JP2007309883A - External force detection system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect a cutting position when a polarization maintaining optical fiber is cut for some certain reason, in addition to a behavior location of an external force (side-thrust). <P>SOLUTION: In an external force detection system, equipped with a polarization maintaining optical fiber which has two polarization modes from which the light propagation velocity differs and a treatment apparatus which detects behavior location of the external force, based on a polarization maintaining optical fiber which has two polarization modes from which the light propagation velocity differs and an optical path difference with light which has a polarization mode of another side which the external force interacts on the light and the polarization maintaining optical fiber which have one polarization mode, and is generated in the polarization maintaining optical fiber; a treatment apparatus detects the cutting position of the polarization maintaining optical fiber, based on the optical path difference of the light which enters to the polarization maintaining optical fiber, and returning the light acquired from the polarization maintaining optical fiber. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an external force detection system.

Non-Patent Document 1 below uses a polarization maintaining optical fiber (also referred to as a polarization maintaining optical fiber) having two polarization modes (fast axis mode and slow axis mode) having different light propagation speeds as a sensor. When a laser beam in one polarization mode is incident on the polarization maintaining optical fiber, the other pressure generated by the side pressure (external force) acting on the laser light in the one polarization mode and the polarization maintaining optical fiber. A technique for measuring a position where an external force is applied by detecting an optical path difference with a laser beam in a polarization mode on one end side of a polarization maintaining optical fiber is disclosed.
Heiyuan et al., 3 others, “One-sided input / measurement of polarization-maintaining optical fiber side pressure sensor by combining optical coherence function”, 21st Sensing Forum of Society of Instrument and Control Engineers (Toyo University, 2B1-1, September 2004), pp.249-253

  By the way, in the prior art disclosed in Non-Patent Document 1, when the polarization maintaining optical fiber is cut for some reason, laser light is acquired from the polarization maintaining optical fiber at one end side of the polarization maintaining optical fiber. Therefore, the cutting position of the polarization maintaining optical fiber cannot be detected. For example, when such a conventional technology is applied to intrusion detection (that is, security related equipment) of a suspicious person or the like, the situation that the cutting position of the polarization maintaining optical fiber cannot be measured is security related equipment. This is an extremely serious problem in terms of personality, and an immediate solution is desired.

  An object of the present invention is to detect a cutting position when a polarization maintaining optical fiber is cut for some reason, in addition to a position where an external force (side pressure) is applied.

  In order to achieve the above object, in the present invention, as a first solution, a polarization maintaining optical fiber having two polarization modes having different light propagation speeds and one polarization mode of the two polarization modes. And having a frequency modulated at a predetermined frequency shift is generated by a light source and emitted to one end of the polarization maintaining optical fiber, and an external force is applied to the light having one polarization mode and the polarization maintaining optical fiber. An external force detection system comprising: a processing device that detects an action position of an external force based on an optical path difference with light having the other polarization mode generated in the polarization maintaining optical fiber. Employs a means for detecting the cutting position of the polarization maintaining optical fiber based on the optical path difference between the light incident on the polarization maintaining optical fiber and the reflected light acquired from the polarization maintaining optical fiber.

  As a second solution, in the first means, the light of one polarization mode provided in the other end of the polarization maintaining optical fiber and propagating through the polarization maintaining optical fiber is changed to the other polarization mode. The processing device is provided with a terminating device that replaces and enters the polarization-maintaining optical fiber, and the processing device acquires return light from the polarization-maintaining optical fiber using an optical coupler provided at one end of the polarization-maintaining optical fiber. A means is adopted in which the action position of the external force is detected based on the optical path difference between both polarization modes of light included in the light.

  As a third solution, in the above first or second means, the processing device may calculate the frequency of the light in the other polarization mode generated by an external force among the light acquired from the polarization maintaining optical fiber. While shifting by a predetermined amount, phase-modulating light in one polarization mode or light generated from a light source obtained from a polarization-maintaining optical fiber, and interfering the frequency-shifted light with the phase-modulated light The means for detecting the action position of the external force and the cutting position of the polarization-maintaining optical fiber is adopted.

