JPH1164119A - Optical fiber thermal distorsion sensor and thermal distorsion measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber thermal distorsion sensor good in both measurement accuracy and sensitivity and capable of measuring simultaneously temperature and distorsion. SOLUTION: A sensing part 6 is made up by forming a reflection-type grating on a core of a PANDA-type polarization-plane maintaining optical fiber, measurement light being sent from a light source 9 thereto, and reflected light from the sensing part 6 is received by a photodetector 10. The sensing part 6 measures an amount of change in the central wavelength, caused by temperature and distorsion, of each of reflected light having two polarization modes normal to each other, and therefrom temperature and distorsion acting on the sensing part 6 are found by computation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber sensor, which is capable of simultaneously measuring temperature and strain.

[0002]

2. Description of the Related Art As an optical fiber sensor capable of simultaneously measuring temperature and strain, a sensor using a single-mode fiber core formed with a long-period grating having a grating period of 150 to 400 μm as a sensing unit has been announced. (V. Bhatia et a
l. , Conference Proceedings
of OFS-11, paper Fr2-5, p70
2, 1996).

In this sensor, light of a certain wavelength determined by the period of the long-period grating and the effective refractive indexes of the core propagation mode and the cladding propagation mode is coupled and converted from the core propagation mode to the cladding propagation mode. Since a loss occurs and a large number of cladding propagation modes exist, a loss occurs at many wavelengths other than the wavelength. And since these loss peak wavelengths have temperature dependency and strain dependency, by measuring the change of these loss peak wavelengths by temperature and strain,
Temperature and strain can be measured.

However, in this sensor, the temperature dependence of the loss peak wavelength is small, and the sensor is not sensitive to temperature changes.
There is a disadvantage of low sensitivity. Further, since the band of the loss peak wavelength is wide, there is a disadvantage that the measurement accuracy of the center wavelength is insufficient and the measurement accuracy of the temperature distortion is not good.

[0005]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical fiber temperature distortion sensor having good measurement accuracy and sensitivity and capable of simultaneously measuring temperature and strain.

[0006]

An object of the present invention is to provide a sensing part in which one or more reflective gratings are formed in the core of a polarization-maintaining optical fiber, and the reflected light of two orthogonal polarization modes in the sensing part. Alternatively, the problem is solved by measuring the amount of change in the center wavelength of the transmitted light due to temperature and strain and measuring the temperature and strain acting on the sensing unit. It is desirable that the mode birefringence of the polarization-maintaining single-mode fiber serving as the sensing unit be 3 × 10 ⁻⁴ or more.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
FIG. 1 schematically shows a sensing unit of an optical fiber temperature distortion sensor according to the present invention. In FIG. 1, reference numeral 1 denotes a PANDA type polarization maintaining optical fiber. The PANDA type polarization-maintaining single-mode fiber 1 has a center core 2 and a pair of stress applying portions 3 symmetrically provided at positions slightly apart from the core 2 and opposed to each other with the core 2 as a center. 3 and these cores 2
And the cladding 4 surrounding the stress applying portions 3 and 3, and preferably has a mode birefringence of 3 × 10 ⁻⁴ or more. If the mode birefringence is less than this value, the difference between the center wavelengths of two reflected lights, which will be described later, will be 0.3 nm or less, and it will be difficult for the photodetector to detect the difference.

This PANDA type polarization maintaining optical fiber 1
The reflective grating 5 is formed in a portion of the core 2 where the center length is about 10 to 20 mm. The grating 5 irradiates the core 2 with, for example, excimer laser light, and forms a constant-period change in the refractive index in the longitudinal direction of the optical fiber by the photorefractive effect. The period Λ is about 0.1 to 1.0 μm. The PANDA-type polarization maintaining optical fiber 1 in which the grating 5 is formed on the core 2 serves as the sensing unit 6.

As shown in FIG. 2, one end of a sensing section 6 is connected to one end of a waveguide fiber 7 for guiding measurement light, and the other end of the waveguide fiber 7 is connected to an optical circulator 8. It is connected. The optical circulator 8 is
The waveguide fiber 7 is connected to the light source 9 and the photodetector 10, respectively, to provide a temperature distortion measuring device of this example. A light source that emits unpolarized light is used as the light source 9. An LD having a narrow emission wavelength is not suitable for a light source, and it is desirable to use light of a certain wavelength range (wavelength range capable of covering a wavelength change due to temperature or strain change described later) (hereinafter referred to as broadband light) with a stable power. A light source that emits light, for example, an LED (light emitting diode), a halogen lamp, an ASE (Amplified spontaneous emission) light source, or the like is used. The photodetector 10 has a wavelength resolution of 0.
An optical spectrum analyzer of about 01 nm is used. As the waveguide fiber 7, a general single mode fiber or the like is used.

