The design and application of optical sources for distributed fibre sensing systems

Abstract

This thesis investigates pulsed optical sources for distributed fibre sensing applications. Such sensors operate on the optical time domain reflectometry (OTDR) principle, and the source requirements depend on the desired sensing range, spatial resolution and nonlinear thresholds.

A number of Q-switched Erbium-doped fibre lasers at 1.5µm optimised for high peak powers and short pulse widths were designed and constructed. Experimental results were compared and contrasted with theoretical predictions.

The successful development of high peak power Q-switched fibre lasers at 1.5µm enabled the generation of broadband Stokes-shifted Raman pulses at the wavelength region of 1.65µm, with approximately 1.4W peak power and 45ns pulse width. Using both the 1.5µm and 1.65µm pulses, a novel technique referred to as delayed Raman amplification was demonstrated to increase the range of an OTDR sensor operating at 1.65µm. An increase in sensing dynamic range of 17.5dB was achieved. A Raman-based distributed temperature sensor was also developed using the 1.65µm source, and had a spatial and temperature resolution of 10m and 4ºC respectively, over a 10km sensing range. Both the OTDR and distributed temperature measurements potentially allow losses and temperature to be monitored in active communication links operating at 1.5µm.

A narrow linewidth amplified and gated semiconductor DFB source was constructed and its suitability for two spontaneous Brillouin-based distributed sensors investigated. The first sensor was a high spatial resolution distributed temperature sensor with a 35cm spatial resolution. The second sensor was a combined distributed strain and temperature sensor which used two Mach-Zehnder interferometers in series as filters to measure the Brillouin intensity and frequency shift. Temperature and strain resolutions of 4°C and strain resolution of 290µε were accomplished over a 15km sensing range.

Finally, investigations into using pulsed fibre sources compared to a semiconductor DFB source were performed. Both unidirectional Q-switched fibre ring lasers and short fibres Bragg grating lasers with stable and narrow linewidths were demonstrated. A stable, robust and high output power DFB Erbium/Ytterbium fibre laser was eventually selected to perform simultaneous strain and temperature measurements.

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