JP4077388B2 - Radio wave sensor - Google Patents

Description

  The present invention relates to a radio wave sensor that searches for an object that cannot be observed with light or X-rays by detecting an electromagnetic wave source in the millimeter wave band with an electro-optic crystal.

  As an example of a sensor for detecting a radio wave, there is a radio wave sensor using an electro-optic crystal (see Patent Documents 1 and 2). FIG. 10 is a configuration diagram illustrating an example of a radio wave sensor. This radio wave sensor includes an electro-optic crystal 1001 having a reflective surface 1001a, lenses 1002a and 1002b, an optical fiber 1003, a laser 1004 that outputs pulse light with a predetermined period, a photodetector 1005, a polarization beam splitter (PBS) 1006, and 1; A half-wave plate (HWP) 1007, a quarter-wave plate (QWP) 1008, an amplifier 1009, and an electric signal measuring device 1010 are provided.

  In this radio wave sensor, first, the polarized light emitted from the laser 1004 is transmitted through the PBS 1006, and the polarization of the other polarized light orthogonal to the transmitted polarized light is changed to the perpendicular direction. The light transmitted through the PBS 1006 is adjusted so as to be in a predetermined polarization state by the QWP 1008 and HWP 1007 arranged at an appropriate angle (see Non-Patent Document 1). The predetermined polarization state is a polarization state in which the sensitivity of the radio wave sensor is maximized.

  The light that has passed through the QWP 1008 and the HWP 1007 is collected by the lens 1002b, coupled to the optical fiber 1003, propagates through the optical fiber 1003, passes through the lens 1002a, and enters the electro-optic crystal 1001.

  The light incident on the electro-optic crystal 1001 propagates in the same direction as the radio wave reflected by the reflecting surface 1001a and detected inside the electro-optic crystal 1001. At this time, the polarization state of the light propagating in the electro-optic crystal 1001 is modulated according to the electric field of the radio wave propagating in the electro-optic crystal 1001. In this way, the light modulated in the electro-optic crystal 1001 passes through the lens 1002a, the optical fiber 1003, and the lens 1002b again, passes through the HWP 1007 and the QWP 1008, and enters the PBS 1006.

  The polarization-modulated light that has entered the PBS 1006 in this way is converted into intensity-modulated light by the optical path being changed by the PBS 1006 in a perpendicular direction, and is incident on the photodetector 1005. The photodetector 1005 outputs an electrical signal proportional to the intensity of incident light. The electric signal output from the photodetector 1005 is amplified by the amplifier 1009 and then processed by the electric signal measuring device 1010. The electric signal measuring device 1010 is, for example, a spectrum analyzer or a lock-in amplifier.

The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
JP2001-050908 JP2003-014801A KJWeingarten, MJWRodwel, and DMBloom, "Picosecond Optical Sampling of GaAs Integrated Circuits," IEEE Journal of Quantum Electronics, Vol.24, No.2, pp.198-220, 1988.

However, the conventional radio wave sensor described above has the following problems.
As shown in FIG. 10, a sensor head 1011 including an electro-optic crystal 1001 and a signal processing unit 1012 that performs signal processing are connected by an optical fiber 1003. Here, when stress is applied to the optical fiber 1003 due to the displacement of the sensor head 1011, the polarization state of light propagating through the optical fiber 1003 changes. For this reason, in the conventional radio wave sensor having the configuration shown in FIG. 10, the polarization state of the light incident on the electro-optic crystal 1001 changes due to the displacement of the sensor head 1011. Therefore, when the sensor head 1011 is moved, the sensitivity decreases. There was a problem.

  The present invention has been made to solve the above problems, and a radio wave sensor configured to propagate an optical signal emitted from an electro-optic crystal housed in a sensor head to a signal processing unit using an optical fiber. An object of the present invention is to suppress a decrease in sensitivity due to displacement of the sensor head.

The radio wave sensor according to the present invention includes a quarter-wave plate that is housed in a sensor head and that uses light source light emitted from a light source and propagated while maintaining a polarization state as first light in a predetermined polarization state. A polarizing element, an electro-optic crystal that changes the polarization of at least a portion of the first light that is contained in the sensor head and is incident and transmitted therethrough in response to a change in the electric field of the received electromagnetic wave, and emits the second light, and a sensor A polarization separation means for extracting the first polarization component from the second light emitted from the electro-optic crystal; an optical fiber for propagating the first polarization component extracted by the polarization separation means; and The sensor head includes at least a photoelectric conversion element that photoelectrically converts the propagated first polarization component into an electric signal, and a signal processing unit that processes the electric signal photoelectrically converted by the photoelectric conversion element. It provided that state, two principal axes perpendicular quarter wave plate makes an angle of 22.5 ° with respect to the two principal axes perpendicular polarized light separating means, and two orthogonal electric electrooptic crystal The target main axis forms an angle of 45 ° with respect to two orthogonal main axes of the quarter-wave plate, and the first light emitted from the polarizing element passes through the space between the polarizing element and the electro-optic crystal. The light is incident on the electro-optic crystal.

