Multi-Gb/s optical cross-connect switch architectures: TDM versus FDM techniques

The authors describe two techniques for implementing an electrooptical cross-connect switch: time division multiplexing (TDM) and frequency division multiplexing (FDM). Both methods hold the promise of realizing switch sizes substantially larger than existing ones with lithium niobate couplers. The critical technologies for the TDM technique are semiconductor mode-locked lasers and optical AND gates, with the latter being the more severe bottleneck. While compressed pulse widths in the picosecond range are achievable, practical optical AND gates capable of the same speed range remain an open research issue. The FDM technique is by far the more promising means for implementing large-size switches. By virtue of the close proximity of the actual components, the stabilization of a comb of laser frequencies can be done easily with a simple Fabry-Perot filter, thereby eliminating the need for an absolute frequency reference. The foremost technical issue is how to design a tunable receiver with a wide frequency range. This could be done either with a coherent receiver with a tunable laser as the local oscillator or with a tunable filter followed by a direct-detection receiver. In a design example, the authors illustrate a two-stage fiber Fabry-Perot filter with a free spectral range of 13650 GHz and a finesses of 4550, yielding a 1000×1000 switch at a 1.7 Gb/s line rate