Apparatus and standards for measuring physical quantities

Abstract

An apparatus for measuring a physical quantity includes a standard (6) whose power spectrum upon reflection includes several power peaks distributed within an operating range, the standard having a free spectral range of 5 nm or less, the standard including a waveguide (34) and a Bragg grating (80) fabricated within the waveguide. The Bragg gratings include at least three identical patterns aligned behind each other along the longitudinal axis of the waveguide and separated from each other by a regular interval. The spacing of the Bragg grating (80) is configured so that its power spectrum exhibits several identifiable harmonics on the order of more than 100 within the operating range. The harmonics thus form the power peaks of the power spectrum of the standard.

Description

[Technical Field]

[0001] The present disclosure relates to an apparatus for measuring a physical quantity and a standard (etalon) for realizing this apparatus. [Background technology]

[0002] These devices are used, for example, to measure temperature, pressure or mechanical deformation.

[0003] An example of such a known measuring device is described in Chinese Patent Publication No. 102879022. This known device includes a standard and an optical transducer. The optical transducer converts fluctuations in the physical quantity to be measured into shifts in the power peaks in the power spectrum of the optical transducer. The standard is used to generate a reference power spectrum, which is used to correct the measurement and thus improve its accuracy.

[0004] To achieve this, the power spectrum of the standard contains closely spaced contiguous power peaks within a given operating range.

[0005] Many different methods have been proposed to realize such standards. For example, it has been proposed to realize such standards using a Fabry-Perot cavity, which is a mirror whose optical interface is connected to the end of an optical fiber. Such mirrors have a high reflectivity of over 90%. Under these conditions, the peaks in the power spectrum of the standard are minute, and the accuracy of the measurement device is high. However, the fabrication of such a Fabry-Perot cavity is complicated, especially because the mirror must be connected to the end of the optical fiber. An example of such a standard is described in WO 2020 / 113147.

[0006] Chinese Patent No. 102879022 proposes to realize a standard by inscribing a series of Bragg gratings into the core of an optical fiber. The fundamental resonant frequency f of each of these Bragg gratings is B Wavelength λ B is different from that of other Bragg gratings. However, the power peak widths of the standards of CN102879022 are generally less fine than those obtained using the standards of WO2020 / 113147. Furthermore, the fabrication of such continuous Bragg gratings is complex and often results in standards that are quite long and therefore bulky.

[0007] Prior art is also known from US Patent Application Publication No. 2019 / 178688 and US Patent Application Publication No. 2020 / 271485. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Chinese Patent No. 102879022 [Patent Document 2] International Publication No. 2020 / 113147 [Patent Document 3] US Patent Application Publication No. 2019 / 178688 [Patent Document 4] US Patent Application Publication No. 2020 / 271485 [Patent Document 5] Chinese Utility Model No. 211603608 [Patent Document 6] French Patent Application Publication No. 2207936 [Patent Document 7] French Patent Application Publication No. 3087008 [Non-patent literature]

[0009] [Non-Patent Document 1] Pengtao Luo et al. “Femtosecond laser plane-by-plane inscribed ultrahigh-order fiber Bragg grating and its application in multi-wavelength fiber lasers”, Optic letter, June 15, 2022 Summary of the Invention [Problem to be solved by the invention]

[0010] The object of the present invention is to provide an apparatus for measuring physical quantities which is highly accurate and at the same time easy to manufacture. [Means for solving the problem]

[0011] The invention is set out in the accompanying claims. [Brief explanation of the drawings]

[0012] The invention will be better understood on reading the following description, which is given by way of non-limiting example only and is made with reference to the drawings, in which: [Figure 1] FIG. 1 is a schematic diagram of the architecture of an apparatus for measuring physical quantities. [Figure 2] FIG. 2 is a schematic diagram of a standard for the measurement device shown in FIG. [Figure 3] FIG. 3 is a partial schematic diagram of a Bragg grating used in the standard shown in FIG. [Figure 4] FIG. 4 is a schematic cross-sectional view of the Bragg grating pattern shown in FIG. [Figure 5] FIG. 5 is a graph showing a portion of the power spectrum of the Bragg grating of FIG. [Figure 6] FIG. 6 is a flowchart of a method for manufacturing the Bragg grating shown in FIG. [Figure 7]FIG. 7 is a flow chart of a method for measuring a physical quantity using the apparatus shown in FIG. [Figure 8] FIG. 8 is a schematic longitudinal partial view of another type of standard of the measuring device shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] In these figures, the same reference numerals are used to denote the same elements, and a detailed description of features and functions well known to those skilled in the art will be omitted here.

[0014] In this specification, detailed examples of embodiments are described with reference to the drawings in Chapter 1. Next, modified examples of these embodiments are introduced in Chapter 2. Finally, advantages of various embodiments are described in Chapter 3.

[0015] Chapter 1: Example of Implementation

[0016] 1 shows a device 2 for measuring a physical quantity, where the physical quantity to be measured is, for example, the temperature of the external environment.

[0017] The device 2 comprises an optical transducer 4 and a standard 6. The optical transducer 4 is subjected to variations in the physical quantity it measures.

[0018] The optical transducer 4 converts fluctuations in the physical quantity being measured into shifts in the power peaks of its power spectrum. In this text, unless otherwise specified, "power spectrum" or "spectrum" refers to the reflected power spectrum. The reflected power spectrum is the power spectrum of an optical signal reflected by an optical component. The peaks in the reflected power spectrum correspond to absorption lines in the transmitted power spectrum of the same optical component.

[0019] The power spectrum of the optical transducer 4 includes, for example, a single power peak within a given operating range. This operating range is wider than 5 nm. Typically, its width is also less than 200 nm or 120 nm. Here, the width of the operating range is equal to 100 nm. The operating range is within the optical region. The optical region refers to a range that includes wavelengths commonly used in optics. More precisely, in the specification, the optical region refers to a range extending from 200 nm to 10,000 nm, often from 200 nm to 5,000 nm or even from 400 nm to 2,000 nm.

[0020] For example, the optical transducer 4 is the same as or similar to that described in Chinese Patent Publication No. 102879022. Thus, the optical transducer 4 here is a Bragg grating realized in the core of the optical fiber 14. The fundamental frequency f of this Bragg grating is B4 Wavelength λ B4 is within a predetermined operating range. B4 is located substantially in the center of the operating range. The wavelength of the fundamental frequency of the Bragg grating is given by the following relation (1): λ B =2*n e *Λ where: -λ B is the wavelength of the fundamental frequency of the Bragg grating, -n e is the effective index of the optical fiber in which the Bragg grating is formed, -Λ is the spacing of the Bragg grating, and The - symbol "*" indicates a scalar multiplication operation.

[0021] Effective propagation exponent n e is also known as the "modal phase constant". This is defined by the following relationship: n g =n e -λdn e / dλwhere n gis the group index, and λ is the wavelength of the optical signal guided by the optical fiber. The effective propagation index of an optical fiber depends on the dimensions of the optical fiber's core, the material from which the core is made, and the dimensions of the optical cladding of the optical fiber. It can be determined by experiment or numerical simulation.

