Optical fibre devices: vibrometer, current monitor & amplifier

Abstract

A practical all-fibre laser vibrometer is described. The design has been optimised for hand-held use and incorporates a frequency-scanned laser diode and single-mode optical fibre coupler. Vibration information is extracted by a pseudo-heterodyne technique. The vibrometer is an ideal portable device for machine vibration analysis, since it is insensitive to external vibrations, is robust and lightweight uses inexpensive components and allows access to difficult targets. Performance has been demonstrated to approach the theoretical limit as surface velocities can be measured in the range 10mm/s to 0.2m/s over the frequency range 50 to 20 x 10³ Hz and with an operating range of 50 to 300mm.

Highly-elliptical birefringent fibres have been fabricated by spinning a linearly-birefringent fibre during the draw. These fibres are particularly interesting for application as Faraday-effect fibre current monitor. Since, in contrast with conventional fibres, they can be wound in small multi-turn coils whilst retaining their sensitivity. The fibre and its exploitation in three optical schemes is modelled using Jones calculus and also experimentally investigated. A simple optical configuration is proposed, combining the elliptically-birefringent fibre and a broad-spectrum light source. An accurate, compact and robust current monitor is obtained. The sensor is characterised by a measurement repeatability of ± 0.5%, a temperature drift of 0.05%/°C and a sensitivity of 1mA rms/Hz½. Further, the performance of this sensor with optimised fibre length for a given measurement bandwidth is predicted.

A detailed characterisation of the erbium-doped fibre amplifier is presented. Spectral measurements of pump excited-state absorption are presented and indicate the optimum pump wavelength to be 980nm. Further, a gain of 24dB was obtained for an amplifier pumped with 11mW of power at this wavelength. Pump noise. multichannel crosstalk and non-linear effects are investigated in these amplifiers and shown to be negligible due to the long (tau ~ 14ms) fluorescence lifetime of Er3+. The AM and FM response of these amplifiers has been characterised for signal modulation frequencies In the range 130MHz to 13GHz. Both the amplifier gain and phase are found to be constant for both amplitude modulated (AM) and frequency modulated (FM) signals. The fundamental noise mechanisms in these amplifiers are investigated and it is shown that they can be operated near the quantum limit with a 3dB excess noise figure. Finally, the performance of the amplifier in a 16-channel coherent broadcast network is described. The amplifier provides a gain of 22dB, and dynamic range of 15dB and thus facilitates a network capacity of 65 Tbit km/s.

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