Francesco Chiavaioli received the M.Sc. degree (summa cum laude) in Telecommunications Engineering and the Ph.D. degree in Information Engineering from the University of Siena, Italy, in 2008 and 2012, respectively. In 2015-2016, he was a Visiting Scientist with the ICFO, Barcelona, Spain. From 2018, he is a permanent Researcher with the National Research Council of Italy (CNR), the Institute of Applied Physics “Nello Carrara” (IFAC). His research activities encompass the ERC-related sectors of PE7_5, PE7_6, PE5_9, PE4_5, PE3_12, LS5_11, LS7_2 and LS9_1. In particular, the research activities include fiber sensing system, fiber gratings, optical biosensors, assay development for biomolecule detection, nano-materials for sensors, Bloch surface wave devices, WGM resonators, SPR-based and lossy mode resonance-based devices, bile/pH fiber optic probes, in-fiber random structures, photonic nanostructures and devices for life science. He worked or is actively working in projects at National, European and International levels. He is author of more than 90 publications on the subjects in ISI Journals, in Conference Proceedings and in book chapters. He presented over 9 invited talks at conferences or as lectures (SPIE Student Chapter, INESC-TEC, Porto, 2015; University of Nottingham, UK, 2019; Jinan University, China, 2019), and earned a Journal Cover from MDPI Biosensors (Vol. 7, No. 2, June 2017). He is part of Editorial board of MDPI Optics and of Program Committee of SPIE Photonics Europe conference (“Optical Sensing and Detection” session), and is jury member of several Ph.D. thesis. He acts as confirmed Reviewer in more than 20 top-level ISI journals (Advanced Science, Light: Science & Applications, Optica, Small, Biosensors and Bioelectronics, etc.). He is Member of ACS, OSA, SPIE, EOS, and SIOF. He was awarded the “Short Term Mobility” program 2017 and 2019 by CNR, and as Outstanding Reviewer by IEEE (2018), OSA (2019) and IOP (2020 and 2021).
Publications (33)
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In this study, we demonstrate a multicore fiber interferometer applied for insulin biosensing. This interferometer was produced by splicing a segment of a multicore fiber (MCF) fused to a standard single-mode fiber (SMF). Our investigation depicted that the interferometer's response to changes in refractive index (RI) was influenced by polarization when a thin gold layer was deposited at the end facet of the multicore fiber (MCF). The most significant sensitivity to RI was achieved when using P-polarized light with a ~10 nm-thick gold-coated sample. This gold-coated interferometer was subsequently functionalized for biosensing purposes and employed in the detection of insulin, successfully identifying insulin at a concentration level of 10−8 g/ml.
Continuous monitoring of pH and pressure is a common procedure followed by physicians to acquire a clinical picture in gastroesophageal reflux diseases but also the presence of the bile in the gastroesophageal apparatus has been demonstrated to provide important information associated to the damaging effects of the biliary salts on the esophageal mucosa. Results and performances of optical fiber sensors for the simultaneous measurement of bile and pH with the optical fibers will be described together with all the necessary steps followed to arrive to clinical applications. The activity related to the integration of the bile and pH sensors in a single catheter and to the development of a single catheter for the simultaneous measurement of all the three important parameters (pH, bile and pressure) will be fully discussed.
Esophageal pressure, bile content and pH are important parameters in gastroesophageal diseases. An all-optical technology is described capable to perform simultaneously oesophageal manometry, pH-metry and bilimetry. The three different sensors were integrated in a single optical fibre catheter for the simultaneous measurement of the three parameters. The optoelectronic prototype for the interrogation of the catheter is constituted by two separate optoelectronic modules for the interrogation of the pressure sensor and for the bile and pH sensors, respectively. The prototype and the optical catheter are compliant with European Directives on medical devices in terms of electromagnetic compatibility, electrical safety and biocompatibility.
A lens-less optical fiber designed for enhanced-fluorescence biosensor applications is presented. In order to obtain the enhanced sensor performances, two elements are essential: a planar antenna that redirects fluorescence emission into a narrow cone and an automated fiber-based optical system for multi-spot analysis. In particular, the potential early diagnosis of sepsis via C-reactive protein (CRP) detection is here demonstrated, reaching a limit of detection of 1.5 ng/mL), which is in the clinical range of interest for such biomarker. Upon the combination with other sepsis biomarkers, the presented sensor can become relevant for the early diagnosis of sepsis. These results validate the developed prototype as a simple, affordable, easy-to-operate, plug&play device with fast turnaround times, compatible with standardized micro-well arrays, and potentially suitable for POC applications with respect to the diagnosis of sepsis. It is also suitable for implementation with other biomarkers and liquid environments.
