Differential group delay monitoring for high-speed data based on a low-frequency radio-frequency tone power measurement

A first-order polarisation mode dispersion (differential group delay (DGD)) monitoring technique using a low-speed detector for 40 Gb/s return to zero data transmission is proposed and experimentally demonstrated. The monitoring technique is based on adding an additional optical carrier which is frequency-shifted with respect to the data carrier at an orthogonal polarisation state. The added carrier beats with the data clock tones generating new RF tones at the photodetector output whose power depends on a sinusoidal function. By properly setting the frequency shift between optical carriers, a low-frequency radio-frequency (RF) tone is generated and used for DGD monitoring of high-speed data, providing a cost-effective solution, high resolution and wide monitoring range compared to traditional RF-tone-based DGD monitoring techniques. Experimental results show successful DGD monitoring from 0 to 25 ps with an RF-tone dynamic range of 18 dB. Insensitivity of the proposed technique to chromatic dispersion and optical signal-to-noise ratio is also demonstrated.