Field of invention

The present invention relates to an optical fiber for sensor operation and a sensor implementing the same. Specifically, the present invention relates to a multimode polymer optical fiber with at least one fiber grating. The present invention further relates to a method of producing the grating in the fiber.

Background of invention

Fiber-optical sensors are well-known and several suitable fibers are commercially available. Most commercial fiber-optical sensors are using silica fibers, because they are so cheap and well developed with low loss. Polymer optical fibers (POF) are another available class of fibers, which has many advantageous properties for sensing, as compared to the more standard silica fiber. The POF is typically more sensitive to stress and compression, is much more flexible and durable, and can be both biocompatible and biodegradable. A huge number of polymers can be used for fiber fabrication, which makes the POF sensor suitable for sensing of a wide range of chemicals.

The most successful fiber-optical sensor type is using a Fiber Bragg Grating (FBG) inscribed into the core for detecting the reflected wavelength and how it moves with a change in the parameter one wants to measure. An essential property of a fiber for accurate robust FBG sensing is that the fiber is single-mode. The standard silica telecom fiber is thin, single-mode and cheap, which is why its use in FBG sensing is so successful.

In contrast, single-mode POFs are not generally commercially available and those few that are have enormous loss. Commercially available, cost-efficient and low loss POFs do exist and are used in home networks, cars, airplanes, etc. However, to make it easy to connect them they are thick and multimode. Thus, when gratings are inscribed in such a POF, the inscribed grating will also be multimode. One example is provided in Krebber et al.:“Smart technical textiles with integrated POF sensors”, Proc. of SPIE Vol. 6933, 69330V, (2008), where a standard PMMA POF with an outer diameter of 1 mm fiber sample was reduced to less 1/10 of its original diameter, i.e. a taper ratio of less than 0.1. This did not result in single mode operation and such low taper ratio leads to unacceptable taper loss. When light is being transmitted in a multimode POF with an FBG, the Bragg reflection spectrum will have several peaks (Bragg reflections), be unstable and fluctuate in a highly unwanted manner. Sensors relying on multimode FBGs therefore suffer from low accuracy and low resolution. For POF FBG sensing technology really to take off there is therefore a need to develop a low loss single-mode POF FBG.

Summary of invention

The purpose of the present invention is to provide a fiber for a sensor that exploits the low cost and low loss advantages of a multimode telecom POF and at the same time also provides the sensing advantages of a single-mode fiber.

The present disclosure therefore relates to an optical fiber for a sensor, comprising a multimode POF having a core diameter of less than 150 pm, more preferably less than 100 pm, even more preferably less than 65 pm and most preferably less than 60 pm, such as 50 pm. The POF comprises at least one tapered section wherein the core diameter - and the outer diameter - has been reduced. Preferably the POF is tapered in the tapered section to allow for single mode operation of the POF at a predefined wavelength. I.e. the tapering ratio R or R _¥re can be selected accordingly in order to allow and/or ensure single mode operation in a predefined wavelength range of light guided in the core of the tapered section. E.g. in the tapered section the core diameter may be less than 15 pm, more preferably less than 13 pm, such as 12 pm. A grating can be inscribed in each tapered section such that single mode operation can be provided at the grating, i.e. a single-peak grating spectrum can be provided by means of the grating in the tapered section. More than one tapered section with grating may be provided along the POF, e.g. two, three, four or more tapered sections, each tapered section configured to ensure single mode operation of the light and each tapered with a grating inscribed.

Another way to define the presently disclosed optical fiber is by means of a taper ratio: An optical fiber for a sensor, comprising a multimode POF having an outer diameter, po, and a core diameter pc; a tapered section having a taper waist outer diameter, rot, and a taper waist core diameter, rot, on the multimode fiber, wherein the tapered section is defined by a taper ratio given by R = rot/ro, and a grating inscribed in the tapered section. The taper ratio may then be between R=0.10 and R=0.35 and po and/or pc and/or rot and/or PCT can further be selected to allow for single mode operation of the fiber in the tapered section resulting in a single-peak grating spectrum in the tapered section. Similarly the core taper ratio R _¥ re = pcr/pc can be used to define the tapered section of the fiber and the core taper ratio R _¥ re can also be selected to be between 0.10 and 0.35.

