Response Of Long-period Gratings Research Articles

CHARACTERIZATION OF LONG PERIOD GRATINGS MANUFACTURED WITH FIBER OPTIC FUSION SPLICER FOR SENSOR DEVELOPMENT

A Long Period Grating (LPG) is a structure inscribed in an optical fiber that is of great interest in the field of communications and sensors. At Centro de Investigaciones Ópticas (CIOp), an LPG manufacturing process was implemented. It is based on use of electric arcs produced with an optical fiber splicer to generate a series of physical disturbances that modify optical fiber behavior as a light transmission guide. Since transmission response of LPG is sensitive to fiber environment conditions, it can be used to detect changes in certain physical and chemical parameters. In particular, it enables the implementation of optical sensors sensitive to temperature, surrounding medium refractive index and axial deformation. Characterization of locally generated LPGs is essential for future design of sensors applicable in many engineering areas. In this article, typical LPG sensitivities to the three mentioned parameters are reported.

Water-Induced Fused Silica Glass Surface Alterations Monitored Using Long-Period Fiber Gratings

Long-period gratings (LPGs) induced in optical fibers show exceptional refractive index (RI) sensitivity and thus they are often applied for label-free biosensing. However, during measurements at changing environmental conditions, the surface of the fiber made of fused silica is subjected to stresses that can cause alterations in its structure. The problem is particularly important when long-term or biosensing measurements are considered and when sensor surface may be exposed to subsequent drying and immersion in aqueous solutions. The aim of this paper was to investigate the influence of sequential drying in air and soaking in water of the fused silica fiber cladding surface on the optical response of LPGs. We demonstrated that transmission spectrum of LPG measured in water before and after drying in air differed significantly and further changed with subsequent drying steps indicating increase of the local RI. All these changes correspond to the alterations in the cladding surface, i.e., its fracturing and pores rearrangement, caused by the stress induced during drying and then immersing in water, as well as possible corrosion and formation of nanosized objects on the cladding surface. The effect can be additionally influenced by washing the samples in organic solvents. We confirmed that measurements done in a flow-cell system, where the sensor was kept wet during the sensing experiment may eliminate the water-induced fiber surface alteration effect and thus minimize the amount of false results. Described findings are highly important for biosensing applications of any optical-fiber-based devices.

Comparative Study of Long-Period Gratings Written in Standard and Fluorine-Doped Fibers by Electric Arc Discharge

In this paper, we present a comparative study of long-period gratings (LPGs) written in the standard Ge-doped fiber and in two different fluorine-doped fibers by means of the electric arc discharge (EAD)-based technique. Concerning Ge-doped fibers, we present an assessment of the EAD procedure by fabricating relatively short LPGs with deep attenuation bands (up to 32 dB) and trivial power losses. We also demonstrate the ability to manage the effect of the single EAD perturbation to select grating length, which in turn acts on the bandwidth of attenuation bands, without compromising depth. Furthermore, for the first time, we produced LPGs in F-doped fibers with attenuation bands deeper than 30 dB, demonstrating the dependence of the spectra on the grating period. Finally, we illustrate a comparative experimental study on the sensitivity of LPGs fabricated in the standard and F-doped fibers with surrounding refractive index (SRI) and temperature changes. We proved that the SRI response of LPGs in F-doped fibers is significantly higher than in the standard fiber and it strongly depends on the type of F-doped fiber considered. LPGs written in F-doped fibers exhibit a slightly lower temperature sensitivity (in the range 20–30 pm/°C for $\lambda _{\mathrm {res2}}$ cladding mode) than the LPG in the standard fiber (45 pm/°C).

Response of Strongly Over-Coupled Resonant Mode of a Long Period Grating to High Refractive Index Ambiance

Over-coupling of the resonant band of interest has been found to have significant influence on the refractive index sensitivity of long period grating (LPG) for surrounding material having refractive index higher than the cladding material. In this paper we report, for the first time to the best of our knowledge, a quantitative analysis using the concept of radiation mode coupling that elaborates the influence of initial coupling strength on the response of LPG to high refractive index surrounding. The simulated results agree well with those obtained experimentally where it could be shown that that a suitably over-coupled resonant mode of LPG can have a sensitivity ~724 dB/RIU in the range of the ambient refractive index which is slightly higher than the cladding where the conventional LPGs have almost no sensitivity.

Theoretical and Experimental Study of Long-Period Grating Refractive Index Sensor

This article presents the theoretical and experimental investigation of the response of long-period gratings as a refractive index sensor. Cladding modes are calculated, and results are compared with Optigrating 4.2.2 (Optiwave Systems Inc., Ottawa, Canada). The response has been checked for refractive indices ranging from 1 to 1.458. Theoretically simulated results are in accordance with the experimental results. It was found that the software package calculated values correctly up to the seventh decimal point. The ambient refractive index response of a long-period grating over a much wider index range has been modeled for values both less and more than the cladding refractive index.

