What is a Laser?
by J. L. Doty
copyright (c) 2009 by J. L. Doty
released: April 9, 2009
A laser is an optical oscillator. It’s an amplifier with positive feedback, meaning it feeds back on itself in a way that makes it unstable at certain frequencies. It’s very similar to that loud squeal that occurs when one places the microphone of an audio system close to a speaker. The input to the microphone is amplified, comes out the speaker, goes into the microphone again, is amplified further, etc. Furthermore, frequency and wavelength are related through the relationship:
where ν is the frequency, c is the velocity of sound (approximately 344 m/s) and λ is the wavelength.
When the distance between the speaker and the microphone (d) is an integral number of wavelengths (d = 1λ, 2λ, 3λ, etc.) positive feedback produces the squeal at the appropriate frequency. One or two frequencies will dominate through a phenomenon called gain saturation, and the result is something very close to a single tone with some nasty harmonics. Note that this frequency-wavelength relationship is identical to that of light, but in that case we use a different velocity for c (approximately 3 x 108 m/s).
A laser is remarkably similar to this; see the block diagram of a laser cavity following this paragraph. The cavity consists of two mirrors that are oriented so that they reflect back upon one another, with an optical amplifier (OA) between them. The light reflects off one mirror, is amplified by passing through the OA, reflects off the other mirror, is amplified further by the OA, etc., feeding back on itself much like the microphone, amplifier and speakers. One mirror is fully reflecting, while the other is partially reflecting, allowing some of the energy to be tapped from the cavity.
The relationship in equation 1 still holds, but because the round-trip path of the light makes a double pass through the cavity, there is a factor of two in the relationship between the separation of the mirrors and the wavelengths (d = 0.5λ, 1.0λ, 1.5λ, etc.) that produce oscillation. The oscillating audio system is an acoustically resonant cavity, while the laser is an optically resonant cavity.
I once read an article about some researchers who used a Budweiser beer can as the resonant cavity for a microwave oscillator. We called it a budwaser.
A bit about the word LASER: an acronym for Light Amplification by Stimulated Emission of Radiation. In the strictest sense EM radiation is light only if it falls in the visible portion of the electromagnetic spectrum, meaning wavelengths from approximately 425 to 675 nm. So the terms infrared and ultraviolet and laser are, strictly speaking, mutually exclusive. But the scientific community commonly uses the word laser for ultraviolet, visible and infrared lasers, so it has become acceptable to do so. On the other hand, perhaps the scientific community should get its act together and be more careful, but then we’d have to call an infrared laser an iaser, and a ultraviolet laser a uaser, and a far infrared laser a . . . Nah, forget it.
The stimulated emission part is what produces the amplification of the light. The electrons in atoms and molecules can only occupy certain, discrete, orbital energy states. The trick is to stimulate an electron to jump to a higher energy state, and the method used is referred to as the pumping mechanism. Frequently this is done by collision with a loose electron in a plasma like that in a neon sign—for all intents and purposes a helium-neon laser is just a neon sign with mirrors—or by hitting it with intense white light, the method used when flash-lamp pumping ruby and other crystalline amplifiers. In a gas dynamic laser the pumping is achieved via the rapid changes in temperature and pressure caused by the exploding gasses in the two-dimensional rocket nozzle.
If nothing else happens, after a few microseconds the molecule will spontaneously relax to the lower energy orbital state, in the process emitting a photon travelling in a random direction and with a wavelength in a rather broad range. However, before spontaneous emission occurs, if a photon of just the right wavelength collides with the excited molecule, it will stimulate emission of an identical twin photon, identical in phase, direction and wavelength, and the two photons will continue on where previously there had been only one: amplification.
The number of molecules in an excited state is also an important factor. Unexcited molecules will absorb the photon during the collision, so if there are more unexcited than excited molecules—the normal state of affairs—the amplifying medium will absorb more than it amplifies. To achieve gain, the pumping method must excite molecules faster than they can spontaneously decay, producing a situation in which there are more excited than unexcited molecules, a state termed population inversion.
Strictly speaking, the optical amplifier without the mirrors is a LASER. Examine the acronym carefully and you’ll see that it merely refers to amplification, not the feedback that produces oscillation that produces laser light. In the acoustic oscillator created with the microphone, amplifier and speaker combination, if you don’t put the microphone close enough to the speaker to get sufficient feedback, you don’t get oscillation and you don’t get the loud squeal. Similarly, with the optical amplifier, only when the mirrors are added does it truly become a laser, but then it’s actually Light Oscillation by Stimulated Emission of Radiation: LOSER.
“The Enterprise fired its loser, badly damaging the Klingon warship.”
No! That book won’t sell.