KEYWORDS: 3D printing, Signal attenuation, 3D modeling, Cladding, Optical fibers, Fabrication, Printing, Single mode fibers, Optical gratings, Structured optical fibers
In this work we present mechanically induced Long Period Gratings (LPGs) realized in different kinds of glass optical fibers by using UV-cured 3D printed periodic grooved plate. The periodic modulation of the geometry and refractive index along the length of the optical fiber is induced by pressure applied transversely to the grooved plate and leads to light coupling between the core mode and co-propagating cladding modes at specific resonance wavelengths. Using the proposed method, we realize and analyze in detail the behavior of the gratings induced in different optical fibers (i.e., SMF-28 and two different double cladding fibers), with and without acrylate coating. Results confirm that the depth of the attenuation bands in the transmission spectrum of the gratings, which were induced by gradually increasing the applied load show high tunability in the range of 1400-1650 nm with the maximum depth of 24 dB. Among the methods used for the fabrication of LPGs, the one studied here is reversible, very low cost, easy to tune, compatible with different kind of fibers and does not require a specific preparation of the fiber. The obtained devices are potentially useful for several application spanning from filters to sensors.
This work proposes an in-fiber Mach-Zehnder interferometer (MZI) device which is fabricated by embedding a short section of double cladding fiber (DCF) in between two standard fibers utilising the core-offset splicing approach. Moreover, the DCF has a W-shaped refractive index (RI) profile, where the outer cladding has RI higher than inner one and core. Consequently, by modifying the thickness of the outer cladding, the mode transition of cladding modes from outer to inner cladding can be induced, with the possibility to tune the sensitivity to surrounding refractive index. Specifically, the outer cladding diameter was decreased by means of chemical etching down to a diameter of 112 µm and a sensitivity of -200 nm/RIU was achieved in the range 1.33-1.39, with a 2.5 gain in comparison to unetched fiber. The proposed sensing device has considerable compactness, low manufacturing cost and simplicity, as well as high sensitivity for future applications in chemical and biosensing domain and other related fields.
The investigation of impact of ionizing radiation on photonic waveguides and devices is of a large interest due to new demanding applications in harsh environments, such as space or high-energy-physics experiments and more. Thus, the effects of gamma radiation on refractive index inducing propagation loss different optical fibers are investigated by means of several approaches including fiber Bragg gratings and long period gratings (LPG). In this work, we report the results on exposure to gamma irradiation up to tens of kGy of LPGs written in single-mode optical fibers with unconventional dopants, such as B and P. The LPGs in a reflective configuration have been written using the electric arc discharge technique. The attention is focused on the real-time measurement of LPG resonance, i.e., wavelength shift and transmission power changes during irradiation, as well as the recovery effects after the irradiation. It has been found that the impact of gamma is significantly dependent on the fiber type and the LPG properties. The LPGs in B/Ge co-doped fiber show both the higher resonant wavelength shift and low power losses. Such results are useful for those working with optical fibers and related sensors in environments exposed to radiations.
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.
Here we report about mode transition phenomenon occurring in uncoated Long Period Grating (LPG) and the subsequent possibility of enhancing the surrounding refractive index sensitivity. For this purpose, we have considered a grating realized in double cladding fiber with W-type refractive index profile. This fiber presents an outer cladding with refractive index higher than inner cladding one; thus, through the chemical etching of the outer diameter, the transition phenomenon from outer- to inner-cladding of some cladding modes can be induced in controllable manner. The effect is illustrated by numerical analysis and it is experimentally verified using an LPG written in pure-silica core with F-doped inner cladding and silica outer cladding fiber, by means of the electric arc discharge technique.
The recent results about the fabrication and characterization of Long Period Gratings (LPGs) in different pure silica optical fibers by means of Electric Arc Discharge (EAD) technique are reported in this work. Nowadays, LPG in standard fiber represents a unique platform for physical, chemical and biological sensing whereas specialty optical fibers permit to extend the use of fiber optic technology to unconventional applications. For instance, pure silica fibers are appealing in high energy applications. Here, we take into consideration two fibers with pure-silica core having significant differences in physical and geometrical design. The first one presents a micro-structured pure-silica cladding, photonic crystal fiber, and the second one shows a solid Fluorine-doped cladding. EAD leads to a point-by-point LPG inscription, due to localized tapering of the transversal size of the core and cladding regions along the fiber, and to changes of the silica refractive index due to the stress relaxation induced by local hot spots. The aim of the work is to identify an appropriate “recipe” for each fiber, to fabricate LPGs with strong and narrow attenuation bands and trivial power loss. Indeed, a proper combination of arc power and duration, as well as fiber tension, allows for the appropriate core and cladding modulation and thus for the desired LPG spectral features. The sensitivity characteristics towards surrounding refractive index (SRI) and temperature changes of these LPGs are also investigated.
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