What is a Laser Diode?

A laser diode is a type of semiconductor device that converts electrical energy directly in to a focused beam of light. It is similar to a LED (light-emitting diode) however is much more focused. The term LASER stands for Light Amplification by stimulated emission of Radiation. Laser diodes can emit light beams ranging from the infrared to the UV spectrum. The laser diodes are ideal for fiber-optic communications, CD/DVD reading/recording, laser scanning, laser printing, and barcode reader applications.

Figure 1: Laser Diode

In a laser diode, there are usually two layers of doped gallium arsenide that forms a PN junction. There is a partially reflective surface at the P end and a highly reflective surface at the opposite (N) end. The P has an external lead which is called an anode and N has an external lead which is called a cathode. The intrinsic un-doped active region between P and N-type semiconductors is responsible for the electron and hole recombinations which results in better a output power. An extra PIN photodiode is also placed with the laser diode chip to monitor the laser diode output and reduce the back reflection to the cavity.  

How does it work?

Figure 2: Stimulated Emission

When a laser diode is forward biased, electron and hole recombinations occur in the PN junction. During recombination, the electrons lying in the conduction band of the N region (higher energy level) fall into the valance band of the P region (low energy level), which releases energy in the form of photons. Now, these emitted photons reflect back and forth between the reflective surfaces and strike other atoms which lead to more recombinations and release more photons with the same phase (stimulated emission). When the photons' activity becomes so intense at a certain point, a strong beam of laser light will come out from the partially reflective surface of the diode. This laser light is coherent in characteristics, i.e., all the emitted photons have the same frequency and phase.

In non-coherent light with a single frequency (for example, an Incandescent lamp), the light has a deviation in phase. But, in a coherent light (for example, laser light), the light has the same frequency as well as phase.

Figure 3: Understanding coherent and non-coherent light

Figure 4: Laser Diode P-I characteristics

Materials used for fabrication:

The choice of direct bandgap semiconductor material used to fabricate the laser diodes decides the wavelength of the emitted beam. For example, InGaN (indium gallium nitrate) will emit the beam of light with a wavelength of 405 nm (visible). InGaAsP (indium gallium arsenide phosphide) will emit the beam of light with a wavelength of 1654 nm (infrared).

Types of laser Diode:

• Double heterostructure laser diode.
• Quantum well laser diode.
• Quantum cascade laser diode.
• Interband cascade laser diode.
• Distributed Bragg reflected laser diode (DBR).
• Distributed feedback laser diode.
• Vertical cavity surface-emitting laser diode (VCSEL).
• Vertical external-cavity surface-emitting laser diode (VECSEL).
• External cavity diode laser.
• Separate Confinement Heterostructures Laser Diode.

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