JP2011106943A - Measurement apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a measurement apparatus for stabilizing the sensitivity of a probe, and surely measuring an electric field strength or a magnetic field strength. <P>SOLUTION: The measurement apparatus includes a laser light source 12, a polarization controller 13 for converting a polarization state of a light, an optical fiber 11 for guiding the light emitted from the polarization controller 13, the probe 19 provided in the optical fiber 11, a photodetector 111 for entering the light emitted from the probe 19 through the optical fiber 11, and a polarization monitor 112 for detecting the polarization state of the light entering into the photodetector 111. The probe 19 has a polarizer 15, an EO crystal 17 for changing a refractive index, and a reflection mirror 18 for reflecting a light transmitted through the EO crystal 17. The polarization controller 13 converts the polarization state of the light entering from the laser light source 12 to cause the polarization state of the light entering into the optical fiber 11 to be a polarization state for canceling a change in the polarization state which is applied to the light within the optical fiber 11. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a measuring device.

  As a device for measuring electric field strength or magnetic field strength, a measuring device using laser light is known. For example, Non-Patent Document 1 shows a configuration of a measuring device in which an electro-optic (EO) crystal or a magneto-optic (MO) crystal is provided at one end of an optical fiber that guides laser light. Has been. In such a measuring device, an electro-optic effect or a magneto-optic effect that changes the polarization state of laser light passing through these crystals by changing the refractive index of the EO crystal by an electric field or the refractive index of an MO crystal by a magnetic field. Is used to measure the electric field of the EO crystal or the magnetic field of the MO crystal.

  That is, in the above-described measuring apparatus, when the EO / MO crystal is included in the detection probe and the detection probe is disposed at the measurement location of the electric field or magnetic field, the refractive index of the EO / MO crystal changes due to the influence of the electric field or magnetic field. To do. Laser light is transmitted through the crystal having a changed refractive index, and the laser light is converted into plane polarized light by an analyzer, so that polarized light having light intensity corresponding to the electric field / magnetic field of the EO / MO crystal is extracted. . The electric field / magnetic field of the EO / MO crystal is detected by converting the change in the polarization state of the plane polarized light into an electric signal by a photodetector.

  FIG. 6 is a schematic diagram of the measurement apparatus 6 described above. The measuring device 6 is configured such that the laser light emitted from the laser light source 62 passes through the polarization controller 63 and enters the probe 69 via the circulator 64. In the probe 69, the laser light reaches the reflection mirror 68 through the polarizer 65, the phase difference plate 66, and the EO crystal 67. Thereafter, the laser light travels in the reverse optical path, and the amount of light is detected by the photodetector 610 via the circulator 64.

  In the EO crystal 67, since the refractive index changes according to the magnitude of the electro-optic effect caused by the electric field at the measurement location, the polarization state of the light incident on the EO crystal 67 changes. Since the amount of light emitted from the probe 69 (the amount of light that can be transmitted through the polarizer 65) changes in accordance with the amount of change in the polarization state, the amount of light is detected by the photodetector 610, so that it is indirectly detected. It is possible to detect the electric field / magnetic field at the measurement location where the probe 69 is disposed.

  The measuring device having such a configuration can downsize the probe portion by using a small EO / MO crystal as compared with a conventional measuring device that does not use an EO / MO crystal. In addition, since no metal part is used for the probe part or cable, it is minimally invasive, and since the EO / MO crystal responsiveness to electric / magnetic fields is in the high frequency band of GHz or higher, it has excellent detection characteristics. It has been known.

  However, in the measurement apparatus having such a configuration, if the intensity of light entering the EO crystal 67 and the polarization state are not constant, the sensitivity of the obtained signal intensity is not stable. However, when the probe 69 is routed, the polarization state of the light transmitted through the optical fiber 61 is not constant due to the stress applied to the optical fiber 61 that connects the circulator 64 and the probe 69 and the temperature change of the measurement environment. Since the intensity of light entering the EO crystal 67 through the slab is not constant, there is a problem that the sensitivity is not stable.

