NL2027050B1

NL2027050B1 - Fiber-Bragg grating sensor

Info

Publication number: NL2027050B1
Application number: NL2027050A
Authority: NL
Inventors: Forsch Moritz; Nieuwland Remco; Offerhaus Herman; Caldarola Martin
Original assignee: Univ Delft Tech
Priority date: 2020-12-04
Filing date: 2020-12-04
Publication date: 2022-07-07
Also published as: WO2022119446A1

Abstract

The invention provides a glass fiber (100) for a strain sensor, wherein the glass fiber (100) comprises a set (110) of fiber-Bragg gratings (111,112,113), wherein the set (110) comprises a first fiber-Bragg grating (111), a second fiber-Bragg grating (112) and a third fiber-Bragg grating (113), wherein the first fiber-Bragg grating (111) and the second fiber-Bragg grating (112) are separated by a first optical cavity (131), wherein the first optical cavity (131) has a length L1 selected from the range of 1 - 1000 mm, and Wherein the second fiber-Bragg grating (112) and the third fiber-Bragg grating (113) are separated by a second optical cavity (132), wherein the second optical cavity (132) has a length L2, wherein 0.83L1/L2Sl2, and wherein |L1-L2|/Ll 2 0.01.

Description

Fiber-Bragg grating sensor

FIELD OF THE INVENTION The invention relates to a glass fiber for a strain sensor. The invention further relates to a measurement device. The invention further relates to a method for measuring a parameter. The invention further relates to use of the glass fiber or the measurement device.

BACKGROUND OF THE INVENTION Fiber-Bragg grating sensors are known in the art. For instance, WO9709644A1 describes a fabrication technique for an electro-optically active fiber segment that can be integrated into optoelectronic devices. The fabrication technique offers a dielectric isolation structure surrounding the fiber to allow high field poling, a pair of electrodes used both for poling and for inducing an electro-optic effect, and ends of the fiber unaffected by the fabrication and available for splicing with additional fiber sections.

SUMMARY OF THE INVENTION A Fiber-Bragg grating (FBG) is a periodic modulation of the refractive index of the core of an optical fiber along the propagation direction in a certain segment. This periodic modulation may create a Bragg grating that reflects a narrow set of wavelengths. If strain is applied on a segment that contains such a grating, for example by extending the fiber, the period may be modified and thus the reflected wavelength may shift, wherein the magnitude of the wavelength shift may depend on the applied strain. Hence, the reflected wavelength may give a quantitative measure for the strain on the fiber. Further, transducers may be used to transform a physical quantity to be monitored to strain on the fiber. Thereby, the physical quantity may be monitored by monitoring reflection of wavelengths in the fiber. One of the limitations of FBGs may be that the presence of the periodic modulation may (locally) reduce the maximum tension the fiber can hold, which may limit the applicability of FBGs. A further limitation of FBGs may be that their sensitivity (to strain) may be limited, which may further limit the applicability of FBGs.

The prior art may describe improvements for the transducers that translate a to be measured physical quantity, such as acceleration, into local strain of the fiber.

However, such transducers may be relatively large, complex and/or expensive.

Hence, it is an aspect of the invention to provide an alternative optical fiber for a strain sensor, which preferably further at least partly obviates one or more of above- described drawbacks.

The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

Hence, in a first aspect, the invention may provide an optical fiber for a strain sensor, especially wherein the optical fiber comprises a glass fiber.

The optical fiber may comprise a set of fiber-Bragg gratings, especially wherein the set comprises a first fiber-Bragg grating and a second fiber-Bragg grating.

In embodiments, the first fiber- Bragg grating and the second fiber-Bragg grating may have (essentially) the same period, which may result in interference of the reflected wavelengths in both FBGs, which may create spectral fringes.

The first fiber-Bragg grating, and the second fiber-Bragg grating may be separated by a first optical cavity, especially wherein the first optical cavity consists of an (unaltered) section of the glass fiber with a constant refractive index.

In embodiments, the first optical cavity may have a length L1 selected from the range of 1 — 1000 mm, especially from the range of 2 - 100 cm, i.e., from the range of 20 — 1000 mm.

The optical fiber of the invention may provide the benefit that the strain may be applied in the first optical cavity, which may locally extend the optical fiber, which may modify the cavity length and thus the reflected spectral pattern.

However, as the strain may be selectively applied in the first optical cavity, the maximally imposed strain is not affected (i.e. reduced) by the presence of the fiber-Bragg gratings.

Hence, the optical fiber may enable using a larger maximal tension, which may facilitate a larger measurement range.

For instance, some methods for inscribing an FBG may require removal of a protective coating (e.g., acrylate or polyimide) of the fiber, which may cause small defects in the fiber.

After grating inscription the stripped section can be recoated, but the maximum break strength may be significantly reduced by, for example, a factor of 10. An alternative method may be to write the FBG through the coating.

Thus, omitting the strip-recoat procedure.

However, also here the break strength may be reduced by, for example, a factor of 1.5-2. In particular, the first optical cavity may be a Fabry-Pérot cavity, such that within the bandwidth of the high reflectivity of the FBG, a modulation in the reflectivity spectra may be obtained following the modes from a Fabry-Pérot cavity. In this geometry, straining a section of fiber between the Bragg gratings forming the FP cavity may change the cavity length. As a result, the comb of cavity resonances may shift spectrally while the envelope corresponding to the FBG-spectrum may remain stationary. This spectral shift can be correlated to strain in the fiber in a similar fashion as it is done currently with FBG- based sensors. However, as the part of the fiber that is strained does not contain an FBG, the maximum strain it supports may be higher. In embodiments, the set may further comprise a third fiber-Bragg grating, wherein the second fiber-Bragg grating and the third fiber-Bragg grating are separated by a second optical cavity, wherein the second optical cavity has a length L2, wherein

0.8<L1/L2<1.2, and wherein L1 is not equal to L2, especially wherein |[L1-L2[/L1 > 0.01.

In further embodiments, L1/L2 > 1.01 or L1/L2 < 0.99. In particular, in embodiments, 0.8 <L1/L2 < 0.99 or 1.01 <L1/L2 < 1.2. Hence, in embodiments, L 1 and L2 have a similar but unequal length.

The third fiber-Bragg grating may result in (further) improvements to the sensitivity of the strain measurement. In particular, the optical cavities may have different lengths, and thereby different reflection spectra. When strain is subsequently provided to one of the optical cavities (see above), the corresponding reflection spectrum shifts due to the strain, while the other reflection spectrum remains (essentially) identical. In particular, each optical cavity may have a characteristic spectrum with a modulation that is given by the distance between the FBGs. If the lengths of the optical cavities differ (slightly), then their frequency combs may be similar but not exactly equal, which may create a beating that has a period of one over the full bandwidth. If one follows one feature of this emergent pattern in the spectrum it may shift over the full reflective bandwidth of the FBG for a shift of one Free Spectral Range (FSR) of one of the cavities. This may happen when strain is applied to the construct. Such embodiments may thus provide an increased detection sensitivity and/or robustness.

In specific embodiments, the invention may provide an optical fiber, especially a glass fiber, for a strain sensor, wherein the glass fiber comprises a set of fiber- Bragg gratings, wherein the set comprises a first fiber-Bragg grating, a second fiber-Bragg grating and a third fiber-Bragg grating, wherein the first fiber-Bragg grating and the second fiber-Bragg grating are separated by a first optical cavity, wherein the first optical cavity has a length L1 selected from the range of 1 — 1000 mm, especially from the range of 2 -

100 cm, and wherein the second fiber-Bragg grating and the third fiber-Bragg grating are separated by a second optical cavity, wherein the second optical cavity has a length L2, wherein 0.8<L1/L.2<1.2, wherein L1 is unequal to L2.

In embodiments, L1 and L2 may differ by 0.1 — 30%, especially by 0.5 — 20%, such as by 1 — 10%. In further embodiments, 0.001 <|L1-L2|/L1 < 0.3, especially

0.005 <|L1-L2)/L1 <0.2, such as 0.01 <|L1-L2[/L1 <0.1. In particular, the difference |L1- L2| may influence the sensitivity of the strain measurement, i.e, a smaller difference may result in a higher sensitivity. However, a higher sensitivity may also imply that — at a given strain — more bandwidth may be needed to cover the full dynamic range. Hence, if [L1- L2/L1 is too small, the full dynamic range may not be covered by the bandwidth of the FBGs, whereas if [L1-L2|/L1 is too big, the sensitivity enhancement may be insufficient to warrant the extra work. Hence, in further embodiments, [L1-L2//L1 > 0.001, such as >

0.005, especially > 0.01, such as > 0.05, especially > 0.1, such as > 0.15, especially > 0.2.

In further embodiments, [L1-L2]/.1 < 0.35, such as < 0.3, especially < 0.25, such as <0.2, especially < 0.15, such as < 0.10, especially < 0.08. The improved properties of the optical fiber, especially the glass fiber, described herein may further facilitate simplification of the transducer, 1.e., the optical fiber may further be advantageous as it can be effectively used with a less complex, smaller and/or cheaper transducer.

