Abstract
We present a fibre optic sensing head for making simultaneous measurements of temperature and strain, which operates over a large temperature range. The configuration is based on the different temperature sensitivities of type I and type IIA gratings written in a fibre with high germanium content. Maximal errors of +/-0.7 degreesC Hz(-1/2) and +/-3.8 muepsilon Hz(-1/2) are reported over 500 degreesC and 1200 muepsilon measurement ranges, respectively.

Abstract
Fibre Bragg grating (FBG) sensors are finding increased usage in experimental mechanics for monitoring service conditions in structures and other equipment and are currently being tested for process monitoring. In FBG sensors, strain and temperature cause a shift in the Bragg wavelength reflected by the grating contained in these fibres. In situ monitoring of strain and temperature during welding processes increases knowledge of the welded material and the welding process itself. In the present work, two welding processes are monitored using FBG sensors and the complete measurement approach including sensor selection, calibration, instrumentation, welding monitoring and result interpretation is presented. Calibration for strain measurements at constant temperature was performed using a four-point bending test, and temperature calibration was carried out using an oven. Results for a sensor length of 5 mm are presented. Both transient and residual strains were recorded during experiments on metal inert gas and friction stir welding and the possible impact of this monitoring technology is discussed in the light of process optimization and subsequent structural health monitoring.

Abstract
A fibre Bragg grating sensing configuration is presented for simultaneous measurement of strain and temperature. The proposed concept relies on writing a single Bragg grating on the splice region of two fibres with different levels of germanium doping. Doing so, a grating structure appears with three resonance peaks, which show distinct temperature sensitivities but similar strain responses. The experimental validation of the concept indicated resolutions of +/-1.5 degreesC Hz(-1/2) and +/-5.6 muepsilon Hz(-1/2) over measurement ranges of 80 degreesC and 1000 muepsilon for temperature and strain, respectively.

Abstract
A novel sensing head for simultaneous measurement of strain and temperature is demonstrated, based on two Bragg gratings arranged in a twisted configuration. Choosing an appropriate twisting period, the grating resonance wavelengths show similar temperature sensitivities but different strain responses, permitting simultaneous discrimination of these parameters with rms deviations of +/- 1.5 degrees C and +/- 4.7 mu epsilon respectively.

Abstract
The results of an investigation of the performance of a time-division-multiplexed (TDM) fiber Bragg grating (FBG) sensor array using a tunable laser source are reported. The system performance is found limited by the extinction ratio of the optical pulse modulator used for pulse amplitude modulation. Formulas that relate the crosstalk to the extinction ratio of the optical pulse modulator, the modulation parameters of the tunable laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 FBG sensors with 3 µe resolution can be realized with a commercially available single Mach-Zehnder type optical pulse modulator of -35-dB extinction ratio.

Abstract
A scheme to frequency multiplex a group of sensors based on all-fiber Michelson interferometers is presented. Lead insensitivity is obtained by using the two fibre leads of the configuration as an extra Michelson interferometer whose differential phase is kept constant by active compensation. Topics concerning the system design, sensor sensitivity, and crosstalk between sensors are investigated. Experimental and numerical computational results are presented.