Patent Description:
Embedding of fiber optic sensors in materials is a known technology to measure in-situ material deformations. In the ingress/egress strategy, one of the main problems is usually to find suitable methods for connecting this embedded fiber with an external fiber. Although several ingress/egress techniques exist, mainly based on feed through designs and external coupling, the resilience and strength of the coupling is not guaranteed.

Optical fiber sensors are normally spliced to a pigtail which connects the sensor with its read-out device. This typically needs some length of fiber, such as a few centimetres, starting from the edge of the composite. Composite structures are usually fabricated by moulding, in which cases embedding of optical fibers in composite laminates can be performed during fabrication. After fabrication and prior to use, trimming of the structure edges is often necessary. By trimming the edges, all fiber ingress points will be lost, and as a consequence splicing is very difficult or impossible to carry out when the fiber is broken at the edge of the composite.

A possible solution is avoiding cutting the fiber. For example, a connection may be provided in the waveguide embedded in the material during the fabrication. In case of composites, for example moulded composites, the solution is cumbersome. The embedded fiber in such system has restricted orientations and positions, and the production mould and established manufacture route needs to be adapted. The fiber is protected using Teflon or other types of tubing or Kapton foil. Sometimes the outcome of the fiber is protected with silicone. It is hardly possible to trim edges or remove mould burrs off the material. Other options include removing partially composite material, for example several layers on the side along the fiber, revealing a portion thereof, and then connecting the fiber in the revealed side. This is a cumbersome method, it requires a separate protective block for the fiber and the removed parts of the composite, and prior knowledge of the position of the fiber is necessary, or at least a reliable method to find it, which is not trivial in many cases.

Document <CIT> describes a method for connection of optical fibers used as optical sensors in e.g. automotive industry, involving embedding fiber in glass-fiber reinforced plastic, and turning front sides of ends of fibers towards each other. A portion of the embedding material, surrounding the fiber, is typically removed before an optical fiber ferrule and sleeve is used to align the embedded fiber with the external fiber. Finally, an arc is used to connect them. This method may damage the optical fiber and its cleave angle may introduce losses. These losses add up to the losses from splicing fibers, which take place especially in the usual case in which both fibers are different (e.g. one may be optimized for FBG sensing, while the other for signal transmission).

In the above discussed existing methods, there is no or very little room for repair or rework of a bad connection.

Document <CIT>, discloses a straight optical connector similar to conventional connectors of the MT type. Two optical fiber cables are arranged facing each other in a main body of an optical connecting member. A gap in the main body is filled by a resin and hardened by photocuring. The optical connector is only suitable for optical fiber cables for which an end portion is free and easily accessible.

Document <CIT>, discloses a branched optical coupling module for establishing an optical link between an optical fiber on the one hand and a light-emitting and a light-receiving element on the other hand. A hollow interior of a connector housing is filled by a resin and hardened by photocuring. The branched optical coupler is only suitable for optical fiber cables for which an end portion is freely accessible.

Publication <CIT> discloses an optical connector located within a gap between a first optical assembly and a second optical assembly. The optical connector includes a contrast layer having at least one cured bridge portion and at least one uncured portion.

It is an object of embodiments of the present invention to provide a reliable, non-invasive optical fiber connection between an embedded optical fiber and an external optical fiber with reduced or negligible losses.

It is an advantage of methods of the present invention that coupling the optical fiber directly at the edge or composite surface is enabled, and it allows connection or repair of the connection at any time.

The present invention provides an optical system according to independent claim <NUM>.

It is an advantage of embodiments of the present invention that a composite with embedded optical fiber can be obtained in a standard manufacturing process, without the need to introduce a connection during the process.

The system may comprise a strain relieve element decoupling the stress acting on the second optical fiber from the optical connection. It is an advantage of embodiments of the present invention that no strain is induced on the intermediate waveguide. It is an advantage of embodiments that a good attachment between the embedded optical fiber and an external optical fiber is provided, and that a smooth optical transition between the two optical fibers is established, whereby coupling between different optical fibers can be obtained by providing a gradual conversion of the mode field diameter.

The optical system may further comprise a ferrule, a block, a connector, a base plate or a combination of these attached to the self-written waveguide. It is an advantage of embodiments of the present invention that a reinforcement of the external optical fiber, such as a reader fiber pigtail, is obtained. The connection may also easily and reliably be manufactured.

