Gain dependence of measured spectra in coherent Brillouin optical time-domain analysis sensors

We report on the effects of large pump pulse powers on Brillouin optical time-domain analysis (BOTDA) sensors based on phase-modulated probe wave and coherent detection. It is found that the large Brillouin gain that comes from the use of high power pulses induces a narrowing of the RF phase-shift spectrum that is measured in these sensors. This narrowing leads to a Brillouin frequency shift measurement error when the sensor is configured for dynamic measurements. However, the effect has been found to be less significant than that observed in dynamic slope-assisted BOTDA sensors based on amplitude.The authors wish to acknowledge the financial support from the Spanish Ministerio de Economía y Competitividad through the project TEC2013-47264-C2-2-R, FEDER funds and Universidad Pública de Navarra