In the radio wave sensor, e Bei polarization maintaining propagation means for propagating the sensor head to hold the state of polarization of the emitted light beam from the light source, polarizer, the polarization maintaining propagation means polarization state There you source light propagated held a predetermined first optical polarization state. The polarization maintaining propagation means may be a polarization maintaining fiber, for example. Alternatively , the light source is housed in the sensor head . The light source light propagated to the electro-optic crystal is in a state where the polarization state is preserved.

  In the radio wave sensor, the electro-optic crystal has a reflecting surface on the surface opposite to the incident surface of the first light, and the polarization separating means transmits light from the light source, and light having a different polarization state from the light source light has different directions in the optical path. The polarization beam splitter extracts the first polarization component from the second light emitted from the electro-optic crystal, and the light source light passes through the polarization beam splitter and enters the polarization element. The emitted first light is incident from the incident surface of the electro-optic crystal, the light incident from the incident surface of the electro-optic crystal is reflected by the reflecting surface and emitted from the incident surface, and the light emitted from the incident surface of the electro-optic crystal. Is sufficient if it passes through the polarizing element and enters the polarizing beam splitter.

  In the radio wave sensor, the electro-optic crystal includes a reflecting surface on a surface opposite to the incident surface of the first light, and the polarization separating unit transmits the light in the first polarization state and the light in the first polarization state and the polarization. The light of the second polarization state having different states is disposed in the optical path between the first polarization beam splitter and the second polarization beam splitter that change the optical path in different directions, and between the first polarization beam splitter and the second polarization beam splitter. The polarization state of the light transmitted through the first polarization beam splitter is set to the first polarization state with respect to the second polarization beam splitter, and the polarization state of the light transmitted through the second polarization beam splitter is set to the first polarization beam splitter. A first optical fiber configured to include a polarization changing unit configured to be in a second polarization state and propagating light whose optical path is changed in a different direction by the second polarization beam splitter; and a first polarization beam A second optical fiber that propagates light whose optical path has been changed in a different direction by a pre-itter, a first photoelectric conversion element that photoelectrically converts light propagated through the first optical fiber into an electrical signal, and propagates through the second optical fiber A second photoelectric conversion element that photoelectrically converts the converted light into an electrical signal, a differential amplifier that differentially amplifies the two electrical signals photoelectrically converted by the first and second photoelectric conversion elements, and the differential amplifier Signal processing means for processing the output electrical signal, and the light source light passes through the first polarizing beam splitter, the polarization changing means, and the second polarizing beam splitter in this order and enters the polarizing element. Then, the first light emitted from the polarizing element is incident from the incident surface of the electro-optic crystal, and the light incident from the incident surface of the electro-optic crystal is reflected by the reflecting surface and is emitted from the incident surface. Light emitted from the surface The first polarization component that has passed through the polarization element, entered the second polarization beam splitter, exited from the incident surface of the electro-optic crystal, and the optical path was changed in a different direction by the second polarization beam splitter is the first light. The second polarization component (differing in polarization state from the first polarization component) propagated by the fiber and emitted from the incident surface of the electro-optic crystal and transmitted through the second polarization beam splitter is transmitted through the polarization changing means and is transmitted through the first polarization component. The light is incident on the beam splitter, the optical path is changed in a different direction by the first polarizing beam splitter, and the light is propagated by the second optical fiber.

  As described above, in the present invention, the polarization element and the polarization separation means are housed in the sensor head together with the electro-optic crystal, and the polarization state when the light source light emitted from the light source is emitted from the light source is maintained. In this state, the sensor head is supplied to reach the electro-optic crystal. For example, when the light source light is propagated by a polarization maintaining propagation means such as a polarization maintaining fiber or the light source is accommodated in the sensor head, the light source light emitted from the light source is deflected when it is emitted from the light source. It can be supplied to the sensor head in a state where the wave state is maintained.

  With these configurations, according to the present invention, even when the sensor head is displaced, the polarization state and intensity of the light incident on the sensor head are kept constant. An excellent effect that the reduction can be suppressed is obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Embodiment 1]
FIG. 1 is a configuration diagram showing a configuration example of a radio wave sensor according to the first embodiment of the present invention. The radio wave sensor includes a sensor head 100 and a signal processing unit 120. First, the sensor head 100 includes an electro-optic crystal 101 including a reflecting surface 101a, a polarizing element 102, a polarizing beam splitter (PBS) 103, a lens 104a, 104b. These are optical systems arranged in a free space inside the sensor head 100.

