Speaker : Kavita Sharma (EE11D038)
Cavity Ring-down Spectroscopy (CRDS) is an absorption spectroscopy technique that increases the effective path length of interaction between light and the sample of interest, thereby improving sensitivity. Apart from this important advantage, CRDS also enables time–domain measurement with cost-effective photo detectors and immunity to intensity fluctuations of the source. Fiber-based implementation of the cavity allows for compact instrumentation with alignment-free optics. Current implementations of fiber based-CRDS systems are limited by high inherent losses due to couplers, connectors, splices and the base loss of the sample cell, leading to a poor range of detection. The other associated issue is poor detection limit ensuing from large noise.
In my first seminar, I discussed the utilization of the multi-pulsing phenomenon, in a Q-switched fiber laser, to improve the detection range. Acetylene gas was used as a sample to demonstrate the practical applicability of the technique. A detection range of over 27 dB with a detection limit of 0.08 dB – (corresponding to ~ 2800 ppm of acetylene) was achieved, limited only by the stability of the source.
In this seminar, I will discuss the modification of conventional CRDS techniques to address issues related to poor detection limit and increased inherent cavity loss. We fabricate tapered fibers and use them as sample heads to achieve improved sensitivity and demonstrate sensing of refractive index and concentration in liquid samples in the 1.55 µm and 2 µm wavelength regions. We compare the relative performance of CRDS in three different modes of all-fiber operations: conventional, amplified and gain-clamped configurations. The results are benchmarked against refractive index measurements made using the evanescent field in tapered fibers. A detection limit of 5.9×10-5 RIU at a refractive index of 1.316 is demonstrated with the gain-clamped CRD technique at 1550 nm. The taper in this case, was 4 mm long and had a diameter of 1.2 µm. The best detection limit achieved experimentally at 1950 nm is 1.96×10-5 RIU at refractive index of 1.307 in conjunction with amplified CRDT. The tapered fiber was of length 4 mm and diameter 2 µm. In the case of wavelengths in the 2 µm region, the detection limit could be further improved with the use of components and fiber that are optimized for minimal loss in these wavelengths.
All are cordially invited