Enhanced sensitivity laser measurements of electromagnetic fields in the RF range based on heterodyne electro-optic detection using a Fabry-Pérot cavity

This thesis presents the evolution of an electro-optic sampling setup from a simple one adapted for measurements of radiated electric fields in the kHz range into a complete high sensitivity setup adapted to the GHz range. During this thesis different systems were tested and compared, and finally an original setup is proposed to improve the frequency range of cavity based electro-optic probes found in the literature. It is a phase modulation (PM) heterodyning setup where an electro-optic crystal is placed in one of two arms of a Mach-Zender interferometer, hence it modulates the phase difference between the two beams of the interferometer. In the other arm an acousto-optic frequency shifter operating at 250 MHz is used to down convert the frequency of the detected signal in order to discriminate from parasites emitted at the external electric field frequency thus allowing the measurements to be made at a different frequency from that of parasites. In order to enhance the sensitivity of the heterodyning technique, the electro-optic crystal was implemented in a Fabry-Pérot cavity. Calculations show that the best sensitivity is obtained with an asymmetrical cavity in reflection mode. This justifies the cavity design that was selected. As compared to cavity based techniques already used in the literature, where the use of a cavity is based on a whole different concept, the PM heterodyning based on a Fabry-Pérot cavity provides a solution to the question of sensitivity enhancing, which in the literature is shown to be at the expense of the frequency bandwidth. The only penalty is the frequency of the external field that shall be a multiple of the oscillation rate of light inside the cavity. Measurements were done both in the kHz and in the GHz range. The smallest electric field that we were able to measure was of 3 mV/m/√Hz.
(PHYS 3) -- UCL, 2013