The ferrule may be adapted to cooperate with an external ferrule of a removable optical fiber, in order to provide optical contact between the second optical fiber and the removable fiber. It is an advantage of embodiments of the present invention that the connection may be detachable, allowing for example different readers to be connected to an embedded fiber sensor.

Both ferrules may have angled termination. For FBG sensors, preferably angled connectors are used to avoid unwanted reflection. In case the connection is not detachable, the use of angled fibers/Ferrules is less important since the fiber facet is embedded in the SWW material which has an index close to that of the fibers. In case the connection is detachable, the detachable interface part needs to have an angled fiber facet to avoid reflection. This is the main advantage of using an angled surface. The optical fibers may be single mode optical fibers. It is an advantage of embodiments of the present invention that the connection may be aligned in an easy way.

The second optical fiber may be an optical fiber adapted for information transmission. It is an advantage of embodiments of the present invention that damages and stress can be detected in a composite, such as a structural composite, during use, without the need of complex fabrication processes for embedding the sensor with connection in the composite.

The present invention also relates to a method for optically coupling an external optical fiber with an optical fiber embedded in a structural composite according to independent claim <NUM>.

It is an advantage of embodiments of the present invention that trimming is not required, obtaining a reliable and stable connection. It is a further advantage that two different waveguides may connect with minimum loss.

Aligning the exposed end-faces of the waveguides may comprise transmitting a signal through the external waveguide and measuring the difference in reflectivity between the end-face of the embedded optical fiber and the composite material, thus revealing the location of the embedded optical fiber. It is an advantage of embodiments of the present invention that good fine alignment can be obtained, ensuring maximum transmission between the optical fibers. The latter can be obtained without needing additional components and in reflection. The embedded optical fiber does not need to be connected.

Obtaining a self-written waveguide comprises the steps of dispensing a UV-light curing optical adhesive between the end-faces of the two optical fibers, forming the core of the self-written waveguide, and forming the cladding of the self-written waveguide. It is an advantage of embodiments of the present invention that good alignment of the core fibers can be obtained. It is a further advantage that a smooth, gradual match between mode field diameters (MFDs) of the embedded and the external optical fibers can be obtained.

Forming the cladding comprises applying flood UV exposure to the UV-light curing optical adhesive.

It is an advantage of embodiments of the present invention that different degrees of curing can be reached in the SWW, allowing flexibility of optical index difference between the core and the cladding of the SWW, thus allowing tailoring the transmission between the external and embedded optical fibers.

Alignment may be performed with support of a base plate. It is an advantage of embodiments of the present invention that mechanical and thermal stability can be obtained, for example during manufacture, by use of a carbon and/or composite base plate.

Where in embodiments of the present invention reference is made to "end-face" of an optical fiber, reference is made to the cross section of the optical fiber, usually perpendicular to the side walls, from which the radiation leaves the optical fiber. In cylindrical optical fibers, for example, the side walls comprise the cladding, and the end-faces of the fiber are the sections at the opposite extremes of the fiber, revealing the fiber core. If the end-face is oblique to the side walls, it receives the name of "cleaved end-face" due to the usual method of obtaining it.

In a first aspect, the present invention relates to an optical system providing a connection between two optical fibers, one of which is embedded in material such as a composite material, the other being an external optical fiber, by an intermediate waveguide providing adhesion and optical connection with low losses between the embedded optical fiber and the external optical fiber.

The embedded optical fiber is completely buried within the material, except for its end-face, which is in contact with the exterior via an aperture or window in the material. The surface of composite material may contain the surface of the end-face, giving a smooth transition between the two different surfaces with no bumps or depressions. The intermediate waveguide is made in contact with the end-face of the embedded optical fiber, it may also be in direct contact with the portion of the composite surrounding the optical fiber, and it is in contact with the external optical fiber.

The material serving as intermediate waveguide is an optical material, e.g. a self-written waveguide (SWW), e.g. with adhesive properties (optical adhesive, for example "NOA68"). Because the embedded optical fiber and the external optical fiber may be different, ideally the optical material should roughly "average" properties of both optical fibers (average optical index, average mode field diameter (MFD)). It was found that the SWW can act as a mode field diameter converter between two different optical fibers and therefore result in a lower loss than a simple butt-coupling connection of those <NUM> fibers.

