Abstract
Fibre optic modal interferometry has been around for long as a sensing concept. Initially mainly supported on the utilization of standard Hi-Bi fibres associated to polarimetric modal interferometry, later this sensing approach evolved to modal interference based on spatial modes propagating in the core, and on spatial modes propagating in the core and in the cladding, with coupling performed by fibre devices such as long period gratings and tapers. More recently the outcome of Photonic Crystal Fibres (PCF) originated a burst of activity around the concept of modal interferometry for sensing. The reasons for that viewed in a historic perspective are presented in this work.

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
In this study an optic fibre system for health monitoring of fibre reinforced plastics was developed. It is based on the detection of acoustic emission (AE) waves in a loaded material. A low-finesse Fabry-Perot interferometer sensor is used as alternative to the conventional piezoelectric transducers for AE waves sensing. An original procedure for optical fibre sensor interrogation is proposed.

Abstract
In this paper, two novel hybrid multimode/single mode fiber Fabry-Perot (FP) cavities were compared. The cavities fabricated by chemical etching are presented as high temperature and strain sensors. In order to produce this FP cavity a single mode fiber was spliced to a graded index multimode fiber with 62.5 mu m core diameter. The multimode fiber was cut approximately 150 mu m away from the splice. Then the tip of the fiber containing the multimode fiber segment was dipped into a solution of 48% of HF during 8 minutes, creating a concavity due to the fact that the reaction between HF and the germanium doped fiber core is much faster than the reaction between HF and the pure silica cladding. By this method a concavity of approximately 100 mu m deep was created at the fiber tip. Two different FP cavities can be fabricated. The first cavity is obtained when a spliced with an identical tip concavity fiber (Sensor A) and the second is created when a tip concavity is spliced to a single mode fiber (Sensor B). The Fabry-Perot cavities were tested as a high temperature sensor in the range between room temperature and 800 degrees C and as strain sensors. A reversible shift of the interferometric peaks with temperature allowed to estimate a sensitivity of 0.75 +/- 0.03 pm/degrees C and 0.98 +/- 0.04 pm/degrees C for the sensor A and B respectively. For strain measurement sensor A demonstrated a sensitivity of 1.85 +/- 0.07 pm/mu epsilon and sensor B showed a sensitivity of 3.14 +/- 0.05 pm/mu epsilon. The sensors demonstrated the feasibility of low cost fiber optic sensors for high temperature and strain.