JP2556912B2 - Voltage detector - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a voltage detecting device, and more particularly to a voltage detecting device including an electro-optic crystal in which the intensity of passing light changes in response to a change in an electric field due to a measured voltage.

2. Description of the Related Art Conventional voltage detecting devices of this type, for example, so-called E-, which use an electro-optical material whose refractive index changes according to the electric field strength, are used.
As the O-voltage detecting device, one disclosed in USP 4,446,425, JP-A-63-300969 or JP-A-64
Some of them are disclosed in Japanese Patent No. 9370. These voltage detection devices are provided with an optical probe made of an electro-optic material having a light reflecting surface at the tip, take out a predetermined polarization component of the reflected light from the light reflecting surface with a polarizing element, and use this with a photodetector. Although it receives light and photoelectrically converts it, it cannot detect S / N well with a normal amplifier because the change in light intensity accompanying a voltage change is weak. Therefore, it is necessary to turn on / off the electric signal to be measured and to narrow-band amplify the signal of the modulation frequency. In the above-mentioned USP 4,446,425 invention, etc., when the electrical signal to be measured is the output signal of the photodetector, the light pulse incident on the photodetector may be turned on and off, which is easily performed. However, in this case, there is a problem that the device under test (electrical signal source under test) is limited to one that generates an electrical signal by light.

[Problems to be Solved by the Invention]

On the other hand, it is possible to directly turn on / off the electric signal under measurement by the electric chop element, but in this case, waveform distortion occurs when the electric signal under measurement passes through the electric chop element, making accurate waveform measurement difficult. There is a problem that Further, considering the case of measuring the DC voltage, if a voltage is applied to the E-O probe, the intensity of light passing through the polarizing element changes, so if the amount of passing light is monitored by a photodetector, even the DC voltage can be measured. You should be able to measure. However, since the sensitivity of the E-O probe is low, the change in the amount of light is extremely small with an applied voltage of only a few V,
Ordinary optical power meters cannot detect because of insufficient dynamic range. Generally, as shown in FIG. 14, it has been proposed to chop an electric signal to be measured and efficiently detect only the modulation component by a lock-in amplifier. However, when measuring the DC voltage, the electric signal cannot be chopped, so it is conceivable to chop the incident light to the optical probe as shown in FIG. However, if the lock-in is detected as it is, only the light intensity itself is measured as shown in FIG. 15 (C), and the change due to the voltage application to the EO probe can be detected. Can not. Further, as described above, a light reflecting surface is formed at the tip of the optical probe, but it is difficult to set the reflectance of this light reflecting surface to 100%. There is a problem that part of the incident light leaks to the side. When a part of the incident light leaks from the light reflecting surface to the measured signal source side, the reflected light from the measured signal source again passes through the light reflecting surface and enters the optical probe, which is a part of the output signal. It will be detected. In the above case, if the light reflectance of the signal source to be measured is constant, signal processing can be performed in consideration of this reflectance and the transmittance of the light reflecting surface to obtain a correct measurement value. However, if the signal source under measurement is, for example, an LSI, the reflectance changes depending on the pattern,
It cannot be easily corrected. The present invention has been made in view of the above conventional problems, and provides a voltage detection device capable of reliably detecting a change in a signal under measurement without turning on / off the signal under measurement. The purpose is to do. It is another object of the present invention to provide a voltage detection device capable of measuring a signal that does not change, such as a DC voltage. It is an object of the present invention to provide a voltage detecting device capable of correcting a measurement error due to light transmitted through a light reflecting surface of an optical probe. It is also an object of the present invention to provide a voltage detection device capable of correcting a measurement error caused by the light transmitted through the light reflecting surface as described above due to the variation in the reflectance of the signal source surface to be measured. And

[Means for Solving the Problems]

