Stimulated Brillouin Scattering in EDFA-Assisted Pulsed Fiber Optic Sensing Links with Non-Monotonic Duty-Cycle Dependence

Stimulated Brillouin scattering (SBS) constrains the launch peak power in long Erbium-Doped Fiber Amplifier (EDFA)-amplified pulsed fiber-optic sensing links, but its dependence on duty cycle is not determined by peak power alone. When the repetition rate is fixed and the pulse width is varied, the duty cycle changes both the pulse-limited Brillouin interaction length in the fiber and the pulse peak power delivered by the EDFA. Here, we treat duty cycle as a coupled SBS operating margin parameter and examine this dependence in a 25 km standard single-mode fiber link operated at 4 kHz. The analysis compares the duty-cycle-dependent SBS threshold, estimated from the pulse-limited interaction length, with the launched on-state pulse power extracted from Amplified Spontaneous Emission (ASE)-filtered signal band average power measurements. A Continuous-Wave (CW) sweep gave a reference SBS threshold of 5.5 dBm. In pulsed operation, the extracted launched pulse peak power decreased by 23.8 dB across the investigated duty cycle range, whereas the measured backward optical power increased by 20.3 dB up to its maximum and then decreased by 7.1 dB at higher duty cycle. Variable Optical Attenuator (VOA)-controlled launch power sweeps showed no 3 dB onset at duty cycle (D) of 0.5% within the measured range, while onsets between 1.76 dBm and 3.49 dBm were extracted for duty cycles from 1% to 50%. These results show that SBS risk in EDFA-assisted pulsed sensing links cannot be assessed only from total EDFA output power. Signal band launch power calibration after ASE rejection is required to estimate the pulse peak power relevant to SBS onset. The validation is limited to power-meter-based backward power measurements and does not spectrally resolve the Brillouin Stokes component.