What is a Dark Laser?

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.

Pulsed Lasers - The Dark Laser

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.

Operation of Dark Laser

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.

A Dark Pulsed Laser - the "Opposite" of a Bright Pulsed Laser

Dark Lasers Used in Telecommunications & Fiber Data Transmission

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.

The Future

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.