The present invention relates to an optical probe which is made of an electro-optical material whose refractive index changes due to an electric field of a predetermined portion of an object to be measured, and has a light reflecting surface formed at its tip, and a light source device for generating a light beam to be incident on the optical probe. And a photodetector that is emitted from this light source device, is incident on the optical probe, and is reflected by the light reflecting surface, receives the light beam emitted from the optical probe, and photoelectrically converts the light beam. From the change of the photodetector output, in the voltage detection device for measuring the voltage of the DUT, the light source device is provided with a chop means for repeatedly turning on and off the light beam,
The second photodetector, which receives and photoelectrically converts a light beam whose polarization state is not changed by the electric field applied to the optical probe, and the output difference between the photodetector and the second photodetector, the chopping means. And a narrow band detecting means for detecting a narrow band at an on-off frequency, and calculating a difference between the output of the narrow band detecting means in a state in which an electric field is not applied to the optical probe and a state in which the electric field is applied to the optical probe, and The above object is achieved by providing a correction circuit that divides the difference by the output of the photodetector in the state where no electric field is applied to the optical probe. Further, the above-mentioned object is achieved by using the light source device as a pulse laser which outputs a laser pulse light, and by making it possible to decompose the voltage waveform to be measured for a long time.

[Action]

In the present invention, not the electrical signal to be measured but the light incident on the optical probe is turned on and off by the optical chopping means. Therefore, compared to the case where the signal to be measured is chopped, Accurate waveform measurement is possible. It is also possible to measure DC voltage. Further, noise due to light leaked from the light reflecting surface of the optical probe is corrected from the difference in the output of the narrow band detection means when the electrical signal to be measured is applied to the optical probe and when it is not applied,
Accurate detection can be performed. Further, the difference between the outputs of the narrow band detection means in the state where the electric field is not applied to the optical probe and the state where the electric field is applied is calculated, and the difference in the output is detected by the photodetector in the state where the electric field is not applied to the optical probe. It is possible to correct the variation in the reflectance of the object to be measured by dividing by the output of.

