Optical fiber sensor system for strain detection

An optical fiber sensor system includes a light source, a modulation unit, an optical coupler, a polarization separator, a first polarization controller optically coupled to the polarization separator, and a first detection unit that includes a first optical detector that receives the first component, converts the first component into a first electrical signal, and detects stress. The first polarization controller controls a polarization state of light input to the polarization separator so that the first electrical signal exhibits a first-order response to the stress.

BACKGROUND OF THE INVENTION

The present invention relates to an optical fiber sensor system.

Japanese Unexamined Patent Publication No. 2008-309776 describes an optical fiber vibration sensor including a light source, a light receiver, a light branching and coupling portion having a polarizer and two couplers, and a fiber loop portion. The light source and the light branching and coupling portion are coupled optically, and light of which a light polarization state is controlled such that it is uniform by the polarizer is output to the fiber loop portion through the light branching and coupling portion. Clockwise light and counterclockwise light are propagated to the fiber loop portion. The clockwise light and the counterclockwise light are recombined and interfere in the light branching and coupling portion, and interference light is obtained. The interference light is received by a light receiver and converted into an electrical signal. When the acoustic signal is applied to the fiber loop, a different phase difference is imparted to the clockwise light and the counterclockwise light and an interference state of the interference light is changed. Stress applied to the fiber loop is detected by the light receiver receiving the interference light of which the interference state has been changed.

SUMMARY OF THE INVENTION

An optical fiber sensor system according to one aspect of the present invention includes a light source that outputs measurement light; a modulation unit that includes a looped optical path and a coil around which the optical path is wound, and modulates light passing through the optical path using stress applied to the coil; an optical coupler that is optically coupled to the light source and both ends of the optical path, receives the measurement light, causes the measurement light to branch into first light and second light of which polarization states are different from that of the measurement light, outputs the first light from the one end of the optical path to the other end, outputs the second light from the other end of the optical path to the one end, combines the first light input to the other end with the second light input to one end, and outputs interference light; a polarization separator that separates the interference light into a first component and a second component of which polarization states are orthogonal to each other and outputs the first component and the second component; a first polarization controller optically coupled to the polarization separator; and a first detection unit that includes a first optical detector that is optically coupled to the polarization separator and receives the first component output from the polarization separator, and converts the first component input to the first optical detector into a first electrical signal to detect a stress, wherein the first polarization controller controls a polarization state of light input to the polarization separator so that the first electrical signal exhibits a first-order response to the stress.

DESCRIPTION OF EMBODIMENTS

Description of Embodiment of the Present Invention

First, details of an embodiment of the present invention will be described. (1) An optical fiber sensor system according to an embodiment of the present invention includes a light source that outputs measurement light; a modulation unit that includes a looped optical path and a coil around which the optical path is wound, and modulates light passing through the optical path using stress applied to the coil; an optical coupler that is optically coupled to the light source and both ends of the optical path, receives the measurement light, causes the measurement light to branch into first light and second light of which polarization states are different from that of the measurement light, outputs the first light from the one end of the optical path to the other end, outputs the second light from the other end of the optical path to the one end, combines the first light input to the other end with the second light input to one end, and outputs interference light; a polarization separator that separates the interference light into a first component and a second component of which polarization states are orthogonal to each other and outputs the first component and the second component; a first polarization controller optically coupled to the polarization separator; and a first detection unit that includes a first optical detector that is optically coupled to the polarization separator and receives the first component output from the polarization separator, and converts the first component input to the first optical detector into a first electrical signal to detect a stress, wherein the first polarization controller controls a polarization state of light input to the polarization separator so that the first electrical signal exhibits a first-order response to the stress.

In this optical fiber sensor system, the first polarization controller controls the polarization state of light input to the polarization separator so that the first electrical signal shows first-order dependence on stress. Accordingly, since a first electrical signal proportional to an amplitude of the acoustic signal due to stress can be obtained, it is possible to detect a weak acoustic signal with high sensitivity.

(2) The above-described optical fiber sensor system may further include a second detection unit including a second optical detector that is optically coupled to the polarization separator and receives the second component output from the polarization separator, the second detection unit converting the second component input to the second optical detector into the second electrical signal; and a control unit that controls the first polarization controller on the basis of the second electrical signal output from the second detection unit. Accordingly, it is possible to control the polarization state of light input to the polarization separator on the basis of the second component.

