1. Field of the Invention
This invention relates to a voltage detecting device for detecting a voltage provided at a predetermined part of an object under measurement, and more particularly to a voltage detecting device utilizing the principle that the polarization of light passing through electro-optical material is changed by a voltage provided to that material
2. Background Information
A voltage detecting device for detecting, in a time resolution of sub-pico seconds, the voltage of an ultra-high-speed photo detector, semiconductor switch or high-speed electronic device is well known in the art.
In FIG. 6(a) is shown a perspective view of a voltage detecting device of the above-described type, which was disclosed by Janis A. Valdmanis et al, "IEEE Journal of Quantum Electronics," Vol. QE-19, No. 4, pp. 664-667 (published in Apr. 1983). Shown in FIG. 6(b) and FIG. 6(c) are a top view and a front view of the voltage detecting device shown in FIG. 6(a).
In the voltage detecting device as shown in FIG. 6(a), an electro-optical material 50 of lithium tantalate (LiTaO.sub.3) is cut perpendicular to the C-axis, an aluminum strip line 52 is provided on a surface 51 of the electro-optical material 50 which is perpendicular to the C-axis, and a predetermined part of an object 53 under test is connected to the strip line 52.
When the object 53 under test is an ultra-high-speed photo detector, for example, with a predetermined part of the photo detector connected to the strip line 52, a voltage pulse VP having a pulse width of the order of ten pico seconds outputted from photodetector 53 moves along the strip line 52 at a speed V.sub.O as shown in FIG. 6(c). As a result, an electric field E is applied to that part of the electro-optical material 50 which is just below the strip line 52 along which the voltage pulse VP moves. The refractive index of that part of material 50 is changed as a result of the electric field. Accordingly, referring to FIG. 6(b), when a linearly polarized light beam PB is applied to one side 54 of the electro-optical material 50 in such a manner that it forms an angle .theta. with the longitudinal axis A--A of the strip line 52, the speed component, V cos .theta., of the light beam PB taken along the axis A--A is equal to the speed V.sub.O of the voltage pulse VP. Thus, the light beam PB passes through the electro-optical material 50 following the refractive index change caused by the voltage pulse VP. The polarization of the light beam will be changed by the well known Pockels effect as it passes through material 50. The light beam will be outputted, as a transmitted light beam, from the opposite side 55 of the electro-optical material 50. If the change in polarization of the transmitted light beam can be detected, then the value of the voltage pulse VP moving along the strip 52 can be achieved without affecting the object under test.
In the voltage detecting device shown in FIG. 6(a), the light beam PB is applied to the electro-optical material 50 in such a manner that its speed component along the longitudinal axis of the strip line 52 is equal to the speed V.sub.O of the voltage pulse VP; that is, the light beam PB and the voltage pulse are allowed to interact with each other, so that the voltage is detected from the change in polarization of the light beam PB. In the voltage detecting device, as described above, the light beam PB is allowed to enter the electro-optical material 50 through one side 54 and to emerge from the opposite side 55. Therefore the optical beam PB and the voltage pulse interact with each other for the period of time T, which represents the time the light beam PB spends crossing the strip line 52. This relationship is given by the following formula: EQU T=W/(V sin .theta.) (1)
where W is the width of the strip line 52, and V is the speed of the light beam PB in the electro-optical material 50.
If the voltage pulse VP is of the order of several kilovolts (kV), the polarization of light beam PB will change to the extent that it can be detected, even if the period of time T is short. However, if the voltage pulse VP is lower than several kilo-volts, then it is necessary to increase the period of time T in order to detect the polarization of the light beam PB. Since the period of time T depends on the width W of the strip line 52, as is apparent from the above equation (1), the ability to design the voltage detecting device shown in FIGS. 6(a) and (b) to increase the period of time T, and to change the polarization of the light beam PB as required, is limited to the practical width W of strip line 52.
Accordingly, an object of this invention is to provide a voltage detecting device which can change the polarization of a light beam in an electro-optical material to the extent that it can be detected, to detect the voltage value of the voltage pulse with high accuracy.