【Example】

Embodiments of the present invention will be described below with reference to the drawings. In the first embodiment of the present invention, as shown in FIG. 1, an electro-optical element in which the intensity of passing light changes in response to a change in a signal under measurement in a signal source under measurement 10 that causes a voltage change. Light intensity changing means 12, a short pulse light source 14 composed of a semiconductor laser, an optical chopper 16 for repeatedly turning on and off the light emitted from the short pulse light source 14, and a light for controlling the optical chopper 16. A light source device 20 including a chopper control circuit 18, a first photodetector 22 for photoelectrically converting the light emitted from the light source device 20 and passing through the light intensity changing means 12, and the light from the light source device 20. In addition, a beam splitter 24 that laterally separates the light before entering the light intensity changing means 12, and a second photodetector 26 that receives the light separated from the beam splitter 24 and photoelectrically converts the light.
And a narrow band detecting means 28 for calculating the difference between the outputs of the first and second photo detectors 22 and 26 and detecting the narrow band at the on / off frequency of the optical chopper 16.
The voltage detection device 30 is configured from the above. The narrow band detection means 28 includes an arithmetic circuit 28A for calculating the difference between the outputs of the first photodetector 22 and the second photodetector 26,
A narrow band filter that detects the narrow band of the output signal of the arithmetic circuit 28A at the ON / OFF frequency of the optical chopper 16.
28B, an amplifier 28C for amplifying the output of the narrow band filter 28B, and the first and the second when there is no change in the light intensity due to the signal under measurement in the light intensity changing means 12.
And an output adjuster 28D that adjusts the arithmetic circuit 28A so that the difference between the outputs of the photodetectors 22 and 26 becomes zero. This output adjuster 28D includes the first and second photodetectors 2
One or both of the output signals of 2 and 26 are adjusted. In the case of this embodiment, the light is turned on / off by the optical chopper 16 without directly turning on / off the signal of the signal source 10 to be measured.
By turning off, the same effect as turning on / off the signal under measurement can be obtained. Therefore, the distortion of the signal waveform due to the ON / OFF of the signal under measurement can be prevented, and the accurate waveform measurement of the signal under measurement becomes easy. Further, the short pulse light source 14 synchronizes this with the signal to be measured, so that the light intensity change amount in the light intensity changing means 12 due to the change of the signal to be measured can be achieved with a gate time of about the light pulse width of the short pulse light source 14. Can be detected. In this case, the first and second photodetectors 22, 26 do not need to be of high speed type. In the first embodiment described above, the light source is a short pulse light source, but if it is not necessary to detect the signal under measurement in a short gate time, a continuous light source may be used. Further, in the above-described embodiment, the second photodetector 26 separates the light before entering the light intensity changing means 12 by the beam splitter 24 and receives the light. ,
As long as it is necessary, any light can be received as long as it can receive light in a state where the light intensity change means 12 does not change the light intensity. Therefore, a beam splitter may be arranged on the light emission side of the light intensity changing means 12 so that the light can be received from here in a state where the light intensity is not changed by the signal under measurement. In the above embodiment, the optical chopper 16 is a mechanical optical chopper, an ultrasonic light modulator, or a light source drive circuit. Further, the narrow band detecting means 28 may be a spectrum analyzer, a lock-in amplifier, or a narrow band band pass amplifier other than the combination of the narrow band filter and the amplifier. Next, a second embodiment of the present invention shown in FIG. 2 will be described. The second embodiment is composed of an electro-optical material whose refractive index changes due to an electric field in a predetermined portion of the object to be measured, and an optical probe 34 having a light-reflecting surface 34A formed at its tip, and the optical probe 3
4, a light source device 36 for generating a light beam to be incident on the light source device 36, emitted from the light source device 36, incident on the optical probe 34, and after being reflected by the light reflecting surface 34A, emitted from the optical probe 34. And a first photodetector 38 for photoelectrically converting the received light beam, and measuring the voltage of the DUT from the change in the output of the first photodetector 38. In 32, the light source device 36 is provided with an ultrasonic optical modulator 36A that repeatedly turns on and off a light beam, and a light beam that is not subjected to a refractive index change due to an electric field applied to the optical probe 34 is supplied to the optical probe 34. A second photodetector 40 that receives and photoelectrically converts light before incidence, and the first and second photodetectors 40.
The output difference of the photodetectors 38 and 40 of the ultrasonic wave modulator 36A
