Stefania Campopiano

Microfluidic sensor based on integrated optical hollow waveguides

A simple integrated optical refractometric sensor based on hollow-core antiresonant reflecting optical waveguides is proposed. The sensor uses the antiresonant reflecting guidance mechanism and permits one to measure the refractive index of a liquid filling the core by simply monitoring the transmitted spectrum. The device has been made with standard silicon technology, and the experimental results confirm numerical simulations performed in one- and two-dimensional geometry. The sensor exhibits a linear response over a wide measurement range (1.3330-1.4450) and a resolution of 9 x 10(-4) and requires a small analyte volume.

Long period grating working in transition mode as promising technological platform for label-free biosensing

We present the development of a platform for label-free biosensing based on overlayered Long Period Gratings (LPGs) working in transition mode. Nano-scale layers of Polystyrene (PS) with different thicknesses were deposited onto the same LPG to test the performances of the device in different working points of its modified sensitivity characteristic. Adsorption dynamic of biotinylated bovine serum albumin (BBSA) onto the PS overlays was on-line monitored as well as a subsequent streptavidin (SA) binding dynamic on the biotinylated sites of the protein ad-layer. Experimental results show that overlayered LPGs are among the most sensitive refractive index transducers to be employed in label-free biochemical detection and that wide margins of further optimization exist.

Sensitivity characteristics in nanosized coated long period gratings

This work presents an experimental analysis of the sensitivity characteristics to the surrounding refractive index (SRI) in long period gratings coated with polymeric overlay with high refractive index and thickness values ranging in hundreds of nanometers. The presence of the coating induces the cladding to overlay mode transition depending on the overlay features and the SRI. The immediate consequence is a drastic modification of the sensitivity characteristics from the sublinear monotone behavior to a resonantlike shape. Here, the sensitivity characteristics of coated long period gratings have been investigated to outline their dependence on the overlay thickness and order mode.

Long-Period Gratings in Hollow Core Fibers by Pressure-Assisted Arc Discharge Technique

We report on the fabrication of long-period gratings (LPGs) in hollow-core air-silica photonic bandgap fibers by using the pressure-assisted electrode arc discharge (EAD) technique. The EAD procedure combined with pressure actuation inside fiber holes enables the modification of hole size and shape in both core and cladding region avoiding holes collapsing and thus acts as a useful tool to impress effective refractive index modulation leading to low loss gratings. Periodically repeated EAD treatments permit the fabrication of LPG-based devices in hollow core optical fibers enabling new functionalities hitherto not possible. Here, the experimental demonstration of LPG prototyping with different characteristics exhibiting attenuation bands with depth up to 12 dB are reported.

Photonic band-gap engineering in UV fiber gratings by the arc discharge technique

Localized heat treatments combined with local non-adiabatic tapering is proposed as suitable tool for the engineering of photonic band-gaps in UV-written fiber Bragg gratings (FBGs). In particular, here, we propose the use of the electric arc discharge to achieve localized defects along the FBG structure, however differently from previously reported works, we demonstrate how this post processing tool properly modified can be exploited to achieve the full control of the spectral characteristics of the final device. Also, we show how the suitable choice of the grating features and the correct selection of the defect geometry can be efficiently used to achieve interesting features for both communication and sensing applications.

Spectral behavior in thinned long period gratings: effects of fiber diameter on refractive index sensitivity

We report the experimental investigation of the sensitivity characteristics to the surrounding refractive index (SRI) in thinned long period gratings (LPGs) for a wide range of fiber diameters and different low-orders cladding modes. Wet chemical etching combined with microscopic and spectral analysis allow us to experimentally retrieve the SRI sensitivity characteristics of thinned LPGs. The obtained results allow us to identify accurately the dependence of the sensitivity characteristics on the fiber radius, taking into account the SRI range and the order mode. This provides a useful tool to identify the thinned structure able to fulfill the sensitivity requirements by maintaining an acceptable robustness level.

Underwater Acoustic Sensors Based on Fiber Bragg Gratings

We report on recent results obtained with a fiber optic hydrophone based on the intensity modulation of the laser light in a FBG (Fiber Bragg Grating) under the influence of the sound pressure. In order to control the behavior of the hydrophone in terms of sensitivity and bandwidth, FBGs have been coated with proper materials, characterized by different elastic modulus and shapes. In particular, new experiments have been carried out using a cylindrical geometry with two different coating, showing that the sensitivity is not influenced by the shape but by the transversal dimension and the material characteristics of the coating.

Broadband Mirrors in the Near-Infrared Based on Subwavelength Gratings in SOI

We describe the design, fabrication, and characterization of high-reflectivity broadband mirrors operating in the near-infrared (700-1000 nm) wavelength range. The mirrors consist of 1-D and 2-D subwavelength resonant gratings (SWGs) fabricated on a silicon-on-insulator (SOI) wafer. A very good agreement between numerical and experimental results is obtained. The mirror response can be tailored by adjusting the geometrical parameters of the gratings, with the grating period as the main parameter. The optimized mirrors reflect strongly (> 95%) over a fractional optical bandwidth Δλ/λ of about 12% and 7.5% for 1-D and 2-D gratings, respectively. The important and somewhat surprising feature of these gratings is that high reflectivities have been achieved, despite the fact that silicon exhibits significant absorption in this wavelength range.

Fabrication and Characterization of Long-Period Gratings in Hollow Core Fibers by Electric Arc Discharge

Recently, the fabrication of long-period gratings (LPGs) in hollow-core air-silica photonic bandgap fibers by means of pressure assisted electrode arc discharge (EAD) technique have been presented. The EAD procedure properly combined with air pressure inside fiber holes enables the localized modification of hole size and shape in both core and cladding region avoiding holes collapsing. LPGs are fabricated with a step-by-step approach by periodically repeated EAD treatment. In this paper, the role of pressure inside the fiber holes as well as the effect of the grating pitch on the transmitted spectra have been experimentally investigated to achieve the design criteria of novel hollow core devices. An appropriate perturbation of fiber structure (core and/or cladding) may change the field profile of the fiber modes and cause light coupling from the fundamental mode to higher order modes. Here, the experimental demonstration of LPG prototypes with different features exhibiting attenuation bands with depth up to 12 dB are reported. Finally, the resonant wavelength dependence on local temperature and strain changes are experimentally investigated. We believe that the fabrication of LPGs-based devices in hollow core optical fibers enable new functionalities hitherto not possible.

A Novel Optochemical Sensor Based on

In this paper, a fiber optic sensing system, designed, and developed for the detection of ammonia in aqueous ambient at room temperature, is presented. The sensor is constituted by a standard silica optical fiber (SOF) coated by a tin dioxide sensitive layer. The SnO2 films have been transferred onto the distal end of the SOF by means of the simple and low-cost electrostatic-spray-pyrolysis deposition technique. The spectral characterization of the fabricated samples has been carried out in the wavelength range 400-1750 nm in order to estimate the thickness of the SnO2 fiber coatings. The morphology and the elemental composition of the deposited layers have also been investigated by means of scanning-electron-microscopy observation and energy-dispersive-spectrometer analysis, respectively. Single-wavelength reflectance measurements have been carried out to test the sensing performances of the realized sensors toward ammonia traces in water. A fiber-Bragg-grating temperature sensor has also been used for monitoring the temperature changes occurring inside the test ambient during the experimental measurements, in order to identify the effects of thermal drifts on the sensor response. The results here presented demonstrate that the developed refractometric chemical sensor is able to provide measurements of ammonia concentration in water and at room temperature with a high sensitivity, response times of few minutes, and a resolution as low as 2 ppm