Voltage detector

Abstract: A voltage detector detects a voltage developing in a selected area of an object such as an integrated circuit by utilizing an electro-optic material equipped in an optical probe. A change in refractive index of the electro-optic material which is caused by the voltage in the object, is detected as a change of polarization of a light beam passing through the electro-optic material. In order to simplify the optical system, the electro-optic material itself has a function to change the light-­ traveling path. In the first aspect, a surface of light-­ incidence side is worked into a lens shape. In the second aspect, the electro-optic material has a graded refractive index profile as a result of ion diffusion.

Description: The present invention relates to a voltage detector in accordance with the generic clauses of claim 1 and 7. 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 launched 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 need exists in the art for detecting such rapidly changing voltage with high precision without affecting the fine-line portions. However, this need has not been fully met by the prior art voltage detectors. With devices of the type that detects 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 employs an electron beam has 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 vacuum and its surface must be exposed at that. In addition, the area to be measured is prone to be damaged by electron beams. 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 EP-A- 293 788) 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. 5. 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 via a condenser lens 60; an optical fiber 92 for guiding reference light from the optical probe 52 into a photoelectric converter 55 via a collimator 90; an optical fiber 93 for guiding output light from the optical probe 52 into a photoelectric converter 58 via a collimator 91; and a comparator circuit 61 for comparing the electric signals form the photoelectric converters 55 and 58. The optical probe 52 is filled with an electro-optic material 62 such as an optically uniaxial crystal of lithium tantalate (LiTaO₃). 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 multilayered dielectric 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. 5 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 beam is polarized by the polarizer 54 and a predetermined polarized component having intensity I is launched into the electro-optic material 62 in the optical probe 52 via 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 an ordinary ray and an 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)sin²[(π/2)V/V₀] 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 V₀ 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)sin²[(π/2)V/V₀] . The intensity of output light, or (I/4)sin²[(π/2)V/V₀] , 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. 5, 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 the same 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 small 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. In the voltage detector 50 shown in Fig. 5, the light beam BM issuing from the light source 53 is converged by a condenser lens (not shown in Fig. 5 but shown in Fig. 6 by 70) and launched into the electro-optic material 62. However, the addition of the condenser lens 70 increases the number of components of the optical system and the adjustment of the latter becomes too complicated to ensure high reliability. Document US-A-4 618 819 discloses an apparatus for measuring an electrical signal, eg. in a micro-circuit, whereby a beam of optical pulses sychronous with said signals to be measured is generated, polarized and converged with the help of a lens. The light beam passes through an electro-optic crystal, adjacent to the sample and is reflected from high reflection coating at the bottom of the crystal. The change of polarization of the light beam, in accordance with the Pockel's effect, is measured with two photo-detectors that measure the intensities of the transmitted and rejected beams at an analyser. The change of polarization correlates with the electrical signal to be measured. Document DE-A-2 010 932 discloses an apparatus for measuring an electric current in a high voltage line, whereby a monochromatic light beam is generated, polarized, reflected on a mirror, and passes a first Faraday glass, an objective, a second Faraday glass with a focusing shape, and has a high reflection metal layer on its rear side to reflect the light beam. The reflected light beam passes again the objective and the first Faraday glass, the mirror, an analyser and a photo-detector. One of the Faraday glasses is positioned in the magnetic field generated by the current of the high voltage line. The second Faraday glass is positioned in a second magnetic field to compensate for the change of polarization of the light beam, due to the first magnetic field. The power of the second compensating magnetic field is proportional to the current to be measured. An object, therefore, of the present invention is to provide a voltage detector capable of reliable detection of the voltage developing in a selected area of an object of interest with a simpler optical system. This object is solved by the features of the characterizing clauses of claims 1 and 5. 