Further physics - What is laser?
Lee Kwok-san and Tong Shiu-sing   
Translation by Janny Leung   

The term laser should not sound alien to you. We often encounter laser in our daily life, examples include the laser pointer used in classrooms, and the CD-ROMs in a computer or in a hi-fi that are used to read the data stored in a CD. In industry, laser is often used for cutting and microscopic processing. For military purposes, laser is used to intercept guided missiles. Scientists have also accurately measured the distance between the Earth and the Moon by using laser; the error involved is only a few centimetres. These are some extensive applications of laser. So actually how is it produced? We will explain the basic principles of laser below.

It took a very long time to develop laser. Renowned physicist Einstein has already discovered its principles in 1917, but it was not until 1958 that laser was successfully produced.

Laser is an acronym of Light Amplification by the Stimulated Emission of Radiation. The full name itself exhibits the major processes involved in laser production. Before we look into these processes, we must, first of all, understand the structure of matter, and the principles of light emission and absorption.

 Schematic diagram of a carbon atom
Fig. 1   Schematic diagram of a carbon atom.

Matter is made up of atoms. Fig. 1 shows a schematic diagram of a carbon atom. At the centre of the atom lies the nucleus; it is constituted of protons and neutrons. A proton carries a positive charge but a neutron does not carry any charge. Outside the atom is a cloud of electrons that carry negative charges; they are in motion surrounding the nucleus. An interesting point to note is that in an atom the energy of an electron is not arbitrary. By quantum mechanics, which describes the microscopic world, each electron stay at a certain energy level, and different energy levels correspond to different energies of the electrons. To simplify the picture, we could imagine the energy levels as some orbits surrounding the nucleus; the farther away they are from the nucleus, the higher their energy would be, as shown in Fig. 1. Moreover, the maximum number of electrons that each orbit can accommodate differs as well. For example, the lowest orbit (the one closest to the nucleus) has a capacity of two electrons, while the higher orbit can hold at most eight. This simplified model is actually not entirely accurate [1], but it can sufficiently help us to explain the basic principles of laser.

Electrons can transit to other energy levels by absorbing or releasing energy. For example, after an electron has absorbed a photon [2], it transits from a lower energy level to a higher one (Fig. 2a). By the same token, an electron at a higher energy level may transit to a lower one if it releases a photon (Fig. 2b). In these processes, the energy of the photon absorbed or released always equals the energy difference between the two levels. Since the energy of the photon governs the wavelength of light, the absorbed or emitted light has a definite colour.

Electron transitions in an atom
Fig. 2  Electron transitions in an atom.

When all electrons of an atom are at the lowest possible energy levels and thus the atom possesses the lowest energy it has, we say that it is at the ground state. Fig. 1 shows the electronic configuration of a carbon atom at the ground state. When one or more electrons are at a higher energy level, we say that the atom is at an excited state. It is mentioned earlier that electrons transit between energy levels by absorbing or emitting light. These transitions are divided into three types:
  1. Spontaneous absorption - an electron transit from a lower energy level to a higher one by absorbing a photon (Fig. 2a)
  2. Spontaneous emission - an electron spontaneously emits a photon to transit from a higher energy level to a lower one (Fig. 2b)
  3. Stimulated emission - photons incident into the matter to stimulate the electrons to transit from a higher energy level to a lower one and to emit a photon. The incident photon and their emitted counterparts have the same wavelength and phase; this wavelength corresponds to the energy difference between the two energy levels. A photon stimulates an atom to emit another photon, and hence two identical photons are resulted (Fig. 2c)
 Schematic diagram of a ruby laser
Fig. 3  Schematic diagram of a ruby laser.

population inversion
Fig. 4  Population inversion is key to producing laser.

Comparing laser and ordinary lamplight
Fig. 5  Comparing laser and ordinary lamplight.

Laser is basically produced by the third transition mechanism. The principles of ruby laser are shown in Fig. 3. It comprises a flash lamp, a laser medium and two mirrors. The laser medium is a ruby crystal containing a slight amount of chromium atoms. At start, the flash lamp injects light into the laser medium, stimulating the chromium atoms in it and exciting the electrons at the outermost layer of the atoms. At this moment, some electrons will return to a lower energy level by emitting photons. The emitted photons will be reflected by the mirrors set at the two ends of the laser medium to stimulate more electrons to undergo stimulated emissions, thus increasing the intensity of the laser. One of the mirrors at the two ends will reflect all the photons while the other will reflect most of them, and the remaining small portion of photons that passes through the latter mirror constitute the laser we see.

There is another feat involved in producing laser: to reach the state of the so-called population inversion. Take the ruby laser as an example (Fig. 4). An atom firstly absorbs energy and transits to an excited state. The atom stays at the excited state only momentarily: after seconds, it falls to an intermediate state called metastable state. It stays at the metastable state for a rather long time: around seconds or more. Its prolonged stay at the metastable state causes the number of atoms at the state being larger than that at the ground state, and such a phenomenon is called population inversion. Population inversion is a key to producing laser, because it ensures that the number of atoms returning from the metastable state to the ground state by stimulated emission is more than that transiting from the ground state to the metastable state by spontaneous absorption, so that the number of photons in the medium will increase and hence there is a laser output.

The laser produced from stimulated emission has the following three major characteristics (Fig. 5):
  1. It is monochromatic. Only light of a single wavelength is produced in the whole process. This differs from ordinary light such as sunshine or lamplight, which are composed of different wavelengths of light, being close to white light.
  2. It is coherent. All photons have the same phase and the same polarization, hence they produce an very high intensity when they superpose. The lights we see in daily life have random phases and polarization, and hence they are relatively much weaker.
  3. It has a very narrow and collimated ray, and hence it is very powerful. In contrast, lamplight diverges towards different directions and has a low intensity.
Based on its power, laser can be divided into three types, the first being low power laser which uses gas as its laser medium. For example, the barcode scanner often used in supermarkets utilizes helium gas and neon gas as its laser medium. The second type is medium power laser, such as the laser pointers used in classrooms. The last type is high power laser which uses semiconductors as laser medium. Its power output can reach 500 mW. The laser used in the thermonuclear fusion experiments can emit momentary but extremely intense laser pulses whose pulsation power reaches W! Such laser could produce a high temperature of a hundred million degrees Celsius and stimulate the deuterium-tritium particle fuels to undergo thermonuclear fusion.

[1] According to quantum physics, electrons do not move on definite orbits surrounding the nucleus, the position of the electrons is predictable only in a probabilistic way by using the Schrodinger equation.
[2] Quantum physics shows that light possesses the properties of particle, especially when it is interacting with an atom. The particles of light are called photons.