Laser Diode

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The   term   laser   stands   for   light   amplification   by   stimulated   emission   of radiation.  Laser  light  is  monochromatic,  which  means  that  it  consists  of  a  single colour and not a mixture of colours. Laser light is referred to as coherent light which     means that a light with a single wavelength. This is opposite to the incoherent light,

which has wide band of wavelengths. The light emitted by LED is an incoherent light   whereas light emitted by laser diode is a coherent light.

Construction and Symbol

The  Fig  shows  the  symbol  and  basic  construction  of  laser  diode.  It  consists  of  pn junction formed by two doped gallium arsenide layers. The two ends of the structure
are flat and parallel with one end mirrored and one partially reflective. The length (L)
of the junction is precisely related to the wavelength of the light to be emitted.




Operation

The operation of laser diode is illustrated in the Fig. When the pa junction is
forward  biased  by  an  external  voltage  source,  the  electrons  move  through  the junction  and  recombines  as  in  an  ordinary  diode.  When  electrons  recombine  with holes,  photons  are  released.  These  photons  strike  atoms,  causing more  photons to be released.


As the forward bias current is increased, more electrons enter the depletion region
and  cause  more  photons  to  be  emitted.  Eventually  some  of  the  photons  that  are randomly drifting within the depletion region strike the reflected surfaces
perpendicularly,  so  that  they  are  reflected  back  along  their  original  path.  These reflected photons are then reflected back again from the other end of the junction.


This movement of photons from one end to another end continues for thousands of     times.  During  this  movement  photons  strike  more  atoms  and  release  additional photons  due  to  the  avalanche  effect.  This  activity  of  reflection  and  generation  of increasing number of photons results in a very intense beam of laser light which is     formed  by  the  photons  that  pass  through  the  partially  reflective  end  of  the  pn junction. Each photon produced in the above explained emission process is identical
to  the  other  photons  in  energy  level,  phase  relationship,  and  frequency.  Thus emission  process  gives  a  intense  beam  of  laser  light  with  a  single  wavelength.  To produce  a  beam  of  laser  light  it  is  necessary  to  have  a  current  through  the  laser diode above certain threshold level. The current below threshold level forces diode to behave as LED, emitting incoherent light.
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