JP2012226016A - Optical fiber for sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber for a sensor, which achieves extension of a range of a measurable physical quantity at a low cost.SOLUTION: An FBG 20 having a plurality of reflecting regions 21 to 2n is formed in a core 11 of an optical fiber 10 for a sensor, and each of the reflecting regions 21 to 2n includes a plurality of grating parts 30 formed in the core 11. Grating periods Λ1 to Λn being intervals of grating parts 30 are different from one another in each of the reflecting regions 21 to 2n, and reflection spectra of respective reflecting regions 21 to 2n have parabolic shapes different from one another. Among reflection spectra of respective reflecting regions 21 to 2n, reflection spectra adjacent to each other in a wavelength direction overlap with each other. Therefore, a reflection spectrum 20 of the entire FBG 20 has reflection intensity gradually increased from the short wavelength side toward the long wavelength side to have a substantially triangular shape.

Description

  The present invention relates to a sensor optical fiber used for a sensor for measuring a physical quantity, and more particularly to a sensor optical fiber having an FBG that reflects light of a specific wavelength with respect to incident light.

  In recent years, for example, an optical fiber having an FBG (fiber Bragg grating) is used as a sensor for measuring a physical quantity such as a temperature or a strain amount of a measured part. The FBG is a diffraction grating in which the refractive index of the core of the optical fiber is changed at a predetermined length period (grating period). The FBG has a specific wavelength (Bragg) corresponding to the grating period with respect to the incident light to the optical fiber. (Wavelength) light is reflected and the remaining light is transmitted. When the FBG expands / contracts according to the physical quantity applied from the part to be measured, the grating period also changes accordingly. Since the Bragg wavelength changes linearly with respect to the change amount of the grating period, the physical quantity applied to the FBG can be measured based on the change amount of the Bragg wavelength.

  Here, a method for measuring a physical quantity using a general FBG will be schematically described with reference to FIG. A parabolic region denoted by reference numeral 100 in FIG. 5 represents the reflection spectrum of the FBG before the physical quantity to be measured is applied. A region denoted by reference numeral 200 indicates a spectrum of incident light incident on the optical fiber from the short wavelength light source, and the FBG reflects this incident light with an intensity S10. When a physical quantity for extending the FBG is applied from the above state, the reflection spectrum of the FBG shifts to the long wavelength side as indicated by the dashed-dotted region 110, and the intensity of the reflected light changes from S10 to S20. Therefore, the physical quantity applied to the FBG is relatively obtained based on the difference between the reflection intensities S10 and S20.

  However, the range of physical quantities that can be measured with a general FBG made of glass is not so wide. For example, when the physical quantity is temperature, the temperature range that can be measured for a reflection spectrum shift of 0.4 nm is about 50 ° C. Become. That is, when the change width of the temperature of the measured part is wider, the FBG reflection spectrum shifts to a position deviating from the incident light spectrum 200 as in the region 120 indicated by the two-dot chain line in FIG. In this case, there is a problem that the temperature cannot be measured because reflection on the FBG does not occur. One means for avoiding such a problem is to shift the wavelength of incident light in accordance with the shift of the reflection spectrum of the FBG. For example, Patent Document 1 discloses that the temperature of the outer panel of an aircraft is FBG. It is described that a wavelength tunable laser is used as a light source of a temperature measurement system to be measured.

  Another means for avoiding the above problem is to widen the reflection band of the FBG. For example, Patent Document 2 discloses a CFBG (chirped fiber Bragg grating) having a reflection band wider than that of a general FBG. Are listed. The CFBG is formed so that the grating period gradually increases while a general FBG is formed with a constant grating period, thereby giving the Bragg wavelength a predetermined bandwidth. ing. Since the reflection spectrum of the CFBG has a substantially trapezoidal shape extending in the wavelength direction because the Bragg wavelength has a predetermined bandwidth, the reflection band is wider than a general FBG whose reflection spectrum is parabolic.

Special table 2009-516855 gazette JP 2003-322736 A

  As described above, when a wavelength tunable laser is used as a light source as in the temperature measurement system described in Patent Document 1, it is possible to perform measurement using a general FBG even if the change amount of the physical quantity is wide. . However, since the wavelength tunable laser is an expensive device as compared with a short wavelength light source such as an LED, for example, there is a problem that the cost required for expanding the range of the physical quantity that can be measured increases.

