Linear cavity tapered fiber sensor using amplified phase-shift cavity ring-down spectroscopy

Phase-shift cavity ring-down spectroscopy (PS-CRDS) enables the measurement of minute fluctuations in optical loss encountered by an intensity modulated laser beam via lock-in demodulation of the accumulated signal phase as the laser beam propagates through the cavity. A linear fiber cavity containing two highly reflective fiber Bragg gratings (FBGs) and a tapered fiber is a favorable cavity geometry for sensor applications due to its compact nature and wide availability of its components at telecom wavelengths. However, due to the optical absorption of water, usage of telecom wavelengths for sensing of aqueous solutions degrades the sensitivity. This problem can be worked around via an optical amplifier placed inside the fiber cavity, compensating for intrinsic cavity losses. In this work, we demonstrate amplified PS-CRDS using such a linear cavity tapered fiber sensor utilizing an optical amplifier that enables the use of thinner tapered fibers, thus achieving higher sensitivities. We also employ continuous laser wavelength sweeps and analyze peak-to-peak PSs of individual cavity modes instead of using laser-cavity mode locking with the Pound–Drever–Hall technique. This enables further simplification of the experimental arrangement without compromising sensor performance. To test the performance of the reported sensor, solutions of varying sucrose concentrations in deionized water were measured systematically by tracking the average peak-to-peak PS ⟨ ϕ pp ⟩ of the cavity modes. Tapered fibers with waist diameters ( D t a p e r ) around 2.2 µm were used during the experiment and limit of detection (LOD) values were measured down to 2.7 µM, corresponding to 1.01 × 10 − 7 R I U . The reported LOD values can be further improved by mechanical stabilization and thermal control of the device by the use of FBGs with higher reflectivity, by applying automatic optical gain control, and by spectral filtering to remove errors caused by amplified spontaneous emission. The presented concentration sensing device can be suitable for developing compact and highly sensitive, label-free optofluidic sensors.