Signed online chromatic-dispersion monitoring by synchronous detection of FM-induced arrival-time modulations in the clock-recovery phase-locked loop

Abstract

Feature Issue on Optical Performance Monitoring (OPM). Chromatic dispersion (CD) in optical single-mode fibers, i.e., the wavelength dependence of the propagation delay, distorts pulses and is a big problem in 10- and 40-Gbit/s transmission systems. Adjustable drop-in CD compensators require an online CD detection. For this purpose, we modulate the optical power of the laser in a 40-Gbit/s transmitter by 1.2% (rms) at a frequency of 5 MHz. In the presence of CD, the associated periodic optical frequency variation modulates the signal arrival time, which is measured by synchronous (lock-in) detection of an error signal in the clock recovery of the receiver. CD as large as -268 to +350 ps/nm is detected including its sign, more than by any comparable technique. The uncertainty of measured arrival-time modulations can be as low as 100 attoseconds. Moreover, no extra optics or high-frequency electronics are needed, which makes this method extremely cheap to implement. Results are given for nonreturn to zero and carrier-suppressed return to zero (CSRZ), intensity modulation, and differential phase-shift keying.