LOCAL-AREA NETWORKS: Wide WDM allows 10-Gbit/s applications

Local-area networks (LANs) are the heart and soul of business operations. Current LAN standards rely on light sources such as light-emitting diodes, Fabry-Perot or distributed-feedback (DFB) lasers, and vertical-cavity surface-emitting lasers (VCSELs) for data-transmission speeds that range from 125 Mbit/s to 1.25 Gbit/s. Multimode fiber is commonly used for distances of less than 500 m, while single-mode fiber spans distances up to 10 km. The next generation of LAN standards, currently under development, will reach 10 Gbit/s.

Guiding the relevant standards development are the IEEE 802.3ae committee, which is developing the 10-Gigabit Ethernet Standard, and the Optical Internetworking Forum, which is an industry group developing low-cost technologies for short-reach 10-Gbit/s transmission (OC-192).

As the data- and telecom industries search for light sources at this speed and distance, several new technologies are being considered, each for different distances and fibers, though some may have multiple uses. For example, a parallel-optics approach could use 850-nm VCSEL arrays to provide ten 1-Gbit/s channels over less than 100 m of multimode fiber. This approach is a straightforward way of producing low-cost components for short-reach interconnects; however, a network would require the installation of expensive cabling. A serial approach based on 1300-nm Fabry-Perot or DFB lasers could reach up to 2 or 10 km, respectively, over single-mode fiber. These technologies have advantages in terms of simplicity but run into expensive packaging issues while requiring significant technical advances.

Alternative approach

Agilent Laboratories (Palo Alto, CA), part of Agilent Technologies, has demonstrated an alternative approach that reaches from 2 m to 10 km and is usable over either single-mode or multimode fiber. At the Optical Fiber Communication conference (Baltimore, MD; March 5-10), researchers presented their prototype transceiver module for wide wavelength-division multiplexing (WDM). Their module is a 20-nm-spaced, four-channel link producing an aggregate bit rate of 10 Gbit/s through 300 m of multimode fiber. Over single-mode fiber approximately 10 km can be supported.

Although widely used in long-haul telecom, WDM is usually a high-component-cost technology with tight channel spacing (<1 nm). Principal project scientist Lisa Buckman said that the Agilent group selected wide channel spacing in their design so that temperature control would not be required, hence lowering the cost. In addition, the four 2.5-Gbit/s DFB lasers in the module do not require a high side-mode-suppression ratio because of the relatively short transmission distances. "Although the prototype was spaced at 20 nm, the final product will be at 24.5 nm," she said. "This will allow us to fully utilize the 1270- to 1355-nm transmission window."

The four DFB lasers were placed on a ceramic submount and aligned to a low-cost, single-mode four-to-one silica waveguide combiner. Buckman said that the final product would use a precision alignment system that is currently being tailored for this transceiver but is widely used in the company's small-form-factor product line.

Also, within the module was a plastic demultiplexer, which used "home-grown" interference filters and an imperfect mold, but the resulting transmission spectrum was excellent.

For the future, Buckman said, "we'll increase the speed per channel before increasing channel density, which is more expensive." Eventually, the researchers see the possibility of producing 100-Gbit/s—and even 1-Tbit/s—transceivers as they find economic ways to bump up the channel transmission rates, then channel counts, and, finally, the fiber counts in a system.