[0022] The standard 6 has a reflected power spectrum with several power peaks distributed within its operating range. The free spectral range across the operating range of the standard 6 is 5 nm or less, preferably 1 nm or less. Hereinafter, this series of peaks is referred to as a "comb of peaks" or simply a "comb." Unlike the optical transducer 4, the standard 6 is configured so that its power spectrum is constant. In particular, the standard 6 is configured so that its power spectrum does not shift as a function of the measured physical quantity or as a function of changes in other physical quantities in the external environment in which the standard 6 is immersed. In particular, the standard 6 is configured so that its power spectrum does not shift as a function of the temperature of the external environment. Here, "not shifting" means that the amplitude of the shift in the power spectrum of the standard 6 is negligible compared to the amplitude Δλ of the shift in the power spectrum of the optical transducer 4, which is observed simultaneously. Here, the amplitude of the shift in the power spectrum of the standard 6 is considered negligible if it is 10 or 100 times the amplitude Δλ.

[0023] The optical transducer 4 is optically connected to an input / output port 10 of an optical coupler 12 via a waveguide 14 . The optical coupler 12 is an input port 16 optically connected to an output port 18 of a spectrum analyzer 20 via a waveguide 22; and It has an output port 24 optically connected to an input port 26 of the spectrum analyzer 20 via a waveguide 28.

[0024] The standard 6 is optically connected to an input / output port 30 of an optical coupler 32 via a waveguide 34 . The optical coupler 32 is It has an input port 36 optically connected to an output port 38 of the spectrum analyzer 20 via a waveguide 42 . The optical coupler 32 is It has an output port 44 optically connected to an input port 46 of the spectrum analyzer 20 via a waveguide 48.

[0025] In this embodiment, all of the above waveguides are optical fibers. Therefore, in the following, the same symbols are used to refer to either waveguides or optical fibers. The optical fiber used here is a single-mode optical fiber, also known as SMF (Simple Mode Fiber).

[0026] The spectrum analyzer 20 measures the spectral responses of the optical transducer 4 and the standard 6 and is able to determine the change in the physical quantity to be measured from these measured spectral responses. To do this, it includes: - tunable laser light source 50, an optical coupler 52 for optically connecting the output port 54 of the laser source 50 to both output ports 18 and 38 simultaneously; two optical sensors 62 and 64 optically connected to the input ports 26 and 46, respectively, for measuring the power of the optical signals received at these input ports; and an electronic processing unit 70 electrically connected to the optical sensors 62, 64 and receiving electrical signals representative of the power of the optical signals measured by the optical sensors 62 and 64, respectively;

[0027] The laser source 50 emits a single frequency optical signal via an output port 54 to the optical transducer 4 and the standard 6. The wavelength λ of the emitted optical signal is s is in the optical domain. The value of the wavelength λ depends on the control signal received at the control port 66 of the laser source 50. More precisely, the wavelength λ s is the wavelength λ for each value of the control signal. sλ is linked to the value of the control signal by a transfer function that relates corresponding values ​​of λ. Usually, this transfer function is not perfectly linear; in this case, it is said to be "nonlinear." Such a laser source 50 is also called a "scanning laser source." It is a laser source that can be scanned at wavelength λ using an appropriate control signal. s This is because the entire operating range is scanned by the wavelength λ smin From wavelength λ smax The width of the operating range is typically determined by the characteristics of the laser source 50. The width of the operating range is determined by the difference λ smax -λ smin In this embodiment, the operating range extends from 1500 nm to 1600 nm.

[0028] The optical sensor 62 measures the optical signal backscattered by the optical transducer 4. In parallel, the optical sensor 64 measures the optical signal backscattered by the standard 6. Here, the optical sensor 62 and the optical sensor 64 are identical. For example, the optical sensors 62 and 64 are each photodiodes. Each of the optical sensors 62, 64 has a spectral range of observation that encompasses its operating range.

[0029] In particular, unit 70 is configured as follows. determining the amplitude Δλ of the peak shift of the optical transducer 4 from the spectral responses of the optical transducer 4 and the standard 6 measured separately by the optical sensors 62 and 64, and then - determining the fluctuation of the physical quantity to be measured from the determined amplitude Δλ;

[0030] To perform these operations, unit 70 includes a programmable microprocessor 72 and a memory 74 containing the data and instructions necessary to operate spectrum analyzer 20 . For example, the memory now contains a sensitivity coefficient SG that defines the variation of the physical quantity from the determined amplitude Δλ. In this example, the coefficient S G is given by the following relationship Δλ / λ Bi4 =S G*Defined by ΔG. -λ Bi4 is the wavelength of the fundamental frequency of the Bragg grating of the optical transducer 4 in the reference state, −Δλ is the amplitude of the change in wavelength of the fundamental frequency of the Bragg grating of the optical transducer 4 obtained in response to the change ΔG in the physical quantity of the measurement target.

[0031] Here, the wavelength λ Bi4 corresponds to the reference wavelength of the power peak of the optical transducer 4. The amplitude Δλ corresponds to the wavelength λ of the fundamental frequency measured for the optical transducer 4. Bm4 and the reference wavelength λ Bi4 It is equal to the difference between the wavelength λ Bi4 Unlike, wavelength λ Bm4 varies as a function of the physical quantity being measured. Bi4 The value of this wavelength λ is stored in the memory 74. Bi4 may also be associated in memory 74 with the corresponding absolute value of the physical quantity being measured.

[0032] Typically, the unit 70 is also connected to a man-machine interface 76 for communicating the measurement results to a human.

[0033] 2 shows in more detail the architecture of standard 6. Standard 6 includes a Bragg grating 80 fabricated in the core of optical fiber 34. Bragg grating 80 is a very high order Bragg grating.

[0034] In this context, "very high order" refers to the fact that the power spectrum of a Bragg grating exhibits distinguishable harmonics of orders greater than N in the optical domain, more precisely in the operating range, where N is an integer greater than 100, preferably greater than 500 or 1000. In other words, the reflection power spectrum of a very high order Bragg grating has k harmonics of orders greater than N, each corresponding to a power peak different from the peaks corresponding to the k-1 and k+1 harmonics. This k peak is higher than the noise. This k peak is located at a wavelength λ defined by the following relation (2): k Located in. λ k =2*n e *Λ / k where k is an integer equal to the harmonic order. e is the effective index of the optical fiber in which the ultra-high-order Bragg grating is realized. Λ is the spacing of the ultra-high-order Bragg grating.

[0035] Here, this kth peak is located within the operating range.

