Whispering Gallery Microcavity Nanosensor with In-Line Interferometer

The application of whispering gallery mode (WGM) microcavities for Nano detection and biosensing is a field of research seeing ongoing attention [1-15]. This method exploits the high quality-factor (Q) of the optical cavity for ultra-sensitive monitoring of shifts in the resonant wavelength. These resonant wavelength shifts are due to modifications in the optical properties of the resonator that can be due to the binding of individual nanoparticles or molecules. The typical procedure for monitoring the resonant wavelength of a microcavity includes scanning a tuneable wavelength laser while photo detecting the optical transmission which is monitored on an oscilloscope [3]. One issue with this method is the inherent uncertainty in the location of the transmission dip due to fluctuations in the laser wavelength, known as laser jitter. To circumvent this issue the use of an interferometer to produce a reference signal that follows the laser jitter has been used with an observed increase in sensitivity [10]. The demonstrated scheme, however, requires a split-off of the probe laser to the interferometer. This limits its application to power sensitive measurement where the maximum injection of probe laser power into the cavity is required. Here, we report that by simply inserting a FC/PC connectorized fibre as a Fabry P ́erot (FP) interferometer (FPI) after the output of a whispering gallery cavity, one may reach similar detection sensitivity as reported in ref. [10] with a much simpler setup and without the necessity of split-off power.