Tunable lasers as the name suggests are lasers whose wavelengths can be tuned
or varied. They play an important part in optical communication networks. Recent
improvements in tunable laser technologies are enabling highly flexible and effective
utilization of the massive increases in optical network capacity brought by large-scale
application of dense wavelength division multiplexing. Several tunable laser technologies
have emerged, each with its own set of tradeoffs with respect to the needs of
particular optical networking applications. Tunable lasers are produced mainly
in 4 ways: The distributed feedback laser (DFB), the external cavity diode laser,
the vertical cavity diode laser and the micro electro mechanical system (MEMS)
technology. Tunable lasers help network administrators to save a lot of cost,
by allowing them to efficiently manage the network with lesser number of spares.
They also enable reliable functioning of the optical network. Changing traffic
patterns, customer requirements, and new revenue opportunities require greater
flexibility than static OADMs can provide, complicating network operations and
planning. Incorporating tunable lasers removes this constraint altogether by allowing
any channel to be added by the OADM at any time.
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
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.
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.
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.
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