Hence, the invention may provide an optical fiber for a strain sensor. The term “optical fiber” may refer to a flexible transparent fiber, and may comprise a glass fiber or a polymer fiber (or: “plastic optical fiber”). In embodiments, the optical fiber may especially comprise a glass fiber. Glass fibers may have a lower transmission loss, which may allow for a longer transmission length, which may enable monitoring a parameter (see below) from a further distance. In addition, glass fibers may have less creeping, especially no creeping, i.e., glass fibers may deform less than polymer fibers.

In embodiments, the glass fiber may comprise a material selected from the group comprising silica, fluorozirconate, fluoroaluminate, chalcogenide glass and crystalline materials, such as sapphire. In further embodiments, the glass fiber may comprise silica. In specific embodiments, the glass fiber may comprise amorphous silicon dioxide. As will be known to the person skilled in the art, the glass fiber may be doped with one or more dopants, such as with a dopant selected from the group comprising germanium dioxide (GeO), aluminum oxide (Al:03), fluorine and boron trioxide (B20s)

The term “strain sensor” may herein especially refer to any device or system configured to sense a strain. In particular, the strain sensor may be a measurement device (see below), such as the measurement device of the invention.

The optical fiber, especially the glass fiber, may comprise a set of fiber- 5 Bragg gratings. The term “fiber-Bragg grating” may herein refer to a periodic modulation of the refractive index of a core of the optical fiber. In particular, the fiber-Bragg grating may provide a wavelength-specific dielectric mirror. As will be known to the person skilled in the art, a fiber-Bragg grating may be provided using, for example, an ultraviolet laser to selectively and locally alter the refractive index of the fiber core.

In embodiments, each fiber-Bragg grating of the set of fiber-Bragg gratings, especially the first fiber-Bragg grating, or especially the second fiber-Bragg grating, or especially the third fiber-Bragg grating (see below), may have (local) variations in the refractive index selected from the range of 10° — 107%, such as from the range of 105 — 10°

3. In particular, Each fiber-Bragg grating of the set of fiber-Bragg gratings may alternate between a first refractive index and a second refractive index, wherein the first refractive index and the second refractive index differ by an amount selected from the range of 10% — 1072, such as from the range of 10% — 107.

In further embodiments, each fiber-Bragg grating of the set of fiber-Bragg gratings, especially the first fiber-Bragg grating, or especially the second fiber-Bragg grating, or especially the third fiber-Bragg grating (see below), may have a reflection spectrum having a spectral width selected from the range of 0.05 — 200 nm, such as from the range of 0.1 — 100 nm, especially from the range of 0.2 — 50 nm. The term “reflection spectrum” may herein especially refer to the range of wavelengths that is reflected by the fiber-Bragg grating. In particular, the spectral width may be defined as the Full Width at Half Maximum (FWHM).

In further embodiments, each fiber-Bragg grating of the set of fiber-Bragg gratings, especially the first fiber-Bragg grating, or especially the second fiber-Bragg grating, or especially the third fiber-Bragg grating (see below), may have a reflection coefficient (or “reflectivity”) selected from the range of 1 — 100%, such as from the range of 1 — 99%, especially from the range of 1-90%, such as from the range of 1-60%. In particular, the reflection coefficient may relate to a wavelength selected from the range of 400 - 1700 nm, especially a wavelength of radiation provided by a radiation source (see below). The term “reflection coefficient” may herein especially refer to the ratio of the intensity of the (reflected) radiation reflected by an element, such as by an FBG or by an optical cavity, to the (intensity of the) (incident) radiation (for a specific wavelength), i.e., the term “reflection coefficient” may herein especially refer to a ratio of the intensity of (reflected) radiation reflected by an element, such as an FBG or an optical cavity, to the intensity of the (incident) radiation provided to the element.

In embodiments, the reflection spectrum of a fiber-Bragg grating may define a peak, wherein the reflection coefficient of the fiber-Bragg grating for the wavelength corresponding to the peak is selected from the range of 1 — 100%, such as from the range of 1 — 99%, especially from the range of 1-90%, such as from the range of 1- 60%. In further embodiments, the reflection spectrum of a fiber-Bragg grating may define a peak having a spectral width, wherein the reflection coefficient of the fiber-Bragg grating for the wavelengths corresponding to the spectral width 1s selected from the range of 1 — 100%, such as from the range of 1 — 99%, especially from the range of 1-90%, such as from the range of 1-60%.

In embodiments, the set of fiber-Bragg gratings (also: “the set”) may comprise a first fiber-Bragg grating and a second fiber-Bragg grating. In further embodiments, the set may further comprise a third fiber-Bragg grating.

In embodiments, the first fiber-Bragg grating and the second fiber-Bragg grating may be separated by a first optical cavity. The first optical cavity may especially consist of a section of the optical fiber, especially of the glass fiber, with an (essentially) homogeneous refractive index (or: “index of refraction”). In particular, in embodiments, the first optical cavity may have a length L 1, wherein the refractive index of the first optical cavity may vary < 107, such as < 10% especially < 10%, such as < 10% especially < 107, such as < 10%, especially < 107", including 0, along at least 80% of the length L1. The length LI may especially be defined as the distance between the first fiber-Bragg grating and the second fiber-Bragg grating along a longitudinal dimension of the optical fiber. In further embodiments, the refractive index of the first optical cavity may vary < 1%, especially < 0.7%, such as < 0.5%, especially < 0.1%, including 0%, along at least 80% of the length L1, such as along at least 90% of the length L1, especially along at least 95% of the length L1, such as along at least 98% of the length L 1, including along 100% of the length L1.

In embodiments, the first optical cavity may have a reflection coefficient selected from the range of 1 — 100%, such as from the range of 1 — 99%, especially from the range of 1-90%, such as from the range of 1-60%. In general, in embodiments, the first optical cavity may consist of an (unaltered) section of the optical fiber with an (essentially) constant refractive index. Further, in embodiments, the first optical cavity may have (on average) a lower refractive index than the fiber-Bragg gratings. In embodiments, the first optical cavity may have a length L1 selected from the range of 1 — 2000 mm, , especially from the range of 1 — 1000 mm, such as from the range of 5 — 1000 mm, such as from the range of 1 — 100 cm, especially from the range of 2 — 100 cm, such as from the range of 3 — 80 cm. In embodiments, the length L1 may be at least 0.1 cm, such as at least 0.2 cm, especially at least 0.5 cm, such as at least 1 cm, especially at least 2 cm, such as at least 3 cm, especially at least 5 cm, such as at least 10 cm. In further embodiments, the length L1 may be at most 500 cm, such as at most 200 cm, especially at most 100 cm, such as at most 80 cm, especially at most 50 cm. In further embodiments, the second fiber-Bragg grating and the third fiber- Bragg grating may be separated by a second optical cavity. Similar to the first optical cavity, the second optical cavity may consist of a section of the optical fiber, especially of the glass fiber, with an (essentially) homogeneous refractive index. In particular, in embodiments, the second optical cavity may have a (second) length L2, wherein the refractive index of the second optical cavity may vary < 1% along at least 80% of the (second) length L2. The (second) length L2 may especially be defined as the distance between the second fiber-Bragg grating and the third fiber-Bragg grating along a longitudinal dimension of the optical fiber. In further embodiments, the refractive index of the second optical cavity may vary < 1%, especially < 0.7%, such as < 0.5%, especially <

0.1%, including 0%, along at least 80% of the length L1, such as along at least 90% of the (second) length L2, especially along at least 95% of the length L2, such as along at least 98% of the length L2, including along 100% of the length L2. In embodiments, the first optical cavity and the second optical cavity may have essentially the same refractive index, i.e., the average refractive index of the first optical cavity and the average refractive index of the second optical cavity may differ < 1%, such as < 0.5%, especially < 0.1%, including 0%.

In embodiments, at least part of the first fiber-Bragg grating may have the same refractive index as the first optical cavity. In particular, the first fiber-Bragg grating, especially a periodic structure of the first fiber-Bragg grating, may have a periodic modulation of the refractive index, wherein the refractive index alternatingly is (i) (essentially) identical to the refractive index of the first optical cavity, and (ii) deviating from the refractive index of the first optical cavity. Similarly, in embodiments, at least part of the second fiber-Bragg grating may have the same refractive index as the first optical cavity, and/or at least part of the third fiber-Bragg grating may have the same refractive index as the first optical cavity.

In embodiments, the second optical cavity has a (second) length L2, especially selected from the range of 1 — 200, especially from the range of 2 — 100 cm, such as from the range of 3 — 80 cm. In embodiments, the length L2 may be at least 0.5 cm, such as at least | cm, especially at least 2 cm, such as at least 3 cm, especially at least 5 cm, such as at least 10 cm. In further embodiments, the length L2 may be at most S00 cm, such as at most 200 cm, especially at most 100 cm, such as at most 80 cm, especially at most 50 cm.

