Voltage detector

A voltage detector comprises a pulse light source, a delay means, a dispersing prism made of an electro-optic material and a linear image sensor. The electro-optic material senses a voltage developing in a selected area of an object as a change of its refractive index. An optical path of a pulse light is changed by the dispersing prism according to the change of its refractive index. An output pulse light emerging from the dispersing prism is detected by a linear image sensor. A waveform of the periodic voltage pulses can be detected by gradually delaying a phase of the pulse light emitted from the light source. The voltage developing in one-dimensional positions in the object can be also detected by employing a dispersing prism extending along the positions to be detected and a two-dimensional detector.

BACKGROUND OF THE INVENTION 
The present invention relates to a voltage detector for detecting the 
voltage developing in a selected area of an object to be measured such as 
an electric circuit. In particular, the present invention relates to a 
voltage detector of the type that detects voltage by making use of the 
change in light polarization that occurs in accordance with the voltage 
developing in a selected area of an object to be measured. 
Various voltage detectors have been used to detect the voltage developing 
in a selected area of objects to be measured such as electric circuits. 
Conventional voltage detectors are roughly divided into two types: in one 
type, the probe is brought into contact with a selected area of an object 
to be measured and the voltage developing in that area is detected; and in 
the other type, the probe does not make contact with a selected area of an 
object to be measured and instead an electron beam is directed into that 
area and the voltage developing in it is detected. 
Voltage changes rapidly in fine-line portions of objects such as integrated 
circuits that are small and complicated in structure, and a strong demand 
exists in the art for detecting such rapidly changing voltage with high 
precision without affecting the condition of the fine-line portions. 
However, this need has not been fully met by the prior art voltage 
detectors. With devices of the type that detect voltage by bringing the 
probe into contact with a selected area of an object to be measured, it is 
difficult to attain direct contact between the probe and a fine-line 
portion of the object of interest such as an integrated circuit. Even if 
this is successfully done, it has been difficult to correctly analyze the 
operation of the integrated circuit solely on the basis of the voltage 
information picked up by the probe. A further problem involved is that 
contact by the probe can cause a change in the operation of the integrated 
circuit. Voltage detectors of the type that employ an electron beam have 
the advantage that they are capable of voltage detection without bringing 
the probe into contact with an object to be measured. However, the area to 
be measured with such voltage detectors has to be placed in and exposed to 
a vacuum. In addition, the area to be measured is prone to be damaged by 
the electron beam. 
The prior art voltage detectors have a common problem in that they are 
unable to operate quickly enough to follow rapid changes in voltage and 
hence fail to achieve precise detection of voltages that change rapidly as 
in integrated circuits. 
With a view to solving these problems, it has been proposed by two of the 
present inventors (Japanese Patent Application No. 137317/1987 filed on 
May 30, 1987) that voltage be detected by making use of the polarization 
of a light beam that changes with the voltage developing in a selected 
area of an object to be measured. 
A voltage detector operating on this principle is schematically shown in 
FIG. 7. The detector generally indicated by 50 is composed of the 
following components: an optical probe 52; a CW (Continuous-Wave) light 
source 53 typically in the form of a laser diode; an optical fiber 51 for 
guiding a light beam from the CW light source 53 into an optical probe 52 
through a condenser lens 60; an optical fiber 92 for guiding reference 
light from the optical probe 52 into a photoelectric converter 55 through 
a collimator 90; an optical fiber 93 for guiding output light from the 
optical probe 52 into a photoelectric converter 58 through a collimator 
91; and a comparator circuit 61 for comparing the electric signals from 
the photoelectric converters 55 and 58. 
The optical probe 52 is equipped with an electro-optic material 62 such as 
an optically uniaxial crystal of lithium tantalate (LiTaO.sub.3). The tip 
63 of the electro-optic material 62 is worked into a frustoconical shape. 
The optical probe 52 is surrounded with a conductive electrode 64 and has 
at its tip 63 a coating of reflecting mirror 65 in the form of a thin 
metal film or a dielectric multilayer film. 