And a narrow band detecting means 42 for detecting a narrow band at an on / off frequency. The light source device 36 includes the ultrasonic light modulator 36A, a semiconductor laser 36B as a light source, and a light collecting unit arranged between the emitting side of the semiconductor laser 36B and the incident side of the ultrasonic light modulator 36A. The lens 36C, the optical isolator 36D, and the semiconductor laser 36B are connected to the electronic device 46 to be measured which is an object to be measured.
Laser drive device 36E for driving in synchronization with
And an optical modulator control device 36F for controlling the ultrasonic optical modulator 36A. Reference numeral 48 in the figure denotes an electronic device driving device for driving the electronic device under test 46, and outputs a signal for synchronizing with the electronic device under test 46 to the semiconductor layer driving device 36E. The ultrasonic light modulator 36A is the optical modulator control device 36F.
The 0th-order light, which is controlled by, is diverted from the optical probe 34 when it is not modulated,
The primary light is directed to the optical probe 34 via the collimator lens 52 when being modulated by ultrasonic waves. A polarization beam splitter 54, a beam splitter 56, a compensator 58 and a condenser lens 60 are arranged in this order between the collimator lens 52 and the optical probe 34. The polarization beam splitter 54 passes through these,
The light that has once entered the optical probe 34 and has been reflected by the light reflecting surface 34A is laterally separated and made incident on the first photodetector 38. The beam splitter 56 laterally splits the primary light before entering the optical probe 34 and guides it to the second photodetector 40. The narrow band detection means 42 includes a variable attenuator 42A for adjusting the output of the second photodetector 40, and the variable attenuator 4A.
2A in response to the outputs of the first and second photodetectors 38, 40 in the state where no electric field is applied to the optical probe 34
A variable attenuator control circuit 42B that controls the outputs of the two photodetectors 38 and 40 to be the same, and a second photodetector 40 that passes through the output from the first photodetector 38 and the variable attenuator 42A. The output from the optical modulator control device 36F, based on the reference signal from the optical modulator 36A, at the frequency of the light on / off by the ultrasonic optical modulator 36A, lock-in amplifier 42C for narrow band amplification, and configured. Has been done. Reference numeral 55 in the figure denotes a recording device that records the output of the lock-in amplifier 42C together with the amount of change in the optical pulse generation timing in the semiconductor laser 36B based on the signal from the semiconductor laser driving device 36E. Reference numeral 57 indicates a display analysis device for displaying and analyzing the record of the recording device 55. Next, the operation of the above embodiment will be described. The electronic device drive 48 allows the electronic device under test to be measured.
46 is operated, and the semiconductor laser driving device 36E drives the semiconductor laser 36B in synchronization with the measured electronic device 46 based on the signal from the electronic device driving device 48. The emitted light from the semiconductor laser 36B enters the ultrasonic light modulator 36A through the condenser lens 36C and the optical isolator 36D. The ultrasonic light modulator 36A is driven by the light modulator control device 36F, and when the incident light is modulated, it becomes primary light, and collimator lens 52, polarization beam splitter 54, beam splitter 56, compensator 58 and collector. Light lens 60
And then enters the optical probe 34. An electric field is applied to the optical probe 34 by the voltage of the electronic device under test 46, and the polarization state of light in the optical probe 34 changes according to the electric field. In the optical probe 34, the incident light is reflected by the light reflecting surface 34A, then reaches the polarization beam splitter 54 via the condenser lens 60, the compensator 58, and the beam splitter 56 again, and is directed to the first photodetector 38 here. Is reflected. On the other hand, when the primary light from the ultrasonic light modulator 36A passes through the beam splitter 56, a part thereof is reflected and reaches the second photodetector 40. The output A of the first photodetector 38 and the output B of the second photodetector 40 are the variable attenuator 42A and the variable attenuator control circuit 42B.
Is automatically and previously adjusted by the variable attenuator 42A so that the output is A = B when no electric field is applied to the optical probe 34, and then the lock-in amplifier 42C is adjusted. Entered in. This lock-in amplifier 42C is based on the reference signal from the optical modulator control device 36F, and based on the reference signal from the optical modulator control device 36F among the signal components A-B, the ultrasonic optical modulator 36A. The on / off modulation frequency component of is amplified in a narrow band. The output of this lock-in amplifier 42C, that is, AB = C, is
When input to the recording device 55, the recording device 55 records the output of the lock-in amplifier 42C together with the optical pulse generation timing change amount in the semiconductor laser driving device 36E, which is displayed and analyzed by the display analysis device 57. . Therefore, in this embodiment, the electronic device under test 46 is
Optical probe 34 without turning on / off the signal at