  The electro-optic crystal 101 emits at least part of the incident and transmitted light with its polarization changed corresponding to the electric field change of the received electromagnetic wave. The electro-optic crystal 101 includes an incident / exit surface facing in parallel with the reflecting surface 101a, and an optical path of light incident from the incident / exit surface is input / exit surface-reflecting surface-input / exit surface. While propagating through this optical path, at least a part of the propagating (transmitting) light changes in polarization corresponding to the electric field change of the electromagnetic wave received by the electro-optic crystal 101.

The PBS 103 is a polarization separation unit that extracts the above-described change in polarization from the light emitted from the electro-optic crystal 101. The PBS 103 transmits the light source light in a predetermined polarization state, and the light emitted from the electro-optic crystal 101 changes the optical path in a different direction, for example, 90 °, for light having a different polarization state from the light source light. The change in polarization is extracted from the inside.
The polarizing element 102 causes the light source light transmitted through the PBS 103 to enter the electro-optic crystal 101 as desired elliptically polarized light.

The signal processing unit 120 includes a laser 121, a photodetector 122, an amplifier 123, and an electric signal measuring device 124. The laser 121 is a light source that generates the light source light described above. The photodetector 122 is a photoelectric conversion element such as a photodiode, for example.
Laser light (light source light) emitted from the laser 121 of the signal processing unit 120 is propagated to the sensor head 100 by a polarization-maintaining fiber (PMF) 131, and an optical signal output from the sensor head 100 is The optical fiber 132 is connected to the photodetector 122 of the signal processing unit 120.

The polarization maintaining fiber 131 is an optical fiber that maintains the polarization state of the guided light. For example, a portion (stress applying portion) having a diameter of about 30 μm made of quartz glass doped with B ₂ O ₃ is used. An optical fiber provided in the clad on both sides of the core. When the sensor head 100 is displaced, the polarization maintaining fiber 131 is deformed, but the polarization state of the light source light propagating through the polarization maintaining fiber 131 is maintained.

  Accordingly, in the radio wave sensor shown in FIG. 1, the light source light is emitted from the laser 121 as the light source, and then passes through the PBS 103 and enters the polarization element 102 while maintaining the polarization state. . The light source light emitted from the polarization maintaining fiber 131 diverges abruptly. Therefore, the light source light is shaped so as to become parallel light by the lens 104a and then incident on the PBS 103.

  As described above, the light source light emitted from the laser 121 and held in the polarization state by the polarization maintaining fiber 131 and propagated to the sensor head 100 is converted into parallel light by the lens 104 a and enters the PBS 103. The light source light emitted from the laser 121 enters the PBS 103 in a linearly polarized state. Here, the light source light incident on the PBS in the linearly polarized state is P-polarized light, and the electric field vector is parallel to the vertical direction (x direction) in FIG. In the following description, the same applies even if the P-polarized light and the S-polarized light are interchanged.

  Almost all of the light source light that is converted into parallel light and enters the PBS 103 is transmitted, is converted into elliptically polarized light by the polarizing element 102, and enters the electro-optic crystal 101 from the incident / exit surface. The incident elliptically polarized light propagates in the electro-optic crystal 101 and reaches the reflecting surface 101a, where it is reflected and emitted from the incident / exiting surface. The incident elliptically polarized light propagates in the left-right direction (parallel to the z-axis) in FIG.

  The reflecting surface 101a is, for example, a mirror that reflects light and transmits radio waves. Accordingly, when the electro-optic crystal 101 is irradiated with radio waves, the radio waves are transmitted through the reflecting surface 101a and propagate in the electro-optic crystal 101 in the -z direction. At this time, the light propagating in the electro-optic crystal 101 (elliptical polarization) undergoes polarization modulation according to the electric field (instantaneous electric field amplitude) of the radio wave propagating in the electro-optic crystal 101.

  In this way, the light emitted from the electro-optic crystal 101 after being subjected to polarization modulation is transmitted through the polarizing element 102 and enters the PBS 103. Of the light emitted from the electro-optic crystal 101 and incident on the PBS 103, the P-polarized light goes straight, the traveling direction of the S-polarized light is changed by 90 °, is condensed by the lens 104b, and enters (couples) the optical fiber 132. Accordingly, light that has undergone polarization modulation by the electro-optic crystal 101 is separated as intensity-modulated light by the PBS 103.

  The intensity modulated light separated by the PBS 103 and coupled to the optical fiber 132 is propagated to the signal processing unit 120. The optical fiber 132 need not be a polarization maintaining fiber. When the modulation frequency by the electro-optic crystal 101 is not so high, a multimode optical fiber that is easy to couple light (light beam) is suitable for the optical fiber 132. On the other hand, when the modulation frequency is high, the optical fiber 132 is preferably a single mode optical fiber.

