Low-cost, signed online chromatic dispersion detection scheme applied to a 2×10 Gb/s RZ-DQPSK optical transmission system

Abstract

Chromatic dispersion (CD) in single-mode optical fibres limits the maximum transmission distance of digital optical transmission systems at 10 and 40 Gb/s. It can be eliminated by a tunable CD compensator, provided that the residual dispersion is detected, including its sign. A low-cost hardware implementation of a signed online CD detection scheme is reported. It is based on synchronous detection of frequency modulation (FM)-induced arrival time modulations in the clock recovery phase locked loop (PLL) of a 2 x 10 Gb/s return-to-zero differential quadrature phase shift keying (RZ-DQPSK) optical receiver. The RZ-DQPSK transmitter laser is modulated by a synthesised 4.732 MHz sinusoidal signal. This results in ≃1% (rms) intensity modulation and an optical FM with ≃105 MHz (rms) deviation. Low-cost components were used to build a synchronous detector for the arrival time signal that yields the fibre CD, including its sign. The total measurement interval is ≃84 μs. The sign of the CD is detected within a range of ±700 ps/nm. This range is limited by the locking range of the clock recovery PLL. The CD measurement readout is fairly linear over the range ±360 ps/nm.