBR, based sensor and a method of manufacturing such DBR based sensor are presented. The DBR based sensor comprises a housing and a DBR comprised within an optical fiber. The housing comprises a cavity. The optical fiber is arranged through the housing. A sensor part of the optical fiber comprising the DBR is arranged stress-free within the housing with the DBR being located within the cavity. Examples of a DBR are a Fiber Bragg Grating, FBG, and a Fabry-Pérot etalon, FPE. +Fig. 7A

Description

A Distributed Bragg Reflector based temperature sensor for object monitoring and a method of manufacturing thereof

Technical field

The present disclosure relates to sensors. More specifically, the present disclosure relates to Distributed Bragg Reflector (DBR) based sensors for monitoring objects. Furthermore, the present disclosure relates to a housing of such DBR based sensor, a method of manufacturing such DBR based sensor, a chainable sensor array of such DBR based sensors, and a measurement system based on such DBR based sensors.

Background

Fiber Bragg Gratings (FBGs) are a type of DBRS, i.e., a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic (such as height) of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. A DBR, in particular an

FBG, can be constructed in a short segment of optical waveguide, such as an optical fiber, to reflect a particular wavelength range of light and to transmit all others (i.e., the DBR is tuned to or is responsive to that particular wavelength range). This is achieved by creating the aforementioned periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror and reflects light. DBRs can be used as sensors (DBR sensors) in an optical waveguide, by emitting light into the waveguide towards those DBRs and analyzing the light reflected from the DBR sensors. A DBR can thus be used as a sensor by the fact that its reflected wavelength is a direct result of DBR material changes from the environment (change in index, geometric variation), such that observing a shift in wavelength response by means of illumination and detection of reflectance or transmission constitutes a sensor measurement of the DBR's environment. This process of detection, analysis or monitoring is often called DBR sensing or interrogation and is a way of measuring.

Typical applications of DBR sensing may include temperature analysis (as the length and dielectric constant of the optical waveguide increases with temperature) and strain analysis of physical bodies when the waveguide is attached to the body to act as an optical strain gauge, e.g., for structural health monitoring, although other types of applications may also be considered.

FBG based sensors find applications in various fields such as structural health monitoring in civil engineering, aerospace, oil and gas infrastructures, chemical industry infrastructures, biomedical sensing, and many more where accurate and reliable sensing in potentially harsh environments is required, or object monitoring for improvement of process efficiency. Non-limiting examples of objects that can be monitored using FBG sensors include pipelines, chemical reactors, dam and high- voltage transformers. FBG sensors offer high accuracy and precision in measuring parameters like strain, temperature, pressure, and more. They can detect very small changes in these parameters with great reliability.

FBG sensors allow multiple sensors to be multiplexed along a single optical fiber.

This means several sensing points can be monitored using a single fiber, reducing complexity and costs in installations.

FBG sensors are typically highly durable and resistant to degradation, making them suitable for long-term monitoring in challenging environmental conditions.

Moreover, FBG sensors can be made very small and lightweight, making them suitable for applications where space and weight are critical factors. DBR based sensors, and in particular FBG sensors, can be very sensitive and therefore susceptible to vibration noise, i.e., noise in sensor reading as a result of unwanted vibrations at the

DBR, which can result in false readings on the measurements. There is a need for

DBR sensors that are less susceptible to vibration noise.

Summary

A summary of aspects of certain examples disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects and/or a combination of aspects that may not be set forth.

The present disclosure aims to overcome the drawbacks identified in the background section. In particular, the present disclosure aims to provide a DBR sensor and methods of manufacturing such DBR sensor. The DBR sensor may be used in a chainable sensor array for monitoring objects, with reduced susceptibility to vibration noise or adjacent DBR sensor readings.

Advantageously, the DBR sensor of the present application can be manufactured cost efficiently and at high volumes. The housing of the DBR sensor may advantageously protect the DBR against external influences, such as vibration noise, but also moist, water, and/or gasses. The housing may be created waterproof without the need for adhesives. Advantageously, the housing may provide temperature isolation and temperature conductivity where needed, for optimal temperature sensing at the DBR within the housing. Moreover, the housing may advantageously provide a fixation for protective tubing.