A particular example of connection between a sensor (e.g. "DTG-sensor") fiber and an external standard telecom (e.g. "SMF-<NUM>") fiber is explained by comparison with splicing. Because the optical properties of both fibers are different (e.g. the core diameter is different), the mode field diameter of the light travelling in both fibers is different which will result in an additional loss when simply splicing those fibers. The idea described here uses an intermediate (self-written) waveguide structure that acts as a mode field converter, thereby minimizing the theoretical achievable losses of the DTG-SMF28 connection.

The external optical fiber is an optical fiber that is not embedded in the embedding material.

The connection of the present invention is highly reproducible, inexpensive, and simple. It does not require removal of composite walls or material which may reduce the durability of the composite.

According to one set of embodiments, the optical system furthermore comprises a mechanical connection between the composite material, directly or indirectly, and the external optical fiber, e.g. a pigtail of that optical fiber. The mechanical connection thereby is established in such a way that there is no direct force acting on the optical connection, e.g. on the self written waveguide, thus decoupling mechanical and optical aspects.

<FIG> shows a schematic cross-section of a simple connection according to an embodiment of the present invention. A first optical fiber <NUM> comprising a fiber core <NUM> is embedded in an embedding material <NUM>. The optical fiber <NUM> is completely embedded in the material at the zone of the connection except for the end-face <NUM> of the optical fiber, the entirety of its side walls <NUM> being in contact with the embedding material <NUM>. The end-face <NUM> of the embedded optical fiber <NUM> is shown in line with the surface <NUM> of the material <NUM>. However, this is not essential in the present invention, and the optical fiber <NUM> may not be completely embedded in its whole length, just in a portion proximal to the end-face. For example, the extreme of the optical fiber <NUM> opposite to the end-face <NUM> may be outside the material, or some areas of the material may reveal the optical fiber. Additionally, a portion of the side walls <NUM> extending from the end-face <NUM> may be also revealed, slightly.

A second, external, optical fiber <NUM> is provided. An intermediate optical material <NUM> is fabricated in contact with the external optical fiber <NUM> and it is also provided at the end-face <NUM> of the embedded optical fiber <NUM>. The intermediate optical material <NUM> is provided as a self-written waveguide <NUM>. Thus, optical connection between the embedded optical fiber <NUM> and the external optical fiber <NUM> can be obtained without splicing.

The intermediate material <NUM> covers the totality of the end-face <NUM>, but it may also extend over part of the surface <NUM> of the material <NUM>. The core <NUM> of the external optical fiber <NUM> and the core <NUM> of the embedded optical fiber <NUM> are preferably optically aligned. The invention, however, is not limited to said configurations. According to embodiments of the present invention, the SWW <NUM> covers at least the core of the embedded optical fiber but may not cover the complete end-face of the embedded optical fiber.

In embodiments of the invention, the embedding material may be a composite such as a composite sheet, and the embedded optical fiber may be an optical fiber buried within and extending underneath the surface. The composite sheet has certain thickness, thus the sheet comprises at least a side surface (e.g. a border surface along the composite thickness), and the end-face of the optical fiber may be comprised in the side surface of the sheet. The optical fiber may comprise a sensor, such as a deformation sensor comprising a fiber Bragg grating (FBG), a sensor for detecting internal damage in the composite, etc..

The external optical fiber may be an optical fiber optimized for telecommunications. In embodiments of the present invention, it may be an optical fiber for providing a connection to a sensor reader (e.g. directly connected to a reader, or adapted in the system as a pigtailing fiber (or pigtail) for providing such connection to a sensor fiber, for example). In general, optical fibers for transmission to a reader and optical fibers for sensing may have different characteristics and standards. For example, the numerical aperture and cut-off wavelength of both optical fibers may be different. For example, the diameter of the core <NUM> of the embedded optical fiber <NUM> may differ from the diameter of the core <NUM> of the external optical fiber <NUM>. In general, it can be said that the mode field diameter (MFD) of the embedded optical fiber <NUM> may be different from the MFD of the external optical fiber <NUM>.

