8.2. WDM COMPONENTS 345
Figure 8.9: Grating-based demultiplexer making use of (a) a conventional lens and (b) a graded-
index lens.
input WDM signal is focused onto a reflection grating, which separates various wave-
length components spatially, and a lens focuses them onto individual fibers. Use of a
graded-index lens simplifies alignment and provides a relatively compact device. The
focusing lens can be eliminated altogether by using a concave grating. For a compact
design, the concave grating can be integrated within a silicon slab waveguide [1]. In a
different approach, multiple elliptical Bragg gratings are etched using the silicon tech-
nology [64]. The idea behind this approach is simple. If the input and output fibers
are placed at the two foci of the elliptical grating, and the grating periodΛis adjusted
to a specific wavelengthλ 0 by using the Bragg condition 2Λneff=λ 0 , whereneffis
the effective index of the waveguide mode, the grating would selectively reflect that
wavelength and focus it onto the output fiber. Multiple gratings need to be etched, as
each grating reflects only one wavelength. Because of the complexity of such a device,
a single concave grating etched directly onto a silica waveguide is more practical. Such
a grating can be designed to demultiplex up to 120 channels with a wavelength spacing
of 0.3 nm [66].
A problem with grating demultiplexers is that their bandpass characteristics depend
on the dimensions of the input and output fibers. In particular, the core size of output
fibers must be large to ensure a flat passband and low insertion losses. For this rea-
son, most early designs of multiplexers used multimode fibers. In a 1991 design, a
microlens array was used to solve this problem and to demonstrate a 32-channel multi-
plexer for single-mode fiber applications [68]. The fiber array was produced by fixing
single-mode fibers in V-shaped grooves etched into a silicon wafer. The microlens
transforms the relatively small mode diameter of fibers (∼ 10 μm) into a much wider
diameter (about 80μm) just beyond the lens. This scheme provides a multiplexer that
can work with channels spaced by only 1 nm in the wavelength region near 1.55μm
while accommodating a channel bandwidth of 0.7 nm.
Filter-based demultiplexers use the phenomenon of optical interference to select