Soft glass fibers are emerging as an attractive alternative to silicate fibers for visible and mid-infrared lasers. The paper reports on the inscription of Bragg gratings with characteristic suitable for the realization of monolithic laser cavities in some soft glass fibers, in particular in a custom developed fluoride fiber. The first results have been obtained at around 1550 nm for simplicity of characterization, but the approach can be extended to other wavelengths
The paper presents an all-optical system for the detection of bacterial contamination in flowing water that combines the readings from a multi-functional fiber Surface Plasmon Resonance (SPR) sensor with fluorescence measurements. The preliminary application to cases of water contaminated with Escherichia coli is discussed.
The paper presents the development and investigation of distributed and a quasi-distributed fiber optic sensors for the real-time monitoring of radiations during cancer treatments. Both sensors rely on ad-hoc developed nanoparticle-doped optical fibers with enhanced sensitivity to radiation. The distributed sensor is interrogated with an OFDR-based instrument and allows the reconstruction of the spatial dose distribution along the fiber. The quasi-distributed sensor is implemented through fiber Bragg gratings inscribed with a femtosecond laser in the few-mode section of a single mode-multi mode-single mode interferometer.
Enhanced Rayleigh backscattering optical fibers, interrogated by an optical frequency domain reflectometer, are used to perform remote real-time measurements of X-ray irradiation profiles, with possible application as dosimeters in radiotherapy treatments. The enhanced Rayleigh backscattering is obtained by proper engineering of the composition of fiber core, either by introduction of Aluminum or Magnesium silicate nanoparticles as radiation-sensitive dopants. A detectable radiation-induced refractive index change can be spatially resolved through the measurement of the frequency shift of the Rayleigh backscattering along the fiber. It is experimentally demonstrated that two mechanisms of radiation-induced refractive index change take place. At doses nearly compatible with those delivered in radiotherapy, a negative refractive index is induced, whereas at high doses the change is positive. This behavior is also confirmed by the shift of Bragg wavelength of a fiber Bragg grating inscribed in the nanoparticles-doped fiber and used as a reference.
A reflectance-based surface plasmon resonance (SPR) fiber sensor with enhanced sensitivity for biochemical sensing is reported after comparing its result with the transmittance-based SPR optical fiber sensors. The fabricated SPR sensor contains a gold-coated multimode fiber with the implementation of a standard source-sensor-spectrometer interrogation system. As the refractive index of the liquid under test is increased, a redshift of the SPR is observed. The coupling of the source to the fiber sensor is optimized by investigating the effect of an intentional misalignment in transmission-based setup. When a fiber tip coated with the silver mirror and the bifurcated fiber bundle is used, an alignment-free disposable sensor probe is achieved. A comprehensive characterization of the proposed reflectance-based SPR probe is discussed. The maximum sensitivity of 3212.19 nm/refractive index unit (RIU) is obtained.
The paper compares two different approaches to design an innovative probe with optimized heated area for
laser ablation of solid tumors: micro-patterning of the fiber delivery tip, and exploitation of the dissipation
of plasmonic waves at the metal-dielectric interface. Both probes integrate a fiber Bragg grating for real-
time monitoring of the obtained temperature increase to provide feedback to surgeons in practical applications.
Experimental characterizations carried out using liver phantoms and ex-vivo porcine livers have demonstrated
that both approaches can be used for the devised application, although further optimizations and tests are still
necessary before clinical assessment.
Multipath interference in bend-insensitive optical fibers is experimentally evaluated in the 1300 nm wavelength range, describing how this phenomenon originates and how it can be measured by a setup based on a tunable laser. The characterization of bend-insensitive fibers from different manufacturers is presented; all the considered fibers exhibit a negligible effect in patchcords longer than 10m, whereas jumpers with length ≤ 2m can produce multipath interference at detrimental levels above -30dB. The phenomenon is even more evident in the cascade of offset-spliced short jumpers made of bend-insensitive fiber; preliminary results showed a large rise of the MPI when two or more jumpers are shortly spaced.
The paper presents our recent results towards the development of a miniaturized all-fiber probe for laser induced thermal ablation of tumor cells, which combines the optimal delivery of a near-infrared high power ablating beam, a low power visible aiming beam and fast Bragg grating (FBG) temperature sensors. Specific combiner and probe end-cap based on dual cladding fibers have been developed to allow the simultaneous handling of the laser beams and of the signal that feeds the temperature sensor. Moreover, a very fast FBG interrogation system has been implemented to track abrupt temperature variations during medical treatment.
