Impact of pump depletion on the determination of the Brillouin gain frequency in distributed fiber sensors

Abstract

The energy transfer between the two interacting optical waves in a distributed sensor based on stimulated Brillouin scattering can lead to a non-uniform spectral distribution of the pumping power after a long propagation. This results in a spectrally distorted gain that biases the determination of the maximum gain frequency. A quantitative analytical model gives an expression for the tolerable pump power change keeping the maximum bias within a given accuracy.

Raman-Assisted Brillouin Distributed Temperature Sensor Over 100 km Featuring 2 m Resolution and 1.2 $^{\circ}$C Uncertainty

“…The carrier frequency is suppressed by properly setting the dc bias of the modulator, and the higher frequency sideband is filtered before detection. Both sidebands propagate in the fiber under test, which compensates the depletion of the pump signal by the detected probe (lower sideband), thus making the system very robust to depletion-induced errors [10].…”

“…We can now set the acceptable depletion to values below 10%. It has been shown that this gives acceptable frequency errors below 1 MHz [10]. For the typical fiber characteristics ( m/W, m ) and km, we can infer that one has to work at power levels in the order of 1 W or below in the Raman-assisted configuration (in conditions of perfect transparency), while one can reach up to 7 W in the conventional nonassisted one.…”

“…Until recently, an analytical model of depletion in BOTDA sensors was not available. In the last months, Foaleng-Mafang et al [10] developed a simple analytical model of pump depletion in conventional BOTDA. Although the model developed in [10] is not adapted to the Raman-assisted case, we can again have an idea of the required parameters in our system by comparing the two cases treated in the previous section: the conventional case and the case of perfect transparency.…”

“…According to [10], the fraction of power lost by the pump through depletion can be directly calculated from the theoretical Brillouin loss induced by the probe; hence (3) where is the pump power in absence of Brillouin interaction, is the Brillouin gain coefficient, and is the effective area of the fiber. This expression comes from the fact that the gain on the probe can be considered negligible in all cases.…”

Rating the performance of a Brillouin distributed fiber sensor

“…Then the FOM is simply defined to be proportional to the signal-to-noise ratio, with some simple added considerations based on the limitations brought by the other nonlinear effects such as modulation instability and forward Raman scattering 17 and by pump depletion 18 . Figure 1.…”