JP4875835B2 - Measuring system - Google Patents

Description

  In the present invention, light is incident on an optical crystal such as an electro-optic crystal, a magneto-optic crystal, and a piezoelectric (photoelastic) crystal to which a physical quantity such as an alternating electric field or an alternating magnetic field is applied, and the light emitted from the optical crystal is detected. The present invention relates to a measurement system that obtains signals corresponding to an AC electric field, an AC magnetic field, a sound pressure, and the like.

  FIG. 4 is a diagram showing a conventional measurement system for detecting an alternating electric field. As shown in the figure, the light beam 3 emitted from the light source 2 of the signal processing unit 1 includes a polarizing beam splitter (PBS) 4, a Faraday rotator (FR) 5, and a half-wave plate. After passing through (Half-wave plate; HWP) 6, polarizing beam splitter 7, quarter-wave plate (QWP) 8, half-wave plate 9, and lens 10, the sensor head is passed through an optical fiber 11. 12 is propagated. The light beam 3 emitted to the space by the sensor head 12 is collimated by the lens 13 and enters an electro-optic (E1ectro-optic; EO) crystal 14. The light beam 3 incident on the electro-optic crystal 14 is reflected by a dielectric mirror 15 formed on the end face of the electro-optic crystal 14 and travels backward in the electro-optic crystal 14.

  On the other hand, the electric field to be measured can enter the electro-optic crystal 14 through the dielectric mirror 15. When an electric field is applied to the electro-optic crystal 14, the birefringence of the electro-optic crystal 14 changes, so that light propagating in the electro-optic crystal 14 undergoes polarization modulation. The polarization-modulated light beam 3 enters the optical fiber 11 again, propagates to the polarization detection system of the signal processing unit 1, is converted into intensity-modulated light, and is detected by photodetectors (PDs) 16 and 17. Then, the electric field to be measured is detected by processing the output electric signals of the photodetectors 16 and 17 by the differential amplifier 18 and the electric signal measuring device 19.

JP 2000-171487 A Japanese Patent Laid-Open No. 2001-50908 JP 2003-14801 A

  However, in such a measurement system, the sensor head 12 including the electro-optic crystal 14 and the signal processing unit 1 are connected by the optical fiber 11, and stress is applied to the optical fiber 11 due to the displacement of the sensor head 12. Since the polarization state of the light beam 3 propagating through the optical fiber 11 changes, the polarization state of the light beam 3 incident on the electro-optic crystal 14 due to the displacement of the sensor head 12 and the polarization of the light beam 3 incident on the signal processing unit 1. Since the state changes, there is a problem that sensitivity changes when the sensor head 12 is moved.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a measurement system that can suppress a decrease in sensitivity due to displacement of an optical crystal.

In order to achieve this object, in the present invention, the light source output linearly polarized light emitted from the light source is transmitted to the optical crystal via the circulator by the polarization maintaining fiber, and the light source output linearly polarized light is transmitted through the optical crystal. after being polarized modulated by the measured physical quantity, polarization modulated light is polarized and modulated is again incident on the polarization maintaining fiber is transmitted to the polarization adjuster by the circulator, the polarization of the polarization-modulated light by the polarization adjuster After being converted and output , the output light from the polarization adjuster is separated into orthogonal first and second separated linearly polarized light by a polarization beam splitter, and the first and second separated linearly polarized lights are respectively separated into the first and second separated linearly polarized light. 2 is converted into an electric signal by the photodetector, and the electric signal is output as a differential signal by the differential amplifier. The differential signal is output by the electric signal measuring instrument. A measurement system which is outputted as an amplitude signal and phase signal of the measured physical quantity, the polarization adjuster has a configuration for adjusting the conversion of the polarization of the polarization-modulated light by control of the control signal processing unit, the first, the first of the second photodetector, the amplitude signal from the second photoelectric current and the electrical signal measuring device is inputted to the control signal processing unit, the control signal processing unit is the first, second light so as to adjust the conversion of the polarization of the polarization-modulated light by the polarizing adjuster so that a current equal and the amplitude signal is maximized.
In this case, the speed at which the polarization adjuster converts the polarization of the polarization-modulated light is set to be slower than the speed at which the light source output linearly polarized light is polarization-modulated by the measured physical quantity through the optical crystal.

  In the measurement system according to the present invention, even if the stress applied to the optical fiber is changed by moving the optical crystal and a random polarization change is applied to the polarization-modulated light, the measured physical quantity is always measured with the maximum sensitivity. Is possible.

  In addition, when the speed at which the polarization adjuster converts to the arbitrary polarized light is slower than the speed at which the light source output linearly polarized light is polarized and modulated by the measured physical quantity via the optical crystal, the polarization is modulated by the measured physical quantity. The measured physical quantity can be reliably measured without canceling the polarization signal.

