Whispering gallery mode biosensor: Dip sensing and power effect

The objective of my thesis research was to develop a method to fabricate a pre-assembled whispering gallery mode (WGM) sensor for dip sensing and vapor-phase sensing. The dip sensor would be a probe for a 96-well plate system. The vapor-phase sensor would integrate an array of pre-assembled sensors for detecting smells. Early experiments during my thesis research were directed towards the vapor sensing. The wavelength of the distributed feedback (DFB) laser (light source) was modulated in a triangular wave by changing the laser diode current. To study the resonance shifts due to vapors of different compounds, a known volume of a volatile liquid was added into a chamber housing the resonator while recording the resonance spectra continuously. When the volatile compound evaporated, the total pressure in the chamber increased, causing red shifts in the resonance peaks. The shifts obtained were expressed in terms of the laser current I. A calibration curve obtained by recording lambda for different I in a static condition is generally applied to convert these shifts to wavelength shifts. However, we realized that the static calibration could not be applied when the laser current was dynamically modulated. Therefore, we developed a dynamic lambda - I calibration method.;During this calibration study we noticed that the resonance spectrum was different, depending on the power of light fed to the resonator. When the laser power was attenuated, several narrow peaks were seen in the spectrum. However, when unattenuated light was fed into the resonator, some peaks broadened, especially at low frequencies of the wavelength modulation. We continued with the calibration process at low input power, circumventing the peak broadening. After the calibration study was completed, we explored the power dependence of the peak profile. We altered possible parameters such as the laser current scan range, the current ramp rate and the laser power. We introduced external disturbances by changing the resonator's temperature and the flow of gas surrounding the resonator while holding the laser wavelength unchanged. It was observed that the resonator held onto a resonance mode at high laser pump-in powers; at low powers, the resonance was more susceptible to these disturbances. A continued high-intensity signal in the photodetector was frequently seen when the pump-in power was high, implying an always-in-resonance state.;This always-in-resonance state helped the resonator to latch onto a specific mode despite environmental disturbances. A literature study indicated involvement of a thermal effect in such stabilization. The thermal effect suggests that, when the pump-in wavelength approaches the resonance wavelength from the blue side, the WGM heats up the resonator, causing the resonance to shift further to a longer wavelength. A thermal equilibrium between the heat generated in the resonator and the heat dissipated into its surroundings dictates the duration of stabilization. We exploited this knowledge and replaced the air surrounding the resonator with gases of different thermal conductivities which can alter the thermal equilibrium. It was hypothesized that a gas with a high thermal conductivity would minimize the thermal effect. A vacuum system was constructed for the experiment to house the resonator and surround it with different commonly. As expected, the thermal effect minimized when surrounded by highly conductive gas, helium.;Finally, we diverted our focus toward developing the pre-assembled WGM sensors for dip sensing. A typical WGM system requires positioning of the resonator relative to the feed and pickup fibers in each experiment, involving a bulky 3-D positioner to optimize the coupling. Operation of the positioner makes the sensor impractical for biologists. The design of the sensor head needs considerable changes to minimize human intervention. We have explored more than a dozen designs and have successfully developed a method to make preassembled sensor heads. Developing a dip sensor is much more difficult, since the thinned tapers and the resonator need to withstand the force experienced in the dipping and withdrawal from water. After making improvements to the preassembled design we succeeded in designing a user-friendly and mechanically robust dip sensor, which can be used in a 96-well plate measurement system. (Abstract shortened by UMI.).