With a grating inscribed in a multimode (MM) fiber the nature of the fiber hinders identification and tracking of a single grating peak consistently when there is an external perturbation to the MM grating. This is because there are several modes propagating in MM fiber core, which results in a significant power coupling among the modes, hence power fluctuations on the desired grating to be tracked. Further, the span and the bandwidth of the MM fiber gratings are significantly wider compared to single-mode (SM) FBGs and therefore MM FBG sensors exhibit low resolution even if they are used over a narrow dynamic range. Despite these limitations as FBG sensor fiber, MM POFs have several other important advantages over SM POFs, such as low attenuation, which is a very important factor for POF sensors, ease of fabrication and handling, they are also mechanically robust and can be relatively easily connectorized and integrated with conventional electronics. This is utilized in the present invention.

The inventors have found a way to combine the main advantages of SM and MM POFs and demonstrate herein that MM POFs can constitute the most efficient solution for the development of high quality FBG sensors, which is realized by fiber tapering and grating inscription at the waist of the taper. I.e. when a multimode optical fiber having a suitable core diameter is tapered appropriately, single-mode operation can be achieved in the tapered section, thereby providing a much higher accuracy and higher resolution than a non-tapered multi-mode POF. Another important aspect of the present invention is that the taper ratio should be selected such as to minimize the unavoidable loss due to leakage of higher-order modes during the down-taper. In principle almost any POF can be tapered to ensure single mode operation and such that a grating can be inscribed in the tapered section, and for a given wavelength range the taper ratio can be accordingly selected, i.e. the presently disclosed principle applies broadly. However, for POFs with large core diameters and/or large numerical apertures (NA), the necessary taper ratio is so low that the taper loss would too high, such that even though it would work and single mode light would be guided in the tapered section, but for any practical / commercial applications the taper loss would be too high. Hence, core diameter of the straight fiber section should be selected to be sufficiently small, such as below 300 pm, to reduce the taper loss to an acceptable level. In that regard a taper loss of less than 2 dB would be acceptable, but a taper loss of less than 1 dB would be preferred, and a taper loss of less than 0.5 dB would be most preferred. In particular with several tapered sections with gratings, a low taper loss of less than 1 dB, preferably less than 0.5 dB, would be preferred. If there are several similar tapered sections following each other along the POF, the taper loss of the second, third, etc., would most likely be less than the taper loss of the first tapered section, because it is in the first tapered section that the most higher order modes of light leak out.

The present inventors have realized that the single-mode FBG inscribed in the tapered section provides a single-peak grating spectrum, meaning that the tapered POF and FBG effectively are single mode thereby obtaining an efficient single-mode Bragg reflector. Hence, the present disclosure provides a POF for FBG sensing that has low loss and low cost and - compared to silica fibers - more robustness as a result of the combination of the normal un-tapered part of the POF and the tapered single-mode section with the grating.

The normal procedure to lower the loss of existing thin single-mode POFs would be to meticulously explore material and fiber fabrication technology. The solution proposed here is to instead use fiber tapering, for example tapering of already existing thick commercial multimode POFs to make it locally thinner and therefore single-mode under appropriate conditions, and then inscribe the FBG in this localized single-mode section of the fiber. I.e. the presently disclosed method is about tapering the fiber and inscribing a grating, e.g., at the waist of the taper, thereby enabling the inscription of a single-mode FBG locally. When a MM fiber is tapered, the diameter of the fiber is reduced and thus the diameter of the fiber core is also reduced, which results in a reduced number of modes to propagate in the core of the fiber at the taper waist region. In addition, as the taper section is thin it allows the inscription laser beam to reach the core faster, hence it also facilitates the inscription process.

The present disclosure further relates to a sensor configured for optically measuring changes of a grating in a tapered POF as described herein.

The present disclosure further relates to a method for producing an optical fiber, comprising the steps of: providing a multimode POF, tapering the POF in a section thereof to provide a tapered section of the POF, and inscribing a grating in the tapered section. The taper ratio is selected to minimize taper loss while allowing for single mode operation of the fiber in the tapered section resulting in a single-peak grating spectrum in the tapered section.

It is demonstrated herein that low loss commercially available POFs, e.g. made of perfluorinated PMMA, can be used for efficient and reliable FBG based sensors. The method that can be used is tapering the fiber by for example the heat and pull technique and inscribing at least one grating at the taper waist section of the taper, e.g., by UV laser and phase-mask inscription or plane by plane using direct writing with a femtosecond laser. Phase mask inscription using an UV laser is a standard processing technique, of. Koerdt et al.,“Fabrication and characterization of Bragg gratings in a graded-index perfluorinated polymer optical fiber”, Procedia Technology, 2014 and Min et al.,“Fabrication and Characterization of Bragg Grating in CYTOP POF at 600-nm Wavelength”, Sensors Letters, vol. 2, no. 3, Sep 2018. Grating inscription by means of a fs laser is also known in the art Kalli et al.,“Plane-by-Plane Femtosecond Laser Inscription Method for Single-Peak Bragg Gratings in Multimode CYTOP

Polymer Optical Fiber”, Journal of lightwave tech, vol. 35, no. 24, Dec 15, 2017.

