Over the last years, fiber optic sensors have been increasingly applied for applications in environments with a high level of radiation as an alternative to electrical sensors, due to their: high ...immunity, high multiplexing and long-distance monitoring capability. In order to assess the feasibility of their use, investigations on optical materials and fiber optic sensors have been focusing on their response depending on radiation type, absorbed dose, dose rate, temperature and so on. In this context, this paper presents a comprehensive review of the results achieved over the last twenty years concerning the irradiation of in-fiber Long Period Gratings (LPGs). The topic is approached from the point of view of the optical engineers engaged in the design, development and testing of these devices, by focusing the attention on the fiber type, grating fabrication technique and properties, irradiation parameters and performed analysis. The aim is to provide a detailed review concerning the state of the art and to outline the future research trends.
In this work, we report about our recent results concerning the fabrication of Long Period Grating (LPG) sensors in several optical fibers, through the Electric Arc Discharge (EAD) technique. In ...particular, the following silica fibers with both different dopants and geometrical structures are considered: standard Ge-doped, photosensitive B/Ge codoped, P-doped, pure-silica core with F-doped cladding, Panda type Polarization-maintaining, and Hollow core Photonic crystal fiber. An adaptive platform was developed and the appropriate "recipe" was identified for each fiber, in terms of both arc discharge parameters and setup arrangement, for manufacturing LPGs with strong and narrow attenuation bands, low insertion losses, and short length. As the fabricated devices have appealing features from the application point of view, the sensitivity characteristics towards changes in different external perturbations (i.e., surrounding refractive index, temperature, and strain) are investigated and compared, highlighting the effects of different fiber composition and structure.
In this work, we report a fiber optic biosensor for the label-free detection of vitamin D, by targeting the major circulating form of D3 in human body, i.e., 25-hydroxyvitamin D3 (25(OH)D3). The ...sensing platform relies on a long period grating (LPG) written into an unconventional fiber, i.e., double cladding fiber (DCF) with W-type refractive index profile. The sensitivity to surrounding medium is enhanced by chemical etching of the outer cladding as the working point of the device is tuned to the mode transition region. The DCF structure allows to combine the enhancement of the sensitivity (up to −1400 nm/RIU) with a good visibility of the grating resonance band (>10 dB) and long-term stability due to an all-silica structure. Moreover, the LPG transducer is coated with a nanosized layer of graphene oxide to provide carboxylic functional groups (-COOH) for the grafting of the biological recognition element for vitamin D3, i.e., a 25(OH)D3 specific antibody. To the best of our knowledge, this is the first evidence of a fiber optic biosensor for the detection of vitamin D3, which was performed using concentrations within the clinically relevant range of 1–1000 ng/mL and achieving a low limit of detection below 1.0 ng/mL in buffer solution that is well comparable with the state of the art. Finally, the performance of the biosensor was also evaluated in a complex medium with interfering proteins at physiological concentration obtaining promising results.
•First evidence of a label-free fiber optic biosensor for the detection of vitamin D.•Optical transducer based on a long period grating (LPG) in double cladding fiber.•Graphene oxide nanosized layer with functional groups for antibody binding.•Detection of 25-hydroxyvitamin D3 (25(OH)D3) concentrations within 1–1000 ng/mL.•Low limit of detection (LOD) below 1 ng/mL.
Optical fibers are well known for their use in high-speed data links and related sensors nowadays find application in different domains, such as structural health monitoring, distributed sensing, but ...also biological and chemical monitoring ...
We report on the investigation of the sensing features of the Long-Period fiber Gratings (LPGs) fabricated in hollow core photonic crystal fibers (HC-PCFs) by the pressure assisted Electric Arc ...Discharge (EAD) technique. In particular, the characterization of the LPG in terms of shift in resonant wavelengths and changes in attenuation band depth to the environmental parameters: strain, temperature, curvature, refractive index and pressure is presented. The achieved results show that LPGs in HC-PCFs represent a novel high performance sensing platform for measurements of different physical parameters including strain, temperature and, especially, for measurements of environmental pressure. The pressure sensitivity enhancement is about four times greater if we compare LPGs in HC and standard fibers. Moreover, differently from LPGs in standard fibers, these LPGs realized in innovative fibers, i.e., the HC-PCFs, are not sensitive to surrounding refractive index.