In this work, we report on the refractometric sensing properties of an interferometer produced by fusion-splicing a segment of a seven coupled-core fiber to a standard single-mode fiber. We demonstrate that for a ~10 nm-thick gold layer deposited on the multicore facet, the interferometer amplitude and response to refractive index changes is polarization dependent. We thereby show that refractometric sensitivity is maximum for the polarization-induced minimum amplitude spectrum.
Esophageal pressure, bile content and pH are important parameters in gastroesophageal diseases. An all-optical technology is described to perform simultaneously oesophageal manometry, pH-metry and bilimetry. The pressure measurement along the oesophagus is performed with 11 fibre optic gratings (FBGs) within a single mode fibre, which ensures monitoring every 2.5 cm along the oesophagus. The bile measurement is based on the direct absorption of bilirubin, the main biliary pigment, using a bundle of plastic optical fibres (POFs) carrying the signal. Regarding the detection of pH, an acid-base indicator that modifies its absorption as a function of pH is immobilized on glass particles with controlled porosity immobilized at the end of two POFs. The sensors are integrated in the same catheter for the simultaneous measurement of the three parameters. The prototype for the catheter interrogation was designed and developed, constituted by the integration of two optoelectronic modules for the interrogation of the pressure sensor and for the bile and pH sensors, respectively.
Esophageal pressure, bile content and pH in the gastroesophageal apparatus are important parameters to be monitored in gastroesophageal diseases. An all-optical device was developed for their simultaneous measurement and utilizes a catheter where plastic optical fibres for the measurement of bile and pH and a glass fibre with FBGs for pressure monitoring are integrated. The interrogation device contains two different modules: one for the pressure monitoring based on the measurement of the wavelength shift of the grating peaks, and the other one for bile and pH measurement, based on absorption changes caused by the esophageal content.
Esophageal pressure, bile content and pH in the gastroesophageal apparatus are important parameters to be monitored in gastroesophageal diseases. An all-optical device was developed for their simultaneous measurement and utilizes a catheter where plastic optical fibres for the measurement of bile and pH and a glass fibre with FBGs for pressure monitoring are integrated. The interrogation device contains two different modules: one for the pressure monitoring based on the measurement of the wavelength shift of the grating peaks, and the other one for bile and pH measurement, based on absorption changes caused by the esophageal content.
Gold-coated tilted fiber Bragg gratings (TFBGs) have been extensively studied over the past years, particularly for biosensing purposes. Surface Plasmon Resonance (SPR) is generated through the deposition of a gold layer of appropriate thickness onto the grating region. The combination of SPR and TFBG permits to create a comb-like spectrum of narrow-band cladding mode resonances, which is usually demodulated by tracking the change of optical features of a selected peak. Here, for the first time to the best of our knowledge, a twenty-fold more sensitive demodulation technique based on the intersection of the upper and lower envelopes of gold-coated TFBG spectra is presented. This method has been successfully applied in biosensing for the detection of HER2 (Human Epidermal Growth Factor Receptor-2) proteins, a crucial breast cancer biomarker. Some practical improvements have also been proposed and assessed: first, a uniform FBG has been superimposed on the TFBG to reduce the read-out wavelength span to 10 nm instead of 70 nm, while keeping the temperature-compensated measurements; second, a microfluidic system has been designed and integrated to inject the samples towards the sensor at controlled flow rates. All these novelties make this sensing platform even more attractive and promising for use in practical applications.
The study of the interaction of fibronectin and phosphorylcholine molecules with surfaces is of high relevance to understand the biological performance of bioactive coatings. To accomplish this task, one-dimensional photonic crystals supporting Bloch surface waves were interrogated in label-free and enhanced fluorescence operation modes. In particular, the enhanced fluorescence mode offers the possibility to confirm the presence of proteins with a sharp improvement of the resolution. Bioactive coatings based on fibronectin/ phosphorylcholine have thus the potential to not only enhance the body acceptance of implanted devices, but also extend the lifetime of such devices.