[0036] The power spectrum of the Bragg grating 80 within its operating range contains a series of peaks corresponding to harmonics of orders greater than N. These peaks are very close together and very fine. Here, "very close together" means that the free spectral range is less than 5 nm, preferably 1 nm or less. "Very fine" means that the half-width of each peak is less than the free spectral range, preferably less than half the free spectral range. Furthermore, because each of these peaks corresponds to a very high-order harmonic, the heights of these peaks are substantially the same throughout the operating range. In other words, the power spectrum of the Bragg grating 80 is a comb of peaks as defined above. An example of such a comb is shown in Figure 3 of the following paper: Pengtao Luo et al.: "Femtosecond laser plane-by-plane inscribed ultrahigh-order fiber Bragg grating and its application in multi-wavelength fiber lasers," Optic Letter, 15 / 06 / 2022. This paper will be referred to hereafter as "LUO2022".

[0037] Ultrahigh-order Bragg gratings differ in a number of ways from standard Bragg gratings commonly used in optics, where the spacing of the standard Bragg grating is chosen such that - fundamental resonance frequency f of the Bragg grating B Wavelength λ B is in the optical region. Only first harmonics of order -20 or less are allowed into the optical region.

[0038] As a result, the spacing of these standard Bragg gratings is systematically smaller than 50 μm or 20 μm, and usually smaller than 10 μm. Under these conditions, a standard Bragg grating cannot be a very high-order Bragg grating. In fact, in this case, even if harmonics of order k larger than 100 are discernible in its power spectrum, the wavelength λ of these harmonics is much smaller than the wavelength λ of the standard Bragg grating. k is not in the optical domain. In other words, the wavelength λ of the harmonic of order k, which is greater than 100, k are all less than 200 nm. Conversely, the spacing of very high-order Bragg gratings is greater than 20 μm or 50 μm, and often greater than 100 μm. Under these conditions, the fundamental resonant frequency f of very high-order Bragg gratings is B Wavelength λ B and wavelengths of harmonics less than 100 orders of magnitude are not in the optical domain.

[0039] Standard Bragg grating patterns are typically fabricated using ultraviolet radiation pulses or CO2 lasers, rather than femtosecond laser pulses. Bragg gratings fabricated without femtosecond laser pulses show that only harmonics less than 20 orders of magnitude can be distinguished. This is likely due to the fact that the refractive index changes in optical fiber obtained using these other known methods are much less sharp than those obtained using femtosecond lasers. Therefore, Bragg gratings fabricated without femtosecond laser pulses are not very high-order Bragg gratings, even if they have spacings greater than 20 μm or 50 μm.

[0040] It should also be emphasized that Bragg gratings should not be confused with juxtapositions of Fabry-Perot cavities along an optical fiber. Indeed, the spectral properties of an optical fiber containing such juxtapositions of Fabry-Perot cavities depend on the length of each Fabry-Perot cavity and the reflectivity of the optical interfaces located at each end of each Fabry-Perot cavity. Unlike Bragg gratings, the optical interfaces are not regularly spaced apart to form a periodic structure.

[0041] Bragg gratings are also frequently used in the field of laser sources to form the end optical interfaces of the Fabry-Pérot cavities of these laser sources. In this case, the spectral response of the cavity is determined primarily by the length of the cavity, not the spectral characteristics of the Bragg grating used. More precisely, as taught in the above-mentioned paper LUO2022, the spectral characteristics of the Bragg grating are used to tune the wavelength of the laser source. This use of very high-order Bragg gratings in laser sources falls outside the scope of the field of measuring physical quantities. In particular, this use does not teach that high-order Bragg gratings can be advantageously used to generate standards for measurement devices.

[0042] Standard 6 is also designed to ensure that the power spectrum of Bragg grating 80 remains constant despite variations in operating conditions. To this end, it includes an insulating structure 82 that isolates Bragg grating 80 from variations in the external environment in which standard 6 is immersed. In this embodiment, insulating structure 82 includes a housing 84 within which Bragg grating 80 is fixed with no degrees of freedom. Housing 84 isolates Bragg grating 80 from variations in the mechanical stresses that the external environment exerts on Bragg grating 80.

[0043] The insulating structure 82 also includes, within the housing 84: - temperature sensor 90, an adjustable heating or cooling element 92 capable of heating or cooling the Bragg grating 80; and a microcontroller 94 configured to control the element 92 as a function of a temperature setpoint Tc and the temperature measured by the sensor 90, and to limit the temperature variation of the Bragg grating 80 around this temperature setpoint Tc.

[0044] Element 92 is, for example, a Peltier module or a set of Peltier modules.

[0045] Microcontroller 94 includes a programmable microprocessor 96 and a memory 98 containing the data and instructions necessary to operate standard 6. In this case, memory 98 includes a pre-stored temperature setpoint Tc and instructions for a servo control module 100. When servo control module 100 is executed by microprocessor 96, the temperature of Bragg grating 80 is controlled by the temperature setpoint Tc stored in memory 98. To do this, microprocessor 96 controls element 92 as a function of the deviation between the temperature setpoint Tc and the temperature measured by sensor 90 to reduce this deviation.

[0046] Figure 3 shows a more detailed example of a Bragg grating 80. The optical fiber 34 extends along a longitudinal axis 108 that is parallel to the Z direction of an orthogonal reference frame XYZ. Figures 3, 4, and 8 are oriented with respect to this XYZ reference frame. For example, the Z direction is horizontal and the Y direction is vertical.

[0047] 3, only the portion of optical fiber 34 that includes Bragg grating 80 is shown. Optical fiber 34 guides an optical signal along a longitudinal axis 108.

[0048] The optical fiber 34 is a core 110 through which the optical signal guided by the optical fiber 34 propagates; an optical cladding 112 made of a material with a refractive index that keeps the optical signal within the core 110 by reflection at the interface between the core 110 and the optical cladding 112; and - Includes a mechanical sheath, typically made of a polymer, that covers the optical cladding 112. To simplify FIG. 3, the mechanical sheath of the optical fiber 34 is not shown.

[0049] Bragg grating 80 is designed to produce a comb of peaks over its operating range, and furthermore, here, Bragg grating 80 is designed so that this comb is formed by harmonics of Bragg grating 80 on the order of 1024.

[0050] For this purpose, the Bragg grating 80 is made up of a series of patterns M arranged behind each other along the longitudinal axis 108. i The index i is the sequence number of the pattern in the Z direction. The index i of the first leftmost pattern of the grating 80 is equal to 1, and the index i of the last rightmost pattern of the grating 80 is equal to p. p is the number of patterns M of the Bragg grating 80. i In Figure 3, only the first two patterns and the last two patterns of the Bragg grating 80 are shown. M2 and M p-1 The presence of intermediate patterns between are represented by small circles on the longitudinal axis 108.

[0051] The number p of patterns is 3 or more, preferably 10 or more. In fact, it has been observed that the larger the number p, the smaller the half-width of each peak. Here, the number p is also selected to be small enough to keep the length of the Bragg grating 80 small, i.e., less than 1 m, preferably less than 10 cm. The length of the Bragg grating 80 is determined by the length of the patterns M1 and M2 measured along the axis 108. p The number p is usually less than 200 or even less than 100.

