  In this work we demonstrate an FPGA-based coherent optical frequency domain
reflectometry setup for cable monitoring. Using coherent detection for
averaging and narrowband filtering, we significantly improve the
signal-to-noise ratio (SNR) compared to traditional intensity-only techniques
while also enabling continuous monitoring of phase and polarization. In
addition, the probe signal has constant power, avoiding the nonlinear
distortions and thereby enabling continuous use without interfering with data
channels. We perform a field demonstration over a trans-oceanic cable using
loopback configurations present in each repeater, demonstrating measurement SNR
exceeding 30 dB for all about 80 repeaters with an averaging window of 1
second. We furthermore compare cable monitoring using coherent processing to
today's solutions using power-only measurements. Our results show how
transitioning to coherent technology for cable monitoring can improve
sensitivity and enable new types of monitoring, exploring knowledge gained from
transitioning from incoherent to coherent data transmission.
