Laser diode are semiconductor device that converts electrical signal to light .
Construction:
They are made up of:
· GaAlAs (Gallium Aluminium Arsenide) for short wavelength.
· InGaAs(Indium Gallium Arsenide) for long wavelength devices.
Conversion of electric current into light is fairly efficient
Little heat is generated when compared to incandescent lamp.
Types of laser diodes:
· Mono-junction: Only one p-n junction
· Hetero-junction: several layers of p and n materials
The two ends of the structure is optimally flat.
It has mirror in one end and partially reflective surface on the other
Operation:
· Length of the junction is related to wavelength of light emitted.
· Input is given from a special current controlled DC supply.
· When the junction is forward biased, electron and holes recombine, during this process they emit photons. These photons reflect back and forth between the mirror surfaces.
· The region between the electrons acts like a cavity that filters light and purify colour. The laser cavity is a fraction of mm length.
· The photons bounce back and forth and induce more recombination. When more recombination happens more new photons are created. This cumulative action causes avalanche effect.
· All the photons generated merge from the partial end
· As temperature increases threshold current also increases.
Advantages:
· Compact
· Relatively efficient when compared to gas lasers.
· Rugged and long life.
Disadvantages:
· Require maintenance in drive electronics.
· Optical performance is less than other type of lasers.
· Coherence length and monochromic properties are less.
· Most laser diodes are not suited for holography.
Applications:
· CD players
· Pointers-Visible lights
· Laser printer
· Fibre optics
· UPC scanners
· Barcode scanners
· CT and MRI Scanners
· DVD technology
· Laser cameras
· Laser typesetters.
