Comparison of detection limit in fiber-based conventional, amplified, and gain-clamped cavity ring-down techniques

Abstract Relative performance and detection limit of conventional, amplified, and gain-clamped cavity ring-down techniques (CRDT) in all-fiber configurations are compared experimentally for the first time. Refractive index measurement using evanescent field in tapered fibers is used as a benchmark for the comparison. The systematic optimization of a nested-loop configuration in gain-clamped CRDT is also discussed, which is crucial for achieving a constant gain in a CRDT experiment. It is found that even though conventional CRDT has the lowest standard error in ring-down time ( Δ τ ), the value of ring-down time ( τ ) is very small, thus leading to poor detection limit. Amplified CRDT provides an improvement in τ , albeit with two orders of magnitude higher Δ τ due to amplifier noise. The nested-loop configuration in gain-clamped CRDT helps in reducing Δ τ by an order of magnitude as compared to amplified CRDT whilst retaining the improvement in τ . A detection limit of 1 . 03 × 1 0 − 4 RIU at refractive index of 1.322 with a 3 mm long and 4.5 μ m diameter tapered fiber is demonstrated with the gain-clamped CRDT.