8.2. WDM COMPONENTS 351
Figure 8.15: A star coupler formed by using the fused biconical tapering method.Several kinds of star couplers have been developed for LAN applications [95]–
[101]. An early approach made use of multiple 3-dB fiber couplers [96]. A 3-dB fiber
coupler divides two input signals between its two output ports, the same functionality
needed for a 2×2 star coupler. Higher-orderN×Nstars can be formed by combining
several 2×2 couplers as long asNis a multiple of 2. Figure 8.14 shows an 8× 8
star formed by interconnecting 12 fiber couplers. The complexity of such star couplers
grows enormously with the number of ports.
Fused biconical-taper couplers can be used to make compact, monolithic, star cou-
plers. Figure 8.15 shows schematically a star coupler formed using this technique. The
idea is to fuse together a large number of fibers and elongate the fused portion to form
a biconically tapered structure. In the tapered portion, signals from each fiber mix to-
gether and are shared almost equally among its output ports. Such a scheme works
relatively well for multimode fibers [95] but is limited to only a few ports in the case
of single-mode fibers. Fused 2×2 couplers were made as early as 1981 using single-
mode fibers [73]; they can also be designed to operate over a wide wavelength range.
Higher-order stars can be made using a combinatorial scheme similar to that shown in
Fig. 8.12 [97].
A common approach for fabricating a compact broadcast star makes use of the
silica-on-silicon technology in which two arrays of planar SiO 2 waveguides, separated
by a central slab region, are formed on a silicon substrate. Such a star coupler was
first demonstrated in 1989 in a 19×19 configuration [98]. The SiO 2 channel wave-
guides were 200μm apart at the input end, but the final spacing near the central re-
gion was only 8μm. The 3-cm-long star coupler had an efficiency of about 55%. A
fiber amplifier can be integrated with the star coupler to amplify the output signals be-
fore broadcasting [99]. The silicon-on-insulator technology has been used for making
star couplers. A 5×9 star made by using silicon rib waveguides exhibited low losses
(1.3 dB) with relatively uniform coupling [100].
8.2.5 Wavelength Routers
An important WDM component is anN×Nwavelength router, a device that com-
bines the functionality of a star coupler with multiplexing and demultiplexing opera-
tions. Figure 8.16(a) shows the operation of such a wavelength router schematically
forN=5. The WDM signals entering fromNinput ports are demultiplexed into in-
dividual channels and directed toward theNoutput ports of the router in such a way