(3) In the above-described optical fiber sensor system, the control unit may control the first polarization controller so that a light intensity of the second component input to the second optical detector is minimized. Thus, it is possible to perform polarization control using the first polarization controller so that the light intensity of the first component is maximized. Therefore, it is possible to detect an acoustic signal with a higher sensitivity.

(4) The above-described optical fiber sensor system may further include a second polarization controller provided midway in the optical path. In this case, it is possible to control the polarization state of light passing through the optical path using the second polarization controller.

(5) In the above-described optical fiber sensor system, the control unit may control the second polarization controller on the basis of the first electrical signal. In this case, by the control unit controlling the second polarization controller, it is possible to control a polarization state of light passing through the optical path.

(6) In the above-described optical fiber sensor system, the control unit may control the second polarization controller so that a DC component of the first electrical signal becomes an average value between a maximum value and a minimum value. In this case, it is possible to increase a component showing first-order dependence on the stress.

(7) In the above-described optical fiber sensor system, the optical coupler may be a two-input two-output optical coupler. In this case, since ports of the optical coupler can be assigned for an input of measurement light, an output of interference light, and one end and the other end of the optical path, occurrence of light loss can be suppressed.

(8) In the above-described optical fiber sensor system, the measurement light may be linearly polarized light, the optical coupler may cause the measurement light to branch into the first light and the second light that are circularly polarized light, the modulation unit may have optical rotation characteristics and convert the first light and the second light into elliptically polarized light, and the optical coupler may combine the first light and the second light that are elliptically polarized light, and output the interference light that is elliptically polarized light different from the first light and the second light. By controlling the polarization state in this way, it is possible to efficiently generate a component exhibiting first-order dependence on the stress.

Detailed Description of Embodiment of Present Invention

A specific example of the optical fiber sensor system according to the embodiment of the present invention will be described with reference to the drawings. The present invention is not limited to such an example, and is intended to include all modifications defined in the claims and falling in a scope equivalent to the claims. In the following description, the same or corresponding elements are denoted with the same reference numerals in the description of the drawings, and repeated descriptions are omitted.

FIG. 1illustrates a configuration of an optical her sensor system1according to an embodiment. The optical fiber sensor system1is, for example, an optical fiber acoustic sensor that is used for diagnosing soundness of a structure, and detects acoustic emission (AE) that is generated when a crack occurs in the structure. The optical fiber sensor system1includes a light source11, an optical coupler12, a modulation unit13, a first polarization controller14, a polarization separator15, a first detection unit16, a second detection unit17, and a control unit18.

The light source11is optically coupled to the optical coupler12. The light source11outputs measurement light L1to the optical coupler12. For example, a broadband light source having a short coherent length can be used as the light source11. In this case, it is possible to increase resistance to reflected light noise or the like.

The optical coupler12receives the measurement light L1output by the light source11. The optical coupler12is a two-input two-output optical coupler having four ports12a,12b,12c, and12d. The port12aand the light source11are optically coupled, and the measurement light L1output from the light source11is input to the port12a. The optical coupler12causes the measurement light L1to branch into clockwise light (first light) L2and counterclockwise light (second light) L1The measurement light L1, the clockwise light L2, and the counterclockwise light L3have different polarization states.

The modulation unit13includes a looped optical path21that couples the port12cto the port12d, a coil22around which the optical path21is wound, a delay path23provided midway in the optical path21, and a second polarization controller24that controls a polarization state of light passing through the optical path21. One end of the optical path21is connected to the port12c, and the other end of the optical path21is connected to the port12d. Further, the delay path23is provided between the port12cand the coil22, and the second polarization controller24is provided between the coil22and the port12d.

The clockwise light L2is output from the port12cand is input to the port12dthrough the delay path23, the coil22, and the second polarization controller24. The counterclockwise light L3is output from the port12d.and is input to the port12cthrough the second polarization controller24, the coil22, and the delay path23.

The optical coupler12combines the clockwise light L2input to the port12dwith the counterclockwise light L3input to the port12cand outputs interference light L4from the port12b. Further, the optical path21includes an optical fiber, and is configured to have optical rotation characteristics. The optical rotation characteristics are realized, for example, by twisting of the optical fibers of the optical path21.

Therefore, the clockwise light L2and the counterclockwise light L3respectively output from the port12cand the port12d, and the interference light L4output from the port12bhave different polarization states. Further, since the clockwise light L2and the counterclockwise light L3propagate through the same optical path21, optical path lengths of the clockwise light L2and the counterclockwise light L3are the same. Thus, the optical coupler12and the modulation unit13of this embodiment constitute a Sagnac interferometer.