Here, we present a novel label-free biosensor based on fiber optic technology which was tested for the detection of a serum inflammatory marker, the C-reactive protein (CRP). The biosensor is based on a long period grating (LPG) inscribed in a double cladding fiber (DCF) having a W-type refractive index profile. Such DCF fiber permits to tune the sensor working point to the so-called mode transition region through etching of the fiber outer cladding. Therefore, a significant enhancement of the refractive index sensitivity, as well as visibility of the grating spectral features were attained since the mode transition was induced in all-silica fiber structure. Subsequently, the so-prepared LPG was coated with a nano-scale layer of graphene oxide, providing carboxylic functional groups for the covalent immobilization of the biological recognition element for the CRP. As a result, a remarkable limit of detection of 320 pg/mL and a large working range of clinical relevance (0.002-100 μg/mL) were achieved during the real time detection of CRP in human serum.
Sepsis, defined as the systemic inflammatory response to a confirmed or suspected source of infection, is the most severe infection-related condition and its identification can be particularly difficult in the initial stages. The importance of having a Point-of-care testing platform capable of measuring sepsis biomarkers for a secure early-stage diagnosis is evident to reduce delay in treatment and hence recovery period for the patient.
We will report on a simple and cost-effective device which also shows high portability. It is based on the optical detection of labeled essays through a fully-automated fiber probe. Efficient signal collection is obtained by replacing the standard glass substrate with a planar metallo-dielectric multilayer which funnels the emission into a narrow cone around the polar axis [1]. Optical interrogation is implemented with a minimized epi-fluorescence monolithic system directly connected to the fiber.
On one hand, optical probes provide the ability to detect low quantities of target molecules without direct contact to the sample; on the other hand, nano-photonics promises to overcome the limitations related to bulk optics with precise and fragile alignment procedures.
We will report on preliminary results obtained for a reference dry essays (IgG/anti-IgG) marked with ATTO647N, which demonstrates sensitivity overcoming the requirements for CRP-based sepsis detection. We will also discuss optimization steps which are expected to bring sensitivity beyond the level required for PRC-based sepsis detection. The proposed device is also prone to implementation in microfluidic-based protocols.
[1] Checcucci S, Lombardi P., Rizvi S., Sgrignuoli F., Gruhler N., Dieleman F.B.C., Cataliotti F.S., Pernice W.H.P., Agio M., and Toninelli C., Beaming light from a quantum emitter with a planar optical antenna, Light: Science and Applications, Vol. 6, e16245 (2017).
D-dimer is a useful diagnostic biomarker for deep vein thrombosis or pulmonary embolism, collectively referred to as venous thromboembolism (VTE). The ability to detect in real-time the amount of D-dimer with a fast and reliable method is a key step to anticipate the appearance of these diseases. The combination of fiber-optic–based platforms for biosensing with the nanotechnologies is opening up the chance for the development of in situ, portable, lightweight, versatile, reliable and high-performance optical sensing devices towards lab-on-fiber technology. The generation of lossy mode resonances (LMRs) by means of the deposition of nm-thick absorbing metal-oxide films on special geometric-modified fibers allows measuring precisely and accurately surface refractive index changes, which are due to the binding interaction between a biological recognition element and the analyte under investigation. This approach enhances the light-matter interaction in a strong way, thus turning out to be more sensitive compared to other optical technology platforms, such as fiber gratings or surface plasmon resonance. Here, the results of a highly specific and sensitive biosensor for the detection of D-dimer based on LMR in fiber-optics are presented by monitoring in real-time the shift of the LMR related to the biomolecule interactions thanks to a conventional wavelength-interrogation system and an ad-hoc developed microfluidics. A detection limit of 100 ng/mL, a value 5-fold below the clinical cutoff value, has been attained for D-dimer spiked in human serum. The comparison of the results achieved with proteomics-based methodologies, which allows for the identification of betaand gamma-chains of fibrinogen, demonstrates the ability of our platform to specifically (<90%) recognize D-dimer.