As described above, the intensity-modulated light propagated from the sensor head 100 to the signal processing unit 120 through the optical fiber 132 is converted into an electric signal proportional to the intensity by the photodetector 122. Therefore, the electrical signal output from the photodetector 122 is loaded with information on the radio wave to be measured detected by the electro-optic crystal 101.
The electric signal output from the photodetector 122 is amplified by the amplifier 123 and then subjected to predetermined signal processing by the electric signal measuring device 124. The electric signal measuring device 124 may be a spectrum analyzer or a lock-in amplifier, for example.

  According to the radio wave sensor of FIG. 1 configured as described above, the light source light emitted from the laser 121 is propagated to the sensor head 100 by the polarization maintaining fiber 131, so that the sensor head 100 is displaced and biased. Even when an external force is applied to the wave holding fiber 131, the state and intensity of polarization of the light source light hardly change. As a result, the polarization state and intensity of the light incident on the electro-optic crystal 101 and the light emitted from the electro-optic crystal 101 are hardly changed. The sensitivity of detection can always be kept high.

[Embodiment 2]
Next, another embodiment of the present invention will be described. FIG. 2 is a configuration diagram showing a configuration example of a radio wave sensor according to another embodiment of the present invention. As in the radio wave sensor shown in FIG. 1, this radio wave sensor is composed of a sensor head 100 and a signal processing unit 120. First, the sensor head 100 includes an electro-optic crystal 101 having a reflective surface 101a, a polarizing element 102, A polarization beam splitter (PBS) 103 and lenses 104a and 104b are provided. In addition, the radio wave sensor shown in FIG. 2 includes a laser 141 that generates light source light inside the sensor head 100. These are optical systems arranged in a free space inside the sensor head 100.

  Therefore, in the radio wave sensor shown in FIG. 2, the signal processing unit 120 includes the photodetector 122, the amplifier 123, and the electric signal measuring device 124, but no laser as a light source is arranged. In addition, the light source light emitted from the laser 141 propagates in the free space inside the sensor head 100 without passing through the polarization maintaining fiber, passes through the PBS 103 or the polarizing element 102, and enters the electro-optic crystal 101. . The other configuration is the same as that of the radio wave sensor of FIG. 2, and an optical signal output from the sensor head 100 is connected to the photodetector 122 of the signal processing unit 120 through an optical fiber 132.

  Also in the radio wave sensor shown in FIG. 2, the light source light is emitted from the laser 141 which is a light source, and then passes through the PBS 103 and enters the polarizing element 102 while maintaining the polarization state. Since the light source light emitted from the laser 141 diverges abruptly, it is shaped so as to become parallel light by the lens 104a and then incident on the PBS 103.

  As described above, the light source light emitted from the laser 141 is converted into parallel light by the lens 104a and is incident on the PBS 103 in a linearly polarized state. The light source light incident on the PBS in the linearly polarized state is, for example, P-polarized light, and the electric field vector is parallel to the vertical direction (x direction) in FIG. Almost all of the light source light that is converted into parallel light and enters the PBS 103 is transmitted, is converted into elliptically polarized light by the polarizing element 102, and enters the electro-optic crystal 101 from the incident / exit surface. The incident elliptically polarized light propagates in the electro-optic crystal 101 and reaches the reflecting surface 101a, where it is reflected and emitted from the incident / exiting surface. The incident elliptically polarized light propagates in the left-right direction (parallel to the z-axis) in FIG.

  When the electro-optic crystal 101 is irradiated with radio waves, the radio waves are transmitted through the reflecting surface 101a and propagate in the electro-optic crystal 101 in the -z direction. At this time, the light propagating in the electro-optic crystal 101 undergoes polarization modulation according to the electric field of the radio wave propagating in the electro-optic crystal 101.

  In this way, the light emitted from the electro-optic crystal 101 after being subjected to polarization modulation is transmitted through the polarization element 102 and incident on the PBS 103, and is separated as intensity-modulated light by the PBS 103. The intensity modulated light separated by the PBS 103 and coupled to the optical fiber 132 is propagated to the signal processing unit 120. As described above, the intensity-modulated light propagated from the sensor head 100 to the signal processing unit 120 through the optical fiber 132 is converted into an electric signal proportional to the intensity by the photodetector 122. The electric signal output from the photodetector 122 is amplified by the amplifier 123 and then subjected to predetermined signal processing by the electric signal measuring device 124.

  According to the radio wave sensor of FIG. 2 configured as described above, since the light source light emitted from the laser 141 propagates in the free space in the sensor head 100, how the sensor head 100 is displaced. However, the light source light does not change its polarization state and intensity. As a result, even in the radio wave sensor shown in FIG. 2, the polarization state and the intensity of the light incident on the electro-optic crystal 101 and the light emitted from the electro-optic crystal 101 are almost unchanged. Even when the head 100 is moved, the sensitivity of detection of radio waves can always be kept high.