The presently disclosed approach will also allow multiplexing several gratings in a single fiber for quasi distributed sensing. However, for this the taper loss should be reduced by minimizing the taper ratio and possibly also by automating the tapering process

Description of drawings

Figs. 1 A-B, 2 and 3 show reflection spectra of gratings inscribed in untapered and tapered fibers, GigaPOF-50SR, which is a graded index perfluorinated POF with a core, cladding, and over-cladding (outer) diameters of 50, 75, and 500 pm,

respectively. It is commercially available from Chromis Fibreoptics.

Figs. 4A-C show humidity response at 50 °C of the grating inscribed in a waist of 0.24 taper ratio of GigaPOF-50SR fiber. Fig. 4A: the grating response to relative humidity (RH) change in time. Fig. 4B: RH versus Bragg wavelength shift. Fig. 4C: Reflection spectrums of the grating at different RH levels.

Figs. 5A-B show temperature response of the grating inscribed in a waist of 0.24 taper ratio of GigaPOF-50SR fiber at 50% RH. Fig. 5A: in the range 20 to 70 °C. Fig. 5B: 15 to 55 °C. Figs. 6A-B shows the strain response at room temperature and RH of a grating inscribed in the waist of a tapered GigaPOF-50SR fiber, tapered with a 0.24 taper ratio. Fig. 6A: Reflection spectrum of the grating at different applied strain levels, Fig. 6B: Strain versus Bragg wavelength shift.

Fig. 7 shows the strain response at room temperature and RH of a grating inscribed in the waist of a tapered GigaPOF-50SR fiber, tapered with a 0.3 taper ratio.

Fig. 8 shows optical microscope images of a section of fiber waist region. The fiber is GigaPOF-50SR where (a) corresponds to the untapered fiber, (b) has a taper ratio of 0.5, (c) has taper ratio of 0.3 and (d) has a taper ratio of 0.24.

Fig. 9 shows measured taper outer diameter versus taper length for a fiber tapered down to a taper ratio of 0.24.

Fig-i o shows optical microscope image of over cladding etched GigaPOF-50SR.

Detailed description of the invention

To provide for single-mode operation in a multimode fiber the core can be tapered down as herein described. The tapering should typically be made as smooth as possible in order to optimize the mode conversion thereby minimizing the loss of the tapered part. To ensure single mode operation in a predefined wavelength the appropriate core diameter for ensuring single mode operation can typically be calculated or experimentally determined but it also depends of the index profile of the POF, for example whether it is a step-index or a graded-index fiber.

In one embodiment, the presently disclosed optical fiber is configured such that the single mode operation is achieved for light with a wavelength of between 1500-1600 nm, preferably around 1550 nm.

In a further embodiment, the presently disclosed optical fiber is configured such that the single mode operation is achieved for light with a wavelength of between 700-900 nm, preferably around 800-850 nm.

In yet a further embodiment, the presently disclosed optical fiber is configured such that the single mode operation is achieved for light with a wavelength of between 500-700 nm, preferably around 550-660 nm. If the core diameter is too large, e.g. core diameters of between 240 pm and 1 mm, as for example known from ESKA POFs, the necessary tapering ratio to obtain single mode operation is so large that the loss induced by the tapering becomes very high. Hence, the core diameter of the presently disclosed multimode POF is preferably less than 300 pm, more preferably less than 250 pm, more preferably less than 200 pm, even more preferably less than 150 pm, even more preferably less than 100 pm, even more preferably less than 75 pm, or most preferably less than 60 pm, such as 50 pm, as known from GigaPOF-50SR from Chromis Fiberoptics.

The core diameter in the tapered section is selected to provide for substantial single mode operation in a given wavelength interval of the light propagating in the fiber. Hence, the taper core diameter, ot, may be more than 4 pm, preferably between 4 pm and 30 pm, more preferably between 4 pm and 20 pm. With core diameters of the untampered region of between 50 pm and 300 pm, this provides a core taper ratio R _¥ re of between 0.013 and 0.4, more preferably between 0.1 and 0.35 In the example of the GigaPOF-50SR with a core diameter of 50 pm, the suitable taper core diameter has been shown to be around 12 pm for 1550 nm operation corresponding to a core taper ratio R core of 0.24.