In embodiments, the first optical cavity and the second optical cavity may have similar lengths. Hence, in embodiments, 0.7<L1/L.2<1.3, such as 0.8<L1/L2<1.2, especially 0.1<L1/L2<1.1. In particular, in embodiments, L1/L2 < 1.3, such as < 1.2, especially < 1.1, such as < 1.05. In further embodiments L1/L2 > 0.7, such as > 0.8, especially > 0.9, such as > 0.95.

In particular, the first optical cavity and the second optical cavity may have similar though different lengths, especially to provide a distinct reflection pattern by the combination of the reflection patterns of each cavity and corresponding FBGs. Hence, in embodiments, L1 may be unequal to L2. In particular, in embodiments, the length difference AL (or: “|L1-L2|”) between L1 and L2 may be selected from the range of 0.001-

0.3*L1, such as from the range of 0.01-0.15*L1. In further embodiments, the length difference AL > 0.001*L 1, such as > 0.005*L 1, especially > 0.01*L1, such as > 0.05*L1, especially > 0.08*L1, such as > 0.1*L1. In further embodiments, the length difference AL < 0.3*L1, such as < 0.25*%L1, especially < 0.2*L1, such as < 0.15*L1, especially <

0.12*L1, such as <0.1*L1, such as < 0.08*L 1.

In embodiments, one or more of the first fiber-Bragg grating, the second fiber-Bragg grating and the third fiber Bragg grating may comprise a periodic structure selected from the group comprising a uniform fiber-Bragg grating, a chirped fiber-Bragg grating, and an apodised fiber-Bragg grating.

Especially, each of the first fiber-Bragg grating, the second fiber-Bragg grating and the third fiber Bragg grating may comprise an (individually selected) periodic structure selected from the group comprising a uniform fiber-Bragg grating, a chirped fiber-Bragg grating, and an apodised fiber-Bragg grating.

The periodic structure may especially be selected from the group comprising a chirped fiber-Bragg grating and an apodised fiber-Bragg grating.

The term “periodic structure” may herein refer to the pattern of modulation of refractive index in the fiber-Bragg grating.

In particular, a periodic structure may be defined by a pattern type, such as a uniform fiber-Bragg grating, a chirped fiber-Bragg grating, and an apodised fiber-Bragg grating, by a period, and by a repetition number.

In embodiments, each fiber-Bragg grating of the set of fiber-Bragg gratings may have a grating length selected from the range of 1 — 1000 mm, such as from the range of 2 — 600 mm, especially from the range of 5 — 200 mm, such as from the range of 8 — 50 IS mm.

The pattern type may refer to the overall shape of the pattern.

For instance, a uniform pattern may have be defined by an arrangement of identically spaced planes arranged perpendicular to a longitudinal dimension (or “longitudinal axis”) of the optical fiber, wherein the refractive index alternates between two values, such as unaltered and altered, between consecutive sets of adjacent planes.

A chirped pattern may, for example, be defined by an additional feature in the refractive index profile of the grating, such as a (linear) variation in the grating period, i.e., the periodic structure may comprise a chirp.

A chirp may broaden the range of reflected wavelengths, especially broadening the main peak, which may facilitate detecting a modulation under the reflection envelope.

Thereby, a chirped pattern may enable the use of a lower resolution (cheaper) spectrometer, may facilitate following a single fringe in the pattern over a larger strain regime i.e., the optical fiber may have a larger dynamic range, and it may enable the use of a larger beat length, i.e, the use of a wider spectral interference pattern, which may increase the sensitivity enhancement provided by interference.

It will be clear to the person skilled in the art that also combinations of these pattern types and further features may be used.

In embodiments, the first fiber-Bragg grating, the second fiber-Bragg grating and the third fiber-Bragg grating may each have the same pattern type selected from the group comprising a uniform fiber-Bragg grating, a chirped fiber-Bragg grating, and an apodised fiber-Bragg grating, In further embodiments, the first fiber-Bragg grating, the second fiber- Bragg grating and the third fiber-Bragg grating may each have a grating period Al.

In embodiments, each fiber-Bragg grating of the set of fiber-Bragg gratings may comprise a chirped fiber-Bragg grating. In particular, thereby the spectral reflection width may be enlarged compared to, for example, uniform FBGs.

The term “grating period” (or: “period™) may herein refer to the length of the shortest repetitive pattern in the fiber-Bragg grating, such as a single section with a first refractive index and a second section with a second refractive index, especially a pattern consisting of an unaltered section and an altered section. In particular, the period may define the central reflection peak in the reflection spectrum of the FBG. In embodiments, the (grating) period may be selected from the range of 50 nm — 2 um, such as from the range of 100 nm — 1.5 pum, especially from the range of 125 nm — 1 un, such as from the range of 250 nm — 750 nm. In further embodiments, the period may be at least 50 nm, such as at least 75 nm, especially at least 100 nm, such as at least 125 nm, especially at least 150 nm, such as at least 200 nm. In further embodiments, the period may be at most 2 pum, such as at most 1.5 pm, especially at most 1 um, such as at most 900 um, especially at most 750 nm, such as at most 500 nm.

Especially, the periodic modulation in the periodic structure may define a Bragg wavelength Ag, wherein the Bragg wavelength Az may be a central wavelength around which a narrow set of wavelengths may be reflected. As will be known to the person skilled in the art, the Bragg wavelength may generally be defined according to Az = 2*neg™ A, wherein Nef is an effective refractive index (of the FBG), and wherein A is the grating period of the FBG.

The repetition number may refer to the amount of the shortest repetitive pattern that the FBG comprises. For example, if an FBG, especially a periodic structure, comprises a pattern ABABABAB, wherein “A” refers to a section with a first refractive index, and wherein “B” refers to a section with a section with a second refractive index, then AB may be the shortest repetitive pattern, and the repetition number — for this example — would be 4. As will be clear to the person skilled in the art, the repetition number may be substantially larger. For instance, if an FBG, especially a periodic structure, has a grating length of about 100 mm, and a grating period of about 1 um, the repetition number may be about 10%. Hence, in embodiments the first fiber-Bragg grating may comprise a periodic structure selected from the group comprising a uniform fiber-Bragg grating, a chirped fiber-Bragg grating, and an apodised fiber-Bragg grating. In further embodiments, the second fiber-Bragg grating may comprise a periodic structure selected from the group comprising a uniform fiber-Bragg grating, a chirped fiber-Bragg grating, and an apodised fiber-Bragg grating. In further embodiments, the third fiber-Bragg grating may comprise a periodic structure selected from the group comprising a uniform fiber-Bragg grating, a chirped fiber-Bragg grating, and an apodised fiber-Bragg grating.

In further embodiments, each of the first fiber-Bragg grating, the second fiber-Bragg grating and the third fiber Bragg grating may comprise the same periodic structure, i.e., they may, for instance, all comprise a chirped fiber-Bragg grating, or they may all comprise an apodised fiber-Bragg grating.

In specific embodiments, one or more of the first fiber-Bragg grating, the second fiber-Bragg grating and the third fiber-Bragg grating may comprise a chirped fiber- Bragg grating. In further specific embodiments, one or more of the first fiber-Bragg grating, the second fiber-Bragg grating and the third fiber-Bragg grating may comprise an apodised fiber-Bragg grating. In particular, for apodised FBG, the refractive index modulation depth may vary over the length of the grating, especially such that the grading of the refractive index approaches zero at the end of the FBG. Apodised FBGs may especially result in a reduction of secondary lobes in the reflection spectrum, which may result in a more well-defined range of reflected wavelengths. In particular, this may be beneficial when multiple sets of fiber-Bragg gratings are included in one optical fiber, as the reduction of the secondary lobes may also reduce cross-talk between different sets of FBGs.

In embodiments, the first fiber-Bragg grating may have a first periodic structure, and the second fiber-Bragg grating may have a second periodic structure. Similarly, in embodiments, the third fiber-Bragg grating may have a third periodic structure.

In further embodiments, the first periodic structure and the second periodic structure may have the same pattern type, especially arranged identically with respect to a longitudinal dimension of the optical fiber (for instance, with respect to a chirped FBG).

In further embodiments, the first periodic structure and the second periodic structure may have (essentially) the same period.

In particular, the first fiber-Bragg grating, especially the first periodic structure, may have a first set first grating period A11, and the second fiber-Bragg grating, especially the second periodic structure, may have a first set second grating period A12, especially wherein a period difference between the first set first grating period All and the first set second grating period A12 differs with less than 5%, ie, |A11-A12| <0.05*¥All, especially < 0.03*A11, such as <0.01*A11, including 0. In further embodiments, the first periodic structure and the second periodic structure may have (essentially) the same repetition number.

