Laser Diodes in Fiber Optic Communication

Laser is an abbreviated form of ”Light Amplification by the Stimulated Emission of Radiation”. A laser is a device that produces optical radiation by the process of optical amplification based on the stimulated emission of electromagnetic readiation. Laser was developed in 1960. Invention of Laser is one of the major milestones in the history of fiber optic communication. Theodore H. Maiman at Hughes Laboratory invented the first Laser, based on teh theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

A Laser emits coherent light and this is the major distinguishable differencce that laser has compared with other light emitting devices used for optical communication. This difference makes laser diodes suitable for long distance fiber optic communication using singlemode fibers.

A laser diode, which is often abbreviated as LD, is an electrically pumped semiconductor laser in which the active laser medium is formed by a p-n junction of a semiconductor diode. This p-n junction of a LD is similar to that found in a light-emitting diode (LED). LDs have found their use in fiber optic communication, barcode readers, laser printing devices, laser scanning, DVD/Blue-ray disc reading and recording and applications where narrow high intensity light beam is required.

From the view point of electrical science, a laser diode is a P-I-N diode. The active region of a laser diode is in the intrinsic region, which is the “I” in the P-I-N. The electrons and holes are pumped into the I region from the N and P regions. These electrons and holes are called carriers.

Initial researches and developments in laser were foucssing on simple P-N diodes. Current trend is to use double-heterostructure, in which the carriers (electrons, holes) and photons are confined so that their chances for light generation and recombination are higher. The idea is to provide more chance for them to get excited and emit light.

In order to achieve the above goal, laser diodes are fabricated using direct bandgap semiconductors. The laser diode epitaxial structure is grown using one of the crystal growth techniques, usually starting from an N doped substrate, and growing the I doped active layer, followed by the P doped cladding, and a contact layer. To provide lower threshold current and higher frequency, the active layer consists ofquantum wells.

A simple laser diode is inefficient to meet most of today’s applications. Simple laser devices require so much power that they can only achieve pulsed operation without damage.Practical application demands customizations in laser diodes and the available types of lasers are Double heterostructure lasers, Quantum well lasers, Quantum cascade lasers, Separate confinement heterostructure lasers, Distributed Bragg Reflector lasers, Distributed feedback lasers, VCSELs, VECSELs and External-cavity diode lasers.

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