JP2006010449A - Optical fiber sensing system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber sensing system capable of performing measurement of a larger number of points than a conventional system in real time. <P>SOLUTION: This optical fiber sensing system is provided with a light source, a light receiving portion, and an optical fiber sensor whose transmitted light is attenuated when it is strained and bent. Optical filters for transmitting or reflecting light in different wavelength bands respectively are attached to both ends of the optical fiber. The difference between the quantities of transmitted light or reflected light of these optical fibers is measured as a strain quantity applied to the optical fiber. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber sensing system that performs strain measurement using an optical fiber.

  In recent years, the structure that has been built rapidly during the high growth period has rapidly deteriorated, and research on the method for evaluating the soundness of the structure has been actively conducted mainly by the government. Conventionally, electrical methods using resistance strain gauges have been the mainstream method for measuring strain in structures, but there are problems such as reliability, lightning strikes, and electromagnetic noise. Measurement (sensing) is drawing attention.

  In the optical fiber sensing system, the microbending method using the attenuation of the transmitted light amount due to the bending of the optical fiber and the Brillouin Optical Time Domain Reflectmeter (hereinafter referred to as BOTDR) using the displacement of the Brillouin scattered light frequency by the extension of the optical fiber. And the FBG method utilizing the fact that the reflection wavelength or the transmission wavelength is displaced by the extension of the fiber Bragg grating (hereinafter referred to as FBG).

  Non-Patent Document 1 proposes a strain sensing system using a microbending method. FIG. 6 is a diagram showing a sensor unit of the microbending method.

  The light incident from one end of the figure passes through the optical fiber sensor 106, and the transmission loss is measured at the light receiving portion at the other end. When the optical fiber sensor 106 is distorted in the direction in which the radius of the optical fiber is reduced, the transmission loss increases. Conversely, transmission loss decreases when subjected to distortion in the direction of increasing radius. In general, transmission loss has an exponential relationship with respect to the amount of distortion, but if expressed as 10 × log (incident light quantity / 10) −10 × log (output light amount), a linear relationship can be derived, and distortion is determined from transmission loss. The amount can be easily derived.

  According to this measuring method, only one point can be measured with one optical fiber, but the position where the distortion is received can be specified by using the light generated from the light source as pulsed light. A measuring instrument using pulsed light is called an optical time domain reflectometer (hereinafter referred to as OTDR), and can measure the amount of reflected light and the amount of transmitted light at an arbitrary point.

  However, since OTDR specifies a position for each generated pulse, real-time measurement cannot be performed and it is not suitable for vibration measurement. Also, when the measurement location is local and multipoint, or in a wide range, it takes time for measurement and is not suitable for measurement that requires an emergency such as a disaster.

  Similarly to OTDR, the BOTDR method uses pulsed light to specify the position, so real-time measurement cannot be performed.

  Non-Patent Document 2 describes an FBG method in which a broadband light source is used and FBGs having a plurality of different reflection wavelengths are performed using a single optical fiber.

  FIG. 7 is a diagram showing the configuration of the FBG method described in Non-Patent Document 2.

  The light derived from the broadband light source 201 passes through the optical splitter 202, passes through the optical fiber 205, and reaches the FBG 203. A plurality of lights having different wavelengths are reflected by the FBG 203, pass through the optical fiber 205, pass through the optical splitter 202, reach the optical spectrum analyzer 204, and detect the reflected wavelength. Since the reflection wavelength of the FBG 203 is expanded and contracted, the distortion amount can be measured as the shift amount of the reflection wavelength if the FBG 203 is strained in the expansion / contraction direction. By using the broadband light source 201 in this way, a single optical fiber can be provided with FBGs having a plurality of different reflection wavelengths, and a plurality of distortion amounts can be measured in real time.

  However, the reflection wavelength band of the FBG 203 is assigned a wide wavelength band of about 4 nm per line in consideration of the shift amount. On the other hand, the wavelength band of the broadband light source is about 80 nm at the maximum, and the FBG 203 can have only about 20 at the maximum for one optical fiber.

The optical fiber sensing system is intended for measurement of structures, and for large-scale measurements and for road structures, it is necessary to provide more sensors for each optical fiber. It has been demanded.
Akira Tsuji et al., JSME Proceedings Vol. 63, No. 615, No. 96-19911, November 1997 Shinji Yamashita et al., Latest Materials on Optical Communication Technology of Optronics, Application to Optical Measurement / Sensor, December 1995

  The conventional techniques described above are not suitable for real-time measurement and have a problem that the number of measurement points necessary for large-scale measurement is insufficient.

