How Do Lasers Work?
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The Atom - Basics of a Laser
What do atoms have to do with lasers? Everything! Atoms are the basic building blocks of all matter in this universe and are an essential part of how lasers work.
Atoms are composed of protons, neutrons, and electrons:
- A proton has an electric charge of +1 and an atomic mass of 1.0073u.
- A neutron has an electric charge of 0 and an atomic mass of 1.0087u.
- An electron has an electric charge of -1 and an atomic mass of 0.0005u.
The center of an atom is called a nucleus. The nucleus is composed of protons and neutrons. Orbiting around the nucleus are electrons. As electrons circle around the nucleus in multiple orbits and sometimes in multiple energy levels, it is often referred to as an “electron cloud”. Let’s take a look at what a simple helium atom looks like.
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Excited States
An Atom is composed of protons, neutrons, and electrons. The nucleus, or the center of an atom, is composed of protons and neutrons. Electrons are constantly in motion orbiting around the nucleus in multiple orbits and energy levels. Believe it or not, the atoms that make up the table you are using are vibrating and in motion! The movements are too minute for you to realize but solids are actually in motion!
The electrons spinning around the nucleus of an atom moves in various energy levels and orbital shapes. How electrons are arranged in an atom is called an atom’s electron configuration. The electron configuration is different for each type of atom and can be visually represented by concentric circles around an atom’s nucleus. Each ring represents an energy level. The inner most circle is the atom’s first energy level (E1) and has enough room for 2 electrons. The second ring (E2) is the atom’s second energy level and has enough room for 8 electrons. The third ring (E3) can hold even more electrons and so forth.
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Stimulated Emission
L.A.S.E.R is an acronym for Light Amplification by Stimulated Emission of Radiation. But what exactly is stimulated emission? More importantly, how does stimulated emission amplify light?
Suppose an electron is in an excited energy level (E2) and a photon comes along with energy equal to the difference between the electron’s energy and a lower energy (ΔE = E2 - E1). What will happen is this photon will stimulate the excited electron to fall into the lower energy state (E1). Before dropping to the lower energy state, a photon with nearly identical properties is emitted in the same direction. This newly released photon is in phase with the original photon and has the same frequency, wavelength and color! There are now a total of two photons! This is called stimulated emission.
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Spontaneous Emission
An atom can become excited when it absorbs enough energy in the form of heat, light, or electricity. However, atoms like to stay in its ground state as it is more stable. Therefore, atoms that are excited usually return to their ground state relatively quickly, within nanoseconds. Before an excited electron can return to its ground state, it must first release the absorbed energy. The energy released is equal to the difference between the two energy levels of transition (ΔE = E2 - E1) and is released in the form of a photon, the basic unit of light! This is known as spontaneous emission. This concept is named rather appropriately as excited electrons “de-excites” quite randomly and the photon released is also non-directional (it shoots out in random directions).
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Population Inversion
In order for lasers to work and for stimulated emission to amplify light, a condition called population inversion must first be met. That is, the amount of excited atoms in a system must be greater than ground state atoms.
Imaging atoms as circles floating around in a glass tube. Lets say the total amount of atoms in this glass tube is equal to N. The number of ground state atoms is equal to N1. The total amount of excited atoms is equal to N2. The number of ground state atoms (N1) plus the number of excited state atoms (N2) is equal to the total of atoms (N) in the glass tube or N = N1 + N2.
An outside energy source causes atoms within the glass tube to become excited where some undergo spontaneous emission and photons are released. These photon can either cause:
- Stimulated Emission. This occurs when a photon with the correct energy passes by an excited state atom. Stimulated emission takes place and a photon with identical properties is released in the same direction for a total of 2 photons.
- Absorption. This occurs when a photon with the correct energy passes by a ground state atom. This photon is absorbed and causes an atom to become excited. Stimulated emission does not occur.
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2 Level Atom Laser
Two-level atoms have two energy states, ground state (E1) and excited state (E2). We have been using the basic 2-level atom to explain the concepts of excited states, spontaneous emission, and stimulated emission. However, in practical applications, 2-level atoms are not used for producing lasers. Let’s take a look at why.
In 2-level atoms, it would be very difficult to achieve population inversion where more atoms are in excited states versus ground states. This is actually true for atoms where the lasing transition (the energy levels where stimulated emission occurs) includes the ground level. Picture this, when 2-level atoms in a laser system is pumped with energy, the 2-level atoms are excited from E1 (ground state) to E2 (excited state). In 2-level atoms, spontaneous emission occurs almost instantly causing the excited atoms to immediately fall back to their ground states. There is no time for excited atoms to gather. With that being said, it would be very difficult to produce population inversion.
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3 Level Atom Laser
The first working laser, the ruby laser demonstrated by Theodore Maiman at Hughes Research Laboratories in 1960, was a 3-level laser! 3-level atoms are better fit for producing lasers versus 2-level atoms, however, most laser systems today utilize atoms with at least 4 or more levels. Let’s take a look at why.
3-level atoms have 3 energy levels, E1 is the ground state, E2 is its first excited state, and E3 is its second excited state. A typical 3-level laser works in the following way:
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Core Components of a Laser
The basic laser only needs three components to function. These principle components are an energy source commonly known as the pump source, the gain medium, and 2 or more mirrors that make up an optical resonator.
The pump source provides energy to the laser system. This is the initial energy needed to excite the atoms within the gain medium to achieve stimulated emission and population inversion needed for light amplification (the laser beam). Examples of pump sources are electrical discharges, flash lamps, arc lamps, light from another laser, chemical reactions and even explosive devices! The type of pump source used principally depends on the gain medium. The gain medium also determines how the pump source is transmitted to the medium. A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture. A Nd:YAG laser uses either light focused from a xenon flash lamp or diode lasers. An excimer lasers use a chemical reaction.
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Ruby Laser
Let’s take a look at how a simple ruby laser works! The ruby laser was the first working laser ever constructed and was demonstrated by Theodore Maiman at Hughes Research Laboratories in 1960.
The ruby laser is a solid state laser and uses a solid ruby rod or cylinder as its gain medium. The ruby mineral, or corundum, is an aluminum oxide with a small amount of chromium (around .05%) which gives it its pink or red characteristics and is responsible for the lasing behavior of the crystal. The red laser beam emitted by the ruby laser has a wavelength of 694.3nm.
Although ruby lasers can be constructed quite easily due to its minimal parts, ruby lasers have been involved in several famous laser ranging experiments where the distance of the moon was accurately measured to within 15cm!
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Wavelength, Frequency and Amplitude
Lasers are composed purely of photons, the basic unit of electromagnetic radiation and light. Photons are always in motion and travel at a constant speed of c= 2.998 x 108 m/s (more commonly known as the speed of light). Photons can be destroyed or created when radiation is either adsorbed or emitted and have zero mass and rest energy.
The fascinating and most astounding thing about photons or light is that they have wave-particle duality. Depending upon the circumstances of the experiment performed, photons can have either particle-like or wave-like properties. Like a particle, photons are capable of interacting with other particles such as in spontaneous emission and stimulated emission. Like waves, photons can be calculated to have frequency, wavelength, amplitude and other properties inherent in wave mechanics.
