JP4243159B2 - FBG sensing system - Google Patents

Description

  The present invention relates to an FBG sensing system that performs strain measurement and temperature measurement using an FBG.

  In the field of optical fiber sensing, FBG (Fiber Bragg Grating) is used in combination with an ASE (Amplified Spontaneous Emission) light source or a wavelength tunable light source, which is a broadband light source, and is used for strain measurement of structures. The FBG reflects specific light determined by the grating interval. The reflection wavelength width (−3 dB width) of the FBG is about 0.1 nm, and the reflection wavelength of the FBG is changed by applying stress to the FBG to expand and contract (FBG is distorted). If the FBG is attached to a measurement location of the structure, ASE light or the like is incident, and the reflection wavelength is measured with a wavelength meter or the like, the distortion of the FBG, that is, the distortion of the structure can be measured. This usage is called a reflective FBG sensor.

  FBG can also be used as a transmissive sensor. In other words, the FBG that reflects a specific wavelength is a blocking body that blocks transmitted light of a specific wavelength. Since the wavelength to be cut off and the wavelength to be reflected are equivalent when viewed as a sensor, it can be used as a transmission type sensor by measuring the wavelength of the cut-off light.

  Furthermore, since the refractive index of the fiber changes with temperature changes, the FBG can also be used for temperature detection. In other words, since strain measurement using FBG is easily affected by temperature, means for eliminating the effect of temperature is necessary for strain measurement.

  In the above optical fiber sensing, an optical fiber can be stretched as far as 30 km, and the amount of distortion at that point can be measured. Since there are optical fiber transmission loss and FBG loss, a high-power and broadband light source is used as a broadband light source or wavelength tunable light source in order to extend the measurement distance and increase the number of measurement points. It is also necessary to measure the reflection wavelength with high accuracy.

  As an ASE light source that is a broadband light source, as shown in FIG. 5 in Patent Document 2, a light source having an optical power spectrum density of −20 dB / nm or more in a wavelength range of about 100 nm has already been disclosed.

  In the case of FBG used at a wavelength of 1550 nm, the change in the reflection wavelength is about 1.2 pm / μ strain. That is, if the change in the reflected wavelength from the FBG is measured with a resolution of 1 pm, distortion can be measured with a resolution of 1 μstrain.

  FIG. 7 shows an FBG sensing system disclosed in Patent Document 1 as a prior art. As a sensor, an FBG is connected in series on one fiber.

  Output light from the broadband light source 90 is guided to the FBG sensor 105 through the optical fiber 140 via the coupler 130. There are a plurality of FBG sensors 105, each having a different reflection wavelength and connected in series. The different reflection wavelengths reflected by the FBG sensor 105 return to the coupler 130 and are guided to the wavelength meter 150 to detect the respective wavelengths.

  FIG. 8 shows the contents described in Patent Document 3 in a simplified manner. A broadband light source 201, a wavelength meter 202, a 3 dB coupler 203 (represented as a directional coupler in Patent Document 3), an optical fiber 204, and an FBG sensor 205 are configured. In order to be able to measure many FBG sensors, they are branched by a 3 dB coupler 203 and used.

FIG. 9 shows an FBG sensing system described in Patent Document 4. The main line fiber 400, the broadband light source 401, the splitter 402, the add / drop filters 411 to 41N, the optical amplifier 403, and the wavelength separation filter 404 are configured. The difference from Patent Documents 1 and 3 is that an add / drop filter is inserted between the FBG sensor (FBG _{1 to} FBG _N ) and the trunk line fiber 400, so that even if one FBG sensor is disconnected, another FBG sensor is disconnected. The signal from can be detected.
US Pat. No. 5,361,130 Japanese Patent Laid-Open No. 2001-111145 Special table 2001-511895 JP 2002-310729 A

  However, when Patent Document 1 is used, since the FBG sensors 105 are connected in series, for example, if one FBG sensor 105 on the coupler 130 side is disconnected, there is a problem that all the FBG sensors cannot be used. In other words, this series connection system was a system with poor safety and reliability. In particular, the FBG sensor is used for strain measurement, and stress may be applied to the sensor portion, so there is a possibility of disconnection.

  Further, even when Patent Document 3 is used, since the FBG sensor 205 is connected in series, it is not reliable as described above. Further, since the insertion loss of transmitted light is 3 dB or more in the 3 dB coupler 203, there is a problem that the output of the broadband light source 201 must be increased to compensate for the loss.

  In order to solve the above problem, there is a method using an add drop filter as described in Patent Document 4, but when the trunk fiber 400 is disconnected, signals from the FBG sensor after the disconnected point cannot be detected. There was a problem of end.

  Here, it is an object to construct a highly reliable and robust system that does not affect the signals of other FBG sensors even if one FBG sensor or main line fiber breaks.