The outer diameter of the presently disclosed multimode POF is not that important, but it may be less than 1 mm, preferably less than 600 pm, more preferably less than 500 pm, or most preferably around 500 pm. Alternatively less than 400 pm, more preferably less than 300 or most preferably around 250 pm.

The taper ratio may be selected to be between R=0.15 and R=0.3, or between R=0.2 and R=0.25, such as around R=0.21 , around R=0.22, around R=0.23, or around R=0.24. This applies in particular to POFs with core diameter of around 50 pm. The single-mode operation of the GigaPOF-50SR with R=0.24 is described in Example 1. Use of this POF with a taper ratio of R=0.24 is demonstrated in Examples 2-3.

Among the modes in which light is guided in a waveguide, such as the core of a POF, the term "single mode" means the mode in which light is guided in a state in which a single wavenumber vector is possessed for the light having a certain frequency. Each waveguide mode has an intrinsic periodic electromagnetic field intensity distribution in the waveguide. As used herein, single moded means that the core of the tapered section(s) only supports one propagating mode for the particular wavelength, or wavelength range, of light. The fiber cutoff wavelength generally delineates the operating wavelengths for which the tapered section(s) will be single moded with operating wavelengths greater than the cut-off wavelength being suitable for single mode propagation.

A step-index fiber with core diameter d, core index , and cladding index n ₂ has a V- number equal to V where l is the wavelength of the light. For the step- index fiber to be single-moded, V should preferably be less than 2.405. Hence, for a given POF in a given wavelength range, the taper ratio can be appropriately selected to ensure single mode operation. I.e. for a given fiber the necessary taper ratio will increase when going to shorter wavelengths, because the core diameter d must be made correspondingly smaller to maintain the ratio d/l such that V is less than 2.405. Hence, for a given fiber the required taper ratio R _¥re at a wavelength l = 1550 nm may be less than 0.24, but reducing the wavelength to 775 nm, the required taper ratio R _¥re is then less than 0.12, which most likely will lead to higher taper loss.

For highly multi-moded fibers with large V numbers the number of modes is

approximately V ² 12. The taper loss is related to the power in the higher-order modes, which is being radiated out when tapering, compared to the power in only the fundamental mode, which should be the only one left in the taper waist. For example if the core diameter of the initial straight highly multi-moded fiber is doubled (e.g. from 500 urn to 1 mm), the number of higher-order modes increases by a factor of four. This does not mean that the power in the higher-order modes will increase by a factor of four - but it illustrates that the power in the higher order modes relative to the power in the fundamental mode will increase and thus the taper insertion loss will increase when increasing the core diameter of the initial POF.

The length of the taper waist region may be between 0.5 and 100 mm, more preferably between 10 and 50 mm, even more preferably between 10 and 30 mm. The length of the taper waist is typically around 20 mm.

Type of fiber

Polymer optical fibers (POFs) share many of the merits that conventional silica optical fibers have for sensing applications, such as immunity to electromagnetic interference, small size and multiplexing capabilities. POFs have unique features over those of silica fibers for many sensing applications. These include high flexibility in bending, non- brittle nature, low Young’s modulus, high elastic strain limits and high fracture toughness. The lower Young’s modulus means that a POF typically requires a weaker force to strain the fiber.

The presently disclosed multimode POF may be a graded index fiber or a step-index fiber. Parts of the fiber or the whole fiber may be perfluorinated.

Despite all these advantages, SM POFs, particularly those made of the polymer material mentioned above, suffer high attenuation, especially in the 1500 nm region. As a result the operating wavelength region for FBG sensors based on these fibers is limited to the 600-850 nm wavelength region. Transmission losses in POFs in the infrared region are dominated by absorptions due to carbon-hydrogen (C-H) bond vibrations. In order to tackle this problem it has been proposed and demonstrated to substitute the hydrogen atoms with heavier elements and push C-H bond vibration to longer wavelengths (7.7pm - 10 pm), and reduce the loss in the visible and near infrared region. By substituting hydrogen atoms with fluorine, MM graded index Perfluorinated POF is made available commercially. The low loss of GigaPOF increases both the usable wavelength range up to infrared, which was limited to the visible range for most POFs made of other polymer materials and also the usable fiber length. This feature enables the use of GigaPOFs in short distance communications along with low-cost components that are commercially available and were originally developed for silica fibers for 1550nm operation.

In spite of this feature, the method used to develop GigaPOF fibers does not allow to realize small core fibers and hence single mode fibers. The smallest core diameter realized for this fiber is 50 pm. Nevertheless, 62.5 pm and 120 pm core size

GigaPOFs have been also fabricated and are available on the market from Chromis Fiberoptics. The fiber basically consist of core/cladding and over-cladding, which is composed of doped/undoped amorphous perfluorinated polymers

(polyperfluorobutenylvinyl ether), and polycarbonate, respectively.