Embodiments of the present invention allow a smooth match between the different optical fibers, irrespective of their MFDs, through the intermediate optical material <NUM>, which may be a self-written waveguide SWW. In the general case of mismatching MFDs, the SWW may offer MFD conversion, gradually adapting the MFD when the optical mode signal travels through the connection, improving the performance in comparison with other techniques such as fusion splice. For example, it may provide a gradual conversion, e.g. a linear conversion, or a step-wise conversion.

<FIG> shows a schematic of a realistic implementation of the concept shown in <FIG>, including the optical connection as well as a mechanical housing <NUM>. The intermediate optical material <NUM>, such as a SWW <NUM>, extends over part of the surface <NUM> of a material <NUM> (e.g. a composite) comprising an embedded optical fiber <NUM>, improving mechanical resilience and stability of the connection. Resilience may be further improved by the addition of an adhesive layer <NUM>, which may have the same or different composition as the intermediate material <NUM> (e.g. the adhesive may comprise material suitable for a SWW <NUM>, but it may also be other type of adhesive). A ferrule <NUM>, for example a ceramic ferrule, is in contact with the external optical fiber, which may be an optical fiber "pigtail" <NUM>. The ferrule <NUM> is mainly used for handling during connectorization. The ferrule <NUM> also provides mechanical resilience and support to the fiber pigtail <NUM>, which may or may not stick out of the ferrule. A reinforcement housing <NUM> may be provided.

In embodiments of the present invention, the optical connection can be provided between single-mode optical fibers, although the present invention may also provide an optical connection between multi-mode or multi-core fibers.

In a second aspect, the present invention relates to a method for obtaining an optical connection according to embodiments of the first aspect of the present invention. The method may comprise providing SWW <NUM> in an egress and ingress method of optical transmission in an embedded optical fiber such as a fiber sensor. The method is suitable for optically coupling an external optical fiber (such as a fiber pigtail of a sensor reader) with an optical fiber embedded in an embedding material (such as the optical fiber of a deformation sensor in an embedding composite), and it may advantageously reduce coupling losses.

<FIG> shows a flowchart with the main steps (full lines) and optional steps (dashed lines) of the present method.

The step of providing <NUM> an optical fiber in a material may comprise embedding an optical fiber (e.g. a fiber sensor, such as a FBG sensor) in a composite material by placing the optical fiber in a mould, without any connectors, and curing the composite. The rest of the process comprises providing a connection (connectorizing) in the material. Other sub-steps such as trimming, removing burrs, and other treatments may be comprised in this step. It is an advantage that the standard fabrication process of the material or composite does not need to be changed in order to provide connectors.

Once the optical fiber, also referred to as embedded optical fiber, is provided <NUM> in the material, a step of exposing <NUM> the end-face of the embedded optical fiber is provided. For example, a cut can be made in the composite after curing, for example using a diamond blade, exposing the end-face of the embedded fiber. Using a diamond blade has the advantages that little or no strain or damage is introduced in the optical fiber or the especially delicate fiber core. Other methods may be used, such as slicing the material with a microtome, removing material by grinding and/or polishing, etc. For example, the end-face may be revealed by grinding away material from the surface that will contain the connections (e.g. the edge of a composite), and then polishing the surface for obtaining a very smooth end-face surface with an undamaged fiber core revealed (for example, using <NUM>/<NUM> grain size grinder paper, or finer polishing). In some cases, no composite material needs to be removed at all (for example, if the fiber sticks out of the material and it can be cut so an end-face is exposed, for example).

The step of providing <NUM> an external optical fiber (for example an optical fiber, such as a pigtail suitable for a sensor reader) comprises providing an optical fiber with at least one end-face for receiving optical signals therein. For example, at least one end-face should be properly obtained with few or no defects. The method may further comprise the optional step of providing <NUM> a reinforcing ferrule, which may be attached to the pigtail, which optionally may stick out of the ferrule, for facilitating further connection with an external sensor, reader, fiber, etc. In embodiments comprising thin embedding materials, the ferrule may be a micro-ferrule. In some embodiments, a number of fibers may be used whereby instead of a ferrule e.g. a block with precise v-grooves can be used.