Next, the measurement principle of the sensor having such a configuration will be described. In the PANDA polarization-maintaining optical fiber 1, the mode of light guided to the core 2 is a polarization mode LP ₀₁ ^X guided along a plane connecting the center points of the pair of stress applying parts 3, 3. Mode (hereinafter referred to as s mode)
And a polarization mode LP ₀₁ ^y mode (hereinafter, referred to as f mode) guided along a plane orthogonal to the above plane. A difference in propagation constant (Δβ = βs−βf) is formed between these two polarization modes, and each polarization mode has a different effective refractive index Nf, Ns corresponding thereto. .

Generally, light incident on a reflection type grating is reflected when the condition of λ = 2N を 満 た す is satisfied.
Here, λ is the center wavelength of the reflected light, N is the effective refractive index, and Λ is the grating period. Therefore, two center wavelengths of the light (polarized light) reflected by the reflection type grating 5 formed in the PANDA type polarization maintaining optical fiber 1 exist in correspondence with the two effective refractive indices Nf and Ns. become,
These are λf and λs. FIG. 3 shows an example of the spectrum of the f-mode and s-mode reflected light reflected from the sensing unit 6, where two sharp reflection peaks are clearly separated, and the short-wavelength side peak is λf , The peak on the long wavelength side becomes λs.

Generally, the refractive index of glass has temperature dependence. Further, since the propagation constant is determined by the waveguide structure of the fiber and the refractive indices of the core and the cladding, the effective refractive index N
f and Ns have different temperature dependences, and therefore, the center wavelengths λf and λs of the reflected light also have different temperature dependences. When strain is applied in the longitudinal direction of the sensing unit 6, the strain expands and contracts, and the grating period Λ changes. In addition, the effective refractive indices Nf, Nf
s also changes. For this reason, the center wavelengths λf, λs of the reflected light
Have different distortion dependencies.

By utilizing this phenomenon to measure the change in the center wavelengths λf and λs of the f-mode and s-mode light reflected from the grating 5 of the sensing unit 6, the temperature dependence and distortion of the grating 5 are measured. By separating the dependence, the temperature and strain acting on the sensing unit 6 can be measured simultaneously.

The rate of change of the center wavelengths λs and λf of the reflected light in the s-mode and f-mode with respect to temperature and the rate of change with respect to strain are ∂λs / ∂T, ∂λs / ∂ε, and ∂λf /
Since ∂T and ∂λf / ∂ε are expressed, the changes Δλs and Δλf of the reflection wavelengths of the s-mode and the f-mode with respect to the temperature change ΔT and the strain change Δε are expressed by the following equation (1).

[0015]

(Equation 1)

The temperature change ΔT and the strain change Δε are
Equation (1) is solved for ΔT and Δε, and is given by the following equation (2).

[0017]

(Equation 2)

In the above equation (2), ∂λs / ∂T, ∂
For λs / ∂ε, ∂λf / ∂T, and ∂λf / ∂ε,
A known temperature change and strain change are separately given to the sensing unit 6 in advance, and λs and λf obtained at that time are measured.
I can find it in the future. FIG. 4 shows λs, λf
Is an example of the temperature dependence of A, where A is λs and B is λs.
The temperature dependence of f is shown. FIG. 5 shows an example of the tension dependency as a specific example of the strain dependency of λs and λf.
Indicates the dependence of λs on the tension, and B indicates the dependence of λf on the tension. The tension F here is a pulling force acting in the longitudinal direction of the fiber 1.

In FIGS. 4 and 5, A to D are represented by substantially straight lines, and from the inclination thereof, the above-mentioned ∂λs / ∂T, ∂
When λs / ∂F, ∂λf / ∂T, and ∂λf / ∂F are obtained and converted from the tension F into strain ε using the Young's modulus of the glass, ∂λs / ∂ε and ∂λf / ∂ε are concrete. Obtained as a numerical value. Therefore, if Δλs and Δλf are obtained by actual measurement from Expression (2), ΔT and Δε will be obtained.
Therefore, if the reflection center wavelengths λso and λfo at a certain reference temperature To and strain εo are known, Δλs and Δλs
By adding ΔT and Δε from λf as described above, the temperature and strain acting on the sensing unit 6 can be obtained simultaneously. The calculation based on the equation (2) can be easily performed using a personal computer or the like. When A to D shown in FIGS. 4 and 5 are represented by straight lines, {λs / ∂
For T, ∂λs / ∂F, ∂λf / ∂T, and ∂λf / ∂F, respectively, a tension at the reference temperature T0 or the reference strain t0 or a partial differential value with respect to the temperature may be obtained.

Hereinafter, specific numerical values will be described with reference to the examples shown in FIGS. From the slopes of the straight lines A to D in FIGS. 4 and 5, ∂λs / ∂T = 0.0095 nm / ° C. ∂λf / ∂T = 0.010 nm / ° C. ∂λs / ∂F = 0.01470 nm / g ∂λf / ∂F = 0.01461 nm / g. When the tension F is converted into the strain ε using the fiber glass Young's modulus of 72.9 GPa, ∂λs / ∂ε = 0.001342 nm / με ∂λf / ∂ε = 0.001334 nm / με. When these values are substituted and calculated, D = −8.
812 × 10 ⁻⁷ (nm ² / με · ° C.).

