JP2006208080A - Optical fiber vibration sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber vibration sensor easy to lay a sensor cable, allowing production at low cost, and capable of detecting vibration applied along a longitudinal direction of the sensor cable, with high resolution. <P>SOLUTION: One portion of an optical fiber loop 12 is used as the sensor cable 12a for measuring the vibration, in a Sagnac interference type optical fiber sensor for making two light signals emitted from a light source 15 and branched propagated in order through the first photocoupler 18, a polarizer 21 and the second photocoupler 19 incident clockwisely and anti-clockwisely into the optical fiber loop 12 including a phase modulator 22, for photoelectrically converting a return signal thereof by a light receiver 16 via the second photocoupler 19, the polarizer 21 and the first photocoupler 18, and for detecting the signal by a signal processing unit 23 to output a data. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber vibration sensor that detects mechanical vibration applied to an optical fiber, and more particularly to a Sagnac interference type optical fiber vibration sensor.

  As means for measuring vibration, there is a sensor using an optical fiber grating (FBG).

  An FBG is a diffraction grating formed by applying a periodic refractive index change in the longitudinal direction to the core of an optical fiber, reflecting a specific wavelength band propagating through the optical fiber and transmitting other wavelength bands. is there.

  The vibration sensor using the FBG uses an optical fiber on which the FBG is formed as a sensor cable. When vibration (distortion) is applied to the sensor cable, the sensor cable expands and contracts, the lattice spacing of the FBG changes, and enters the FBG. Since the reflected wavelength of the light is shifted in accordance with the grating interval, the distortion applied to the FBG is measured from the change in the reflected wavelength at the FBG (see, for example, Patent Document 1).

  Using this, a system has been proposed in which a sensor cable in which a plurality of FBGs having different diffraction grating intervals are connected to each other is installed on a fence or the like to detect an intruder in the fence.

JP 2003-202272 A JP-A-6-194179

  However, the vibration sensor using FBG has the following problems.

  The vibration sensor using the FBG is formed by interspersing the FBG on a long sensor cable. Therefore, since vibration (distortion) that can be measured by the FBG is a point, in order to detect vibration in the longitudinal direction of the sensor cable, it is necessary to apply a uniform tension to the sensor cable and hold it. Furthermore, since the sensor cable expands and contracts due to temperature changes, a mechanism for applying tension and its management become complicated.

  When manufacturing FBGs, a high-power ultraviolet laser and a finely processed phase mask are required, and complicated processes such as optical fiber coating removal, recoating, and optical adjustment are required, resulting in high costs. turn into.

  Since the output of light reflected by the FBG is small, the resolution of the optical fiber sensor is not improved.

  Since the vibration sensor is configured using a light source having a wide spectral width such as an ASE (amplified spontaneous emission) light source or an optical waveguide type wavelength filter, the vibration sensor is expensive.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical fiber vibration sensor that solves the above-described problems, can be easily laid, and can be manufactured at low cost, and can detect vibration applied in the longitudinal direction of the sensor cable with high resolution. There is.

  In order to achieve the above object, the invention according to claim 1 is directed to a phase modulator that converts two optical signals emitted from a light source and sequentially propagated through a first optical coupler, a polarizer, and a second optical coupler into branches. The return optical signal is photoelectrically converted by the light receiver through the second optical coupler, the polarizer, and the first optical coupler, and the signal is detected by the signal processing unit. In a Sagnac interference type optical fiber sensor that outputs data, the optical fiber vibration sensor uses a part of an optical fiber loop as a sensor cable for vibration measurement.

  According to the second aspect of the present invention, two optical signals emitted from a light source, propagated through an optical coupler and branched are incident left and right into an optical fiber loop including a phase modulator, and the return optical signal is transmitted through the optical coupler. In a Sagnac interference type optical fiber sensor that performs photoelectric conversion with a light receiver, detects a signal with a signal processing unit, and outputs data, an optical fiber vibration using a part of the optical fiber loop as a sensor cable for vibration measurement It is a sensor.