[0052] Two immediately consecutive patterns M in the Z direction i and M i+1 The spacing Λ between 80 is constant regardless of the index i. Therefore, the interval Λ 80 is two immediately consecutive patterns M i and M i+1 is equal to the distance along axis 108 between

[0053] where the interval size Λ 80 is k c The wavelength of the order k harmonic is calculated to be equal to or very close to the center of the operating range, where k is the order k. c is chosen to be equal to 1024.

[0054] For this purpose, the interval Λ 80 is 0.9*[k c *λ c / (2*n e )] and 1.1*[k c *λ c / (2*n e )], preferably between 0.98*[k c *λ c / (2*n e )] and 1.02*[k c *λ c / (2*n e )], where n e is the effective index of the optical fiber 34, and λ c is the wavelength at the center of the operating range, where wavelength λ c is 1550 nm.

[0055] By way of example, the optical fiber 34 is made from the optical fiber sold by Corning® under the reference SMF-28. The effective propagation index n e is approximately equal to 1.4676. Under these conditions, the term k c *λ c / (2*n e ) is equal to approximately 540.8 μm, where the spacing Λ 80 is chosen to be equal to 540.8 μm.80 With this choice of value for , only harmonics of orders between 317 and 7936 are in the optical region, and only harmonics of orders between 993 and 1058 are within the operating range. In particular, the wavelength λ of the fundamental frequency of the Bragg grating 80 B80 is not in the optical region.

[0056] This interval Λ 80 For the value of L, the length of the Bragg grating is 80 80 The number of patterns p is selected to be less than 185 so that the length L of the Bragg grating 80 is less than 10 cm, where p is selected to be equal to 120. 80 is approximately equal to 65 mm.

[0057] Pattern M i are all structurally identical to one another and differ only from one another in their position along the axis 108. Therefore, in the following, the pattern M i Only the pattern M will be explained in detail. i is mainly a plane P perpendicular to the axis 108 i Therefore, this plane P i are parallel to the X and Y directions. In Figure 3, the pattern M 1、 M2, M p-1 and M p The plane P on which each of these extends 1、 P 2、 P p-1 and P p Only the following is shown.

[0058] Figure 4 shows the pattern M i 4 shows a more detailed example of the core 110. In FIG.

[0059] Each pattern M i reflects a portion of the incident optical signal. Another portion of the incident optical signal is reflected by the pattern M. i Finally, for each pattern M i scatters some of the energy of the incident optical signal, but this energy is i No reflection or transmission occurs through each pattern Mi This energy spread by the pattern M results in insertion losses caused by the presence of the Bragg grating 80 in the core 110 of the optical fiber 34. To minimize these insertion losses, i Cross-sectional area S Mi is the cross-sectional area S of the core 110 110 It occupies less than half of the area S Mi is the plane P i Upper pattern M i The area of ​​the orthogonal projection of the surface area S is equal to 110 is equal to the cross-sectional area of ​​the core 110. Typically, the surface area S 110 is constant along the entire length of the optical fiber 34.

[0060] Preferably, the surface area S Mi is 0.1 x S 110 , 0.05×S 110 or 0.01 x S 110 where the surface area S Mi is 0.05 x S 110 is less than.

[0061] Limit the number of patterns p and therefore the length L of the Bragg grating 80 80 Pattern M is sufficient to limit i To obtain a reflectance of , the surface area SMi must be 0.016 μm 2 The plane P of a larger spherical bubble, i.e., 100 nm in diameter i In this embodiment, the surface area SM i is 0.032 μm 2 That's all.

[0062] For this purpose, the pattern M i There are some bubbles B j The index j is composed of the same pattern M i Among all other bubbles in Bubble B j The index j is an integer between 1 and q, and q is the number of elements in the pattern M. i Air bubble B jThe number q is equal to 2 or 4 or greater, where q is equal to 6.

[0063] In this embodiment, all the bubbles B j are structurally identical to each other. They are i can only be distinguished from one another by their position in

[0064] Each bubble B j introduces a significant change in the refractive index of the core 110 in the direction of propagation of the optical signal. To achieve this, the refractive index of the core 110, n r110 and Bubble B j Refractive index n rB The difference between the refractive index n and the refractive index n is 0.3 or 0.4 or more. r110 and n rB It is empty or substantially empty, depending on the difference between

[0065] Furthermore, to ensure that the change in refractive index is abrupt, each bubble B j Diameter D j is less than 200 nm, preferably less than 100 nm. j are also larger than 10 nm or 50 nm.

[0066] Each bubble B j are mainly spherical. Therefore, bubble B j Diameter D j Bubble B j It is equal to the diameter of the smallest sphere that contains the whole. Here, this diameter D j is less than 100 nm.

[0067] Each bubble B j The center of the plane P i Included in.

[0068] In this embodiment, the bubbles B j is discontinuous. j do not overlap with each other and are not fluidly connected.

[0069] Pattern M i is centered on axis 108. To achieve this, bubble B j is pattern M i The center of mass of at least one bubble B is located within 100 nm of the axis 108. j are arranged side by side so that their centers are located within 100 nm of the axis 108.

[0070] In this embodiment, pattern M i The center of gravity of the pattern M lies on the axis 108. i is symmetric about axis 108.

[0071] Bubble B j The center of the plane P intersects with the axis 108. i Axis A belongs to i They are located behind each other on the top. Therefore, pattern M i consists of a row of separate bubbles. In this case, this row of separate bubbles forms what is referred to in the text as a "dotted line." Here, axis A i are parallel to the Y direction. In this embodiment, bubbles B3 and B4 are located above and below axis 108, respectively. The centers of bubbles B3 and B4 are less than 100 nm from axis 108.

[0072] Axis A i Two bubbles B immediately adjacent to each other along the j ,B j+1 The distance between the axes is constant. i A pair of bubbles B immediately adjacent to each other along the j ,B j+1 Whatever the distance separating the centers of these two bubbles is the same.

[0073] Figure 5 shows the power spectrum of the Bragg grating 80 between 1545 nm and 1555 nm. The reflectivity of the resulting comb peak reaches -21 dBm.

[0074] 6 illustrates a method for manufacturing an optical fiber 34. The method begins at step 120, in which an optical fiber is provided, the core 110 of which is initially devoid of a Bragg grating. For example, the optical fiber provided is the optical fiber sold by Corning® Incorporated under the reference SMF-28.

[0075] Here, the mechanical sheath of this optical fiber is transparent to the femtosecond laser pulse, so that the pattern M i There is no need to remove this mechanical sheath where it is formed.

[0076] Next, in step 122, a Bragg grating 80 is formed in the core 110. To do this, a pattern M is formed in the core 110 of the supplied optical fiber. i The operation 124 for forming such a pattern M i is repeated at each location where

[0077] In operation 124, each bubble B j is generated by a single femtosecond laser pulse. More precisely, in operation 124, the femtosecond laser beam is directed to generate a bubble B j and emit a pulse of duration less than 500 fs or 250 fs, which is focused at the center of bubble B. j Then, a point in the core 110 where the center of the bubble B is located is irradiated. j The optical fiber is then moved relative to the femtosecond laser, and the femtosecond laser beam passes through the next bubble B j+1 A new femtosecond laser pulse is emitted, focused at the center of the

[0078] In this embodiment, bubble B j are generated one after another.