  With respect to this problem, Patent Documents 1 and 2 show a configuration for stabilizing the sensitivity. In either case, feedback control is performed using the output detected by the photodetector, and polarization control is performed on the optical path between the circulator and the photodetector, thereby suppressing a decrease in sensitivity of the probe.

JP 2006-266711 A JP 2005-292068 A

S. Wakana, T. Ohara, M. Abe, E. Yamazaki, M. Kishi and M. Tsuchiya, “Fibera-Edge Electrooptic / Magnetooptic Probe for Spectral-Domain Analysis of Electromagnetic Field,” IEEE Trans. Microw. Theory Tech. , vol. 48, no. 12, pp.2611-2616

  However, the configuration of the above patent document has the following problems. That is, in the configuration of Patent Document 1, it is necessary to adjust the control system depending on the observation conditions, and it is difficult to stabilize the detection sensitivity of the probe. In particular, when measuring a minute signal, it is necessary to narrow the frequency span of the spectrum analyzer and slow down the sweep time. Therefore, the time required for stabilization is slowed by the conditions. Moreover, since the configuration of Patent Document 2 is a complicated control system using a lock-in amplifier, the measurement is limited.

  The present invention has been made in view of such circumstances, and is a measurement that can reliably measure the electric field strength or magnetic field strength by canceling the fluctuation of the polarization state in the optical fiber and stabilizing the sensitivity of the probe. An object is to provide an apparatus.

  In order to solve the above-described problems, the measurement apparatus of the present invention is directed to a light source, a polarization conversion unit that converts a polarization state of light emitted from the light source, and light emitted from the polarization conversion unit. An optical fiber, a measurement probe provided at one end of the optical fiber, a light amount detection unit in which light emitted from the measurement probe enters through the optical fiber, and a polarization state of the light incident on the light amount detection unit A polarization state detecting unit that converts the light guided through the optical fiber into linearly polarized light and a refractive index that changes due to an electro-optic effect or a magneto-optic effect. An optical crystal on which light through the polarizing element is incident, and a reflection unit that reflects the light transmitted through the optical crystal toward the optical fiber, and the polarization conversion unit includes: Based on the polarization state of the light detected by the light state detector, the polarization state of the light incident on the optical fiber is a polarization state that cancels a change in the polarization state received by the light in the optical fiber. The polarization state of light incident from a light source is converted.

  Further, in the present invention, a configuration may be adopted in which a beam splitter that maintains a polarization state of the light incident on the light amount detection unit and separates a part thereof and guides the light to the polarization state detection unit may be employed.

  In the present invention, the polarization conversion unit includes an electro-optic crystal on which the light is incident, and controls a polarization state of the light by controlling a state of application to the electro-optic crystal. be able to.

  Further, in the present invention, the measurement probe transmits the optical crystal between the polarizing element and the optical crystal so that the polarization state of the light incident on the reflecting portion is circularly polarized light. It can be set as the structure which has a phase difference plate which provides a phase difference to the light which injected from the polarizing element.

  In the present invention, as the measurement probe, a first measurement probe that measures an electric field in the x-axis direction, a second measurement probe that measures an electric field in the y-axis direction, and a third measurement that measures an electric field in the z-axis direction. Each of the first measurement probe, the second measurement probe, and the third measurement probe is provided with the optical fiber, and the optical fiber is emitted from the polarization conversion unit to each optical fiber. It can be set as the structure provided with the optical switch which switches and inject | emits light.

  In the measuring apparatus according to the present invention, a change in the polarization state generated in the light transmitted through the optical fiber is detected from the polarization state of the light emitted from the optical fiber, and the incident light to the probe is converted so as to cancel the change. Therefore, it is possible to provide a measuring apparatus that can stabilize the sensitivity of the probe and perform reliable measurement.

It is a lineblock diagram of the measuring device concerning a 1st embodiment of the present invention. It is the schematic which shows the probe in the measuring apparatus of 1st Embodiment. It is the figure which showed the change of the polarization by the fiber effect. It is a lineblock diagram showing the modification of the measuring device concerning a 1st embodiment of the present invention. It is a block diagram of the measuring device which concerns on 2nd Embodiment of this invention. It is the block diagram which showed the prior art example of the measuring device.

