Whispering Gallery Mode Microresonators: Applications from Optical Sensing to Communications
Date of Award
Doctor of Philosophy (PhD)
High quality factor, small footprint and ease of fabrication of whispering gallery mode micro-resonators make them ideal platforms for applied and fundamental studies ranging from cavity QED to sensing and photonic circuits. In this thesis, whispering gallery mode (WGM) microcavities are explored as building blocks of photonic circuits and as sensitive ultrasound transducers. We have carried out a detailed theoretical and experimental analysis to use Whispering Gallery Mode Resonators (WGMRs) for implementing add-drop filters (ADF) that are essential elements of optical communication networks. ADFs are required
to multiplex dierent communication channels over a single ber and route optical signals of dierent wavelengths to dierent users. We showed explicit relation between dierent loss mechanisms in the ber-taper coupledWGMRs and the various gures of merit of ADFs, such
as drop eciency, bandwidth and crosstalk. Using a passive silica micro toroidal WGMR, we built an ADF with thermal tuning capabilities and with enhanced robustness to mechanical instabilities commonly seen in ber-taper coupledWGMRs. The key to this was to introduce micro-scale walls adjacent to the WGMRs to place ber-taper couplers for improved stability
without sacricing the coupling eciencies. In a second work along this direction, we built ADFs using ber-coupled rare-earth-ion (Erbium and Ytterbium) doped silica microspheres in which gain provided by the ions used to compensate a portion of the losses. As a result, we improved drop eciency by 4-fold and decreased crosstalk by 10-fold over an ADF without gain medium. We showed that by tuning the gain, we can nely control the characteristics of the ADF.
Enhanced light intensity in WGMRs lead to photon radiation pressure induced mechanical oscillations, known as opto-mechanics. In strongly driven opto-mechanical systems, optical signal experiences chaos which is in general not desirable in communication systems.
However, today it is well-known that chaotic signal can be used for secure communication provided that the receiver and the transmitter are synchronized apriori. In opto-mechanical system, chaotic signal can be generated if the system is driven by a high intensity light eld. For very weak elds, neither periodic oscillations nor chaotic ones are possible. For secure communication, it may be interesting to route a weak optical signal to chaos by the help of a strong eld residing in the same opto-mechanical system. Therefore, we investigated chaos generation and synchronization in an opto-mechanical system based on WGMRs. We
demonstrated that a weak probe signal follows a similar route to chaos as a strong pump signal which induces chaos, clearly demonstrating chaos synchronization.
We also investigated the capability of WGMRs for ultrasound sensing and demonstrated that very high sensitivity and low detection limit can be achieved using a micro-toroid resonator encapsulated in allow refractive index polymer. We also show that one can lower the detection limit and increase the sensitivity by designing the micro-toroid such that it has an
intrinsic mechanical resonance near the frequency of the incoming ultrasound signal. Using optical resonators for ultra sound sending promises an all-optical photo-acoustic imaging system.
Viktor Gruev, Sahin Ozdemir, Srikanth Singamanen