Bidirectional transceivers — devices that simultaneously receive incoming signals and transmit outgoing ones — are a crucial component of any passive optical network (PON), and this type of network is the fastest growing technology in the telecom marketplace. The transceivers provide the interface between the end user and the optical network. In practice, they receive information at wavelengths of 1490 and 1550 nm and transmit back at a single wavelength, 1310 nm. Thus, every end user requires a triplexer to separate these three wavelengths (Figure 1).Figure 1. A triplexer separates the three wavelengths used in a passive optical network. The challenge is to find the least expensive method of fabricating reliable devices.Recently, scientists at Enablence Technologies Inc. in Ottawa and at Kotura Inc. in Monterey Park, Calif., designed and demonstrated a triplexer based on a silicon-on-insulator (SOI) planar waveguide. They believe it is the first demonstration of an SOI device containing a monolithically integrated, cascaded interferometer and a planar reflective grating.The planar-waveguide triplexer comprises a three-stage Mach-Zehnder interferometer and a planar reflective grating (Figure 2). The input/output port at the top left connects to the single-mode fiber from the PON, and the three other ports are available to the end user. The two incoming channels from the PON exit from the lower port of the interferometer chain, while the outgoing channel enters the chain’s upper port and emerges from the same port in which the incoming channels entered. Meanwhile, the two incoming channels are separated from each other by the planar grating and emerge from the triplexer’s two output ports. Figure 2. The functionality of the device shown in Figure 1 is implemented in this silicon-on-insulator planar waveguide circuit. Reprinted with permission of IEEE Photonics Technology Letters.Of course, separating and combining multiple wavelengths carried on a single optical fiber is a common task in today’s dense wavelength-division-multiplexing (DWDM) world. An observer might well wonder why the elegant solutions used there — arrayed wavelength gratings, for example, or fiber Bragg gratings — would not be better and simpler than the seemingly complex device diagrammed in Figure 1. The reasons, explained Serge Bidnyk of Enablence, are multiple and complex, but foremost is probably the large flat passbands required in a PON triplexer. Although DWDM channels are narrow and closely packed, a PON’s three channels are necessarily quite broad and widely spaced. Neither arrayed-wavelength nor fiber Bragg gratings can readily handle the bandwidths required in a PON.Figure 3. The transmission through the 1310-nm channel shows a broad, 100-nm (1 dB) transmission window. Reprinted with permission of IEEE Photonics Technology Letters.In practice, nearly all of today’s PON triplexers are based on tiny bulk-optics components and rely on thin films to separate and combine wavelength channels. Thus, the SOI triplexer not only introduces monolithic optics, but sets the stage for future integration of the triplexer with optical sources and receivers.Although a single-stage Mach-Zehnder could separate the incoming 1490- and 1550-nm channels from the outgoing 1310-nm channel, the scientists designed their triplexer with three sequential stages so that they could tolerate fabrication nonuniformities such as variations in the etch depth. They achieved fabrication yields of 95 percent with the three-stage devices.They provided a 17- to 2.6-μm tapered waveguide port to mate with the PON fiber to minimize coupling loss. In the fabricated device, the total fiber-to-waveguide-to-fiber loss was 2.9 dB, including 1.6 dB resulting from uncoated waveguide facets. To enhance the reflectivity of the grating, the scientists coated it with a 100-nm layer of aluminum, lowering its loss to about 1.8 dB. Total insertion loss for the 1310-nm channel was 3.2 dB, and 5 dB for the 1490- and 1550-nm channels.Figure 4. The 1490- and 1550-nm channels show >32-dB isolation from each other and 45 dB from the outgoing 1310-nm channel. Reprinted with permission of IEEE Photonics Technology Letters.To quantify the spectral behavior of their device, the scientists connected a tunable broadband source to the input/output port of Figure 1 and measured the output at each of the other ports as the input was scanned across its wavelength range. The transmission of the 1310-nm channel showed a broad peak, which is necessary to accommodate the inexpensive, uncooled lasers used to transmit an end user’s output (Figure 2). Adjacent-channel isolation in the 1490- and 1550-nm channels was better than 32 dB (Figure 3). Isolation between the input channels and the output channel was greater than 45 dB.The 5-dB insertion loss in the incoming channels is the only performance specification that does not measure up to the requirements for a commercial triplexer. The scientists believe that this loss can be reduced by applying antireflection coatings to the facets and by otherwise improving the fiber-to-waveguide coupling.IEEE Photonics Technology Letters, Nov. 15, 2006, pp 2392-2394.