Study and investigation of long period grating as refractive index sensor

In this paper, we have investigated the response of long period grating (LPG) as refractive index sensor. The response has been studied for refractive index variation ranging from 1 to 1.45. In this work, we found that the sensing mechanism is based on two different aspects. First is the change of coupling power from the guided core mode to other co-propagating cladding modes and second is the wavelength shift of the peak resonant wavelengths from their original positions due the change of refractive index of the environmental (external) medium surrounding the cladding of the grating.

Investigation of Long Period Grating as refractive index sensor

In this paper, we have theoretically and experimentally investigated the response of Long Period Grating (LPG) as refractive index sensor. The response has been checked for the refractive index ranging from 1 to 1.458. It was found that the experimental results are very close to the theoretically simulated results. The ambient refractive index response of an LPG over a much wider index range has been modeled for values both less and more than the cladding refractive index.

Monitoring the quality of frying oils using a nanolayer coated optical fiber refractometer

The analysis of the quality of food oils is of paramount importance, because the degradation of oils can lead to formation of harmful substances to the human organism. With the increase of the degradation of oils an increase of its refractive index occurs. The objective of this work is to develop and to characterize optical fiber refractometers sensitive to variations of refractive index of food oil samples. The optical fiber refractometers thanks to the intrinsic characteristics make them suitable for monitoring the quality of frying oils. They possess the advantages to require small volumes of sample for analysis, do not contaminate the sample, and supply the response in real time. In this work a long period grating (LPG) as refractometer is used because of their sensitivity to refractive index of the external media: degraded and not degraded frying oil samples. The oil samples had been characterized by the analysis of total polar components. The refractive index of oil is above 1.47, this region the LPG does not show enough sensitivity, a nanolayer of an organic material was coated onto the fiber. Using the Langmuir–Blodgett technique the response of LPG is modified according to the refractive index and thickness of the film. The deposition of the film modifies the rates effective modes of cladding, thus improving the response of the changes in the refractive index of the external media higher than that the refractive index of the cladding ( n = 1.457).

Chloride ion optical sensing using a long period grating pair

In this work, an investigation into the use of a compact non-coated long period grating (LPG) pair applied to chloride ion monitoring in known aqueous solutions over the few parts per million (ppm) region is presented. The LPG pair used in this work (LPGP) comprised two 2.5 dB single LPGs, with a physical centre-to-centre separation of around 30 mm, fabricated using an excimer ultraviolet laser via a 250 μm amplitude mask. The basis of the measurement using the LPGP was refractive index monitoring, with the sensor being calibrated using solutions of known refractive indices (RIs). Tests were carried out on aqueous solutions, containing sodium chloride (NaCl) at various concentrations ranging from 0 to 23.2 g/100 ml. Cross comparison with the response of a single LPG (LPG1) under similar testing conditions was also carried out. The results have shown that through monitoring the induced wavelength shift, the LPGP could measure a RI change of ±0.003, reaching a precision of ∼6 times better than what was achieved with a single LPG. In addition, it was shown that the LPGP can provide additional information, through the induced spectral variations of the interference fringes created, as a result of the small RI changes arising from the changes in the NaCl concentration. The spectral analysis was carried out by applying a Fast Fourier Transform to the spectra under study and small quantities of chloride ions (10 ppm) can thus be detected using the LPGP sensor. The work has shown the potential for applications in early stage corrosion monitoring (and indeed prevention), for example for marine or transport structures, due to chloride ion presence and ultimate ingress.

Long period grating pair chloride ion sensor for early corrosion prevention

A novel approach to chloride ion monitoring in aqueous solutions using a long period grating (LPG) pair is reported, with the LPG pair used (LPGP) comprising two 2.5dB single LPGs and a physical centre-to-centre separation of around 30mm. An ultra-violet excimer laser was used to create the pair through a 250μm amplitude mask. The refractive index change due to the presence of the chloride ion (from an aqueous NaCl solution) was monitored, with the sensor having previously been calibrated using solutions of known refractive indices (RIs). A cross-comparison with the response of a single LPG (LPG1) under similar testing conditions was also carried out, and the results have shown that the LPGP could measure a RI change of ±0.003, achieving an improvement in the precision obtained (by ∼ 6 times) using a single LPG. The focus of the work is on the underpinning technology for early stage corrosion monitoring (and indeed prevention) in structures, arising due to salt and water ingress.

Modification of the refractive index response of long period gratings using thin film overlays

The response of a fibre optic long period grating (LPG) to the surrounding refractive index is shown to be modified by the deposition of a thin film of thickness of order 100 nm onto the cladding. The LPG becomes sensitive to surrounding refractive indices greater than that of silica, and exhibits an enhanced sensitivity to external refractive indices lower than that of silica.