According to an aspect of the present disclosure, a DBR based sensor is presented. The DBR based sensor may include a housing and a DBR included within an optical fiber. The housing may include a cavity. The optical fiber may be arranged through the housing. A sensor part of the optical fiber including the DBR may be arranged stress-free within the housing, with the DBR being located within the cavity.

According to an aspect of the present disclosure, a housing of a DBR based sensor is presented. The housing may include a cavity. The housing may be arranged for accommodating an optical fiber through the housing. The housing may be arranged for accommodating a sensor part of the optical fiber. The sensor part may include the

DBR that is arranged stress-free within the housing. Hereto, the DBR may be located within the cavity of the housing.

In an embodiment, the sensor part of the optical fiber may have a curvature within the cavity.

In an embodiment, the sensor part of the optical fiber does not touch the housing.

In an embodiment, the housing may include two parts. Each part of the housing may include a part of the cavity. The two parts may be put together to form the cavity and the housing.

In an embodiment, the two parts of the housing may be identical.

In an embodiment, at least one of the two parts of the housing may include an aluminum window on the outside of the housing and touching the cavity.

In an embodiment, at least part of the housing may be colored for a specific temperature response.

In an embodiment, a part of the housing facing the object to be measured may be colored for a specific temperature response.

In an embodiment, the housing may further include a ring arranged around the cavity and over the optical fiber. The ring may hold the optical fiber in place within the housing. Furthermore, the ring may increase water resistance and/or reduce environmental influences towards the DBR. Alternatively, another fixation means may be used to fixate the optical fiber in place within the housing, such as one or more pads.

In an embodiment, one or both of the two parts may include a groove for accommodating the ring.

In an embodiment, one or both of the two parts may include one or more studs for keeping the ring in place.

In an embodiment, the DBR based sensor may be an FBG based sensor.

In an embodiment, the DBR based sensor may be a Fabry-Pérot etalon (FPE) based sensor.

According to an aspect of the present disclosure, a method of manufacturing a

DBR based sensor is presented. The method may include providing a housing. The housing may include a cavity at a middle part of the housing and through holes extending from the outside of the housing to the cavity. The method may further include applying an optical fiber into the housing, such that a sensor part of the optical fiber comprising the DBR is arranged in a curvature within the cavity. The method may further include applying a protective tubing around the optical fiber on a part of the optical fiber extending outside the housing and leaving the sensor part of the optical fiber free from the protective tubing. The sensor part of the optical fiber including the

DBR may be arranged stress-free within the housing with the DBR being located within the cavity.

In an embodiment, the method may include providing a first part of the housing.

The first part of the housing may include a cross section of the through holes and a cross section of the cavity. The method may further include inserting the optical fiber into the first part of the housing by guiding the optical fiber through the through holes 5 and the cavity, such that the sensor part of the optical fiber including the DBR is arranged in a curvature within the cavity. The method may further include fixating a second part of the housing onto the first part of the housing. The second part of the housing may include a complementary cross section of the through holes and a complementary cross section of the cavity. The sensor part of the optical fiber including the DBR may be arranged stress-free within the housing with the DBR being located within the cavity after fixating the second part of the housing onto the first part of the housing.

In an embodiment, the step of inserting the optical fiber may make use of a tool holding the optical fiber in the curvature.

In an embodiment, the method may further include applying a ring around the cavity and over the optical fiber. Alternatively, another fixation means may be applied to fixate the optical fiber in place within the housing, such as one or more pads.

In an embodiment, the ring may be applied into a groove surrounding the cavity.

In an embodiment, the ring may be kept in place by one or more studs arranged around the cavity.

In an embodiment, the method may further include removing the tool after the ring has been applied.

In an embodiment, the protective tubing may be fixated to the housing by clamping the protective tubing within the through holes.

According to an aspect of the present disclosure, a chainable sensor array is presented. The chainable sensor array may include one or more DBR based sensors having one or more of the above-described features. The chainable sensor array may include one or more further other types of DBR based sensors.