The optical probe 52 further includes the following components: a 
collimator 94; condenser lenses 95 and 96; a polarizer 54 for selectively 
extracting a light beam having a predetermined polarized component from 
the light beam passing through the collimator 94; and a beam splitter 56 
that splits the extracted light beam from the polarizer 54 into reference 
light and input light to be launched into the electro-optic material 62 
and which allows the output light emerging from the electro-optic material 
62 to be directed into an analyzer 57. The reference light is passed 
through the condenser lens 95 and thence launched into the optical fiber 
92, whereas the output light emerging from the electro-optic material 62 
is passed through the condenser lens 96 and thence launched into the 
optical fiber 93. 
Voltage detection with the system shown in FIG. 7 starts with connecting 
the conductive electrode 64 on the circumference of the optical probe 52 
to a predetermined potential, say, the ground potential. Then, the tip 63 
of the probe 52 is brought close to the object to be measured such as an 
integrated circuit (not shown), whereupon a change occurs in the 
refractive index of the tip 63 of the electro-optic material 62 in the 
probe 52. Stated more specifically, the difference between refractive 
indices for an ordinary ray and an extraordinary ray in a plane 
perpendicular to the light-traveling direction will change in the 
optically uniaxial crystal. 
The light beam issuing from the light source 53 passes through the 
condenser lens 60 and is guided through the optical fiber 51 to be 
directed into the collimator 94 in the optical probe 52. The light beam is 
polarized by the polarizer 54 and a predetermined polarized light having 
intensity I is launched into the electro-optic material 62 in the optical 
probe 52 through the beam splitter 56. Each of the reference light and the 
input light, which are produced by passage through the beam splitter 56, 
has an intensity of I/2. As already mentioned, the refractive index of the 
tip 63 of the electro-optic material 62 varies with the voltage on the 
object being measured, so the input light launched into the electro-optic 
material 62 will experience a change in the state of its polarization at 
the tip 63 in accordance with the change in the refractive index of the 
latter. The input light is then reflected from the reflecting mirror 65 
and makes a return trip through the electro-optic material 62, from which 
it emerges and travels back to the beam splitter 56. If the length of the 
tip 63 of the electro-optic material 62 is written as l, the state of 
polarization of input light launched into that material will change in 
proportion to the difference between refractive indices for the ordinary 
ray and the extraordinary ray and to the length 2l as well. The output 
light sent back into the beam splitter 56 is thence directed into the 
analyzer 57. The intensity of the output light entering the analyzer 57 
has been decreased to I/4 as a result of splitting with the beam splitter 
56. If the analyzer 57 is designed in such a way as to transmit only a 
light beam having a polarized component perpendicular to that extracted by 
the polarizer 54, the intensity of output light that is fed into the 
analyzer 57 after experiencing a change in the state of its polarization 
is changed from I/4 to (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ] in the analyzer 
57 before it is further fed into the photoelectric converter 58. In the 
formula expressing the intensity of output light emerging from the 
analyzer 57, V is the voltage developing in the object to be measured, and 
V.sub.0 is a half-wave voltage. 
In the comparator circuit 61, the intensity of reference light produced 
from the photoelectric converter 55, or I/2, is compared with the 
intensity of output light produced from the other photoelectric converter 
58, or (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ]. 
The intensity of output light, or (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ], will 
vary with the change in the refractive index of the tip 63 of the 
electro-optic material 62 that occurs as a result of the change in 
voltage. Therefore, this intensity can be used as a basis for detecting 
the voltage developing in a selected area of the object to be measured, 
say, an integrated circuit. 
As described above, in using the voltage detector 50 shown in FIG. 7, the 
tip 63 of the optical probe 52 is brought close to the object to be 
measured and the resulting change in the refractive index of the tip 63 of 
the electro-optic material 62 is used as a basis for detecting the voltage 
developing in a selected area of the object of interest. Therefore, the 
voltage developing in fine-line portions of a small and complicated object 
such as an integrated circuit which are difficult to be contacted by a 
probe or which cannot be contacted by a probe without affecting the 
voltage being measured can be effectively detected by the detector 50 
without bringing the optical probe 52 into contact with such fine-line 
portions. If desired, a pulse light source such as a laser diode that 
produces light pulses of a very short pulse width may be used as a light 
source to ensure that rapid changes in the voltage on the object to be 
measured are sampled at extremely short time intervals. Rapid changes in 
the voltage on the object of interest can be measured with a very high 
time resolution by using a CW light source and a quick-response detector 
such as a streak camera. Either method is capable of precision detection 
of rapid changes in voltage. 