The target of the present research is the design, implementation and characterization of a portable device based on an all-optical technology capable to perform simultaneously esophageal manometry, pH-metry and bilimetry by means of a single or combined catheter (OPTIMO project). It will provide physicians a compact and reliable tool to perform exhaustive diagnosis in gastroesophageal reflux pathologies. It would be definitely an innovative product in the gastroesophageal diagnostic scenario with high potential for worldwide market penetration and application developments. In particular, pressure measurement along the esophagus is performed with an array of optical fiber gratings, which ensures the monitoring along a length of about 30 cm with high spatial resolution. The sensors for the measurement of pH and bile are based on the change in absorption caused by the parameter under investigation. In the case of pH, an acid-base indicator that changes its absorption as a function of pH is immobilized on controlled pore glasses suitable coupled to plastic optical fibers. As for bile detection, the measurement is based on the direct absorption of bilirubin, the main biliary pigment, and a bundle of plastic optical fibers carries the signal. With a planned clinical assessment of the realized device, the achievement of this main objective implies the accomplishment of the following sub-tasks: i) development of optical fiber sensor for the measurement of pH; ii) development of the optical fiber sensor for bile detection; iii) development of the optical fiber manometer; iv) integration of the optical fiber sensors within a single catheter; v) realization of the portable interrogation unit for all the three parameters; vi) clinical assessment of the final device on both volunteers and patients.
Sepsis, defined as the systemic inflammatory response to a confirmed or suspected source of infection, is the most severe infection-related condition and its identification can be particularly difficult in the initial stages. The importance of having a POCT platform capable of measuring sepsis biomarkers for a secure early-stage diagnosis is evident since traditional methods of pathogen determination delay treatment and also increase the recovery period for the patient. The biggest advantage of optical probes is the ability to detect low quantities of target molecules without direct contact to the sample. Nanophotonics-based sensing promises to build on the advantages of optical sensing, while overcoming its limitations by providing a high sensitivity, specificity, dynamic range, as well as the possibility for easy integration into simple and affordable devices. The project FASPEC (Fiber-based planar antennas for biosensing and diagnostics) aims at developing and prototyping a high-performance fluorescence-based molecular assay for in-vitro diagnostics that integrates lab-on-a-chip and optical readout functionalities within a single, fully automated platform. The key biophotonics innovation of the project is the replacement of the bulk optics used for collecting the fluorescence signal with a suitably designed optofluidic chip. The latter shall function as an optical antenna to direct fluorescence towards the sensor head, hence enhancing the sensitivity of the fluorescence-based assay by orders of magnitude. Application-specific lab-on-a-chip systems equipped with our high-throughput and ultrasensitive detection scheme have been envisioned.
An ultra-sensitive plasmonic fiber-optic photothermal anemometer is proposed and demonstrated. The device consists of a highly-tilted fiber Bragg grating (TFBG) sensor, which is coated with a gold layer exciting surface plasmon resonance (SPR) and then carbon nanotubes as the photothermal conversion element. The carbon nanotubes deposited on the sensor surface efficiently convert light from the heating laser, which is wavelength matched to the SPR signature, into heat. Air flow draws away the surface heat, thus inducing both a strong SPR wavelength shift and a changed of the power modulation. Using this approach, the proposed anemometer accounts for a dynamic range from 0.05 m/s to 0.65 m/s for wind speed measurement. In addition, the real-time monitoring of wind speed has been proved by measuring the intensity of the heating laser source. This device is a valuable candidate for a wide range of potential applications in both scientific research and industrial production, given its simplicity and robustness in structure.
The combination of fiber-optic–based platforms for biosensing with nanotechnologies is opening up the chance for the development of in situ, portable, lightweight, versatile, reliable and high-performance optical sensing devices. The route consists of the generation of lossy mode resonances (LMRs) by means of the deposition of nm-thick absorbing metaloxide films on special geometric-modified fibers. This allows measuring precisely and accurately the changes in surface refractive index due to the binding interaction between a biological recognition element and the analyte, with very high sensitivity compared to other optical technology platforms, such as fiber gratings or surface plasmon resonance. The proposed methodology, mixed with the use of specialty fiber structures such as D-shaped fibers, allows improving the light-matter interaction in a strong way. The shift of the LMR has been used to monitor in real-time the biomolecule interactions thanks to a conventional wavelength-interrogation system and an ad-hoc developed microfluidics. A big leap in performance has been attained by detecting femtomolar concentrations in real samples of human serum. The biosensor regeneration has been also studied by using a solution of sodium dodecyl sulphate (SDS), proving the device reusability. Therefore, this technology possibly represents a paradigm shift in the development of a simple, high-specificity and label-free biosensing platform, which can be applied to speed up diagnostic healthcare processes of different diseases toward an early diagnostic and personalized treatment system.