Once both optical fibers are obtained, the step of aligning <NUM> an end-face of the external optical fiber with the exposed end-face of the embedded optical fiber, so there is optical connection between the optical fibers, is provided. In case of optical fibers, the core <NUM>, <NUM> of the optical fibers <NUM>, <NUM> should be aligned with respect to each other, as shown in <FIG>. Due to the small size of the optical fibers (usually less than <NUM> microns), this process is not trivial. Although the use of a camera may be an option, the embedded optical fiber may be difficult to see due to low contrast. This also has the disadvantage that an additional camera would be required. In general, the aligning <NUM> comprises setting the external optical fiber at an appropriate distance (e.g. <NUM> microns) of the surface of the material, introducing a signal through one of the optical fibers and scanning the surface of the material with the external optical fiber, studying the signal detected in a detector attached to the external optical fiber. The present invention shows two exemplary methods of alignment, in transmission and in reflection.

For the method in transmission, in order to find an end-face of the optical fiber in the composite surface, a signal is introduced through the opposite side of the embedded optical fiber and the signal is searched on the surface. Specifically, if the embedded optical fiber is connectorized on the side opposite to the end-face <NUM> to be attached to the SWW, it can be connected to a laser source. The external optical fiber is then connected to a detector, and the surface of the composite is scanned. When the external optical fiber detects an intensity maximum, the external optical fiber and the embedded optical fiber are aligned.

For the method in reflection, the signal is introduced in the external optical fiber, illuminating the relevant portion of surface of the material in which the end-face of the embedded optical fiber is expected. The reflectivity of the composite and the end-face of the optical fiber are typically different, and can be known. Thus, the external optical fiber scans the surface, and once the maximum reflection typical from the embedded optical fiber is detected, the external and the embedded optical fibers are aligned.

The present invention may use one or another method, or may use a combination of both. For example, a method in reflection may be used for aligning a first end of an embedded optical fiber, connection between external and embedded optical fibers may be done and subsequently be used to align a second end of the embedded optical fiber.

Optionally, the alignment may comprise performing <NUM> coarse alignment and/or performing <NUM> fine alignment. Fine or micro alignment can be used for an optical fiber sensor (e.g. a fiber comprising a FBG), by scanning over the edge of the embedding material, as before. From the external optical fiber, a signal (e.g. broadband light) is launched, matching what is required for reading out the FBG sensor. The external optical fiber is now scanned over the edge of the composite material comprising the embedded optical fiber with FBG sensor, so that the highest reflection peak is obtained when both optical fibers are aligned. This technique is mainly suitable when both optical fibers are already slightly aligned, for example via a coarse alignment, and only require final fine scanning to exactly align both optical fibers.

For performing <NUM> the optional, coarse, alignment (e.g. over millimeter range) a faster technique can be used, in which the external optical fiber scans the surface and analyzes the difference in reflectivity of glass (embedded fiber) and composite material, which displays a "signature" from which the location of the embedded optical fiber can be revealed. This "signature" may be a smoother and more uniform reflection profile when mapping the area of the optical fiber than when mapping in the composite surface. A peak of reflection intensity may also be obtained at the edge between the composite and the optical fiber, and if the optical fiber comprises a FBG, a reflection peak in the center, allowing fine alignment. In any case, the large step size of a coarse alignment makes this option advantageously faster.

Once alignment is performed, the method comprises the steps of providing <NUM> optical material in the gap between both optical fibers and obtaining <NUM> an intermediate waveguide by forming a SWW. The gap may be approximately <NUM> microns. A small blob of material of <NUM> or <NUM> microns of diameter may be provided, the present invention not being limited thereto. The optical material should cover the end-faces of the optical fiber, and may extend over the sides of the external optical fiber, and/or over any reinforcement structure (e.g. a ferrule). The material may extend over the wall of the composite. The optical fibers may be brought to contact for a reference measurement, then separated at the distance required for the intermediate optical material, and then obtaining <NUM> an intermediate waveguide. Writing the waveguide may comprise introducing an UV signal in the optical material via the external optical fiber such as a <NUM> laser signal, thereby forming the core of the SWW. Writing the waveguide may further comprise UV flooding the surface of the optical material, thereby forming the cladding of the SWW. This curing step may be tailored, so the optical index in the core is known and different from the index in the cladding. The difference of optical indices influences the MFD. Thus, the SWW may be tailored for a certain gradient of MFD and transition. Typically, maximum transmission between the external optical fiber and the embedded optical fiber is desired, but other applications may aim at lower transmissions.