Specifically, the light source11outputs the measurement light L1that is linearly polarized light. The optical coupler12causes the measurement light L1to branch into the clockwise light L2and the counterclockwise light L3that are circularly polarized light, and outputs the clockwise light L2and the counterclockwise light L3to the coil22. Further, the modulation unit13has the optical rotation characteristics (for example, 45° optical rotation characteristics or an odd multiple thereof), converts the clockwise light L2and the counterclockwise light L3into elliptically polarized light, and outputs the elliptically polarized light to the optical coupler12. The optical coupler12combines the clockwise light L2and the counterclockwise light L3that are the elliptically polarized light, and outputs interference light L4that is the elliptically polarized light (for example, a polarization state thereof is rotated by 90°) different from the clockwise light L2and the counterclockwise light L3.

As illustrated inFIG. 2, the coil22has a core22aaround which the optical path21is helically wound. The coil22is attached to the structure S that is a measurement target, and functions as a sensor head of the optical fiber sensor system1. The structure S emits an acoustic signal A due to mechanical disruption or the like to expand and contract the optical path21through the coil22. In this case, due to stress that is applied to the coil22, a refractive index of the optical fiber constituting the optical path21is changed and the optical path length of the clockwise light L2and the counterclockwise light L3is changed. As a result, phase modulation occurs in the clockwise light L2and the counterclockwise light L3.

Referring back toFIG. 1, the delay path23imparts a delay to the clockwise light L2and the counterclockwise light L3. The counterclockwise light L3to which the delay has been imparted is directly output to the port12cof the optical coupler12. On the other hand, the clockwise light L2to which the delay has been imparted is directly output to the port12dof the optical coupler12through the coil22and the second polarization controller24. Therefore, a timing of the modulation applied to the clockwise light L2by the stress and a timing of the modulation applied to the counterclockwise light L3by the stress are different from each other.

In this embodiment, the delay path23is arranged on an outbound path for the clockwise light L2. Therefore, the acoustic signal A is propagated to the clockwise light L2with a delay of a predetermined time in comparison with the counterclockwise light L3. Further, the second polarization controller24is provided midway in the optical path21to control the polarization state of light passing through the optical path21.

The first polarization controller14is optically coupled to the port12bof the optical coupler12. The interference light L4output from the port12bis input to the first polarization controller14. The first polarization controller14controls the polarization state of the interference light L4input from the port12b. The control of the polarization state in the first polarization controller14and the second polarization controller24will be described in detail below.

The polarization separator15is optically coupled to the first polarization controller14. The light L5output from the first polarization controller14is input to the polarization separator15. The polarization separator15separates the input light L5into a first component L6and a second component L7having different polarization states and outputs the separated light. For example, the first component L6and the second component L7are orthogonal to each other.

The first detection unit16is optically coupled to the polarization separator15. The first component L6output from the polarization separator15is input to the first detection unit16, and a first electrical signal E1that is a detection signal for detecting an acoustic signal is generated. The first detection unit16includes, for example, a first optical detector16aand a first amplifier16b. The first optical detector16aconverts a light intensity of the input first component L6into a voltage value. The first amplifier16bgenerates the first electrical signal E1from the voltage value obtained by the first optical detector16aand outputs the first electrical signal E1.

The second detection unit17is optically coupled to the polarization separator15. A second component L7output from the polarization separator15is input to the second detection unit17, and a second electrical signal E2that is a detection signal for detecting an acoustic signal is generated. The second detection unit17includes, for example, a second optical detector17aand a second amplifier17b. The second optical detector17aconverts a light intensity of the input second component L7into a voltage value. The second amplifier17bgenerates the second electrical signal E2from the voltage value obtained by the second optical detector17aand outputs the second electrical signal E2.

The control unit18controls the first polarization controller14on the basis of the second electrical signal E2output from the second detection unit17. The control unit18controls the first polarization controller14so that the light intensity of the second component L7input to the second detection unit17is minimized. The control unit18controls the polarization state of the light L5using the first polarization controller14to add a component proportional to the amplitude of the acoustic signal A to the interference light L4.

Further, the control unit18controls the second polarization controller24on the basis of the first electrical signal E1output from the first detection unit16. The control unit18controls the second polarization controller24so that the DC component of the first electrical signal E1is an average value between the maximum value and the minimum value. Thus, the control unit18performs polarization control of the light passing through the optical path21using the second polarization controller24. Therefore, it is possible to increase a component proportional to the amplitude of the acoustic signal A.