  The present invention has been made to solve these problems. In an optical fiber sensing system comprising a light source, a light receiving unit, and an optical fiber in which transmitted light attenuates when bent by bending, both ends of the optical fiber sensor are provided. Are provided with optical filters that transmit or reflect light of different wavelength bands, and a difference in the amount of light transmitted or reflected by these optical filters is measured by the light receiving unit as an amount of distortion applied to the optical fiber sensor. .

  Furthermore, it is provided that there are a plurality of portions on a single optical fiber where the transmitted light attenuates when subjected to strain, and the optical filters provided at both ends transmit or reflect light of different wavelength bands. Features.

  Furthermore, the light source is a broadband light source, and the light receiver can detect light quantities in a plurality of wavelength bands. Alternatively, the light source is a wavelength tunable light source, and the light receiver determines a wavelength band in synchronization with a search speed of the wavelength tunable light source.

  Further, the light source is disposed at one end of the optical fiber sensor in which transmitted light is attenuated when bent by receiving the strain, and the light receiver is disposed at the other end, and the transmitted light of the optical filter is measured. Alternatively, the light source and the light receiver are disposed at one end of an optical fiber sensor that attenuates transmitted light when bent under the strain, and the other end is terminated, and output light from the light source and return light to the light receiver. A branching device for branching is provided, and the reflected light of the optical filter is measured.

  Further, the optical filter is a fiber Bragg grating.

  Further, the coating of the optical fiber sensor in which transmitted light attenuates when bent by receiving the strain is carbon coated.

  Further, the bending loss characteristic of the optical fiber sensor in which the transmitted light attenuates when bent under the strain is generated even when the bending diameter is 20 mm or more.

  Further, the refractive index difference between the core and the clad of the optical fiber sensor in which the transmitted light attenuates when bent by receiving the strain is within 0.5%.

  Further, the optical fiber sensor in which transmitted light attenuates when bent under the strain is a photonic crystal fiber or holey fiber.

  As described above, according to the present invention, in an optical fiber sensing system including a light source, a light receiving unit, and an optical fiber sensor in which transmitted light is attenuated when bent and bent, both ends of the optical fiber sensor have different wavelength bands. An optical filter that transmits or reflects light is provided. Real-time measurement is possible by measuring the difference in the amount of transmitted light or reflected light of these optical filters as the amount of distortion applied to the optical fiber sensor by the light receiving unit. It is possible to provide an optical fiber sensing system capable of more multipoint measurement than the measurement method. Further, by using a carbon coated fiber or a fiber having a large bending loss characteristic, it is possible to provide an optical fiber sensing system that can perform real-time measurement and multipoint measurement with higher reliability.

  Hereinafter, an optical fiber sensing system according to the present invention will be described. FIG. 1 is a diagram showing a configuration of an optical fiber sensing system of the present invention.

  It is composed of a broadband light source 1, an optical spectrum analyzer 4 as a light receiving unit, an optical branching device 2, an optical filter 3a, an optical filter 3b, an optical fiber 5, and an optical fiber sensor 6. Each component is coupled with an optical fiber. . The optical fiber sensor 6 is an optical fiber in which transmitted light is attenuated when bent and bent, and the optical filters 3a and 3b arranged at both ends thereof have transmission bands and reflection bands of different wavelength bands.

  The optical filters 3a and 3b may be of a dielectric multilayer film, but the FBG is desirable because it can be easily formed in a narrow wavelength band and has high wavelength utilization efficiency. FBGs are intended to irradiate an optical fiber with ultraviolet light and periodically change the refractive index to form a diffraction grating. However, it is desirable that the refractive index be distributed and apodized to narrow the wavelength band. Further, since the FBG is made of an optical fiber and is small, the sensor can be miniaturized.

  It is desirable that the optical fiber sensor 6 and the optical filters 3a and 3b are close to each other and bonded. As each distance increases, the transmission loss of the optical fiber used for bonding becomes more likely to change due to the influence of temperature and strain. For this reason, the optical fiber sensor 6 and the optical filters 3a and 3b are preferably integrated modules. For example, a plurality of portions to be the optical fiber sensor 6 can be formed on one optical fiber, and FBGs to be the optical filters 3a and 3b can be formed on both ends of each.

