  Photonic sensors based upon high-quality optical microcavities have found a
wide variety of applications ranging from inertial sensing, electro- and
magnetometry to chemical and biological sensing. These sensors have a dynamic
range limited by the linewidth of the cavity mode transducing the input. This
dynamic range not only determines the range of the signal strength that can be
detected, but also affects the resilience of the sensor to large deteriorating
external perturbations and shocks in a practical environment. Unfortunately,
there is a general trade-off between the detection sensitivity and the dynamic
range, which undermines the performance of all microcavity-based sensors. Here
we propose an approach to extend the dynamic range significantly beyond the
cavity linewidth limit, in the nonlinear modulation regime, without degrading
the detection sensitivity for weak signals. With a cavity optomechanical
system, we experimentally demonstrate a dynamic range six times larger than the
cavity linewidth, far beyond the conventional linear region of operation for
such a sensor. The approach demonstrated here will help design
microcavity-based sensors to achieve high detection sensitivity and a large
dynamic range at the same time, a crucial property for their use in a practical
environment.