FIG. 3is a graph illustrating a relationship between stress applied to the coil22and an intensity of the electrical signals E1and E2. As illustrated inFIG. 3, if the second component L7on the side on which the output is minimized is dark polarization and the first component L6orthogonal to the dark polarization is bright polarization, the signal intensity of the dark polarization is proportional to a square of the stress, and the signal intensity of the bright polarization is proportional to the first power of stress when the weak signal is input due to the stress.

FIG. 4is a graph illustrating time-series data of the intensity of the electrical signal E1in each of bright polarization and dark polarization. When the DC component of the bright polarization has been adjusted to a maximum side or a minimum side, a component proportional to a square of the stress in the electrical signal E1increases. On the other hand, when the second polarization controller24controls the polarization state of the light passing through the optical path21and the bright polarization is adjusted to an average value between the maximum value and the minimum value, a component proportional to the stress can be maximized. Both of the minimum value and the maximum value described above are values that vary with the resolution of the second polarization controller24, and indicate measurable minimum and maximum values.

Next, effects obtained from the optical fiber sensor system1will be described in greater detail.

In the optical fiber sensor system1, the light L5of which the polarization state has been controlled is output from the first polarization controller14so that the interference light including a component exhibiting a first-order dependence on the acoustic signal A is output. In the polarization separator15, the first component L6exhibiting the first-order dependence on the acoustic signal A is output as a branch and is input to the first optical detector16a. The first polarization controller14controls the polarization state of the interference light L4so that the first electrical signal E1exhibits first-order dependence on stress. Therefore, it is possible to detect a weak acoustic signal A with high sensitivity.

Further; the optical fiber sensor system1includes the second detection unit17that includes the second optical detector17athat is optically coupled to the polarization separator15and receives the second component L7output from the polarization separator15, and converts the second component L7input to the second optical detector17ainto the second electrical signal E2and the control unit18that controls the first polarization controller14on the basis of the second electrical signal E2output from the second detection unit17. Thus, the polarization state of the light L5input to the polarization separator15can be controlled on the basis of the second electrical signal E2.

Further, in the optical fiber sensor system1, the control unit18controls the first polarization controller14so that a light intensity of the second component13input to the second detection unit17is minimized. Accordingly, since the first polarization controller14can be controlled so that the light intensity of the first component L6is maximized, it is possible to detect the acoustic signal A with high sensitivity.

Further, the optical fiber sensor system1includes a second polarization controller24provided midway in the optical path21. Therefore, the polarization state of light passing through the optical path21can be controlled by the second polarization controller24.

Further, in the optical fiber sensor system1, the control unit18controls the second polarization controller24on the basis of he first electrical signal E1. Thus, the control unit18controls the second polarization controller24, making it possible to control the polarization state of light passing through the optical path21.

Further, in the optical fiber sensor system1, the control unit18controls the second polarization controller24so that a DC component of the first electrical signal E1becomes an average value of a maximum value and a minimum value. Accordingly, since the first electrical signal E1exhibiting first-order dependence on stress can be output, it is possible to detect a weak acoustic signal A with high sensitivity.

Further, in the optical fiber sensor system1, the optical coupler12is a two-input two-output optical coupler. Thus, the ports12ato12dof the optical coupler12can he assigned for an input of the measurement light L1, an output of the interference light L4, and one end and the other end of the optical path21, and occurrence of light loss can be suppressed.

Further, in the optical fiber sensor system1, the measurement light L1is linearly polarized light, and the optical coupler12causes the measurement light L1to branch into the clockwise light L2and the counterclockwise light L3that are circularly polarized light. The modulation unit13has optical rotation characteristics and converts the clockwise light L2and the counterclockwise light L3into elliptically polarized light. The optical coupler12combines the clockwise light L2and the counterclockwise light L3that are elliptically polarized light, and outputs the interference light L4that is elliptically polarized light different from the clockwise light L2and the counterclockwise light L3. By constituting the optical coupler12and the modulation unit13in this way, it is possible to efficiently add a component having a polarization state exhibiting a first-order response to stress to the interference light L4. Therefore, it is possible to efficiently generate a component exhibiting the first-order response to the stress.

An embodiment of the present invention has been described, but the present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the gist of the present invention. For example, in the above-described embodiment, the delay path23is arranged in an outbound path for the clockwise light L2. However, an arrangement aspect of the delay path23is not limited to the above embodiment and, for example, the delay path23may be arranged in an outbound path for the counterclockwise light L3.