  However, when a plurality of optical fiber sensors 6 are used, if the distance between the optical fiber sensors is short, the optical filters 3a and 3b can be shared with the adjacent optical fiber sensors 6, and the total number of the optical filters 3a and 3b may be reduced. it can.

  In addition, a broadband light source 1 and an optical spectrum analyzer 4 are arranged on one end side of the optical fiber sensor 6 via an optical splitter 2. The optical branching device 2 may be a two-branch coupler or an optical circulator. The optical spectrum analyzer 4 only needs to detect the wavelength and the amount of light.

  As a combination of the light source and the light receiver, in addition to the combination of the broadband light source 1 and the optical spectrum analyzer 4, a combination of a wavelength variable light source and an optical power meter may be used. If the wavelength search speed of the wavelength tunable light source and the light reception of the optical power meter are synchronized, equivalent measurement is possible. Although the real-time property in a strict sense is lost, the search speed of the wavelength tunable light source is as fast as several hundred Hz, and sufficiently high-speed measurement is possible.

  FIG. 2 is a diagram showing the configuration of the optical fiber sensor 6. As shown in the figure, an optical fiber having a diameter of about 20 mm is installed in a semicircular shape, and distortion is applied in a direction in which the radius changes, so that transmitted light is attenuated when bending due to distortion.

  FIG. 3 is a diagram showing the relationship between the distortion amount and the transmission loss. When distortion in the direction in which the radius of the optical fiber sensor 6 is reduced is added, the transmission loss increases. On the contrary, if distortion is applied in the direction in which the radius increases, the transmission loss decreases. The transmission loss here is one transmission loss. In this embodiment, the transmission loss is transmitted through the optical fiber sensor 6 in a reciprocating manner. Therefore, the loss can be assumed to be twice that of FIG.

  Further, although the optical fiber sensor 6 is semicircular, it may be circular, coiled, ¼ circle or 上 circle, and the loss is proportional to the winding length. Although it depends on the mounting method, the measurement accuracy is expected to increase as the winding length increases. It is also possible to change the transmission loss by twisting the optical fiber by making the optical fiber stranded with several dummy fibers and expanding and contracting the stranded portion.

  Next, a detection method using the optical fiber sensing system of the present invention will be described.

  The light output from the broadband light source 1 passes through the optical splitter 2, passes through the optical fiber 5, and reaches the optical filter 3a. Light of a specific wavelength is reflected by the optical filter 3a. Wavelengths other than the specific wavelength of the optical filter 3a are transmitted, transmitted through the optical fiber sensor 6, the transmission amount is attenuated according to the distortion amount, reaches the optical filter 3b, and light having a specific wavelength different from that of the optical filter 3a is reflected. The The light reflected by the optical filter 3b is transmitted and attenuated again through the optical fiber sensor 6, passes through the optical filter 3a, is combined with the light reflected by the optical filter 3a, passes through the optical fiber 5, and passes through the optical splitter 2. The light passes through and reaches the optical spectrum analyzer 4, and detects the amount of light for each wavelength of the optical filter 3 a and the optical filter 3 b.

  Here, since the light quantity of the optical filter 3b passes through the optical fiber sensor 6 twice, the light quantity difference between the peak wavelengths of the optical filter 3a and the optical filter 3b is twice the transmission attenuation amount by the optical fiber sensor 6. I understand.

  FIG. 5 is a diagram showing the relationship between the optical filter 3a and the optical filter 3b derived from FIG.

  When the distortion applied to the optical fiber sensor 6 is ± 1500 με, the transmission attenuation is ± 0.2 dB.

  Here, when the optical filters 3a and 3b are FBGs, the full width at half maximum is about 0.25 nm, and the maximum transmission attenuation in the optical fiber sensor 6 is sufficiently smaller than the half value (3 dB), so that both wavelengths are recognized. 0.25 nm is sufficient for. When the bandwidth of the broadband light source 1 is 80 nm, approximately 320 points can be measured.

  However, in practice, the dynamic range of the output of the broadband light source 1 and the sensitivity of the optical spectrum analyzer 4 is about 70 dB, and measurement cannot be performed when the sum of transmission losses exceeds this range. In this embodiment, the optical fiber sensor 6 is semicircular, the initial transmission loss is 0.5 dB, and the maximum number of measurement points is 140 (70 / 0.5). If the optical fiber sensor has a ¼ or ８ shape, the number of measurement points can be easily increased. Further, the number of measurement points can be increased by improving the output of the broadband light source 1 or improving the sensitivity of the optical spectrum analyzer 4. If a wavelength variable light source is used as the light source and an optical power meter is used as the light receiving unit, the dynamic range can be 100 dB or more and the number of measurement points can be increased.

