A new type of pulsed laser has
been recently developed- the dark laser. The technology is available with the
aid of quantum dot diode lasers and has a great potential in the fields of
telecommunications and fiber data transmission applications.
A laser device can either
produce a continuous beam of light or short intense pulses of light (pulsed or
mode-locked laser). Mode-locked lasers produce very short bright pulses that
last from a few femtoseconds (1ps = 10-15 s) to a few picoseconds (1fs = 10-12
s).
A new type of pulsed laser was
recently developed in the National Institute of Standards and Technology
(NIST), JILA-a joint institute of NIST and the University of Colorado. This new
device called a “dark laser" or “dark pulse laser," produces a train
of ultrafast dark pulses or periodic intensity decreases, in the steadily
transmitted beam of light. The use of quantum dots - clusters of atoms that
emit light when excited – has made possible this new development that may be
most promising for the future of fiber data transmission and telecommunications.
Dark laser's pulses can be
correlated to dark solitons. Solitons are pulses of light that do not disperse
after passing through a special optical material, and therefore do not loose
energy with distance. Dark solitons are difficult to generate and, when this is
the case, only a small number are produced through shaping systems outside the
laser.
The production of dark pulses
from within the laser is possible through a rare operating regime of the
quantum dot diode laser. This regime is described by the Haus equation. The
positive solutions of this equation correspond to bright pulses, while the
negative are related to dark pulses.
The operation is based on the
passing of electrical current through the junction of a p-type semiconductor
(surplus of positively charged holes) and an n-type semiconductor (surplus of
negatively charged electrons). As the moving electrons reunite with the
existing holes (exciton effect),
the released energy is taking the form of light inside the quantum dots. The
cavity of the laser is used to amplify the light generated by the quantum dots.
The complicated dynamics of the dots result in a repeated (periodic) drop in
light intensity of about 70%. This is actually a gray pulse and not a black
pulse, which would be the result of a 100% drop in intensity (black laser).
In the case of the laser
developed by NIST, indium gallium arsenide quantum dots were used with a 10nm
diameter. The dots were put in the core of a 5-mm-long gallium arsenide ridge
waveguide that acted as the gain medium. After the dots were stimulated by
electrical currents, they emitted light of the same frequency due to their same
size.
The dark laser is a good
candidate for fiber optics telecommunications. Information can be encoded in
dark pulses the same way it is done with bright pulses and travel through long
distances without being distorted or degraded in quality. Bright pulses travel
along the existing fiber optic networks that have been designed to reduce the
loss of energy over the distance. Similarly, a new modified fiber optic network
should be developed especially for dark pulses.
The dark laser can also be
used for short timescale measurements on the infrared frequency, since its
ultrafast pulses are in the order of femtoseconds (10-15 sec) and picoseconds
(10-12 sec). Other applications include optical atomic clocks and optical
frequency combs. The lack of dispersity and the pulse linearity are two
properties that make the above applications possible. Both properties give
special advantage to the dark laser; however, its future role is yet to be
determined.
Although the development of
dark laser technology seems to be very promising in the fields of networking and
telecommunications, it is too early to assess any possible impact. This is due
to the equally impressive advances in the field of bright pulsed lasers, which
may eventually surpass the advantages of this new technology.