[First Embodiment]
Hereinafter, the measurement apparatus 1 according to the first embodiment of the present invention will be described with reference to FIGS. In all the drawings below, the dimensions and ratios of the constituent elements are appropriately changed in order to make the drawings easy to see.

  FIG. 1 is a configuration diagram of a measuring apparatus 1 including a probe having an electro-optic crystal according to the present embodiment. The measuring device 1 includes a laser light source (light source) 12 that generates a stable linearly polarized optical signal, a polarization controller (polarization converter) 13, a circulator 14, a probe (measurement probe) 19, a beam splitter 110, A photodetector (light quantity detection unit) 111, a polarization monitor (polarization state detection unit) 112, and a control signal processing unit 113 are included.

  The laser light emitted from the laser light source 12 passes through the polarization controller 13, changes its polarization state, passes through the circulator 14, and travels toward the probe 19 at the tip of the optical fiber 11.

  FIG. 2 is a schematic view showing the probe 19. As shown in the drawing, the probe 19 has a polarizer (polarizing element) 15, a phase difference plate 16, an EO crystal 17, and a reflection mirror 18 made of a dielectric multilayer film having a high reflectance.

  The retardation plate 16 is a λ / 4 plate, for example, and has a function of converting incident linearly polarized light into circularly polarized light. As the EO crystal 17, PZT, PZLT, DAST, BSO, and LTO crystals having a large EO effect (Pockels effect) can be suitably used.

  Here, the transmission optical axis of the polarizer 15, the phase difference plate 16, and the fast axis and slow axis of the EO crystal 17 are respectively adjusted to the states shown in the figure. That is, the fast axis and the slow axis of the phase difference plate 16 and the EO crystal 17 are oriented in the same direction, respectively, and the transmission optical axis of the polarizer 15 is arranged so as to intersect the fast axis and the slow axis. .

  In the probe 19 having such a configuration, incident laser light ((a) in the figure) is first extracted from the linearly polarized light by the polarizer 15 ((b) in the figure) and then transmitted through the phase difference plate 16. Thus, linearly polarized light is converted into circularly polarized light ((c) in the figure). This circularly polarized light is incident on the EO crystal 17 whose refractive index has changed due to the influence of the electric field at the measurement location, and undergoes a change in polarization state.

  The light reflected by the reflection mirror 18 and emitted from the EO crystal 17 enters the phase difference plate 16 as elliptically polarized light different from that before entering the EO crystal 17 ((d) in the figure). Since it is elliptically polarized light, the light emitted from the phase difference plate 16 is also converted into elliptically polarized light and enters the polarizer 15 ((e) in the figure). In the polarizer 15, linearly polarized light in the same direction as the transmitted optical axis is extracted from the incident elliptically polarized light and emitted ((f) in the figure). That is, the amount of linearly polarized light that is extracted changes according to the amount of change in the refractive index that the EO crystal 17 is affected by the influence of the electric field.

  Returning to FIG. 1, the light emitted from the probe 19 passes through the optical fiber 11 and the circulator 14 again and is separated by the beam splitter 110. A part of the separated light is directed to the photodetector 111 for signal measurement, and the rest is directed to the polarization monitor 112.

  The polarization monitor 112 reads the polarization state of the light, and the control signal processing unit 113 obtains the inverse conversion of the conversion for changing the linearly polarized light to the read polarization state based on the read polarization state, and the inverse conversion is converted to the linearly polarized light. Calculate the polarization state when applied to. Then, the control signal processing unit 113 adjusts the polarization controller 13 so that the light emitted from the polarization controller 13 is in a polarization state when the inverse transformation is applied to linearly polarized light. By applying this control, it is possible to cancel the fluctuation of the polarization state generated in the optical fiber 11.

  FIG. 3 is an explanatory diagram for explaining a mechanism that can cancel the fluctuation of the polarization state generated in the optical fiber 11 by adjustment applied to the polarization controller 13 based on the calculation of the control signal processing unit 113.

