Tunable lasers

Lasers

          Lasers are devices giving out intense light at one specific color. The kinds of lasers used in optical networks are tiny devices — usually about the size of a grain of salt. They are little pieces of semiconductor material, specially engineered to give out very precise and intense light. Within the semiconductor material are lots of electrons — negatively charged particles. Not just one or two electrons, but billions and billions of them. Some of these electrons can be in what is known as an “excited” state, meaning that they have more energy than regular electrons. An electron in an excited state can just spontaneously fall down to the regular “ground” state. The ground state has less energy, and so the excited-state electron must give out its extra energy before it can enter the ground state. It gives this energy out in the form of a “photon” — a single particle of light.

Tuning With Micromechanical Elements

          In tuning VCSELs, the technique used is based on mechanical modification of the laser cavity using micro electro mechanical systems (MEMS) technology. With MEMS, a movable mirror can be fabricated at one end of the laser cavity. This approach enables VCSEL/MEMS devices to achieve a relatively wide tuning range--preliminary specifications from manufacturers quote tuning ranges of 28-32 nm, enough to cover 35-40 channels at the standard 0.8-nm channel spacing.

Introduction

In a wavelength-division multiplexed (WDM) network carrying 128 wavelengths of information, we have 128 different lasers giving out these wavelengths of light. Each laser is designed differently in order to give the exact wavelength needed. Even though the lasers are expensive, in case of a breakdown, we should be able to replace it at a moment's notice so that we don't lose any of the capacity that we have invested so much money in. So we keep in stock 128 spare lasers or maybe even 256, just to be prepared for double failures. The devices themselves are still semiconductor-based lasers that operate on similar principles to the basic non-tunable versions. Most designs incorporate some form of grating like those in a distributed feedback laser. These gratings can be altered in order to change the wavelengths they reflect in the laser cavity, usually by running electric current through them, thereby altering their refractive index. The tuning range of such devices can be as high as 40nm, which would cover any of 50 different wavelengths in a 0.8nm wavelength spaced system. Technologies based on vertical cavity surface emitting lasers (VCSELs) incorporate moveable cavity ends that change the length of the cavity and hence the wavelength emitted. Current designs of tunable VCSELs have similar tuning ranges.

The Distributed Bragg Reflector (DBR)

          A variation of the DFB laser is the distributed Bragg reflector (DBR) laser. It operates in a similar manner except that the grating, instead of being etched into the gain medium, is positioned outside the active region of the cavity. Lasing occurs between two grating mirrors or between a grating mirror and a cleaved facet of the semiconductor.

Simplified Capacity Planning and Expansion

          Similarly, as conditions in the rest of the network change, service providers need the flexibility to change wavelength assignments in the network in order to ensure non- locking pathways. If the system relies on fixed-wavelength lasers, then nodes along the route must all be equipped with unique line cards for every possible channel, even though only a limited number will be used at any one time. With tunable lasers in the network, the nodes would only need to be stocked with the number of line cards needed to meet actual demand. This simplifies capacity planning to forecasting overall demand and allocating the appropriate number of needed channels.

Future Of Tunable Lasers

          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. IDC 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.

Conclusion

          Recent advances in tunable laser technology have brought the promise of tunable networks into clear focus. Widespread adoption of tunable lasers will not only eliminate logistical and inventory problems and the associated costs that result from fixed- wavelength line cards— but will also enable novel network architectures with dynamic functionality such as dynamic add-drop, thus enabling new value-added services and creating new sources of top-line revenue for system providers.