Agostino Iadicicco

Microstructured Fiber Bragg Gratings

The first fiber Bragg gratings were accidentally written in a Ge-doped silica fiber using a high power argon-ion laser [Hill 1978]. Following this first evidence of photosensitivity in optical fibers, a huge effort was put into fiber gratings: improving their fabrication (they are now all externally inscribed), obtaining complex profiles, optimizing their performance, and incorporating them into devices and systems. On this line, the last decade was characterized by the birth of an emerging class of fiber gratings which can be defined as ldquomicrostructured fiber Bragg gratings.rdquo They refer to two main categories: one relies on short period gratings writing in microstructured optical fibers, whereas the second deals with standard short period gratings where structural defects at microscale are properly created within the hosting fiber by postprocessing techniques. This paper reviews the fabrication processes at the basis of this new technology as well as its properties and applications. Emphasis will be placed on principles of operation, technological developments and applications discussing perspectives, and challenges that lie ahead.

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.

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.

Structured Chirped Fiber Bragg Gratings

In this paper, a theoretical and numerical analysis of novel in-fiber photonic devices based on a structured chirped fiber Bragg gratings (CFBGs) for sensing and communication applications is presented. The investigated structure consists in a CFBG with single or multiple defects obtained by a deep and localized stripping of the cladding layer along the grating structure. The thinning of the cladding layer, partial or total, changes the core propagation features and thus leads to a significant modification of the grating spectral features. The effect of the local thinning, properly exploited, basically consists in the formation of one or more passbands within the original grating bandwidth and in one or more stopbands out of the original grating bandwidth. In addition, due to spatial encoding of the Bragg wavelength in CFBGs, the spectral position of each channel exclusively depends on the features of its own defect in a well defined location along the grating. Thus, the spectral properties of each channel are not affected by additional defects located elsewhere along the grating structure, enabling the possibility to develop independent multichannel devices by exploiting a single grating element. The spectral behavior exhibited by the microstructured device has been here numerically analyzed in dependence on the thinned region parameters. In addition a simple theoretical model has been extracted in order to easily design the device according to the desired spectral features for specific applications.

Resonant Hydrophones Based on Coated Fiber Bragg Gratings

In this paper, we report on recent experimental results obtained with fiber-Bragg-grating (FBG) hydrophones for underwater acoustic detection. The optical hydrophones under investigation consist of FBGs coated with ring-shaped polymers of different size and mechanical properties. The coating materials were selected and designed in order to provide mechanical amplification, via judicious choice of their acousto-mechanical properties and by exploiting selected resonances occurring in different frequency ranges. Our underwater acoustic measurements, carried out within the range 4–35 kHz, reveal the resonant behavior of these optical hydrophones, as well as its dependence on the coating size and type of material. These experimental data are also in good agreement with our previously published numerical results. By comparison with bare (i.e., uncoated) FBGs, responsivity enhancements of up to three orders of magnitude were found, demonstrating the effectiveness of polymeric coatings in tailoring the acoustic response of FBG-based hydrophones.

Experimental Study of the Refractive Index Sensitivity in Arc-induced Long Period Gratings

This paper presents an experimental study of the sensitivity characteristics to the surrounding refractive index (SRI) in arc-induced long period gratings (LPGs), in order to outline their dependence over the fabrication parameters. Several LPGs were fabricated, with spectral features that are appealing for chemical sensing applications, e.g., negligible power losses, bands depth greater than 20 dB, and total length smaller than 35 mm. In addition, low spatial periods Λ, which are close to the limit of the arc-based fabrication technique, were selected in order to excite high-order cladding modes. In particular, the period was chosen in range 350-500 μm to focus attention on the same cladding modes for the gratings under investigation. Accordingly, a wide experimental analysis was then carried out to investigate the dependence of the sensitivity to SRI changes on the period in order to derive design criteria for LPG fabrication. In the wavelength range 1100-1700 nm and taking into consideration the attenuation bands related to the LP05 and LP06 cladding modes, an SRI sensitivity enhancement up to one order of magnitude can be achieved by proper selection of the grating period.

Micro-structured fiber Bragg gratings: optimization of the fabrication process

This work has been devoted to present and demonstrate a novel approach for the fabrication of micro-structured fiber Bragg gratings (MSFBGs) with enhanced control of the geometric features and thus of the spectral properties of the final device. The investigated structure relies on the localized stripping of the cladding layer in a well defined region in the middle of the grating structure leading to the formation of a defect state in the spectral response. In order to fully explore the versatility of MSFBGs for sensing and communications applications, a technological assessment of the fabrication process aimed to provide high control of the geometrical features is required. To this aim, here, we demonstrate that the optimization of this device is possible by adopting a fabrication process based on polymeric coatings patterned by high resolution UV laser micromachining tools. The function of the polymeric coating is to act as mask for the HF based chemical etching process responsible for the cladding stripping. Whereas, UV laser micromachining provides a valuable method to accurately pattern the polymeric coating and thus obtain a selective stripping along the grating structure. Here, we experimentally demonstrate the potentiality of the proposed approach to realize reliable and cost efficient MSFBGs enabling the prototyping of advanced photonics devices based on this technology.

Comparative Study of Long-Period Gratings Written in Standard and Fluorine-Doped Fibers by Electric Arc Discharge

In this paper, we present a comparative study of long-period gratings (LPGs) written in the standard Ge-doped fiber and in two different fluorine-doped fibers by means of the electric arc discharge (EAD)-based technique. Concerning Ge-doped fibers, we present an assessment of the EAD procedure by fabricating relatively short LPGs with deep attenuation bands (up to 32 dB) and trivial power losses. We also demonstrate the ability to manage the effect of the single EAD perturbation to select grating length, which in turn acts on the bandwidth of attenuation bands, without compromising depth. Furthermore, for the first time, we produced LPGs in F-doped fibers with attenuation bands deeper than 30 dB, demonstrating the dependence of the spectra on the grating period. Finally, we illustrate a comparative experimental study on the sensitivity of LPGs fabricated in the standard and F-doped fibers with surrounding refractive index (SRI) and temperature changes. We proved that the SRI response of LPGs in F-doped fibers is significantly higher than in the standard fiber and it strongly depends on the type of F-doped fiber considered. LPGs written in F-doped fibers exhibit a slightly lower temperature sensitivity (in the range 20-30 pm/°C for λres2 cladding mode) than the LPG in the standard fiber (45 pm/°C).