Subject :- Applied Physics 2.

Topic :- Laser.

Difficulty :- Medium.

A semiconductor diode laser is a specially fabricated P-N junction diode that emits a coherent light when it is forward biased.

Construction:

The diode is small in size. The junction lies in the horizontal plane through the center. The top and bottom faces are metallized and ohmic contacts are provided to pass current through the diode. The front and rear faces are polished parallel to each other and perpendicular to the plane of junction.

The polished surface constitutes the resonator. The other two surfaces are roughened to prevent lasing action in that direction. The active region consists of a layer of about 1micrometr thickness. A simple way of achieving population inversion I to use it in the form of heavily doped PN junction and to forward bias it.

Energy band diagram:

For an unbiased PN junction, fermi level of p and n type semiconductor coincides. For a heavily doped n type semiconductor fermi level lies well within the conduction band. And for P type it lies well within the valence band.

When forward bias is applied, the energy level shift and the new distribution takes place. Electrons and holes are injected in to the depletion region which as a result decreases in width. The injected electrons and holes appear in high concentration in this transition region. At low forward current, the electron hole recombination causes spontaneous emission of photons and the junction acts as LED.

As the current is increased, the intensity of light increases linearly. When the current reaches threshold value, the carrier concentration in the depletion region will reach very high value. The upper levels in the depletion regions are having high population density of electrons while lower levels in the same region are vacant. This is state of population inversion.

The narrow region where population inversion is achieved is called inversion or active region. The forward bias plays the role of pumping agent. Art room temperature GaAs laser emit light of wavelength 9000 AU in IR region. GaAsP emits 6000AU in the visible region

Advantages:

• It is simple, compact and highly efficient
• Requires very little power and little auxiliary equipment is required
• It is tunable that means the wavelength of the emitted LASER can be regulated

Disadvantage:.

• It is highly temperature sensitive

Applications:

• Used in fiber optic communication
• Used in satellite communication
• Used in LASER printers, copiers
• Used in CD players, optical floppy discs