Semiconductor Electronics: Materials, Devices and Simple Circuits

Laser Diode

A laser diode is a semiconductor instrument identical to a light-emitting diode (LED). It uses a p-n junction to produce coherent light where all waves are at the same frequency and phase. This coherent light is emitted by a laser diode using a process “Light Amplification by Stimulated Radiation Emission,” referred to as LASER. As it uses p-n junction to produce laser light, hence the name laser diode.

Laser Diode

Laser Diode

How does a Laser Diode work?

There are three basic phenomena from which the atom can emit light energy.

  1. Absorption
  2. Spontaneous emission and
  3. Stimulated emission.

Absorption:

The laser diode consists of a p-n junction where there are holes and electrons. (Here, a hole is, the absence of an electron). When a specific voltage is applied, at the p-n junction, the electrons absorb energy and transfer to a higher energy level. And in the original position of the excited electron, holes are formed. The electrons live in this exciting state without merging with holes for a short period, referred to as “recombination time” or “upper-state life.” The recombination time for most laser diodes is around a nanosecond.

Spontaneous Emission:

After the life of excited electrons in the upper-state, they recombine with holes. When electrons fall from a higher energy level to a lower energy level, the energy difference converts into photons or electromagnetic radiation. This same method generates light in LEDs. The difference between the two energy levels determines the frequency of the emitted photon.

Stimulated Emission:

In stimulated emission, photons strike electrons at a higher energy level and, these photons produce from an external light source. As these photons reach the electrons, the electrons absorb energy and recombine with the holes and emit an extra photon. As a result, one incident photon induces another photon to release. Hence, it is known as stimulated emission.

Construction of Laser Diode:

Materials Required:

  • Metal Contact
  • P-type Material
  • Active/Intrinsic Region (N-type Material)
  • N-type Material

Construction:

The input terminals are attached to metal plates sandwiched to n-type and p-type layers. This type of laser diode is the “Homojunction Laser Diode.” The intrinsic zone between p- and n-type material is used to increase the volume of the active region so that more holes and electrons can accumulate at the junction. It causes more electrons to recombine with holes at any moment, resulting in improved output power. The laser light emits out of the elliptical region. This beam from the laser diode can further be focused using an optical lens. This entire PIN diode (P-type, Intrinsic, N-Type) structure is enclosed in a metal structure.

Types of Laser Diodes

There are several types of laser diodes, such as:

  1. Double Heterostructure Laser Diode
  2. Quantum Well Laser Diode
  3. Separate Confinement Heterostructure Laser Diode
  4. Vertical Cavity Surface Emitting Laser Diode (VCSEL)
  5. Quantum Cascade Laser Diode
  6. Interband Cascade Laser Diode
  7. Distributed Bragg Reflector Laser Diode
  8. Distributed Feedback Laser Diode
  9. External Cavity Diode Laser
  10. Vertical External Cavity Surface Emitting Laser Diode (VECSEL)

Major Categories of Laser Diode

There are two categories of the laser diode.

  1. Injection Laser Diode
  2. Optically Pumped Semiconductor Laser Diode

Injection Laser Diode: This is similar to LEDs except, LEDs use large channels of semiconductors while laser diodes use narrow channels. In this, the light beam passes through the waveguide, and the diode itself serves as a waveguide. The continuous stimulated emission intensifies the light beam.

Optical Pumped Semiconductor Laser: In this, the injection laser diode serves as an external pump. The semiconductor materials of the III & V  group serves as a base. The amplification is performed by stimulating emission. It provides some benefits, such as the preventing interference caused by the structure of the electrode. It also gives a good wavelength selection.

Applications of Laser Diode

Consumer Electronics: CD/DVD players, Laser printers, Wireless Communications, Barcode Scanners, etc.

Medical: Laser diodes are used in medical machines to extract unwanted tissues, kill cancer cells, non-invasive and cataract surgery, etc.

Autonomous Vehicles: Laser diode technology is useful in the development of LIDAR devices for autonomous driving.

Scientific Instrumentation: Remote contactless measurement, spectrometry, rangefinders, etc.

Commercial Applications: As means of a high-intensity laser beam for the accurate cutting of materials. They are also useful for 3D printing to soften the substrate.

FAQs about Laser Diode

Q1. What are the advantages of Laser Diode?

Answer. The advantages of Laser Diode are:

  1. Low power consumption device.
  2. Economical as its cost of production and operation is low.
  3. It can work for long durations.
  4. Portable because of its small size and its internal architecture.
  5. Highly stable and very efficient.

Q2. What are the disadvantages of Laser Diode?

Answer. The disadvantages of Laser Diode are:

  1. Their temperature is influenced by the shifts in the operating temperature, as they are temperature-dependent.
  2. It is not ideal for high power applications.
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