Difference between LED and Laser Diode:

The Difference between LED and Laser Diode is given below:

As already mentioned LASER is the shortened form of light amplification by stimulated emission of radiation. A laser emits radiation of essentially one wavelength, or a very narrowband of wavelengths. This means that the light has a single colour i.e., a monochromatic. Laser light is referred to as coherent light as opposed to light made up of a wide band of wavelengths, which is termed incoherent.

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 beam contains sufficient energy to weld metals or to destroy cancerous growths. It can also be applied to precise measurements, to guidance of industrial machinery, to optical fiber communication techniques.

In the LED, the source of light is the energy emitted by electrons which recombine with holes (at a lower energy level). In the case of an LED, the light is incoherent i.e., it is made up of a wide spectrum of wavelengths.

In a laser the atoms are struck by photons (or packets of energy) which are exactly similar to the photons of energy emit­ted when recombination occurs. This triggers energy emission and results in two identical photons for each recombination : the incident photon and the emitted photon. The photons produce further emission of similar photons, which in turn generate more similar photons. The result is an emission of energy in the form of a beam of coherent light. LEDs can be designed so as to produce coherent light with a very narrow bandwidth.

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 area and the concentration of injected carriers (holes and electrons) is much higher.

The active region of a laser diode is enclosed by two aluminium-enriched layers of lower refractive index to act as optical reflectors. Because of the confinement caused by these reflector, light can exit only from the front or back faces of the laser diode. These faces are semitransparent 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 losses from the faces and the operating mode of the junction changes from the random type of the LED output to an organized, coherent, stimulated emission of a laser.

The threshold at which this change occurs is between 50 mA and 150 mA, depending upon the laser diode material (in contrast to the LED, which has a very low current threshold of operation). The amount of emitted light decreases with increasing temperature, so the laser diodes must be kept cool with the use of heat sink and other mechanical schemes. Laser diodes can produce large amounts of optical output power (20 to 100 mW), 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. This makes them suitable for use with the thin monomade fibers. Laser diodes can operate at rates exceeding GHz but need more complex circuitry to drive them than that of LEDs.

Because the high energy density, a laser beam can be quite dangerous. Eve protection must be worn when working with these devices.

Laser diodes are used in a large variety of applications-telecommunications, data communications, broadband access, industrial, aerospace, test and measurement, medical and defence industries. They are also used in laser printers and consumer products requiring large-capacity optical disk systems, such as compact disk (CD) and digital video disk (DVD) players. In broadband communication, they are employed with fiber-optic cables for increasing the internet speed.

New applications are being found as the lasing wavelength is pushed lower into the visible spectrum with visible laser diodes (VLDs). Also, near-infrared diodes are being used in machine vision systems, sensors, and security systems.