An additional layer of optical material, or adhesive, may be provided to cover the intermediate waveguide and provide mechanical stability. Mechanical effects, such as variations of volume due to heating during the UV curing, are prejudicial for the very sensitive alignment between the optical fibers, but these effects may be counteracted by the use of a base plate, such as a carbon or composite removable base plate, which provides mechanical and thermal stability. Providing an additional layer of optical material or a base plate may be done sequentially or simultaneously with the step of aligning <NUM> the optical fibers. For example, SWW may be obtained while monitoring in reflection the alignment (e.g. by observing the grating spectra). Monitoring also provides information regarding the curing process.

Other reinforcements can be applied. For example, the external optical fiber (e.g. the pigtail fiber) may comprise a ferrule, which provides resilience to the connection during manufacture and use, even in an ultrathin foil embedding.

As an optional final step, structural reinforcement such as a housing, strain reliefs, etc. may be provided <NUM>.

Embodiments of the present invention advantageously ensure an easy-to-use and reliable mechanical connection. This can for example be obtained by use of a fiber ferrule, by fixing the fiber in or on a block, a connector, a base plate or a combination of these. It ensures a more stable mechanical connection with the composite (owing to the larger diameter compared to the fiber itself). A standard, ceramic or otherwise, fiber ferrule may be used, which provides compatibility with standard connectors. This step may be performed even before obtaining the SWW, for example it may be done at the point of providing <NUM> the external optical fiber, so when the SWW-based optical connection is made, the pigtail optical fiber is already mounted in the ferrule.

In an embodiment, the external pigtailing fiber or external optical fiber is permanently attached to the ferrule, using a proper mechanical part as reinforcement. This mechanical reinforcement part may be applied after providing <NUM> optical material between the optical fibers or after obtaining <NUM> the SWW (e.g. by sliding it over the already attached ferrule), or may be mounted at the same time than the SWW is fabricated <NUM>. In case of the latter, the complete assembly (ferrule and reinforcement) is then used during alignment and mounted simultaneously.

In a further embodiment, with reference to <FIG>, the external pigtailing optical fiber/ferrule assembly <NUM> is mounted in a sleeve or holder <NUM> during the connectorizing process in such a way that the holder <NUM> can still be removed and, for example, replaced by a different connector (e.g. standard FC/APC). In some embodiments, the holder <NUM> is replaced by the final connector housing and everything is mounted as a complete assembly. After the process, the removable ferrule <NUM> can be disconnected and replaced by the desired fiber patch cord.

The method may comprise further steps, such as removing the detachable ferrule <NUM> not fixed to the SWW and the temporary connector <NUM> (if such devices were used during the previous steps), providing a connection housing, which may include providing a holder for the SWW material, providing a strain relief fixation for reducing bending and strain on the pigtail optical fiber and/or the external optical fiber, and/or adding a cable fixation structure and a dust cover. <FIG> shows a finalized connection comprising a housing <NUM>, which may have been provided after removal of the holder <NUM> and the removable ferrule <NUM>.

An advantage of embodiments of the present invention is the possibility of coupling two different types of optical fibers with very low losses.

An example of setup is shown in <FIG>, for monitoring optical coupling loss in real-time during the connection process. A monitoring source <NUM> and one or two detectors <NUM>, <NUM> may be used. For example, a superluminescent LED (SLED) may be a suitable monitoring source <NUM>, and the detectors, a photodiode <NUM> and an Optical Spectrum Analyzer (OSA) <NUM>, can used in transmission, but other detectors and sources may be used. The SLED and OSA can be substituted by other detectors if the alignment is performed in reflection. For example, a fiber interrogator may be used. The setup is controlled by a processing unit <NUM> such as a computer, which may also control triggering of the source <NUM>, for example a <NUM> laser source, allowing precise and automatic data acquisition which facilitates process optimization. The pigtailing fiber <NUM> is provided, which may be the same material as the embedded fiber or a different material and may be the same or different type of fiber. Between the pigtail <NUM> and the composite embedded fiber <NUM>, optical material <NUM> is provided and alignment can be done. The <NUM> source may be introduced in the fiber upon alignment, and a cladding may be formed on the optical material <NUM> via UV flood <NUM>. Further optical components shown are a splitter <NUM> and a circulator <NUM>.

