Photonic crystals could provide compact multiplexers

A theoretical investigation of photonic crystal (PC) waveguide couplers suggests that they could be two orders of magnitude smaller (shrinking from dimensions of hundreds of micrometers to tens of micrometers). Masanori Koshiba at Hokkaido University (Sapphoro, Japan) used a time-domain beam-propagation method based on a finite element scheme and a simple coupled-mode theory to model a multiplexer/demultiplexer (mux/demux) made using photonic bandgap technology for use in coarse WDM (CWDM) systems.

The CWDM system was recently proposed as a cost-effective approach for access networks offering multiplexed services. Unlike dense WDM systems, which require highly accurate wavelength control in both the light source and mux/demux components, CWDM has a relaxed wavelength arrangement, with channel spacing of about 20 nm.

Photonic crystals could potentially control lightwave propagation for these and other applications with benefits such as smaller devices and incorporating tight turns without radiation loss. They are attractive for WDM applications because they can be made into optical filters.

Koshiba designed a two-channel mux/demux based on PC waveguide couplers and investigated the WDM properties. Then he investigated a four-channel multiplexer by cascading two such devices with different coupling coefficients. The time-domain beam-propagation method he used is based on a finite-element scheme (FETD-BPM) that can employ nonuniform and nonorthogonal meshes. The analysis suggests that waveguides created in two-dimensional PCs of disconnected dielectric pillars (modeled) with infinite length would suffer from prohibitive diffraction losses. A preferable option is using PCs made of cylindrical pores in a dielectric material.

Koshiba cautions that further studies on mux/demux using real, three-dimensional PCs are necessary, as is an efficient method of coupling light from traditional waveguides into and out of PC waveguides.

For more information, contact Masanori Koshiba at

Yvonne Carts-Powell


  1. M. Koshiba, J. Lightwave Tech. 19(12), 1970 (December 2001).
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