  Despite existing for millennia, tuberculosis (TB) remains a persistent global
health challenge. A significant obstacle in controlling TB spread is the need
for a rapid, portable, sensitive, and accurate diagnostic test. Currently,
sputum culture stands as a benchmark test for TB diagnosis. Although highly
reliable, it necessitates advanced laboratory facilities and involves
considerable testing time. In this context, we present a rapid, portable, and
cost-effective optical fiber sensor designed to measure lipoarabinomannan
(LAM), a TB biomarker found in patients' urine samples. Our sensing approach is
based on the applications of phase shift-cavity ringdown spectroscopy (PS-CRDS)
to an optical fiber cavity created by two fiber Bragg gratings. A tapered fiber
is spliced inside the optical cavity to serve as the sensing head. We
functionalize the tapered fiber surface with anti-LAM antigen CS-35 through a
unique chemistry, creating a strong affinity for LAM molecules. We measure the
phase difference between the cavity transmission and the reference modulating
signal at the cavity output. The measured phase is directly proportional to the
injected LAM concentrations in aqueous solutions over the sensing head. Our
demonstrated sensor provides a detection limit of 10 pg/mL and a sensitivity of
2.6$^\circ$/ng/mL. This sensor holds promise for numerous applications in the
healthcare sector, particularly in low-resource settings.