According to the present invention, the polarization-maintaining optical fiber has the same detection principle as the detection of the acting position of the external force based on the optical path difference between the light incident on the polarization-maintaining optical fiber and the reflected light acquired from the polarization-maintaining optical fiber. The cutting position can be detected.
Therefore, it is possible to solve the problem of the prior art that the cutting position cannot be specified because return light cannot be obtained from the polarization maintaining optical fiber.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the present embodiment, the external force detection system according to the present invention is applied to an intrusion detection system that detects intrusion of a suspicious person into a predetermined monitoring area.

  1 and 2 are system configuration diagrams of the intrusion detection system. This intrusion detection system has an external force detection mode and a cutting detection mode as operation modes. FIG. 1 shows a state where the intrusion detection system is set to the external force detection mode, and FIG. 2 shows a state where the intrusion detection system is set to the cutting detection mode.

  As shown in these drawings, the external force detection system includes a processing device 1, a sensing fiber 2 (external force detection optical fiber), and a termination device 3. The processing device 1 includes a laser light source 1a, a first 3 dB coupler 1b, an optical switch 1c, a polarization beam splitter 1d, an optical phase modulator 1e, a phase-modulated signal generator 1f, a frequency shifter 1g, a local signal generator 1h, 2 3 dB coupler 1 i, light receiver 1 j, filter circuit 1 k, signal processing device 1 m, and light source control circuit 1 n.

  First, the sensing fiber 2 will be described. The sensing fiber 2 is birefringent and has two polarization modes (fast axis mode and slow axis mode) that are orthogonal to each other and have different light propagation speeds. It is a wave holding optical fiber. A polarization-maintaining optical fiber has a property that, when an external force (side pressure) acts and is distorted, mode coupling of light occurs between the fast axis mode and the slow axis mode. That is, the sensing fiber 2 propagates the laser light while maintaining the polarization direction when, for example, laser light having the polarization direction in the fast axis mode is incident. Although mode mode laser light is not generated, when an external force is applied, the polarization state is lost, and laser light having the slow axis mode as the polarization direction is generated.

Further, in the sensing fiber 2, since the light propagation speed is different between the fast axis mode and the slow axis mode, the laser light in the fast axis mode and the laser light in the slow axis mode have an optical path difference. When the two are compared, the phases are different. By using such a sensing fiber 2, it is possible to specify a position where an external force is applied based on an optical path difference (that is, a phase difference) between the fast axis mode laser beam and the slow axis mode laser beam.
The technique for measuring the position where the side pressure is applied using the polarization maintaining optical fiber is disclosed in detail in Non-Patent Document 1 described above.

  Such a sensing fiber 2 is laid on the outer periphery of the monitoring area in order to detect a suspicious person who is about to enter the monitoring area, and the measurement light is incident from the processing device 1 connected to one end. Return light is incident from the terminating device 3 connected to the other end. The measurement light propagates in the sensing fiber 2 from one end to the other end and enters the termination device 3, while the return light propagates in the sensing fiber 2 from the other end to the one end. Is incident on.

  The laser light source 1a is a semiconductor laser that oscillates a laser beam (measurement light) having a predetermined wavelength and emits it to the first 3 dB coupler 1b. The oscillation wavelength can be varied according to the injection current input from the light source control circuit 1n. Is. That is, the laser light source 1a emits frequency-modulated light (wavelength-modulated light) whose frequency (wavelength) changes in a step shape by the step-wave shaped injection current input from the light source control circuit 1n to the first 3 dB coupler 1b. To do. This wavelength-modulated light is an optical signal whose center wavelength is, for example, 1.55 μm and the wavelength changes with a predetermined wavelength width, and its polarization direction is set to the fast axis mode of the sensing fiber 2. The light source control circuit 1n generates a step-wave shaped injection current based on the synchronization signal input from the signal processing device 1m, and outputs it to the laser light source 1a.

  The first 3 dB coupler 1b is an optical coupler having two pairs of optical input / output ports, and emits wavelength-modulated light incident from the laser light source 1a to the sensing fiber 2 as measurement light and to the optical switch 1c as reference light. On the other hand, the return light input from the sensing fiber 2 is output to the polarization beam splitter 1d. The optical switch 1c is selectively controlled by the signal processing device 1m to selectively select either the reference light incident from the first 3 dB coupler 1b or the return light of the sensing fiber 2 incident from the polarization beam splitter 1d. And output to the optical phase modulator 1e.

