Abstract
In distributed optical fibre sensors, distributed amplification schemes have been investigated in order to increase the measurement range while avoiding the limitation imposed by the fibre attenuation and the nonlinear effects. Recently, the use of Raman amplification with an engineered intensity modulation has been demonstrated as an efficient way to produce a virtually lossless trace employing a single-end configuration. In this paper, we propose the combination of this technique with a simultaneous second order Raman pumping scheme for increasing the measurement range. The optimal modulation profile has been numerically analyzed and we experimentally demonstrate a sensor able to detect perturbations along 70 km of fibre, with a minimal SNR penalty along the total length. Thanks to this new approach, the sensitivity in the worst point is considerably improved, and the ASD noise floor is also reduced. The measurement range is extended approximately 15 km compared with the equivalent first order pumping case.
Highlights
Distributed optical fibre sensors (DOFS) are a promising technology for vibration sensing, as low-cost and widespread telecommunication cable can be efficiently transformed into an acoustic sensor [1]
The semiconductor laser current is driven by an arbitrary waveform generator (AWG) and, its amplitude is modulated
One important limitation when these lasers are used for distributed amplification is the Relative Intensity Noise (RIN) transfer between the pumps and the signal [15]
Summary
Distributed optical fibre sensors (DOFS) are a promising technology for vibration sensing, as low-cost and widespread telecommunication cable can be efficiently transformed into an acoustic sensor [1]. Several Raman pumping schemes, including the use of bidirectional pumps or higher order schemes [7,8], have been investigated in order to achieve a close-to-perfect compensation of the attenuation losses These schemes can minimize simultaneously the amplified spontaneous emission (ASE) noise and the nonlinear impairments. Single-end implementations of virtual transparency in DOFS has been proposed [9,10] These configurations use Raman and Brillouin amplification in which perfect compensation is obtained by means of engineering the time-domain gain through the modulation of the pump intensity. Thanks to these configurations, the nonlinear effects and the ASE are reduced and the pump energy is saved. As semiconductor lasers, limited to 400-500 mW output power, are typically used as the pump laser, the extra gain produced by the second order pumping scheme cannot be obtained with a first order configuration
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