[Embodiment 3]
Next, another embodiment of the present invention will be described.
FIG. 3 is a configuration diagram illustrating a configuration example of the radio wave sensor according to the present embodiment. As in the radio wave sensor shown in FIG. 1, this radio wave sensor is composed of a sensor head 100 and a signal processing unit 120. The sensor head 100 includes an electro-optic crystal 101 having a reflective surface 101a, a polarizing element 102, and a polarized beam. A splitter (PBS) 103a and lenses 104a and 104b are provided.

In addition, in the sensor head 100 shown in FIG. 3, a polarizing beam splitter (PBS) 103b, a Faraday rotator (FR) 105, and a half-wave plate are provided in an optical system between the PBS 103 and the polarizing element 102. A polarization changing means composed of (HWP: half-wave plate) 106 is provided. Therefore, in the radio wave sensor shown in FIG. 3, the polarization separation means is configured by the two polarization beam splitters 103a and 103b and the polarization changing means.
On the other hand, the signal processing unit 120 of the radio wave sensor shown in FIG. 3 includes two photodetectors 122a and 122b and a differential amplifier 123a that differentially amplifies the outputs in addition to the laser 121 and the electric signal measuring device 124. .

  In the radio wave sensor shown in FIG. 3, the light source light emitted from the laser 121 of the signal processing unit 120 is propagated to the sensor head 100 by the polarization maintaining fiber 131 as in the radio wave sensor of FIG. 1. In the radio wave sensor shown in FIG. 3, first, the intensity-modulated light separated by the PBS 103a is collected by the lens 104b, coupled to the optical fiber 132a, propagated through the optical fiber 132a, and detected by the signal processing unit 120. Connected to the device 122a. The intensity-modulated light separated by the PBS 103b is collected by the lens 104c, coupled to the optical fiber 132b, propagates through the optical fiber 132b, and is connected to the photodetector 122b of the signal processing unit 120.

  Hereinafter, an operation example of the radio wave sensor shown in FIG. 3 will be described with reference to FIG. Note that x and y shown in FIG. 4 indicate x and y shown in FIG. 3, and FIG. 4 is arranged in a plane perpendicular to the paper surface of FIG. 3, in other words, in the internal free space of the sensor head 100. The cross section of the optical path of an optical system is shown. Also, the alternate long and short dash line in FIG. 4 indicates the arrangement angle of the HWP 106, and the thick line with arrows at both ends indicates the polarization state of light propagating through the optical system.

The light source light emitted from the laser 121 and held in the polarization state by the polarization maintaining fiber 131 and propagated to the sensor head 100 is converted into parallel light by the lens 104a and enters the PBS 103a. The light source light emitted from the laser 121 enters the PBS 103a in a linearly polarized state. Here, the light source light incident on the PBS in the linearly polarized state is, for example, P-polarized light, and the electric field vector is in a state parallel to the x- axis as shown in FIG. 3 is parallel to the vertical direction (x direction).

  Almost all of the light source light that is converted into parallel light by the lens 104a and enters the PBS 103a is transmitted and enters the FR 105. The light source light (P-polarized light) incident on the FR 105 is transmitted through the FR 105 and rotated by + 45 ° in the xy plane as shown in FIG. Next, the light source light transmitted through the HWP 106 rotates by −45 ° and returns to P-polarized light as shown in FIG. The light source light that has returned to the P-polarized light in this manner passes through the PBS 103 b and travels straight and enters the polarizing element 102.

  The light source light incident on the polarizing element 102 is converted into elliptically polarized light by the polarizing element 102 and enters the electro-optic crystal 101 from the incident / exit surface. The incident elliptically polarized light propagates in the electro-optic crystal 101 and reaches the reflecting surface 101a, where it is reflected and emitted from the incident / exiting surface. When the electro-optic crystal 101 is irradiated with radio waves, the radio waves are transmitted through the reflecting surface 101a and propagate in the electro-optic crystal 101 in the -z direction. At this time, the elliptically polarized light propagating in the electro-optic crystal 101 is subjected to polarization modulation according to the electric field of the radio wave propagating in the electro-optic crystal 101.

  In this way, the light emitted from the electro-optic crystal 101 after being subjected to polarization modulation is transmitted through the polarizing element 102 and enters the PBS 103b. Of the light emitted from the electro-optic crystal 101 and incident on the PBS 103b, the P-polarized light travels straight and the traveling direction of the S-polarized light is changed by 90 °, and is condensed by the lens 104c and incident (coupled) on the optical fiber 132b. Accordingly, the light that has undergone polarization modulation by the electro-optic crystal 101 is separated as intensity-modulated light by the PBS 103b.

  In addition, the light traveling straight in the PBS 103b is P-polarized light parallel to the x-axis as shown in FIG. 4D, and passes through the HWP 106, as shown in FIG. 4E. And + 45 ° in the xy plane. Further, the light that has passed through the FR 105 and returned straight after traveling through the PBS 103b is rotated by + 45 ° and becomes S-polarized light, as shown in FIG. 4 (f). As a result, the light transmitted through the PBS 103b, the HWP 106, and the FR 105 is polarized by 90 ° in the traveling direction by the PBS 103a, collected by the lens 104b, and incident on the optical fiber 132a.