It is possible to obtain the same effect as turning on / off the light incident on the ultrasonic wave modulator 36A to turn on / off the signal under measurement itself, and to turn on / off the electronic device under test 46. Based on this, the distortion of the signal waveform is eliminated. Further, in this embodiment, since the semiconductor laser 36B is driven in synchronization with the signal of the electronic device under test 46, the signal waveform in the electronic device under test 46 is changed by sequentially changing the optical pulse generation timing. Can be measured accurately. As described above, this embodiment makes it possible to measure a high-speed electric waveform by synchronizing the semiconductor laser 36B with the electronic device under test 46, but the present invention is not limited to this. The present invention can also be applied to the case where a DC voltage is applied to the electronic device under test 46. In this case, the light source in the light source device does not need to be pulsed light, and may be a DC light source such as a He Ne laser. In the case of measuring the DC voltage, it was impossible in the past as shown in FIGS. 14 and 15, but in this embodiment, as shown in FIG. 3, the input light is chopped and locked. With the differential input (AB) in the in-amplifier 42C, only the change due to the voltage application to the optical probe 34 can be lock-in detected as shown in FIG. 3 (D). Further, by providing a transparent back electrode on the surface of the optical probe 34 shown in FIG. 5 which faces the light reflecting surface 34A and applying a DC signal thereto, the absolute voltage can be measured. Next, a third embodiment of the present invention shown in FIG. 4 will be described. In the third embodiment, a correction circuit 44 is provided on the output side of the lock-in amplifier 42. Since the other structure is the same as that of the second embodiment of FIG. 2, the same parts are designated by the same reference numerals as those in FIG. 2 and the description thereof is omitted. The correction circuit 44 calculates a difference in the output of the lock-in amplifier 42C in a state in which the electric field is not applied to the optical probe 34 and a state in which the electric field is applied, and the difference in the output is supplied to the optical probe 34. The first without electric field applied
Of the photodetector 38 and is divided by the output of the photodetector 38. Next, the operation of this embodiment will be described. Normally, the reflectance of the light reflecting surface 34A at the tip of the optical probe 34 is set to 100.
% Is impossible. Therefore, a part of the incident light to the optical probe 34 reaches the DUT surface through the light reflecting surface 34A, and is reflected there, enters the optical probe through the light reflecting surface 34A again, This is detected as noise by the photodetector and causes a measurement error. Not only that, when the reflectance of the surface of the object to be measured is uneven, the measurement error due to the reflected light from this surface also changes with the change of the reflectance, which makes further accurate measurement difficult. The correction circuit 44 eliminates the influence of the error due to the transmitted light based on the incompleteness of the reflectance of the light reflecting surface 34A, and further the error due to the variation of the reflectance of the DUT surface,
It enables accurate measurement. As shown in FIG. 5, first and second regions 46A and 46B having different reflectances on the surface of the measured object, for example, the electronic device to be measured 46 described above, and their reflectances R1 and R2.
Suppose The incident light intensity is 2I ₀ , and the reflectance of the light reflecting surface 34A is R ₀ . Here, when R ₀ = 1 is set, the basic output I from the optical probe 34 is expressed by the following equation (1). However, the expression (1) is an approximate expression when V is smaller than Vπ. I = I ₀ (1 + π / Vπ · v) (1) where Vπ is a half-wave voltage, and v is a measured voltage. On the other hand, in a state where the reflectance of the light reflecting surface 34A is R ₀ <1, the output of the light reflected by the light reflecting surface 34A and the light reflecting surface 34A, the light reflected by the area 46A, and the light reflecting surface 34A again are reflected. When the output of the transmitted light is I _x and I _y , respectively, I _x = I ₀ R ₀ (1 + π / Vπ · v) (2) I _y = (1−R ₀ ) ² R1I ₀ (1 + π / Vπ · v) v) (3) I in the above equation (1) is almost equal to the sum of I _x and I _y . Therefore I
Becomes as shown in the following equation (4). I = I _x + I _y = I ₀ (1 + π / Vπ · v) {R ₀ + (1−R ₀ ) ² R1} (4) Here, both I _x and I _y are the light amounts in the polarization beam splitter 54. Is assumed to be halved. As described in the case of the second embodiment, the narrow band detection means 42 includes the variable attenuator 42A and the variable attenuator control circuit 4
2B, the outputs of the first and second photodetectors 38 and 40 are adjusted to be equal when no voltage is applied to the optical probe 34, and the lock-in amplifier is adjusted in this state.
From 42C, A-B = C differential output I _A -I _B i.e. second view is output. When measuring the first region 46A and the second region 46B, I _A , I _B, and I _AB in each region when the voltage is not applied to the optical probe 34 and when the voltage is applied are as follows. . That is, in the first area 46A, the following (5) to (9)
It looks like an expression. Here, (◯) indicates that voltage is applied, and (×) indicates that voltage is not applied. I _A (×) = I ₀ {R ₀ + (1-R ₀ ) ² R1} (5) I _A (○) = I ₀ {R ₀ + (1-R ₀ ) ² R1} (1 + π / Vπ・ V) (6) I _B = I _A (×) = I ₀ {R ₀ + (1-R ₀ ) ² R1} (7) I _AB (×) = I _A (×) -I _B = _{0 ... (8) I AB (} ○) = I a (○) -I B = I 0 {R 0 + (1-R 0) 2 R1} π / Vπ · v ... (9) the second region 46B In, the following equations (10) to (13) are obtained. I _A (×) = I ₀ {R ₀ + (1-R ₀ ) ² R2} (10) I _A (○) = I ₀ {R ₀ + (1-R ₀ ) ² R2} (1 + π / Vπ · v) ... (11) I AB (×) = I A (×) -I B = I 0 (1-R 0) 2 (R2-R1) ... (12) I AB (○) = I A (○ _{) -I B = I 0 [(} 1-R 0) 2 (R2-R