The paper reports - with simulations and experiments - on the development of low-cost Surface Plasmon Resonance fiber probes for bio-sensing. The simulations analyze the impact of non-uniform metal layer depositions, a situation that would typically occur in a simplified fabrication process, whereas the experimental section discusses some preliminary results of the characterization of large-core fiber probes used as refractometers of aqueous solutions.
This work presents the comparison between the fiber Bragg grating technology and a vibration-measurement technique based on the detection of polarization rotation (polarimetric sensor) in a standard optical fiber, applied to the dynamic structural monitoring of carbon reinforced composites for the automotive industry. A carbon reinforced composite test plate in a 4-layer configuration was equipped with fiber Bragg gratings and polarimetric fiber sensors, then it was mechanically stressed by static and dynamic loads while monitoring the sensors response. The fiber Bragg grating setup exhibited 1.15±0.0016 pm/kg static load response and reproduced dynamic excitation with 0.1% frequency uncertainty, while the polarimetric sensing system exhibited a sensitivity of 1.74±0.001 mV/kg and reproduced the dynamic excitation with 0.5% frequency uncertainty. It is shown that the polarimetric sensor technology represents a cheap yet efficient alternative to the fiber Bragg grating sensors in the case of vibration-monitoring of small structures at high frequency.
Thulium-doped fiber lasers are gaining in popularity since they emit at about 2 μm, a wavelength particularly interesting
for many industrial, sensing and medical applications, and, moreover, in the so-called “eye-safe” spectral region. Despite
the many advantages, however, thulium-doped fiber lasers with power high enough to allow practical applications have
still limited deployment mainly due the high cost per emitted watt. The paper investigates alternative paths to high power
CW emission at about 2 μm by exploring two complementary approaches: the development of specific pump combiners
and the study of new pumping schemes that take advantage of co-doped fibers. The developed pump combiners are
based on fused fiber technology and are characterized either by the use of “non-standard” fiber dimensions to allow
pumping through an ytterbium-doped fiber laser or by a large number of input ports (up to 39) to provide adequate levels
of pump power through the efficient coupling of several fiber pigtailed diodes with emission wavelength suitable for
pumping thulium. On the other hand, a co-doped ytterbium-thulium fiber is also studied to analyze the possibility of
using ytterbium ions as pump source for thulium ions. The use of ytterbium, either as co-dopant or as laser source, is
particularly interesting because it allows taking advantage of the remarkable advancements made in the pump diodes for
such a laser system, and specifically of the favorable cost per emitted watt. Preliminary experimental results have
demonstrated the feasibility of the proposed approaches and have shown that the joint use of the “ad-hoc” pump
combiners and of the ytterbium-thulium co-doping can lead to the development of lasers with power suitable for
industrial applications, although the efficiency needs further improvements.
We present a non-contact low-cost displacement sensor based on plastic optical fiber (POF), designed for industrial
applications. The proposed system is based on bifurcated fiber bundle (BFB) approach. The bundle is composed by a
transmission fiber, used both to illuminate the target and collect backreflected power, and an additional receiving fiber that allows implementing target reflectivity and input power compensation. A Monte Carlo ray-tracing technique has been developed in order to evaluate system performance and tolerances to geometrical misalignments and target surface alteration. Experimental results show an accuracy of 0.04 mm on displacement readout, on the 0-1.2 mm range, assessing reflectivity compensation.
A low-cost optical fiber Bragg grating-based microphone is presented. The sensing system makes use of an intensitybased
interrogation approach, with a fixed-wavelength laser source employed as optical source. The optical sensor is
complemented by signal processing, consisting of adaptive filters and a Capon power spectral density estimator. System
characterization has been carried out, showing maximum sensitivity in the 120-450 Hz range, with 0.45 Hz repeatability
on single-tone detection. Application for condition-based maintenance of industrial plants, based on a simulated model,
is discussed.
A fiber laser operating at 1.94μm in pulsed regime has been developed in a MOPA configuration. The seed consists of a custom-developed board hosting a laser diode, whose current is modulated to achieve the desired pulse shape, duration and repetition rate. The pulses are amplified through a thulium-doped fiber amplifier pumped at 793 nm. The design of the amplifier stage has been performed by dynamic simulation of a rate-equations model and compared to the experimental measurements. Simulations and experimental measurements have exhibited comparable results, devising the realization of an effective pulsed laser system whose parameters can be easily tuned through the seed.
We present a fast high-resolution fiber Bragg grating sensing system for weigh-in-motion (WIM) application. The
proposed system makes use of standard telecom photonics components operating at high speed and with insufficient
resolution; then, using signal processing we artificially improve the accuracy of the system down to 1 με. This way, the
proposed architecture overcomes the state of the art of optical systems for WIM, which cannot cope with both high
resolution and high frequency requirements. The developed system has been applied to a prototype weigh-in-motion
device, which consists of a road speed bump. Structural deformations of the bump when perturbed by a thin-footmark
load are well reproduced. Using multiple Bragg grating sensors, it is possible to unambiguously determine position and
weight of a moving load on the bump with accuracy of 0.2 - 1.2 kg.
