Limitation in Brillouin Time-domain Analysis Due to Raman Scattering

In the past few years, Brillouin sensing using either spontaneous or stimulated Brillouin scattering has proven to be an efficient tool for the measurement of physical quantities such as temperature, strain, local birefringence or core dopant concentration along a fiber. The Brillouin effect is the non-linear process showing the lowest threshold; however an increase of the involved optical powers turns out to be necessary in order to achieve better performances in terms of spatial resolution and range. In our pump-and-probe analyser configuration, an erbium-doped fiber amplifier is used to boost the pulses produced by a standard DFB laser diode at 1550 nm, giving a peak power in the range of a few Watts for pulses from 10 to 100 ns. Taking into account the very low silica absorption at this wavelength, such a high optical power results in a metric resolution over a very long range. It turns out that such ideal conditions are not practically fulfilled, as shown in Fig. 1. This figure shows the actual amplification of the probe signal through Brillouin interaction as a function of the position in a 25 km fiber. As it can be seen, this gain falls abruptly to zero at 10 km; after this position, varying between 10 and 20 km depending on the amount of pump power and the type of fiber, the measurement of the Brillouin frequency shift, and thus the determination of local temperature or strain, are no longer possible. Moreover the initially square pump pulses undergo strong distorsions. Neither the linear absorption nor the pump depletion due to Brillouin scattering can explain this behaviour.