FIG. 1 is a block diagram showing a measurement system according to the present invention, and FIG. 2 is a diagram showing a configuration example of the measurement system shown in FIG. As shown in the figure, a light source 21, a circulator 23, an optical sensor 25, a polarization adjuster 30, a polarization beam splitter 34, first and second photodetectors 35 and 36, a differential amplifier 37, a lock-in amplifier 38, a display Part 44 and a control signal processing part 39. In order to maintain the polarized light from the light source 21 to the circulator 23, the circulator 23 to the optical sensor 25, and the circulator 23 to the polarization adjuster 30, polarization-maintaining fibers (PMFs) 22, 24, 29 is used. A polarization maintaining fiber 32 is used as the output fiber of the polarization adjuster 30. The optical sensor 25 includes a lens 26, an optical crystal 27, a mirror 28, and a dielectric tube 45. The optical crystal 27 to be used differs depending on the physical quantity to be measured. When an electric field is measured, an electro-optic crystal such as CdTe, ZnTe, or DAST is used. When a magnetic field is measured, a magnetic material such as YIG or diluted magnetic semiconductor (DMS) is used. using an optical crystal, when measuring the sound pressure using a photoelastic crystal such as quartz and Te0 ₂ crystals.

  The light source output linearly polarized light emitted from the light source 21 is transmitted to the optical crystal 27 via the circulator 23 by the polarization maintaining fibers 22 and 24. For example, the polarization-modulated light that is polarized and modulated by the instantaneous applied electric field in the optical crystal 27 is obtained. The light is again incident on the polarization maintaining fiber 24, the polarization mode dispersion is superimposed by the polarization maintaining fibers 24 and 29, and is transmitted to the polarization adjuster 30 of the polarization signal processing unit 46 by the circulator 23. The polarization-modulated light transmitted to the polarization adjuster 30 is converted into arbitrary polarized light such as linearly polarized light and elliptically polarized light by the polarization adjuster 30. If the response speed of the polarization adjuster 30 is larger than the displacement speed of the electric field to be measured, the polarization signal that has been polarization-modulated by the electric field to be measured is canceled. For this reason, the speed at which the polarization adjuster 30 converts the light into arbitrary polarized light is sufficiently slower than the speed at which the light source output linearly polarized light is polarized and modulated by the measured physical quantity via the optical crystal 27. That is, the response frequency of the polarization adjuster 30 is made smaller than the vibration frequency of the physical quantity to be measured. The response speed of the polarization adjuster 30 differs depending on the physical quantity to be measured. The response frequency of the polarization adjuster 30 when measuring an electromagnetic field of microwave or higher must be 300 MHz or less, and when measuring an electromagnetic field of VHF or higher, it must be 30 MHz or less. When measuring an ultrasonic wave Must be 20 kHz or less. In the measurement system, the polarization change targeted by the polarization adjuster 30 is the superposition of the polarization mode dispersion determined by the lengths of the polarization maintaining fibers 24 and 29, and the axial deviation of the light source output linear polarization incident on the optical crystal 27. The polarization change caused by the rotation of the polarization plane due to the above, the temperature change of the polarization maintaining fibers 24 and 29, and the bending stress.

  The polarization adjuster 30 includes a half-wave plate, a quarter-wave plate, and a motor 31 that rotates the wave plate. It is preferable to use a linear drive motor or a motor using an air bearing as the motor 31 so that the polarization axis is not inclined or moved by the rotation operation of the wave plate. Further, the rotational angle accuracy of the wave plate needs to be about 1 ° or less, and it is preferable that infinite rotation is possible. GP-IB is useful for simply controlling the motor 31, but a direct control circuit is required when high-speed control is required.

  The arbitrary polarized light output from the polarization adjuster 30 is separated by the polarizing beam splitter 34 into orthogonal p-polarized light (first separated linearly polarized light) and s-polarized light (second separated linearly polarized light). In the measurement system shown in FIGS. 1 and 2, the spatially propagated light from the output fiber 32 of the polarization adjuster 30 is collimated by the lens 33 and then incident on the polarization beam splitter 34. When an optical waveguide type polarization beam splitter is used, the polarization adjuster 30 and the polarization beam splitter, and the polarization beam splitter and the photodetectors 35 and 36 can be connected by an optical fiber. In this case, the optical fiber may be either a polarization maintaining fiber or a single mode fiber (SMF), and it is cheaper to use a single mode fiber that can be easily aligned.