However, in the voltage detector 50 shown in FIG. 7 where the voltage 
developing in a selected area of an object of interest is measured by 
making use of the change in the polarization of a light beam in the 
electro-optic material 62, it is necessary to extract a predetermined 
component of linear polarization not only from the light beam from the 
light source 53 by means of the polarizer 54 but also from emerging light 
from the electro-optic material 62 by means of the analyzer 57. In order 
to satisfy these needs, the efficiency of utilization of light beams is 
inevitably reduced. Another problem of the voltage detector 50 is that 
because of the use of the beam splitter 56 the analyzer 57 can receive 
only a weak emerging beam whose intensity is much smaller than that of the 
light beam issuing from the light source 53 and this has put limits on the 
precision of voltage detection. Furthermore, the polarizer 54, analyzer 57 
and beam splitter 56 add to the number of components of the optical system 
and have put limits on the effort to improve its precision significantly. 
If a streak camera instead of a photoelectric converter is used as the 
detector in the voltage detector shown in FIG. 7, the change in the 
voltage occurring in a selected area of the object to be measured is 
detected as an one-dimensional distribution of light intensity on the 
phosphor screen of the streak camera. Therefore, in order to determine the 
waveform of the voltage, a predetermined conversion processing must be 
performed on the resulting one-dimensional distribution of light 
intensity. 
The method of detecting the voltage in a selected area of the object of 
interest in accordance with the change in the polarization of light has 
one more serious problem; that is, this method is only capable of 
detecting the absolute value of the voltage and is unable to determine its 
polarity (i.e., whether it is positive or negative). 
SUMMARY OF THE INVENTION 
An object, therefore, of the present invention is to provide a voltage 
detector that employs a simple optical system and yet is capable of direct 
and precise detection of a voltage waveform at a selected area of an 
object to be measured. 
Another object of the invention is to provide a voltage detector which can 
detect voltages in one-dimensional positions of an object with relatively 
simple processing. 
A further object of the invention is to provide a voltage detector which 
can detect a voltage in an object including its polarity. 
These objects of the present invention can be attained by a voltage 
detector which generally comprises a pulse light source for emitting pulse 
light, a delay means for gradually delaying the pulse light, an optical 
path changing means that is formed of an electro-optic material and allows 
the pulse light from the delay means to be outputted with its optical path 
inside the material corresponding to a refractive index of the material, 
and a detection means for detecting the output pulse light from the 
optical path changing means. This voltage detector measures by sampling 
the voltage developing in a selected area of the object of interest. 
In the present invention, pulse light issuing from the pulse light source 
is gradually delayed by the delay means and is thereafter launched into 
the optical path changing means, say, a dispersing prism, formed of an 
electro-optic material. The incident pulse light emerges from the optical 
path changing means either as transmitted light or as reflected light, 
which travels on a optical path in accordance with the refractive index of 
said optical path changing means. The refractive index of the optical path 
changing means varies with the electric filed produced by the voltage 
developing in a selected area of the object to be measured. Thus, the 
optical path of the transmitted light or reflected light emerging from 
said optical path changing means will also vary with the voltage in a 
selected area of the object of interest. By detecting the resulting change 
in the optical path of the emerging transmitted or reflected light with 
the detection means, say, a linear image sensor, part of the waveform of 
the voltage passing through a selected area of the object of interest can 
be measured by sampling. By repeating similar sampling measurements on the 
pulse light that is gradually delayed with the delay means, the waveform 
of the voltage to be measured can be detected on a serial time basis. The 
detected voltage waveform is such that one is able to determine the 
polarity of the voltage (whether it is positive or negative). 
Other and further objects, features and advantages of the invention will 
appear more fully from the following description taken in connection with 
the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Two embodiments of the present invention are described hereinafter with 
reference to the accompanying drawings. 
