Temperature Dependence of the Thermo-Optic Coefficient of SiO2 Glass.

This paper presents a thorough analysis on the temperature dependence of the thermo-optic coefficient, dn/dT, of four bulk annealed pure-silica glass samples (type I-natural quartz: Infrasil 301; type II-quartz crystal powder: Heraeus Homosil; type III-synthetic vitreous silica: Corning 7980 and Suprasil 3001) from room temperature down to 0 K. The three/four term temperature dependent Sellmeier equations and respective coefficients were considered, which results from fitting to the raw data obtained by Leviton et al. The thermo-optic coefficient was extrapolated down to zero Kelvin. We have obtained dn/dT values ranging from 8.16 × 10-6 up to 8.53 × 10-6 for the four samples at 293 K and for a wavelength of 1.55 µm. For the Corning 7980 SiO2 glass, the thermo-optic coefficient decreases monotonically, from 8.74 × 10-6 down to 8.16 × 10-6, from the visible range up to the third telecommunication window, being almost constant above 1.3 µm. The Ghosh's model was revisited, and it was concluded that the thermal expansion coefficient only accounts for about 2% of the thermo-optic coefficient, and we have obtained an expression for the temperature behavior of the silica excitonic bandgap. Wemple's model was also analyzed where we have also considered the material dispersion in order to determine the coefficients and respective temperature dependences. The limitations of this model were also discussed.

High Sensitivity Cryogenic Temperature Sensors Based on Arc-Induced Long-Period Fiber Gratings.

In this paper, we investigated the evolution of the dispersion curves of long-period fiber gratings (LPFGs) from room temperature down to 0 K. We considered gratings arc-induced in the SMF28 fiber and in two B/Ge co-doped fibers. Computer simulations were performed based on previously published experimental data. We found that the dispersion curves belonging to the lowest-order cladding modes are the most affected by the temperature changes, but those changes are minute when considering cladding modes with dispersion turning points (DTP) in the telecommunication windows. The temperature sensitivity is higher for gratings inscribed in the B/Ge co-doped fibers near DTP and the optimum grating period can be chosen at room temperature. A temperature sensitivity as high as -850 pm/K can be obtained in the 100-200 K temperature range, while a value of -170 pm/K is reachable at 20 K.

Simulation of the Transmission Spectrum of Long-Period Fiber Gratings Structures with a Propagating Acoustic Shock Front.

In this paper, we investigate modification of transmission spectra of long-period fiber grating structures with an acoustic shock front propagating along the fiber. We simulate transmission through inhomogeneous long-period fiber gratings, π-shift and reflective π-shift gratings deformed by an acoustic shock front. Coupled mode equations describing interaction of co-propagating modes in a long-period fiber grating structures with inhomogeneous deformation are used for the simulation. Two types of apodization are considered for the grating modulation amplitude, such as uniform and raised-cosine. We demonstrate how the transmission spectrum is produced by interference between the core and cladding modes coupled at several parts of the gratings having different periods. For the π-shift long-period fiber grating having split spectral notch, the gap between the two dips becomes several times wider in the grating with the acoustic wave front than the gap in the unstrained grating. The behavior of reflective long-period fiber gratings depends on the magnitude of the phase shift near the reflective surface: an additional dip is formed in the 0-shift grating and the short-wavelength dip disappears in the π-shift grating.

Optical Fiber Interferometers Based on Arc-Induced Long Period Gratings at INESC TEC.

In this work, we review the most important achievements of an INESC TEC long-period-grating-based fiber optic Michelson and Mach-Zehnder configuration modal interferometer with coherence addressing and heterodyne interrogation as a sensing structure for measuring environmental refractive index and temperature. The theory for Long Period Grating (LPG) interferometers and coherence addressing and heterodyne interrogation is presented. To increase the sensitivity to external refractive index and temperature, several LPG interferometers parameters are studied, including order of cladding mode, a reduction of the fiber diameter, different type of fiber, cavity length and the antisymmetric nature of cladding modes.

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part II: Properties and Applications in Optical Communications and Sensing.

In this work, we review the most important achievements of INESC TEC related to the properties and applications of arc-induced long-period fiber gratings. The polarization dependence loss, the spectral behavior at temperatures ranging from cryogenic up to 1200 °C and under exposure to ultraviolet and gamma radiation is described. The dependence of gratings sensitivity on the fabrication parameters is discussed. Several applications in optical communications and sensing domains are referred.

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part I: Fabrication, Characterization and Mechanisms of Formation.

In this work, we reviewed the most important achievements of INESC TEC related to the fabrication of long-period fiber gratings using the electric arc technique. We focused on the fabrication setup, the type of fiber used, and the effect of the fabrication parameters on the gratings' transmission spectra. The theory was presented, as well as a discussion on the mechanisms responsible for the formation of the gratings, supported by the measurement of the temperature reached by the fiber during an electric arc discharge.

Fiber optic-based refractive index sensing at INESC Porto.