  As can be seen from a comparison between FIG. 1 and FIG. 2, the optical switch 1c is controlled by the signal processing device 1m to select the return light incident from the polarization beam splitter 1d in the external force detection mode. On the other hand, in the cut detection mode, the reference light incident from the first 3 dB coupler 1b is selected and emitted to the optical phase modulator 1e.

The polarization beam splitter 1d transmits / reflects the return light according to the deflection direction. That is, the polarization beam splitter 1d transmits, for example, the light component in the polarization direction that matches the fast axis mode of the sensing fiber 2 out of the return light incident from the first 3 dB coupler 1b and emits it to the frequency shifter 1g. The light component in the polarization direction that matches the slow axis mode is reflected and emitted to the optical switch 1c.
The polarization direction of the return light (slow axis mode component) emitted from the polarization beam splitter 1d to the optical switch 1c matches the polarization direction of the return light (fast axis mode component) emitted to the frequency shifter 1g. Have been adjusted so that.

  The optical phase modulator 1e is configured to phase the laser light (either reference light or return light) input from the optical switch 1c based on the modulated signal (electric signal) input from the phase modulated signal generator 1f. The phase modulated light generated by the phase modulation is emitted to the second 3 dB coupler 1i. Under the control of the signal processing device 1m, the phase-modulated signal generator 1f generates a step-wave-like phase-modulated signal and outputs it to the optical phase modulator 1e. As shown in FIG. 3, the phase-modulated signal is synchronized with the change of the injection current and corresponds to the position (segmented region) of the sensing fiber 2.

  The frequency shifter 1g shifts (shifts) the frequency of the return light incident from the polarization beam splitter 1d by a minute fixed amount based on the local signal (electric signal) input from the local signal generator 1h. Displacement light is used, and the frequency displacement light is emitted to the second 3 dB coupler 1i. The local signal generator 1h generates the local signal and outputs it to the frequency shifter 1g under the control of the signal processing device 1m.

  The second 3 dB coupler 1 i is an optical coupler having two pairs of optical input / output ports, and combines the phase-modulated light incident from the phase-modulated signal generator 1 f and the frequency-shifted light incident from the frequency shifter 1 g. At the same time, the combined light generated by the combining is emitted to the light receiver 1j and the terminator. Here, the combined light is a combination of phase-modulated light and frequency-displaced light. By generating a step-wave shaped injection current and driving the laser light source 1a, a coherence function is synthesized, Since the interference at the position is realized, a beat signal is generated only when an external force is applied. The frequency shifter 1g described above is provided so that the beat in the combined light is not a beat at zero frequency (zero beat) but a beat at a specific frequency.

  That is, the optical switch 1c, the polarization beam splitter 1d, the optical phase modulator 1e, the phase modulated signal generator 1f, the frequency shifter 1g, the local signal generator 1h, and the second 3 dB coupler 1i described above are a kind of optical interferometer. It is composed. This optical interferometer determines the interference state between two optical signals to be measured, that is, the fast axis mode component of the return light and the slow axis mode component of the return light (or reference light = measurement light) by means of the injected current. The detection is performed according to the determined frequency (wavelength) of the measurement light and the position of the sensing fiber 2 selected by the modulated signal.

  The light receiver 1j receives the combined light, which is an optical signal, and outputs a detection signal, which is an electrical signal, to the filter circuit 1k. This detection signal is an electric signal indicating a voltage corresponding to the light intensity of the combined light. The filter circuit 1k is a low-pass filter for removing unnecessary high frequency components from the detection signal, and outputs the detection signal from which unnecessary high frequency components are removed to the signal processing device 1m. The terminator is a non-reflective end that does not reflect the combined light.

  The signal processing device 1m synchronously controls the light source control circuit 1n, the optical switch 1c, the phase-modulated signal generator 1f, and the frequency shifter 1g based on a predetermined control / signal processing program and is input from the filter circuit 1k. Signal processing is performed on the detection signal. Details of control processing and signal processing by the signal processing device 1m will be described later.