  The p-polarized light and the s-polarized light separated by the polarization beam splitter 34 are photoelectrically converted by the photodetectors 35 and 36 as intensity-modulated light that fluctuates in opposite phases according to the amplitude of the electric field to be measured. , 36 outputs an electrical signal. Note that the photodetectors 35 and 36 must have sufficient conversion efficiency up to a frequency higher than the fluctuation frequency of the physical quantity to be measured.

  The differential amplifier 37 receives an electrical signal from the photodetectors 35 and 36 and outputs a signal proportional to the difference between the electrical signals from the photodetectors 35 and 36. The output signal of the differential amplifier 37 is a signal proportional to the electric field to be measured and is detected by the lock-in amplifier 38. That is, the differential signal is output by the lock-in amplifier 38 as an amplitude signal and a phase signal of the physical quantity to be measured.

The first and second photocurrents I _p1 and I _p2 of the photodetectors 35 and 36 and the amplitude signal (intensity signal) V from the ronquin amplifier 38 are sent to the control signal processing unit 39 via signal lines 40 to 42. Then, the control signal processing unit 39 controls the polarization adjuster 30 by controlling the motor 31 via the signal line 43 so that the photocurrents I _p1 and I _p2 are equal and the amplitude signal V is maximized. Adjust the polarization.

FIG. 3 shows the standardized output electrical signals V ₁ (t) and V ₂ (t) of the photodetectors 35, 36 and the standardized output electrical signal ΔV (t) of the differential amplifier 37. . Here, t represents time. When the polarization-modulated light is kept circularly polarized by the polarization adjuster 30, the waveforms of the output electric signals V ₁ (t) and V ₂ (t) are shown in FIGS. 3 (a) and 3 (b), respectively. As shown in FIG. 3C, the amplitude of the output electric signal ΔV (t) is sufficiently large and the DC component becomes zero. If the polarization adjuster 30 is not used, the polarization state of the polarization-modulated light randomly changes in the fiber, and the waveforms of the output electrical signals V ₁ (t) and V ₂ (t) are generally shown in FIG. d) and (e), and the waveform of the output electric signal ΔV (t) has a small amplitude as shown in FIG. Further, since a DC component is present in the waveform of the output electric signal ΔV (t), the differential amplifier 37 is saturated and does not operate normally, and there is a demerit that noise increases.

That is, when the photocurrents I _p1 and I _p2 of the photodetectors 35 and 36 are matched, as shown in FIGS. 3A and 3B, the output electric signals V ₁ (t) and V ₂ (t) The DC voltages can be matched. Since the photocurrents I _p1 and I _p2 of the photodetectors 35 and 36 are average values of excitation currents due to input light, they can be referred to as DC voltages. By matching the DC voltage, the noise level of the differential amplifier 37 is minimized. In the reference to the photocurrents I _p1 and I _p2 , only the amount of polarized light divided by the polarization beam splitter 34 matches, and the polarization adjuster 30 may be adjusted to linearly polarized light or elliptically polarized light. There is. Therefore, referring to the amplitude signal V from the lock-in amplifier 38, if the polarization is adjusted so that the polarization adjuster 30 forms circularly polarized light, that is, the amplitude signal is maximized, the output from the differential amplifier 37. The maximum signal is obtained, the S / N ratio is maximized, and the maximum sensitivity is always obtained.

  As described above, in the measurement system shown in FIGS. 1 and 2, the maximum sensitivity is always obtained. Therefore, when the optical sensor 25 is moved, the stress applied to the polarization maintaining fiber 24 changes, and the polarization modulated light is randomized. Even if a change in polarization is added, the physical quantity to be measured can be measured with the maximum sensitivity. In addition, since the speed at which the polarization adjuster 30 converts the light into the arbitrary polarized light is sufficiently slower than the speed at which the light source output linearly polarized light is polarized and modulated by the measured physical quantity via the optical crystal 27, the measured electric field such as the measured electric field is measured. The polarization signal modulated by the physical quantity is not canceled out, and the measured physical quantity can be reliably measured.