In this paper, we report about the fabrication of long period gratings (LPGs) directly into an Erbium-doped fiber, by using the electric arc discharge technique. The attention is focused on the ...writing process and the resulting properties, by considering gratings with different periods and measuring their spectra over a wide wavelength range. The LPGs show high order attenuation bands with tunable resonance wavelengths and depths up to 30 dB, while the lengths of the final devices are in range 20-45 mm. The polarization-dependent loss of these LPGs is also measured, for the first time in this kind of fiber. As further novelty, the influence of surrounding refractive index, applied strain, and temperature is investigated and discussed in comparative manner. Based on the achieved results, this fabrication procedure can be adapted to a specific application, for example in optical communications, signal processing, and sensing fields.
In this paper, we present the design and analysis of chirped fiber Bragg grating (CFBG) sensors, optimized for temperature measurements. The transfer matrix method has been used to simulate the CFBGs ...and to study their thermal response. The simulations have been carried out with different temperature profiles in order to understand how the CFBG thermal response varies as a function of the grating design parameters and, thus, to optimize the design for the specific application. Finally, to assess the numerical simulations, a series of experimental tests was performed showing very good agreement.
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.
Bones are a frequent site of metastases that cause intolerable cancer-related pain in 90% of patients, making their quality of life poor. In this scenario, being able to treat bone oncology patients ...by means of minimally invasive techniques can be crucial to avoid surgery-related risks and decrease hospitalization times. The use of microwave ablation (MWA) is gaining broad clinical acceptance to treat bone tumors. It is worth investigating temperature variations in bone tissue undergoing MWA because the clinical outcomes can be inferred from this parameter. Several feasibility studies have been performed, but an experimental analysis of the temperature trends reached into the bone during the MWA has not yet been assessed. In this work, a multi-point temperature study along the bone structure during such treatment is presented. The study has been carried out on ex vivo bovine femur and tibia, subjected to MWA. An overall of 40 measurement points covering a large sensing area was obtained for each configuration. Temperature monitoring was performed by using 40 fiber Bragg grating (FBGs) sensors (four arrays each housing 10 FBGs), inserted into the bones at specific distances to the microwave antenna. As result, the ability of this experimental multi-point monitoring approach in tracking temperature variations within bone tissue during MWA treatments was shown. This study lays the foundations for the design of a novel approach to study the effects of MWA on bone tumors. As consequence, the MWA treatment settings could be optimized in order to maximize the treatment effects of such a promising clinical application, but also customized for the specific tumor and patient.
This work reports on the use of Fiber Bragg Grating (FBG) sensors integrated with innovative composite items of aircraft landing gear for strain/stress monitoring. Recently, the introduction of ...innovative structures in aeronautical applications is appealing with two main goals: (i) to decrease the weight and cost of current items; and (ii) to increase the mechanical resistance, if possible. However, the introduction of novel structures in the aeronautical field demands experimentation and certification regarding their mechanical resistance. In this work, we successfully investigate the possibility to use Fiber Bragg Grating sensors for the structural health monitoring of innovative composite items for the landing gear. Several FBG strain sensors have been integrated in different locations of the composite item including region with high bending radius. To optimize the localization of the FBG sensors, load condition was studied by Finite Element Method (FEM) numerical analysis. Several experimental tests have been done in range 0-70 kN by means of a hydraulic press. Obtained results are in very good agreement with the numerical ones and demonstrate the great potentialities of FBG sensor technology to be employed for remote and real-time load measurements on aircraft landing gears and to act as early warning systems.
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