  The amount of change ΔΦ in the polarization state that occurs through the optical fiber 11 is the same regardless of the light traveling direction. For this reason, in the case of the probe 19 of the present embodiment, the polarization state at the exit of the optical fiber 11 (the polarization state of the laser light Lb) is measured, so that light that is linearly polarized from the entrance of the optical fiber 11 is in front of the polarizer 15. It is possible to know the change in polarization state (change in polarization state received by the laser beam La).

  The light emitted from the probe 19 is linearly polarized because it passes through the polarizer 15. Therefore, by monitoring the ellipticity and polarization axis of the polarization state of the light detected by the polarization monitor 112, the change in the polarization state received by the linearly polarized light transmitted through the optical fiber 11 can be monitored. This change in polarization state is due to factors such as bending, thermal expansion, and non-uniformity of the optical fiber 11.

  Therefore, if the change in the polarization state can be known, it is possible to keep the linearly polarized light just before the polarizer 15 by giving the inverse transformation to the original linearly polarized light.

  The polarization state of a certain light can be uniquely expressed by a Poincare sphere in which one point on the spherical surface corresponds to the polarization state. In addition, when the polarization state is represented by a Jones vector, a state transition when changing from one polarization state to another polarization state can be represented using a determinant. That is, the control signal processing unit 113 can obtain information on the state change received by passing through the optical fiber 11, and information on a matrix corresponding to the change received from the optical fiber 11 can be obtained from the information.

  The control signal processing unit 113 performs inverse transformation of the matrix corresponding to the change received from the optical fiber 11, and sends the polarization adjustment information corresponding to the new matrix obtained by the inverse transformation to the polarization controller 13. The polarization state of the light incident on is converted.

  Since the polarization controller 13 has a half-wave plate and a quarter-wave plate, the incident laser light can be changed to a polarized light in a free polarization state. Further, in place of these wave plates, a fiber crank, a fiber coil, a Faraday rotator, an electro-optic crystal, or the like can be used. In particular, when the polarization state is controlled using an electro-optic crystal, the polarization state can be converted by electrical control. Therefore, unlike mechanical control, polarization can be converted at a fast response speed of GHz or higher, which is preferable. .

  The light converted by the polarization controller 13 undergoes the same conversion as described above when passing through the optical fiber 11 and becomes just linearly polarized light before the polarizer 15. Therefore, a fixed amount of light can be incident on the probe 19 and the measurement sensitivity can be stabilized.

  In such a control system, the signal measurement system for measuring the amount of light and the control system for controlling the polarization state are almost independent, and can be added without significant changes to the conventional system. Therefore, a highly reliable control system can be easily obtained.

  According to the measuring apparatus 1 configured as described above, it is possible to cancel the fluctuation of the polarization state in the optical fiber, stabilize the sensitivity of the probe, and reliably measure the electric field strength.

  In the measurement apparatus 1 of the present embodiment, the phase difference plate 16 is provided in the probe 19 and the influence of the EO crystal 17 is easily detected. However, the configuration is not limited thereto. In the configuration without the phase difference plate 16, the light incident on the EO crystal 17 is linearly polarized light and is converted into elliptically polarized light by passing through the EO crystal 17. Such elliptically polarized light is partially blocked by the polarizer 15 when emitted from the probe 19, and the amount of emitted light changes. Therefore, similarly to the above-described configuration, the change in the amount of light can be measured by the photodetector 111, and the target measuring device can be obtained.

  Further, in the present embodiment, the configuration is such that a part of light is separated by the beam splitter 110 and guided to the polarization monitor 112 to detect the polarization state. However, the present invention is not limited to this. A device having a function as a state detection unit and capable of detecting the polarization state simultaneously with the amount of light incident on itself may be used.

[Second Embodiment]
FIG. 4 is an explanatory diagram of a measuring apparatus according to the second embodiment of the present invention. The measurement device of this embodiment is partially in common with the measurement device 1 of the first embodiment. The difference is that instead of the probe 19 having the EO crystal 17, a probe having an MO crystal is used.

  FIG. 4 is a configuration diagram of the probe 49 included in the measurement apparatus. The probe 49 includes a polarizer 41, an MO crystal 42, and a reflection mirror 43. As the MO crystal 42, a garnet crystal such as YIG or Bi-substituted YIG having a large MO effect (Faraday effect) can be preferably used.

