1.  Field of the Invention
The present invention relates to a voltage sensor that is used in an optical PT (Power Transformer) that detects a voltage to ground or a voltage of a driving power source for motors and the like. 
2.  Description of the Related Art
An intensity-modulated voltage sensor is arranged to include, for example, as shown in FIG.  10, along the optical axis, successively from the side of incident light, a first polarizer 35 of set angle 90 degrees, a 1/4 wavelength plate 38, a Pockels cell 2, and a second polarizer 36 of set angle 95 degrees, and a voltage to be measured is applied to Pockels cell 2.  Since a change in the output intensity of second polarizer corresponds to the polarization state of the output light of Pockels cell 2, which varies depending on the voltage to be measured, the voltage to be measured can be determined by monitoring the change in the output power of the polarizer with an optical receiver.  Here, the modulation depth of an output power is defined as the ratio of the AC component of the output power to the DC component of the output power. 
However, a change in the loss of connection in each optical part and a change in the intensity of the incident light due to the external environment generate a gradual change in the output power apart from the above modulation of the output power due to the voltage to be measured.  In particular, if the frequency of the voltage to be measured is near DC, it becomes impossible to distinguish the AC component of the voltage to be measured from the AC component due to the above secondary causes, so that an exact measurement can not be achieved.