Abstract
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) 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> - 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.
Highlights
Optical buffers are widely investigated in the last decade because of their potential applications in alloptical communication systems [1]–[4]
The optical delay lines (ODL) is composed of Mach–Zehnder interferometer (MZI) switches and waveguide delay pairs with an incremental length difference
Major factors that degrade the buffered optical signal and restrict the buffering capacity include crosstalk induced by the limited extinction ratios (ERs) of MZI switches and the waveguide loss
Summary
Optical buffers are widely investigated in the last decade because of their potential applications in alloptical communication systems [1]–[4]. Several approaches have been proposed to realize chip-scale optical buffering devices [9]. Coupled resonator structures (CRS) and optical delay lines (ODL) are the two most popular methods. In the CRS-based optical buffers, microrings [10]–[12] or photonic crystals [13]–[15] are used to delay optical signals. By changing the refractive index of the resonators, a continuously tunable optical buffer can be realized. For ODL-based optical buffers, optical signals are switched among waveguides with different lengths to make a discretely tunable delay [16]–[18]. ODL outperforms CRS in terms of the quality of transmitted signal but at the expense of a larger footprint [1], [9]
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