Abstract
In this work, a multiwavelength Raman fiber laser based on a highly birefringent photonic crystal fiber is presented. A laser resonator is formed when the Raman amplification with cooperative Rayleigh scattering in a dispersion compensating fiber is used as a distributed mirror and combined with a highly birefringent photonic crystal fiber loop mirror. The multiwavelength Raman fiber laser presents 11 stable channels per nm with a peak power of similar to 1.5mW. Stable multiwavelength lasing at room temperature is achieved due to the low sensitivity to temperature and environmental noise of the highly birefringent photonic crystal fiber based fiber loop mirror.

Abstract
An inclinometer sensor based on optical fibre-taper-modal Michelson interferometer is demonstrated. The magnitude of the tilt (bending angle of the fibre taper interferometer) is obtained by passive interferometric interrogation based on the generation of two quadrature phase-shifted signals from two fibre Bragg gratings. Optical phase-to-rotation sensitivity of 1.13 rad/degree with a 14 mrad/root Hz resolution is achieved.

Abstract
A long-period grating written in the SMF-28 fiber was heat treated at 1000 degrees C for 15 days. The spectrum of the grating shifted to longer wavelengths and the amplitude of the cladding mode resonances decreased as a result of structural relaxation. The background loss increased considerably for time longer than 200 h, and this loss is caused by devitrification of the fiber.

Abstract
It is reported a new dynamic electrical interrogation technique based on the modulation of two Bragg gratings structures located in the readout unit that permits to attenuate the effect of the 1/f noise of the electronics in the resolution of the LPG-based curvature sensors. The new concept is tested to detect large variations of curvature being achieved a resolution of 6.56 × 10-3 meters.

Abstract

Abstract
Acoustic emission monitoring is often used in the diagnosis of electrical and mechanical incipient faults in high-voltage apparatus. Partial discharges are a source of failure in power transformers, and the differentiation from other sources of acoustic emissions is of the utmost importance. This paper reports the development of a new sensor concept-mandrel-based fiber-optic sensor-for the detection of incipient faults in oil-filled power transformers, taking direct measurements inside a transformer. These sensors can be placed in the inner surface of the transformer tank wall, not affecting the insulation integrity of the structure, and improving fault detection and location. The applicability of these acoustic sensors in air, water, and oil is investigated and the paper presents the promising results obtained, which will enable the industrial development of practical solutions.