A planned approach to photonics design is accelerating network development
Long hailed as the key to unlimited bandwidth, fiber optics has taken some heat for a perceived inability to keep up with demand. Fiber can cost as much as $50,000 per mile, and the explosion in usage has made keeping up a chore. To avoid steep installation costs and to satisfy customers, companies are increasingly depending on photonics innovations to improve the bandwidth of
existing fiber.
One of the most important methods is multiplexing. The ability to tune or select a specific wavelength, or channel, out of many frequencies of light on a single fiber has greatly expanded the flexibility and usability of existing networks.
A typical arrayed waveguide grating made by NeoPhotonics Corp. contains 34 devices and combines up to 40 optical data streams onto one optical fiber. Image: NeoPhotonics Corp.
Dense wavelength division multiplexing (DWDM) and coarse wavelength division multiplexing (CWDM) are allowing transmission rates on a fiber of up to 440-880 GBps, but controlling this throughput has required similar advances in planar lightwave circuit (PLC) technology. Devices such as arrayed waveguide gratings (AWGs) are able to separate and combine signals—much like the diffraction grating in a spectrometer—for use with DWDM and CWDM systems. Manufacturers are capitalizing on advances in semiconductor fabrication and nanotechnology to build sensitive multiplexing equipment.
“Currently these are the dominant types of systems for long haul and core metro networks. With (PLC) technology, the costs are coming down, and DWDM is being used further into edge and access networks. There’s even talk of using them in passive optical networks (PONs) to the home,” says Ferris Lipscomb, vice president of marketing for NeoPhotonics Corp., San Jose, Calif.
Tunable optics are driving new industry standards. Accelerometers, gyroscopes, sophisticated sensors, imaging equipment, and ever-faster networks will depend on a well-organized approach to integrated optics development. NeoPhotonics has joined with several other major optoelectronic manufacturers to set standards that govern the size of crucial devices. Multiplexing (and demultiplexing) waveguides are important parts of DWDM systems, and a new small form factor put forward this year as part of a long-standing multi-source agreement (MSA) illustrates rapid innovations in PLC technology.
This new standard will govern the package size of thermally stabilized AWGs and will speed PLC innovation by system integrators.
The power of planar
Optimization of fiber bandwidth depends greatly on precise control of fiber launch conditions. A silicon substrate, which has good heat dissipation and structural characteristics, is the material of choice for integrated PLCs in large part because it draws on the volume capabilities of the semiconductor industry.
As a result, silica-on-silicon technologies have taken off. Lasers, photodiodes, and others can all be mounted onto a single silicon slice—as many as 30 or more individual functions on 200 mm2 of silicon—and the optoelectronics themselves have gotten more efficient and much smaller. Fiber amplifiers, for example, were once made as discrete and bulky devices. The waveguide amplifiers that replaced them are easily small enough to fit on a PLC. The elimination of hermetic packaging for individual photonics components have in some cases yielded size savings of a factor of 10.
There are some drawbacks to PLCs, however. Even the best have higher absorption and scattering coefficients than fiber. In addition to lossy behavior, circuits that use devices such as AWGs are highly sensitive to polarization and thermal conditions.
“The key development for integrated optics is maintaining the same quality with higher index contrast. You want a higher differential in index in waveguides and still keep that high quality as you make smaller devices,” says Lipscomb. “As devices become much more sensitive to imperfections, fabrication must be controlled on a nanometer level.”
Manufacturers are now looking at ways to reduce size but not so as to limit PLC builders. As with the IC industry, development depends on compact designs that are standardized enough to promote the development of integrated packages.
Dense networks depend on AWGs
Compared to the standard of a few years ago, new packaged thermally stabilized AWGs are now 60% of the size of the previous standard. This is mostly due to improvements in package design rather than changes in the chip design, but according to Lipscomb, both factors will be important as integrated optics continue to advance.
“The purpose of these devices is to split and combine signals. Filters do it one at a time, but AWGs do multiple channels in parallel. It’s really an integrated circuit made on silicon. It’s just like an IC, made with layers of doped silica on silicon wafers,” says Lipscomb.
Thermally stabilized AWGs were chosen for standardization because they are crucial building blocks for devices used in long haul and metro grids. NeoPhotonics’ designs can multiplex or demultiplex 40 or more separate wavelengths. Current MSA AWG offerings include channel spacings of 50 GHz, 100 GHz, and 200 GHz, covering the C and L bands. NeoPhotonics has also developed AWGs that combine 80 channels at 50 GHz, effectively expanding the carrying capacity of a single fiber strand by a factor of 80.