In particular, the first fiber- Bragg grating, especially the first periodic structure, may have a first repetition number nl, and the second fiber-Bragg grating, especially the second periodic structure, may have a second repetition number n2, especially wherein [n1-n2| <0.05*n1, especially < 0.03*nl, such as <0.01*n1, including 0. Similarly, in further embodiments, the first periodic structure and the second periodic structure may have the same pattern type, especially arranged identically with respect to a longitudinal dimension of the optical fiber (for instance, with respect to a chirped FBG). In further embodiments, the first periodic structure and the third periodic structure may have (essentially) the same period.

In particular, the third fiber-Bragg grating, especially the third periodic structure, may have a first set third grating period A13, especially wherein a period difference the first set first grating period A11 and the first set third grating period A13 differ with less than 5%, i.e, |[A11-Al3] < 0.05*All, especially <0.03*A11, such as < 0.01*A11, including 0. In embodiments, the fiber-Bragg gratings of the set may (each) have a grating period Al, indicated with All, A12, (and A13), respectively.

In particular, A1=A11=A12(=A13), ie.

All and A12 (and A13) may have (essentially) the same value.

In embodiments, the glass fiber may comprises a second set of fiber-Bragg gratings, wherein the second set comprises three fiber-Bragg gratings separated by a second set first optical cavity and a second set second optical cavity, wherein the second set first optical cavity has a length L1,, selected from the range of 2 - 100 cm, and wherein the fiber-Bragg gratings of the second set have a second grating period A2, wherein |[A1- A2| is selected from the range of 0.1 — 1000 nm.

Hence, in embodiments, the glass fiber may comprises a second set of fiber- Bragg gratings wherein the second set comprises a (second set) first fiber-Bragg grating, a (second set) second fiber-Bragg grating, and optionally a (second set) third fiber-Bragg grating. In further embodiments, the second set first fiber-Bragg grating and the second set second fiber-Bragg grating may be separated by a second set first optical cavity, wherein the second set first optical cavity has a length L1, selected from the range of 1 — 200, especially from the range of 2 — 100 cm, such as from the range of 3 — 80 cm. In embodiments, the length L1: may be at least 0.5 cm, such as at least 1 cm, especially at least 2 cm, such as at least 3 cm, especially at least S cm, such as at least 10 cm. In further embodiments, the length L12 may be at most 500 cm, such as at most 200 cm, especially at most 100 cm, such as at most 80 cm, especially at most 50 cm. In particular, the (second set) first fiber-Bragg grating, the (second set) second fiber-Bragg grating (and the (second set) third fiber-Bragg grating) may have a second grating period A2, especially wherein |A1-A2| = 5* AA, wherein AA is the bandwidth of the FBGs, especially the bandwidth of the first set of fiber-Bragg gratings.

In further embodiments, |[A1-A2| > 3* AX, wherein AA is the bandwidth of the FBGs, especially of the first set of fiber-Bragg gratings. Especially, |A1-A2| > 4% AR, such as |A1-A2| > 5* Aj, especially |[A1-A2| > 6* AA, such as |[A1-A2| > 7* A. In further embodiments, |A1-A2| < 20% A}, especially [A1-A2]| < 15% AX, such as [A1-A2] < 12% Ai, especially |[A1-A2| < 10* AA, such as [A1-A2]| < 7* Aj, especially |A1-A2] <6* A.

In particular, |A1-A2| may be selected based on the bandwidth of the fiber- Bragg gratings, which may be determined by the length of the FBGs, the chirp rate (if any) and the refractive modulation depth. In embodiments, the first set of fiber-Bragg gratings may have a (first) bandwidth AA, wherein |[A1-A2| > 3*A2, such > 4**Aà, especially > 5*AA „such as > 6*A), especially > 7*AÀ. In further embodiments, |A1-A2| < 20*AA, such as < 15*AA, especially < IOA.

In further embodiments, the first set of fiber-Bragg gratings may have a first bandwidth B1 selected from the range of 0.05 — 200 nm, such as from the range of 0.1 — 100 nm, especially from the range of 0.2 — 80 nm, such as from the range of 0.5 — 50 nm, especially from the range of 1 — 20 nm.

In further embodiments, the second set may comprise a (second set) third fiber-Bragg grating. In such embodiments, the (second set) second fiber-Bragg grating and the (second set) third fiber-Bragg grating may be separated by a (second set) second optical cavity, wherein the (second set) second optical cavity has a length L2; wherein

0.8<L1,/1.2;<1.2, wherein L 1; is unequal to L2>. In particular, the glass fiber may comprise a plurality of sets of fiber-Bragg gratings, wherein each set comprises a first fiber-Bragg grating, a second fiber-Bragg grating, and optionally a third fiber-Bragg grating, wherein the fiber-Bragg gratings (of a respective set) have the same grating period A, and wherein the fiber-Bragg gratings of different sets have different grating periods A. In particular, the grating periods of different sets may differ at least 0.1 nm, such as at least 0.2 nm, especially at least 0.5 nm, such as at least 1 nm, especially at least 2 nm, such as at least 5 nm, especially at least 10 nm. In further embodiments, the grating periods of different sets may differ at most 1000 nm, such as at most 500 nm, especially at most 300 nm, such as at most 200 nm, especially at most 150 nm, such as at most 100 nm, especially at most 50 nm, such as at most 30 nm. In further embodiments, the glass fiber may comprise a plurality of sets of fiber-Bragg gratings, wherein each set has a respective first cavity length, wherein the largest first cavity length and the smallest first cavity length differ by at most 10% (especially relative to the largest first cavity length), such as at most 5%, especially at most 3%, such as at most 1%, including 0%. In further embodiments, the first periodic structure and the third periodic structure may have (essentially) the same repetition number. In particular, the third fiber- Bragg grating, especially the third periodic structure, may have a third repetition number n3, especially wherein |nl-n3| < 0.05*nl, especially < 0.03*nl, such as < 0.01%*nl, including 0. Hence, in embodiments, the first fiber-Bragg grating and the second fiber- Bragg grating (and the third fiber-Bragg grating) may have (essentially) the same periodic structure. In a second aspect, the invention may provide a measurement device for measuring a parameter. The measurement device may comprise an optical fiber, especially a glass fiber, and a measurement element. The optical fiber may comprise a set of fiber- Bragg gratings, especially wherein the set comprises a first fiber-Bragg grating, and a second fiber-Bragg grating. In embodiments, the first fiber-Bragg grating and the second fiber-Bragg grating may be separated by a first optical cavity, especially wherein the first optical cavity has a length L1 selected from the range of 1 — 200 cm. In further embodiments, the measurement element may be functionally coupled to the glass fiber at a first location and at a second location, especially wherein the first location and the second location are arranged in the first optical cavity, i.e., the first location and the second location may be arranged (in the optical fiber) between the first fiber-Bragg grating and the second fiber-Bragg grating. The measurement element may especially (be configured to) impose (or: “direct”) a strain (exclusively) on the optical fiber on the basis of (a value of) the parameter, especially (exclusively) on a section of the optical fiber between the first location and the second location on the basis of (the value of) the parameter. Hence, the invention may provide a measurement device for measuring a parameter, especially wherein the measurement device comprises the optical fiber according to the invention and a measurement element. The measurement element may especially be functionally coupled to the optical fiber at a first location and a second location arranged in the first optical cavity. In particular, the measurement element may be configured to impose a strain on the optical fiber on the basis of (a value of) the parameter. Thereby, the parameter influences the strain imposed on an (unaltered) section of the optical fiber (arranged between the first location and the second location), which strain affects the distribution of reflected wavelengths by the fiber-Bragg gratings, which may be accurately measured, wherein the (unaltered) section may have a higher resistance to strain than the bordering fiber-Bragg gratings.

In specific embodiments, the invention may provide a measurement device for measuring a parameter, wherein the measurement device comprises a glass fiber and a measurement element, wherein the glass fiber comprises a set of fiber-Bragg gratings, wherein the set comprises a first fiber-Bragg grating, and a second fiber-Bragg grating, wherein the first fiber-Bragg grating and the second fiber-Bragg grating are separated by a first optical cavity, wherein the first optical cavity has a length L1 selected from the range of 1 - 1000 mm, especially from the range of 2 - 100 cm, wherein the measurement element is functionally coupled to the glass fiber at a first location and at a second location, wherein the measurement element is configured to impose a strain on the glass fiber between the first location and the second location on the basis of the parameter, wherein the first location and the second location are arranged in the first optical cavity.

In embodiments, the first location and the second location may be separated by a distance dc, especially wherein dc is selected from the range of 0.05*L1 — 0.95*L 1, such as from the range of 0.1*L1-0.9*L1, especially from the range of 0.2*L1-0.8*L1. In embodiments, dc may be at least 0.05*L1, such as at least 0.1*L1, especially at least

0.2*L1, such as at least 0.3*L1, especially at least 0.4*L1, such as at least 0.5*L1. In further embodiments, dc may be at most 0.95*L1, such as at most 0.9*L1, especially at most 0.8*L1, such as at most 0.7*L1, especially at most 0.6*L1.