The present invention is for solving these problems, and includes a first optical coupler having a first port and a second port on one end side and a third port and a fourth port on the other end side, An optical fiber for deriving output light from a broadband light source or a wavelength tunable light source is connected to the first port, and the first and second ports of the second optical coupler are connected to the third and fourth ports of the optical coupler, respectively. Thereafter, n-th optical couplers (n is an integer of 2 or more) are similarly connected in series, and an FBG sensor is disposed at any or all of the optical couplers, and the first port of the first optical coupler. And / or the second port is a signal detection port.

  Further, the present invention is characterized in that the second port of the first optical coupler is unused and a 1 × 2 port optical coupler is formed.

Furthermore, the present invention is characterized in that the first to nth optical couplers have a branching ratio of 50%.

  Further, the present invention is characterized in that a polarization controller is added to the fiber of the first optical coupler.

  According to the present invention, in an FBG sensing system, a 2 × 2 optical coupler is connected in series, and an FBG sensor is disposed between any or all of the optical couplers, so that an FBG sensor or an optical fiber connected in series can be connected. Even if there is a disconnection, measurement by the FBG sensor is possible, and a highly reliable and robust system can be constructed.

  Hereinafter, an FBG sensing system according to the present invention will be described. FIG. 1 is a block diagram showing a first embodiment of the present invention.

  The FBG sensing system of the present invention includes a broadband ASE light source 1, an optical fiber 2, optical couplers 3 to 4, FBG sensors 6 to 7 having different reflection wavelengths, a wavelength meter 9, and a polarization controller 9. Further, the FBG sensors 6 to 7 each reflect light having different specific wavelengths in the light incident direction, and transmit light other than the reflected wavelength. Optical couplers 3 to 4 are optical fiber type couplers made by fusing and drawing optical fibers. As the wavelength meter 9, a commercially available optical spectrum analyzer or optical wavelength meter can be used.

  The output light from the broadband ASE light source 1 passes through the optical fiber 2, is divided into two by the first optical coupler 3, and branches to the third port 3c and the fourth port 3d.

The wavelength of the light reflected by the FBG sensor 6 among the light branched to the third port 3c is measured by the wavelength meter 9. The light transmitted through the FBG sensor 6 and the light on the fourth port 3d side of the optical coupler 3 are guided to the second optical coupler 4 and combined to be further branched into two ports. The light reflected by the FBG sensor 7 is branched into two by the second optical coupler 4 and further coupled through the third port 3c and the fourth port 3d of the first optical coupler 3, respectively, and further the second port. Extracted from 3b and measured by the wavelength meter 9.

  With such a configuration, the reflection wavelength from the FBG sensors 6 to 7 is measured, and the distortion amount of the FBG sensor can be converted from the change in wavelength.

  Here, consider a case where the FBG sensor 6 is disconnected due to an accident. In this case, the third port 3c of the optical coupler 3 cannot be used, and the reflected wavelength of the FBG sensor 7 can be measured via the fourth port 3d. However, in this case, the signal strength of the reflected signal intensity of the FBG sensor 7 is reduced because there is no signal corresponding to the third port 3c. Thus, even if the FBG sensor is disconnected or a part of the optical fiber is disconnected, the measurement can be continued by the optical signal of the uncoupled port of the optical couplers 3 to 4.

  However, since the configuration in which the first optical coupler 3 and the second optical coupler 4 are connected in series is a Mach-Zehnder interferometer configuration, for example, the reflected light of the FBG sensor 6 interferes with the first optical coupler 3. There is also a danger that it cannot be observed with the wavelength meter 9. In order to avoid this risk of interference, the light source is preferably a low coherent light such as an ASE light source.

  The polarization controller 10 is used to avoid the above-described interference, and is adjusted so that the reflected spectrum intensity from the FBG sensor 7 does not become small.

  Also, interference can be avoided if care is taken so that the arm lengths (two fiber lengths between the optical coupler 3 and the optical coupler 4) are not equal.

  The right ends A and B of the optical coupler 4 may be open ends, but it is better to have non-reflective terminations. This is to prevent the reflected light from the fiber end face from becoming noise with respect to the reflected signal of the FBG sensor.

  FIG. 2 is a block diagram showing a second embodiment of the present invention. The difference from the first embodiment is that the optical coupler 3 is a 1 × 2 port coupler, the optical circulator 11 is connected to the first port of the optical coupler 3 in order to detect the signal light from the FBG sensor, and the output port of the optical circulator 11 Is connected to the wavelength meter 9. In order to increase the number of FBG sensors, the third optical coupler 5 is provided and the FBG sensor 8 is provided. As shown in FIG. 1, by connecting optical couplers in series, even if the FBG sensor is disconnected or a part of the optical fiber is disconnected, measurement can be continued using the optical signals of the uncoupled ports of the optical couplers 3 to 5. It becomes.