According to an aspect of the present disclosure, a measurement system is presented. The measurement system may include a chainable sensor array as described above. The measurement system may further include an interrogator device coupled to the chainable sensor array.

Brief description of the Drawings

Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbol indicate corresponding parts, in which:

Fig. 1 shows an example DBR sensor based measurement system based on a chainable sensor array;

Fig. 2 shows a three-dimensional (3D) model of an example embodiment of an

DBR sensor;

Fig. 3 shows an abstract two-dimensional (2D) representation of an example

DBR sensor;

Figs. 4A-4B show a 3D model of an example housing of a DBR sensor;

Fig. 5 shows a 3D model of an example housing of an FBG sensor including an aluminum window;

Fig. 6 is a graph showing the influence of the color of the housing on the time response of an example FBG sensor;

Figs. 7A-7B show example assembly steps for assembling a DBR sensor;

Figs. 8A-8B show 3D models of example parts of a housing of a DBR sensor; and

Fig. 9 shows alternative example assembly steps for assembling an FBG sensor including an aluminum window.

The figures are intended for illustrative purposes only, and do not serve as restriction of the scope of the protection as laid down by the claims.

Detailed description

It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the present disclosure is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present disclosure should be or are in any single example of the present disclosure. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present disclosure.

Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same example.

Furthermore, the described features, advantages, and characteristics of the present disclosure may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the present disclosure may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the present disclosure. Reference throughout this specification to "one embodiment,” "an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present disclosure. Thus, the phrases "in one embodiment," "in an embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. In a practical implementation of the present disclosure, at least some of, preferably all of, the DBRs may, e.g., be FBGs. An FBG is a type of distributed

Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the optical fiber core, which generates a wavelength- specific dielectric mirror.

A fundamental principle behind the operation of an FBG is Fresnel reflection, where light traveling between media of different refractive indices may both reflect and refract at the interface. The refractive index will typically alternate over a defined length. The reflected wavelength is called the Bragg wavelength (Ag).

Known applications of FBG technology include optical communication applications, sensing applications and wavelength-specific reflector applications. The present disclosure uses FBG technology in sensing applications.

The Bragg wavelength is known to be sensitive to strain and to temperature, and

FBGs have been developed that can be used as sensing elements in optical fiber sensors. In an FBG sensor, the measurand causes a shift (AAs) in the Bragg wavelength. The relative shift (AAg/Ag) in the Bragg wavelength due to an applied strain (¢) and/or a change in temperature (AT) may be approximately by the following equation: AAg/ Ag= Cst + CAT. Here, Cs is a coefficient of strain, which is related to a strain optic coefficient, and C: is a coefficient of temperature, which depends on a thermal expansion coefficient of the optical fiber and a thermo-optic coefficient.

An FBG sensor may thus be used to detect the applied strain (£) and/or changes in the temperature (AT) at the FBG sensor. Information obtained from the FBG sensor is particularly useful when obtained periodically over time, enabling pressure sensing applications, temperature sensing applications and/or vibration sensing applications.

Alternatively or additionally, at least some of, preferably all of, the DBRs may, e.g., be FPEs.

DBR sensors may be used in terminated sensor arrays and chainable sensor arrays.

Terminated sensor arrays include one or more DBRs protected by a stiff tube or jacket decoupling strain artefacts. A limitation of terminated sensor arrays is the inability to chain multiple arrays and the limited maximum length of the stiff element.

Chainable sensor arrays typically include of one or more DBRs, each encapsulated into a transducer element. Known chainable sensor arrays have high production costs and have a limited operating temperature range. Moreover, known chainable sensor arrays typically cannot accurately measure temperature distribution over longer length at a low-cost. Available chainable solutions are typically designed for small size applications requiring a complex manufacturing process, resulting in a suboptimal temperature operating range and higher costs. Moreover, the manufacturing process of known DBR sensors is often complex, further contributing to higher costs.

