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Refereed/Peer-reviewed Whispering gallery modes (WGMs) in micro-resonators are of interest due to their high Q-factors. Ultra-thin fiber tapers are widely deployed to couple light into micro-resonators but achieving stable and practical coupling for out-of-lab use remains challenging. Here, a new WGM coupling scheme using an exposed-core silica fiber (ECF) is proposed, which overcomes the challenge of using fragile fiber tapers. Microspheres are deposited onto the exposed channel for excitation via the evanescent field of the fiber's guided modes. The outer jacket of the ECF partially encapsulates the microspheres, protecting them from external physical disturbance. By varying the mode launching conditions in this few-mode ECF, in combination with a Fano resonance effect, we demonstrate a high degree of tunability in the reflection spectrum. Furthermore, we show multi-particle WGM excitation, which could be controlled to occur either simultaneously or separately through controlling the ECF mode launching conditions. This work can bring value towards applications such as optical switches and modulators, multiplexed/distributed biosensing, and multi-point lasing, integrated in a single optical fiber device that avoids fiber post-processing.

A method for determining the diameters and shell thicknesses of microbubble resonators is presented; it entails simulating whispering gallery mode (WGM) spectra using a newly developed finite-difference time-domain (FDTD)-based toolkit. Spectra for a range of shell thicknesses are simulated using FDTD, assuming a linear dependence of the free spectral range on the diameter, and the free spectral ranges and positions of the prominent modes are matched to those of the measured spectrum. This method improves upon existing techniques for extracting the diameter and thickness, such as SEM imaging, which typically require the microbubble to be dissected or otherwise rendered unusable for subsequent use. The model allows a variety of methods of mode excitation to be simulated. Dye coatings are simulated by placing a layer of dipole sources on the surface of the resonator, yielding mode couplings comparable to those measured in experiments. The model is tested for a smalldiameter silica glass microbubble, with the free spectral range being simulated for a range of diameters and shell thicknesses. The numerically simulated spectra are then compared to the experimentally measured spectrum. The ability to determine the geometric parameters of such resonators directly from their WGM spectra represents a step forward in the characterization of microbubble resonators. Furthermore, the model opens the way to previously unstudied spectral behavior of microbubbles with small diameters and thin shell thicknesses. Refereed/Peer-reviewed

We derive a unified model for calculating the power spectrum and resonance positions of multilayer microsphere resonators. The model may include a dipole in any layer, or a uniform distribution of dipoles resembling an active layer.

We explore the scope for engineering dispersion in whispering gallery mode silica microbubbles for nonlinear applications, such as optical frequency comb generation. In particular, the zero dispersion wavelength is shown to be highly tunable by changing the thickness of the shell. Using a small diameter and small wall thickness, dispersion equalization within the visible is predicted. This opens up the possibility of realizing visible frequency combs for a range of different applications. Refereed/Peer-reviewed

The opportunities for engineering dispersion in whispering gallery mode microbubbles are explored. Using certain materials it is shown that dispersion equalization can be realized at interesting wavelengths such as deep within the visible or mid-infrared. Refereed/Peer-reviewed

Whispering gallery modes (WGMs) within microsphere cavitiesenable highly sensitive label-free detection of changes in the surroundingrefractive index. This detection modality is of particular interest forbiosensing applications. However, the majority of biosensing work utilizingWGMs to date has been conducted with resonators made from either silicaor polystyrene, while other materials remain largely uninvestigated. Byconsidering characteristics such as the quality factor and sensitivity of theresonator, the optimal WGM sensor design can be identified for variousapplications. This work explores the choice of resonator refractive indexand size to provide design guidelines for undertaking refractive indexbiosensing using WGMs. Refereed/Peer-reviewed

The independent excitation and detection of spatial modes in few-mode fiber networks presents major challenges. This paper presents novel spatial mode-division-multiplexing techniques based on planar couplers and asymmetric Y-junctions. These developments may play a role in future high-capacity few-mode fiber telecommunications. usc Refereed/Peer-reviewed

The excitation and separation of individual modes in a few-mode optical fiber network can be realized using mode-selective couplers. For excitation at the beginning of the fiber, two-core mode-selective couplers can be used, while at the end of the fiber, either two- or three-core mode-selective couplers are required for demultiplexing of the field symmetric or field asymmetric modes, respectively. Both analytical and numerical solutions are presented to quantify the mode-selective functionality. Refereed/Peer-reviewed

The theory of mode-sorting in bimodal asymmetric Y-junctions is extended to multimode asymmetric Y-junctions with multiple output arms. This theory allows for the optimization of these mode-sorting planar structures. Asymmetric Y-junctions provide unique opportunities for spatial mode division multiplexing (MDM) of optical fiber. Spatial MDM is considered paramount to overcoming the bandwidth limitations of single-mode fiber. The design criteria presented in this paper facilitate their design. Refereed/Peer-reviewed