  The CFBG described in Patent Document 2 has a wider reflection band than a general FBG, but its reflection spectrum has a substantially trapezoidal shape with a constant peak value of reflection intensity. In other words, the CFBG described in Patent Document 2 does not change the reflection intensity with the shift of the reflection spectrum. Therefore, for the sensor application in which the physical quantity is relatively measured from the difference in the reflection intensity before and after the application of the physical quantity. It had a problem that it could not be used.

  The present invention has been made to solve these problems, and an object of the present invention is to provide an optical fiber for a sensor that can extend the range of measurable physical quantities at low cost.

  An optical fiber for sensor according to the present invention is an optical fiber for sensor comprising a core through which incident light propagates and an FBG in which the refractive index of the core is periodically changed along the direction in which incident light propagates. Has a plurality of adjacent reflection regions along the direction in which incident light propagates, and the plurality of reflection regions change the refractive index of the core at different periods, and the reflection spectrum of the plurality of reflection regions. Of these, reflection spectra that are adjacent to each other in the wavelength direction overlap at least partially.

  The FBG is divided into a plurality of reflection areas, and these reflection areas are configured to change the refractive index of the core at different periods, so that the reflection spectra of the reflection areas are different from each other, and these are assembled. Becomes the reflection spectrum of the entire FBG. The reflection spectrum of each reflection region is substantially parabolic with the wavelength corresponding to the period of the refractive index change of the core as the center wavelength and the reflection intensity at the center wavelength as the peak value. Here, when the period of the refractive index change of the core is shortened, it is common that the bandwidth of the reflection spectrum is widened and the peak value of the reflection intensity is low. On the contrary, when the period of refractive index change is lengthened, the bandwidth of the reflection spectrum is narrowed and the peak value of the reflection intensity is increased. Further, the center wavelength of the reflection spectrum becomes longer as the refractive index change period becomes longer.

  Among the reflection spectra of each reflection region, reflection spectra that are adjacent to each other in the wavelength direction overlap at least partially, so that the reflection spectrum of the entire FBG is substantially increased in reflection intensity from the short wavelength side toward the long wavelength side. It will be triangular. That is, since the reflection intensity with respect to incident light changes before and after application of the physical quantity, the physical quantity can be measured based on the difference in reflection intensity. In addition, since the central wavelengths of the reflection spectrum of each reflection region are different from each other, the reflection band of the reflection spectrum of the entire FBG is expanded, that is, the change range of the measurable physical quantity is expanded. Temperature measurement can be performed using a short wavelength light source. Therefore, in the sensor optical fiber, the measurable physical quantity range can be expanded at low cost.

  According to this invention, in the optical fiber for sensors, it is possible to expand the range of measurable physical quantities at low cost.

It is the schematic which shows the structure of the optical fiber for sensors which concerns on Embodiment 1 of this invention. 3 is a partially enlarged view schematically showing the configuration of the FBG in the sensor optical fiber according to Embodiment 1. FIG. 4 is a graph illustrating an FBG reflection spectrum in the sensor optical fiber according to the first embodiment; 4 is a graph for explaining a physical quantity measurement method using the sensor optical fiber according to the first embodiment. It is a graph for demonstrating the measuring method of the physical quantity using the conventional FBG.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
Embodiment 1 FIG.
First, the configuration of the optical fiber for sensor according to the first embodiment will be described by taking as an example a case where it is applied to a temperature measurement sensor of a measured part.
As schematically shown in FIG. 1, the sensor optical fiber 10 includes a core 11 through which incident light L <b> 1 emitted from a light source (not shown) connected to one end thereof is propagated, and a cladding 12 that covers the outer periphery of the core 11. I have. The sensor optical fiber 10 includes an FBG 20 that is a diffraction grating in which the refractive index of the core 11 is periodically changed along the direction in which the incident light L1 propagates. The FBG 20 is a part provided in a measurement target (not shown), and reflects light having a wavelength corresponding to the temperature applied from the measurement target as reflected light L2 with respect to the incident light L1, and the remaining light as transmitted light L3. It has the property of transmitting. The FBG 20 has n reflection regions 21, 22, 23,..., 2n formed so as to be adjacent to each other along the direction in which the incident light L1 propagates.

