Widely Tunable Lasers Give More Band for Buck - Technology Information

Telecommunications, April, 2001 by Kevin Affolter

Providers need a powerful, versatile laser technology to respond to changing capacity demands.

People are logging on to the Internet by the millions, but today's numbers are tiny compared to projections for Internet traffic in just a few years. IDG estimates the number of Internet users will increase from 150 million today to more than 500 million in four years. Those users won't want to hear about huge growth posing major challenges for service providers. They'll expect excellent, no-excuse service for low prices. In this demanding environment, providers must maximize capacity in every part of their networks to deliver more bandwidth for the buck.

Fiber-optic networks are now able to carry extremely high capacity, greater than 1 Tbps, on a single fiber thanks to DWDM and ever increasing line rates. Multiple wavelengths, or channels, can now be transmitted over long distances operating at 10 Gbps. While 10 Gbps has become the bit rate of choice, this will soon migrate to 40 Gbps and higher. Since the first DWDM systems were deployed in the mid-1990s, the number of wavelengths supported on a fiber has increased from less than 16 to more than 160. Each wavelength currently requires a dedicated, fixed wavelength laser. This approach is scalable and satisfies the raw demand for bandwidth but does not give service providers the level of flexibility they require in bandwidth allocation and provisioning.

Because of this market's dynamic nature, providers have very little reliable information about future demand, making accurate forecasts nearly impossible. Yet, the inflexible nature of fixed lasers requires providers to plan networks far in advance and then live with the result, with little or no margin for growth or change.

Getting on the Right Track

Narrowly tunable lasers, which can tune across 20 to 30 channels (approximately 12 nm), are being used now to reduce network planning complexity, lower the vast inventory of fixed-wavelength laser spares and better accommodate changing network demands.

With narrowly tunable lasers, service providers no longer need to have the wavelength-forecast accuracy required previously Systems vendors and component suppliers can easily accommodate last-minute changes in network demand and can keep their revenue streams on track in the face of customer changes. At the same time, providers are able to turn up services with greater certainty and are less likely to end up with stranded inventory.

Narrowly tunable lasers ease the burden of sparing. With fixed-wavelength lasers, providers need to keep a spare for each wavelength they have in operation. The cost of implementing a successful sparing strategy can typically be measured in tens of millions of dollars. With narrowly tunable lasers, spares can be shared around the network. A business can hold just a few narrowly tunable lasers in reserve to replace any laser wavelength that operates within its tunable range. The financial resources that were tied up in inventory can be put to more productive use.

As network demands change, narrowly tunable lasers offer providers flexibility. Although wavelengths are predetermined depending on the pattern of capacity demand, fluctuating traffic can shift the pattern, and a provider may find it can't use its fixed-wavelength laser because that wavelength is already in use on a fiber. A narrowly tunable laser, however, can be switched to a different channel and continue to carry traffic. In practice, this is very dependent on the system architecture. For example, if the laser is launched through a fixed filter the fiber from the laser will need to be connected to a different filter to allow the new wavelength to be launched onto the line. Likewise, it can offer some flexibility to drop and add wavelengths dynamically at more remote sites throughout a network, changing traffic capacity and patterns at network pinch points on the fly. For this flexibility to be realized there is a dependence on the availability of tunable add/drop multiplexers. Major service providers inc reasingly require this level of flexibility.

Taking the Next Step

Narrowly tunable lasers offer the right approach for cost-effectively maximizing bandwidth, but they don't go far enough. As bandwidth demands explode and networks increase in size, even these lasers aren't sufficiently flexible to ensure maximum capacity use. Providers need a more powerful and versatile laser technology that can support long distances and an almost unlimited number of channels. The high-power, widely tunable lasers coming onto the market have been endorsed as the way to eliminate having some network channels underutilized, while others are overloaded.

Widely tunable lasers are being used by optical networking vendors to help service providers maximize existing network resources, streamline the planning process for network growth, and dynamically provision bandwidth to millions of Internet users around the globe. Widely tunable lasers can instantly move traffic from overcrowded channels to unused channels on the fly.