As explained with reference to <FIG> and <FIG>, the fiber tips may at this point slightly stick out of the ferrule. This facilitates initial tests, but is not essential and other configurations may be used, such as a standard polished fiber-ferrule assembly. If no fiber sticks out, the connection may present improved mechanical resilience, e.g. similar to commercial assemblies in which the fiber is not sticking out.

An embodiment of the method of the present invention may comprise the following procedure:.

The insertion loss is measured continuously during the course of the experiment. This serves as a direct feedback for the success of the connection process and gives insight on the SWW formation process. In such a way, parameter optimization becomes also easier. Alternatively or in addition thereto also the signal in reflection could be employed as a figure of merit, in case the embedded fiber has no connector.

In an example of method of the present invention, an embedded commercial fiber sensor can be coupled to a commercial telecommunication fiber. For example, the embedded fiber may be a DTG <NUM>-<NUM>, optimized for <NUM> wavelength window, and with a diameter of <NUM> microns, a core index of <NUM> and a refractive index contrast (RIC) of <NUM> (RIC represents the difference of optical indices between the core and the cladding). The telecommunication fiber may be e.g. an SMF-<NUM> fiber with a core index of <NUM> and RIC of <NUM>. Use of SWW according to embodiments of the present invention brings better coupling performance than direct "DTG/SMF-<NUM> splice" method. Match between the MFD of the DTG-SWW interface and the SWW-SMF-<NUM> interface should be as close as possible. Since mode field diameters in DTG and SMF28 fibers are clearly different (~<NUM> microns compared to ~<NUM> microns respectively), the SWW will need some sort of average MFD of both fibers. The ideal parameters would be a RIC between <NUM> and <NUM> and an MFD of <NUM> microns. In this case it acts as an MFD-converter, gradually adapting the MFD when the optical mode travels through the connection. This justifies the better performance which can be obtained compared to a fusion splice. <FIG> shows a simulation of the fraction of transmitted power T for different RIC values through a SMF-<NUM>/SWW/DTG transition, representing the "mode size-converting" behaviour of a SWW section. Because RIC represents the difference between core and cladding optical indices, different values may be obtained for different curing types and curing times. If the RIC in the SWW is similar to that in the SMF-<NUM> fiber (bottom power distribution plot in <FIG>), the MFD of a mode propagating from the SMF-<NUM> fiber side presents only a small variation. When the RIC is increased, the maximum value of the power (in the center of the waveguide/fiber) is increased. The total power is conserved; thus the MFD is reduced (higher mode peak power). The minimum achievable loss for this specific simulation case is about <NUM>. It is possible that a more optimized RIC profile of the SWW may still reduce this value. For example, a gradual change in RIC will likely reduce the loss, but this is more difficult to obtain in practice. The loss between an SMF-<NUM> fiber - (uncured NOA68) - DTG fiber at nearly <NUM> micron separation was simulated to be about <NUM>. 9dB, which is similar to the expected splice losses.

Claim 1:
An optical system, the optical system comprising
- a first optical fiber (<NUM>) embedded in a composite material (<NUM>), the first optical fiber (<NUM>) comprising side walls extending in a longitudinal direction in contact with the composite material (<NUM>),
- a second, external optical fiber (<NUM>) having a different optical mode field diameter than the first optical fiber, and
- a self-written intermediate waveguide (<NUM>) in optical contact with the first and second optical fibers (<NUM>, <NUM>) thus forming an optical connection between the first optical fiber (<NUM>) and the second optical fiber (<NUM>), wherein the self-written intermediate waveguide (<NUM>) is formed in a UV-light curing optical adhesive, the first optical fiber (<NUM>) comprises an optical fiber sensor for detecting deformations in the composite material (<NUM>), and only an end face perpendicular to the side walls of the first optical fiber (<NUM>) is exposed to the outside of the composite material (<NUM>), said end face of the first optical fiber being in contact with the self-written intermediate waveguide (<NUM>).