KEYWORDS: Waveguides, Luminescence, Absorption filters, Interference filters, Optical filters, Fluorescence spectroscopy, Signal detection, Microfluidics, Signal to noise ratio
Filtering strategies are a crucial aspect for signal detection in many fluorescence based systems such as chemical and/or biochemical sensors. The design, fabrication and characterization of a new waveguide absorption filter for the optimization of the fluorescence signal collection, thanks to its high numerical aperture, is here presented. The absorption filter is designed to work as an optical waveguide in order to increase the optical path and, consequently, the absorption of the excitation light. A comparison of the performances of the absorption filter and a conventional interference filter, with particular emphasis on the angular dependence of the spectral features, is also reported. We experimentally demonstrate, for what regards the attenuation capability of the excitation signal, the failure of the interference filter for incidence angles greater than 15° and the validity of the absorbing waveguide filter for large incidence angles. Finally, preliminary results performed in fluorescence on an IgG labelled/ anti-IgG assay show the improvement in detected fluorescence intensity collected by means of the proposed absorption filter compared to that measured with the interference filter. This suggests that the filtering strategy based on the waveguide absorption filter can greatly simplify fluorescence detection systems and find interesting applications in different areas of sensing, from Point of Care Testing (POCT) to environmental monitoring.
The paper illustrates both review and original simulation results obtained via the modelling of different set-ups based on optical microresonators for applications in optical sensing, lasing and spectroscopy. Passive microbubbles and microspheres coupled via long period fiber gratings (LPGs) and tapered fibers are designed and/or constructed for sensing of biological fluids in the near infrared (NIR) wavelength range. Rare earth doped chalcogenide glass integrated microdisks are designed for active sensing in the medium infrared (MIR) wavelength range. A home-made numerical code modelling the optical coupling and the active behavior via rate equations of ion population is employed for a realistic design, by taking into account the most important active phenomena in rare earths, such as the absorption rates, the stimulated emission rates, the amplified spontaneous emission, the lifetime and branching ratios, the ion-ion energy transfers and the excited state absorption. Optical coupling is obtained by employing ridge waveguides, for micro-disks, and tapered fibers, for microspheres and microbubbles. Different dopant rare earths as Erbium (Er3+) and Praseodymium (Pr3+) are considered.
The last twenty years have seen the increasing demand by physicians of devices able to carry out fast and reliable measurements of chemical and biochemical parameters beside the patient’s bed so as to allow the formulation of a rapid and reliable diagnosis and/or the choice of the most appropriate therapy, avoiding the need for analysis of centralized laboratories. These are the so-called Point of Care Testing (POCT) devices that are becoming essential for the analysis of many diseases, where a quick medical attention is crucial for the patient's life.
Optical biosensors and chemosensors can definitely play a fundamental role in this area and the use of optical fibers as optical links can also lead to invasive continuous measurements within the human body. The determination of one single parameter is sometimes sufficient, but it is important to emphasize that it is often necessary to monitor a panel of biomarkers associated to the onset and/or to the development of a definite pathology and, in this context, the optical biochip can play an essential role in the development of POCT equipment. The activity developed at the Institute of Applied Physics in this field in strict collaboration with physicians is described with particular attention to the measurement of bile-containing refluxes in the gastroesophageal apparatus in non- hospitalized patients, to the detection of gastric carbon dioxide in intensive care patients, to the simultaneous measurement of sepsis biomarkers in serum samples and to the measurements of immuno-suppressants in transplanted patients.
The first example of an optical sensor platform based on surface plasmon resonance (SPR) in a plastic optical fiber (POF) integrated into a thermo-stabilized flow cell for biochemical sensing applications is proposed. In this work, an IgG/anti-IgG assay was implemented as model bioassay, with the IgG biolayer deposited on the sensor gold surface and the biological target, anti-IgG, transported through a new thermo-stabilized flow cell. The experimental results show that the proposed device can be successfully used for label-free biochemical sensing. This complete optical sensor system can be used for the future reduction of the device cost and dimension, with the possibility of integrating the POF-SPR sensing platform with microfluidic and optoelectronic devices.
A novel optical fiber coupler to whispering gallery mode (WGM) micro-resonators, which allows frequency selective addressing of different micro-resonators along the same fiber, is proposed. The coupling unit is based on a pair of identical long period fiber gratings (LPGs) and a thick adiabatic taper (>15 μm in waist) in between, where evanescent coupling from cladding modes to WGMs takes place. This robust unit can be replicated more times along the same fiber, simply cascading LPGs with different bands. Independent addressing of two different resonators along the same fiber is demonstrated.
