Broadband linear distributed-feedback laser chirp generation based on high-order optoelectronic phase–locked loop for FMCW reflectometry

We present the generation of a broadband, highly linear laser chirp utilizing a distributed-feedback laser with an optoelectronic feedback loop. A high-order optoelectronic phase–locked loop effectively suppresses both sweep nonlinearity and 1/f frequency noise during the laser chirp. With the loop closed, the frequency error relative to linearity is ∼62 kHz over an 80 GHz laser chirp in 50 ms, compared with 21 MHz when the loop is open. We achieve a Fourier transform–limited spatial resolution of 1.85 mm across a 1200 m fiber link, ∼12 times the intrinsic laser round trip coherence length. The demonstrated configuration is adaptable to nearly any semiconductor laser.