  Here, a broadband ASE light source is used as the light source, but a wavelength variable light source may be used. At this time, as the wavelength tunable light source, a fiber ring laser type is preferable because interference hardly occurs inside the Mach-Zehnder interferometer. This is because the fiber ring laser type is less coherent than the wavelength tunable light source using a semiconductor laser and interference is difficult to occur.

  Thus, the configuration in which the optical couplers are connected in series has an effect that the measurement of the FBG sensor can be continued even if some of the optical fibers and the FBG sensor are disconnected. In addition, when an optical coupler with a 50% branching ratio is used, the loss of the light amount due to the optical coupler is not accumulated 50%, but repeats merging / branching, so a smaller value (excess loss (5 for optical couplers) % Or less), and even if optical couplers are connected in multiple stages, no significant optical loss occurs.

  The FBG sensing system of the present invention shown in FIG. 1 was created.

  A broadband light source 1 having the same configuration as that shown in Patent Document 2 was used. The central reflection wavelengths of the FBG sensors 6 to 7 are set to 1557 nm and 1554 nm, respectively. A commercially available optical spectrum analyzer is used as the optical wavelength meter 9. The optical couplers 3 and 4 are 2 × 2 port optical fiber couplers prepared by fusing and stretching optical fibers.

  3A and 3B are transmitted light spectra at the A end and B end of the system. It can be confirmed that almost the same transmitted light intensity is obtained at the A and B ends in the entire band. The amount of transmitted light loss due to the FBG sensor 7 (wavelength 1554 nm) is 17 dB or more, whereas the amount of transmitted light loss due to the FBG sensor 6 (wavelength 1557 nm) is about 3 dB. Thus, the amount of transmitted light loss by the FBG sensor 6 (wavelength 1557 nm) is as small as about 3 dB because the light from the fourth port 3 d of the optical coupler 3 is multiplexed by the optical coupler 4.

  FIG. 4A shows a reflected light spectrum when this system is operating normally. The reflection spectra of the FBG sensors 6 to 7 are obtained with substantially the same intensity.

  4B shows the reflected light spectrum when the C section is disconnected, FIG. 5A shows the reflected light spectrum when the D section is disconnected, and FIG. 5B shows the E spectrum when the E section is disconnected. Even when the C part and the E part are disconnected, the wavelength meter 9 can also measure the reflected light intensity from the FBG sensor 7 (wavelength 1554 nm). The amount of decrease in light intensity was expected to be 3 dB, but was actually about 6 dB, which was a large loss. Such a state occurs when the polarization controller 10 is adjusted so that the reflection spectrum intensity of the FBG sensor 7 in the normal state (FIG. 4A) is maximized.

  Further, when the D section is disconnected (FIG. 5A), the signal of the FBG sensor 6 cannot be obtained, but the signal of the FBG sensor 7 can be measured.

It is a block diagram which shows 1st Embodiment of the FBG sensing system in this invention. It is a block diagram which shows 2nd Embodiment of the FBG sensing system in this invention. (A) (b) is a figure which shows the transmitted light spectrum in this invention Example. The reflection spectrum in the Example of this invention is shown, (a) is at the time of normal operation, (b) is at the time of a disconnection. (A) (b) is a figure which shows the reflection spectrum at the time of a disconnection in this invention Example. It is a spectrum waveform of an ASE light source. It is a figure which shows the conventional FBG sensing apparatus. It is a figure which shows the conventional FBG sensing apparatus. It is a figure which shows the conventional FBG sensing apparatus.

Explanation of symbols

1: Broadband light source 2: Optical fibers 3-5 Optical fiber couplers 6-8: FBG sensor 9: Optical wavelength meter 10: Polarization controller 11: Optical circulator 90: Broadband light source 105: FBG sensor 130: Coupler 140: Optical fiber 150 : Wavelength meter 201: Broadband light source 202: Wavelength meter 203: 3 dB coupler 204: Optical fiber 205: FBG sensor 400: Main line fiber 401: Broadband light source 402: Splitter 403: Optical amplifier 404: Wavelength separation filters 411 to 41 N: Add drop filter

Claims (4)

A first optical coupler having a first port and a second port on one end side and a third port and a fourth port on the other end side is provided, and output light from a broadband light source or a wavelength tunable light source is supplied to the first port. An optical fiber to be led out is connected, and the first and second ports of the second optical coupler are connected to the third and fourth ports of the optical coupler, respectively. Thereafter, the nth optical coupler (n is an integer of 2 or more) ) Are similarly connected in series, and an FBG sensor is arranged in any or all of the optical couplers, and the first port and / or the second port of the first optical coupler is used as a signal detection port. FBG sensing system. 2. The FBG sensing system according to claim 1, wherein the second port of the first optical coupler is unused and is a 1 × 2 port optical coupler. 3. The FBG sensing system according to claim 1, wherein the first to nth optical couplers have a branching ratio of 50%. The FBG sensing system according to any one of claims 1 to 3, wherein a polarization controller is added to the fiber of the first optical coupler.

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