[0079] Bubble B jThe values ​​of the various parameters of the femtosecond laser for generating bubbles like B depend on the characteristics of the optical fiber supplied and the characteristics of the femtosecond laser used. j The adjustment of these different parameters to produce B is a subject of skill in the art. For example, for illustrative purposes, the reader may imagine that an air bubble B is introduced into the core of an optical fiber. j Reference can be made to the application China Utility Model No. 211603608, which details an example of an apparatus for forming such bubbles. Here, the optical fiber 34 was manufactured using the following parameters: -The central wavelength of the femtosecond laser pulse is 512 nm. -Each femtosecond laser pulse has a duration of 160 fs. -The power of each femtosecond laser pulse is equal to 45 nJ.

[0080] Next, the operation of the measuring device 2 will be described with reference to the method shown in FIG.

[0081] In step 130, unit 70 calculates the wavelength λ s is the wavelength λ smin From wavelength λ smax To this end, unit 70 sends to laser source 50 a control signal generated from an estimate of the transfer function of laser source 50.

[0082] The optical signal emitted by the laser source 50 is directed by the optical coupler 52 and the optical fibers 22, 14, 42, and 34 to the optical transducer 4 and the standard 6. The optical transducer 4 and the standard 6 then reflect portions of the incident optical signal. These reflected portions of the optical signal correspond to the signals backscattered by the optical transducer 4 and the standard 6, respectively.

[0083] In parallel with step 130, in step 132, optical sensor 62 measures only the optical signal backscattered by optical transducer 4, and optical sensor 64 measures only the optical signal backscattered by standard 6. More precisely, optical sensors 62, 64 each generate an electrical signal whose amplitude represents the power of the measured optical signal. The electrical signals generated by optical sensors 62, 64 are transmitted to and acquired by unit 70.

[0084] Once the electrical signal is acquired by unit 70, in step 134 unit 70 determines the amplitude Δλ.

[0085] wavelength λ s is the wavelength λ of the optical transducer 4 Bm4 The power of the optical signal backscattered by the optical transducer 4 passes through a maximum value when the wavelength λ is equal to s Since varies linearly with time, the moment t at which this maximum value occurs m is the wavelength λ Bm4 Similarly, the reference wavelength λ Bi4 is the reference instant t i In step 134, unit 70 measures the time t m and the reference time t i Calculate the deviation of the wavelength over time λ s Since is linear, the deviation t m -t i is proportional to the amplitude Δλ. m -t i The proportionality coefficient of the amplitude Δλ is s The target slope α of the line representing the change over time c This target gradient α c is a predetermined known constant. Thus, in step 134, unit 70 calculates m and t i Δλ is calculated from the deviation measured between

[0086] Then, in step 136, the unit 70 determines the variation ΔG of the measured physical quantity from the amplitude Δλ. For example, the variation ΔG can be determined by the following relationship: Δλ / λ Bi4 =S G *Calculated using ΔG, where S G is a sensitivity coefficient pre-stored in the memory 74. Wavelength λ Bi4 is associated in memory 74 with the corresponding absolute value of the measured physical quantity, unit 70 also calculates this absolute value of the measured physical quantity in step 136.

[0087] Now, in step 140, each time the power spectrum of the standard 6 is measured by the optical sensor 64, the unit 70 establishes a new estimate of the transfer function of the laser source 50. To do this, for example, the unit 70 calculates the time t at which each k-th order peak occurs in the comb of the standard 6. k,m The unit 70 then records the recorded time t k,m The wavelength λ of this k-th order peak k Associate with time t k,m and wavelength λ k The combination of horizontal axis λ k and vertical axis t k,m The point (λ k ;t k,m ) is then calculated for the point (λ k ;t k,m ) corresponds to a curve that passes through the set

[0088] This estimated transfer function is then calculated using the target gradient α c wavelength λ s is used to generate a control signal for the laser source 50 that achieves a linear change in time of k,m is the wavelength λ s The change in is perfectly linear, and the slope α c The theoretical moment t at which the kth peak should have occurred k,t Therefore, the theoretical time tk,t is the wavelength λ of the kth peak k and a predetermined known target gradient α c The deviation t is calculated from k,m -t k,t If the amplitude of exceeds a predetermined threshold, the control signal is set to a value at the moment t in order to limit the amplitude of this deviation. k-1,t and t k,t For example, the deviation t k,m -t k,t If is positive, this means that at time t k,m is time t k,t In this case, the control signal is delayed from the moment t k-1,t and t k,t Between wavelength λ s is corrected to grow faster. Conversely, the deviation t k,m -t k,t If is negative, this means that at time t k,m is time t k,t In this case, the control signal is k-1,t and t k,t Between wavelength λ s is modified to increase more slowly.

[0089] Therefore, in this embodiment, the measured spectrum of the standard 6 is used to determine the wavelength λ s This linearization improves the accuracy of the measurement and also compensates for drifts in the laser source 50. In this way, the measurement of the spectrum of the standard 6 therefore participates in the determination of the amplitude Δλ.

[0090] Figure 8 shows a standard 150 that can be used in place of standard 6. Standard 150 is identical to standard 6 except that a second Bragg grating 152 is formed in optical fiber 34. To simplify Figure 8, only core 110, grating 80, and second Bragg grating 152 are shown.

[0091] The second Bragg grating 152 has a fundamental frequency wavelength λ B152is a standard Bragg grating within the operating range. B152 The absolute value of is known. Here, the pattern of the second Bragg grating 152 is B152 The second Bragg grating 152 is configured so that the amplitude of its power peaks at is greater, preferably 1.5 or 2 times, than the amplitude of the comb peaks of the grating 80 within its operating range.

[0092] For example, second Bragg grating 152 may be formed in core 110 at the same location as grating 80, but may be radially offset from grating 80 so that its pattern does not interfere with the pattern of grating 80. For example, second Bragg grating 152 may be formed in the upper part of core 110, and grating 80 may be formed in the lower part of core 110. The upper part of core 110 is above a horizontal plane containing axis 108, and the lower part is below this horizontal plane.

[0093] The power spectrum of standard 150 is equal to the superposition of the spectrum of grating 80 and the spectrum of second Bragg grating 152. Thus, in addition to the comb of peaks, the spectrum of standard 150 has a wavelength λ B152 This additional peak is easily identifiable because its amplitude is greater than the amplitude of the comb peaks of grating 80.

[0094] When standard 6 is replaced by standard 150, unit 70 is modified to further determine the absolute value of the wavelength corresponding to each of the comb peaks of grating 80. To do this, unit 70 finds the maximum power peak in the measured spectrum of standard 150, i.e., the peak corresponding to second Bragg grating 152. The absolute value of the wavelength at which this maximum peak appears in the power spectrum of standard 150 is known, and is determined to be wavelength λ B152The positions of the comb peaks of grating 80 relative to the maximum peak are determined from the measured spectrum of standard 150. Then, for each peak of the comb of grating 80, unit 70 determines the absolute value of the wavelength corresponding to this peak from: - the number of free spectral ranges separating the maximum peak, and -wavelength λ B152 The absolute value of .