  The terminating device 3 is composed of a polarization beam splitter 3a and a terminating optical fiber 3b as shown in the figure. The polarization beam splitter 3a transmits the measurement light incident from the sensing fiber 2 and emits the measurement light to the termination optical fiber 3b, while emitting return light incident from the termination optical fiber 3b to the sensing fiber 2.

  The terminal optical fiber 3b is an optical fiber in which a 90-degree twisted connection portion is formed at an intermediate position. That is, the terminating optical fiber 3b emits, as return light, the polarization beam splitter 3a that has the polarization direction changed by 90 degrees with respect to the measurement light incident from the polarization beam splitter 3a. As described above, in this embodiment, the polarization direction of the measurement light matches the fast axis mode of the sensing fiber 2, so the polarization direction of the return light matches the slow axis mode of the sensing fiber 2.

Next, the operation of the intrusion detection system configured as described above will be described in detail.
As described above, the intrusion detection system includes an external force detection mode and a cutting detection mode. The external force detection mode is a normal operation mode for detecting an external force acting on the sensing fiber 2, that is, an intrusion of a suspicious person or the like. On the other hand, the cut detection mode is a predetermined time for detecting the cut of the sensing fiber 2. This is a non-normal operation mode that is set for a short period at regular intervals.

FIG. 3 shows the relationship between the injection current input from the light source control circuit 1n to the laser light source 1a, the phase-modulated signal input from the phase-modulated signal generator 1f to the optical phase modulator 1e, and the segmented region of the sensing fiber 2. FIG. Injection current, a tone burst signal having a predetermined repetition period T _S with the current value changes stepwise at a predetermined level change period T _L. The phase-modulated signal is a staircase signal which has a number of steps corresponding to the resolution for the length of the sensing fiber 2 with synchronized with the repetition period T _S of the injection current.

In the laser light source 1a, the laser light having a wavelength corresponding to the injection current input from the light source control circuit 1n under the control of the signal processing device 1m, that is, the wavelength changes stepwise in synchronization with the level change period _TL. the laser beam is repeated for each period T _S repeat the change (measurement light) emitted to the first 3dB coupler 1b oscillates with. For such a laser light source 1a, the optical phase modulator 1e phase-modulates the return light based on the phase-modulated signal input from the phase-modulated signal generator 1f under the control of the signal processing device 1m. To do. The frequency modulation (wavelength modulation) of the measurement light and the phase modulation of the return light are synchronized by the light source control circuit 1n and the phase-modulated signal generator 1f being synchronously controlled by the phase-modulated signal generator 1f. Done.

  First, the external force detection mode will be described. The signal processing device 1m sets the intrusion detection system to the external force detection mode by controlling the optical switch 1c so as to select the return light incident from the polarization beam splitter 1d. In the external force detection mode, the laser light emitted from the laser light source 1 a to the first 3 dB coupler 1 b is measured light whose polarization direction matches the fast axis mode of the sensing fiber 2 from the first 3 dB coupler 1 b to the sensing fiber 2. And is propagated again in the sensing fiber 2 as return light whose polarization direction has been changed in the same direction as the slow axis mode by the terminating device 3, and passes through the first 3 dB coupler 1b. Then, the light enters the polarizing beam splitter 1d.

  In this manner, the laser light (measurement light or return light) propagating twice in the sensing fiber 2 does not cause mode coupling in the state where no external force (side pressure) acts on the sensing fiber 2, and therefore returns to the measurement light. Light does not interfere with each other. Therefore, in such a state, the return light incident on the polarization beam splitter 1d includes only the laser light having the slow axis mode as the polarization direction, and is reflected by the polarization beam splitter 1d to cause the optical switch 1c to be reflected. Through the optical phase modulator 1e and not into the frequency shifter 1g.

  The return light that has entered the optical phase modulator 1e is phase-modulated by the modulated signal described above and is incident on the second 3 dB coupler 1i. However, since the return light is not incident on the frequency shifter 1g, No return light is incident on the 3 dB coupler 1i from the frequency shifter 1g, so only the return light emitted from the optical phase modulator 1e is detected by the light receiver 1j. Therefore, the detection signal output from the light receiver 1j is an electric signal that includes a relatively small noise component but has a relatively stable amplitude.

