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

Abstract

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 self-heterodyne detection. These sensors are particularly suitable to perform dynamic strain and temperature measurements because the radio-frequency (RF) signal that is obtained when the probe wave is detected has a phase-shift spectrum that is independent to first order of the Brillouin gain. Therefore, a fixed optical frequency separation between pump and probe wave can be deployed and uses the stable RF phase-shift spectrum to obtain the Brillouin frequency shift (BFS) from measured changes in the probe RF phase-shift. However, in this paper, it is found that there is a narrowing of the RF phase-shift spectrum that depends on the Brillouin gain. This effect becomes significant when very high power pulses are used and the resultant large gain induces a narrowing of the RF phase-shift spectrum. This narrowing leads to a BFS measurement error when the sensor is configured for dynamic measurements. We analyze, theoretically and experimentally, the origins and the magnitude of the narrowing of RF phase-shift spectra for high pump pulses in a coherent BOTDA sensor. Furthermore, this spectral shape change is compared to the broadening of the gain spectrum that has been recently discovered in conventional direct-detection BOTDA sensors, which is linearly dependent on the pulse peak power injected to the fiber, finding that the spectral shape change is less significant in coherent BOTDA sensors. Finally, we quantify the BFS measurement error that it can induce and find the trade-offs to keep it below a certain threshold. It is found that, from a practical point of view, this effect is significant for short fibers, where nonlinear effects are negligible and large pump pulses can be used.