  As shown in FIG. 2, the FBG 20 is a part in which a plurality of lattice portions 30 having a refractive index different from the refractive index of the core 11 are formed with a predetermined length cycle along the axial direction of the core 11. That is, the periodic change in the refractive index of the core 11 in the FBG 20 is caused by forming the plurality of grating portions 30. Here, although the number of the plurality of grating portions 30 included in each of the reflection regions 21 to 2n is the same, the interval between the lattice portions 30 adjacent to each other in these regions, that is, the period of the refractive index change in the core 11. (Grating period) is different for each of the reflection regions 21 to 2n.

  More specifically, the grating portion 30 is formed in the reflective region 21 located on the most upstream side in the direction in which the incident light L1 propagates so that the grating period is Λ1. Further, in the reflection region 22 adjacent to the downstream side of the reflection region 21, the grating portion 30 is formed so as to have a grating period Λ2 longer than the grating period Λ1 of the reflection region 21. Similarly, the grating period after the reflection area 23 (see FIG. 1) is also gradually increased from the upstream side toward the downstream side, and the grating period Λn in the reflection area 2n located on the most downstream side is the longest. .

  The wavelength (Bragg wavelength) of the reflected light L2 that the FBG 20 reflects with respect to the incident light L1 changes according to the grating period. Here, the reflection spectrum of the FBG having a constant grating period is substantially parabolic with the Bragg wavelength as the center wavelength and the reflection intensity at the Bragg wavelength as the peak value. Generally, when the grating period of the FBG is shortened, the bandwidth of the reflection spectrum is widened and the peak value of the reflection intensity is lowered. Conversely, when the grating period is lengthened, the bandwidth of the reflection spectrum is narrowed and the peak value of the reflection intensity is increased. The Bragg wavelength, which is the center wavelength of the reflection spectrum, becomes longer as the grating period becomes longer.

  That is, the reflection regions 21 to 2n of the FBG 20 have a parabolic reflection spectrum because their grating periods Λ1 to Λn are constant. Further, the reflection spectra of the reflection regions 21 to 2n have different peak values of the center wavelength, the bandwidth, and the reflection intensity because their grating periods Λ1 to Λn are different from each other. Referring to FIG. 3, the reflection spectrum 41 of the reflection region 21 formed with the shortest grating period Λ1 has the center wavelength λ1 on the shortest wavelength side, and has the widest bandwidth and the lowest peak value. It will have.

  Further, since the reflection region 22 adjacent to the downstream side of the reflection region 21 is formed with a grating period Λ2 longer than the grating period Λ1 of the reflection region 21, the center wavelength λ2 of the reflection spectrum 42 is the center wavelength of the reflection spectrum 41. Longer wavelength side than λ1. Further, the reflection spectrum 42 has a narrower bandwidth and a higher peak value than the reflection spectrum 41. The same applies to the subsequent reflection regions 23 to 2n, and the peak value of the center wavelength, bandwidth, and reflection intensity of the reflection spectrum sequentially changes as the grating period becomes longer. The reflection spectrum 4n of the reflection region 2n formed with the longest grating period Λn has the center wavelength λn on the longest wavelength side, and has the narrowest bandwidth and the highest peak value.

  Here, as shown in FIG. 3, the grating periods Λ1 to Λn of the reflection regions 21 to 2n are set so that reflection spectra adjacent to each other in the wavelength direction overlap at least partially. Therefore, the reflection spectrum 40 of the entire FBG 20 that is a collection of the reflection spectra 41 to 4n has a substantially triangular shape in which the reflection intensity gradually increases from the short wavelength side toward the long wavelength side. Further, since the center wavelengths λ1 to λn of the respective reflection spectra 41 to 4n are different from each other, the reflection spectrum 40 of the entire FBG 20 is in a state of spreading in the wavelength direction, that is, a state in which the reflection band is widened.

  The sensor optical fiber 10 is made of a material such as quartz glass, and has a positive coefficient of thermal expansion. Further, as an example, each lattice unit 30 is formed by irradiating the core 11 of the sensor optical fiber 10 with ultraviolet rays or the like.

Next, a method of measuring the temperature of the part to be measured using the sensor optical fiber 10 according to Embodiment 1 of the present invention will be described.
First, the FBG 20 shown in FIG. 1 is provided in a part to be measured which is a target of temperature measurement. Next, on one end side of the sensor optical fiber 10, a short wavelength light source (not shown) for making the incident light L1 incident on the sensor optical fiber 10, and a measuring instrument (not shown) for monitoring the reflected light L2 from the FBG 20 are provided. Is connected. For example, an LED or the like is used as the short wavelength light source that emits the incident light L1.