_{1) + {R 0 + (} 1-R 0) 2 R2} π / Vπ · v] ... (13) Note that, I _B are as (7). Next, when the difference between the differential outputs (AB) with and without the voltage applied to the optical probe 34 is obtained, the following (1
It becomes as shown in the equation 4). I _AB (○) -I _AB (×) = I ₀ {R ₀ + (1-R ₀ ) ² R1} π / Vπ · v (14) Also, in region 46B, as shown by the following formula (15) become. I _AB (○) = I _AB (x) = I ₀ {R ₀ + (1-R ₀ ) ² R2} π / Vπ · v] (15) In the above formulas (14) and (16), R1 and Since R2 is different, these cannot be said to be completely corrected (6th
See figure). Here, the arc-shaped portions in FIGS. 6C and 6D depend on the voltage distribution when a voltage is applied to the two electrodes 46B.
-B shows the distribution of the output. That is, it is an output when the reflectance R0 of the optical probe is assumed to be 100%. In general, when the same voltage is applied to two electrodes and the voltage is measured by an optical probe, the output distribution does not have the electrode shape itself. Because there is no electrode that determines the potential outside the electrodes, the voltage distribution depends on the surrounding electrode configuration. Specifically, the voltage distribution gradually decreases at both ends and between the electrodes. The output is influenced by both. This is illustrated in the arc-shaped output distribution. When the equations (14) and (15) are divided by the output I _A (×) in the state without voltage change, the following equations (16) and (17) are obtained. It is proportional to the applied voltage c without depending on the reflectances R1 and R2 in the regions 46A and 46B. _{{I AB (○) -I AB} (×)} / I A (×) = π / Vπ · v ... (16) {I AB (○) -I AB (×)} / I A (×) = π / Vπ · v (17) As described above, the correction circuit 44 outputs the differential signal of the lock-in amplifier 42C by the output of the first photodetector 38 in the state where the voltage is not applied to the optical probe 34. Divides the output, so
As a result, equations (16) and (17) are obtained. Here, for example, the reflectance R ₀ = 90% of the light reflecting surface 34A,
Further, considering the worst condition in the electronic device under test 46, assuming that the reflectance R1 of the first region 46A is R1 = 0 and the reflectance R2 of the second region 46B is 100%, R ₀ + (1 -R ₀ ) ²
R1 = _R0 = 0.90, in region 46B _R0 + (1- _R0 ) ² R2 = _R0 +
(1-R ₀ ) ² = 0.91, and the equation (14) and (1
There is a difference of about 1% between the corrected output of equation (5) and the corrected output of equations (16) and (17). Therefore, the measurement accuracy in this embodiment is about 1%. In the above embodiment, the semiconductor layer driving device 36E is
It is operated on the basis of a signal from the electronic device driver 48, which is provided with a reference clock generator 62, for example, as shown in FIG. 7, from which the clock signal is supplied to the semiconductor laser driver 36E and the electronic device. Drive 48
You may make it output to. Further, in the above embodiment, the light is reflected from the polarization beam splitter 54 to the first photodetector 38 and from the beam splitter 56 to the second photodetector 40, respectively. For example, as shown in FIG. 8, a half mirror 64 is arranged on the incident light, a part of the incident light passing through the polarizer 66 is reflected on the second photodetector 40, and the optical probe
A part of the light returned from 34 may be input to the first photodetector 38 via the analyzer 68. Further, as shown in FIG. 9, a polarization beam splitter 54 may be arranged instead of the half mirror 64 in the embodiment of FIG. In the above embodiment, the first photodetector 38 and the second photodetector 38
The output of the photodetector 40 is controlled by the variable attenuator 42A and the variable attenuator control circuit 42B so that both outputs become equal when no voltage is applied to the optical probe 34. For example, as shown in FIG. 10, a variable amplifier 70 that amplifies the first photodetector 38, and a variable amplifier that controls the variable amplifier 70 according to the outputs of the first and second photodetectors 38 and 40. It may be controlled by the amplifier control circuit 72. Further, as shown in FIG. 11, the outputs of the first and second photodetectors 38 and 40 may be input to the differential amplifier 74, and the outputs may be input to the lock-in amplifier 42C. In this case, the differential amplifier 74 is the first and second photodetectors in the state where the voltage is not applied to the optical probe 34 in advance.
Adjust so that the outputs of 38 and 40 are equal. Also, as shown in FIG. 12, the lock-in amplifier 42C
The division circuit 76 may be provided on the output side of the. This division circuit 76 uses the differential output of the lock-in amplifier 42C as
It is divided by a part of the output of the second photodetector 40, thereby canceling the intensity fluctuation of the light source in the light source device 36. Furthermore, in each of the above embodiments, the ultrasonic light modulator 36A or the light source light is turned on / off by the optical chopper 16. This is, for example, the semiconductor laser 36B in the above embodiment, a semiconductor laser driving device. The 36E may electrically turn on and off to form a state similar to that when light is turned on and off by the optical modulator. Further, in this case, as shown in FIG. 13, the on / off signal of the semiconductor laser driving device 36E is supplied to the lock-in amplifier 4
It may be used for the reference signal in 2C.

