Analysis of a Silicon Reconfigurable Feed-Forward Optical Delay Line

We analyze a silicon reconfigurable feed-forward optical delay line (ODL) composed of cascaded Mach-Zehnder interferometer (MZI) switches and waveguide delay pairs. Optical delay is step-tuned by switching among optical routes with an incremental length difference. The crosstalk caused by limited extinction ratios (ERs) of MZI switches and the waveguide loss restricts the ODL buffering capacity. A method to suppress the crosstalk and hence expand the buffering capacity is proposed by inserting variable optical attenuators (VOAs) between successive MZI switches. A design example shows that a seven-stage ODL without VOAs can delay 8-98 bits with a tuning step of 1 bit for a 40 Gbit/s non-return-to-zero (NRZ) 2¹³ - 1 pseudo-random bit sequence (PRBS) optical signal. When VOAs with a 10 dB attenuation ratio are included, the signal-to-crosstalk ratio (SCR) of the output signal is increased by 17 dB on average, making the ODL capable of delaying 8-135 bits at its maximum.