  A third aspect of the present invention is the optical fiber vibration sensor according to the first or second aspect, wherein an optical fiber portion excluding a central portion of the optical fiber loop is used as a sensor cable for vibration measurement.

  According to a fourth aspect of the present invention, the signal processing unit determines an output level of vibration data detected by a light receiver, and an alarm device or the like is activated when an output level greater than a preset value is detected. The optical fiber vibration sensor according to claim 1, further comprising a processing unit that outputs a signal to be transmitted.

  The optical fiber vibration sensor according to any one of claims 1 to 4, wherein the light source, the light receiver, the polarizer, the optical fiber connected to the optical coupler, and the optical fiber loop are made of a polarization maintaining optical fiber. It is.

  According to the present invention, the sensor cable can be easily laid, can be configured at low cost, and exhibits excellent effects such that vibration applied in the longitudinal direction of the sensor cable can be detected with high resolution.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIGS. 1A and 1B are schematic views showing a preferred embodiment of an optical fiber vibration sensor according to the present invention.

  As shown in FIGS. 1A and 1B, an optical fiber vibration sensor 10 includes a vibration sensor main body 11 and an optical fiber loop 12 connected to the vibration sensor main body 11 to form an optical closed circuit. Prepare. In the present embodiment, the vibration sensor main body 11 and the optical fiber loop 12 are connected by two optical connectors 13 and 13. A part of the optical fiber loop 12 in which an optical closed circuit is formed by an optical fiber becomes a sensor cable 12a for vibration measurement.

  The vibration sensor main body 11 includes a light source 15, a light receiver 16, a first optical coupler 18, a polarizer 21, a second optical coupler 19, a connecting optical fiber 17 that optically connects them, and a signal processing unit 23. Is housed in the housing 14.

  Optical fibers 17a and 17b are connected to the light source 15 and the light receiver 16, respectively. Both optical fibers 17a and 17b are connected to an optical coupler 18 that is an optical branching and coupling element. The optical couplers 18 and 19 are optical fiber couplers having 1 × 2 input / output ports. Optical fibers 17a and 17b are connected to two input / output ports on one end side of the first optical coupler 18, respectively. An optical fiber 17c is connected to one input / output port on the other end side of the optical coupler 18, and a part of the optical fiber 17c forms a polarizer and is connected to one input / output port of the second optical coupler 19. ing. The two input / output ports on the other end side of the second optical coupler 19 are connected to both ends of a long optical fiber, respectively, and this optical fiber is a loop (closed circuit) optical fiber loop 12.

  A phase modulator 22 provided at one end of the optical fiber loop 12 applies phase modulation with a relatively time delay between light waves propagating through the optical fiber loop 12 in opposite directions. In the present embodiment, the phase modulator 22 is formed by winding a part of the optical fiber loop 12 around a cylindrical PZT (piezoceramic) serving as a vibrator, and wound around the PZT by a voltage applied to the PZT. The phase of propagating light is modulated by expanding and contracting.

  The signal processing unit 23 drives the light source 15, processes an electric signal obtained by photoelectric conversion of the optical signal detected by the light receiver 16, controls the modulation level of the phase modulator 22, outputs the processing result, and the like. The light source 15, the light receiver 16 and the optical phase modulator 22 are electrically connected.

  The polarizer 21 is a fiber-type polarizer in which a part of the optical fiber 17c that connects the first and second optical couplers 18 and 19 is formed in a coil shape, and the birefringence of the core is increased.

  The optical fiber 17 and the optical fiber loop 12 connected to the light source 15, the light receiver 16, the polarizer 21, and the optical couplers 18 and 19 are preferably composed of polarization-maintaining optical fibers.

  The optical fiber vibration sensor 10 according to the present embodiment is an application of an optical fiber gyro based on the Sagnac effect. A part of the optical fiber loop 12 connected to the vibration sensor main body 11 is used for vibration measurement. It is characterized in that it is used as the sensor cable 12a.