  As described above, in the radio wave sensor shown in FIG. 3, in the light emitted through the electro-optic crystal 101, S-polarized light is propagated to the signal processing unit 120 by the optical fiber 132b, and P-polarized light is It is propagated to the signal processing unit 120 by the optical fiber 132a. In this way, the light propagated through the optical fiber 132a is photoelectrically converted by the photodetector 122a, and the light propagated through the optical fiber 132b is photoelectrically converted by the photodetector 122b. The two electrical signals photoelectrically converted by the photodetectors 122a and 122b are differentially amplified by the differential amplifier 123a, and predetermined signal processing is performed by the electrical signal measuring device 124.

FIG. 5 is a characteristic diagram showing the states of the output electrical signals of the photodetectors 122a and 122b and the differential amplifier 123a. (A) shows the states of the output electrical signals of the photodetector 122a. Indicates the state of the output electrical signal of the photodetector 122b, and (c) indicates the state of the output electrical signal of the differential amplifier 123a. The average intensity of the S-polarized component and the P-polarized component in the light emitted from the electro-optic crystal 101 is adjusted by the polarizing element 102 to be equal immediately before entering the PBS 103b. Therefore, the output electric signals of the two photodetectors 122a and 122b change according to the measured radio wave, centering on a value (0.5V ₀ ) corresponding to half of the total light intensity (FIG. 5 (a), FIG. 5B).

  Since the total light intensity is always constant, the amplitude of the corresponding portion of the output electrical signal of the photodetector 122b decreases at the moment when the amplitude of the output electrical signal of the photodetector 122a increases. That is, the output electrical signals of the photodetectors 122a and 122b change in opposite phases when the measured radio wave is detected. As shown in FIG. 5C, the output electric signal of the differential amplifier 123a has no DC component, and the amplitude is twice that of the output electric signal of one photodetector. Further, the influence of the intensity noise of the light detected by the photodetector changes in phase in the two photodetectors 122a and 122b. Therefore, the influence of the noise is reduced in the two output electric signals that are differentially amplified by the differential amplifier 123a.

  Also in the radio wave sensor of FIG. 3 configured as described above, the light source light emitted from the laser 121 is propagated to the sensor head 100 by the polarization maintaining fiber 131, so that the sensor head 100 is displaced and polarized. Even if an external force is applied to the holding fiber 131, the state and intensity of the light source light hardly change. As a result, the polarization state and intensity of the light incident on the electro-optic crystal 101 and the light emitted from the electro-optic crystal 101 are hardly changed. The sensitivity of detection can always be kept high.

[Embodiment 4]
Next, another embodiment of the present invention will be described. FIG. 6 is a block diagram showing a configuration example of the radio wave sensor according to the present embodiment, in which the polarization changing means is composed of only the FR 105 so as to obtain the above-described operational effect by the differential amplification. FIG. 6 shows a state in which the optical axis of the optical system is parallel to the z-axis direction in the free space inside the sensor head 100. In this radio wave sensor, the PBS 103a is arranged in a state rotated by 45 ° around the z axis.

  Therefore, as shown in the polarization direction (1) in the perspective view of FIG. 6B, the light source light converted into parallel light by the lens 104a becomes P-polarized light with respect to the PBS 103a when transmitted through the PBS 103a. Thereafter, the light source light transmitted through the FR 105 is rotated by 45 ° and becomes P-polarized light with respect to the PBS 103b as shown in the polarization direction (2).

On the other hand, of the light emitted from the optical crystal 101, the P-polarized light in the state indicated by the polarization direction (3) that passes through the PBS 103b is rotated by 45 ° as shown in the polarization direction (4) by passing through the FR 105. Thus, it becomes S-polarized light with respect to the PBS 103a. As a result, the traveling direction of this S-polarized light is changed by 90 ° by the PBS 103a.
Thus, in the radio wave sensor shown in FIG. 6, it is possible to omit HWP.

  Next, a configuration example of the optimum polarizing element 102 will be described. For example, as shown in FIG. 7A, the polarizing element 102 is composed of a quarter-wave plate (QWP) 102a and a half-wave plate (HWP) 102b. That's fine. In order to perform high-sensitivity detection, the polarization state of the light beam incident on the electro-optic crystal 101 should be adjusted to an appropriate state.

  The polarization state of light to be incident on the electro-optic crystal 101 depends on various factors such as the type, plane orientation, arrangement angle, and direction of the electric field of the measured radio wave. By adjusting the angles of both the QWP 102a and the HWP 102b, the light incident on the electro-optic crystal 101 can be in an arbitrary polarization state. Therefore, by adopting the configuration shown in FIG. 7A, by adjusting the QWP 102a and the HWP 102b, high-sensitivity measurement can always be performed regardless of the conditions of the electro-optic crystal 101 and the electric field direction of the measured radio wave. Is possible.

