Optical Network Components

Aileen Sansone

Since 1999, the fiber component industry has shifted its developmental resources from volume production of leading-edge components to delivering the best component value for the equipment manufacturer’s dollar.

This shift lies at the confluence of several other trends. Among them is the move toward implementing equipment in the metropolitan and access portions of the network, which will emphasize small components that are low in cost and electrical power dissipation. Integration of optics and electronics into higher-level modules also adds value.

Efforts to standardize components for optical networking have produced such devices as multisource agreement transponder modules that, in their second generation, resemble a 1/2-in.-thick credit card in dimension. Their size signifies a 60 percent reduction from first-generation modules. Also, new devices require only 7 W of electrical power, compared with 9.5 W for their predecessors. Courtesy of JDS Uniphase Corp.

As network demands increase value, system requirements are calling for components that enable more flexible architectures, longer transmission spans and greater spectral efficiency. Together, these trends will drive development of new component technologies.

To reduce component size and cost, manufacturers are combining multiple functions in single packages. New optical amplifier gain blocks, for example, integrate the pump laser, erbium-doped fiber and any necessary couplers and isolators into a module measuring just 70 x 90 x 12 mm and providing 24 dB of gain with 15 dBm of maximum output power.

Uncooled pump lasers are a recent breakthrough in this regard. Cooling technology is expensive and bulky, and often requires more power than the component being cooled. An uncooled pump can reduce the power consumption in a gain block such as that described from about 4.5 W to less than 1 W. The result is a module that is less expensive and smaller.

Semiconductor technology enables even smaller optical amplifiers that can fit into a 14-pin butterfly package. Historically, they have been polarization-sensitive, but innovations have addressed this drawback, making these devices candidates for switching and receiver applications as well as amplification.

The modularization of optical functions and the development of more flexible system architectures has initiated standardization and cooperative industry efforts. The results include multisource agreement transponder modules, first available about a year ago. At 10 Gb/s, these transponders established optical, electrical and mechanical standards. Since then, the second- and third-generation models have reflected the industry trends of reduced size, cost and electrical power. Third and future generations of these products should advance these attributes.

Modules are becoming more common in equipment vendors’ systems because of their interchangeability and reduced cost, but they must become even more flexible to meet the needs of metropolitan and access applications. One challenge will be to include tunable lasers instead of fixed-wavelength lasers without compromising the modules’ compactness and low power requirements.

The drive to miniaturize also has led to developments in optical switches. Typical are micro-optical mechanical switches and microelectromechanical systems (MEMS) with dimensions approximately half of those on the market a year ago. Vendors are addressing reliability concerns with MEMs switch arrays, and some versions have passed Telcordia qualification.

Greater flexibility

Leading-edge developments are focused on technologies that increase network flexibility. A variety of new products are required to enable remote or automatic bandwidth management, or to select and route signals. Products such as tunable lasers and filters are becoming available. More sophisticated flexible switches, optical add/drops, variable optical attenuators, dynamic gain equalizers, transient-suppressed amplifiers and optical network monitors are also needed for reliable and effective operation of these networks.

Optical monitoring equipment will be critical to the success of transparent, flexible optical networks because the status of multiplexed signals at each element must be tracked.

The monitoring needs are trending toward simple, lower-performance devices that not only can determine which dense wavelength division multiplexing (DWDM) channels are present, but also can measure the optical signal-to-noise ratio in each channel and the optical power. The need also is increasing for optical monitors that can perform extended sets of tests on the DWDM signals, including more accurately assessing the signal-to-noise ratio and/or bit error rate.

Raman amplification continues to power longer transmission spans. Emerging products will combine Raman and erbium-doped fiber amplification, both of which have the potential to select the gain profile in each amplifier very accurately to optimize overall system performance.

Alternate modulation formats also will increase the potential for longer transmission spans. Nonreturn-to-zero transmission formats have been the norm for most optical systems, but return-to-zero formats are beginning to enable ultralong-haul distances. Modulator products for either format are increasing in efficiency.

Spectral efficiency

Network architects are investigating two approaches to higher spectral efficiency: increasing the data rate and/or increasing the channel density.

For higher data rates, the industry is developing technology to enable 40-Gb/s transmission. Modulators, photodetectors, tunable chromatic dispersion compensators, polarization mode dispersion compensators, the electronics to drive these components and the test equipment to verify performance are all available.

Denser WDM transmission depends on the successful development of filters for narrower channel spacings as well as steeper filtering to skip fewer channels in banded architectures. Filters with channel spacings of 25 GHz are on the market, and 12.5-GHz filters are on the way. Components that operate in the L-band have also emerged.

As we enter 2002, the fast pace of technological change, invention and development in fiber optic components will continue. Innovation is still the name of the game. Component manufacturers will further refine their technologies in devices with increased value, lower cost, smaller dimensions and more efficient power consumption.

Meet the author

Aileen Sansone is manager of technology development for JDS Uniphase Corp. in Bloomfield, Conn.