This dissertation addresses two important aspects regarding the sensing of radio-frequency electromagnetic fields using integrated optical ring resonator devices. The first topic involves the theoretical design, fabrication and demonstration of a new field sensor based on electro-optically (EO) active integrated optical ring resonators. The second topic addresses the problem of enhancing the response from a single-mode ring resonator of a given ring waveguide loss through modifications in the device geometry.
The miniature integrated optical EO ring resonator sensor consists of low-dielectric constant polymers, is metal-free and is supported by a thin, flexible substrate. The low-invasive platform is achieved through the development of a new fabrication process. The waveguide cores of the devices are constructed of polycarbonate doped with the EO chromophore Disperse Red 1 and are poled using the contact poling method. The measured loaded quality factors of the poled EO rings are between 15,600 and 18,900.
The fields emanating from a microstrip resonator circuit at 3.9 GHz are measured. It is determined that the measured modulation from the four-ring linear array is largest when the optical wavelength is biased on the steep slopes of the resonance lineshapes as theoretically predicted. Using electric field values obtained from electromagnetic simulations of the microstrip circuit, the EO coefficient is 0.72 pm/V. The sensitivity for electric fields in free-space field is 142.2 V / (m Hz0.5). The sensitivity is obtained for an off-resonance optical power of -9 dBm at an optical wavelength near 1550 nm, a photoreceiver conversion gain of 900 V/W, and a system impedance of 50 ohm. Also, sensing from asymmetric lineshapes due to the bistable effect in the ring resonators is also demonstrated. This EO field sensing demonstration is the first reported using EO ring resonator sensors built on a metal-free flexible integrated optics platform.
The second part of this dissertation addresses the problem of enhancing the response from a single-mode ring resonator of a given ring waveguide loss. A two-mode bus waveguide coupled to a single-mode ring resonator device is investigated. The Fano-shaped output lineshapes are shown to depend on the coupling coefficients, the input mode power distribution, and the relative phase difference of the two input modes. Theoretical analysis and numerical parameter sweeps are used to determine optimal coupling coefficients to obtain maximum lineshape slope of the difference of the two mode power transmissions. The maximum lineshape slope that is theoretically obtainable from the device is 1.3 times that of an optimally coupled single-mode-coupled resonator with the same round-trip ring waveguide loss.
A device is fabricated in a polystyrene-silicon dioxide material system for demonstration purposes. Measurement results of the two-mode-coupled resonator device show that near-optimal coupling is achieved. The measured slope is found to be 1.28 times larger over an optimally coupled all-single-mode device. This work is the first experimental demonstration of enhanced lineshape slopes from the two-mode coupled ring resonator.