Further, as shown in FIG. 7B, the polarizing element 102 may be configured by only the QWP 102a. For example, the electrical spindle of the electro-optic crystal 101 if already known, only QWP102a adjusted to a specific angle as a polarizing element, it is possible to perform highly sensitive measurements.

  FIG. 8 is a characteristic diagram in which the detection sensitivity is indicated by contour lines with the angle of the QWP 102a and the angle of the electrical principal axis of the electro-optic crystal 101 as variables in the configuration shown in FIG. 7B. In FIG. 8, the white area indicates the most sensitive state. As shown in FIG. 8, for example, when the angle of the QWP 102a is 22.5 ° and the angle of the electrical principal axis of the electro-optic crystal 101 is −22.5 °, it can be seen that the sensitivity is high.

  FIG. 9A is an explanatory diagram showing an arrangement relationship between the QWP 102a and the electro-optic crystal 101 that can obtain the above-described high sensitivity state. An axis S and an axis F indicate two main axes orthogonal to the QWP. The main axis S forms an angle of 22.5 ° with the x axis. In such an arrangement state, the polarization state of the light incident on the electro-optic crystal is in a state indicated by an ellipse in FIG. Note that the two orthogonal electrical principal axes of the electro-optic crystal are X and Y, and the X axis forms an angle of −22.5 ° with the x axis.

  FIG. 9B is an explanatory diagram showing the positional relationship between the QWP and the electro-optic crystal when an electro-optic crystal 901 having a zinc blende type crystal structure is used. Light is incident from the (110) plane of the electro-optic crystal 901. The electro-optic crystal 901 is sensitive to radio waves having an electric field component perpendicular to the (1-10) plane. When the direction of the electric field vector of the measurement target radio wave is known, the arrangement state of the electro-optic crystal 901 may be set so that the electric field vector is perpendicular to the (1-10) plane of the electro-optic crystal 901. . In this state, the maximum sensitivity can be obtained.

As the above-described electro-optic crystal, a compound semiconductor crystal such as GaAs, ZnTe, or CdTe, or an oxide crystal having a sillenite structure such as Bi ₁₂ SiO ₂₀ or Bi ₄ Ge ₃ O ₁₂ can be used.

It is a block diagram which shows the structural example of the electromagnetic wave sensor in the 1st Embodiment of this invention. It is a block diagram which shows the structural example of the electromagnetic wave sensor in other embodiment of this invention. It is a block diagram which shows the structural example of the electromagnetic wave sensor in other embodiment of this invention. FIG. 6 is an explanatory diagram showing a polarization state of light propagating through an optical path inside the sensor head 100. It is a characteristic view which shows the state of the output electric signal of photodetector 122a, 122b and the differential amplifier 123a. It is a block diagram which shows the structural example of the electromagnetic wave sensor in other embodiment of this invention. 3 is a configuration diagram illustrating a configuration example of a polarizing element 102. FIG. FIG. 6 is a characteristic diagram in which detection sensitivity is indicated by contour lines with the angle of the QWP 102a and the angle of the electrical principal axis of the electro-optic crystal 101 as variables. It is explanatory drawing which shows the arrangement | positioning relationship between QWP and electro-optic crystal which can obtain a highly sensitive state. It is a block diagram which shows an example of a conventional radio wave sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Sensor head, 101 ... Electro-optic crystal, 101a ... Reflecting surface, 102 ... Polarizing element, 103 ... Polarizing beam splitter (PBS), 104a ... Lens, 104b ... Lens, 120 ... Signal processing part, 121 ... Laser, 122 ... Photodetector, 123... Amplifier, 124... Electrical signal measuring device, 131... Polarization maintaining fiber (PMF), 132.

Claims (4)

