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Stephen Shirodkar
Stephen Shirodkar

How Laser Diodes Work and Why They Matter: A Guide to Their Principles and Applications

Laser Diodes History: How They Were Invented and How They Evolved

Laser diodes are one of the most common and versatile types of lasers in the world. They are used in a wide range of applications, from optical communication and data storage to laser printing and scanning, from medical and cosmetic treatments to industrial and military uses. But how did these tiny devices come to be? What were the scientific discoveries and technological innovations that led to their creation and improvement? In this article, we will explore the history of laser diodes, from their origins in the 1950s to their current state-of-the-art performance.

Laser diodes history


What are laser diodes and how do they work?

A laser diode is a type of semiconductor device that emits coherent light when an electric current passes through it. A semiconductor is a material that can conduct electricity under certain conditions, such as when it is doped with impurities or when it is exposed to light or heat. A laser diode consists of two layers of semiconductors with different doping levels, called the p-type and the n-type, sandwiched between two electrodes. When a voltage is applied across the electrodes, electrons from the n-type layer move to the p-type layer, creating a region with no charge carriers, called the depletion region. As more electrons cross the depletion region, they recombine with holes (the absence of electrons) in the p-type layer, releasing photons (light particles) in the process. This is called spontaneous emission.

However, not all photons are emitted in the same direction or wavelength. To create a coherent beam of light, some photons need to stimulate other electrons to emit photons with the same phase, direction, and wavelength. This is called stimulated emission. To achieve this, a laser diode has two mirrors at each end of the semiconductor layers, forming a cavity that reflects photons back and forth. This increases the chances of stimulated emission and amplifies the light intensity. The photons that escape from one of the mirrors form the output beam of the laser diode. The wavelength of the laser diode depends on the band gap (the energy difference between the valence band and the conduction band) of the semiconductor material, which can be varied by changing its composition or structure.

Why are laser diodes important for modern technology?

Laser diodes have many advantages over other types of lasers, such as gas lasers or solid-state lasers. Some of these advantages are:

  • They are small and compact, making them suitable for integration into various devices and systems.

  • They are efficient and low-cost, requiring less power and maintenance than other lasers.

  • They are tunable and modulable, allowing them to change their wavelength and intensity according to different needs and signals.

  • They have a high brightness and coherence, enabling them to transmit information over long distances or focus on small spots.

Because of these features, laser diodes have revolutionized many fields and industries, such as optical communication and data storage, laser printing and scanning, medical and cosmetic treatments, industrial and military uses, and more. We will discuss some of these applications in more detail later in this article.

The Origins of Laser Diodes: The First Attempts and Breakthroughs

The concept of stimulated emission and the maser

The idea of stimulated emission was first proposed by Albert Einstein in 1917, as part of his theory of quantum mechanics. He predicted that an atom in an excited state (with a higher energy level) could be induced to emit a photon with the same energy and phase as an incoming photon, resulting in two identical photons. However, this phenomenon was not observed experimentally until 1954, when Charles Townes and his colleagues at Columbia University built the first device that used stimulated emission to amplify microwaves, called the maser (microwave amplification by stimulated emission of radiation).

The maser consisted of a cavity filled with ammonia gas, which had three energy levels: a ground state, a lower excited state, and a higher excited state. A beam of microwaves with a specific frequency was directed into the cavity, causing some ammonia molecules to jump from the lower excited state to the ground state, emitting photons with the same frequency as the beam. These photons stimulated other ammonia molecules to do the same, creating a chain reaction that amplified the microwave signal. The amplified signal was then extracted from the cavity by a horn antenna. The maser was the first device that demonstrated the principle of coherent light amplification, paving the way for the development of lasers.

The invention of the ruby laser and the helium-neon laser