The term LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser diode is an optoelectronic device that converts electrical energy into light energy to produce high-intensity coherent light. In laser diodes, the p-n junction of the semiconductor diode acts as the laser or active medium.
The emitted radiation may be of the same frequency and phase or have a very narrow bandwidth. It is used in high-speed fiber optic communications. It produces a high-intensity coherent and monochromatic light.
Construction of Laser Diode
The input terminals are attached to a metal plate sandwiched between the n-type and p-type layers. This type of laser diode is a “homo junction laser diode”. The intrinsic region between the p-type and n-type materials is used to increase the volume of the active region, allowing more holes and electrons to accumulate at the junction. This allows more electrons to recombine with gaps all the time, improving the output. Laser light is emitted from the elliptical part. This beam from a laser diode can be further focused using an optical lens. This entire PIN diode structure (P-type, intrinsic-type, N-type) is encapsulated in a metal structure.
Working of Laser Diode
There are three basic phenomena by which atoms can emit light energy.
- Absorption
- Spontaneous emission and
- Stimulated emission.
Absorption
A laser diode consists of a hole-electron p-n junction. (where a hole is the absence of an electron). When a certain voltage is applied, electrons in the PN junction absorb energy and move to higher energy levels. A hole is then formed at the original position of the excited electron. Electrons live in this exciting state without combining with holes for a short period of time, called the “recombination time” or “upper-state lifetime.” Most laser diodes have a recombination time of about 1 ns.
Spontaneous Emission
After the life of the excited electron in the upper state, it recombines with the hole. When an electron falls from a higher to a lower energy level, the energy difference is converted into photons or electromagnetic radiation. Create light with LEDs in the same way. The difference between the two energy levels determines the frequency of the emitted photons.
Stimulated Emission
In stimulated emission, photons meet electrons at higher energy levels and these photons are produced by an external light source. When these photons reach electrons, the electrons absorb energy and recombine with holes, releasing additional photons. As a result, an incident photon induces the emission of another photon. Therefore, it is called stimulated emission.
Types of Laser Diode
- Double Heterostructure Laser Diode
- Quantum Well Laser Diode
- Separate Confinement Heterostructure Laser Diode
- Vertical Cavity Surface Emitting Laser Diode (VCSEL)
- Quantum Cascade Laser Diode
- Interband Cascade Laser Diode
- Distributed Bragg Reflector Laser Diode
- Distributed Feedback Laser Diode
- External Cavity Diode Laser
- Vertical External Cavity Surface Emitting Laser Diode (VECSEL)
Symbol of Laser Diode
Application of Laser Diode
- Consumer Electronics
- Medical
- Autonomous Vehicles
- Scientific Instrumentation
- Commercial Applications