In embodiments, the first location and the second location may each be removed from the (respectively) nearest fiber-Bragg grating by a distance independently selected from the range of at least 0.05*L1, such as from the range of at least 0.1*L1, especially from the range of at least 0.15*L1, such as from the range of at least 0.2*L1.

The term “measurement device” may herein especially refer to any device that may be used to, especially be configured to, determine (the value of) a (physical) parameter.

The parameter may essentially be any parameter that can be converted to a strain. For instance, in embodiments, the parameter may be selected from the group comprising an external strain, a tilt, a yaw, a displacement, a consolidation, a deformation, a stress, a chemical change, a temperature, a pressure, especially an acoustic pressure, and an acceleration. It will be clear to the person skilled in the art that also other (physical) parameters may be converted to a strain, and that the measurement device of the invention may also be suitable to measure such parameters.

In embodiments, the measurement device, especially the measurement element, may comprise a transducer, especially wherein the transducer is configured to convert (a value of) the parameter to (a value of) the strain imposed on the optical fiber. Hence, the measurement device may comprise a transducer, wherein the transducer is configured to convert a parameter selected from the group comprising an external strain, a tilt, a yaw, a displacement, a consolidation, a deformation, a stress, a chemical change, a temperature, a pressure, especially an acoustic pressure, and an acceleration, to a strain, especially a parameter selected from the group comprising a tilt, a yaw, a displacement, a consolidation, a deformation, a stress, a chemical change, a temperature, a pressure, especially an acoustic pressure, and an acceleration, especially wherein the strain is imposed on the optical fiber.

For instance, the transducer may comprise a mass on a spring configured to translate an acceleration to displacement. Alternatively, the transducer may comprise a pressure chamber with a piston where the piston position may depend on the pressure; hence, a shift in pressure may result in a shift in the piston position, which may be used to strain the fiber.

In embodiments, the measurement element may be configured to impose (or: “direct”) the strain on the optical fiber between the first location and the second location on the basis of the parameter, especially exclusively on the section of the optical fiber between the first location and the second location on the basis of the parameter.

In embodiments, the set may comprise a third fiber-Bragg grating, wherein the second fiber-Bragg grating and the third fiber-Bragg grating are separated by a second optical cavity, especially wherein the second optical cavity has a length L2, and especially wherein 0.8<L1/L2<1.2. In embodiments, L1 may be unequal to L2, especially, wherein IL1-L2|/L1 > 0.01.

In further embodiments, the measurement device may further comprise a radiation source, a radiation sensor and/or a control system. In particular, the measurement device may have an operational mode comprising a radiation stage, a sensing stage, and an analysis stage.

In embodiments, in the radiation stage the radiation source may (be configured to) provide radiation to the optical fiber. It will be clear to the person skilled in the art that the range of suitable wavelengths may depend on the distribution of wavelengths modulated by the fiber-Bragg gratings and the first optical cavity (and the second optical cavity). In particular, the wavelength(s) may be selected such that the radiation may be modulated during use of the glass fiber. For example, in embodiments, the radiation may comprise a wavelength selected from the range of 200 - 2000 nm, such as from the range of 300 — 1800 nm, especially from the range of 400 — 1700 nm. In embodiments, the radiation may comprise a wavelength of at least 200 nm, such as at least 300 nm, especially at least 400 nm, such as at least 500 nm, especially at least 600 nm, such as at least 700 nm. In further embodiments, the radiation may comprise a wavelength of at most 2000 nm, such as at most 1900 nm, especially at most 1800 nm, such as at most 1700 nm, especially at most 1600 nm, such as at most 1500 nm, especially at most 1300 nm, such as at most 1000 nm.

In embodiments, the radiation may be selected in view of the Bragg wavelength(s) As of the set of fiber-Bragg gratings. Hence, the Bragg wavelength of one or more fiber-Bragg gratings, especially of all the fiber-Bragg gratings, in the set of wavelengths may be selected from the range of 200 - 2000 nm, such as from the range of 300 — 1800 nm, especially from the range of 400 — 1700 nm.

In further embodiments, each fiber-Bragg grating in the set of fiber-Bragg gratings may have (essentially) the same Bragg wavelength As.

In further embodiments, the radiation source may comprise a broadband radiation source, i.e., a radiation source configured to provide broadband radiation.

Broadband radiation may be more informative with respect to shifts in the distribution of reflected wavelengths as broadband radiation may comprise a continuous spectrum of wavelengths, rather than one or more specific wavelengths. In such embodiments, in the radiation stage, the radiation source may (be configured to) provide broadband radiation comprising wavelengths in the range of 200 - 2000 nm, such as from the range of 300 — 1800 nm, especially from the range of 400 — 1700 nm, i.e., the radiation may comprise an (essentially) continuous spectrum in the range of 200 — 2000 nm. In further embodiments, the broadband radiation may comprise wavelengths in range of Amin — Amax. In embodiments, Amin may be selected from the range of 200 — 800 nm, such as from the range of 300 — 700 nm, especially from the range of 350 — 500 nm. In further embodiments, Amax may be selected from the range of 1400 — 2000 nm, such as from the range of 1500 — 1900 nm, especially from the range of 1600 — 1800 nm.

In embodiments, in the sensing stage the radiation sensor may (be configured to) measure (reflected) radiation reflected, especially refracted, by the set of fiber-Bragg gratings and especially provide a related (reflection) signal to the control system. The radiation sensor may especially (be configured to) sense (reflected) radiation in the same wavelength range as emitted by the radiation source. Hence, in embodiments, the radiation sensor may (be configured to) sense radiation comprising a wavelength selected from the range of 200 - 2000 nm, such as from the range of 300 — 1800 nm, especially from the range of 400 — 1700 nm. In embodiments, the radiation sensor may (be configured to) sense radiation comprising a wavelength of at least 200 nm, such as at least 300 nm, especially at least 400 nm, such as at least 500 nm, especially at least 600 nm, such as at least 700 nm. In further embodiments, the radiation sensor may (be configured to) sense radiation comprising a wavelength of at most 2000 nm, such as at most 1900 nm, especially at most 1800 nm, such as at most 1700 nm, especially at most 1600 nm, such as at most 1500 nm, especially at most 1300 nm, such as at most 1000 nm.

In particular, in embodiments, in the sensing stage the radiation sensor may (be configured to) measure radiation downstream from the set of fiber-Bragg gratings, and especially provide a related (reflection) signal to the control system. Hence, the radiation sensor may sense radiation downstream from, especially reflected by, or especially transmitted by, the set of fiber-Bragg gratings, but it may also (or “instead”) measure radiation that has passed through the set of fiber-Bragg gratings.

In further embodiments, the related (reflection) signal may comprise time- dependent data corresponding to a measurement duration, i.e., the sensing stage may last for a measurement duration, and the radiation sensor may provide a related (reflection) signal comprising measurements over time.

In particular, the related (reflection) signal may comprise (essentially) continuous measurement data.

In embodiments, the measurement duration may be at most 5 seconds, such as at most 2 seconds, especially at most 1 second, such as at most 0.5 seconds.

In further embodiments, the measurement duration may be at least 0.01 seconds, such as at least 0.1 seconds, especially at least 0.5 seconds.

The related (reflection) signal may comprise raw and/or processed data.

Hence, in embodiments, the radiation sensor may provide raw measurement data to the control system.

In further embodiments, the radiation sensor may be configured to (pre- )process the measurement data and provide (pre-)processed measurement data to the control system.

In embodiments, in the analysis stage the control system may (be configured to) determine (a value of) the parameter based on the related (reflection) signal.

In particular, the control system may (be configured to) determine the parameter based on the related signal, wherein the related signal comprises time-dependent data corresponding to a measurement duration.

In further embodiments, the operational mode may further comprise a strain application stage.

In the strain application stage, the measurement element may (be configured to) impose (or: “direct”) the strain on the optical fiber between the first location and the second location on the basis of the parameter, especially exclusively on the section of the optical fiber between the first location and the second location on the basis of the parameter.

In a further aspect the invention may provide a method for measuring a parameter using an optical fiber, especially a glass fiber.

The optical fiber may especially be the optical fiber of the invention.

In particular, the optical fiber may comprise a set of fiber-Bragg gratings, especially wherein the set comprises a first fiber-Bragg grating and a second fiber-Bragg grating.

The first fiber-Bragg grating and the second fiber-Bragg grating may be separated by a first optical cavity, especially wherein the first optical cavity consists of an (unaltered) section of the optical fiber with an (essentially) constant refractive index. In embodiments, the first optical cavity may have a length L1 selected from the range of 1 — 1000 mm, especially from the range of 2 - 100 cm. In embodiments, a measurement element may be configured to impose a strain on the glass fiber between a first location and a second location on the basis of the parameter, wherein the first optical cavity comprises the first location and the second location, In embodiments, the method may comprise one or more of a radiation stage, a sensing stage, and an analysis stage.