  By replacing the probe 19 of the measurement apparatus 1 of the first embodiment with such a probe 49, a measurement apparatus capable of measuring a magnetic field with stable sensitivity can be configured.

[Third Embodiment]
FIG. 5 is an explanatory diagram of a measuring apparatus according to the third embodiment of the present invention. As shown in the drawing, the measuring device 5 of the present embodiment is partially in common with the measuring device 1 of the first embodiment.

  In the measuring device 5, in order to be able to measure a triaxial electric field, an optical switch 55 is disposed immediately after the optical circulator 14, and an x-axis direction, a y-axis direction, and a z-axis that are orthogonal to each other via an optical fiber 51 beyond that. X-axis direction measurement probe (first measurement probe) 59x, y-axis direction measurement probe (second measurement probe) 59y, z-axis direction measurement probe (third measurement probe) 59z capable of measuring respective electric fields in the axial direction. Is connected.

  With such a configuration, it is possible to cancel the influence of the polarization fluctuation generated in the optical fiber 51 that is guided by being switched by the optical switch, to stabilize the measurement sensitivity, and to measure the triaxial electric field. be able to.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but it goes without saying that the present invention is not limited to such examples. Various shapes, combinations, and the like of the constituent members shown in the above-described examples are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

1, 5, 6, Measuring device 11, 51, 61 Optical fiber 12, 62, Laser light source (light source)
13, 63, Polarization controller (polarization converter)
14, 64, Circulator 15, 41, 65 Polarizer (polarizing element)
16, 66 Phase difference plate 17, 67 EO crystal (optical crystal)
18, 43, 68 Reflection mirror (reflection part)
19, 49, 69 Probe (measurement probe)
42 MO crystal (optical crystal)
55 Optical switch 59x x-axis direction measurement probe (first measurement probe)
59y Y-axis direction measurement probe (second measurement probe)
59z z-axis direction measurement probe (third measurement probe)
111,610 Photodetector (light intensity detector)
110 Beam splitter 112 Polarization monitor (polarization state detection)
113 Control signal processor

Claims (5)

A light source;
A polarization converter that converts the polarization state of the light emitted from the light source;
An optical fiber through which light emitted from the polarization converter is guided;
A measurement probe provided at one end of the optical fiber;
A light amount detection unit on which light emitted from the measurement probe enters through the optical fiber;
A polarization state detection unit that detects a polarization state of the light incident on the light amount detection unit,
The measurement probe includes a polarizing element that converts the light guided through the optical fiber into linearly polarized light, and an optical device in which the refractive index changes due to an electro-optic effect or a magneto-optic effect and light that enters the polarizing element enters. A crystal, and a reflection part that reflects the light transmitted through the optical crystal toward the optical fiber,
The polarization conversion unit is a polarization in which a polarization state of light incident on the optical fiber cancels a change in polarization state received by the light in the optical fiber based on the polarization state of the light detected by the polarization state detection unit. A measuring apparatus that converts a polarization state of light incident from the light source so as to be in a state.   The measuring apparatus according to claim 1, further comprising a beam splitter that maintains a polarization state of the light incident on the light amount detection unit, separates a part thereof, and guides the light to the polarization state detection unit.   The polarization conversion unit includes an electro-optic crystal on which the light is incident, and controls a polarization state of the light by controlling a state of application to the electro-optic crystal. The measuring device described in 1.   The measurement probe transmits light through the optical crystal between the polarizing element and the optical crystal, and reflects light incident from the polarizing element so that the polarization state of the light incident on the reflecting portion is circularly polarized light. The measuring apparatus according to claim 1, further comprising a phase difference plate that imparts a phase difference. The measurement probe includes a first measurement probe that measures an electric field in the x-axis direction, a second measurement probe that measures an electric field in the y-axis direction, and a third measurement probe that measures an electric field in the z-axis direction. ,
The optical fiber is provided in each of the first measurement probe, the second measurement probe, and the third measurement probe,
5. The measuring device according to claim 1, wherein an optical switch that switches and emits light emitted from the polarization conversion unit is provided for each optical fiber. 6.

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