  As shown in FIG. 2, when an optical fiber (optical fiber loop) 26 closed in a loop shape is rotated, the optical fiber gyro 25 has a light propagating clockwise through the optical fiber 26 and a light propagating counterclockwise. The rotational angular velocity of the optical fiber loop 26 is measured using the Sagnac effect in which a phase difference proportional to the rotational angular velocity is generated between the two.

  Similar to the rotational angular velocity, the optical fiber loop 26 of the optical fiber gyro 25 is sensitive to external rapid temperature changes, vibrations, sounds, and the like. As a countermeasure to reduce the phase change due to factors other than these rotational angular velocities, a symmetric winding technique is usually used in which the optical fiber loop 26 is wound S (right) on one side from the center of the loop and Z (left) on the other side. Has been given. Resin potting is performed to fix the optical fiber loop 26 with a thermosetting resin or the like so as to buffer vibrations and shocks applied to the optical fiber loop 26.

  The optical fiber vibration sensor 10 of this embodiment generates a phase difference between left and right light due to vibration of an optical fiber loop, and outputs vibration data corresponding to the rotational angular velocity of the optical fiber gyro.

  The optical fiber vibration sensor 10 does not require sensitivity to the rotational angular velocity of the sensor cable 12a. Therefore, the optical fiber loop 12 is wound clockwise on one side with respect to the central portion 12c, left-handed on the other side, and both left-handed and right-handed. It is preferable that the area surrounded by the optical fiber loops is equal.

  Next, the operation of the present embodiment will be described.

  The light L1 emitted from the light source 15 propagates through the first optical coupler 18, is linearly polarized by the polarizer 21, and enters the second optical coupler 19. In the second optical coupler 19, the light L <b> 1 is branched into two and enters the different ends of the optical fiber loop 12. Here, one of the two lights incident on the optical fiber loop 12 is a clockwise light Lcw and the other is a counterclockwise light Lccw.

  The left and right bi-directional light Lccw and Lcw are modulated in phase by the phase modulator 22, travel around the optical fiber loop 12, and enter the second optical coupler 19 again.

  The left and right bi-directional light Lccw and Lcw incident on the second optical coupler 19 interfere with each other in the second optical coupler 19 to become interference light L2. The interference light L <b> 2 propagates through the polarizer 21 and enters the first optical coupler 18, and is branched again into two lights. One of the branched lights is detected by the light receiver 16.

  If the two lights Lcw and Lccw that interfere with the second optical coupler 19 have the same phase, the light L1 incident on the second optical coupler and the interference light L2 emitted from the second optical coupler are the optical coupler 18. , 19 except for radiation loss etc., the light intensity is detected equally.

  On the other hand, if vibration occurs in any part of the sensor cable 12a while light propagates through the optical fiber loop 12, the phase of the propagation light changes due to the expansion and contraction of the optical fiber at the part where the vibration occurs.

  When the phases of the two lights incident on the second optical coupler 19 are different (a phase difference occurs), the intensity of the light received by the light receiver 16 is detected differently from the intensity of the light combined in the same phase. . That is, the light receiver 16 always detects a substantially constant light intensity (zero point) when there is no vibration in the sensor cable 12a, and vibrates with the sensor cable 12a when a light intensity different from the constant light intensity is detected. Is detected.

  In the present embodiment, by configuring the optical fiber vibration sensor 10 to be a Sagnac interference type, it can be manufactured at a low cost and the sensor cable 12a can be easily laid. Further, the intensity (output) of the optical signal detected by the light receiver 16 is large, and vibration can be measured with high resolution. Furthermore, vibration applied in the longitudinal direction of the sensor cable can be detected by the entire length of the sensor cable 12a, so that a wide range can be sensed. Further, the vibration of the sensor cable 12a can be accurately detected without applying a certain tension to the sensor cable 12a.

