Communication network demand drives growth of the planar waveguide market
09/01/2002
Stephen Montgomery, ElectroniCast Corp., San Mateo, CA
The growth of optical planar waveguide technology for communications applications has been driven by a dramatic increase in bandwidth demand beyond the limits of copper. Technological advances in fiber optics assure the migration of fiber closer to the end user, translating into new lower-priced and higher-density devices.
 |
Telecommunication system operators are upgrading their links from 10Gbps to 40Gbps and beyond transmission speeds and using dense wavelength division multiplexing (WDM) to increase network capacity. Planar waveguide components, including planar lightwave circuits (PLCs), systems that contain waveguides, emitters (light sources), and detectors (photodiodes) placed in the plane of a single circuit; waveguide grating routers (WGR); splitters/ couplers; optoelectronic IC substrates; and array waveguide gratings (AWGs) will be alternatives to mass optic fiber and fused biconic taper in future network applications. Global consumption of optical planar waveguides for communications is forecast in the figure.
Planar waveguides fall into two categories: integrated multifunction waveguides and discrete planar waveguide circuit (PWC) devices, including AWGs; photonic switches; variable optical attenuators (VOAs); splitters/couplers; lithium niobate modulators; and interconnect waveguides. According to Lightwave Microsystems Corp., San Jose, CA, a designer and manufacturer of waveguide components and integrated devices, demand is for AWGs for multiplexing and demultiplexing, and VOA-multiplexers for channel balancing. Demand is building for dynamic gain equalizers and for reconfigurable add/drop multiplexers.
Waveguide technology
Fabrication. Lightwave Microsystems describes a planar waveguide circuit as an optical circuit constructed of waveguides laid out on a silicon wafer. First, a lower cladding layer of index ncl (n = refractive index; cl = cladding) is deposited, followed by a core layer of index nco (co=core), with nco typically <1% larger than ncl. The core layer is patterned using standard photo lithographic techniques, and a channel waveguide pattern is transferred to the core layer using various etching techniques. An upper cladding layer is then deposited. The typical planar device is made up of these three layers.
Lightwave Microsystems sets the dimensions of a single-mode silica waveguide at 6-10æm × 6-10æm. Exact dimensions depend on the index difference and on the required degree of match to optical fiber. Dimensions will differ using a material other than silica.
Optical waveguide devices can be processed photolithographically by depositing silica on a glass, semiconductor, or ceramic substrate, or by selectively etching silica, silicon, or III-V compound substrates. Glass-on-silicon is a leading technique for fabricating passive optical waveguide elements on a silicon planar substrate, to support hybrid inte gration of optical components for a variety of optical signal-processing functions. Laser diode devices are mounted on silicon submounts and optical fibers are coupled to the optical waveguide through V-grooves pro cessed in the silicon substrate.table-Planar waveguide circuit global consumption market forecast by region (value basis).
Planar waveguides can also be processed in polymeric film by mask exposure or by laser beam tracing. The development of waveguide-in-polymer processing techniques has been substantially supported with funding from various parallel optical interconnect link development projects funded by the US DARPA and independent laboratory efforts. These waveguide interconnects have a high startup cost, but low incremental cost when produced in high volume.
Optical interconnect parts. These passive optical elements transmit and/or modify light beams. In PLCs, they connect light beams from one optical element to another or from an optical element to an external optical waveguide. They can also modify the beam by focusing, bending
eflecting, splitting/combining, wavelength filtering, or coupling to another waveguide.
The technology of optical interconnect parts has advanced greatly over the past decade. Most advances have been in designs for high-volume, low-unit-cost production. Miniature molded or diffraction lenses, for example, have a substantial front end fixed cost to establish production; thereafter, the incremental cost is a few cents per part.
Process developments
The market for transceivers and other optoelectronic components is price-elastic. Falling prices will be more than offset by growth in quantities shipped. Component producers, therefore, are focusing on ways to drastically reduce production costs.
The price of packaged PWCs or PLCs is projected to include 25-75% of a factory's cost outlay in the alignment, positioning, anchoring, testing, and package closure required. Planar devices are typically more difficult to assemble than standard optoelectronic devices, because they require active alignment of multiple input and output channels to fibers; it is often necessary to validate proper light launching into the device before its active optimization; and the need to manipulate multiple fibers/fiber arrays increases the difficulty of handling fragile devices.
Automation. Generally, the higher the performance requirements, the higher the packaging cost, most of which is money for human labor. Several developers have shown that the biggest payoff in cost reduction for optical components will come from module assembly automation.
Automation has its own price. Establishing an automated facility to produce several hundred thousand hybrid optoelectronic IC (OEIC) modules/year, for example, is estimated to be in the $5-15 million range for packaging, assembly, and test of the devices and parts in the module, depending on the complexity and precision of the components, and excluding the OEIC die processing cost. Photolithographic wafer processing; precision pick-and-place equipment; laser welding; reflow soldering; automated testing; and a fully equipped development laboratory are major investments that must be amortized in product prices within a few years.
Beyond lower cost, assembly auto mation provides other advantages. Quality control becomes more effective with the elimination of human vagaries. Wide swings in output volume are more easily accommodated. Disadvantages include some relaxation of device dimensional tolerances and related modest reduction in component performance.
Automated assembly technology has the potential to achieve im proved throughput and yield. Since the cost of an automated assembly facility is not much different for the highest-data-rate, most expensive OEICs (e.g., those with speeds of 2.5 and 10Gbps) than for moderate-data-rate OEICs (155 and 622Mbps), automation will extend to these highest-priced units. 50,000 transceivers/year at $2000 can amortize investment as quickly as a million transceivers/year at $100 each.
Miniaturization. The expected 20-30×expansion of global network throughput capacity over the next decade will require central office, access node, and subscriber equipment with greatly increased capabilities. Yet installation space is already crowded, and expensive to increase. End customers want greater throughput without increasing the size of switches, transport terminals, etc. Part of this will be achieved by using fewer but faster components. There is also heavy pressure to reduce component size through integration and miniaturized packages.
State of the art
Recent advances in planar optics technology, specifically for use in optical splitters, switches, modulators, and WDMs, support increases in production throughput, consistency, and yield with substantial cost reduction. Many of the development efforts funded by the US DARPA and other defense organizations have had a strong focus on military applications. Significant transfer to the commercial sector, however, is now being achieved.
Stephen Montgomery is president, as well as director of the Fiber Optics Components Group and the Network Communication Products Group at ElectroniCast Corp., 800 So. Claremont, Ste. 105, San Mateo, CA 94402; ph 650/343-1398,[email protected].
ElectroniCast forecasts for the communication network, fiber optic, photonic, and optoelectronic industries. For more information, visit www.electronicast.com.