A review of refractive index measurement based on different types of optical fiber sensor configurations and techniques is presented. It addresses the main developments in the area, with particular focus on results obtained at INESC Porto, Portugal. The optical fiber sensing structures studied include those based on Bragg and long period gratings, on micro-interferometers, on plasmonic effects in fibers and on multimode interference in a large spectrum of standard and microstructured optical fibers.

Fiber optic hot-wire flowmeter based on a metallic coated hybrid long period grating/fiber Bragg grating structure.

In this work an all-optical hot-wire flowmeter based on a silver coated fiber combining a long period grating and a fiber Bragg grating (FBG) structure is proposed. Light from a pump laser at 1480 nm propagating down the fiber is coupled by the long period grating into the fiber cladding and is absorbed by the silver coating deposited on the fiber surface over the Bragg grating structure. This absorption acts like a hot wire raising the fiber temperature locally, which is effectively detected by the FBG resonance shift. The temperature increase depends on the flow speed of the surrounding air, which has the effect of cooling the fiber. It is demonstrated that the Bragg wavelength shift can be related to the flow speed. A flow speed resolution of 0.08 m/s is achieved using this new configuration.

Characterization of the response of a dual resonance of an arc-induced long-period grating to various physical parameters.

We present results on the characterization of the response of a dual resonance observed in the spectrum of a single long-period grating arc-induced in a B/Ge co-doped fiber to different physical parameters. The dual resonance is formed by two overlapping resonances corresponding to coupling of the core mode to symmetric and antisymmetric cladding modes. The behavior of the resonances is studied when the grating is subject to strain, bending, torsion, temperature, or external refractive-index changes. The strain, bending, and torsion sensitivities of the two resonances differ, whereas the temperature sensitivities are almost the same. The sensitivities to variation in external refractive index are the same for the two resonances when the long-period grating is straight and differ when the fiber with the grating is curved.

Two types of resonances in long-period gratings induced by arc discharges in boron/ germanium co-doped fibers.

We demonstrate that under certain conditions it is possible to fabricate in a B/Ge co-doped fiber an arc-induced long-period grating whose spectrum contains a dual set of resonances. These two sets of resonances are formed by distinct mechanisms and are caused by coupling to cladding modes of different symmetries. They behave differently at high temperatures: the set produced by symmetric perturbation disappears during annealing at a temperature of 800 degrees C, while the other set produced by an antisymmetric mechanism can withstand temperatures above 1000 degrees C.

Simultaneous temperature and strain measurements performed by a step-changed arc-induced long-period fiber grating.

A compact sensor based on step-changed arc-induced long-period fiber gratings was implemented to discriminate between temperature and strain. The proposed sensor consists of a single long-period grating with two sections written consecutively in the SMF-28 fiber using the electric arc discharge technique. The two sections have the same period but different fabrication parameters. The operation of the sensor relies on the existence of a difference between the values of temperature and strain sensitivity of two neighboring resonances observed in the spectrum of the step-changed grating. The temperature and strain resolutions obtained for the sensor are 0.2 degrees C and 35 micro epsilon, respectively.

Refractometric sensor based on a phase-shifted long-period fiber grating.

A refractometric sensor based on a phase-shifted long-period fiber grating written by electric-arc discharges is presented. Transmission and reflective configurations for refractive index measurements are studied. It is observed that the reflective topology permits better performance compared with the transmission one, which is the approach normally utilized in the context of long-period fiber sensing. The resolution achieved in the measurement of refractive index enables the application of this sensing head structure in demanding situations, such as the measurement of the level of salinity of water.

Demonstration of coupling to symmetric and antisymmetric cladding modes in arc-induced long-period fiber gratings.

The symmetry of cladding modes excited in microbend and arcinduced long-period fiber gratings is investigated. An optimization technique is developed to determine the fiber parameters and to associate grating resonances with cladding modes of a particular symmetry. Using this optimization procedure, we show that the gratings induced in a standard fiber by arc discharges and microbends couple light to the antisymmetric cladding modes. In the case of a boron-germanium codoped fiber, the cladding modes excited by arc-induced gratings are found to be symmetric. Measurements of the near-field intensity distribution of cladding modes confirm the mode symmetry ascertained by the optimization technique.

Effect of ionizing radiation on the properties of arc-induced long-period fiber gratings.

We experimentally study the effect of ionizing radiation on the properties of long-period gratings fabricated in two pure-silica-core fibers with the arc-discharge technique. It is observed that the spectra of the gratings remain almost unchanged after being subjected to doses in excess of 0.5 MGy. The results also show that the gratings' temperature and strain sensitivities are not affected by gamma radiation.

Arc-induced long-period gratings in aluminosilicate glass fibers.

Permanent long-period gratings were written using arc discharges in two aluminosilicate fibers, one of which was doped with erbium. Reversible gratings were also mechanically induced in both fibers. The thermal behavior of the arc-induced gratings was investigated at up to 1100 degrees C. It was found that the shift of the resonant wavelengths exhibited a well-defined linear dependence on temperature up to 700 degrees C.