Abstract
We report, for the first time to our knowledge, the excellent optical performance of long-period gratings written in three standard libers and in two nonstandard fibers, namely, S-doped and N-doped fibers, using the flexible and low-cost electric are technique. The fundamental mechanism for their formation is proposed. The gratings thermal behavior and the effect of fiber preannealing prior to writing have been investigated. We have demonstrated a good stability of these gratings at high temperatures.

Abstract
We present an overview of the applications of the electric arc technique related to optical fibre technology. The use of arc discharges ranges from the well-known fibre splicing, going through the fabrication of basic devices such as fibre tapers and microspheres, to tailoring the spectra of UV-induced gratings such as in the apodization of fibre Bragg gratings and also in the fabrication of phase-shifted and sampled fibre Bragg gratings. However, in the past decade a topic more intensively investigated was probably long-period fibre gratings. Therefore, some devices based on arc-induced gratings, namely, phase-shifted and step-changed gratings and bandpass filters are discussed. We also present an electrically insulated thermocouple assembled in situ using arc discharges. This sensor is very useful in the determination of the temperature attained by the fibre during an arc discharge, this property being fundamental for the discussion of the mechanisms of formation and for the understanding of the thermal properties of arc-induced devices.

Abstract
Long-period gratings were written using arc discharges in pristine nitrogen doped fibres. Comparison of the resulting gratings spectra show that the resonant positions of gratings in fibres pre-annealed at 1050 degrees C for 30 min are shifted towards shorter wavelengths and their coupling strength is considerably higher. The refractive index and residual stress profiles were measured before and after annealing, as well as the two-dimensional stress profiles inside the grating region. Arc discharges induce periodic and strong asymmetric stresses in the fibre core of the pre-annealed nitrogen doped fibre.

Abstract
Long period gratings arc-induced in different types of fibre were annealed at 1000 degrees C for 24 h. During the annealing, the spectrum of the gratings written in Ge-doped fibres shifted towards longer wavelengths whereas an opposite shift was observed for gratings written in Ge-free fibres. A discussion on the mechanisms responsible for that behaviour is presented.

Abstract
In this article, we present detailed results on the fabrication and characterization of mechanically induced gratings produced by winding a string around a fiber/tube set. A comparison between the mechanical and arc-induced gratings is made, which allowed its to draw some conclusions on the formation mechanisms of the latter. The usefulness of mechanical gratings in research labs is discussed and two typical applications are. also presented. (C) 2008 Wiley Periodicals, Inc.

Abstract
The formation mechanisms and properties of arc-induced gratings are intrinsically related to the temperature attained by the fiber during arc exposure. In this work, we present further results oil the estimation of the fiber temperature based oil the use of electrically insulated thermocouples. Computer simulations show that under typical arc discharge conditions, the fiber reaches steady-state thermal equilibrium in less than half a second, having a peak temperature of about 1350 degrees C (C) 2008 Wiley Periodicals, Inc.