For temperature readings it may be important to avoid interference from strain on the DBR, in particular with FBGs. Strain, and in particular changes in strain, can cause noise on the Bragg wavelengths that are measured for the temperature readings. Thin metal, plastic or ceramic tubes protecting the FBG sensor from strain typically only reduce the strain response rather than completely void it. Achieving a strain relief by fixation of the optical fiber by directly gluing it inside a tube can also result in nonlinear responses of the sensor.

The present disclosure presents a method of manufacturing a DBR sensor and the resulting DBR sensor, which may be used for temperature sensing and which may be used in chainable sensor arrays, without the above identified drawbacks.

Fig. 1 shows an example of a measurement system 100 according to an aspect of the present disclosure. The measurement system 100 may include an interrogator device 110 coupled to an optical fiber 102 including a chainable sensor array 120. In this example, the chainable sensor array 120 includes three DBR, e.g., FBG, sensors 122, 124, 126, but there can be any number of DBR sensors included in the chainable sensor array 120, e.g., 1, 50, 64, 100, hundredths or thousands of DBR sensors. The chainable sensor array 120 may include different types of DBR based sensors, such as a mix of FBG and FPE based sensors.

The interrogator device 110 may include a broadband light source 112 for generating an input spectrum of light into the optical fiber 102. For example, each FBG sensor 122, 124, 126 may reflect part of the input spectrum, which may be detected at a wavelength detection system 114 of the interrogator device 110 as Bragg wavelengths.

The DBR sensor 122, 124, 126 may be used to measure temperature fluctuations overtime. If, e.g., a first Bragg wavelength Ag: corresponds to the reflected light from a first FBG sensor 122, the Bragg wavelength Ags may be analyzed to determine if and to what extent the temperature changes at the first FBG sensor 122 in time. Hereto,

the Bragg wavelength As: may be converted to a digital signal using any known analog- to-digital converter (ADC) system and the digital signal may be processed using any suitable computer system (not shown in Fig. 1).

The periodicity of the measurements may depend on the measurement application. In an environment with frequently fluctuating temperatures, the measurements may be performed continuously, at a periodicity in the order of microseconds, at a periodicity in the order of seconds, at a periodicity in the order of minutes, at a periodicity in the order of hours, or at any other suitable frequency. In a preferred embodiment, the measurements are performed at a periodicity in the order of 20 microseconds to 1 millisecond, which may be considered a continuous monitoring.

The chainable sensor array 120 may be used for monitoring objects. In an example use case, the chainable sensor array 120 may be used for pipeline monitoring, where fluctuations in temperature along the pipeline may be measured using a plurality of DBR sensors 122, 124, 126 along the pipeline. Preferably, but not necessarily, some or all of the DBR sensors 122, 124, 126 are applied equidistantly in the chainable sensor array.

When analyzing the information obtained from the wavelength detection system 114, further sensory data may be taken into account to, e.g., compensate for fluctuations in temperature of a fluid flowing through an object (e.g., fluctuations in the temperature of a liquid in a pipeline) and/or compensate for fluctuations in temperature around the object (e.g., environmental temperature fluctuations). The analysis of the data, including the analysis of the data from the Bragg wavelengths Ag+ and/or any further sensory data, is outside the scope of the present disclosure and will not be further described.

According to a further aspect of the present disclosure, a DBR sensor 122, 124, 126 is presented that is designed for optimal heat transfer towards the DBR of the

DBR sensor 122, 124, 126. Where known DBR sensors are typically designed to follow the temperature of a substrate and/or require the DBR to follow the temperature of the optical fiber, the DBR sensor 122, 124, 126 of the present disclosure has been designed differently. It has been found that conductivity is not a driving factor of heat transfer, but thermal radiation is a key component of the heat transfer. With this knowledge, an improved DBR sensor 122, 124, 126 is designed, which enclosure (herein also referred to as ‘housing’) is not directly fixated to the DBR sensor part including the DBR of the DBR sensor 122, 124, 126. This DBR sensor can measure temperature changes by detecting thermal radiation, typically including a heat exchange via thermal convection. Thus, the sensor part including the DBR is not fixated to a substrate, which advantageously enables very effective strain decoupling, while enabling the DBR sensor 122, 124, 126 to operate as an effective temperature sensor.

