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.
What if we have a multifunctional laser for the optical network that could be adapted to replace one of a number of lasers out of the total 128 wavelengths? Think of the money that could be saved, as well as the storage space for the spares. What is needed for this is a ?tunable laser,?
Tunable lasers are still a relatively young technology, but as the number of wavelengths in networks increases so will their importance. Each different wavelength in an optical network will be separated by a multiple of 0.8 nanometers (sometimes referred to as 100GHz spacing. Current commercial products can cover maybe four of these wavelengths at a time. While not the ideal solution, this still cuts your required number of spare lasers down. More advanced solutions hope to be able to cover larger number of wavelengths, and should cut the cost of spares even further.
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.
??????????? 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.
In a laser we want lots of light to come out. If we just wait for electrons to spontaneously ?decay? from the excited state to the ground state, we are not going to get much light out at all. So what we need to do first is to get lots of electrons into the excited state. To do this we apply an electric current to the laser, which puts lots of electrons up into this excited state (sometimes referred to as ?population inversion?).
So we now see more and more spontaneous emission of photons caused by electrons decaying from the excited state to the ground state. But this is still not enough light for what we need. We want lots of these electrons to decay at the same time to give lots of light out, and we want this to be happening all the time so that we have a steady stream of light.
We want to catch, or ?confine,? the spontaneously emitted photons within the laser. We want them to travel back and forth through the laser time and time again, because these photons can encourage other excited electrons to fall to the ground state and give out more photons. These photons are stimulating emission of further photons, and therefore effectively amplify the light within the device. And all the time an electric current is putting more electrons into the excited state where they wait to fall to the ground state and give out light. Hence we have a LASER ? Light Amplification by Stimulated Emission of Radiation (the radiation in this case is light).
Different materials can be used to obtain different wavelengths from the laser. In actual fact, most lasers used in optical networks will operate at wavelengths of around 1300nm or 1550nm, as these are points of minimum loss within optical fibers.
??????????? The operation of a ruby laser illustrates the basic lasing principle. When optically "pumped" by light from the flash tube, the ruby rod becomes a gain medium with a huge excess of electrons in high-energy states. As some electrons in the rod spontaneously drop from this high-energy level to a lower ground state, they emit photons that trigger further stimulated emissions. The photons bounce between the mirrors at the ends of the ruby rod, triggering ever more stimulated emissions. Some of the light exits through the half-silvered mirror.
NEED FOR TUNABLE LASERS
??????????? Today, single fiber-optic strands carry multiple wavelengths of infrared radiation across entire continents, with each wavelength channel carrying digital data at high bit-rates. Known as wavelength-division multiplexing (WDM), this process greatly expands the capacity of fiber-optic communications systems. Currently, WDM transponders, which include the laser, modulator, receiver, and associated electronics, incorporate fixed lasers operating in the near-infrared spectrum, at around 1550 nm. A 176-wavelength system uses one laser per wavelength, and must store 176 additional transponders as spares to deal with failures. These devices therefore account for a high percentage of total component costs in an optical network.
Tunable lasers offer an alternative. A single tunable laser module can serve as a backup for multiple channels, so that fewer transponders need to be stocked as spares. The result: cost savings and simplification of the entire sparing process, including inventory management. While applications in inventory reduction will drive much of the initial demand for tunable lasers, the real revolution will come when they are applied to make optical networks more flexible.
Fiber-optic networks today are essentially fixed: the optical fibers are connected into pipes with huge capacity but little re-configurability. It is well-nigh impossible to change how that capacity is deployed in real time. Part of the problem is the difficulty of choosing a wavelength for a channel: as traffic is routed through a network, certain wavelengths may be already in use across certain links. Tunable lasers will ease a switch to alternative channels without swapping hardware or reconfiguring network resources.
Tunable lasers can also provide flexibility at multiplexing locations, where wavelengths are added to and dropped from fibers, by letting carriers remotely reconfigure added channels as needed. Such lasers can help carriers more effectively manage wavelengths throughout a network, based on different customer requirements. The benefits gained are a far greater degree of flexibility in provisioning bandwidth and a reduction in the time it takes to actually deliver new services.
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