【The invention's effect】

Since the present invention is configured as described above, the voltage detection device has an excellent effect that an error due to light leaked from the light reflecting surface of the optical probe can be corrected and an accurate measurement can be performed. Further, in this case, there is an excellent effect that it is possible to correct variations in reflectance of the surface of the object to be measured and perform accurate measurement.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the voltage detecting device according to the present invention, FIG. 2 is a block diagram showing a second embodiment of the present invention, and FIG. 3 shows the operation of the second embodiment. 4 is a block diagram showing an essential part of the third embodiment of the present invention, FIG. 5 is an enlarged sectional view showing the vicinity of the optical probe in the third embodiment, and FIG. 6 is the same embodiment. Diagram showing the measurement results by
7 to 13 are block diagrams showing modified examples of each part in the second embodiment, and FIGS. 14 and 15 are cases where an electric signal is chopped and input light is chopped in a conventional voltage detecting device. It is a diagram which compares and shows the detection state in a case. 10 ... Signal source to be measured, 12 ... Light intensity changing means, 14 ... Short pulse light source, 16 ... Optical chopper, 20, 36 ... Light source device, 22, 38 ...
1st photodetector, 24 ... Beam splitter, 26, 40 ... 2nd photodetector, 28, 42 ... Narrow band detection means, 30 ... Voltage detection device, 32 ... Voltage detection device, 34 ... Optical probe, 34A … Light reflecting surface, 36A… Ultrasonic light modulator, 36B… Semiconductor laser, 42
C ... Lock-in amplifier, 44 ... Correction circuit, 46 ... Electronic device under test.

Claims (2)

(57) [Claims] 1. An optical probe made of an electro-optical material whose refractive index is changed by an electric field at a predetermined portion of an object to be measured and having a light-reflecting surface at its tip, and a light source for generating a light beam incident on the optical probe. A light detector emitted from the light source device, incident on the optical probe, reflected by the light reflection surface, and then received by the light probe, and photoelectrically converting the light beam. ,,
In a voltage detection device that measures the voltage of an object to be measured from a change in the output of the photodetector, the light source device includes a chopping unit that repeatedly turns on and off a light beam, and the polarization state of an electric field applied to the optical probe A second photodetector for receiving a light beam that does not change and photoelectrically converting it;
Narrow band detecting means for narrowing the output difference between the photo detector and the second photo detector at the on-off frequency by the chopping means, and a state in which no electric field is applied to the optical probe and a state in which an electric field is applied to the optical probe. And a correction circuit for calculating the output difference of the narrow area detection means and dividing the output difference by the output of the photodetector in a state where no electric field is applied to the optical probe. A voltage detection device characterized by the above. 2. The voltage detecting device according to claim 1, wherein the light source device is a pulse laser that outputs a laser pulse light, and the voltage waveform to be measured can be decomposed at high time.