Derivation of a reflectogram using digital lock-in detection in coherent FMCW reflectometry based on a uniformly pumped Brillouin grating

We used digital lock-in detection to demodulate in-phase and quadrature components from a balanced mixer output in coherent FMCW reflectometry based on a uniformly pumped Brillouin grating. All the frequency components contributing to a reflectogram were extracted without attenuation by using a rectangular window in the frequency domain. We succeeded in reshaping a deformed reflectogram into a step-wise reflectogram for a single mode fiber that consisted of two fibers with different Brillouin frequencies. We found multiplicative noise to be dominant in the retrieved reflectograms, whose signal levels typically varied between ±7 and ±10%. We compared the observed changes with those estimated from power fluctuations in the amplified probe and pump light waves.