How does a laser work

One must have seen and if not seen then must have heard the term laser. But if you have seen the laser light emitting from a source, then there should be natural question how does a laser work? Therefore let us discuss how the laser works?

As already discussed in my earlier articles, that the full form of laser is “Light amplification by stimulated emission of radiation”. Therefore it means that in laser light is amplified through a process called stimulated emission. Thus it means that stimulated emission is that process through which laser can be achieved. That is the reason the stimulated emission is known as the principle of laser.

Concepts to learn how does a laser work: Continue reading “How does a laser work”

Pumping Source

As we have discussed in the previously, that principle of laser is stimulated emission and for it to take place, population inversion has to be achieved and maintained. For this, there must be a source of external energy, which can continuously supply energy to energy to laser medium, so that population inversion can be achieved. Such a source of external energy is called pump or pumping source and the process of supplying external energy to laser medium so as to achieve the population inversion is called pumping.

Next time we will discuss the various types of pumping source.

Active medium of laser

When energy is given to laser medium (solid, liquid or gas), then only a small fraction of laser medium shows lasing action. This part of laser medium is called active medium or active centre. Thus, due to this reason, the laser medium is also called the heart of a laser. For example, in case of ruby laser, Al2O3 is doped with Cr2O3. The laser is due to doped chromium ions. Thus Cr3+ ions are active centres. In He-Ne laser, laser is produced due to Ne atoms, therefore, Ne atoms are active centres.

Note: Dear students, next time, we will learn about optical pumping and its types.

Types of laser

The lasers can be classified on the basis of state as explained below:

a)      State of laser medium: According to the state of laser medium, we have solid state lasers like ruby laser,gas lasers like He-Ne laser and carbon dioxide laser, semiconductor laser and liquid laser like dye laser.

b)      Mechanism of pumping: According to it, we have optical pumping based lasers like ruby laser, electric discharge based lasers like He-Ne laser. Pumping can also be done through chemical reaction.

c)      Nature of output: According to nature of output, we have pulsed lasers like ruby laser and continuous wave lasers like He-Ne laser.

d)      Spectral region (wavelength) of output: According to it, we have ultra violet, visible and infrared lasers.

Note: Do you know what is pumping or how atoms/electrons are excited?

Metastable State

It is also the excited state but having life time of 10-5 to 10-3 seconds. Population inversion occurs only in between the metastable state and lower state. The two states in between the population inversion occurs and laser is achieved, the upper one is known as upper laser level (ULL) and lower is known as lower laser level (LLL).

Note: Two levels lasers cannot be constructed. Can you explain why? Atleast three levels are required to produce laser.

Population Inversion in Lasers

Normally the population density of atoms/electrons is more in the ground state than the excited state. But if the process of stimulated emission dominates over the process of spontaneous emission, then it may be possible that N2 > N1.

Where N1 is the number of atoms in the ground state and

N2 is the number of atoms in the excited state.

The process of achieving greater population density of atoms in the higher energy state as compared to lower energy state is called population inversion. The atoms from lower energy states are raised to excited states by external energy.

Kindly Read This Also :

Note from winnerscience: If you want the e-notes of all the laser  articles that will include the basics of lasers like stimulated absorption, difference between spontaneous and stimulated emission, Einstein  Coefficients, properties and applications of lasers, complete construction and working of lasers like Ruby laser, He-Ne laser, Carbon dioxide laser, Nd:YAG laser, dye laser, semiconductor laser, holography and additional articles of Q-switching and mode locking, then please contact [email protected]. You can post your queries also there.

Stimulated Emission or Induced Emission

If the atom is in the excited state E2 and a photon of energy exactly equal to E2 – E1 = hυ is incident on it, then the incident photon interacts with the atom in the excited state and then it stimulates or induces the atom to come down to the ground state E1. A fresh photon is emitted in this process. Therefore, when an atom ejects a photon due to its interaction with a photon incident on it, the process is called stimulated emission (or induced emission).

Characteristics of stimulated emission:

a)      For each incident photon, there are two outgoing photons moving in the same direction.

b)      As the emitted photon has exactly the same energy, phase and direction as the incident photon, we will achieve an amplified and unidirectional coherent beam.

Note: The laser is based on the principle of stimulated emission.

Spontaneous Emission

If the atom in the excited state automatically decays to the ground state by emitting a photon of energy (E2 – E1 = hυ), then this process is called spontaneous emission. Generally an atom/electron in excited state can stay for

10-9 – 10-8 seconds.

Characteristics of spontaneous emission:

a) The emitted photon of energy E2 – E1 = hυ can move in any random direction.

b) There will not be any phase relationship between the photons emitted from various atoms.

Hence, the radiations coming out due to spontaneous emissions are incoherent.

Reference: The content of all the articles of lasers posted here is referred from my book: Optics and Lasers by Amit Sarin and Sameer Kalia, published by kalyani publishers.

Kindly Read This Also :

Note from winnerscience: If you want the e-notes of all the laser  articles that will include the basics of lasers like stimulated absorption, difference between spontaneous and stimulated emission, Einstein  Coefficients, properties and applications of lasers, complete construction and working of lasers like Ruby laser, He-Ne laser, Carbon dioxide laser, Nd:YAG laser, dye laser, semiconductor laser, holography and additional articles of Q-switching and mode locking, then please contact [email protected]. You can post your queries also there.

Stimulated Absorption

When a photon of light having energy E2 – E1 = hυ is incident on an atom in the ground state, the atom in the ground state E1 may absorb the photon and jump to higher energy state E2. This process is called stimulated absorption or induced absorption. This is called so because the incident photon has stimulated the atom to absorb the energy.

Characteristics of Laser beam

A laser beam has the following important characteristics:

1. Directionality: An ordinary source of light emits light waves in all the directions and is highly divergent. But the divergence or angular spread of a laser is very small.

2. Monochromaticity: It means that all the laser rays have same wavelength and frequency when they are emitted from the same source.

3. Brightness: A laser beam has brightness many times in magnitude greater than that of conventional sources due to high directional property of laser beam.

4. Coherence:Two or more light waves are said to be coherent if they bear a constant phase relation among themselves. Coherence can be classified into two ways:

a) temporal coherence

b) spatial coherence.

Do you know the difference between the two?

Do you know about the spontaneous and stimulated emission?

Think, we will discuss it later on!!