Abstract
It is reported a LPG dynamic interrogation technique based on the modulation of fibre Bragg gratings 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 sensing head. The concept is tested to detect variations of the external refractive index and a resolution of 2.0x10 (4) NIR was achieved without system optimization. Additionally, the effect in the sensor resolution when introducing Erbium and Raman optical amplification is experimentally investigated.

Abstract
A novel interrogation technique for multiplexing LPG-fiber loop mirrors has been proposed and experimentally demonstrated. Each displacement sensor is formed by a fiber loop mirror combined with a long period grating. This high reflectivity variable mirror is suitable to be interrogated by an OTDR and to be multiplexed in a crosstalk free setup with temperature independence.

Abstract
The measurement of micro-displacement or bending using a long-period fibre grating (LPG) in a self-referenced fibre optic intensity sensor is presented. The sensing head is based on a LPG whose attenuation peak changes as a function of its micro-displacement or bending and the fibre optic intensity sensor is referenced in frequency. Two other LPGs were used as rejection filters. located at the output of the optical source to shorten its spectral width. The implemented experimental set-up is described and, the results are presented being considered the measurement range and the intrinsic resolution.

Abstract
A quasi-distributed displacement sensor for structural monitoring using an optical time domain reflectometer is demonstrated. Four displacement sensing heads are placed along a standard single mode optical fibre in several locations with different intervals. Their configurations introduce power loss through the decrease of their fibre loop radius when displacement is applied. The decrease of the light intensity with displacement variation is reported. Losses of 9 dB for a similar to 120 mm displacement with a sensitivity of similar to 0.027 dB/mm are reported. The quasi-distributed configuration is able to address sensors with similar to 1 m distance resolution between them.

Abstract
This work presents an alternative solution for simultaneous measurement of strain and temperature. The sensing head is formed by a long-period fiber grating combined with a high-birefringence fiber loop mirror resulting in a configuration capable of temperature and strain discrimination. These optical devices have opposite sensitivity responses when a variation of temperature and/or strain is applied. Maximum errors of +/- 0.8 degrees C and +/- 21 mu epsilon are reported over 60 degrees C and 700-mu epsilon measurement ranges, respectively.

Abstract
We present a new configuration based on a fiber ring laser sensor with four-wave mixing effect generation for simultaneous measurement of strain and temperature. The fiber ring laser sensor, corresponding to the signal wavelength, is based on an erbium-doped fiber amplifier, a highly nonlinear bismuth-based optical fiber, and a Bragg grating filter. Four-wave mixing is generated when an external pump laser is combined with a ring laser sensor. Two new peaks appear and are named the converted and satellite wavelengths. With efficiency conversion, which corresponds to the optical power ratio between the converted signal and ring laser input signal, and a Bragg wavelength it is possible to discriminate strain and temperature. Maximum errors of +/- 1.8 degrees C and +/- 12 mu epsilon are reported over 100 degrees C and 1000 mu epsilon measurement ranges, respectively. (c) 2007 Society of PhotoOptical Instrumentation Engineers.