[0095] For example, if the comb peak of grating 80 is separated from the maximum peak by a free spectral range of 5.5, the absolute value of the wavelength at which this peak appears is λ B152 +5.5*ISL 80 where ISL is equal to 80 is the known value of the free spectral range of the grating 80.

[0096] Then, from the absolute value of the wavelength of the comb peak, unit 70 calculates the wavelength λ s A transfer function for the laser source 50 can be estimated that relates the absolute value of the wavelength λ as a function of time to a particular value of the control signal. s This simplifies the generation of a control signal that linearly varies the wavelength λ. Bm4 It is also possible to directly establish the absolute value of . Then, the unit 70 calculates the wavelength λ Bm4 Establish the absolute value of the physical quantity being measured from the known relationship between the absolute value of and the absolute value of the physical quantity.

[0097] Chapter 2 Variations:

[0098] Standard variations:

[0099] Harmonic order k at the center of the operating range c is selected to be greater than 100, preferably greater than 500 or 1000. c can be larger than 2000, 4000, or 10000. In theory, this order k c However, from the relation (2), the degree k cIt can be seen that the higher the k, the larger the Bragg grating spacing Λ and therefore the longer the Bragg gratings of very high orders. c The upper limit is imposed by the desired maximum length of the Bragg grating, which is taken here to be 1 m.

[0100] Similarly, the minimum value of the spacing Λ is greater than 20 μm, typically greater than 50 μm, so that very high harmonics are included in the optical domain. In theory, there is no maximum value for the spacing Λ. In practice, whatever the value chosen for the spacing Λ, the wavelength λ c The order k that places the center of the operating range c However, the larger the spacing Λ, the longer the Bragg grating will be. Therefore, in practice, it is also the maximum desired length of the Bragg grating that imposes an upper limit on the value of the spacing Λ.

[0101] For example, by applying the teachings given in Chapter 1, it is possible to obtain combs for all operating ranges. This applies in particular to operating ranges centered around wavelengths commonly used in optical systems, such as 800 nm, 1000 nm, 1300 nm, or 1500 nm.

[0102] The operating range can be wider than 100 nm. For example, the width of this operating range can alternatively be greater than 200 nm or 300 nm. There is no upper limit to the width of this operating range, but it must be within the optical domain and be able to be scanned by the laser light source of the spectrum analyzer.

[0103] The various variants of the ultra-high order Bragg grating patterns described in French Patent Application No. 2207936, filed by the applicant on July 29, 2022, are applied to the ultra-high order Bragg grating patterns of the measurement device described herein.

[0104] The ultra-high-order Bragg grating patterns generated in the optical fiber core can have different shapes. For example, in one embodiment, each pattern contains a single bubble. In another embodiment, as described in the above-mentioned paper LUO2022, each pattern has an elliptical shape.

[0105] Optical fibers other than SMF-28 can be used. For example, the optical fiber can be a multi-mode optical fiber or MMF (Multi-Mode Fiber).

[0106] The core of the optical fiber does not have to be specifically doped: for example, the described manufacturing method can be used with optical fibers whose cores are made of germanosilicate, pure silica, rare-earth doped aluminosilicate, or sapphire.

[0107] In certain embodiments, the properties of the optical fiber 34 from which the standard 6 is fabricated differ from the properties of the optical fiber 14, so that the sensitivity of the standard to changes in the physical quantity being measured is less than the sensitivity of the optical transducer 4 to these same changes in the physical quantity.

[0108] Variations of insulation structure:

[0109] Other types of insulating structures are possible: for example, the grating 80 can be isolated from changes in mechanical stress by implementing the teachings of patent application FR 3 087 008 A1.

[0110] The isolation structure may also be designed to isolate the grating 80 from changes in hydrostatic pressure.

[0111] In a simplified embodiment, the insulating structure is not an active insulating structure but a passive insulating structure, i.e., an insulating structure that does not consume electrical energy to insulate the optical fiber 34 from changes in the external environment. For example, the passive insulating structure includes a material with a very low thermal expansion coefficient in which the optical fiber is fixed without any degrees of freedom. Typically, this material with a very low thermal expansion coefficient is in the range of 5×10-6 K -1 The material has a coefficient of thermal expansion less than 0.05 W / m / K. For example, this material is an iron-nickel alloy, such as an Fe-Ni alloy with 36% atomic nickel. This alloy is known as Invar®. In another example of a passive isolation structure, the optical fiber is embedded within a material with a thermal conductivity less than 0.05 W / m / K.

[0112] In another embodiment, the insulating structure includes a material that provides mechanical stress to the optical fiber to compensate for the effects of thermal expansion of the optical fiber in response to temperature changes.

[0113] In particular, if the mechanical stresses exerted on the grating 80 by the external environment do not change or only change very slightly, the housing 82 can be omitted.

[0114] Optical transducer variations:

[0115] The optical transducer does not necessarily have to include a Bragg grating. Alternatively, the optical transducer can include a Fabry-Perot interferometer instead of a Bragg grating. Like a Bragg grating, such a Fabry-Perot interferometer has a power spectrum with a peak whose position varies as a function of temperature, elongational stress (tensile stress), and hydrostatic pressure acting on the Fabry-Perot cavity. Such a Fabry-Perot interferometer can be implemented in the core of an optical fiber. In another variation, the optical transducer is a gas cell whose transmitted power spectrum contains an absorption line. The position of this absorption line in the power spectrum varies, for example, as a function of temperature.

[0116] When the optical transducer is a Bragg grating, the power spectrum of the optical transducer shifts in response to changes in temperature, longitudinal deformation of the optical fiber core, or changes in hydrostatic pressure. In this manner, all of the above-described embodiments can be adapted to measure a physical quantity selected from the group consisting of temperature, longitudinal deformation of the optical fiber core, and changes in hydrostatic pressure. Measurement of any of these physical quantities can be used to derive measurements of other physical quantities, such as vibration, acceleration, or acoustic wave detection.

[0117] The physical quantity to be measured may be a physical quantity other than temperature, longitudinal deformation, or hydrostatic pressure. All that is required is that the optical transducer be sensitive to this other physical quantity. For example, the optical transducer may be sensitive to a dose of radiation. By way of example, the core of the optical fiber 14 is made of a photosensitive material. Here, the core is made of germanosilicate. First, a Bragg grating is generated in the core of the optical fiber 14. This Bragg grating is then converted into a Bragg grating sensitive to the radiation dose to be measured. To do this, the fabricated Bragg grating is exposed to ultraviolet light to generate colored cores resulting from the recombination of bonds between germanium and silica. When subjected to the radiation dose to be measured, these colored cores change, resulting in a change in the wavelength λ of the Bragg grating of the optical transducer. Bm4 shifts.