In embodiments, the radiation stage may comprise providing radiation to the optical fiber, wherein the radiation comprises a wavelength selected from the range of 200 - 2000 nm, such as from the range of 300 — 1800 nm, especially from the range of 400 — 1700 nm. In embodiments, the radiation stage may comprise providing radiation comprising a wavelength of at least 200 nm, such as at least 300 nm, especially at least 400 nm, such as at least 500 nm, especially at least 600 nm, such as at least 700 nm. In further embodiments, the radiation stage may comprise providing radiation comprising a wavelength of at most 2000 nm, such as at most 1900 nm, especially at most 1800 nm, such as at most 1700 nm, especially at most 1600 nm, such as at most 1500 nm, especially at most 1300 nm, such as at most 1000 nm. In embodiments, the sensing stage may comprise measuring radiation downstream from, especially reflected by, or especially transmitted by, the set of fiber- Bragg gratings and providing a related (reflection) signal. In further embodiments, the sensing stage may comprise measuring radiation having a wavelength selected from the range of 200 - 2000 nm, such as from the range of 300 — 1800 nm, especially from the range of 400 — 1700 nm. In embodiments, the sensing stage may comprise sensing radiation comprising a wavelength of at least 200 nm, such as at least 300 nm, especially at least 400 nm, such as at least 500 nm, especially at least 600 nm, such as at least 700 nm. In further embodiments, the sensing stage may comprise sensing radiation comprising a wavelength of at most 2000 nm, such as at most 1900 nm, especially at most 1800 nm, such as at most 1700 nm, especially at most 1600 nm, such as at most 1500 nm, especially at most 1300 nm, such as at most 1000 nm. In particular, in embodiments, the sensing stage may comprise measuring radiation downstream from the set of fiber-Bragg gratings, and especially providing a related signal to the control system. Hence, the sensing stage may comprise sensing

(reflected) radiation reflected by the set of fiber-Bragg gratings, but it may also (or “instead”) comprise measuring (transmitted) radiation that has transmitted by the set of fiber-Bragg gratings.

Hence, in embodiments, the sensing stage may comprise measuring radiation transmitted by the fiber-Bragg grating, 1.e., the sensing stage may comprise measuring non-reflected radiation.

In embodiments, the analysis stage may comprise determining (a value of) the parameter based on the related (reflection) signal.

It will be clear to the person skilled in the art how the various stages described herein can be beneficially temporally arranged. In particular, the strain application stage, the radiation stage, and the sensing stage may at least partially overlap in time, especially (essentially) fully overlap in time.

In specific embodiments, the invention may provide a method for measuring a parameter using a glass fiber, wherein the glass fiber comprises a set of fiber- Bragg gratings, wherein the set comprises a first fiber-Bragg grating and a second fiber- Bragg grating, wherein the first fiber-Bragg grating and the second fiber-Bragg grating are separated by a first optical cavity, wherein the first optical cavity has a length L1 selected from the range of 1 — 1000 mm, especially from the range of 2 - 100 cm, wherein the method comprises (i) imposing a strain on the glass fiber between a first location and a second location on the basis of the parameter, wherein the first optical cavity comprises the first location and the second location, 1.e., wherein the first location and the second location are arranged (in the optical fiber) between the first fiber-Bragg grating and the second fiber-Bragg grating; (11) providing radiation to the glass fiber, wherein the radiation comprises a wavelength selected from the range of 400 - 1700 nm; measuring reflected radiation reflected by the set of fiber-Bragg gratings and providing a related signal; determining the parameter based on the related signal.

In embodiments, the method may further comprise a strain application stage. Especially, the strain application stage may comprise imposing a strain on (a section of) the optical fiber (exclusively) between a first location and a second location on the basis of (a value of) the parameter, especially wherein the first optical cavity comprises the first location and the second location. In embodiments, the strain application stage may comprise subjecting the optical fiber to a condition wherein a strain is imposed between the first location and the second location

In embodiments, the parameter may be selected from the group comprising an external strain, a tilt, a yaw, a displacement, a consolidation, a deformation, a stress, a chemical change, a temperature, a pressure, especially an acoustic pressure, and an acceleration. It will be clear to the person skilled in the art that also other (physical) parameters may be converted to a strain, and that the method of the invention may also be suitable to measure such parameters.

In embodiments, the method may comprise providing radiation and measuring downstream radiation during a measurement duration, wherein the imposing of the strain between the first location and the second location starts during the measurement duration, and especially wherein the related signal comprises time-dependent data. In particular, the method may comprise detecting moving between fringes of the set of fiber- Bragg gratings during the measurement duration. Specifically, different values of strain, and thus different values of the parameter, may result in an (essentially) identical distribution of reflected wavelengths. In particular, the distribution of reflected wavelengths may vary as a function of strain according to a repeating pattern (see, for example Fig 3A below). Hence, it may be challenging to accurately determine the strain imposed on the optical fiber based on a snapshot measurement. Hence, in embodiments, the method may comprise measuring the radiation during the measurement duration, and especially tracking the distribution of reflected wavelengths over time. Thereby, movement between the fringes can be accounted for when determining the amount of strain that corresponds to the wavelengths measured at any given timepoint.

Hence, in further embodiments, the determining of the parameter based on the related signal may comprise determining moving between fringes of the set of fiber- Bragg gratings during the measurement duration.

In a further aspect, the invention may provide a use of an optical fiber, especially a glass fiber, to measure a parameter, especially wherein the optical fiber comprises a set of fiber-Bragg gratings, wherein the set comprises a first fiber-Bragg grating, and a second fiber-Bragg grating, wherein the first fiber-Bragg grating and the second fiber-Bragg grating are separated by a first optical cavity, and especially wherein the first optical cavity may have a length L1 selected from the range of 1 - 1000 mm, especially from the range of 2 - 100 cm. In embodiments, the use may comprise imposing a strain on between a first location and a second location on the basis of (a value of) the parameter, especially wherein the first optical cavity comprises the first location and the second location. The terms “upstream” and “downstream” relate to an arrangement of items or features relative to the propagation of the radiation from a radiation generating means (here especially the radiation source), wherein relative to a first position within a path of the radiation from the radiation generating means, a second position in the path of the radiation closer to the radiation generating means is “upstream”, and a third position within the path of radiation further away from the radiation generating means is “downstream”. In particular, if a radiation sensor is arranged at the same side of a set of fiber-Bragg gratings as a radiation source, and the radiation sensor is configured to measure radiation reflected by the set of fiber-Bragg gratings, then the radiation sensor is configured downstream from the set of fiber-Bragg gratings (with respect to the path that the radiation travels). The embodiments described herein are not limited to a single aspect of the invention. For example, an embodiment describing the method may, for example, further relate to the operational mode of the measurement device. Similarly, an embodiment describing the optical fiber may further relate to the optical fiber in the measurement device, as well as to the optical fiber used in the method. In particular, an embodiment of the method describing an operation may indicate that the measurement device may, in embodiments, be configured for and/or be suitable for the operation. The invention may be applied for monitoring of a parameter, such as in the context of structural health monitoring, underwater acoustic measurements such as in a hydrophone, pressure sensing to determine liquid levels or storage tanks, etc.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig. 1A-C schematically depict embodiments of the optical fiber. Fig. 2 schematically depicts an embodiment of the measurement device. Fig. 3A-C depict simulated and experimental results. Fig. 4 depicts experimental results. The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1A schematically depicts an embodiment of an optical fiber for a strain sensor, wherein the optical fiber comprises a glass fiber 100. The glass fiber 100 comprises a set 110 of fiber-Bragg gratings 111,112,113, wherein the set 110 comprises a first fiber- Bragg grating 111, and a second fiber-Bragg grating 112. The first fiber-Bragg grating 111 and the second fiber-Bragg grating 112 are separated by a first optical cavity 131. In particular, the first optical cavity may consist of an (unaltered) section of the glass fiber 100 with a constant refractive index. In the depicted embodiment, the first optical cavity 131 has a length L1 selected from the range of 1-1000 mm. During use of the glass fiber 100 (also see below), strain corresponding to a value of a parameter may especially be applied between a first location 10 and a second location 20, wherein the first location 10 and the second location 20 are arranged in the first optical cavity 131. Thereby, the strain can be applied to a part of the glass fiber 100 that is structurally strong, while still modulating the length of the first optical cavity 131, which may result in a change in the distribution of reflected wavelengths.

Fig. 1B schematically depicts a further embodiment of the glass fiber 100, wherein the glass fiber 100 further comprises a third fiber-Bragg grating 113. In the depicted embodiment, the second fiber-Bragg grating 112 and the third fiber-Bragg grating 113 are separated by a second optical cavity 132. The second optical cavity may consist of an (unaltered) section of the glass fiber 100 with a constant refractive index. In the depicted embodiment, the second optical cavity 132 has a length L2, wherein 0.8<L1/L2<1.2, and especially wherein L1 is unequal to L2, such as wherein , and wherein [L1-L2|/L1 > 0.01.

