Ultra-long range optical frequency domain reflectometry using a coherence-enhanced highly linear frequency-swept fiber laser source

We report on an ultra-long range optical frequency domain reflectometry (OFDR) using a coherence-enhanced highly linear frequency-swept fiber laser source based on an optoelectronic phase-locked loop (OPLL). The frequency-swept fiber laser is locked to an all-fiber-based Mach-Zehnder interferometer (MZI) to suppress sweep nonlinearity and enhance the laser coherence, leading to a high coherence linear frequency sweep of 1 GHz in the duration time of 25 ms. This enables the OFDR to realize an ultra-long range measurement with a high spatial resolution. As a result, we obtain a 10 cm transform-limited spatial resolution at a 20 km fiber within 25 ms measurement time, and a 72 cm spatial resolution over an entire 200 km fiber link within 5 ms measurement time. The proposed reflectometry provides a high-performance solution with both high spatial resolution and ultra-long measurement range for field real-time fiber network monitoring and sensing applications.