[0118] Alternatively, the optical transducer comprises successive Bragg gratings generated one after the other in the core of the same optical fiber, in which case the wavelength λ of each of these Bragg gratings is preferably Bi4 are different. This allows the same optical transducer to measure physical quantities at different locations. In this embodiment, the power spectrum of the optical transducer includes several power peaks within its operating range.

[0119] In another variant, the measuring device includes several optical transducers optically connected in parallel to the spectrum analyzer. Such a configuration of multiple optical transducers is exemplified, for example, in Chinese Patent Application Publication No. 102879022.

[0120] Alternatively, the optical transducer is not integrated into the optical fiber 14, but is simply optically connected to the distal end of the optical fiber 14. For example, the optical transducer is a Fabry-Perot cavity formed between two reflective mirrors, which are fabricated outside the optical fiber 14.

[0121] Spectrum analyzer variants:

[0122] Alternatively, the light source is not tunable. For example, the light source may be a broad laser light source, i.e., a laser light source that emits an optical signal whose power spectrum simultaneously covers the entire operating range. In this case, the emitted optical signal is not at a single frequency. Furthermore, for each spectral response measured, the spectrum analyzer includes multiple photodetectors that simultaneously measure the power of the spectral response for a number of different wavelengths. For example, in this case, each photosensor 62, 64 is an array spectrometer. In such an embodiment, the wavelength λ is used to scan the entire operating range. s There is no need to change . Then the estimation of the laser source transfer function can be omitted.

[0123] The light source does not necessarily have to be a laser source. For example, the light source may be a tunable Fabry-Perot cavity. In this case, the control signal causes a displacement of at least one optical interface of this Fabry-Perot cavity. The displacement of this optical interface then causes a change in the natural resonant frequency of the cavity and thus the wavelength λ. s causes changes in

[0124] Other configurations of the unit 70 are possible for determining the amplitude Δλ. In particular, the amplitude Δλ is determined by the time difference t m -t iFor example, the amplitude Δλ can alternatively be determined without using the wavelength λ Bi4 and measurement wavelength λ Bm4 This method is described, for example, in the "Absolute Frequency Measurement" section of WO 2020 / 113147.

[0125] In another variation, the estimated transfer function of the laser source 50 is used to calculate the wavelength λ Bm4 to obtain a more realistic correction wavelength. For example, to do this, the correction wavelength λ Bm4 is calculated using the following relationship: Bm4 =[(λ k+1 -λ k ) / (t k+1 , m -t k,m )]*(t m -t k,m )+λ k -where λ k and t k,m are respectively, t k,m is the moment t measured in step 134 m The point (λ) of the transfer function estimated in step 140 immediately preceding k ;t k,m ) are the abscissa and ordinate of -λ k+1 and t k+1,m are respectively, t k+1, m The point (λ) of the transfer function estimated in step 140 is immediately after the instant tm. k+1 ;t k+1、m ) are the abscissa and ordinate of In this variant, two consecutive points (λ k ;t k、m ) and (λ k+1 ;t k+1,m ) is linearly interpolated between the two points. However, non-linear interpolation of the transfer function between the two points is also possible.

[0126] In the above case, standard 6 is the wavelength λ of the light source. sis not used to linearize the change in

[0127] In a simplified variant, the unit 70 determines only the change in the measured physical quantity, but not its absolute value. In this case, the wavelength λ Bi4 It is not necessary to know the value of the measured physical quantity corresponding to

[0128] In another embodiment, the spectral response of the standard 6 is measured first, and only thereafter, the spectral response of the optical transducer 4 is measured. In this case, the optical switch is first placed in a calibration position, where the spectrum analyzer 20 is optically connected only to the standard 6 to measure the spectral response of the standard. The optical switch is then switched to a measurement position, where the spectrum analyzer 20 is optically connected only to the optical transducer 4 to measure the spectral response of the optical transducer 4. Typically, the spectral response of the standard 6 is then measured only intermittently, not every time the spectral response of the optical transducer 4 is measured. In this embodiment, the optical signal illuminating the standard 6 is not necessarily exactly the same as the optical signal illuminating the optical transducer 4, because they are emitted at two different times.

[0129] Alternatively, at least one, and preferably both, optical sensors are connected to the distal ends of the optical fibers 14 and 34. In this case, the spectrum analyzer 20 measures the optical signal that has passed through the optical transducer 4 and the standard 6. As a result, the power spectrum of the measured signal is a transmitted power spectrum, not a reflected power spectrum. However, everything that has been described for the specific case of a reflected power spectrum can be adapted to the case of a transmitted power spectrum without any particular difficulty.

[0130] The optical couplers 12, 32 and 52 can be replaced by a single multi-channel optical coupler, which performs the functions of all three optical couplers 12, 32 and 52.

[0131] Manufacturing method variations:

[0132] There are many variants of methods for producing very high order Bragg gratings, in particular all of the methods and variants described in the French patent application FR 2207936 filed on July 29, 2022 by the applicant can be used to produce the grating 80. The method described in the above-mentioned paper LUO2022 can also be used.

[0133] Other variations:

[0134] The waveguides do not necessarily have to be optical fibers. Everything stated in this text in the specific case of optical fibers also applies when the waveguides are realized on a photonic chip. For example, in the latter case, the core of each waveguide may be made of single-crystal silicon or other semiconductor material, and the cladding may be made of a material commonly used in silicon optics, such as silicon oxide.

[0135] k c Next peak wavelength λ c All that has been stated above for the specific case where λ is between 200 nm and 5000 nm c is between 5000 nm and 10000 nm, especially when the wavelength λ c This also applies when the wavelength λ is in the infrared region. c If the wavelength is in the infrared region, the core of the optical fiber is made of, for example, chalcogenide glass.

[0136] Some of the above-mentioned variations may be combined in a single embodiment.

[0137] Section 3: Advantages of the Described Embodiments

[0138] The very high order Bragg gratings make it possible to obtain a comb of peaks using a single Bragg grating rather than a series of several Bragg gratings as described in CN Patent No. 102879022. Therefore, compared with the standard described in CN Patent No. 102879022, the standard described in the previous chapters is simpler and less cumbersome to manufacture.

[0139] Furthermore, very high-order Bragg gratings produce combs of peaks identical to those obtained using a Fabry-Perot cavity such as that described in WO 2020 / 113147. On the other hand, for the same performance, very high-order Bragg gratings are easier to fabricate and require less space.

[0140] The small footprint of high-order Bragg gratings not only reduces the size of the measurement equipment, but also makes it easier to build insulating structures. Indeed, it is much easier to thermally isolate high-order Bragg gratings less than 10 cm in length compared to Fabry-Perot cavities several meters long.

[0141] Therefore, a measurement device with a standard fabricated using a very high order Bragg grating would be simpler to manufacture for the same performance.

[0142] Using the spectral response of the standard 6 to obtain the linear variation of the wavelength emitted by the laser source 50 over the entire operating range simplifies the construction of the optical sensors 62, 64 and therefore the construction of the measurement device.