Fig. 2 shows a 3D model of an example embodiment of a DBR sensor 200, which may be similar or identical to the DBR sensor 122, 124, 126 of Fig. 1. The DBR sensor 200 includes a housing 210 and a cavity 212 within the housing 210. An optical fiber in a protective tubing 222 is fed through the housing 210, such that the sensor part 220 of the optical fiber including the DBR without the protective tubing 222 resides within the cavity 212. While the tubing 222 may be fixated, e.g., clamped, to the housing 210, the sensor part 220 is not fixated to the housing 210 and preferably loosely resides within the cavity 212, i.e., without any pulling force from the tubing 222 to minimize strain on the sensor part 220. Preferably, the sensor part 220 is arranged within the cavity 212 without touching the housing 210 to avoid external vibrations resulting in vibration noise to reach the sensor part 220 through the housing 210.

In an example embodiment, initially, the protective tubing 222 may be present all over the optical fiber, including the sensor part 220. In this example, the protective tubing may be cut away from the sensor part 220 of the optical fiber, e.g., using window stripping. The protective tubing, or parts of the protective tubing covering the sensor part 220, may be transparent or partially transparent to ease locating the sensor part 220 where the protective tubing is to be removed. Fig. 3 is an abstract 2D representation of an example DBR sensor 300, which may be similar or identical to the DBR sensor 200 of Fig. 2. A housing 310 including a cavity 312, a tubing 322 and a sensor part 320 are shown. The DBR of the sensor part 320 is depicted as S1 in Fig. 3.

At and between the locations F2 and F3, the optical fiber, which is routed inside the tubing through the cavity 312, resides stress-free inside the cavity 312 of the housing 310, preferably without touching the housing 310.

The tubing 322 may be fixated to the housing 310 by using the housing 310 as crimping element. By using the housing 310 as crimping tool, the tubing 322 may be fixated to the DBR sensor 300 without the need of intermediate assembly steps. In the example of Fig. 3, the tubing 322 is fixated to the housing 310 at locations F1 and F4 using the clamping force of the housing 310 itself.

While fixating the tubing 322 to the housing 310 using clamping force is preferred, other fixation methods may be used. For example, the tubing 322 may be glued to the housing 310 at locations F1 and F4.

The tubing 322 may be a stiff tubing, e.g., made from Polytetrafluoroethylene (PTFE).

Fig. 4A is a 3D model of an example embodiment of a housing 400, which may be similar or identical to the housing 310 of the DBR sensor 300. In this example, the housing 400 includes two parts 410, 420. Features of one of these parts 410 will be described. In this example, the other part 420 may be identical to the first part 410 and the two parts 410, 420 may be clamped together. Part 410 is shown in isolation in Fig. 4B. When the two parts 410, 420 are put together, the housing 400 includes a cavity 430, similar to the cavity 212 of Fig. 2.

The part 410 may include one or more protrusions 412 located near location F1, preferably laterally arranged to a through hole 416 for accommodating a tubing such as tubing 322. In the example of Fig. 4A and 4B, the part 410 has two protrusions 412.

The protrusions 412 may a ridged surface for improved fixation to the other part 420.

The part 410 of the housing 400 may further include one or more protrusion receptors 414 located near location F4, preferably laterally arranged to through hole 418 for accommodating a tubing such as tubing 322. In the example of Fig. 4A and 4B, the part 410 has two protrusion receptors 414.

The protrusion receptors 414 are arranged to receive the protrusions of the other part 420. The protrusions 412 are arranged to be inserted into the protrusion receptors of the other part 420. When the two parts 410, 420 are put together, the two parts 410, 420 will be clamped together by the protrusions 412 frictionally fixed within the protrusion receptors 414. At the same time, a tubing in the through holes 418, 418 will be clamped within the through holes 416, 418 and thus to the housing 400.