Hence, the fiber used herein may be made of any polymer, such as PMMA, perfluorinated PMMA, poly-carbonate, Zeonex, Topas, or any combination thereof. The fiber may alternatively be a fluoropolymer, preferably comprising an amorphous structure. The grating in the tapered section

The presently disclosed optical fiber and the inscribed grating are preferably configured such that single mode operation in the tapered section results in a single-mode FBG reflection spectrum with a single main peak with a Full-Width-at- Half-Maximum (FWHM) of less than 5 nm, preferably less than 3 nm, most preferably around or less than 1 nm.

The sensor and the light source therefor

The presently disclosed sensor based on multimode POFs disclosed herein may be suitable for sensing (a change in) strain, pressure, temperature, humidity, and/or for sensing of biomolecules and/or bio-chemical substances, and/or bio-related parameters, such as pH, oxygen, cortisol, and/or salinity. The sensor may comprise a light source configured for transmitting light into the fiber and to the Bragg grating, such that the light being transmitted through the fiber is reflected when incident on the Bragg grating, the reflected signal comprising the wavelength of the Bragg wavelength. At least one detector can then be provided and configured to detect changes in the Bragg wavelength that can be the result of the fiber being exposed to for example changes in strain, pressure, temperature, and/or humidity. Sensing of strain, temperature and humidity is exemplified in the examples below.

The light source is advantageously configured for transmitting light in the wavelength range where the tapered section is single mode, e.g. in the range from 1500 nm to 1600 nm, more preferably around 1550 nm. Alternatively in the range from 700 nm to 900 nm, or in the range from 500 to 700 nm. The light source may for example be a supercontinuum laser or an LED.

The method for producing the optical fiber

The presently disclosed method may further comprise a step of annealing the fiber and the tapered section before inscribing the grating. Annealing may be performed at more than 50 degrees, preferably at more than 60 degrees, more preferably at more than 70 degrees, most preferably around 80 degrees. For example, fabricated tapers might be annealed at 80‘O for 48 hours in a conventional oven. The annealing temperature can be determined based on the glass temperature of the polymer from which the fiber is made of. In relation to providing the tapered section, the average temperature in the heated zone / section used for tapering the fibers may be about 1 15 -C. Tapered sections with different ratios may be produced by manually pulling the fiber for different total length with the help of 3D stages on both side of the heating zone.

The grating may be inscribed by means of a femtosecond laser operating at less than 600 nm, preferably less than 520 nm, most preferably around 517 nm. For example operating with a repetition rate of around 1 kHz. This may allow for rapid inscription.

The repetition rate may be synchronized to motion of a stage, whereon the fiber is mounted.

The grating may alternatively be inscribed by means of a laser operating at a wavelength between 190 and 350 nm, e.g., a HeCd laser operating at around 325 nm or an excimer laser, such as a Krypton fluoride (KrF) excimer laser, operating at around 248 nm.

In a preferred embodiment the grating is inscribed by means of a phase mask.

Example 1

The fiber used in this example is GigaPOF-50SR from Chromis Fiberoptics. The fiber core, cladding and over-cladding diameters are ~50 pm, ~75 pm, and 500 pm, respectively. The core and cladding glass transition temperature (T _g ) is 108 °C and that of the over cladding is 144 °C. The fibers are tapered by the heat and pull method known in the optical fiber community. The heating zone consists of a 20 cm long U- shaped aluminum block, which was fixed on a hot plate. This configuration allows easy handling of the fiber during insertion and extraction of the fiber into and out of the heating zone. Several experiments have been made to determine a suitable average heating temperature for tapering the fibers with our set up and this was finally found to be around 1 15 ^q C. Tapers with different taper ratios have been produced by manually pulling the fibers for different total length with the help of 3D stages on both side of the heating zone. The tapering ratio is defined as the ratio of the diameter of the untapered fiber to the diameter of the waist of the tapered fiber. The total length of the fiber to be pulled versus the tapering ratio is predicted by an exponential law [Villatoro, J.;

Monzon-Hernandez, D.; Luna-Moreno, D.“In-line optical fiber sensors based on cladded multimode tapered fibers”. Appl. Opt., 43, 5933-5938(2004)], [Xue, S.; van Eijkelenborg, M.A.; Barton, G.W.; Hamble, P.“Theoretical, numerical, and experimental analysis of optical fiber tapering”, J. Lightwave Technol., 25, 1 169-1 176 (2007)], [Birks, T.A.; Li, Y.W.“The shape of fiber tapers”, J. Lightwave Technol., 10, 432^138 (1992)]. For the given furnace length, L, the total fiber elongation, Z, and the taper ratio, R, are related by,

Z = —(2 L Zn(ff))

After several fiber tapers are fabricated for different taper ratio they have been inspected with an optical microscope. The inspection results confirmed that the tapers were fabricated with a success rate of 65 % for a tapering ratio ranging from 0.5 to 0.24. The manual pulling and the open heat-zone configuration resulted in a small non-uniform temperature distribution, see fig. 9.

