Do you need a tunable laser for resonant cavity optical sensors?

Abstract

By measuring the shift in the resonant frequency of whispering gallery modes in an optical microcavity, it is possible to obtain very high sensitivity to changes in the properties of the surrounding medium and the sensor surface. However, a narrow linewidth tunable laser is typically required in order to track the frequency shift. This significantly increases the cost and complexity of such systems. Phase shift cavity ring down spectroscopy (PS- CRDS) represents an alternative approach. In PS-CRDS the interrogating optical signal is sinusoidally modulated and the shift in the phase of the detected signal (rather than the shift in the cavity resonant wavelength) provides information about changes in the cavity properties. PS-CRDS has previously been successfully implemented in resonant optical microcavities, but a tunable laser was still used in order to maintain coupling to the cavity resonance. Here we consider the use of a broadband optical source (e.g. a diode laser or LED) to interrogate the cavity using the PS-CRDS principle. The spectrum of the source always spans more than one cavity resonance and so does not need to be tuned as the cavity resonances shift. We undertake an analytical and experimental investigation to evaluate the effectiveness of this approach and compare it to traditional interrogation methods in terms of sensitivity and signal-to-noise ratio. We focus in particular on the implementation of a resonant cavity biosensor in silicon photonics ring resonators. The results of the study show that the sensitivity to changes in cavity mode loss is slightly lower than when a narrow linewidth source is used, and that sensitivity to changes in the effective refractive index is very significantly reduced. We will discuss the implications of these results in terms of suitable applications of this technique, and the improved potential for integration that the low coherence source brings.