KEYWORDS: Fiber Bragg gratings, Signal to noise ratio, Signal processing, Sensors, Filtering (signal processing), Interference (communication), Signal detection, Multiplexing, Acoustics, Data acquisition
This paper presents a multipoint fiber Bragg grating (FBG) sensing system operating as a precision microphone. This
instrument aims to become the best performing technology for condition-based maintenance (CBM) of critical elements,
like ball bearings and cogwheels, embedded in industrial manufacturing machineries. The system architecture is based
on the simple matched-laser principle, leading to a low-cost and high-sensitivity system, operating in time and
wavelength multiplexing mode. Then, heavy signal processing is applied, providing an outstanding performance
improvement of 59 dB in terms of signal-to-noise ratio. A demonstration of condition-based maintenance operation has
been performed using standard models of ball bearing sound spectra. Compared to traditional microphones applied to
CBM, the signal processing-powered FBG system provides remarkable advantages in terms of sensitivity and rejection
of environment noise, providing an improvement of cost-effectiveness of CBM.
A custom fabrication procedure for pump combiners matched to a wide variety of double-clad fibers used in high power
fiber lasers has been developed. Both combiners with and without feedthrough can be fabricated with an improved
repeatability thanks to a specific mechanical setup that simplifies placing the fibers in their proper location within the
input bundle before the tapering and fusion processes. Extensive characterization results on 7-to-1 and on (6+1)-to-1
combiners are confirming the validity of the proposed approach.
A setup for core-pumping of Yb-doped optical fibers has been developed to induce photodarkening and benchmark their
suitability as a long-life active medium in high power fiber lasers for industrial applications. The measurements setup, its
reliability and preliminary tests on PD affected and PD free fibers are here presented. Repeatability of measurements has
also been carried out.
A low-cost optical sensing system to measure vibrations up to high frequencies (40 kHz) is presented and validated with
experiments. The developed system uses a sensing head made with plastic optical fibers to illuminate the vibrating
surface and collect the reflected light; a non-demanding data processing is used to measure the amplitude and frequency
of the vibration and to compensate the reflectivity of the vibrating surface. Preliminary measurements, performed on
vibrating targets with different surface reflectivity have been performed to assess the performance of the system.
In this paper, we discuss the realization of an optical microphone array using fiber Bragg gratings as sensing elements.
The wavelength shift induced by acoustic waves perturbing the sensing Bragg grating is transduced into an intensity
modulation. The interrogation unit is based on a fixed-wavelength laser source and - as receiver - a photodetector with
proper amplification; the system has been implemented using devices for standard optical communications, achieving a
low-cost interrogator. One of the advantages of the proposed approach is that no voltage-to-strain calibration is required
for tracking dynamic shifts. The optical sensor is complemented by signal processing tools, including a data-dependent
frequency estimator and adaptive filters, in order to improve the frequency-domain analysis and mitigate the effects of
disturbances. Feasibility and performances of the optical system have been tested measuring the output of a loudspeaker.
With this configuration, the sensor is capable of correctly detecting sounds up to 3 kHz, with a frequency response that
exhibits a top sensitivity within the range 200-500 Hz; single-frequency input sounds inducing an axial strain higher than
~10nε are correctly detected. The repeatability range is ~0.1%. The sensor has also been applied for the detection of
pulsed stimuli generated from a metronome.
A fiber Bragg grating sensor for simultaneous static and dynamic strain detection is hereby presented. The principle of
operation of the interrogator is based on direct intensity detection: a fixed-wavelength laser source is filtered through the
sensing element, and the output power is detected with a photodiode. Multiple sensing for matched-wavelength gratings
is performed by splitting the source into multiple channels. The exploitation of cheap components for optical
telecommunications results in a low-cost hardware solution that matches several budget-constrained applications. The
optical sensor is complemented by signal processing techniques (adaptive filters, spectral estimation, data modeling),
capable of improving performances of the system without changing the optical layout.
The system has been tested both in static interrogation, as a temperature sensor, and as vibration detector in a typical
structural monitoring context. The maximum interrogation range is ~200με, depending on the grating shape, with a
resolution <<1 με, and a repeatability of ~1%. A 1% stability over long time has been assessed with a long-term test. The
adaptive filtering improves the signal-to-noise ratio of 5.3 dB. The resolution-unlimited spectral estimator resolves
resonance peak detection for a vibration of 0.1με.
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