FIG. 1 shows schematically the composition of a voltage detector according 
to the first embodiment of the present invention. 
The voltage detector shown in FIG. 1 uses a dispersing prism 1 that is made 
of an electro-optic material, say, lithium tantalate (LiTaO.sub.3) in an 
optically uniaxial crystal form. The refractive index of the prism 1 
changes in accordance with the voltage in a selected area of the object to 
be measured (not shown) when the downward apex 2 of the prism 1 is brought 
close to said object. 
The dispersing prism 1 receives pulse light that emanated from a pulse 
light source 2 and passed through a collimator 4, a delay means 7 and a 
mirror 20. The incident pulse light travels through the prism 1 on an 
optical path that is determined by the refractive index of the prism 1 and 
thereafter emerges from the prism 1 as transmitted light. If, as shown in 
FIG. 2, pulse light is launched from a surrounding space (refractive 
index, n.sub.1) into the prism 1 (refractive index, n.sub.2) at an angle 
of .theta. with respect to the normal to the entrance face of the prism 1, 
transmitted light emerges from the prism 1 at an angle of .psi. that 
satisfies the following relationship: 
##EQU1## 
where .alpha. is the apex angle of the prism and n is the relative 
refractive index (n.sub.2 /n.sub.1). If the apex angle .alpha. is 
60.degree., 
##EQU2## 
If the refractive index of the dispersing prism 1 increases with voltage 
application, this causes an increase in the angle of emergence .psi.. 
The transmitted light from the prism 1 is reflected by a given angle from a 
mirror 5 and is launched into a linear image sensor 9 consisting of an 
one-dimensional array of imaging elements. 
In the voltage detector having the composition described above, pulse light 
from the pulse light source 2 passes through the collimator 4, delay means 
7 and mirror 20 and is launched into the dispersing prism 1. 
When no voltage is applied to the prism 1, the pulse light launched into 
the prism 1 travels through it on an optical path T1 and emerges therefrom 
as transmitted light. The exit beam is reflected from the mirror 5 and 
enters an imaging element 9-m in one-dimensional array in the linear image 
sensor 9. 
When a voltage, say, a positive voltage developing in a selected area of 
the object to be measured is applied to the prism 1, the refractive index 
of the prism 1 made of the electro-optic material is changed and the pulse 
light from the light source 2 travels through the prism 1 on an optical 
path T2. The transmitted light emerging from the prism 1 is reflected from 
the mirror 5 and enters an imaging element 9-n in the linear image sensor 
9. 
When a negative voltage is applied to the prism 1, the pulse light from the 
light source 2 travels through the prism 1 on an optical path T3. The 
transmitted light emerging from the prism 1 is reflected from the mirror 5 
and enters an imaging element 9-1 in the linear image sensor 9. 
In the embodiment under discussion, the voltage occurring in a selected 
area of the object to be measured consists of periodically repeating 
pulses and the pulse light from the light source 2 is in synchronism with 
the repeating period of these voltage pulses. Therefore, by gradually 
delaying a phase of the pulse light with the delay means 7, the pulses of 
voltage in a selected area of the object of interest can be measured 
through sampling at the linear image sensor 9. FIG. 3 shows timing charts 
that illustrate how the voltage pulses are measured by sampling. FIG. 3(a) 
shows the periodic occurrence of voltage pulses. In this case, pulse light 
is delayed in time by an increment of .DELTA.t as shown in FIG. 3(b). As a 
result, in response to the incidence of pulse light beams P1, P2 and P3, 
parts W1, W2 and W3 of voltage pulses are measured on a serial time basis 
with the linear image sensor 9. In accordance with the embodiment being 
discussed, the polarity of the parts of voltage pulses (W1 and W2 are 
positive and W3 is negative) can be determined in terms of the change in 
the optical path of pulse light P1, P2 or P3. 
As described above, in the first embodiment of the present invention, the 
refractive index of the dispersing prism 1 changes in accordance with the 
voltage developing in a selected area of the object to be measured and 
this change causes a corresponding change in the optical path of pulse 
light that is transmitted through the prism. By making use of this change 
in optical path, the waveform of the voltage varying at a selected area of 
the object of interest can be detected in such a way that the polarity of 
the detected voltage can also be determined. 
