What is Laser Diode?

Laser diode
Laser diode
 
Laser Diode is a PN Junction semiconductor diode which converts electric energy into light when external voltage is applied. LASER is the abbreviation of  Light Amplification by Stimulated Emission of Radiation.

Symbol of Laser Diode:


Symbol of Laser diode
Symbol of Laser diode


The symbol of laser diode is similar to Light emitting diode. The arrow shows the light generated and emitted.

 

Construction of Laser Diode:



Construction of Laser diode
Construction of Laser diode
 

                             
The construction mainly consists of metal contact, P type material N type material,  intrinsic active region. The P type and N type material is formed by doping aluminium or silicon with Gallium arsenide. The active region is made up of undoped gallium arsenide. This increases the active region, so that holes and electrons combining area increases. In the mirror polished surface more holes and electrons get reflected and combine with each other which results in more radiation.

Working of Laser diode:


Energy Absorption:


Energy absorption
Energy absorption

 

The Laser diode consists of both holes and electrons . when the external voltage is applied at the PN Junction the electrons absorbs some  energy and get excited to higher energy level from lower energy level. The electrons remains excited for few nano seconds before they combine with holes. This time is called recombination time.

Spontaneous emission:


Spontaneous emission
Spontaneous emission

         
             
After the electrons get excited and recombine with the holes they again travel to lower energy level. While travelling to lower energy level they emit photon in the form of electromagnetic wave and the energy of the photon is the difference between the higher energy level and lower energy level.

Stimulated emission:


Stimulated emission
Stimulated emission

 
As we need more coherent photons, reflecting mirrors are used at the either side of the PN Junction diode. So the photons gets trapped inside the junction and they stimulate the excited electrons to recombine with the holes before the recombination time and thus more photons are emitted . when the photon concentration reaches the threshold they escape through the partially reflecting mirror . This results in bright monochromatic coherent light.


Properties of Laser Light:

 
Properties of Laser light
Properties of Laser light



Monochromatic: 

Emits photons of only one wave length

Coherent: 

All emitted photons have same time and phase relationship.

Directional: 

Emitted photons are not diverted , highly directional.

The light from the sun and from most of the artificial light have waves with multiple wavelength and their phases differ from each other. The light from the monochromatic light have waves with same single frequency but their phases differ. 
The light from the laser light have same frequency and the peaks of the phases are same. So the laser light are very sharp and focused to a tiny point.


Advantages


  • Less expensive and compact
  • Uses less power and gives high power output
  • Easy to manufacture

Disadvantages


  • It has heating problem
  • For large source they need costly optics
  • Need high current to drive large laser


Application of Laser diode:


  • In medicine
(e.g) Bloodless surgery, cancer treatment, eye lens , cosmetic treatment etc
  • In communication
(e.g) Under water , space, satellite, long distance etc
  • In industries
(e.g) Cut glass and quartz, trimming ICs, photolithography etc
  • In science and technology
(e.g) Detecting earth quakes, underwater nuclear blast, computer printers etc