  As described above, while the number of FBG type measurement points that can be measured in real time at multiple points has been 20 in the past, the present invention can greatly increase the number of measurement points and provide a larger scale. An optical fiber sensing system capable of measuring points in real time can be provided.

  In the above embodiment, the reflected light of the optical filter 3 is used, but transmitted light can also be used. In this case, a light source may be disposed on one end side of the optical fiber sensor 6 and a light receiving unit may be disposed on the other end side, and the number of times of transmission through the optical fiber sensor 6 is one, so that a larger dynamic range can be obtained. The present invention can be used more effectively.

  Now consider the reliability of the optical fiber sensor 6 against bending.

  The optical fiber used for the optical fiber sensor 6 is generally made of quartz for optical communication. However, the smaller the bending radius, the larger the strain of the outermost layer of the clad and the higher the probability of breakage. Although it depends on the manufacturing method and the initial sorting method, in the case of an optical fiber having a diameter of 20 mm, the fracture probability after 20 years is as large as 1/1000 per 1 m.

  Here, in order to alleviate the distortion of the outermost layer of the clad, it is desirable to apply carbon coating instead of the coating of UV resin that is usually used, because the fracture probability is improved by an order of magnitude and the reliability is improved.

  In addition, if an optical fiber having a large loss characteristic with respect to bending is used, the bending diameter can be increased, and the distortion of the outermost cladding layer can be reduced, thereby improving the reliability. By changing the bending diameter from 20 mm to 30 mm, the probability of fracture by about two digits is reduced, and the reliability can be greatly improved.

  In general, a quartz fiber for optical communication is not suitable for the optical fiber sensor 6 used in the present invention because bending loss does not occur when the bending diameter becomes 20 mm or more.

  Here, the mechanism of the bending loss of the optical fiber will be described.

  FIG. 4 is a diagram showing the structure of the optical fiber 10. As shown in FIG. 4, the optical fiber 10 includes a core 11, a clad 12, and the like. Germanium, phosphorus, or the like is added to the core 11 so that the refractive index of the core 11 is higher than that of the cladding 12. Thus, the light propagating by providing a difference in refractive index between the core 11 and the clad 12 causes total reflection at the interface between the core 11 and the clad 12 and is confined in the core 11. The total reflection condition is determined by the difference in refractive index and the incident angle 13 to the interface. When the optical fiber 10 is bent, the incident angle 13 at the interface between the core 11 and the cladding 12 is displaced, and the total reflection does not occur and a part leaks into the cladding 12. Loss occurs.

  When used for optical communication, it is desirable that the optical fiber 10 be resistant to bending loss and that the difference in refractive index between the core 11 and the cladding 12 be large. However, since a foreign substance is added to the core 11 through which light propagates, the refractive index difference has a limit, which is about 0.5% to 1%.

  On the other hand, when used in an optical fiber sensor as in the present invention, it is desirable that the difference in refractive index is small. If the difference in refractive index is designed to be 0.5% or less, a loss occurs even for a bending with a diameter of 20 mm or more. It becomes like this.

  In recent years, photonic crystal fibers and holey fibers have attracted attention as optical fibers 10 for optical communication. This is because an effective refractive index difference can be increased and bending loss can be reduced by providing a gap in the cladding 12 and providing an air layer having a low refractive index. The core 11 of the optical fiber 10 used here is made of the same material as that of the clad 12, and if the gap is set to 0, there is no difference in refractive index. If the porosity is controlled, the bending diameter can be adjusted, and a loss occurs even for a bending having a diameter of 20 mm or more.

  For example, when a holey fiber in which four gaps of about φ3 μm are provided in the clad 12 along the circumference of the core 11 is used, a characteristic equivalent to the strain transmission loss characteristic shown in FIG. It is done.

  As described above, the number of measurement points of the FBG method, which has conventionally been capable of real-time measurement with multiple points, was 20, but the present invention can greatly increase the number of measurement points, and thus the scale is larger. It is possible to provide an optical fiber sensing system that can measure multiple points in real time, and to provide a highly reliable optical fiber sensing system by using carbon coated fiber or holey fiber as the optical fiber of the optical fiber sensor. be able to.

