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Liquid phase sensing applications at 1550~nm are highly desirable due towidely available off-the-shelf components. Generally, liquids at 1550~nm inducea high absorption loss that limits the overall sensor's sensitivity anddetection limit. One solution is to use an active fiber loop in conjunctionwith cavity ring down spectroscopy to overcome these absorption losses.However, the amplifier inside the fiber loop suffers from inherent gainfluctuations that limit the sensing system's overall performance. Here, weprovide a novel sensor using the wavelength-scanned phase shift-cavity ringdown spectroscopy (PS-CRDS) in conjunction with a linear active fiber cavitythat potentially offers a more sensitive solution than traditional fiber loopsensors. We use a tapered fiber as a sensing head inside the active cavitybuilt from fiber Bragg gratings. We derive a theoretical phase shift expressionfor our system and simulate it using the finite element method to determineoptimum tapered fiber diameter for glucose sensing in DI water. Compared to anon-amplified system, we find that our amplified system can increase thesensitivity by fourteen times via the amplifier gain tuning. We also conductexperimental measurements using 0-15.5~mM glucose solutions and find them inexcellent agreement with our theoretical predictions. Experimentally we obtainthe sensor's sensitivity of 0.768~$^o$/mM (1164~$^o$/RIU) and detection limitof 0.75~mM ( 4.5~$\times$~10$^{-4}$~RIU) without any temperature stabilizationin the system. We anticipate that the present work will find a wide range ofsensing applications in fiber cavities, ring resonators, and other microcavitystructures.

Phase shift-cavity ring down spectroscopy in linear and active fiber cavities for sensing applications at 1550 nm