  When measuring a measured physical quantity that sine vibrates using a CW light source, when measuring a measured physical quantity that changes at a high frequency, such as a millimeter wave band or a THz band, or a measured physical quantity that is repeated in a pulsed manner, light detection is performed. Since the bandwidths of the devices 35 and 36 and the differential amplifier 37 are insufficient, the physical quantity to be measured cannot be measured accurately. Therefore, the light source output linearly polarized light is intensity-modulated with a frequency which is a positive fraction of the vibration frequency of the physical quantity to be measured, and then incident on the polarization maintaining fiber 22, and the lock-in amplifier 38 converts the component of the modulation frequency of the differential signal By outputting and making the response frequency of the polarization adjuster 30 smaller than the modulation frequency of the differential signal, the physical quantity to be measured can be accurately measured. Further, after intensity-modulating the light source output linearly polarized light with a frequency obtained by adding a frequency that is a positive fraction of the vibration frequency of the physical quantity to be measured and a beat frequency smaller than the above frequency, the light is incident on the polarization maintaining fiber 22; If the lock-in amplifier 38 outputs the component of the beat frequency of the differential signal and makes the response frequency of the polarization adjuster 30 smaller than the beat frequency, the measured physical quantity can be accurately measured. The intensity-modulated light can be easily obtained by direct modulation for modulating the injection current of the photodetectors 35 and 36. It can also be generated by external modulation that intensity-modulates CW light using an intensity modulator. However, since the amount of light that can be input to the modulator is limited, the system becomes complicated, such as requiring an optical amplifier in the subsequent stage. When measurement is performed using such intensity-modulated light, the response frequency of the polarization modulator 30 is made smaller than the modulation frequency of the differential signal. For example, when using a beat frequency of 10 kHz, the response frequency of the polarization modulator 30 is set to be smaller than 10 kHz.

  In the above-described embodiment, the polarization adjuster 30 including the motor 31 that rotates the half-wave plate, the quarter-wave plate, and the wavelength plate is used as the polarization adjuster. It can also be configured by a mechanism that applies pressure in the outer diameter direction of the optical fiber. When pressure is applied in the outer diameter direction of the optical fiber, a difference in photoelastic constant between a direction in which the pressure is applied and a direction orthogonal to the direction is generated, and birefringence occurs. By applying pressure from three directions inclined by 45 ° from each other, polarization mode dispersion of 360 ° (one wavelength) or more can be given to orthogonal polarized light, and the polarized light can be rotated infinitely. Although the magnitude of birefringence is proportional to the applied pressure, the optical fiber is broken when a certain pressure is exceeded. Therefore, it is practical to rotate the polarized light by applying pressure from four directions shifted by 45 °.

  Further, in the above-described embodiment, the polarization maintaining fiber 32 is used as the output fiber of the polarization adjuster 30, but a single mode fiber may be used as the output fiber of the polarization adjuster. In the above-described embodiment, the lock-in amplifier 38 is used as the electric signal measuring device, but a spectrum analyzer, an oscilloscope, or the like may be used as the electric signal measuring device.

It is a block diagram which shows the measurement system which concerns on this invention. It is a figure which shows the structural example of the measurement system shown in FIG. It is a graph which shows the output electric signal by which the photodetector and the differential amplifier were standardized. It is a figure which shows the measuring system which has the electric field sensor using the electro-optic crystal which detects the conventional alternating current electric field.

Explanation of symbols

DESCRIPTION OF SYMBOLS 21 ... Light source 22 ... Polarization-maintaining fiber 23 ... Circulator 24 ... Polarization-maintaining fiber 27 ... Optical crystal 29 ... Polarization-maintaining fiber 30 ... Polarization regulator 34 ... Polarization beam splitter 35 ... First photodetector 36 ... Second Photo detector 37 ... differential amplifier 38 ... lock-in amplifier 39 ... control signal processor

Claims (1)

The light source output linearly polarized light emitted from the light source is transmitted to the optical crystal via the circulator by the polarization maintaining fiber, and the light source output linearly polarized light is polarized and modulated by the physical quantity to be measured via the optical crystal, and then polarization modulated. The polarization-modulated light is re-incident on the polarization-maintaining fiber, transmitted to the polarization adjuster by the circulator, the polarized light of the polarization-modulated light is converted and output by the polarization adjuster , and is output from the polarization adjuster. The output light is separated into orthogonal first and second separated linearly polarized light by a polarization beam splitter, and the first and second separated linearly polarized lights are converted into electrical signals by the first and second photodetectors, respectively. The electrical signal is output as a differential signal by a differential amplifier, and the differential signal is output as an amplitude signal and a phase signal of the measured physical quantity by an electrical signal measuring instrument. Speed A is the measuring system, the polarization adjuster which includes a structure for adjusting the conversion of the polarization of the polarization-modulated light by control of the control signal processing unit, the polarization regulator to convert the polarization of the polarization-modulated light Is slower than the speed at which the light source output linearly polarized light is polarized and modulated by the physical quantity to be measured through the optical crystal, and the first and second photocurrents of the first and second photodetectors and the electric signal. said amplitude signal from the measuring instrument is input to the control signal processing unit, the control signal processing unit is the first, controls the polarization adjuster as equal second photocurrent and said amplitude signal is maximized And adjusting the polarization conversion of the polarization- modulated light .

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