Advantageously, a DBR sensor including a housing 400 as shown in Fig. 4A may be assembled in a time efficient manner because of the few production steps involved in putting together such DBR sensor. Moreover, because both sides 410 and 420 may be similar or identical, production of the DBR sensor is cost efficient, as the DBR sensor will include few different components.

Although less preferred, the housing may be designed differently, e.g., as two parts that are screwed or glued together.

In an alternative example embodiment (not shown), the housing may be manufactured through a process of over moulding, wherein the housing may be moulded around the DBR sensor part.

The housing 400 may include one or more fixation aids for fixating the housing to an object. Fixation aids may be used to accommodate, e.g., a screw, bolt or tire wrap for fixating the housing 400 to an object. The housing may be without any fixation aids, in which case the housing may be applied to the object in any other suitable manner, e.g., using tire wraps around the housing and the object.

The housing of the DBR sensor, such as housing 210, housing 310 or housing 400, may be mountable on a mounting bracket. The housing together with the bracket may then be applied to an object. A mounting bracket may be used to, e.g., ease installation of the chainable sensor array 120 to the object to be monitored. Mounting brackets may be attached to the object first, followed by clicking the DBR sensors into the mounting brackets.

In an example embodiment, a part of the housing may be made from aluminum.

An example of a housing 510 including an aluminum window 550 is shown in Fig. 5.

The aluminum window 550, or at least part thereof, may be in direct contact with the cavity within the housing 510, such as cavity 212, 312, 430. The aluminum window 550 is located in a part of the housing facing the object when installed on the object, e.g., inthe part 410 of Fig. 4 with the other part 420 being without an aluminum window.

The aluminum window 550 may increase the performance of the DBR sensor, and in particular that of an FBG sensor, by increasing thermal conductance through the housing at the aluminum window 550 and therewith to the DBR sensor within the housing. Response time of the DBR sensor to temperature changes may thus be improved. Moreover, the most temperature sensitive part of the housing 510 will be the part with the aluminum window 550, making the DBR sensor less sensitive to temperature changes of the environment through the other parts of the housing.

The material used for the housing, other than the optional aluminum window, may be adapted for improved thermal response. For example, using a thermal isolating material for the housing together with the aluminum window may result in improved thermal response through the aluminum window.

In an example embodiment, the housing of the DBR sensor may be colored black.

It has been found that a black colored housing improves black body radiation, thereby increasing thermal radiation through the housing and to the sensor part including the

DBR. The effect of the color of the housing to the performance of an FBG sensor is illustrated in Fig. 6.

Fig. 6 is a graph 600 showing the influence of the color of the housing on the time response of an example FBG sensor according to the present disclosure. The x- axis 602 indicates a time from the start of the measurements. The y-axis 604 indicates a wavelength shift (in picometer, pm) in a Bragg wavelength as a result of thermal radiation on the FBG of the FBG sensor. Line 606 corresponds to a white colored housing; line 608 corresponds to a black colored housing.

For the measurements resulting in graph 600, a surface temperature of the FBG sensor, i.e., at the outside of the housing, is stepwise increased using a hotplate by 26°C. The time response is estimated by using the time to reach 63.2% (depicted 610) of the equilibrium value (depicted 612). The graph 600 shows a temperature response of 2m 16s using a white housing (806) and a time response of 3m 06s using a black housing (608). In fact, all time responses are shown to be larger for the black housing (608) before reaching equilibrium. This shows that the color of the housing may impact the time response of the FBG sensor, with a white color resulting in a quicker response compared to a black color. Other colors may have response times between the black and white colors.

In an example, the housing may be blue. In another example, parts of the housing may be colored differently from other parts of the housing. For example, the part of the housing facing the object may be colored white and the other parts may be colored black or blue.

Optionally, the cavity 212, 312 of the DBR based sensor 200, 300, 500 may be filled with a liquid, such as a silicon gel, oil or aqua gel.

The DBR sensor may be assembled within a housing in various manners. In Figs. 7A-7B a non-limiting example of an assembly method 7000 is shown.

