Laser is the shortened form of light amplification by stimulated emission of radiation. a laser emits radiation of essentially one wave length or a very narrow band of wavelengths. This means that the light has a single colour. ie; monochromatic

The unique property of light produced by a laser is that the emission is in the form of a very narrow beam without significant divergence.

The light generating process of a laser diode is similar to that of an LED and the materials used are often the same. The difference is that the laser diode uses a much smaller junction and the concentration of injected carriers is much higher.

The active region of a laser diode is enclosed by two aluminum enriched layers of lower refractive index to act as optical reflectors. Because of the confinement caused by these reflectors, light can exit only from the front or back faces of the laser diode. These faces are semi-transparent and form a resonator cavity for the light. At a certain current density within the active region of the junction, the optical gain of photons generated exceeds the loses from the faces and the operating mode of the junction changes from random type of the LED output to an organised, coherent, stimulated emission of a laser. The threshold at which this change occurs is between 50mA and 150mA, depending upon the laser diode material. The amount of emitted light decreases with increase in temperature, so the laser diode must be kept in cool with the help of Heat sink and other mechanical schemes. Laser diodes can produce large amount of optical output power with a very narrow output spectrum and hence little dispersion and overlap.
They produce a very tightly directed beam with small Numerical Aperture (NA) which can be directed into the fiber with little loss. Laser diodes can operate at rates exceeding GHz but need more complex circuitary to drive them than that of LEDs.

Because of high Energy density, a laser beam can be quite dangerous. Eye protection must be worn when working with these devices.

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