How Does a Laser Work

You will find lasers everywhere today. Right from CD and DVD drives to industrial and surgical applications, lasers are used in many fields of human endeavor.

The name 'LASER' is actually an acronym that stands for 'Light Amplification by Stimulated Emission of Radiation'. Lasers work through light amplification, achieved by the quantum mechanical phenomenon of stimulated emission. Before we try to understand how this technology works, we must understand what these devices are capable of.

They are devices which give a monochromatic (single wavelength), coherent, and highly amplified output of radiation. These properties make them useful devices in many applications, where monochromatic, coherent light is required.

To understand how these devices work, one must understand a bit about the light emission and absorption properties of atoms, as well as their electronic structure. Every atom has a nucleus, with electrons revolving around it at different distances. The electrons are bound to the nucleus, by electrical attraction, that exists between positive protons in the nucleus and the negatively-charged electrons.

The electrons are in specific and well-defined orbits, while revolving around the nucleus. These well-defined orbits have specific energies associated with them. One can think of these energy levels as steps on a ladder, which are not equally spaced, however. Each higher step on the energy ladder has a higher energy, than the one below.

The first few steps on this energy ladder are the ground states and all the rest are 'excited' energy levels. These orbits are not well-defined but spread out in space due to the uncertainty principle of quantum physics.

That is, we cannot say where exactly an electron is, but only calculate probabilities of it being in an energy level, which is spread out in space. There is a fuzzy cloud of electrons around the nucleus. This is called the electron cloud model. Electrons keep hopping between these energy steps or levels.

Light can be perceived to be made up of a stream of photons. Every photon is a packet of electromagnetic energy, which is one of the different forms of energy.

When an electron gains energy through absorption of a photon, it moves to a higher excited state. When an electron jumps from an energy level, to a lower energy level, it loses energy and emits a photon, that has energy, which is equal to the difference between the two levels.

When an electron makes a spontaneous transition from a higher to a lower energy state, a photon is emitted. Normally, at room temperature, very few electrons are in the higher excited states, so very few atoms emit electrons.

The working principle of a laser is based on the concept of population inversion and stimulated emission. As I said before, normally very few atoms have electrons in the excited state. Population inversion is all about inverting the population of electrons in ground states, so that maximum atoms have electrons in the excited state. This is achieved by bombardment of photons on the atoms, so that many electrons absorb energy and rise to higher excited states. It is known as pumping.

Stimulated emission, unlike spontaneous emission is biasing the probability of electron transition to lower states, causing emission of photons. It is achieved by bombarding of atoms with photons, which have exactly the same energy, as the difference between the transition energy levels.

To get a monochromatic and coherent light output from a bunch of atoms, many electrons need to be in a specific excited state, at the same time and they must make the transition to a lower state, in unison. The light output is monochromatic or single wavelength, as all the atoms emit photons of same energy and it's coherent as the photon emissions occur in unison.

A laser consists of a 'Gain Medium', placed in a resonant cavity, with facility for pumping the gain medium. The gain medium, also known as 'lasing medium', is a bunch of atoms, molecules, or a mixture of atoms, selected according to the wavelength of light, that is needed.

The gain medium is selected after studying spectroscopic emission properties of various atoms. A resonant cavity is an enclosure with reflecting mirror on one side and semi reflecting mirror on the other side.

The gain medium is first pumped, that is, the atoms are excited by bombardment of photons. These atoms have electrons in excited states, which emit photons. These photons keep bouncing around in the gain medium, as they are reflected due to the mirrors. This causes stimulated emission, which increases the number of excited electrons.

When these electrons get de-excited, they release photons of the same energy and wavelength. This creates a concentrated, monochromatic output of photons, which escapes out of the semi-silvered mirror, giving a laser output. This light beam can be concentrated by the usage of lenses, giving it directionality, which is a feature of laser light.

According to the lasing medium, the wavelength of laser output changes. There are many types of lasers, differentiated by the gain medium used. To fully understand how lasers work in detail, one must understand some concepts of quantum mechanics and a lot of electronics.