In step 7002, an optical fiber 102 including a DBR sensor may be placed in a housing, e.g., the first part 410 of a housing. Optionally, a tool 702 may be used to create the curvature of the optical fiber within the cavity of the housing and keep the

DBR sensor at the correct location during assembly. Alternatively, the housing may be manufactured through a process of over moulding, wherein the housing may be moulded around the DBR sensor part.

In step 7004, a ring 704, e.g., rubber ring, may be placed on the optical fiber 102, e.g., covering locations F2, F3 of the optical fiber as shown in Fig. 3. The ring may be located around the cavity. The ring 704 may be used to fixate the optical fiber 102 to the first part 410 of the housing. Additionally or alternatively, the ring 704 may be used to increase water resistance and/or reduce environmental influences towards the DBR.

As an alternative to a ring, one or more pads or other fixation means may be applied on the optical finer 102, e.g., covering the locations F2, F3, for fixating the optical fiber.

With the optical fiber 102 fixated to the first part of the housing, the tool 702 may be removed as shown in step 7006. The DBR sensor part 220 has become visible in step 7006.

In step 7008, protective tubing 322 may be placed around the optical fiber. The protective tubing 322 preferably does not extend into the cavity, but enter the housing at locations F1, F4 as shown in Fig. 3.

In step 7010, the second part 420 of the housing may be placed on the first part 410 of the housing. In this example, the first part 410 and the second part 420 are fixed using small bolts and nuts 708. Alternatively, the parts may be fixed by clamping force, such as shown in Fig. 4A.

The ring 704 applied in step 7004, may be kept in place within the housing in various manners. An example embodiment is shown in Fig. 8A, where the part 410 of the housing may include a groove 802 for accommodating the ring 704. The other part 420 of the housing may be identical to the first part 410 and also include such groove

802. In another example embodiment shown in Fig. 8B, the part 410 of the housing may include one or more protrusions or studs 804 for keeping the ring 704 in place.

The part 410 may further include one or more stud receptors 806 for receiving the studs from the other part 420 when put together. The example embodiment of Fig. 8B may further include a groove 802.

With reference to Fig. 9, the housing may include an aluminum window, such as aluminum window 550 of Fig. 5. In this case, one part of the housing, in this example the second part 420, may include an opening 902 and/or a recess 904 for accommodating the aluminum window 550. Step 7010 of Fig. 7B may then be replaced with step 9002 of Fig. 9, where the second part 920 of the housing is placed on the first part 410.

In a step 9004, the aluminum window 550 may be placed in the opening 902 and/or onto the recess 904. The aluminum window may be fixated by clamping force or gluing to the second part 920 of the housing.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof.

Claims

CONCLUSIONS

A Distributed Bragg Reflector, DBR, based sensor (200, 300, 500) comprising a housing (210, 310, 400, 510) and a DBR (220, 320) contained within an optical fiber (102), wherein the housing includes a cavity (212, 312) having the optical fiber disposed therethrough, and wherein a sensor portion of the optical fiber includes the DBR stress-relieved within the housing, the DBR being located within the cavity.

2. The DBR-based sensor of claim 1, wherein the sensor portion of the optical fiber has a curvature within the cavity.

3. The DBR-based sensor according to any preceding claim, wherein the sensor portion of the optical fiber does not touch the housing.

The DBR-based sensor of any preceding claim, wherein the housing comprises two parts (410, 420), each housing part including a portion of the cavity, and the two parts being joined together to form the cavity and the housing.

5. The DBR-based sensor of claim 4, wherein the two parts of the housing are identical.

6. The DBR based sensor of claim 4, wherein one of the two portions of the housing comprises an aluminum window on the outside of the housing and contacting the cavity.

7. The DBR-based sensor of any preceding claim, wherein at least a portion of the housing is colored for a specific temperature response.

8. The DBR-based sensor of claim 7, wherein a portion of the housing facing the object to be measured is colored for a specific temperature response.

9. The DBR-based sensor of any preceding claim, wherein the DBR-based sensor is one of: a Fiber Bragg Grating, FBG, based sensor; and a Fabry-Pérot etalon, FPE, based sensor.