  In the loop center portion (right end side in the figure) 12c of the optical fiber loop 12, the clockwise light Lcw and the counterclockwise light Lccw branched by the second optical coupler 19 pass through the loop center portion 12c at the same time, so that vibration occurs. The phase difference due to is difficult to occur. Therefore, it is preferable to lay the loop central portion 12c of the optical fiber loop 12 in a place where it does not need to function as a sensor. By using the portion of the optical fiber loop 12 excluding the central portion as the sensor cable 12a, a phase difference due to vibration applied to the sensor cable 12a does not appear and high reliability is achieved.

  When the optical fiber loop 12 and the connecting optical fiber 17 are formed of a conventional single mode optical fiber, the polarization of the light propagating through the optical fiber rotates (the polarization state changes), and the left-right light does not interfere. There is. Therefore, by forming the optical fiber loop 12 and the connecting optical fiber with polarization-maintaining optical fibers, the polarization of the left-right light can be made to interfere with each other without rotating.

  In addition, the phase difference between the left and right light Lccw and Lcw can be calculated from the output vibration data, and the location where the sensor cable 12a vibrates can be detected.

  The optical fiber vibration sensor 10 of the present embodiment can be applied as a sensor for security purposes such as intrusion detection by being attached to a fence or the like.

  For example, as shown in FIG. 3, a storage box 32 is provided at one end of the fence 31, and the vibration sensor main body 11 is stored in the storage box 32. The sensor cable 12a formed by connecting both the return of the optical fiber loop 12 and the return of the optical fiber loop 12 is fixed to a range detected by the fence 31 (for example, a wire mesh portion). The central portion of the optical fiber loop 12 is housed in a termination box 33 provided on the fence 31. When fixing the sensor cable 12a, it is not necessary to apply a uniform tension to the sensor cable 12a, and it is sufficient to fix the sensor cable 12a so that the sensor cable 12a vibrates when the fence 31 vibrates.

  FIG. 4 shows an example of vibration data output from the signal processing unit.

  In FIG. 4, the vibration data to be output is displayed in decibels with the vibration level normalized. If there is vibration in the sensor cable 12a, vibration can be detected from the output data. The figure shows that the fence 31 vibrates three times within a certain time.

  Furthermore, since the optical fiber vibration sensor 10 can detect a wide range of vibrations with a single optical cable, the number of surveillance cameras for security purposes such as intrusion detection can be reduced.

  An alarm device may be connected to the signal processing unit 23, and the alarm device may be provided near the fence, and a processing unit that outputs a signal for operating the alarm device to the signal processing unit 23 may be provided. In the processing unit, the output level of the vibration data detected by the light receiver 16 is determined, and the alarm device can be activated when an output level equal to or higher than a preset value is detected.

  Further, the vibration data to be output is obtained by outputting the vibration in the time domain as shown in FIG. 4, but this may be subjected to Fourier transform, and the cause of the vibration may be analyzed from the frequency characteristics. Analyzing the frequency characteristics obtained by vibrations caused by natural phenomena such as rain and wind in advance and the frequency characteristics obtained by vibrations caused by human factors, etc., if the cause of the vibrations can be estimated based on this analysis, a more effective vibration sensor is obtained. be able to.

  The embodiment of the present invention is not limited to the above-described embodiment.

  For example, by using an SLD (super luminescent diode) as the light source 15, interference noise generated by interference between light (return light) returning from the sensor cable 12 toward the light source 15 and Rayleigh scattered light is generated. Can be reduced.

  The optical couplers 18 and 19, the polarizer 21, and the phase modulator 22 are fiber-type ones, but they are connected to the optical fiber using a waveguide-type optical coupler, polarizer, or phase modulator. Thus, an optical fiber vibration sensor may be configured.

  As the first and second optical couplers 18 and 19, optical fiber couplers having 1 × 2 input / output ports are used, but optical fiber couplers having 2 × 2 input / output ports may be used.

  In this embodiment, a phase modulator 22 is formed using a part of the optical fiber loop 12, and an open loop type optical fiber gyro that modulates the phase of propagating light by applying a sine wave voltage of a predetermined frequency is provided. Although the optical fiber sensor main body 11 is configured based on the above, the vibration sensor main body may be configured based on a closed loop type optical gyro that applies feedback to the phase modulator from the phase of the detected propagation light.

