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

Liquid phase sensing applications at 1550~nm are highly desirable due to widely available off-the-shelf components. Generally, liquids at 1550~nm induce a high absorption loss that limits the overall sensor's sensitivity and detection limit. One solution is to use an active fiber loop in conjunction with cavity ring down spectroscopy to overcome these absorption losses. However, the amplifier inside the fiber loop suffers from inherent gain fluctuations that limit the sensing system's overall performance. Here, we provide a novel sensor using the wavelength-scanned phase shift-cavity ring down spectroscopy (PS-CRDS) in conjunction with a linear active fiber cavity that potentially offers a more sensitive solution than traditional fiber loop sensors. We use a tapered fiber as a sensing head inside the active cavity built from fiber Bragg gratings. We derive a theoretical phase shift expression for our system and simulate it using the finite element method to determine optimum tapered fiber diameter for glucose sensing in DI water. Compared to a non-amplified system, we find that our amplified system can increase the sensitivity by fourteen times via the amplifier gain tuning. We also conduct experimental measurements using 0-15.5~mM glucose solutions and find them in excellent agreement with our theoretical predictions. Experimentally we obtain the sensor's sensitivity of 0.768~$^o$/mM (1164~$^o$/RIU) and detection limit of 0.75~mM ( 4.5~$\times$~10$^{-4}$~RIU) without any temperature stabilization in the system. We anticipate that the present work will find a wide range of sensing applications in fiber cavities, ring resonators, and other microcavity structures.