Long period fiber gratings (LPGs) have recently been proposed as label-free biosensors. A biochemical interaction
occurring along the grating region can be evaluated as a refractive index (RI) change, which modifies the transmission
spectrum of the fiber. This is an emergent, alternative choice with respect to other label-free optical systems, such as
surface plasmon resonance, interferometric and in-fiber configurations, and resonating structures. In this work, various
types of not-coated LPGs, in which the coupling occurs with increasing cladding mode orders, were manufactured for
increasing the RI sensitivity of these sensors. After the functionalization of the fiber surface using Eudragit L100
copolymer, a label-free IgG/anti-IgG bioassay was realized for analyzing the antigen/antibody interaction following the
same model assay. A comprehensive feasibility study was carried out among the different LPGs in order to assess and
compare the biosensor performance, highlighting the advantages and the disadvantages of each type. Experimental
results proved an improvement in the RI sensitivity and in the biosensor performance in the case of high-order cladding
mode LPGs, with values of detection limit lower than 50 ng mL-1 (330 pM). The performance enhancement can be
explained with the increase in the penetration depth of the evanescent field due to the increase of the cladding mode
order. The sensor response was also studied using complex matrices made up of human serum.
Soluble urokinase plasminogen activator receptor (suPAR) is an inflammatory protein present in blood and a marker of
disease presence, severity and prognosis. A heterogeneous sandwich assay is proposed for quantifying suPAR by
employing a capture antibody from rat and a biotinylated detection antibody from mouse. Optical detection was achieved
by a successive exposure of the biotinylated sandwich to streptavidin labelled with ATTO647N. The heterogeneous
assay was implemented on a multichannel polymethylmetacrylate (PMMA) optical biochip, potentially capable of the
simultaneous detection of more than one analyte. Capture antibody was immobilized on the PMMA surface of the
microfluidic channel and the assay was performed with the following protocol: i) surface blocking with BSA, ii)
incubation with suPAR or PBS, iii) incubation with biotinylated suPAR detection Ab and iv) incubation with
streptavidin-ATTO647N. Promising preliminary results were obtained with this protocol. Moreover, an improved optical
setup is proposed which avoids the mechanical scanning of the chip and consequently the in-series fluorescence
excitation and read out, allowing the simultaneous measurement of the fluorescence on all the channels of the
microfluidic chip.
A novel method based on long period fiber gratings (LPGs) for coupling light to high-Q silica whispering gallery mode
(WGM) resonators is presented. An LPG couples the fundamental mode of a fiber to higher order LP cladding modes at
selected frequencies. At an adiabatically tapered section of the fiber following the LPG we demonstrated effective
coupling of these cladding modes to WGMs both in silica microspheres and microbubbles. The taper is about one order
of magnitude thicker than standard tapers used for the same purpose. Therefore this new method offers improved
robustness for practical applications.
A novel configuration of long period fiber grating (LPFG), based on a specially designed refractive index (RI) profile, was manufactured and studied. The internally manufactured geometric structure is characterized by grating planes tilted at increasing angles, as moving away from the center of symmetry of the structure towards its both edges. This structure reproduces the bending of an optical fiber and improves the RI sensitivity of an LPG to the external surrounding medium. A three-fold enhancement in the RI sensitivity was experimentally proved, thus giving a further contribution towards the development of more sensitive RI sensors based on optical fiber LPGs.
Full exploitation of the unique properties of high quality factor micro-optical Whispering Gallery Mode (WGM) resonators requires a controllable and robust coupling of the light to the cavity, either for fundamental investigations or even more for practical applications. Fiber tapers are ideal phase-and-mode-matched couplers and are typically used for lab demonstrations in silica based micro-resonators or in low-index crystalline disks. Prism-based coupling basically adapts to any material and offers improved robustness and reliability for the implementation of devices based on larger resonators. We present the results of our studies on alternative methods based on integrated waveguides with specific reference to the coupling to lithium niobate disk resonators. We also demonstrate efficient coupling from fiber tapers to higher order azimuthal modes in coated microspheres and for third harmonic generation in silica microspheres. We finally propose a new method based on fiber gratings for improved robustness in biosensing applications.