In the depicted embodiment, the first fiber-Bragg grating 111, the second fiber-Bragg grating 112 and the third fiber-Bragg grating 113 have (essentially) the same periodic structure 50.

Fig. IC schematically depicts an embodiment of the glass fiber 100, wherein the glass fiber 100 comprises a plurality of sets of fiber-Bragg Gratings. In particular, in the depicted embodiment, the glass fiber 100 comprises a second set 120 of fiber-Bragg gratings, wherein the second set 120 comprises three fiber-Bragg gratings 121,122,123 separated by a second set first optical cavity 141 and a second set second optical cavity 142, wherein the second set first optical cavity 141 has a length L1;, especially wherein L1: is selected from the range of 1 — 1000 mm, more especially selected from the range of 2 - 100 cm, and especially wherein the second set second optical cavity has a length L23, especially wherein 0.8 <L1:/L2: <1.2, wherein L1; is unequal to L2:.

In further embodiments, the fiber-Bragg gratings of the set 110 have a grating period Al, and the fiber-Bragg gratings of the second set 120 have a second grating period A2. In the depicted embodiment, A2 > Al. In particular, in embodiments, |A1-A2| > 3% AX, where AA is the bandwidth of the FBGs. Especially, |A1-A2| > 4* AX, such as |AI-A2| = 5* A), especially |A1-A2| > 6% Aà, such as |[A1-A2| > 7* AA. In further embodiments, |A1-A2| < 20* Aj, especially [A1-A2] < 15% A2, such as [A1-A2] < 12% AX, especially |[A1-A2| < 10% AA, such as [A1-A2| < 7* AX, especially [A1-A2| <6* AA.

In embodiments wherein the glass fiber 100 has a plurality of sets of fiber- Bragg gratings, it may be beneficial if the optical cavities have similar lengths. Hence, in embodiments, 0.8<L1/L1:<1.2. In the depicted embodiment, L1 may be essentially equal to L 1a.

Fig. 2 schematically depicts a measurement device 200 for measuring a parameter. In the depicted embodiment, the measurement device 200 comprises a glass fiber 100 and a measurement element 210. In particular, in the depicted embodiment, the measurement device 200 may comprise the glass fiber of Fig. 1A. In further embodiments, the measurement device 200 may comprise the glass fiber of Fig. 1B or Fig. 1C. In embodiments, the measurement element 210 may be functionally coupled to the glass fiber 100 at a first location 10 and at a second location 20, especially wherein the measurement element 200 is configured to impose a strain on the glass fiber 100 between the first location 10 and the second location 20 on the basis of (a value of) the parameter.

In the depicted embodiment, the measurement device 200 further comprises a radiation source 220, a radiation sensor 230 and a control system 300.

In embodiments, the measurement device 200 may have an operational mode wherein: (i) the radiation source 220 provides radiation to the glass fiber 100, wherein the radiation comprises a wavelength selected from the range of 400 - 1700 nm; (ii) the radiation sensor 230 measures (reflected) radiation reflected by the set 110 of fiber- Bragg gratings and provides a related (reflection) signal to the control system 300; and (iii) the control system 300 determines (a value of) the parameter based on the related signal.

In embodiments, the radiation source 220 may comprise a broadband radiation source, especially wherein the radiation source (is configured to) provide (broadband) radiation comprising wavelengths in the range of 400 — 1700 nm.

In embodiments, the parameter may be selected from the group comprising an external strain, a tilt, a yaw, a displacement, a consolidation, a deformation, a stress, a chemical change, a temperature, an (acoustic) pressure and an acceleration.

In the depicted embodiment, the measurement device 200, especially the measurement element 210, may comprise a transducer 250 configured to convert (a value of) the parameter to a strain. Further, the measurement element 210 may be configured to impose the on the glass fiber 100 between the first location 10 and the second location 20. In particular, in the depicted embodiment, the transducer 250 may comprise a metal plate configured to impose a strain between the first location 10 and the second location 20 as a function of temperature; metal may have a relatively high expansion value and may thereby convert temperature to strain. For example, the transducer 250, especially the metal plate, may be glued to the glass fiber 100 at the first location 10 and the second location 20. Fig. 2 further schematically depicts a method for measuring a parameter using a glass fiber 100, wherein the glass fiber 100 comprises a set 110 of fiber-Bragg gratings, wherein the set 110 comprises a first fiber-Bragg grating 111 and a second fiber- Bragg grating 112, wherein the first fiber-Bragg grating 111 and the second fiber-Bragg grating 112 are separated by a first optical cavity 130, wherein the first optical cavity 130 has a length L1 selected from the range of 1 -1000 mm, such as from the range of 2 - 100 cm, especially wherein a measurement element 200 is configured to impose a strain on the glass fiber 100 between a first location 10 and a second location 20 on the basis of the parameter, wherein the first optical cavity 130 the first location 10 and the second location 20, wherein the method comprises: (ii) providing radiation to the glass fiber 100, wherein the radiation comprises a wavelength selected from the range of 400 - 1700 nm; (iii) measuring radiation downstream from the set 110 of fiber-Bragg gratings and providing a related signal; and (iv) determining the parameter based on the related signal.

In further embodiments, the method may comprise (1) imposing a strain on the glass fiber 100 between a first location 10 and a second location 20 on the basis of the parameter, especially via (or “using”) the measurement element 200.

Similarly, Fig. 2 schematically depicts a use of a glass fiber 100 to measure a parameter, wherein the glass fiber 100 comprises a set 110 of fiber-Bragg gratings, wherein the set 110 comprises a first fiber-Bragg grating 111, and a second fiber-Bragg grating 112, wherein the first fiber-Bragg grating 111 and the second fiber-Bragg grating 112 are separated by a first optical cavity 130, wherein the first optical cavity 130 has a length L1 selected from the range of 1 — 1000 mm, such as from the range of 2 - 100 cm, and wherein the use comprises imposing a strain on the glass fiber 100 between a first location 10 and a second location 20 on the basis of the parameter, wherein the first optical cavity 130 comprises the first location 10 and the second location 20. Experimental results The simulated data depicted in Figures 3A and 3C was obtained using the open software Python implementing a transfer matrix formalism.

To keep the computational cost low, each of the three gratings was modeled as a single, lossless interface with reflectivity of R= 40%, which is in agreement with the reflectivities that are commonly obtained with fiber-Bragg gratings.

Transfer matrices were assigned of the form: 1,4; = — (3 ) The spaces between the reflective interfaces were assigned a transfer matrix of the form tmat(d) = (* Ck qa) exp Cn | 0) where k is the wave vector of the light and d the distance between the reflective elements.

The combined transfer matrix of a structure combining three reflective elements where the first and second are separated by a distance d1 and the second and third are separated by a distance d2 is then given by: Tiotar = Ymat X tmac (AL) X Tmat X tmar(d2) X Tmat.

The reflection coefficient Tcoefr was determined according to Tcoeff = pa, and the reflected power Pr according to P. = |reoers| Fig. 3A depicts simulated reflection spectra of a glass fiber with a set comprising two fiber-Bragg gratings, wherein the first optical cavity has a length L1 =40 mm when a variable strain is applied to a 30 mm long section of fiber in the first optical cavity.

Specifically, Fig. 3A depicts the reflectivity R for different wavelengths A (in nm) as a function of applied strain S (in pe). The slope of the dark fringes indicates that the sensitivity of the first optical cavity is about 1.25 pm pe”, which may be comparable with state of the art sensitivities.

However, here the strain is exclusively applied to a relatively strong section of the glass fiber.

As described above, (approximately) the same distribution of reflected wavelengths may be observed for different values of strain S.

Hence, in embodiments, the method may comprise providing radiation and measuring downstream radiation during a measurement duration, wherein the imposing of the strain between the first location and the second location starts during the measurement duration, and especially wherein the related signal comprises time-dependent data.

Further, the determining of the parameter based on the related signal may comprise determining moving between fringes of the set 110 of fiber-Bragg gratings during the measurement duration. In particular, by tracking the distribution of reflected wavelengths over time, the strain S may be accurately determined despite (approximately) identical distributions of reflected wavelengths for different values of the strain S.

Fig. 3B shows an experimentally measured spectrum corresponding to the same glass fiber as described for Fig. 3A. In particular, Fig. 3B depicts that an overall envelope that may correspond to the reflection spectra corresponding to the (periodic structures (50)) of the fiber-Bragg gratings 111, 112, whereas the features inside the envelop may correspond to the length of the first optical cavity 131. In particular, Fig. 3B may show how the (Fabry-Pérot) fringes are modulated by the fiber-Bragg grating reflection spectra. In particular, in Fig. 3B, the Bragg wavelength Az may lie at about

1548.0 nm, and the set of wavelengths reflected by the FBG may span approximately from about 1547.8 nm to about 1548.2 nm.