The taper loss for the one which is tapered down to a taper ratio of 0.24 was ~1.67dB (excluding the fiber propagation loss). This actually again can be improved by using a better taper fabrication method, e.g., by using two motors to pull the fibers from both sides, i.e. it will be able to bring the taper loss below 1 dB and most likely even below 0.5 dB. Among the fabricated fiber tapers we choose tapers with a tapering ratio of 0.5, 0.3, and 0.24 and untapered fibers for grating inscription in the waist of the tapers. The measured characteristics of the three fiber tapers are summarised in table 1 . The core diameters of the tapers are estimated under the assumption that the ratio of the core diameter to the outer diameter of the fiber remains unaltered by tapering. The optical microscope images of a section of their waist region are shown in fig. 8, where (a) corresponds to the untapered fiber, (b) has a taper ratio of 0.5, (c) has taper ratio of 0.3 and (d) has a taper ratio of 0.24.

Table 1 : Measured dimensions and taper losses of three different GigaPOF-50SR fiber tapers with taper ratio of 0.5, 0.3 and 0.24.

Fiber Bragg gratings inscription

The fabricated tapers were annealed at 80 'Ό for 48 hours in a conventional oven before grating inscription. The annealing temperature was decided by the T _g of the core/ cladding, which is 108 °C. A fourth order gratings with a pitch of 2.3pm were inscribed by direct-write, plane-by-plane (Pl-b-PI) inscription method using a femtosecond laser system (HighQ laser femtoREGEN) operating at 517 nm, emitting 220-fs pulses at a 2-kHz repetition rate for pulse energies 80 nJ. The fiber was mounted on an air bearing translation system (Aerotech) for accurate two-axis motion and the laser beam was focused from above using a long working distance x50 objective (Mitutoyo). The number of inscribed grating planes and their width in an untapered fiber and in the waist of each fiber tapers are summarized in Table 2 and their corresponding grating reflection spectrums are shown in Figs. 1 -3.

Table 2: Grating parameters and their corresponding characteristics for FBGs inscribed in untapered and 0.5, 0,3 and 0.24 taper ratio GigaPOF-50SR fibers.

Figs. 1-3 show reflection spectra resulting from propagating broadband 1450-1650 nm light through a multimode POF with a core size of 50 pm with a 1550 nm FBG inscribed in a section of the fiber. The different spectra illustrate the situation with different tapering ratios.

Fig. 1 A is the reflection spectrum of an un-tapered multimode POF with an FBG inscribed in an un-tapered section of the fiber. The plurality of peaks in the spectrum is a result of the multimode operation.

Fig. l B is the reflection spectrum for a tapered fiber, where the taper ratio is R=0.5 resulting in a core size of 25 pm, i.e. still multimode

Fig. 2 is the reflection spectrum for a tapered fiber, where the taper ratio is R=0.3 resulting in a core size of 15 pm. The number of reflection peaks are now considerably reduced indicating that fewer modes are propagating in the fiber. However, the plurality of peaks shows that single mode operation is not achieved. Fig. 3 is the reflection spectrum for a tapered fiber, where the taper ratio is R=0.24, i.e. a resulting core diameter of 12 pm where single-mode operation is achieved and the result is a single well defined peak in the reflection spectrum.

Humidity and temperature characterization

As can been seen from figs. 1 -3 the grating inscribed in the untapered and the waist of 0.5 taper ratio fibers have several nanometers span and multiple peaks and obviously these grating cannot be used for sensing. Therefore we characterized only the gratings inscribed in the waist of 0.3 and 0.24 taper ratio fibers for environmental

measurements.

The humidity and temperature response of the grating inscribed in the waist of a 0.3 taper ratio fiber were characterized. The fiber was first connectorized and connected to a single mode silica patch cable for robust operation and then annealed in a climate chamber at 80 °C and 90 % RH for stable humidity and temperature operation. Unlike PMMA POFBGs significant grating blue shift has not been observed during humidity assisted annealing; this is because the fiber was annealed already before the grating inscriptions at 80 °C for 48 hours. In addition, the annealing temperature is way below the T _g of the over-cladding PC (145 ^q C) and do not cause a significant compression on the fiber core even if it is humidity sensitive to some extent although not as strong as PMMA. Further, the core/cladding material is hydrophobic and humidity did not facilitate the annealing processes.

