Laser Basics Lasers are a relatively recent invention, having been made first at AT&T Bell Laboratories in the late 1950's. They are based on ideas worked out by Albert Einstein almost 80 years ago, and we will describe briefly in this paper the principles and uses of some of the more common lasers.

The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. To create laser light, several steps need to occur within the laser that will cause light to be formed and amplified in a highly concentrated beam. The three essential parts to a laser are the "pumping system", the lasing material, and the optical cavity. The heart of the laser is the lasing material, which is the substance that will be 'excited' to glow and produce the laser light.

Stimulated Emission

Ordinarily, the lasing material (like all matter) is in an 'unexcited' state, with the overwhelming majority of its atoms in the substance having their electrons in the lowest available energy state (the 'ground state'). When an atom absorbs energy from some external source, one or more of its electrons will utilize that energy to make a transition up into a higher-energy 'excited' state. Once the atoms are excited, they will attempt to de- excite rather quickly and return to their ground state. The energy released in this downward transition is usually carried away as a photon, or 'chunk' of light, in a process known as 'spontaneous emission'. In lasers, however, the lasing material de-excites by 'stimulated emission' (from which we get the S and E in LASER), a different process entirely.

Lasing materials must have at least three energy levels available: a ground state, an excited state, and an intermediate 'metastable state', so named because electrons can exist in this state for much longer times than are customary for excited states. In order to prepare the lasing medium for stimulated emission, the atoms of the medium must be excited in such a way that they will decay to the metastable state before de-exciting to the ground state. When this happens, a substantial fraction of the lasing material's atoms will be in the metastable state, and a 'population inversion' results (as shown in Figure 1).

Figure 1. Population Inversion. = atom.

Once the lasing material has undergone a population inversion, the process of stimulated emission (conceived by Einstein in 1917) can release the excitation energy as laser light. In this process, a photon incident on an atom in an excited state can induce de- excitation if the photon's energy matches the atom's transition energy exactly. When that occurs, the photon released by the atom in its de-excitation matches the incident photon in energy, frequency, phase, and direction (Figure 2).

Figure 2. Stimulated Emission.

Each of the emitted photons can subsequently induce stimulated emission in similarly excited atoms, resulting in four identical photons. As the process continues, we get eight, then 16, then 32 ... such photons in a chain reaction: the Light has been Amplified (the L and A in LASER).

In real lasers, the process proceeds something like this:

1. Start with unexcited lasing material.

2. Provide energy into lasing material to induce excitation. Typical ways to do

this include zapping it with another laser, passing electrical current through

it, or illuminating it with a bright pulse from a 'flashlamp'.

3. After the initial excitation decays to the atoms' metastable state and a population inversion is induced, a chance de-excitation of one of the atoms

will induce the chain reaction of stimulated-emission photons, resulting in

light amplification.

4. The now de-excited atoms are re-excited by step 2.

Laser Gain

The photons released in the stimulated emission chain-reaction are, for the most part, kept contained within the medium. The way this is done is by placing inward-facing mirrors at the ends of the sample of lasing material, which is usually made in a cylindrical shape as shown in Figure 3.

Figure 3. The 'Laser Cavity'.

The reason this is done is to keep some of the light - which is known to be of the appropriate wavelength since it was emitted in the chain reaction - within the lasing medium in order to initiate more stimulated emission. In general, the longer this 'laser cavity' of lasing material between the reflecting ends, the more powerful the laser will be. A single photon can be responsible for creating several identical stimulated-emission photons during one pass through the sample of lasing material, and the number of photons thus produced is known as the 'gain' of the medium. Note that a continuous (or repeated) supply of energy is necessary to keep the population inversion present within the lasing medium. If the inversion were not present, the photons reflected back into the lasing medium by the end mirrors would merely be absorbed in inducing upward electron transitions in the atoms of the material. No light would be emitted in that case.

We get laser light out of the lasing material bu making one of the mirrors somewhat 'leaky', allowing a few percent or so of the stimulated-emission photons to escape the laser cavity. All of those photons - which make up the intense laser beam we actually see - have the same wavelength, frequency, phase (that is, they are 'in step' with each other), and direction.

Visible Light vs. Laser Light

Visible Light:

Broad range of wavelengths and frequency; polychromatic

Caused by spontaneous emissions - emission of a photon without outside stimulation

Incoherent

Laser Light:

Contains one wavelength and frequency; monochromatic

Cause by stimulated emmission - emission of a photon which is stimulated by a photon which is stimulated by a photon of the same energy

Coherent - light waves are 'in step'

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