Weizhong Zhao

Short cavity single frequency fiber laser for in-situ sensing applications over a wide temperature range

A novel Er-doped silica fiber, with heavy Er doping, was specially developed for application to a single frequency fiber laser. Two high temperature-sustainable fiber Bragg gratings, written into Bi-Ge codoped photosensitive fiber, were chosen for the application and spliced to the specialist Er doped silica fiber to form a compact, linear cavity, fiber laser. The fiber laser retained single mode oscillation over a wide temperature range, from room temperature to 400 degrees C. The wavelength of the laser output could be tuned smoothly, without mode hopping being observed, when the temperature was changed. A narrow linewidth of less than 1 kHz was measured at the output of fiber laser and this indicates the potential of the fibre laser sensing system with extremely high sensitivity and resolution over this wide range.

Thermal decay characteristics of strong fiber Bragg gratings showing high-temperature sustainability

Strong fiber Bragg gratings (FBGs) with high-temperature sustainability were fabricated by writing the gratings into several specially developed photosensitive fibers. The thermal decay characteristics of these gratings were investigated over a temperature range from room temperature to 950°C. A cation-hopping model is presented to account for the obvious differences between the FBGs in terms of their thermal properties. A related cation-oriented trap distribution model is also used to simulate the decay properties of the gratings during high-temperature annealing and is found to yield a good fit to the experimental data. An accurate operating lifetime of these specially fabricated gratings can be predicted by using the cation-oriented trap distribution simulation.

Fluorescence decay characteristic of Tm-doped YAG crystal fiber for sensor applications, investigated from room temperature to 1400/spl deg/C

An yttrium aluminum garnet (YAG) crystal fiber with a thulium-doped end tip was specially grown by means of the laser heated pedestal growth approach and designed to be incorporated in a fiber-optic temperature probe. The fluorescence decay characteristics of the crystal fiber, including the temperature dependence of both the fluorescence lifetime and intensity, were comprehensively investigated. Experimental results indicated that the crystal fiber showed a monotonic relationship between the fluorescence lifetime and temperature with an average lifetime sensitivity of 3 /spl mu/s /spl deg/C over a wide temperature range, taking measurement from room temperature to 1200/spl deg/C. Good stability (up to 1400/spl deg/C) was observed with high repeatability of the fluorescence lifetime during the annealing process carried out on the fiber over this temperature range. The fiber was found to be an excellent candidate material to be used as a fluorescence decay-based fiber thermometer probe and the results are presented on its performance.

Measuring of exhaust gas emissions using absorption spectroscopy

This paper describes an optical fibre sensor for the detection of NOx (NO2 and NO) and CO2 in the exhaust system of a road vehicle. The measurement is based on a free path interaction zone which is interrogated using UV and NIR light guiding optical fibres and collimated lenses. Results are presented in the absorption spectra of the gases in the UV region for the NOx gases and NIR region for CO2. These demonstrate that using this method it is feasible to identify the individual CO2, NO and NO2 species as well as other gases in the exhaust system. Measurement of concentrations to the level of tens parts per million (ppm) have been demonstrated for the NOxx gases.

Fibre Bragg grating sensors for reinforcement corrosion monitoring in civil engineering structures

Fibre optic strain sensors offer a number of advantages over the current electrical resistance type gauges, yet are not widely used in civil engineering applications. The use of fibre optic strain sensors (with a cross comparison with the output of electrical resistance gauges) to monitor the production of corrosion by-products in civil engineering concrete structures containing reinforcement bars has been investigated and results reported.

Vibration-insensitive temperature sensing system based on fluorescence decay and using a digital processing approach

A fluorescence-based temperature sensor system using a digital signal processing approach has been developed and evaluated in operation on a working automotive engine. The signal processing approach, using the least-squares method, makes the system relatively insensitive to intensity variations in the probe and thus provides more precise measurements when compared to a previous system designed using analogue phase-locked detection. Experiments carried out to determine the emission temperatures of a running car engine have demonstrated the effectiveness of the sensor system in monitoring exhaust temperatures up to 250 °C, and potentially higher.

Corrosion induced strain monitoring through Fibre Optic Sensors

The use of strain sensors is commonplace within civil engineering. Fibre optic strain sensors offer a number of advantages over the current electrical resistance type gauges. In this paper the use of fibre optic strain sensors and electrical resistance gauges to monitor the production of corrosion by-products has been investigated and reported.

Temperature monitoring of vehicle engine exhaust gases under vibration condition using optical fibre temperature sensor systems

Two optical approaches, comprising and contracting both the fluorescence decay lifetime and the fibre Bragg grating (FBG) methods, were developed and evaluated for temperature monitoring of exhaust gases for use on a vehicle engine. The FBGs used in the system were written into specially designed Bi-Ge co-doped photosensitive fibres, to enable them to sustain high temperatures to over 800°C, which is far beyond that of FBGs written into most commercial photosensitive fibres. The sensors were subjected to a range of vibration tests, as a part of an optical exhaust monitoring network under development, and results from the test carried out are reported.

Fiber-optic sensor system for heat-flux measurement

Two different types of fiber-optic sensors were used in an experiment to measure the heat flux in a simulated refractory lining material. The results obtained with a sensor based on fluorescence lifetime detection and a sensor based on the peak wavelength shift of a fiber Bragg grating are presented and compared. Analysis of the results of the measurements taken indicates that these fiber-optic sensor systems are capable of performing multipoint temperature, and thus the heat flux, measurements. An approach is also presented for the measurement of the temperature dependence of the conductivity coefficient of the materials concerned, using the sensor systems detailed.

Growth characteristics and potential applications in optical sensors of composite Cr4+:yttrium–aluminum–garnet (YAG)–Nd3+:YAG crystal fiber

Composite Cr4+:yttrium–aluminum–garnet (YAG)–Nd3+:YAG crystal fiber was grown by means of the laser heated pedestal growth method. By using a high quality Nd3+:YAG as the seed and a sintered powder rod as the source rod, it was shown that Cr4+:YAG can be readily grown on a Nd3+:YAG end surface. The crystal fiber thus produced was processed and preliminary results showed that the crystal fiber demonstrated laser performance as a compact passively Q-switched laser device. A Q-switched laser output showing a 9 ns pulse duration, a 19 mW average output and a 10 kHz repetition rate was achieved. The use of the fiber in high temperature optical sensor application is discussed.