Analysis Of Optical Signal Delay In A Feed-forward Optical Buffer

Optical buffers are widely investigated in the last decade because oftheir potential applications in all-optical communication systems. With thedevelopment of silicon photonics, as one of the most promising opticalplatforms for future optical communications and interconnects, it isfeasible to implement on-chip silicon optical buffers and integrate theminto photonic integrated circuits (PIC). Several approaches have beenproposed to realize chip-scale optical buffering devices. Coupled resonatorstructures (CRS) and optical delay lines (ODL) are the two most popularmethods. However, the CRS has large group delay dispersion aroundresonance, the delay-bandwidth product of the delayed optical signal isinevitably limited. ODL outperforms CRS in terms of the quality oftransmitted signal but at the expense of a larger footprintWe analyze a silicon reconfigurable feed-forward ODL composed ofcascaded p-i-n diode modulated Mach-Zehnder interferometer (MZI)switches and waveguide delay pairs. Optical delay is step-tuned byswitching among optical routes with an incremental length difference. Thecrosstalk caused by limited extinction ratios (ER) of MZI switches and thewaveguide loss restricts the ODL buffering capacity. A method to suppressthe crosstalk and hence expand the buffering capacity is proposed byinserting variable optical attenuators (VOA) between successive MZIswitches.Based on the analysis and discussion of the feed-forward ODL, asimulation example shows that a7-stage ODL without VOAs can delay8-101bits with a tuning step of1bit for a40Gbit/s non-return-to-zero(NRZ)213-1pseudo-random bit sequence (PRBS) optical signal. When VOAs with a10dB attenuation ratio are included in the short waveguideof the last two stages, the signal-to-crosstalk ratio (SCR) of the outputsignal is increased to15.7dB on average, making the ODL capable ofdelaying8-135bits at its maximum.