Evre kaydırmalı çınlaç boşalım izgegözlem yöntemi ve sündürülmüş optik lif temelli doğrusal çınlaç algılayıcısı

This dissertation presents the linear optical cavity sensor based on fiber Bragg grat- ings and tapered optical fiber. The sensing and detection approach used in this thesis is phase shift cavity ring down spectroscopy. It is because of linear nature of the cavity and detection mechanism applied, that the whole study is named as "linear cavity tapered fiber sensor using phase shift cavity ring down spectroscopy". First part of the thesis is dedicated to the study of experimental investigations that are available in the literature for the purpose of optical sensing. Among the reviewed techniques are the surface plasmon resonance (SPR), resonant wavelength shift, direct absorption spectroscopy, cavity ring down spectroscopy (CRDS) and its vari- ants like cavity enhanced absorption spectroscopy (CEAS). Then these techniques are sorted out depending upon the advantages that they have on each other and limitations that put constraint on their performance. For example, surface plasmon resonance is a single pass approach, which is its major constraint. But multi-pass method dubbed as resonant wavelength shift, although it is sensitive enough but suf-fers from laser intensity changes. On the other hand, cavity ring down spectroscopy, is free from such errors but it is expensive and complicated technique. The sensing technique called phase shift cavity ring down spectroscopy not only combines the advantages of CRDS and resonant wavelength shift but also provide solutions to their short comings. That is, it is economically feasible and not affected by the noise related to laser intensity. In the manuscript under study, mathematical foundation of the phase shift cavity ring down spectroscopy is also given due attention. In addition to this, fabrication of fiber Bragg gratings, their character- ization, tapered optical fiber and its evanescent field are the topics which are also discussed in the same thesis.In the second part of the thesis, an experimental investigation has been reported by using tapered fiber based cavity. Here we employed a tapered fiber-based linear fiber cavity for the demonstration of a highly sensitive sensor for sucrose concentration in water using PS-CRDS. This, linear fiber cavity has a small cavity length (∼1.25 m) that enables the observation and tracking of individual cavity modes in transmission and phase spectra recorded during laser sweeps. Hence, it eliminates the need for Pound–Drever–Hall locking of the laser source to the cavity resonance and thus provide a simpler experimental scheme. The analysis of the recorded data sets was performed to track the changes in phase shifts observed only when the laser wavelength is in resonance with the cavity modes. Such a mode-tracking PS-CRDS approach reveals limit of detection values less than around 400 µM, better than the performance of previously demonstrated PS-CRDS sucrose concentration sensors employing fiber loop resonators. The sensitivity of our sensor critically depends on the fiber taper diameter and can reach up to around 6◦/1mM Suc. for 3.2 µm taper diameter at 6 MHz modulation frequency using fiber Bragg gratings (FBGs) with reflectivities around 86%. This value can be further increased by employing FBGs with higher reflectivities or fiber tapers with smaller diameters provided that the cavity loss due to water absorption is compensated with an amplifier. It is well known fact common sources of optical losses are scattering, optical components, splicing or water absorption (for liquid phase measurements). These losses become more prominent at small taper waist diameters. So, by com- pensating these losses, working at the taper diameter lesser than 2 µm is possible. Last section of the thesis explains the working of the system for above said idea. Basically, to compensate for the optical losses, a booster optical amplifier (BOA) was added in to the previously designed simple PS-CRDS experimental scheme. Af- ter carefully adjusting the gain, this optical amplifier, provides enhanced ring down time and more round trips of light into the cavity. Which ultimately manifests itself in the form of improved sensitivity and detection limit. Some exemplary measurements performed with the amplified PS-CRDS showed the sensitivity of 4.84◦/1mM Suc. for 2 µm taper diameter at 2 MHz modulation fre- quency. Higher sensitivity can also be achieved by adjusting booster optical amplifier at higher gains and using even more thinner tapers. The thesis ends with the con- cluding remarks and future directions. It highlights the potential applications of PS-CRDS and suitable candidates to replace tapered optical fiber as a sensing head. Implementation of these changes further enhance the performance of sensing device.