Palas Biswas

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.

Sol–Gel-Based Titania–Silica Thin Film Overlay for Long Period Fiber Grating-Based Biosensors

An evanescent wave optical fiber biosensor based on titania-silica-coated long period grating (LPG) is presented. The chemical overlay, which increases the refractive index (RI) sensitivity of the sensor, consists of a sol-gel-based titania-silica thin film, deposited along the sensing portion of the fiber by means of the dip-coating technique. Changing both the sol viscosity and the withdrawal speed during the dip-coating made it possible to adjust the thickness of the film overlay, which is a crucial parameter for the sensor performance. After the functionalization of the fiber surface using a methacrylic acid/methacrylate copolymer, an antibody/antigen (IgG/anti-IgG) assay was carried out to assess the performance of sol-gel based titania-silica-coated LPGs as biosensors. The analyte concentration was determined from the wavelength shift at the end of the binding process and from the initial binding rate. This is the first time that a sol-gel based titania-silica-coated LPG is proposed as an effective and feasible label-free biosensor. The specificity of the sensor was validated by performing the same model assay after spiking anti-IgG into human serum. With this structured LPG, detection limits of the order of tens of micrograms per liter (10(-11) M) are attained.

Regenerated Fibre Bragg Gratings

Silica remains the key optoelectronic and photonic medium, the essence of nearly all modern optical transport systems. Engineering of silica in its various forms ranges from 1 to 3-dimensional waveguide and periodic structures, including recent interest in 3-D photonic crystals. Most of the processing methods involve complex vapour deposition and various co-dopants, which have an advantage of overcoming the lack of finesse involved with general formation of glass structure through high temperature processing and quenching. Nevertheless, to obtain micron or sub-micron precision over the processing of glass for device purposes, invariably post processing methods are commonly used, ranging from etching of systems with dopants, often through patterned masks, to laser processing using UV to mid IR lasers. Concrete examples of micron scale laser processing of glass include direct written waveguides, Bragg gratings in waveguides and optical fibres and photonic crystals. The drawback with these post-processing techniques is that they often produce glass that is structurally less stable than the starting phase. For many applications the thermal stability of laser induced glass changes determines the limits in which they can operate – an excellent example which will form the basis for this chapter, is the optical fibre Bragg grating. Fibre Bragg gratings are used in many industrial and technological applications. Within standard telecommunications applications, for example, type I fibre Bragg gratings that can operate to 80°C for 25 years are required – such gratings can in principle operate for lengthy periods up to 300°C. Gratings that can operate at temperatures well above standard telecommunication requirements are critical to the success of many real time sensing applications. In the oil and gas industries, an alternative application, although standard oil bores are typically quoted as having an environment no more than ~(180-250)°C [Schroeder et al. 1999; Kersey 2000], variations can occur and the increasing depth of the next generation bores suggest sensors that can operate to 400°C or more are desirable for long term or permanent operation. In industries involving high temperature furnaces, such as aluminium smelting or coal based power stations, it would be of interest to be able to monitor temperatures in excess of 1000°C. Similar temperature requirements span many

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.

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 Superstructure Fiber Bragg Grating With Efficient Mode Coupling for Simultaneous Strain and Temperature Measurement With Low Cross-Sensitivity

An efficient fiber-optic sensor based on a superstructure fiber Bragg grating (SFBG) for simultaneous strain and temperature measurement is numerically and experimentally demonstrated. The stain-insensitivity property of a specific LPG resonance band has been used to overcome the effect of cross-sensitivity. The strain sensitivities of the FBG and LPG peaks of the proposed SFBG sensor have been obtained as 0.79 and -0.41 pm/με, and their temperature sensitivities are found as 8.67 and 40.13 pm/°C, respectively. The resolutions for the strain and temperature measurements have been noted as ±12.83 με and ±1.02 °C based on the wavelength resolution of an optical spectrum analyzer (FBG peak: 0.01 nm; LPG peak: 0.1 nm) and the deviations of measured values from applied ones of measurands. The proposed sensor has shown good agreement between the applied and measured measurands with good linearity and reduced cross-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.