Long period fiber gratings (LPFGs) have been proposed as label-free optical biosensor for a few years. Refractive index changes, which modify the fiber transmission spectrum, are still used for evaluating a biochemical interaction that occurs along the grating region. A turn-around point (TAP) LPFG was manufactured for enhancing the refractive index sensitivity of these devices. Considering the simplicity and the fast process with respect to the silanization procedure, the functionalization of the fiber was carried out by Eudragit L100 copolymer. An IgG/anti-IgG immunoassay was implemented for studying the antigen/antibody interaction. A limit of detection lower than 100 μg L-1 was achieved. Based on the same model assay, we compared the resonance wavelength shifts during the injection of 10 mg L-1 anti-IgG antigen between the TAP LPFG and a standard non-TAP one, in which the coupling occurs with a lower order cladding mode, as performance improvement of the LPFG-based biosensors.
Monitoring pH for long periods, usually 24 h, in the stomach and in the esophagus may be essential in the diagnosis of gastro-esophageal diseases. The clinical range of interest is quite extended, between 1 to 8 pH units. Methyl red, after its covalent immobilization on controlled pore glass (CPG), is characterized by a working range which fits well with the clinical one. A novel probe, suitable for gastro-esophageal applications, was designed in order to optimize the performances of the colored CPG. This leads to a very simple probe configuration characterized by a very fast response.
Long period gratings have been recently proposed as label-free optical devices for biochemical sensing. A biochemical interaction along the grating region changes the biolayer refractive index and a change in the fiber transmission spectrum occurs. The fiber biofunctionalization was performed with a novel chemistry using Eudragit L100 copolymer as opposed to the commonly-used silanization procedure. An IgG/anti-IgG bioassay was carried out for studying the antigen/antibody interaction. The biosensor was fully characterized, monitoring the kinetics during the antibody immobilization and achieving the calibration curve of the assay. To compare the biosensor performance, two LPG-based biosensors with distinct grating periods were characterized following the same bioassay protocol. Experimental results demonstrated an enhancement of the biosensor performance when the fundamental core mode of a single-mode fiber couples with a higher order cladding mode. Considering an LPG manufactured on a bare optical fiber, in which the coupling occurs with the 7-th cladding mode, a dynamic signal range of 0.33 nm, a working range of 1.7 – 1450 mg L-1 and a LOD of 500 μg L-1 were achieved
The combination of a long period grating and a fiber Bragg grating written on the same fiber is described as method to
reduce noticeably the interferences caused by strain and temperature in the measurement of refractive index. The hybrid
LPG and FBG optical fiber sensor is manufactured and located in a small volume flow cell. The whole system with its
flow cell and the gratings fabrication are extensively described as well as both the acquisition and data processing. The
maximum sensor sensitivity and resolution are about 3120 nm/RIU and 2 x 10-5 RIU, respectively.
KEYWORDS: Fiber Bragg gratings, Refractive index, Temperature metrology, Sensors, Aluminum, Polymethylmethacrylate, Chemical elements, Signal attenuation, Chemical analysis, Water
Long period gratings (LPGs) have been recently proposed as sensing elements of chemical/biological compounds,
exploiting their sensitivity to the refractive index changes in the surrounding environment. One of the difficulties of their
utilization for this purpose is their strong dependence also to strain and temperature effects. An intrinsic optical feedback
able to eliminate these effects was developed by manufacturing on the same fiber the LPG and a fiber Bragg grating
(FBG) which is immune from external refractive index changes and is influenced by strain and temperature. An accurate
temperature measurement system is utilised to eliminate or in any case to reduce to a minimum the interferences coming
from temperature changes. A KrF excimer laser is used to write both the gratings into the same photosensitive fiber. The
period of the LPG and FBG gratings are 615 μm and 530 nm, respectively and the attenuation at their resonance
wavelengths (1570 nm for LPG and 1534 nm for FBG) was of the order of 15-20 dB. The same source, a broadband
superluminescent diode with emission peak at 1550 nm, is used to interrogate both the gratings. The transmission spectra
is acquired by means of an optical spectrum analyzer (OSA) controlled by a PC and an in-house software identifies the
attenuation band in the FBG and LPG transmission spectra and calculates the minimum values. A suitable thermostabilized
flow cell with a volume of 50 μL containing the fiber with the two gratings, has been developed and
characterized.
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