  On the other hand, when an external force (side pressure) is applied at any position of the sensing fiber 2, mode coupling occurs between the measurement light and the return light at the external force application position (external force application point). Therefore, in this case, the return light incident on the polarization beam splitter 1d includes, as components, light having the fast axis mode of the sensing fiber 2 as the polarization direction and light having the slow axis mode of the sensing fiber 2 as the polarization direction. It will be a thing. Of such return light, a component whose polarization direction is the slow axis mode is converted into phase modulated light by the phase modulated signal in the optical phase modulator 1e and then incident on the second 3 dB coupler 1i. The component whose polarization direction is the fast axis mode is converted into frequency-shifted light that is frequency-shifted by a predetermined frequency in the frequency shifter 1g and then enters the second 3 dB coupler 1i.

  Here, since there is a certain speed difference between the light propagation speed of the fast axis mode and the light propagation speed of the slow axis mode, the return light of the fast axis mode propagated from the external force acting point to the polarization beam splitter 1d and A phase difference corresponding to the optical path length occurs between the slow axis return light and the light.

  In this intrusion detection system, in order to specify the optical path length determined by the external force action point, that is, the external force action point, the relationship between the injection current and the phase-modulated signal shown in FIG. 3 is used. That is, when an external force (side pressure) is applied at any position of the sensing fiber 2, the wavelength of the measurement light sequentially changes within a predetermined range by the injected current, and the slow axis mode component of the return light is the phase-modulated signal. The phase-modulated light is step-modulated by the optical axis, and the combined light generated by combining the phase-modulated light based on the slow-axis mode component and the frequency-shifted light based on the fast-axis mode component of the return light is a sensing fiber. 2 in the phase-modulated signal corresponding to each segmented region on 2, the beat (beat) is generated by interference only in the segmented region including the external force action point, and an optical signal whose light intensity is extremely increased is obtained. FIG. 3 shows, as an example, a state in which an external force is applied in a segmented region corresponding to the third stage of the phase-modulated signal.

  In the signal processing device 1m, which divided region on the sensing fiber 2 corresponds to the point of action of the external force based on the relationship between the detection signal input from the light receiver 1j via the filter circuit 1k and the phase-modulated signal. And outputs the result as a measurement result to the outside.

Next, the operation of the disconnection detection mode of the intrusion detection system will be described in detail.
As shown in FIG. 2, the signal processing device 1m controls the optical switch 1c to select the measurement light (reference light) incident from the first 3 dB coupler 1b, thereby disconnecting the intrusion detection system from the disconnection detection mode. Set to.

  That is, in this cut detection mode, a component of the return light whose polarization direction is the fast axis mode of the sensing fiber 2 is incident on the frequency shifter 1g, and measurement light is incident on the optical phase modulator 1e. Then, the combined light incident on the light receiver 1j from the second 3 dB coupler 1j is a phase-modulated light based on the frequency-shifted light in which the component whose polarization direction is the fast axis mode of the sensing fiber 2 in the return light and the measurement light. Are combined.

  Here, when the sensing fiber 2 is cut, the intensity of the return light is significantly reduced. That is, only a slight intensity of reflected light reflected from the cutting point enters the first 3 dB coupler 1b from the sensing fiber 2 and enters the polarizing beam splitter 1d from the first 3 dB coupler 1b. . Of the reflected light having such a small intensity, only the component whose polarization direction coincides with the fast axis mode is transmitted through the polarization beam splitter 1d and incident on the frequency shifter 1g to become frequency displacement light.

  Therefore, in the same manner as the specific principle of the position of the external force acting point in the external force detection mode described above (more precisely, the segmented region on the sensing fiber 2), the frequency displacement light based on the reflected light and the phase modulated light based on the measurement light Based on the phase difference (optical path difference), it is possible to specify the position of the cutting point (the segmented region on the sensing fiber 2).