[0143] By simultaneously measuring the spectral responses of the optical transducer 4 and the standard 6 as they interact with the same optical signal, it is ensured that the measured spectral responses are in fact those obtained in response to the same optical signal, thereby improving the accuracy of the measurement system.

[0144] The standard 150 isB152 The fact that also includes a second Bragg grating 152 within its operating range makes it possible to identify the absolute value of the wavelength associated with each peak of the comb of the grating 80. Thus, the absolute value of the wavelength at which the power peak of the optical transducer 4 occurs can be measured, and thus traced back to the absolute value of the measured physical quantity.

[0145] The fact that the standard is made of optical fiber simplifies the manufacture of the measurement device.

[0146] Each pattern M i The use of one or more bubbles in the pattern results in a smaller pattern and therefore a substantial reduction in insertion loss.

[0147] Using several isolated bubbles, we can create a pattern M that is reflective enough to reduce the number of patterns p. i Thus, the compactness of the grating 80 can be maintained while limiting insertion loss. Indeed, when bubbles overlap, the overlapping regions between some bubbles are irradiated with several successive femtosecond laser pulses. It has been observed that the regions of the optical fiber core irradiated with several femtosecond laser pulses are degraded. This degradation increases the diffusion loss. Conversely, when the bubbles are separated, no such overlapping regions exist, and insertion loss can be reduced.

Claims

1. 1. An apparatus for measuring a physical quantity, comprising: an optical transducer (4) whose power spectrum has at least one power peak whose position varies as a function of the physical quantity to be measured while remaining within a predetermined operating range, said predetermined operating range being a wavelength range, said operating range being between 200 nm and 10,000 nm; a standard (6;150) whose power spectrum in reflection contains several power peaks distributed within the working range, and whose free spectral range is 5 nm or less; a spectrum analyzer (20); The standard device is a first waveguide (34) including a core (110) extending along a longitudinal axis, such that an optical signal guided by the first waveguide can propagate along the longitudinal axis of the first waveguide; A first Bragg grating (80) is created in the core of the first waveguide, said first Bragg grating having at least three identical patterns (M １ , M ２ , M Ｎ－１ , M Ｎ the first Bragg grating comprising: an insulating structure (82) capable of isolating the first Bragg grating from fluctuations in temperature and mechanical stresses imposed on the standard by the external environment; The spectrum analyzer (20) The spectral response of the optical transducer within its operating range and the spectral response of the standard within the same operating range are measured separately, and then determining the amplitude of the shift of the peak of the optical transducer from the spectral responses of the optical transducer and the standard measured separately from each other; determining a variation in a physical quantity from the determined amplitude of the peak shift of the optical transducer; the first spacing of the first Bragg grating (80) is configured such that the power spectrum of the first Bragg grating has several distinguishable harmonics on the order of greater than 100 in an operating range; the harmonic forms a power peak in the power spectrum of the standard at a known wavelength. Device.

2. the spectrum analyzer (20) includes a tunable light source (50) capable of emitting a single-frequency optical signal that interacts with the optical transducer and the standard, the light source being tunable by a control signal to vary the wavelength of the emitted optical signal, the wavelength of the emitted optical signal being related to the control signal by a nonlinear transfer function; The spectrum analyzer (20) estimating a nonlinear transfer function of a tunable light source from the measured spectral response of the standard and the known wavelengths at which power peaks of the power spectrum of the standard occur over an operating range; and constructing a control signal from the estimated transfer function that provides a more linear change in wavelength of the emitted optical signal as a function of time over the entire operating range; and also configured to control the light source using the constructed control signal.

10. The apparatus of claim 1.

3. The spectrum analyzer a laser light source (50) having an output port (54) from which an optical signal is emitted; an optical coupler (52) for simultaneously optically connecting the optical transducer and the standard to the output port; the spectrum analyzer is capable of simultaneously measuring the spectral responses of the optical transducer and the standard in response to an optical signal emitted at the output port (54); 3. The device according to claim 1 or 2.

4. the first waveguide (34) is an optical fiber; The device according to any one of claims 1 to 3.

5. Each pattern (M １ , M ２ , M Ｎ－１ , M Ｎ ) extends primarily in a plane perpendicular to the longitudinal axis of the first waveguide, referred to as the "pattern plane"; Each pattern is made up of one or more bubbles (B) arranged side by side in the plane of the pattern. １ -B ６ ) the area of ​​the orthogonal projection of all bubbles of the pattern onto the pattern plane is less than 50% of the cross-sectional area of ​​the core (110) of the first waveguide; The device according to any one of claims 1 to 4.

6. Each pattern comprises a plurality of discontinuous cells (B) arranged side by side in the plane of the pattern. １ -B ６ ) consisting of 8. The apparatus of claim 7.

7. the first interval is 20 μm or more, The difference between the refractive index of the core (110) of the first waveguide and the refractive index of each pattern of the first Bragg grating is greater than 0.3; The device according to any one of claims 1 to 6.

8. Each pattern (M １ , M ２ , M Ｎ－１ , M Ｎ ) is generated using femtosecond laser pulses, The device according to any one of claims 1 to 7.

9. The physical quantity is selected from the group consisting of temperature, mechanical deformation, and hydrostatic pressure. An apparatus according to any one of claims 1 to 8.

10. the operating range is between 200 nm and 5000 nm; An apparatus according to any one of claims 1 to 9.

11. A standard for realizing the device according to any one of claims 1 to 10, comprising: The reflection power spectrum of the standard includes several power peaks distributed within a predetermined operating range, the free spectral range of the standard is 5 nm or less, the predetermined operating range is a wavelength range, the operating range is between 200 nm and 10,000 nm, and the standard is a first waveguide (34) extending along a longitudinal axis and including a core (110) through which an optical signal guided by the waveguide can propagate along the longitudinal axis of the waveguide, the waveguide being configured to be optically coupled to the spectrum analyzer; a first Bragg grating (80) formed in the core of the first waveguide, said first Bragg grating including at least three identical patterns arranged behind each other along a longitudinal axis of the first waveguide and separated from each other by a first regular interval; an insulating structure (82) capable of isolating the first Bragg grating from temperature fluctuations and mechanical stress fluctuations imposed on the standard by the external environment, the insulating structure comprising: Temperature sensor (90) an adjustable heating or cooling element (92) for heating or cooling the first waveguide; and a microcontroller (94) configured to control the heating element as a function of a temperature setpoint and a temperature measured by the sensor to limit temperature variation of the first waveguide about said temperature setpoint. the first spacing of the first Bragg grating (80) is configured such that the power spectrum of the first Bragg grating has several distinguishable harmonics on the order of greater than 100 in the operating range, and these harmonics therefore form power peaks in the power spectrum of the standard at known wavelengths; Standard instrument.

12. a second Bragg grating (152) formed in the core of the first waveguide; the second Bragg grating includes at least three identical patterns arranged behind each other along the longitudinal axis of the first waveguide and separated from each other by a second constant spacing; the second spacing is configured such that the wavelength of the fundamental resonant frequency of the second Bragg grating is within an operating range. The standard according to claim 10.

Images