Patent ID: 7859666

Claim:
An electric field sensor comprising: a light source; an electro optic crystal which is applied with an electric field based on a signal under test, in which a birefringent index changes according to the electric field, and which changes a polarization state of light incident from said light source according to the birefringent index and emits the light; a detector that detects an electric signal according to the change of the polarization state of the light emitted from said electro optic crystal; a first electrode that is provided close to said electro optic crystal, and that applies the electric field based on the signal under test to said electro optic crystal; a second electrode that is provided close to said electro optic crystal, thereby forming a pair with said first electrode; and an auxiliary electrode that is electrically connected to said second electrode, wherein, a first capacitance between said auxiliary electrode and a ground is larger than a second capacitance between said first electrode and said second electrode, the first capacitance and the second capacitance being arranged in series.