Simplified BOTDA System Based on Direct Modulation of a Laser Diode With an Extended Measurement Range

We demonstrate a simplified Brillouin optical time-domain analysis system based on direct current modulation of a laser diode which is configured and optimized for long-range measurement. A precompensated low-frequency RF wave is applied for time-division generation of Brillouin pump and probe waves without using any microwave device. Low-speed frequency dithering of the laser diode is additionally applied for the suppression of the coherent noise originating from Rayleigh scattering of the leakage probe wave. In experiments, distributed measurements of Brillouin frequency shift along a 25 km single-mode fiber are presented with a spatial resolution of 1.2 m and measurement accuracy (σ) of 0.45 MHz, respectively.