The humidity measurement was done at constant 50 ^Q C. The chamber was

programmed to change the relative humidity (RH) from 10 % to 90 % and back to 10 % with a step of 20 % RH every 45 minutes. Figs. 4A-C show the result of this process. The total wavelength change observed as the RH increased from 10 to 90 % or decreased from 90 to 10% was -539 pm and the sensitivity measured was 6.73 pm/%RH. As can been seen from fig. 5B the grating retain its SM characteristics as the RH was varied in time.

The temperature characterization was done at a fixed 50 % RH in the range 20 to 70 ^Q C and its responsivity is shown in fig. 5A. The result shows that the grating was not linearly following the temperature change like other POFBGs that have been presented, rather it was a random fluctuation. Another set of experiment on the same grating has also demonstrated random response to temperature (see fig. 5B). From figure 5A the total wavelength change for the 50 °C temperature change was only 166 pm. If we assume the fluctuations is linear and divide the total wavelength change to the total amount of temperature change, we can obtain 3.32 pm/°C which is very small compared to the known magnitude of SM POFBGs temperature response.

Strain, temperature and humidity sensitivities comparison of the SM grating inscribed in the waist of 0.24 taper ratio GigaPOF-50SR with gratings inscribed in SM POFs made of PMMA, Topas 5013, Zeonex 480R and PC is given in Table 3. As most of the work done for these fibers are in the 850 nm region there is no data available at 1550 nm from the literature, so we linearly scaled up to obtain the sensitives at 1550 nm.

Table 3: Sensitivity comparison of common environmental measurands for an SM grating inscribed in the waist of a 0.24 taper ratio GigaPOF-50SR taper with that of other SM POFBGs.

As can been seen from Table 3 the response of the grating in a tapered GigaPOF- 50SR has a unique feature over the others. Both its humidity and temperature sensitivity are very small so that the cross sensitivity due to these parameters is reduced significantly when it is used as a strain sensors. Therefore, it can be used as an ideal strain sensor. We actually believe that the 6.73 pm/% RH humidity sensitivity is due to the PC over-cladding as perfluorinated polymers are hydrophobic. Thus the humidity sensitivity can be further reduced by etching the PC over-cladding material. We have tried to study the response of the grating by etching away the PC over- cladding with dichloromethane. It was easily etched away in less than 5 mins but the grating disappeared gradually within 30 mins. So we believe that the etching could be done before the grating inscription as it is shown in figure 10. However, it could be difficult to handle etched tapered fiber as it becomes very thin. Strain characterization

The strain response of the gratings was studied by mechanically elongating the grating and monitoring its reflection spectrum by clamping the fiber tapers to two micro translation stages. Every time a new strain was loaded or unloaded to the gratings 5 minutes were given for the gratings to get stable. The fibers with 0.24 and 0.3 taper ratios were longitudinally strained up to 2.5 % and 2%, respectively, with steps of 0.5%. A supercontinuum source (SuperK Extreme, NKT Photonics) was used as a light source and an optical spectrum analyzer (Ando AQ6315A, Yokogawa) was used to record the FBGs reflection spectrum during the characterisation. Figure 6 shows the strain response of the grating inscribed in the waist of the 0.24 taper ratio fiber taper at room temperature and relative humidity (RH). The grating presented a linear response with an R-square value of 0.999 and a sensitivity of 20 nm / % strain. Furthermore, no changes in the shape and bandwidth of the grating have been observed, which confirmed that the grating was SM.

Figure 7 shows the strain response of the grating inscribed in 0.3 taper ratio fiber taper at room temperature and relative humidity (RH). As can be been seen from the figure it is difficult to accurately determine the response of the spectrum as there wassignificant mode coupling while the grating was strained and the shape of the spectrum was also changing.

These results show that tapering a multimode POF such that single mode operation around an inscribed FBG is obtained can provide for very efficient and sensitive sensor fibers.

Items

1. An optical fiber for a sensor, comprising:

- a multimode polymer optical fiber (POF) having an outer diameter po and a core with a core diameter pc;

- at least one tapered section of the multimode POF wherein the outer diameter is reduced to rot, and the core diameter is reduced to rot , as defined by the tapering ratio R = rot/ro, and

- a grating inscribed in the tapered section(s),

wherein the taper ratio R is selected to ensure single-mode operation in a predefined wavelength range of light guided in the core of the tapered section.

