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Laser Diode

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What is Laser Diode?

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A semiconductor laser that is used for signal transmission in optical fiber is the laser diode, where LASER stands for Light Amplification by Stimulated Emission of Radiation.

A laser diode is like a light-emitting diode that emits a high-powered light through a glass lens to reduce the signal loss.

A laser diode is also called an injection laser diode. It comprises the following components:

  • Photodiode

  • Laser

  • Glass lens at the front

  • A fiber

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Common types of laser diodes used in telecommunication are:

  • Double heterostructure

  • Quantum well

  • Quantum cascade

  • Separate confinement heterostructure

  • Distributed feedback

  • VCSEL

  • VECSEL

  • External cavity

Laser Diode 

Laser diodes are used as light sources in optical communication. The first laser was built by Theodor H. Maiman at Hughes Research lab in 1960.

We can use the two classes of laser diode - one produces light emissions by itself and another one uses an external source.

A laser diode has the following components about which we will discuss in detail by the construction of the laser diode:


Construction of Laser Diode

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  • A laser diode is made of two semiconductor layers viz: P-type and N-type semiconductor. These semiconductors are made of Gallium arsenide and they are doped with Selenium, Aluminium, or Silicon.

The construction of laser diodes is similar to LED, the only difference is that an active layer is made between these two semiconductors. This active layer often consists of a Quantum well and is made of an intrinsic semiconductor (intrinsic semiconductor means undoped or pure or natural semiconductor).

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  • A depletion layer is between the two semiconductors because by applying a voltage to the diode, the electron-hole combination occurs.

  • The front is completely polished; this surface acts as a reflecting surface or a mirror while the side view is partially polished which acts as a partially reflecting surface.

  • The front face is attached with metal contacts to allow biasing in the semiconductor. Biasing means supplying power to the diode.

  • Laser diodes provide a light of the same frequency.

Now, let’s talking about its working:


Working Principle of a Laser Diode

A laser diode works on the principle of stimulated emission and so emissions occur in three types:

  • Stimulated absorption

  • Stimulated emission

  • Spontaneous emission

Generally, an electron while migrating to the lower energy level releases energy equal to the gap between them and chances are they come back after absorbing energy.

However, a laser diode works on stimulated emission. In this process, we strike a  photon  with an electron to get the light that we can see in the image below:

   

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So, what happens here is when we strike a photon (in the form of light energy) with the electron in the conduction band (CB). Already one electron is reaching the valence band; however, because of striking with a photon, an extra electron also reaches the valence band. In this way, we get two photons emitting out in the form of light.


Characteristics of Laser Diode

A laser has several scientific, commercial, and medical applications. The laser has the following characteristics:

  • Highly directive - Light emitted by laser diodes is directed into a narrow beam that can be easily launched through an optical fiber.

  • Monochromaticity - Laser diodes emit a narrow light containing a single color.

  • Lasers have the same frequency and wavelength.

  • A laser is a high-intensity beam.

  • Lasers are coherent - Light emitted is of a single wavelength.

  • Lasers have the same phase and frequency.

  • Lasers can travel large distances.

Applications of Laser Diode

We find applications of laser diodes in our day-to-day life; let’s look at some of these:

  • As a barcode reader.

  • Laser printing and laser screening.

  • Optical fiber communication.

  • In the medical field, doctors use lasers as an alternative to the scalpel for cutting tissues.

  • Instruments like a rangefinder, spectrometer, and contactless measurements are possible with lasers.

  • Laser diodes with 1.3 and 1.55-micrometer bands are used as the main source of light in telecom. As the band changes, laser diodes are used as optical amplification.

  • In CD and DVD players.

  • Because lasers are high-intensity beams, in industries, people use lasers for cutting, drilling, and welding purposes.

Advantages of Laser Diode

We find various advantages of using laser diodes; let’s have a look at these:

  • Low power consumption.

  • Low cost of manufacturing and operations.

  • Operable for long hours.

  • Easily portable due to small size and internal architecture.

  • Highly reliable.

  • Generates highly-efficient light.

Disadvantages of Laser Diode

Just like the coin has two sides, a laser diode also does, it has few advantages with a big list of advantages, as nothing is perfect. So let’s look at a few disadvantages:

  • Expensive as compared to other light-emitting devices.

  • Lasers are harmful to the eyes.

  • A laser diode is a temperature-dependent device, and its operations get affected by the rising temperature.

  • Laser diodes are not suitable for high-power operations.

FAQ (Frequently Asked Questions)

Question 1: What is Spontaneous Emission?

Answer: After applying the voltage to the laser diode, the doped p-n transitions allow for the recombination of electrons with holes. As electrons from higher energy levels migrate to the lower one, radiations in the form of photons are generated; this is spontaneous emission.


During spontaneous emission, an electron descends to the lower energy level, i.e., to the valence band by releasing the energy equal to the forbidden energy gap.

Question 2: What is Stimulated Absorption?

Answer: When an electron migrates from the valence band to the conduction band, it absorbs energy. The excitation of the electron to the higher energy level by absorbing energy is the stimulated absorption.

Question 3: How are Lasers Used in Diagnosis?

Answer: Lasers are used to shrink and destroy tumor/precancerous growth.

Question 4: How do we Obtain Light from a Laser Diode?

Answer: As the electron reaches the lower level, after forward-biasing the semiconductor, the released electron gets a push,  they cross the depletion region and jump to the higher energy level, i.e., CB.


After reaching the higher energy level, they recombine with holes in the p-type semiconductor. During recombination, they release a monochromatic light.