Now, when the reference temperature is 20 ° C. and the reference strain is 0 με, λf = 1535.07 nm and λs = 153.
5.551 nm is set as a reference value. When the sensing unit 6 is placed under a certain temperature condition and a strain is applied, λf measured at 1537.518 nm and λs is 1537.0.
Assuming 61 nm, Δλf = 1.967 nm, Δλs =
1.990 nm. By substituting this value and each of the above values into the equation (2), a solution of ΔT = 52.88 ° C. and Δε = 1091 με is obtained, and the temperature acting on the sensing unit 6 is 7
It can be seen that the strain is 2.88 ° C. and the strain is 1091 με.
This measured value is in good agreement with the value measured by another strain sensor and a thermocouple, and the difference is 0.2 ° C. in temperature,
The strain was 10 με. As described above, the optical fiber temperature strain sensor can measure temperature and strain simultaneously with high accuracy and high sensitivity.

FIG. 6 shows another embodiment of the optical fiber temperature distortion sensor according to the present invention. The sensor of this embodiment is of a type for measuring transmitted light. That is, the measurement light from the light source 9 is incident on one end of the sensing unit 6 from the waveguide fiber 7, and the transmitted light from the other end of the sensing unit 6 is
Is transmitted by the waveguide fiber 7. FIG. 7 shows an example of a transmission spectrum of the reflection type optical fiber grating. In the sensor of this example, the point where the shape of the peak of the spectrum of the transmission light measured by the photodetector 10 is inverted and becomes a valley is slightly increased. Although different, the wavelength value of each peak is the same, and the measurement principle is the same as that of the previous example.

In the optical fiber temperature distortion sensor of the present invention, the sensing section 6 is formed by forming the gratings 5 on the single PANDA-type polarization maintaining optical fiber 1 at a plurality of intervals along the longitudinal direction thereof. It is also possible. However, in this case, the grating periods の of the respective gratings 5 are different from each other so that one grating 5
Therefore, it is necessary that the center wavelengths λf and λs of the two reflected lights generated by the above do not overlap with each other, and that an interval equal to or larger than the difference between a pair of λf and λs is required. By providing such a plurality of gratings 5.5 along the longitudinal direction of the fiber 1, it is possible to simultaneously measure the temperature and strain at many different positions.

In the above-mentioned example, the PANDA type optical fiber is used as the polarization maintaining optical fiber. However, the present invention is not limited to this, and various types of polarization maintaining optical fibers such as bow tie type and elliptical cladding type are used. Needless to say, can be used.

[0025]

As described above, the optical fiber temperature distortion sensor of the present invention can simultaneously measure the temperature and distortion acting on the sensing section with high accuracy and high sensitivity. In addition, since the sensing unit is small, such as about 10 mm, the sensing unit can be mounted in a narrow space such as an electric cable, and local temperature and distortion can be measured. Further, there are effects such that the number of necessary devices is small and the device can be inexpensive.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an example of a configuration of a sensing unit of a sensor according to the present invention.

FIG. 2 is a configuration diagram illustrating an example of a configuration of a sensor according to the present invention.

FIG. 3 is a spectrum showing an example of a spectrum of reflected light reflected from a reflection grating according to the present invention.

FIG. 4 is a graph showing an example of the temperature dependence of λf and λs in the present invention.

FIG. 5 is a graph showing an example of the dependence of λf and λs on tension in the present invention.

FIG. 6 is a configuration diagram showing another example of the configuration of the sensor of the present invention.

FIG. 7 is a spectrum illustrating an example of a spectrum of reflected light reflected from a reflection grating.

[Explanation of symbols]

1: PANDA type polarization maintaining optical fiber, 2: core, 5
... Reflection type grating, 6 ... Sensing part, 9 ... Light source, 10 ... Photodetector

Continuing from the front page (72) Inventor Kuniharu Himeno 1440, Mukurosaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant Co., Ltd. Person Michihiro Nakai 1440 Rokuzaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant

Claims (3)

[Claims] 1. A sensing part comprising a reflection grating formed on a core of a polarization-maintaining optical fiber, wherein the sensing part has a center wavelength of each of reflected light or transmitted light of two orthogonal polarization modes. An optical fiber temperature distortion sensor that measures a change amount due to temperature and strain and simultaneously measures temperature and strain acting on a sensing unit. 2. The optical fiber temperature distortion sensor according to claim 1, wherein a plurality of reflection type grading formed in the sensing portion are formed in a longitudinal direction of the polarization maintaining optical fiber. 3. An optical fiber temperature distortion sensor according to claim 1 or 2, a light source for transmitting measurement light to the optical fiber temperature distortion sensor, and receiving reflected light or transmitted light from the optical fiber temperature distortion sensor. A light detector for detecting a change amount of a center wavelength of the reflected light or the transmitted light; and a change in a center wavelength of the reflected light or the transmitted light of each of the two orthogonal polarization modes due to temperature and distortion in the sensing unit. A temperature distortion measuring device that measures an amount and simultaneously measures temperature and distortion acting on a sensing unit.