In the first embodiment of the present invention, a polarizer and an 
analyzer need not be used to extract a predetermined polarization 
component, that is, a linearly polarized light beam. Furthermore, this 
embodiment does not require a beam splitter and the resulting improvement 
in the efficiency of utilization of light is such that the linear image 
sensor 9 as a detector can be supplied with transmitted light that is 
substantially equal in intensity to the pulse light issuing from the pulse 
light source 2. As a further advantage, the elimination of a polarizer, an 
analyzer and a beam splitter results in a corresponding decrease in the 
number of components of the optical system. As a consequence of these 
advantages, voltage detection with improved precision and sensitivity can 
be accomplished in the first embodiment of the present invention. 
FIG. 4 shows schematically part of the composition of a voltage detector 
which is a modification of the voltage detector shown in FIG. 1. In the 
voltage detector shown in FIG. 4, pulse light from a pulse light source 2 
is collimated with a beam expander 25 and is passed through a delay means 
8 and a mirror 26 to be launched into a dispersing prism 27. Parallel 
beams, BM1, . . . BMk, . . . and BMl launched into the prism 27 travel 
through it on optical paths that depend on the change which occurs in the 
refractive index of the prism in accordance with the voltage at 
one-dimensional positions of the object of interest in a direction of an 
axial line F. The beams emerging from the prism 27 are launched into a 
two-dimensional detector 10 such as a CCD camera or a vidicon camera. 
The light beam BM1 is launched into either one of the imaging elements 
33-11, . . . , 33-1m, . . . and 33-1n in the two-dimensional detector 10 
in accordance with the change in the refractive index of the prism 27; the 
light beam BMk is launched into either one of the imaging elements 33-k1, 
. . . , 33-km, . . . and 33-kn; and the light beam BM1 is launched into 
either one of the imaging elements 33-l1, . . . , 33-lm, . . . and 33-ln. 
In the voltage detector having the composition described above, when the 
voltage developing in the object to be measured changes in the direction 
of the axial line F, the refractive index of an area of the prism 27 that 
corresponds to selected positions of the object also changes and this 
causes a corresponding change in the optical paths of light beams BM1, 
BMk, and BMl that are to emerge from the prism 27. The change in the 
optical paths of light beams BM1, . . . , BMk, . . . and BMl which are 
launched into the two-dimensional detector 10 reflects the results of 
sampling of part of the one-dimensional waveform of the voltage pulse 
developing in the object of interest. Therefore, by gradually delaying the 
light beams BM1, . . . , BMk, . . . and BMl with the delay means 8, the 
one-dimensional waveform of periodically varying voltage pulses in the 
object of interest can be sampled on a serial time basis with the 
two-dimensional detector 10. 
Thus, in accordance with the modification shown in FIG. 4, the voltages 
occurring at one-dimensional positions of the object of interest can be 
detected simultaneously. 
In the above modification, the positions of the beam expander 25 and the 
delay means 8 may be interchanged. 
FIG. 5 shows schematically the composition of a voltage detector according 
to the second embodiment of the present invention. 
The voltage detector shown in FIG. 5 is common to that shown in FIG. 1 in 
that it includes a pulse light source 2, a delay means 7 and a linear 
image sensor 9 as an one-dimensional detector. 
What is unique about the voltage detector shown in FIG. 5 is that the pulse 
light from the pulse light source 2 is passed through the delay means 7 
and mirror 20 to be launched into an electro-optic material 21 in an 
optically uniaxial crystal form that is furnished with a reflection mirror 
22 in the form of a thin metal film or a multilayered dielectric film. The 
pulse light launched into the electro-optic material 21 is reflected by 
the mirror 22 and emerges from the material 21 to be supplied to the 
linear image sensor 9. 
