The aging of most of the components of the National transmission and distribution system can potentially influence the
reliability of power supply in a Medium Voltage (MV) network. In order to prevent possible dangerous situations,
selected diagnostic indicators on electrical parts exploiting reliable and potentially low-cost sensors are required. This
paper presents results concerning two main research activities regarding the development and application of innovative
optical sensors for the diagnostic of MV electrical components. The first concerns a multi-sensor prototype for the
detection of pre-discharges in MV switchboards: it is the combination of three different types of sensors operating
simultaneously to detect incipient failure and to reduce the occurrence of false alarms. The system is real-time controlled
by an embedded computer through a LabView interface. The second activity refers to a diagnostic tool to provide
significant real-time information about early aging of MV/Low Voltage (LV) transformers by means of its vibration
fingerprint. A miniaturized Optical Micro-Electro-Mechanical System (MEMS) based unit has been assembled for
vibration measurements, wireless connected to a remote computer and controlled via LabView interface. Preliminary
comparative tests were carried out with standard piezoelectric accelerometers on a conventional MV/LV test transformer
under open circuit and in short-circuited configuration.
In this work we present the possibility to obtain long-period fiber gratings (LPFGs) working in inverted mode through
simple and cost effective all-fiber ring shaped illumination. Permanently and locally bent single-mode optical fibers are
used to provide a ring shaped illumination with core mode depletion depending on the bend features. When this kind of
illumination interacts with a tapered LPFG, light power transfer from input cladding modes towards the core mode
occurs depending on the grating characteristics. The final result is a transmission spectrum with zero transmission in the
whole spectral range except for the optical wavelengths corresponding to mode coupling. Two important aspects of the
proposed structure can be envisaged. The first one relies on the possibility to additionally obtain mode coupling
involving asymmetric cladding modes by modifying the illumination symmetry acting on the bend shape. The second
one is the possibility to tune the visibility of the peaks corresponding to the coupled cladding modes by changing the
surrounding medium on the straight fiber located after the bent region without involving the LPFG section. Here, both
aspects have been experimentally demonstrated and their effectiveness proved in sensing applications involving
refractive index measurements. The proposed approach thus opens up new sensing architectures as well as new fluidic
filters for communication applications.
A novel optical fiber sensing scheme based on evanescent wave interaction is proposed. It involves locally and
permanently bent single mode optical fibers. Permanent bends induce significant power coupling between core and
cladding modes. Order and number of excited cladding modes depend on bend features and determine the field profile at
the output of the bent region. This in turn constitutes a simple mechanism to tailor the field distribution in single mode
optical fibers useful for spatial light modulation. Moreover, since guided cladding modes are strongly influenced by the
surrounding refractive index, the power transmitted at the output of the bent region as well as its dependence on the
optical wavelength are strongly sensitive to the SRI opening new scenarios in sensing applications.
Long-period fiber gratings (LPFGs) represent an attractive fiber grating-based technological platform because of their
selective spectral features together with the intrinsic sensitivity to surrounding refractive index (SRI). Unfortunately,
their main limitation relies on the necessity to opportunely coat the glass substrate when sensitivity enhancement and/or
specific functionalization are required. Here, we investigate the possibility to realize a self-functionalized and high-sensitivity
LPFG by evanescent-wave interaction of the propagating light with a periodically patterned overlay. In
particular, a D-shaped optical fiber is considered because of its peculiar geometrical features. First chemical etching is
used to allow the evanescent-wave interaction of the propagating light with the surroundings with the desired sensitivity.
Successively a uniform atactic polystyrene overlay is deposited onto the flat surface of the structure by dip-coating
technique. Finally the overlay is opportunely patterned by laser-micromachining techniques in order to create a LPFG-like
structure. The reported results demonstrate the spectral features of the realized device and confirm the LPFG-like
behavior with high SRI-sensitivity. The flexibility of the adopted fabrication method could allow the realization of
innovative LPFGs to be adopted for a multitude of sensing applications, depending on the nature of the material
deposited onto the flat surface of the etched D-fiber.