  When the incident light L1 is incident on the sensor optical fiber 10 at a normal time before the temperature change occurs in the measured part, the FBG 20 reflects the reflected light L2 corresponding to the temperature at the normal time with respect to the incident light L1. The reflected light L2 is monitored by a measuring instrument (not shown). Here, since a short wavelength light source such as an LED is used as the light source of the incident light L1, as shown in FIG. 4, the bandwidth of the spectrum 50 of the incident light L1 is the bandwidth of the reflection spectrum 40 of the entire FBG 20. Narrower. That is, the FBG 20 in the steady state reflects the incident light L1 having a short wavelength with the reflection intensity S1.

  When the temperature of the part to be measured rises from the steady state, the FBG 20 having a positive coefficient of thermal expansion expands in accordance with the amount of temperature rise of the part to be measured. Therefore, the grating period in each of the reflection regions 21 to 2n of the FBG 20 Λ1 to Λn (see FIG. 2) also become longer. As the grating periods Λ1 to Λn become longer, the Bragg wavelengths λ1 to λn of the reflection regions 21 to 2n shift to the longer wavelength side, respectively. That is, the reflection spectrum 40 of the entire FBG 20 is shifted to the long wavelength side as indicated by reference numeral 40 ′ in FIG. 4.

  Here, the reflection spectrum 40 of the entire FBG 20 is a collection of the reflection spectra 41 to 4n (see FIG. 3) of the respective reflection regions 21 to 2n, and the reflection intensity gradually increases from the short wavelength side toward the long wavelength side. It is a generally triangular shape that rises. Therefore, the intensity of the reflected light L2 changes from S1 to S2 when shifted to the reflection spectrum 40 'on the long wavelength side as the temperature of the measured part increases. The measuring instrument connected to the sensor optical fiber 10 can measure the intensity of the reflected light from the FBG 20, and the temperature of the part to be measured is calculated based on the difference between the reflection intensities S1 and S2.

  As described above, the FBG 20 is divided into a plurality of reflection regions 21 to 2n, and these reflection regions 21 to 2n are configured to change the refractive index of the core 11 with different grating periods Λ1 to Λn. The reflection spectrums 41 to 4n of the reflection regions 21 to 2n are different from each other, and a collection of these becomes the reflection spectrum 40 of the entire FBG 20. Of the reflection spectra 41 to 4n of the reflection regions 21 to 2n, the reflection spectra adjacent to each other in the wavelength direction overlap at least partially, so that the reflection spectrum 40 of the entire FBG 20 has a reflection intensity from the short wavelength side to the long wavelength side. It becomes the thing of the substantially triangular shape which becomes high gradually toward.

  That is, since the reflection intensity with respect to the incident light L1 changes before and after application of temperature, the temperature can be measured based on the difference in reflection intensity. In addition, since the center wavelengths λ1 to λn of the reflection spectra 41 to 4n of the reflection regions 21 to 2n are different from each other, the reflection band of the reflection spectrum 40 of the entire FBG 20 is widened, that is, the change width of the measurable temperature. Therefore, temperature measurement can be performed using an inexpensive short wavelength light source such as an LED. Therefore, in the sensor optical fiber 10, the measurable physical quantity range can be expanded at low cost.

  10 sensor optical fiber, 11 core, 20 FBG, 21 to 2n reflection region, 30 grating part, 41 to 4n reflection region reflection spectrum, L1 incident light, Λ1 to Λn grating period (period of refractive index change of core).

Claims (4)

A core through which incident light propagates;
In an optical fiber for a sensor comprising an FBG in which the refractive index of the core is periodically changed along the direction in which the incident light propagates,
The FBG has a plurality of reflection regions adjacent along the direction in which the incident light propagates,
The plurality of reflective regions change the refractive index of the core at different periods,
Of the plurality of reflection regions, the reflection spectra adjacent to each other in the wavelength direction overlap at least partially.   2. The optical fiber for a sensor according to claim 1, wherein the refractive index of the core is changed by forming a plurality of grating portions having a refractive index different from the refractive index of the core in the core.   The optical fiber for a sensor according to claim 2, wherein the plurality of reflection regions respectively include the plurality of lattice portions that are the same number.   4. The optical fiber for a sensor according to claim 1, wherein a period of a refractive index in each of the plurality of reflection regions is sequentially increased from an upstream side to a downstream side in a direction in which the incident light propagates.

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