Synchronized Fabry-Perot cavity used for enhanced sensitivity electro-optic measurements of electric fields in the microwave range.

An electro-optic (EO) sensing system for measuring free-space electric fields in the microwave range has been developed. The system is based on a phase modulation heterodyning technique using a Mach-Zehnder interferometer. In one of the arms of the interferometer, an acousto-optic frequency shifter is used to downconvert the frequency of the detected signal in order to discriminate it from parasites emitted at the external electric field frequency. The sensing part is a LiTaO crystal placed in a Fabry-Perot cavity. The cavity aims at enhancing the sensitivity of the measurements. Cavity-based EO setups already used in the literature propose this sensitivity enhancement at the expense of the frequency bandwidth, whereas our setup allows this without a major impact on the frequency bandwidth. Electric fields are measured at both 15 kHz and 2.4 GHz with cavities of two different finesses; the best EO phase retardation gains obtained with the cavity are 34 and 60, respectively. The minimum detectable electric field is 0.003  V⋅m⋅Hz.