In designing 10 and 40Gbit/s systems it is crucial to limit dispersion by selecting the right technology platform. A good understanding of the physical phenomena and the use of high-resolution accurate measurement equipment are essential to achieve predictable dispersion performance during manufacturing.
The main parameters for the calculation of both polarisation mode dispersion (PMD) and chromatic dispersion (CD) are group delay (GD, the difference in arrival time between wavelengths) and differential group delay (DGD, the difference in time between principal polarisation states). GD and its derivative, the CD, show how the phase changes with wavelength. DGD and its RMS value, the PMD, show how a device reacts to several polarisation states travelling within it. The PMD is due to the birefringence of the waveguide and leads to pulse broadening.
For interleavers based on silica-on-silicon photonic lightwave circuits (PLCs) the PMD is less than 0.5ps. For a 50GHz interleaver, CD is less than 10ps/nm, much lower than in the fibre-based technology. Polarisation-dependent loss is lower than 0.3dB. So, compared to fibre-based technology, the PLC is the most suitable technology platform for the design of interleavers for 10 and 40Gbit/s.
Silica-on-silicon photonic lightwave circuits enable very attractive flat-top and wide-band high-port interleavers, available for the C and L bands and 25, 50 and 100GHz with high channel count.
Unlike fibre-based technology, the insertion loss (IL) is not dependent on the number of ports (the same for 1x2, 1x4 and 1x8 interleavers) — typically 4dB for the gaussian and 6dB for the flat-top in the 1dB bandwidth.
The bandwidth and IL correlate and can be customised during the mask-design phase. The wider the passband, the higher the insertion loss, such as the 50/400GHz 1x8 interleaver has an IL of 5dB and at least 20GHz clear channel bandwidth at 1dB. Total crosstalk is better than 20dB and return loss greater than 50dB.
Many system makers are planning to upgrade DWDM systems from 100 to 50GHz channel spacing by using 50/100, 50/200, 50/400 or 50/500GHz high-port interleaver. But using a fibre-based 50/400GHz interleaver requires the cascade of seven discrete components: one 50/100GHz 1x2, two 100/200GHz 1x2 and four 200/400GHz 1x2 interleavers (Fig. 2).
In contrast, use of a single PLC-based 1x8 interleaver limits the number of connections, avoids central frequency misalignments, and cuts cost by over 50% and footprint by 75%. For example, a 1x4 PLC-interleaver is four times smaller than a fibre-based package and has better optical performances.
A 4x4 PLC interleaver can be used to upgrade a reconfigurable optical add/drop module (ROADM). Incoming light is separated into S, C and L bands and then into 400GHz-spaced bands using two stages of CWDM demuxes. Switches then enable dropping or adding of these bands. This includes an interleaver that demuxes into output ports with 100GHz-spaced wavelengths in every 400GHz wideband. The use of NxN interleavers cuts by a factor of four the number of VOAs and switches needed. Further, an NxN cyclic interleaver with a loop-back configuration and switches can be used as a low-cost ROADM.
Also, a loop-back configuration with delay lines can realise optical code division multiplexing, which can offer routing operations in a single channel without optical switches).
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A traditional 1x2 interleaver sequentially demultiplexes a high-channel-count incoming wavelength stream into two lower-channel-count streams with double-width channel spacing. By contrast, a 1x4 interleaver sequentially demultiplexes a high-channel-count incoming wavelength stream into four output ports with quadruple-width channel spacing, and so on (Fig. 1).
Thus, a 50/100GHz 1x2 interleaver splits 50GHz-spaced channels into two sets of 100GHz-spaced channels, a 50/200GHz 1x4 interleaver into four sets of 200GHz-spaced channels and a 50/400GHz into eight sets of 400GHz-spaced channels. Another benefit is that, since interleavers are bi-directional, they can be used as multiplexers.