10. A housing (210, 310, 400, 510) of a Distributed Bragg Reflector, DBR, based sensor (200, 300, 500), wherein the housing comprises a cavity (212, 312), wherein the housing is adapted to dispose an optical fiber through the housing, and wherein the housing is adapted to accommodate a sensor portion of the optical fiber comprising the DBR such that it is stress-free within the housing, with the DBR located within the cavity.

The housing of claim 10, wherein the housing comprises two portions (410, 420), each portion of the housing comprising a portion of the cavity, and the two portions being joined together to form the cavity and the housing.

12. The housing of claim 11, wherein the two parts of the housing are identical.

13. The housing of claim 11, wherein one of the two portions of the housing comprises an aluminum frame on the exterior of the housing and contacting the cavity.

The housing of any one of claims 10 to 13, wherein at least a portion of the housing is colored for a specific temperature response.

15. The housing according to claim 14, wherein a portion of the housing facing the object to be measured is colored for a specific temperature response.

The housing of any of claims 11 to 15, further comprising a ring (704) disposed about the cavity and over the optical fiber or one or more pads disposed over the optical fiber.

The housing of claim 16, wherein one or both of the two parts includes a groove (802) for receiving the ring.

The housing of claim 16 or claim 17, wherein one or both of the two parts includes one or more studs (804) to retain the ring in place.

19. A method (7000) for manufacturing a Distributed Bragg Reflector, DBR, based sensor (200, 300, 500), comprising: providing a housing (210, 310, 400, 510), the housing comprising a cavity (323, 312) in a middle portion of the housing and through-holes (416, 418) extending from the exterior of the housing to the cavity; disposing (7002) an optical fiber (102) within the housing such that a sensor portion of the optical fiber comprising the DBR is disposed with a bend in the cavity; disposing (7008) of a protective tube (222, 322, 522) around the optical fiber on a portion of the optical fiber extending outside of the housing and retaining the sensor portion of the optical fiber clear of the protective tube; and wherein the sensor portion of the optical fiber comprising the DBR is stress-free disposed within the housing, the DBR being located within the cavity.

20. The method of claim 19, comprising: providing a first portion (410) of the housing (210, 310, 400, 510), the first portion of the housing comprising a cross-section of the through holes and a cross-section of the cavity;

inserting (7002) the optical fiber (102) into the first part of the housing by guiding the optical fiber through the through-holes and the cavity, such that the sensor part of the optical fiber comprising the DBR is arranged with a curvature in the cavity; and fixing (7010) a second part (420) of the housing to the first part of the housing, the second part of the housing comprising a complementing cross-section of the through-holes and the cavity, wherein the sensor part of the optical fiber comprising the DBR is arranged stress-free in the housing, the DBR being located within the cavity after fixing the second part of the housing to the first part of the housing.

The method of claim 20, wherein inserting (7002) the optical fiber uses a tool (702) that holds the optical fiber in the bend.

22. The method of claim 20 or claim 21, further comprising: disposing (7004) a ring around the cavity and over the optical fiber or one or more pads disposed over the optical fiber.

23. The method of claim 22, wherein the ring is disposed in a groove (802) surrounding the cavity.

24. The method of claim 22 or claim 23, wherein the ring is held in place by one or more studs disposed around the cavity.

25. The method of any of claims 21-24, further comprising removing (7006) the tool (702) after the ring is installed.

26. The method according to any of claims 20 to 25, wherein the protective tube is fixed to the housing by clamping the protective tube in the through holes.

27. The method of claim 19, wherein the housing is reinforced by overmolding.

28. The method of any of claims 19 to 27, wherein the DBR-based sensor is one of: a Fiber Bragg Grating, FBG, based sensor; and a Fabry-Pérot etalon, FPE, based sensor.

29. A chainable array of sensors (120) comprising one or more DBR-based sensors according to any of claims 1 to 9.

A measurement system (100) comprising a chainable array of sensors (120) according to claim 29 and an interrogation device (110) connected to the chainable array of sensors.