Fig. 3C schematically depicts simulated results of a glass fiber comprising a set of three fiber-Bragg gratings. In particular, the glass fiber comprises three identical fiber-Bragg gratings separated by two optical cavities of unaltered fiber sections with lengths LI = 40 mm and L2 = 44 mm, respectively. Fig. 3C depicts the full spectrum for different applied strains on the first optical cavity (unaltered glass fiber). If one follows one of the fringes (indicated by the lines), the strain applied to the fiber can be detected with high sensitivity. In this specific case, the fringe has a slope of about 10 pm pe, which may correspond to a substantial improvement in sensitivity respect to the conventional FBGs.

Fig. 4 depicts experimental observations obtained with a glass fiber 100 comprising a set 100 of three fiber-Bragg gratings 111, 112, 113. The glass fiber 100 was attached to mounting plates such that only one the first optical cavity 131 was subjected to strain. Specifically, the glass fiber was glued such that the gratings themselves were not subjected to strain. Subsequently, 2g weights were applied to one end of the first optical cavity 131, straining the center of the first optical cavity 131 in the process. The estimated fiber strain resulting from each of the 2g weights is = 22 pe. Figure 4 depicts a set of reflectivity spectra measured as a function of the applied strain on the system. Specifically, Fig. 4 clearly depicts the static reflection envelope of the individual fiber-Bragg gratings, as well as a set of moving fringes. The dashed lines highlight the feature that shifts with high sensitivity. In particular, the beat — the average R value — may much faster than the individual interference lines. The term “plurality” refers to two or more. Furthermore, the terms “a plurality of” and “a number of” may be used interchangeably.

The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms “about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

The term “comprise” also includes embodiments wherein the term “comprises” means “consists of”.

The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of" but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species".

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. 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.

The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device,

apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

The various aspects discussed in this patent can be combined in order to provide additional advantages.

Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined.

Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

Conclusions

A fiber optic (100) for a voltage sensor, the fiber (100) comprising a set (110) of Bragg gratings (111,112,113), the set (110) a first fiber Bragg lattice (111), a second fiber Bragg grating (112) and a third fiber Bragg grating (113), wherein the first fiber Bragg grating (111) and the second fiber Bragg grating (112) pass through a first optical cavity ( 131) are separated, wherein the first optical cavity (131) has a length L1 selected from the range of 1 - 1000 mm, and wherein the second fiber Bragg grating (112) and the third fiber Bragg grating (113) separated by a second optical cavity (132), wherein the second optical cavity (132) has a length L2, where 0.8<L 1/L2<1.2, and wherein [L1-L2//L 120.01 .

The fiber optic (100) of claim 1, wherein one or more of the first fiber Bragg grating (111), the second fiber Bragg grating (112) and the third fiber Bragg grating (113) has a chirp fiber Bragg grating (“chirped fiber Bragg grating”).

The fiber optic (100) of any preceding claim, wherein one or more of the first fiber Bragg grating (111), the second fiber Bragg grating (112) and the third fiber Bragg grating (113 ) comprise an apodized fiber Bragg lattice.

The fiber optic (100) of any preceding claim, wherein the first fiber Bragg grating (111), the second fiber Bragg grating (112) and the third fiber Bragg grating (113) each have a same pattern type selected from the group consisting of a uniform fiber Bragg lattice, a chirp fiber Bragg lattice, and an apodized fiber Bragg lattice, wherein the first fiber Bragg lattice has a first repeat number nl, and wherein the second fiber Bragg grating has a second repeat number n2, and wherein the third fiber Bragg grating has a third repeat number n3, where [n1-n2|<0.05*n1, and where In1-n3|£<0, 05*n.

The fiber optic (100) of any preceding claim, wherein the fiber Bragg gratings of the set (110) have a lattice period Al, and wherein the fiber (100) has a second set (120) of fiber Bragg gratings, wherein the second set (120) includes three fiber Bragg gratings (121,122,123) separated by a second set of first optical cavity (141) and a second set of second optical cavity (142), wherein the second set of first optical cavity ( 141) a length L1; selected from the range of 1 - 1000 mm, and wherein the fiber Bragg gratings of the second set (120) have a second grating period A2, where [|A1-A22 5 * AR, where AA is the bandwidth of the fiber -Bragg grids.

The glass fiber (100) of claim 5, wherein 0.8<L1/L1:51.2, and wherein L1 is selected from the range of 20 - 1000mm.

A measuring device (200) for measuring a parameter, the measuring device (200) comprising a fiber optic (100) and a measuring element (210), the fiber optic (100) a set (110) of fiber Bragg gratings wherein the set (110) comprises a first fiber Bragg lattice (111) and a second fiber Bragg lattice (112), the first fiber Bragg lattice (111) and the second fiber Bragg lattice (112) are separated by a first optical cavity (131), the first optical cavity (131) having a length L1 selected from the range of 1 - 1000 mm, the measuring element (210) at a first location (10) and operably coupled to the fiber (100) at a second location (20), wherein the measurement element (200) is configured to apply a voltage to the fiber between the first location (10) and the second location (20) based on the parameter (100), wherein the first location (10) and the second location (20) are arranged in the first optical cavity (130).

The measuring device (200) of claim 7, wherein the set (110) comprises a third fiber Bragg grating (113), the second fiber Bragg grating (112) and the third fiber Bragg grating (113 ) through a second optical cavity (132)

separated, wherein the second optical cavity (132) has a length L2, where 0.8<L 1/L2<1.2, and where |[L1-L2|/L1>0.01.

The measuring device (200) of any one of the preceding claims 7-8, wherein the measuring device (200) further comprises a radiation source (220), a radiation sensor (230) and a control system (300), the measuring device (200) having an operational mode in which: - the radiation source (220) provides radiation to the fiber optic (100), the radiation comprising a wavelength selected from the range of 400-1700 nm; - the radiation sensor (230) downstream of the set (110) of fiber Bragg gratings measures radiation and provides a related signal to the control system (300); and - the control system (300) determines the parameter based on the related signal.

The measuring device (200) according to any one of the preceding claims 7-9, wherein the parameter is selected from the group comprising an external strain, a tilt, a directional deviation, a displacement, a consolidation, a deformation, a stress, a chemical change , a temperature and an acceleration.

A method of measuring a parameter using a fiber (100), wherein the fiber (100) comprises a set (110) of fiber Bragg gratings, the set (110) a first fiber Bragg lattice (111) and a second fiber Bragg grating (112), the first fiber Bragg grating (111) and the second fiber Bragg grating (112) being separated by a first optical cavity (130), wherein the first optical cavity (130) has a length L1 selected from the range of 1 - 1000 mm, wherein a measuring element (200) is configured to apply a tension to the fiber optic (200) between a first location (10) and a second location (20). 100) based on the parameter, the first optical cavity (130) comprising the first location (10) and the second location (20), the method comprising:

- providing radiation to the glass fiber (100), the radiation comprising a wavelength selected from the range of 400-1700 nm; - measuring radiation downstream of the set (110) of fiber Bragg gratings and providing a related signal, - determining the parameter based on the related signal.

The method of claim 11, wherein the parameter is selected from the group consisting of an external strain, a tilt, a direction deviation, a displacement, a consolidation, a deformation, a stress, a chemical change, a temperature, a pressure and an acceleration. .

A method according to any one of the preceding claims 11-12, wherein the method comprises: - providing radiation and measuring radiation during a measurement duration, wherein the imposition of the strain between the first location (10) and the second location ( 20) during the measurement time.

The method of claim 13, wherein determining the parameter based on the related signal comprises determining moving between fringes of the set (110) of fiber Bragg gratings during the measurement duration.

A method according to any one of the preceding claims 11-14, wherein the method comprises applying a voltage on the basis of the parameter to the optical fiber (100) between a first location (10) and a second location (20), wherein the first optical cavity (130) includes the first location (10) and the second location (20).

The method of any one of claims 1-15, wherein the set (110) comprises a third fiber Bragg lattice (113), the second fiber Bragg lattice (112) and the third fiber Bragg lattice (113) are separated by a second optical cavity (132), wherein the second optical cavity (132) has a length L2, where 0.8<L1/L2<1.2, and where [L1-L2/|/L 120.01.

17. Using a fiber (100) to measure a parameter, the fiber (100) comprising a set (110) of fiber Bragg gratings, the set (110) a first fiber Bragg lattice (111) , and comprising a second fiber Bragg grating (112), wherein the first fiber Bragg grating (111) and the second fiber Bragg grating (112) are separated by a first optical cavity (130), the first optical cavity (130) has a length L1 selected from the range of 1 - 1000 mm, and the use imposes it on the optical fiber (100) based on the parameter between a first location (10) and a second location (20) of a voltage, wherein the first optical cavity (130) comprises the first location (10) and the second location (20).