  The optical fiber sensing system of the present invention shown in FIG. 1 was produced.

  As the broadband light source 1, an ASE light source that outputs light of −10 dBm / nm or more in a band from 1525 nm to 1605 nm was used. The optical spectrum analyzer 4 is generally used in optical communication and has a light receiving sensitivity of −80 dBm / nm. As the optical branching device 2, a circulator having a transmission loss of about 0.5 dB and a wavelength characteristic of the transmission loss of less than 0.1 dB was used. FBG was used for the optical filters 3a and 3b. The reflectance was 95% or more, the full width at half maximum of the reflection spectrum was 0.25 nm or less, and the reflection peak wavelengths at 25 ° C. were 1550 nm and 1555 nm, respectively. The optical fiber sensor 6 was installed in a semicircular shape as shown in FIG. 2, and a strain of ± 1500 με was applied at the maximum.

  The transmission loss was measured from the amount of light at the respective peak wavelengths of the optical filters 3a and 3b, and the difference between the two was used as the transmission loss amount of the optical fiber sensor 6. As shown in FIG. 5, a result similar to the expected value could be derived, and the effectiveness of the present invention was verified.

It is a figure which shows the structure of the optical fiber sensing system of this invention. It is a figure which shows the structure of an optical fiber sensor. It is a figure which shows the relationship between distortion and transmission loss. It is a figure for demonstrating the mechanism of the bending loss of an optical fiber. It is a figure which shows the relationship between the light quantity difference of the peak wavelength of an optical filter, and the distortion amount of an optical fiber sensor. It is a figure which shows the sensor part of a microbending method. It is a figure which shows the structure of FBG method.

Explanation of symbols

1: Broadband light source 2: Optical splitter 3a, 3b: Optical filter 4: Optical spectrum analyzer 5: Optical fiber 6: Optical fiber sensor 10: Optical fiber 11: Core 12: Clad 13: Incident angle 106: Optical fiber sensor 201: Broadband light source 202: optical splitter 203: FBG
204: Optical spectrum analyzer 205: Optical fiber

Claims (11)

In an optical fiber sensing system comprising a light source, a light receiving portion, and an optical fiber sensor that attenuates transmitted light when bent and bent, an optical filter that transmits or reflects light in different wavelength bands at both ends of the optical fiber sensor The optical fiber sensing system is characterized in that the light receiving unit measures the difference in the amount of transmitted light or reflected light of these optical filters as the amount of distortion applied to the optical fiber sensor. A single optical fiber is provided with a plurality of portions where transmitted light is attenuated when subjected to strain and bent, and the optical filters provided at both ends transmit or reflect light of different wavelength bands. The optical fiber sensing system according to claim 1. The optical fiber sensing system according to claim 1, wherein the light source is a broadband light source, and the light receiver can detect light amounts in a plurality of wavelength bands. The optical fiber sensing system according to claim 1 or 2, wherein the light source is a wavelength tunable light source, and the light receiver determines a wavelength band in synchronization with a search speed of the wavelength tunable light source. 2. The optical fiber sensor in which transmitted light is attenuated when bent by receiving the strain, the light source is disposed at one end and the light receiver is disposed at the other end, and the transmitted light of the optical filter is measured. The optical fiber sensing system in any one of -4. The light source and the light receiver are arranged at one end of an optical fiber sensor that attenuates transmitted light when bent under the strain, the other end is terminated, and the output light from the light source and the return light to the light receiver are branched. The optical fiber sensing system according to claim 1, further comprising: a branching unit that measures reflected light of the optical filter. The optical fiber sensing system according to claim 1, wherein the optical filter is a fiber Bragg grating. The optical fiber sensing system according to any one of claims 1 to 7, wherein a coating of an optical fiber sensor in which transmitted light is attenuated when bent by receiving the strain is carbon coated. The optical fiber sensing system according to any one of claims 1 to 8, wherein a bending loss characteristic of an optical fiber sensor in which transmitted light is attenuated when bent by receiving the strain occurs even at a bending diameter of 20 mm or more. The optical fiber sensing system according to any one of claims 1 to 8, wherein a difference in refractive index between a core and a clad of an optical fiber sensor in which transmitted light is attenuated when bent under the strain is within 0.5%. . The optical fiber sensing system according to any one of claims 1 to 8, wherein the optical fiber whose transmitted light attenuates when bent under the strain is a photonic crystal fiber or a holey fiber.

Images