  According to the present embodiment, in the external force detection mode, the position of the external force action point is specified based on the phase difference between the phase-modulated light based on the return light and the frequency displacement light based on the return light, but in the cutting detection mode, Generates phase-modulated light based on measurement light instead of phase-modulated light based on return light, and is similar to the external force detection mode based on the phase difference between the phase-modulated light based on the measurement light and the frequency displacement light based on the return light The cutting position of the sensing fiber 2 can be specified by the measurement principle.

Next, FIG. 4 is a system configuration diagram showing a modification of the above embodiment.
The operating principle of the intrusion detection system according to this modification is exactly the same as that of the intrusion detection system according to the above-described embodiment. By drawing the tip of the fiber 2 into the processing apparatus 1, transmitted light (laser light emitted from the tip after the measurement light propagates through the sensing fiber 2) emitted from the tip is used.

  In other words, the intrusion detection system according to this modification has a configuration in which the second optical switch 1p is newly added to the intrusion detection system according to the above embodiment. The second optical switch 1p selectively selects either the reflected light incident from the first 3 dB coupler 1b or the transmitted light incident from the tip of the sensing fiber 2 under the control of the signal processing device 1m. The transmitted light is emitted to the polarizing beam splitter 1d, and the transmitted light is selected in the external force detection mode, and the reflected light is selected in the cut detection mode.

In the intrusion detection system configured as described above, in the external force detection mode, the external force action point is measured based on the fast axis component and the slow axis component of the transmitted light, while in the cut detection mode, the reflection generated at the cutting point is measured. A cutting point is measured based on the light and the measurement light.

1 is a first system configuration diagram of an intrusion detection system according to an embodiment of the present invention. It is a 2nd system block diagram of the intrusion detection system concerning one Embodiment of this invention. It is a wave form diagram for demonstrating operation | movement of the intrusion detection system concerning one Embodiment of this invention. It is a system configuration | structure figure of the intrusion detection system concerning the modification of one Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Processing apparatus, 1a ... Laser light source, 1b ... 1st 3 dB coupler, 1c ... Optical switch, 1d ... Polarizing beam splitter, 1e ... Optical phase modulator, 1f ... Phase modulated signal generator, 1g ... Frequency shifter, DESCRIPTION OF SYMBOLS 1h ... Local signal generator, 1i ... 2nd 3dB coupler, 1j ... Light receiver, 1k ... Filter circuit, 1m ... Signal processing apparatus, 1n ... Light source control circuit, 2 ... Sensing fiber (external force detection optical fiber), 3 ... Termination device, 3a ... Polarizing beam splitter, 3b ... Termination fiber

Claims (3)

A polarization maintaining optical fiber having two polarization modes with different light propagation speeds;
Light having one of the two polarization modes and frequency-modulated with a predetermined frequency shift is generated by a light source and emitted to one end of the polarization-maintaining optical fiber. And a processing device for detecting an action position of the external force based on an optical path difference between the light having a polarization mode and the light having the other polarization mode generated in the polarization maintaining optical fiber by an external force acting on the polarization maintaining optical fiber; An external force detection system comprising:
The processing apparatus detects a cutting position of the polarization maintaining optical fiber based on an optical path difference between light incident on the polarization maintaining optical fiber and reflected light acquired from the polarization maintaining optical fiber. External force detection system. A terminating device is provided at the other end of the polarization maintaining optical fiber, and replaces the light of one polarization mode that has propagated through the polarization maintaining optical fiber with the other polarization mode to enter the polarization maintaining optical fiber. ,
The processing device acquires return light from the polarization maintaining optical fiber using an optical coupler provided at one end of the polarization maintaining optical fiber, and determines the optical path difference between both polarization modes included in the return light. The external force detection system according to claim 1, wherein an external force application position is detected based on the external force detection position.   The processing device shifts the frequency of the light of the other polarization mode generated by the external force among the light acquired from the polarization maintaining optical fiber by a predetermined amount, while the light acquired from the polarization maintaining optical fiber One of the polarization mode light or the light generated by the light source is phase-modulated, and the frequency-shifted light and the phase-modulated light are made to interfere with each other to detect the position where the external force is applied and the position where the polarization-maintaining optical fiber is cut. The external force detection system according to claim 1 or 2, wherein

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