Kamal Dasgupta

A study of regenerated gratings produced in germanosilicate fibers by high temperature annealing

In light of recent proposals linking structural change and stresses within regenerated gratings, the details of regeneration of a seed Type-I Bragg grating written in H2 loaded germanosilicate fiber annealed at high temperatures (~900°C) are systematically explored. In particular, the influence of the strength of the grating, the effect of GeO2 doping concentration and the annealing conditions on regeneration are studied. We show that the role of dopants such as Ge and F contribute nothing to the regeneration, consistent with previous results. Rather, they may potentially be detrimental. Strongest regenerated gratings with R ~35% from a 5mm seed grating could be obtained in fibres with the lowest GeO2 concentrations such as standard telecommunications-compatible grade fibre.

Towards sensitive label-free immunosensing by means of turn-around point long period fiber gratings

Long period fiber gratings have been effectively used in the field of biochemical sensing since a few years. Compared to other well-known label-free optical approaches, long period gratings (LPGs) take advantage of the typical peculiarity of optical fibers. Coupling the propagating core mode with a high-order cladding mode near its turn-around point (TAP) was the strategy adopted to achieve good performances without additional coatings, except for the sensing and selective biolayer deposited on the fiber. Both the modeling and manufacturing of TAP LPGs were discussed. After the functionalization of the fiber surface with the deposition of a Eudragit L100 copolymer layer followed by immunoglobulin G (IgG) covalent immobilization, an IgG/anti-IgG bioassay was implemented along the grating region and the kinetics of antibody/antigen interaction was analyzed. A quantitative comparison between a TAP LPG and a non-TAP LPG was carried out to highlight the improvement of the proposed immunosensor. The real effectiveness and feasibility of an LPG-based biosensor were demonstrated by using a complex matrix consisting of human serum, which also confirmed the specificity of the assay, and a limit of detection of 70 μg L(-1) (460 pM) was achieved.

Modeling and analysis of an extrinsic Fabry–Perot interferometer cavity

A schematic representation of optical feedback between two resonator mirrors undergoing a phase shift each round trip as a function of the separation of the mirrors is studied. A transfer function modeling of the extrinsic Fabry-Perot interferometer (EFPI) is presented. Nyquist analysis has been used to forecast the operational stability and possibility of interference in an EFPI. The analysis with two perfectly parallel surfaces of the cavity shows efficient interference. The performance when there is some tilt between the two mirrors in the cavity is also studied and is presented. In this case some restricted interference is found.

Sensitivity Enhancement of Turn-Around-Point Long Period Gratings By Tuning Initial Coupling Condition

Long period grating (LPG) at turn-around-point (TAP) has been studied with a view to enumerate the dependence of sensitivity of a particular resonant mode at the TAP to surrounding refractive index on the initial coupling strength. It has been shown theoretically and also validated experimentally that sensitivity can be enhanced significantly by tailoring the coupling strength of the cladding mode at the resonant wavelength near the TAP. Sensitivity characteristics have been studied for surrounding refractive index in the range 1.335-1.360, which is of interest in the field of biosensors, where the sensitivity of conventional LPGs is relatively small. We could attain a sensitivity of ~1850 nm/RIU using a TAP-LPG with ~3-dB attenuation at resonance.

Empirical Relations for Design of Linear Edge Filters Using Apodized Linearly Chirped Fiber Bragg Grating

Apodized linearly chirped Bragg grating (CFBG) filters have been studied with a view of determining an optimal set of grating parameters to design and fabricate linear edge filter for Bragg grating sensor demodulation. A rigorous numerical computation towards understanding the relation of all the grating parameters with filter characteristics like the linear bandwidth and slope efficiency produced some simple empirical formula for the design of edge filters of specific desired characteristics. The results are corroborated with experiments.

Evaluation of the performance of high phosphorous with germanium codoped multimode optical fiber for use as a radiation sensor at low dose rates

We propose a GeO2–P2O5-codoped step index multimode (SIMM) fiber having a core diameter of around 50 μm with numerical aperture of around 0.21–0.22. The proposed SIMM fiber shows excellent linear radiation response behavior with sensitivity of around 0.69–0.97 dB/m/100 rad at a 505 nm wavelength within the dose rate range of 10–100 rad/h, as well as very low recovery at room temperature using a Co60 gamma radiation source. This enables its practical application in fiber optic personal dosimeters for measurement of low dose gamma radiation.

Application of strongly overcoupled resonant modes of long-period fiber gratings to measure the adulteration of olive oil

We have shown that strongly overcoupled resonant modes of long-period fiber gratings (LPFGs) can be used to assess the adulteration of olive oil. In this background, we investigate the response characteristics of strongly overcoupled resonant modes of different orders to a surrounding medium, the refractive index (RI) of which is greater than that of the fiber cladding, and in the range where a precise refractive index measurement is immensely useful for inspecting the quality of olive oils and other edible oils. A theoretical simulation that would help in designing a sensor with suitable sensitivity and range of measurement has been presented in detail and also validated with experimental results. It was interesting to observe that in a high RI surrounding, a lower order overcoupled resonant mode is much more sensitive as compared to a higher-order one having a similar coupling coefficient. A quantitative analysis demonstrates that for a particular LPFG, the sensitivity of a strongly overcoupled LP06 mode was found to be ∼2000  dB/RIU, while that of the LP07 mode having similar coupling strength was ∼550  dB/RIU in the surrounding refractive index range from 1.458 to 1.520. The results have been validated experimentally.

Response of Strongly Over-Coupled Resonant Mode of a Long Period Grating to High Refractive Index Ambiance

Over-coupling of the resonant band of interest has been found to have significant influence on the refractive index sensitivity of long period grating (LPG) for surrounding material having refractive index higher than the cladding material. In this paper we report, for the first time to the best of our knowledge, a quantitative analysis using the concept of radiation mode coupling that elaborates the influence of initial coupling strength on the response of LPG to high refractive index surrounding. The simulated results agree well with those obtained experimentally where it could be shown that that a suitably over-coupled resonant mode of LPG can have a sensitivity ~724 dB/RIU in the range of the ambient refractive index which is slightly higher than the cladding where the conventional LPGs have almost no sensitivity.

Design of Palladium-Coated Long-Period Fiber Grating for Hydrogen Sensing

We present a detailed numerical analysis that describes the influence of palladium (Pd) layer thickness on the spectral characteristics of long-period fiber gratings (LPFGs) and their response to the uptake of hydrogen. The investigation is carried out with a view of determining an optimal layer thickness to design high-sensitivity LPFG-based hydrogen sensors. Coupled differential equations for a four-layer waveguide structure have been solved using a matrix method considering a layer of Pd with finite thickness on the cladding. Response of higher order cladding modes of the Pd-coated LPFG at turn-around-point and also at mode transition could be computed. It has been shown that if properly designed, the resonant wavelength of a desired mode may shift by about 20 nm for 1% uptake of hydrogen. There is good match between simulations and experiments for LPFGs with coupling to higher order cladding modes.

Regeneration of complex Bragg gratings

Complex regenerated Bragg gratings, seeded by complex type-I gratings in H2 loaded germanosilicate optical fibre is reported. By this means, dual channel grating filters which are stable beyond 1000°C are produced. These high temperature stable co-located dual gratings have potential application in sensing and multi-wavelength high power lasers.