Dynamic channel equalization improves OADM/optical switch designs

The slowdown in telecommunications spending clearly indicates that increased bandwidth capacity will not automatically translate into increased demand for the optical components and subsystems needed to enhance the network infrastructure in place today. Instead, the industry debates continuously which architecture, technology and equipment will ultimately provide the speed, performance and economy required in second-generation optical networks.

Even though the shift from point-to-point networks to a more meshed architecture is inevitable, it is difficult to predict with any accuracy how the dynamic nature of this new architecture will evolve. Numerous component and subsystem manufacturers are aggressively pursuing the challenge of developing innovative products that control and manipulate light in optically reconfigurable networks.

The ability of component vendors to provide highly flexible, scalable, integrated solutions for managing complex optically reconfigured networks will determine what succeeds in terms of second-generation optical networks. Products based on diffractive microelectromechanical systems (MEMS) have emerged as the optimal platform for achieving enhanced dense wave-division multiplexing (DWDM) operational efficiency and economy, as well as the best platform for low-cost, high-volume manufacturing. The technology will enable the network to transition to dynamic architecture, allowing carriers to optimize bandwidth and implement additional revenue-generating services.

Dynamic components complement the traditional active and passive components by controlling and purifying light for optimum link performance. These components can be software-configured and electronically controlled to compensate for changes in performance by enabling switching between optical paths, equalizing channel power, managing link dispersion and optimizing amplifier chain performance. By optimizing link performance, service providers can quickly reconfigure optical paths while also maintaining maximum network reliability.

Second-generation optical systems require dynamic gain and dynamic channel-equalization applications in metro core and long-haul DWDM optical networks. Dynamic gain equalizers (DGEs) are powerful new components that are able to reduce the attenuation of the individual wavelengths within the light band. Their uses include power control in dynamic erbium-doped fiber amplifiers (EDFAs) and power equalization in metro, long-haul and ultralong-haul transmission, to flatten EDFA gain profiles or control gain shape changes caused by add/drops, thus minimizing the need for optical-electrical-optical regeneration. The device is invaluable in its ability to compensate for the nonlinearity (nonflatness) of EDFA optical amplifiers.

High SNRs

DGEs compensate amplifier gain profiles, which in turn enables high signal-to-noise ratios for all channels in an optical amplifier system. Dynamic gain compensation improves system performance, enhances system flexibility and increases the distance between amplifiers in long-haul and ultralong haul transmission systems. Designers can deploy DGE devices with EDFAs and Raman amplifiers in the C, L or Extended bands.

Dynamic channel equalizers (DCEs) are able to equalize or block individual channels across the C, L or Extended bands. Applications for DCEs include long-haul and ultralong-haul transport, DWDM optical add/drop multiplexers and wavelength selective switching. When used in a feedback loop, the DCE is able to compensate for variations in transmission losses on individual channels. Designers may also use DCEs to block or attenuate a channel completely.

DCEs are electronically controlled and software-configured, offering agility in responding to changing network or system conditions, as well as simplicity in control and configuration. The DCE's ability to control the power of 100+ wavelengths in one compact module eliminates the need for multiple discrete components to control each individual channel.

The high-channel-count device provides seamless attenuation control, a wide dynamic range for blocking and 4X reduction in size and power compared to existing discrete architectures. DCEs also selectively control output power for any pass-through channel.

Its ability to package more powerful and complex functions in smaller packages makes MEMS the ideal technology for building dynamic optical components. Diffractive MEMS technology has proven to be the most flexible platform for this process. The technology is based on D-MEMS actuators; a series of silicon-nitride ribbons suspended above a silicon substrate, which utilize the wave properties of light to provide varying levels of attenuation.

Fabricated devices generally are quite reliable since, with these small deflections, the material experiences negligible fatigue. Devices have been actuated more than 100 billion times with no measurable change in performance. An important added advantage is that these devices offer immunity to ambient acoustic noise and vibration.

The full version of this article was presented at the 2002 Communications Design Conference.

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