  For the phase modulator used in the closed-loop type optical fiber vibration sensor, see References (Kumaya, Yubara, Tsujioka “Development of Mass-Production Type Optical Fiber Gyroscope and Its Applications”, The Review of Laser Engineering, Vol1, No.4 April 1998. , pp.304-309), and the like.

  FIG. 5 is a schematic view showing a second preferred embodiment of the optical fiber vibration sensor according to the present invention.

  As shown in FIG. 5, the basic configuration of the optical fiber vibration sensor 50 of the present embodiment is substantially the same as the optical fiber vibration sensor 10 of FIG. The second optical coupler 19 and the polarizer 21 in FIG. 1 are omitted, and the light source 15, the light receiver 16, and the optical fiber loop 12 are connected by one optical coupler 18. Is different.

  The optical fiber vibration sensor 50 of the present embodiment also has the same function and effect as the optical fiber vibration sensor 10 of FIG.

  The optical fiber vibration sensor 50 of the present embodiment can be configured more easily because the number of parts constituting the vibration sensor main body is smaller than that of the optical fiber vibration sensor 10 of FIG.

  However, since the optical fiber vibration sensor 50 of the present embodiment has no polarizer, the polarization state of the incident light to the sensor cable 12 is unstable, and the stability of the detected zero point of the interference light intensity is shown in FIG. It becomes lower than the optical fiber vibration sensor 10. Therefore, for example, the optical fiber loop 12 and the connecting optical fiber 17 are polarization plane preserving optical fibers, a polarizer is inserted between the light source 15 and the optical coupler 18, and a linearly polarized optical signal is incident on the optical fiber loop 12. In this way, the stability of the zero point of the interference light intensity may be improved.

FIG. 1A is a perspective view showing an optical fiber vibration sensor according to a preferred embodiment of the present invention. FIG. 1B is a schematic diagram illustrating the internal configuration of the vibration sensor main body. It is a schematic diagram which shows the structure of an optical fiber gyro. It is a top view which shows the example of installation of an optical fiber vibration sensor. It is a graph which shows an output when vibration generate | occur | produces in a sensor cable. It is a schematic diagram which shows the optical fiber vibration sensor of suitable other embodiment which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical fiber vibration sensor 11 Vibration sensor main body 12 Optical fiber loop 12a Sensor cable 14 Case 15 Light source 16 Light receiver 18, 19 Optical coupler 21 Polarizer 22 Phase modulator 23 Signal processing unit

Claims (5)

  Two optical signals emitted from the light source, sequentially propagated through the first optical coupler, the polarizer, and the second optical coupler and branched are incident on the optical fiber loop including the phase modulator left and right, and the return light. In a Sagnac interference type optical fiber sensor that photoelectrically converts a signal with a light receiver via a second optical coupler, a polarizer, and a first optical coupler, detects a signal with a signal processing unit, and outputs data. An optical fiber vibration sensor using a part of a loop as a sensor cable for vibration measurement. Two optical signals emitted from the light source, propagated through the optical coupler, and branched are incident on the optical fiber loop including the phase modulator, and the return optical signal is photoelectrically converted by the light receiver via the optical coupler. In the Sagnac interference type optical fiber sensor that detects the signal by the signal processing unit and outputs the data,
An optical fiber vibration sensor, wherein a part of the optical fiber loop is used as a sensor cable for vibration measurement.   The optical fiber vibration sensor according to claim 1 or 2, wherein an optical fiber portion excluding a central portion of the optical fiber loop is used as a sensor cable for vibration measurement.   The signal processing unit determines the output level of vibration data detected by the light receiver, and outputs a signal for operating an alarm device or the like when an output level equal to or higher than a preset value is detected. The optical fiber vibration sensor according to claim 1, wherein the optical fiber vibration sensor is provided. 5. The optical fiber vibration sensor according to claim 1, wherein the optical fiber and the optical fiber loop connected to the light source, the light receiver, the polarizer, and the optical coupler are made of polarization-maintaining optical fibers.

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