How Does a Laser Work

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

What are Lasers?

The name 'LASER' is actually an acronym that stands for 'Light Amplification by Stimulated Emission of Radiation'. The answer to how lasers work, is right in its name itself. Lasers work through light amplification, achieved by the quantum mechanical phenomenon of stimulated emission. Before we try to understand how lasers work, we must understand what are these devices capable of. Lasers are devices which give a monochromatic (single wavelength), coherent and highly amplified output of radiation. These properties of light emitted by them make them useful devices in many applications where monochromatic, coherent light is required. Knowing the working of a laser, will enable you to understand how laser pointers, laser eye surgery, laser hair removal and even how laser printers work. That is because laser forms a major working part, on which these devices and techniques are based.

How Do They Work?

To understand how these devices work, one must understand a bit about the light emission and absorption properties of atoms, as well as the electronic structure in atoms. Every atom has a nucleus with electrons revolving around it at different distances. The electrons are bound to the nucleus by the 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 energy 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 higher energy than the one below. The first few steps on this energy ladder are the ground states and all the next ones are 'excited energy'. 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 equal to the difference between the two levels. When an electron makes a spontaneous transition from a higher to a lower energy state, normally at room temperature, very few electrons are in the higher excited states, so very few atoms emit electrons.

The laser working principle 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 electron have atoms 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 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!

Now that I have given you some idea about the laser working principles, let us have a look at the working of laser. 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 laser 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.