Compact coupled optical resonators and their applications [electronic resource]
Keywords: Applied Physics.

Optical resonators can slow down light by utilizing the interference between light waves. This makes them a technology of great significance. Thanks to the emergence of silicon photonics, on-chip optical resonator devices have seen an unprecedented reduction in footprint and a high level of integration. Coupled resonator optical waveguides (CROW), which consist of a chain of optically coupled cavities, are one of the widely studied and most promising structures. They exhibit unique abilities to support broadband slow light and dynamic tunability, which have important applications in many branches of photonics. These features, however, generally require coupling a large number (N) of resonators, which compromises compactness and thermal stability. In this work, we introduce two new classes of optical interferometers that exhibit properties similar to CROWs, while occupying a much smaller area (1/N) and being consequently more stable against temperature gradient. First, we propose the coupled spiral interferometer (CSPIN), which consists of a planar waveguide coiled into a spiral. The distance between the spiral's adjacent arms is small enough that light is continuously coupled between them. This distributed coupling gives CSPINs a distinct set of properties, some similar to those of ring resonators and CROWs, and others unique. Their actual behavior depends on the number of arms and the intra-arm coupling distribution. Besides the main advantage of greater compactness and stability, simulations show that as a result of the waveguides' inherent mode-index dispersion, any intra-arm coupling coefficient acts as critical coupling (maximum energy storage) at selected wavelengths. Similarly, a CSPIN can be designed as a sensor with a sensitivity that is maximum for any value of the intra-arm coupling, provided the wavelength of the light interrogating the sensor is suitably selected. Second, we study the nested coupled resonant optical waveguides (nested CROW). It consists of a set of concentric rings with decreasing radii. The rings are located inside each other with very small spacing, so light can continuously couple between them. A nested CROW behaves like a ring resonator when the intra-ring coupling is weak, and like a CSPIN and a CROW when the coupling is strong. A unique aspect of the nested CROW is that its resonant wavelengths depend on the intra-ring coupling. When the propagation loss is relatively small, any value of strong coupling is the critical coupling for a set of resonant wavelengths. Increasing the number of rings creates more resonance peaks. By selecting appropriate intra-ring couplings, a nested CROW can also exhibit broadband transmission window that has flat top and sharp roll-off. In this thesis work we compared the sensitivity of a CSPIN and a nested CROW to existing resonators, including a CROW and a single-ring resonator with the same radius and loss coefficient. After optimizing each device individually for maximum sensitivity (by selecting the optimum intra-ring or intra-arm coupling and optimum interrogating wavelength), we show that coupling resonators together (as in a CROW, a CSPIN or a nested CROW) does not increase the device's sensitivity. These resonators all exhibit the same optimal sensitivity as a single-ring resonator with the same radius and loss. When applied to the particular case of rotation sensing using the Sagnac effect, we confirmed through numerical simulations that the sensitivity to rotation is independent of the number of arms in the CSPIN, and exactly the same as the sensitivity to rotation of a ring resonator. The last part of this thesis describes the fabrication and characterization of the first experimental CSPINs. The devices are made with silicon waveguides with sub-micron transverse dimensions on a silica substrate, with a spiral radius of 150 µm and either two or three arms. They were found to exhibit the resonance spectra predicted by theory. Measurements confirm that the sensitivity of a CSPIN with modal index dispersion is independent of the intra-arm coupling. This property gives the CSPIN a significant edge over other resonator-based sensors, which require careful adjustment of the coupling ratio, a difficult task with low reproducibility in practice.