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Dual-operation Q-switched erbium-doped fibre laser for distributed fibre sensing Souza, D.O.Culverhouse, and T.P.Newson


A Q-switched erbium-doped fibre laser has been developed and configured to provide narrowband and broadband operation using an all-fibre optical switch for distributed fibre sensing based on the Landau-Placzek ratio. Narrowband operation allows filtering of the Brillouin and Rayleigh signals and broadband operation minimizes coherent Rayleigh noise.

In recent years, Brillouin scattering has received considerable interest for distributed temperature and strain sensing. The Landau-Placzek ratio method [1,21] has recently been demonstrated in a distributed optical fibre temperature sensor (DOFTS), in which the Brillouin signal is referenced to the Rayleigh signal. A pulse of light is propagated down the sensing fibre and the backscattered Rayleigh and spontaneous Brillouin signals are interrogated. The Rayleigh signal is insensitive to temperature changes and is used to compensate for fibre attenuation and splice losses. The ratio between the Rayleigh and Brillouin signal intensity provides the Landau-Placzek ratio (LPR).

The LPR method requires the use of two sources having different bandwidths. A narrowband source is needed to allow optical filtering of the Rayleigh signal from the closely symmetrically spaced Stokes and anti-Stokes Brillouin components. However, the narrowband Rayleigh signal is noisy due to coherent Rayleigh noise (CRN) which would result in a LPR with degraded temperature resolution. The CRN is dependent on the temporal pulse width, the pulse linewidth and the detector bandwidth. Preliminary investigations indicate that a broadband source of not less than 2.5nm (~320GHz) is required to achieve a temperature resolution of 1°C for a system offering a spatial resolution of 40m.

This paper describes a Q-switched Erbium-doped fibre laser source that can be rapidly switched from operating as either a narrowband source (~700MHz) or a broadband source (~375GHz) ideally suited to distributed sensing. This was achieved by using an all-fibre optical switch within the laser cavity to select either a narrow or broad bandwidth cavity end reflector.

Electronics Letters (1997) Vol.33(24) pp.2040-2042

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