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. Fig. 1(a) is a diagram showing schematically the composition of a voltage detector according to the first embodiment of the invention; Fig. 1(b) is a diagram showing a refractive index profile of an electro-optic material used in the voltage detector shown in Fig. 1(a); Fig. 2 is a diagram showing the composition of a voltage detector according to the second embodiment of the invention; Figs. 3(a), 3(b) and 3(c) are diagrams showing the refractive index profiles of the electro-optic material used in the voltage detector shown in Fig. 2 with the respective profiles being taken on x-, y- and z-axes; Fig. 4 is a diagram showing the composition of a voltage detector having been proposed in Japanese Patent Application No. 137317/1987; and Fig. 5 is a diagram showing schematically a part of a voltage detector using a condenser lens. Various embodiments of the present invention are described hereinafter with reference to the accompanying drawings. Fig. 1(a) is a diagram showing schematically the composition of a voltage detector according to the first embodiment of the present invention. In the voltage detector shown in Fig. 1(a), ions, e.g. alkali metal ions, are diffused in an electro-optic material 5 in an optical probe 4 to make the material 5 itself a lens with graded refractive index profile. The refractive index profile of the electro-optic material 5 as a function of a position in the transverse direction is graded as shown in Fig. 1(b). Reference numerals 15 and 16 are an optical fiber and a condenser lens, respectively. In the voltage detector having the composition described above, a light beam BM issuing from a light source travels through an optical fiber 15 and is launched into the electro-optic material 5 in the optical probe 4. The launched light beam BM is refracted by the electro-optic material 5 serving as a lens with graded refractive index profile and travels until it reaches the reflection mirror 65. In the absence of any voltage applied to the electro-optic material 5, the launched light beam BM falls on the reflection mirror 65 vertically and travels back on the same path after reflection from this mirror, for example along a path I₀. The light beam BM then emerges from the electro-optic material 5. Therefore, the detected output light has substantially the same intensity as the input light. When a voltage developing in a selected area of an object is applied to the electro-optic material 5 in Fig. 1(a), the refractive index profile of the electro-optic material 5 is changed. In the case of the electro-optic material 5 with the graded refractive index profile, a new phenomenon of a change in a light-traveling path on account of the voltage application is obtained, as well as the change in polarization. The first embodiment of the invention utilizes this change in the light-traveling path. That is, when the voltage developing in the object to be detected is applied, the refractive index profile of the electro-optic material 5 is changed, so that the launched light beam travels through the material 5 for example along an optical path I indicated by a dashed line in Fig. 1(a) and then reaches the reflection mirror 65. In this case, the light beam BM does not encounter the reflection mirror 65 perpendicularly to its surface, so the reflected light travels on a different path I' and emerges from the electro-optic material 65. As a result, the detected output light (signal light) has a smaller intensity than the input light and the voltage developing in a selected area of the object of interest can be detected by examining the intensity of the output light. In the first embodiment described above, the voltage developing in a selected area of the object to be measured is not detected by using the change in the state of polarization of the light beam BM but by directly measuring the change in the intensity, or quantity, of the light beam BM. Therefore, in addition to the elimination of the condenser lens 70 shown in Fig. 5 the polarizer 54 and the analyzer 57 shown in Fig. 4 can also be eliminated and an optical system even simpler than that employed in the first embodiment can be realized. Fig. 2 is a diagram showing the composition of a voltage detector according to the second embodiment of the present invention. In the voltage detector shown in Fig. 2, a light beam BM passes through a beam splitter 6 (or half-mirror) before it is launched into an electro-optic material 7 serving as a lens with graded refractive index profile. The electro-optic material 7 has a unidirectional (x-axis) refractive index profile as is apparent from the graphs shown in Figs. 3(a) to 3(c). Therefore, the light beam BM launched into the electro-optic material 7 is converged on the reflection mirror 65 along a straight line indicated by A-A. The linearly converged light beam BM is reflected from the mirror 65 and emerges from the electro-optic material 7 and thence passes through the beam splitter 6, an analyzer 8 and a cylindrical lens 9 to be converged on the photocathode 11 of a streak camera 10 along a straight line indicated by B-B. In other words, the light beam converged in the electro-optic material 7 along the line indicated by A-A is projected on to the photocathode 11 of the streak camera 10 along the line indicated by B-B. Since the line B-B in the photocathode 11, along which the light beam is projected on, is perpendicular to the sweep direction of the streak camera 10, the voltage change in the one-dimensional area of the object of interest lying beneath the reflection mirror 65 can be measured in an extremely short period with the streak camera 10. When a streak camera is used as the detector in the foregoing embodiments, it is sometimes the case that the reference light is not used. In the embodiments described above, the light beam BM is converged on the reflection mirror 65 but part of the light beam BM might fall on the surface of the optical probe 1 or 4 to be scattered by reflection. Such scattering light will lower the precision of voltage detection and should desirably be eliminated. To this end, a coat of a suitable anti-reflection material such as a black paint may be formed on an inner surface of a light-splitting and extracting part of the optical probe 1 or 4 and an outer surface of the electro-optic material so as to absorb the incident light beam BM and hence to prevent the occurrence of light scattering. In all of the embodiments described above, the tip of the electro-optic material is brought close to the object to be measured but not as close as to make contact with this object. If desired, the tip of the electro-optic material may be held in contact with the object of interest during voltage detection.