When no voltage is applied to the electro-optic material 21 in the voltage 
detector having the composition described above, the pulse light from the 
light source 2 travels through the electro-optic material 21 on an optical 
path S1 and the emerging reflected light is launched into an imaging 
element 9-m in the linear image sensor 9. When a positive (or negative) 
voltage is applied to the electro-optic material 21, the pulse light from 
the light source 2 travels through the electro-optic material 21 on an 
optical path S3 (or S2) and the emerging reflected light is launched into 
an imaging element 9-1 (9-n). In this way, the light beam traveling 
through the electro-optic material 21 changes its optical path in 
accordance with the voltage being applied to said material 21 and the 
reflected light from the material 21 is launched into an imaging element 
that corresponds to a particular optical path. 
As a result, the voltage detector shown in FIG. 5, like the one shown in 
FIG. 1, ensures that the waveform of voltage pulses varying at a selected 
area of the object of interest is measured by sampling on a serial time 
basis in such a way that the polarity of the detected voltage can also be 
determined. Furthermore, a polarizer, an analyzer and a beam splitter are 
eliminated to reduce the number of components of the optical system, 
thereby enhancing the efficiency of utilization of light to such an extent 
that very precise and sensitive voltage detection can be achieved. 
FIG. 6 shows schematically part of the composition of a voltage detector 
which is a modification of the voltage detector shown in FIG. 5. 
In the voltage detector shown in FIG. 6, pulse light from a pulse light 
source 2 is collimated with a beam expander 25 and is passed through a 
delay means 8 and a mirror 26 to be launched into an electro-optic 
material 28 in an optically uniaxial crystal form that is furnished with a 
reflection mirror 28 in the form a thin metal film or a dielectric 
multilayer film. Parallel beams BM1 to BMl launched into the electro-optic 
material 29 travel through it on optical paths that depend on the change 
which occurs in the refractive index of said material 29 in accordance 
with the voltage at one-dimensional positions of the object of interest in 
a direction of an axial line F. The beams emerging from the electro-optic 
material 29 on their respective optical paths are launched into a 
two-dimensional detector 12 such as a CCD camera or a vidicon camera. The 
light beam BM1 is launched into either one of the imaging elements 38-11 
to 38-1n in the detector 12 in accordance with the change in the 
refractive index of the electro-optic material 29; and the beam BM1 is 
launched into either one of the imaging elements 38-l1 to 38-ln. 
In accordance with the voltage detector having the composition described 
above, by gradually delaying parallel beams of pulse light with the delay 
means 8, the waveform of voltage pulses in a selected area of the object 
of interest can be measured by sampling in terms of the change in the 
optical paths of light beams BM1 to BMl, which enables them to be detected 
on a serial time basis by means of the two-dimensional detector 12. Thus, 
as in the case of the voltage detector shown in FIG. 4, the voltages 
occurring at one-dimensional positions of the object of interest can be 
detected simultaneously in accordance with the modification shown in FIG. 
6. 
In the above modification, the beam expander 25 and the delay means 8 may 
be replaced with each other. 
The description of the foregoing embodiments assumes that each of the 
dispersing prisms 1 and 27 and electro-optic materials 21 and 29 is made 
of an optically uniaxial crystal. However, none of these embodiments rely 
for their operation on the change in the polarization of light beam, so 
the prisms 1 and 27 and electro-optic materials 21 and 29 need not to be 
made of an optically uniaxial crystal and may instead be formed of an 
isotropic crystal. 
The description of the foregoing embodiments also assumes that the downward 
apex of the dispersing prism or the tip of the electro-optic material is 
not brought into contact with the object to be measured. If desired, they 
may be placed in contact with the object of interest. 
As described on the foregoing pages, the voltage detector of the present 
invention gradually delays the pulse light from the pulse light source and 
allows the incident light on the optical path changing means to emerge 
from it on the optical path that depends on its refractive index, with 
each of the emerging beams traveling on their respective optical paths 
being detected by sampling with the detection means. As a result, the 
waveform of voltage pulses that varies at a selected area of the object to 
be measured can be directly detected with the detection means in such a 
way that the polarity of the detected voltage can also be determined. In 
addition, as the number of components of the optical system is decreased 
and the light beam from the light source is effectively utilized for 
detection purposes without suffering any substantial loss in its 
intensity, the voltage detector of the present invention ensures precise 
and sensitive detection of the voltage developing in a selected area of 
the object to be measured.