By now, fiber Bragg gratings (FBGs) represent a well assessed technology in both communications and sensing fields. In
particular, thanks to their small signal bandwidth and wavelength encoded information, they allow simple measurements
in reflection for sensing purposes and also easy multiplexing capability in realizing sensors arrays. Unfortunately, to
make FBGs sensitive to surrounding refractive index (SRI), hosting fiber structuring is needed. In last years, also tilted
FBGs (TFBGs) - intrinsically SRI sensitive structures - have been proposed as promising technological platform for
several sensing applications. However, complex spectral features combined with the difficulty to be configured as quasi
distributed or multi-point sensors network limit the practical exploitation of this assessed and mature technology. It
would be extremely useful to merge the peculiar spectral characteristics of both grating types. To address this issue, here,
we propose a hybrid cavity involving two unbalanced uniform FBGs written at both sides of a TFBG to form an all-fiber
interferometer. The proposed configuration provides a wavelength gated reflection signal with interference fringes
depending on the cavity features modulated by spectral dips associated to the wavelength dependent optical losses due to
cladding mode coupling occurring along the TFBG. Such a structure preserves the advantages of uniform FBGs in terms
of interrogation methods and allows the possibility to contemporarily measure multiple parameters.
This work is devoted to present and demonstrate a novel approach for the fabrication of micro-structured fiber Bragg
gratings (MSFBGs). The MSFBG consists in a localized stripping of the cladding layer in a well defined region in the
middle of the grating. The introduction of a perturbation along the grating leads to the formation of a defect state in the
FBG spectral response that is tunable through the surrounding medium refractive index.
Here, a two steps MSFBG fabrication technique, based on arc-discharge technique as fiber pre-treatment and maskless
wet chemical etching to sensitize FBG to external refractive index, is proposed. Compared to the lithographic fabrication
approach, previously proposed by the same authors and based on laser micromachining tool, this new simple and lowcost
technique overcomes some technological drawbacks related to the presence of a mask and consequent undercutting
etching. Furthermore, we experimentally demonstrate the potentiality of the presented approach to realize reliable
MSFBGs enabling the prototyping of advanced photonics devices based on this technology.
This work numerically and experimentally investigates the spectral modifications due to non-uniform azimuthally
symmetric high refractive index (HRI) nano-coatings deposited on long-period fiber gratings (LPFGs). First, a uniform
overlay was deposited on a LPFG by using electrostatic self-assembling technique, which guarantees a fine control on
the coating thickness. Successively, UV (&lgr;=193nm) laser micromachining was applied to locally and selectively remove
the coating with high spatial resolution and preserving the azimuthal symmetry of the structure. As the overlay removal
was performed starting from the middle of the grating, strong modifications of the LPFG transmission spectrum
occurred. Phase-shift phenomenon and selective fringes generation in correspondence of all the attenuation bands can be
observed. The two-effects occurring are ruled by the longitudinal length of the uncoated region and the overlay features
(thickness and optical properties). The wavelength selectivity, combined with the effects of the HRI coatings on the
cladding modes distribution, can be successfully applied for the development of advanced and high performances
sensing devices.
In this work, an experimental analysis on the spectral effects induced by depositing a uniform high refractive index
(HRI) thin coating on weakly tilted fiber Bragg gratings (TFBGs) is carried out. First, two weakly TFBGs presenting
different tilt angles were fabricated. Successively, by using dip-coating technique, two different coating thicknesses were
deposited on each of them. The transmission spectra of the obtained structures were collected as the surrounding
refractive index (SRI) changed in the range 1÷1.47. The coated gratings present some differences in their spectral
evolution, that can be opportunely exploited. The coated configuration could improve some aspects of previous
demodulation techniques exploited in the case of bare TFBGs. Moreover, an important effect of HRI coatings is to
enhance the SRI sensitivity of the spectral position of the dips corresponding to cladding modes coupling, leading to a
different interrogation method for sensing applications.
In this work, we propose a novel fabrication method as technological assessment for the development of microstructured
fiber Bragg gratings (FBGs) tailorable for specific applications. Micro-structured FBG (MSFBG) 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. The key feature of this class of devices is the strong dependence of
the defect state generated in the grating spectrum on the optical properties of the medium surrounding the device. In fact,
this property enables the development of tunable devices as well as in fiber refractometers and chemical sensors. One of
the main drawbacks of these devices is the fabrication process requiring a well precise control of the stripped region
dimensions. Here, we demonstrate that the optimization of this device is possible by adopting a fabrication process based
on polymeric coatings and UV laser micromachining. These method combined with wet chemical etching allows the
easy prototyping of MSFBGs with the desired features for specific applications.
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