Robust, signed, online chromatic dispersion detection in a 40-Gbit/s optical transmission link

Chromatic dispersion in optical single-mode fibers, i.e., the wavelength dependence of the propagation delay, distorts pulses and is the biggest obstacle in 40-Gbit/s transmission systems. Drop-in chromatic dispersion compensators require its online detection. In a 40-Gbit/s transmitter we modulate the power of the laser at 5 MHz by 1.2% rms. In the presence of chromatic dispersion, the associated periodic frequency variation modulates the signal arrival time, which is measured with a 100-as uncertainty by lock-in detection of the oscillator control voltage in the receiver clock recovery. Dispersion from 268 to +350 ps/nm is detected including its sign, more than by any comparable technique. Moreover, no extra optics or high-frequency electronics are needed, which makes this method extremely cheap to implement.