The advantages of AWGs are manifold. Tunable, multi-channel control helps maximize fiber installations. Second, they are built using monolithic fabrication methods perfected in the semiconductor industry. This keeps costs down. Third, they can be readily integrated with other devices to perform switching, variable power control, and monitoring.
“AWGs are an important product line for us. It is a direct revenue source and is used in many other product lines as well,” says Lipscomb.
Thermally stabilized AWGs in particular are among the most advanced fiber control devices because of the precise optical demands of DWDM networks. These waveguides typically feature launch wavelengths of just 1.5 µm and require wavefront quality of just a few nanometers. Temperature differentials could easily affect the frequency enough to block light.
“The index of refraction of glass changes as the temperature of the glass is adjusted. The way the filter functions is where channels are exposed to a change in temperatures there comes some change in the frequency of the signal. If you’ve got a bandpass channel at another frequency then you’ve got a problem,” says Lipscomb.
Proposals have been made to implement AWGs still further. Dense wavelength fiber to the home could be a major development in the future, but the industry must first justify the overall cost for such a step to happen.
Promoting progress with MSAs
Multi-source agreements exist for a variety of industries, from semiconductor lithography to medical equipment. The purpose of the agreements—often administered by oversight committees—is to rationalize technological progress and maintain balance between suppliers and developers.
In photonics, MSAs have been adopted to help meet industry goals, such as smooth transitions to higher data rate standards. In the case of PLCs, an MSA can allow significant packaging advances.
“Two things make up the size of the product, the chip and the package. With the new small form factor, there’s a reduction in package size, but not so much chip size,” says Lipscomb. However, nanometer-scale metrology and nanometer-scale tolerance requirements are involved in making the smaller AWGs, and the interfaces, while standardized to the new form, are not traditional.
The new modules will allow designers to count on a one-third footprint reduction for AWGs when designing DWDM optical equipment. This sort of predictability allows manufacturers to choose among several compatible suppliers, reducing lead times and cost. Inventory requirements accordingly lessen.
“In a lot of industries it’s very expensive for companies to design from multiple vendors. They are all different products and different form factors,” says Lipscomb. “With the MSA, the customer designs one board and plugs in products from various vendors as needed. It saves both the customer’s and the component companies’ money.”
Of course, product differences still exist. The MSA defines package dimensions, bolt holes, electrical pin positions and assignments, fiber positions, heater resistance, and firmware. Where companies differ is in optical performance parameters. Insertion loss, crosstalk, and passband are left to the discretion of individual makers. The agreement also allows for two choices of input and output locations and either internal or external electronic control.
Competitors adhere to photonics standards
Standardization agreements in the photonics industry have global reach. Several companies leading the way in photonics designs are conforming to a multi-source agreement (MSA) or component design to facilitate the development of integrated optics. Most recently, a new small form factor for arrayed waveguide gratings (AWGs) has been established. The MSA includes the following manufacturers: Fitel, Peachtree City, Ga., 800-274-8335, www.fitel.com Gemfire Corp., Fremont, Calif., 510-438-7501, www.gemfire.com Hitachi Cable Inc., New York City, N.Y., 914-993-0990, www.hitachi-cable.com NEC FiberOptech, Cupertino, Calif., 408-863-2000, www.nec-fiberoptech.com NEL America Inc., Saddle Brook, N.J., 201-556-1770, www.nel-world.com NeoPhotonics Corp., San Jose, Calif., 408-232-9200, www.neophonotics.com
Participating companies make dozens of varieties of multiplexers and demultiplexers, ranging from eight to 80 channels. So far, only thermally stabilized AWGs are affected by the small form factor. Athermal AWGs are more convenient for passive network assemblies that do not require temperature control and a power source.
NeoPhotonics makes mechanically compensating filters for athermal AWGs as well, and according to Lipscomb these units will eventually adhere to new MSA standards. The need for thermal stabilization is common to the industry and will increase as the DWDM networks grow.
The original MSA, established in 2002, will continue to be supported. But the photonics industry is quickly changing. Potential innovations such as nanowires able to carry single photons could profoundly change the fiber systems architecture. In the near term, the industry is already moving to a new level of chip design, says Lipscomb.
“The limit may not be reached for quite some time. There’s a lot of work being done on smaller chips,” he says.
—Paul Livingstone
Resources
• NeoPhotonics Corp., San Jose, Calif.,
408-232-9200, www.neophonotics.com