2. The optical fiber according to any of the preceding items, wherein the core diameter, pc, of the (untapered) multimode POF is less than 300 pm, more preferably less than 250 pm, more preferably less than 200 pm, even more preferably less than 150 pm, even more preferably less than 100 pm, even more preferably less than 75 pm, or most preferably around 50 pm.

3. The optical fiber according to any of the preceding items, wherein the core diameter in the tapered section is more than 4 pm, preferably between 4 pm and 30 pm, more preferably between 4 pm and 20 pm, more preferably between 8 pm and 16 pm, more preferably between 10 pm and 14 pm, or most preferably around 12 pm. 4. The optical fiber according to any of the preceding items, wherein the outer diameter, po, of the (untapered) multimode POF is less than 1 mm, preferably less than 600 pm, more preferably less than 500 pm, or most preferably around 500 pm, or less than 400 pm, even more preferably less than 300 pm, or most preferably around 250 pm.

5. The optical fiber according to any of the preceding items, wherein the POF is a graded index fiber.

6. The optical fiber according to any of the preceding items, wherein parts of the POF is perfluorinated.

7. The optical fiber according to any of the preceding items, wherein the POF is made of any polymer, such as PMMA, perfluorinated PMMA, poly-carbonate, Zeonex, and Topas, or any combination thereof.

8. The optical fiber according to any of the preceding items, wherein the POF is a fluoropolymer, preferably comprising an amorphous structure.

9. The optical fiber according to any of the preceding items, wherein the POF is configured such that single mode operation is achieved in the tapered section for light with a wavelength of between 1500-1600 nm, preferably around 1550 nm. The optical fiber according to any of the preceding items, wherein the POF is configured such that the single mode operation is achieved in the tapered section for light with a wavelength of between 700-900 nm, preferably around 800-850 nm. The optical fiber according to any of the preceding items, wherein the POF is configured such that the single mode operation is achieved in the tapered section for light with a wavelength of between 500-700 nm, preferably around 550-660 nm. An optical sensor comprising the optical fiber according to any of the preceding items. The sensor according to item 12, comprising:

- a light source configured for transmitting light into the core of the optical sensor fiber such that part of the light transmitted through the fiber is reflected when incident on the FBG, the reflected signal corresponding to the Bragg wavelength of the grating; and

- a detector configured for detecting a change in the Bragg wavelength. The sensor according to item 13, further comprising

- a processing unit configured for transforming the detected change in Bragg wavelength to a change in strain, pressure, temperature, and/or humidity, and/or any other measurand. The sensor according to any of the items 13-14, wherein the light source is configured for transmitting near-infrared light, preferably in the range from 1500 nm to 1600 nm, more preferably around 1550 nm, or in the range from 700 nm to 900 nm, or in the range from 500 to 700 nm. The sensor according to any of the items 13-15, wherein the light source is a supercontinuum laser or any form of LED 17. A method for producing an optical sensor fiber, comprising the steps of:

- providing a multimode POF having an outer diameter po and a core with a core diameter pc;

- providing a tapered section of the multimode POF having a taper waist diameter rot and a taper core diameter rot wherein the tapered section is defined by a taper ratio given by R = por/po and

- inscribing a grating in the tapered section, wherein the tapering ratio R is selected to ensure single mode operation in a predefined wavelength range of light guided in the core of the tapered section.

18. The method according to item 17, further comprising a step of annealing the POF and the tapered section before inscribing the grating.

19. The method according to item 18, wherein the step of annealing is performed at more than 50 degrees, preferably at more than 60 degrees, more preferably at more than 70 degrees, most preferably around 80 degrees.

20. The method according to any of the items 17-19, wherein the step of inscribing the grating is provided by means of a femtosecond laser operating at less than 600 nm, preferably less than 520 nm, most preferably around 517 nm.

21. The method according to any of the items 17-20, wherein the step of inscribing the grating is by using a femtosecond laser operating with a repetition rate of around 1 kHz, the repetition rate preferably is synchronized to motion of a stage, whereon the POF is mounted.

22. The method according to any of the items 17-21 , wherein the step of inscribing the grating is provided by means of a HeCd laser operating at around 325 nm.

23. The method according to any of the items 17-22, wherein the step of inscribing the grating is provided by means of laser with a wavelength shorter than 325 nm, such as a Krypton fluoride (KrF) excimer laser operating at around 248 nm.

24. The method according to any of the items 17-23, wherein the step of inscribing the grating is provided by means of a phase mask.