Resonant enhancement of plasmonic nanostructured fiber optic sensors

Improving the sensing capabilities of miniaturized probes nanofabricated on the tip of optical fibers is crucial for a large number of sensing applications in remote and harsh environ-ments. While most research efforts are directed towards developing improved fabrications techniques or more sensitive structures, we show that the performance of these sensors can be significantly enhanced by designing an appropriate interrogation strategy. In our scheme, a nanostructured plasmonic fiber probe is included in an optical fiber resonator and interrogated with a single-mode laser. A wavelength shift of the plasmonic resonance generates a mismatch of the resonator optical impedance, which is monitored and actively compensated by an optoelectronic gain element. We characterize the technique using samples with different refractive index and compare its performance to a conventional sensing scheme. Our cavity-enhanced configuration leads to a 50-fold improvement of the probe's refractive index resolution (corresponding to a detection limit of 10 RIU, the lowest reported so far for this type of sensors). The demonstrated measurement scheme can be devised with any fiber-optic sensor that responds with a resonance shift or with an attenuation of the transmitted/reflected intensity.