Claims: A voltage detector (50) for detecting a voltage developing in a selected area of an object to be measured comprising a light source (53) for emitting a light beam, a voltage-sensing part including reflection means (65) for reflecting an input light beam with a first predetermined polarized component and an electro-optic material (7) for sensing said voltage in said object in the form of a change of refractive index, beam splitting and beam extracting means (56), and detection means (55,58,61) for determining that voltage developing in said selected area of said object on the basis of intensity of said output light beam with a second predetermined polarized component, characterized in that the electro-optic material (7) has a graded refractive index profile in which said refractive index profile is substantially symmetric to a plane including a central axis of said electro-optic material (7) and said refractive index steadily decreases from said symmetric plane, and said input light beam is linearly converged on a reflection surface (65) of said reflection means. A voltage detector (50) as claimed in claim 1, wherein said splitting and extracting means (56) further splits said light beam with a first predetermined polarized component into a reference light beam and an input light beam which is launched into said electro-optic material (7); and said detection means (55,58,61) determines said voltage on the basis of intensity of said reference light beam and intensity of said output light beam with a second predetermined polarized component. A voltage detector, as claimed in claim 1, wherein said detection means (55,58,61) comprises a streak camera (10) for detecting said output light beam with said second predetermined polarized component, and lens means (9) for linearly converging said output light beam with said second predetermined polarized component on a photocathode (11) of said streak camera (10). A voltage detector as claimed in claim 1, wherein said graded refractive index profile is formed as a result of ion diffusion. A voltage detector (50) for detecting a voltage developing in a selected area of an object to be measured comprising a light source (53) for emitting a light beam, a voltage-sensing part including reflection means (65) for reflecting an input light beam, and an electro-optic material (5) for sensing said voltage in said object in the form of a change of refractive index, beam splitting means (56), and detection means (55,58,61) for determining that voltage developing in said selected area of said object on the basis of intensity of said output light beam characterized in that the electro-optic material (5) provides different input and output light beam paths, whereby a refractive index profile of said electro-optic material (5) is substantially axially symmetric to a central axis of said electro-optic material and said refractive index steadily decreases from said symmetric axis. A voltage detector as claimed in claim 5, wherein said splitting means (56) further splits said light beam introduced from said light source (53) into a reference light beam and said input light beam which is launched into said electro-optic material; and said detection means (55,58,61) determines said voltage on the basis of intensity of said reference light beam and intensity of said signal light beam. A voltage detector as claimed in claim 5, wherein said refractive index profile is formed as a result of ion diffusion.

Inventors: AOSHIMA SHINICHIRO, TAKAHASHI HIRONORI, TSUCHIYA YUTAKA, AOSHIMA, SHINICHIRO, TAKAHASHI, HIRONORI, TSUCHIYA, YUTAKA
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1988
Main classification: G01R1
IPC codes: G01R19, G01R15, H01L21, G01R31, G01R1