Tunable lasers add agility, functionalities, and profits

Boosting the service provider's competitive edge, tunable lasers improve network efficiency and simplify network planning with more manageable inventory and maintenance procedures. Aldo Panessidi

As the telecommunications service industry rapidly moves towards powerful terabit networks, service provider profits are being seriously threatened. Amid intensely mounting competition, pricing wars, increasing operational costs, and the ever-narrowing window to market new services, service providers clamor for solutions to reduce complexities and costs of deploying and managing traffic for multiple services across the network.

One emerging concept holds the promise of welcome relief for service providers using DWDM systems: the evolving tunable laser technology. Partially because of the still-nascent technology—as well as the incremental evolutionary migration toward an all-optical network—tunable lasers have a first market application targeted at alleviating the congested fixed-laser sparing strategy.

Commercial lasers currently deployed in DWDM optical networks are designed to operate on specific wavelengths (see Fig. 1). Typically, each WDM wavelength needs to be fitted with an individual fixed laser manufactured for the appropriate frequency. Further complicating network planning and forecasting of future wavelength requirements, their narrow line design introduces a substantial capital expenditure, as a suitable sparing inventory must be maintained. In fact, when one considers the network's steady evolution to accommodate 160-wavelength capability in the next-generation all-optical network, it becomes quite apparent that fixed lasers are not the most cost-effective alternative for these emerging ultrahigh-capacity networks.

Tunable lasers, on the other hand, offer an immediate, low-risk method to alleviate the need for a large depository of sparing inventory while facilitating the operational planning of the optical-layer wavelength scheme by allowing a tunable spectral range to be manipulated through a single laser. By simply selecting the wavelength, a service provider can best leverage network capacity to meet emerging multiservice market demands, while taking much of the operating expense and complexity out of the transport core. It further represents substantial cost reductions associated with field deployment and spare inventory, as well as with simplifying network planning and forecasting of future wavelength requirements.

EVOLVING TUNABLE TECHNOLOGY
There exists a variety of technologies governing tunable laser design; among them are distributed feedback (DFB) and vertical-cavity surface-emitting lasers (VCSELs). Both of these lasers utilize different technological approaches to enable selectable tuning over a range of wavelengths. To this end, the particular technology provides for different tuning ranges and optical power capabilities.

Presently, the DFB- and VCSEL-based tunable laser technologies offer the leading and most attractive commercial solutions for delivering tunable 10-Gbit/s solutions (see Fig. 2). The technology is evolving from tuning over 20 individual wavelengths in the C- and L-bands to full-band coverage, while meeting the full transmission requirements within a 10-Gbit/s system.

To achieve cost-effective plug-and-play capability, the tunable laser and associated components are self-contained in a transmitter module that interconnects with existing DWDM and crossconnect systems. In addition to a tunable laser, the transmitter module includes wavelength lockers, which ensure that the laser delivers the specified wavelength with a guarantee to maintain same until the unit's end-of-life, plus wavelength modulators, which create the light pulses required to carry traffic.

As the network migrates to increased channel counts, the ability to flexibly manipulate wavelengths on demand becomes of utmost importance to service providers. In fact, complete turnkey solutions specifically designed to leverage emerging agile wavelength technology are in progress. These versatile solutions will encompass both required processing circuits and the controlling software for monitoring wavelength signals at both the optical and physical layer.

AGILE WAVELENGTH
Existing networks are beginning their transition toward the service-oriented management platform of the "automatic switched optical network," with scalability and flexibility that bring dynamic switched optical connectivity to the network's service layer.

Designed to interoperate with optical crossconnect switches, the tunable laser will play a strategic role in the dynamic process of rerouting wavelengths discretely and arbitrarily around the entire network. Service providers can assess and leverage network capacity to provision new services "on the fly" that most profitably address market demands. The technology will allow service providers to substantially reduce capital and operating costs by simplifying network planning.

Volume production to meet the demand of both the long-haul and metro markets has already started. Initially, the tunable laser will have a first long-haul-market application to alleviate the costly sparing inventory required with fixed lasers. Its inherent agile design will provide the service velocity needed to enable service providers to introduce flexibly and rapidly new client-driven optical services—creating additional revenue streams. Tunable laser technology also can be utilized in tandem with photonic switches, providing a solid foundation for all-optical networking.

Aldo Panessidi is brand manager for OPTera Network Solutions, Service Provider and Carrier, at Nortel Networks, 2351 Alfred Nobel, St. Laurent, Que., Canada H4S 2A9. He can be contacted at [email protected].