Resonance-based Electro-Optic Sensing: Innovative Techniques for Sensitivity and Bandwidth Enhancement in Minimally Invasive Vector Microwave Field Imaging.

Abstract

The electro-optic sensing/sampling (EOS) technique has gained great attention over the last decade as a valuable diagnostic tool for high-speed circuits and microwave sensing/imaging due to unique features such as inherently low invasiveness, ultra-broad bandwidth, and high spatial and temporal resolution for the detection of vector microwave fields. Despite these beneficial aspects of EOS, there still remain challenges to be overcome in the existing methodology. The primary one is low sensitivity, originating from the inherently tiny figure of merit in EO materials. Furthermore, the conventional reflective mode of the EOS configuration also demands the arguably cumbersome polarization management and optical alignment associated with an expensive and delicate mode-locked pulsed laser. This dissertation presents novel methods to address the various challenges of the unique features in the EOS systems. The sensitivity is enhanced by > 10 dB with Fabry-Perot resonance-based techniques, and this is accomplished with simplification of the optical implementation compared to the conventional methodology. For high frequency sensing of vector microwave fields, a novel electrical down-conversion technique is presented and compared with the recently introduced and thus more conventional photonic technique. Both down-mixing techniques are optimized through the application of a new carrier suppression method, enhancing the signal-to-noise ratio up to 10 dB, and the merits and shortcomings of the two techniques are discussed. The results suggest employing the photonic down-conversion scheme for higher frequency sensing (i.e, the broader bandwidth). Another technique to expand the bandwidth, utilizing higher-order harmonics of the photonic local oscillator, is also presented. Up to three times the sensing bandwidth, using the second- or third-order harmonics can be readily achieved. Such expansion of the bandwidth is enough to cover even up to the millimeter-wave regime with additional carrier suppression to increase the signal-modulation depth. Finally, the least invasive fiber-based probe that has been envisioned to date, with greater accessibility to devices tested with millimeter-wave sensing, has been developed through miniaturization of the probe size. This dissertation demonstrates a number of innovative techniques that realize the most simple and efficient, minimally invasive, high frequency EO measurement system developed to date.