A polarizing element comprising a quarter-wave plate that is housed in a sensor head and is emitted from a light source and propagated while maintaining a polarization state; the first light having a predetermined polarization state;
An electro-optic crystal that is accommodated in the sensor head, changes the polarization of at least a portion of the first light that is incident and transmitted therethrough in response to a change in the electric field of the received electromagnetic wave, and emits the second light;
Polarization separation means for taking out the first polarization component from the second light contained in the sensor head and emitted from the electro-optic crystal;
An optical fiber that propagates the first polarization component extracted by the polarization separation means;
A photoelectric conversion element that photoelectrically converts the first polarization component propagated through the optical fiber into an electrical signal;
Signal processing means for processing an electrical signal photoelectrically converted by the photoelectric conversion element;
Polarization maintaining and propagating means for propagating to the sensor head while maintaining the polarization state of the light source light emitted from the light source ,
The sensor head is provided in a movable state;
Two orthogonal principal axes of the quarter-wave plate make an angle of 22.5 ° with respect to two orthogonal principal axes of the polarization separating means, and two orthogonal electrical principal axes of the electro-optic crystal are An angle of 45 ° with respect to two orthogonal principal axes of the quarter-wave plate,
The polarizing element uses the light source light propagated while the polarization state is maintained by the polarization maintaining / propagating means as first light in a predetermined polarization state,
The radio wave sensor, wherein the first light emitted from the polarizing element passes through a space between the polarizing element and the electro-optic crystal and enters the electro-optic crystal. A polarizing element comprising a quarter-wave plate that is housed in a sensor head and is emitted from a light source and propagated while maintaining a polarization state; the first light having a predetermined polarization state;
An electro-optic crystal that is accommodated in the sensor head, changes the polarization of at least a portion of the first light that is incident and transmitted therethrough in response to a change in the electric field of the received electromagnetic wave, and emits the second light;
Polarization separation means for taking out the first polarization component from the second light contained in the sensor head and emitted from the electro-optic crystal;
An optical fiber that propagates the first polarization component extracted by the polarization separation means;
A photoelectric conversion element that photoelectrically converts the first polarization component propagated through the optical fiber into an electrical signal;
Signal processing means for processing an electrical signal photoelectrically converted by the photoelectric conversion element;
Comprising at least
The sensor head is provided in a movable state;
The light source is housed in the sensor head;
Two orthogonal principal axes of the quarter-wave plate make an angle of 22.5 ° with respect to two orthogonal principal axes of the polarization separating means, and two orthogonal electrical principal axes of the electro-optic crystal are An angle of 45 ° with respect to two orthogonal principal axes of the quarter-wave plate,
The radio wave sensor , wherein the first light emitted from the polarizing element passes through a space between the polarizing element and the electro-optic crystal and enters the electro-optic crystal . The radio wave sensor according to claim 1 or 2 ,
The electro-optic crystal includes a reflective surface on a surface facing the incident surface of the first light,
The polarized light separating means changes the optical path of light having a different polarization state from the light source light through the light source light, and changes the optical path in a different direction, so that the second light emitted from the electro-optic crystal is the first light. A polarization beam splitter that extracts a polarization component of 1;
The light source light passes through the polarizing beam splitter and enters the polarizing element,
The first light emitted from the polarizing element is incident from an incident surface of the electro-optic crystal,
Light incident from the incident surface of the electro-optic crystal is reflected by the reflecting surface and emitted from the incident surface.
A radio wave sensor , wherein light emitted from an incident surface of the electro-optic crystal passes through the polarizing element and enters the polarizing beam splitter . The radio wave sensor according to claim 1 or 2 ,
The electro-optic crystal includes a reflective surface on a surface facing the incident surface of the first light,
The polarization separation means includes
A first polarization beam splitter and a second polarization beam splitter that transmit light in the first polarization state and light in the second polarization state that is different in polarization state from the light in the first polarization state;
The polarization state of light disposed in the optical path between the first polarization beam splitter and the second polarization beam splitter and transmitted through the first polarization beam splitter is set to the first polarization state with respect to the second polarization beam splitter, Polarization changing means for changing the polarization state of the light transmitted through the second polarization beam splitter to the second polarization state with respect to the first polarization beam splitter;
Consisting of
A first optical fiber for propagating light whose optical path is changed in a different direction by the second polarizing beam splitter;
A second optical fiber that propagates light whose optical path is changed in a different direction by the first polarizing beam splitter;
A first photoelectric conversion element that photoelectrically converts the light propagated through the first optical fiber into an electrical signal;
A second photoelectric conversion element that photoelectrically converts the light propagated through the second optical fiber into an electrical signal;
A differential amplifier that differentially amplifies the two electric signals photoelectrically converted by the first and second photoelectric conversion elements;
Signal processing means for processing the electric signal output from the differential amplifier;
With
The light source light passes through the first polarization beam splitter, the polarization changing means, and the second polarization beam splitter in this order and enters the polarization element.
The first light emitted from the polarizing element is incident from an incident surface of the electro-optic crystal,
Light incident from the incident surface of the electro-optic crystal is reflected by the reflecting surface and emitted from the incident surface.
The light emitted from the incident surface of the electro-optic crystal passes through the polarizing element and enters the second polarizing beam splitter,
The first polarization component emitted from the incident surface of the electro-optic crystal and having its optical path changed in a different direction by the second polarization beam splitter is propagated by the first optical fiber,
A second polarization component having a polarization state different from that of the first polarization component emitted from the incident surface of the electro-optic crystal and transmitted through the second polarization beam splitter is transmitted through the polarization changing means and transmitted through the first polarization component. A radio wave sensor that is incident on a polarizing beam splitter, is changed in an optical path in a different direction by the first polarizing beam splitter, and is propagated by a second optical fiber .