The term "LASER" is an acronym. It stands for "Light Amplification by Stimulated Emission of Radiation." Thus the laser is a device which produces and amplifies light. The mechanism by which this is accomplished, stimulated emission, was first postulated by Albert Einstein in 1917. The light which the laser produces is unique, for it is characterized by properties which are very desirable, but almost impossible to obtain by any means other than the laser.
The natural light by which we see is also the essential energy source by which our planet survives: it controls plant growth, the climate and the weather. The light rays from the sun. which travel at about 300 000 km/s and take approximately 8.5 minutes to arrive on the earth's surface, impart their energy according to the sun's different light wave lengths and the substances on which they fall. Naturally selected light wave lengths when absorbed by the molecules of certain substances excite the molecules causing them to vibrate and generate heat. A steel plate beside. or even placed behind a plain glass plate in the full sunlight, on a hot summer's day, will absorb more energy from the light waves than the glass plate. The steel will become so hot that it will be uncomfortable to touch. whereas the glass will not be significantly warmer than the air through which the light rays passed. Glass absorbs less heat because it is transparent to most of the light wave lengths which make up the sun's rays. The results of this comparison. however, can be changed if a specific wave length is artificially produced. Steel, for example, is partially transparent to certain X-rays. These artificial rays are the same type as light, but of much shorter wavelength. The ability of the different light wave lengths to give up their energy in the form of heat when absorbed by different substances, coupled with the fact that light can be transmitted long distances and then be optically focused to a small spot and cause a massive increase in its power density (W/mm2), has stimulated scientists and engineers to develop special light making devices called lasers.
Lasers
Lasers produce a collimated and coherent beam of light (coherent: waves of one wave length all in phase). This light is quite different from the incoherent light of the sun, The almost parallel, single wave length, light rays which make up the collimated laser beam have a considerably higher power density and can be focused to a much smaller spot size than the randomly radiated rays of the sun. Consequently. a much more efficient power density is achieved with a laser.
The word LASER is an acronym, it stands for: (L) light (A) amplification (S) stimulated by the (E) emission of (R) radiation. and refers to the way in which the light is generated. The basic principle of how a laser works is presented in the following paragraph.
All lasers are optical amplifiers which work by pumping (exciting) an active medium placed between two mirrors, one of which is partially transmitting. The active medium is a collection of specially selected atoms. molecules or ions which can be in a gas, liquid or solid form and which will lase, i.e. emit radiation as light waves (referred to as photons) when excited by the pumping action. Pumping of liquids and solids is achieved by flooding them with light from a flash lamp and gases are pumped by applying an electrical discharge.
The term photon is used instead of light wave when describing the production of laser light, because the photon carries with it a precisely defined amount of energy in relation to its wave length. Whatever the active medium consists of: atoms. molecules or ions there are billions of them and they absorb energy when pumped. which they hold for a very short but random life time. When their life time expires they give up their energy in the form of a photon and return to their former state until pumped again. The release of photons in this manner is called spontaneous emission. The photons released travel in all directions in relation to the optical axis of the laser. If a photon collides with another energized atom, etc, it causes it to release its photon prematurely and the two photons will travel along in phase until the next collision. thus building a stream of photons of increasing density. This action of releasing a photon prematurely is called stimulated emission. Photons which do not travel parallel to the optical axis of the laser are quickly lost from the system. Those which do travel parallel to the axis have their path length considerably extended by the optical feed back provided by the mirrors. before leaving the laser, through the partially transmitting mirror. This action not only serves as an amplifier for photon generation by stimulated emission to achieve the required power level, but also to provide the highly collimated coherent light beam that makes the laser so useful.
The power density across the diameter of a laser output beam is not uniform and is dependent on the laser's active medium, its internal dimensions, optical feed back design and the excitation system employed. The transverse cross sectional profile of a laser beam, which shows its power distribution, is called the 'transverse electromagnetic mode. (TEM). Many different TEM's can he designed for and each type is rated by a number. In general. the higher the number the more difficult it is to focus the laser beam to a fine spot to achieve a high power density. Some lasers produce several different modes and these are usually referred to as having a multi-mode operation.
Laser light has three unique properties. First, it is collimated which means it travels in a single direction with very little divergence even over long distances Ordinary light waves spread and lose intensity quickly.
Second, laser light is monochromatic, consisting of one color or a narrow range of colors . Ordinary light has a much wider range of wavelengths or colors.
Third, laser light is coherent, which means all of the light waves move in phase together in both time and space. When compared with incoherent light from a light bulb or flashlight, one can see the ordinary light is composed of a mixture of frequencies all out of the step with each other and traveling in different directions.
Physical background : Exciting atoms or molecules
In a laser, the atoms or molecules of a crystal, such as ruby or garnet or of a gas, liquid, or other substance are excited in what is called the laser cavity so that more of them are at higher energy levels than are at lower energy levels. Reflective surfaces at both ends of the cavity permit energy to reflect back and forth, building up in each passage. (See figure below)
In a ruby laser, light from the flash lamp, in what is called "optical pumping", excites the molecules in the ruby rod, and they bounce back and forth between two mirrors until coherent light escapes from the cavity.
If a photon whose frequency corresponds to the energy difference between the excited and ground states strikes an excited atom, the atom is stimulated as it falls back to a lower energy state to emit a second photon of the same (or a proportional) frequency, in phase with and in the same direction as the bombarding photon.
This process is called stimulated emission. The bombarding photon and the emitted photon may then each strike other excited atoms, stimulating further emission of photons, all of the same frequency and phase. This process produces a sudden burst of coherent radiation as all the atoms discharge in a rapid chain reaction.
A generalized laser consists of a lasing medium, a "pumping" system and an optical cavity. The laser material must have a metastable state in which the atoms or molecules can be trapped after receiving energy from the pumping system. Each of these laser components are discussed below:
1. Pumping Systems:
The pumping system imparts energy to the atoms or molecules of the lasing medium enabling them to be raised to an excited "metastable state" creating a population inversion.
a. Optical pumping uses photons provided by a source such as a Xenon gas flash lamp or another laser to transfer energy to the lasing material. The optical source must provide photons which correspond to the lower transition levels of the lasing material.
b. Collision pumping relies on the transfer of energy to the lasing material by collision with the atoms (or molecules) of the lasing material. Again, energies which correspond to the allowed transitions must be provided. This is often done by electrical discharge in a pure gas - or gas mixture - in a tube.
c. Chemical pumping systems use the binding energy released in chemical reactions to raise the lasing material to the metastable state.
2. Optical Cavity:
An optical cavity is required to provide the amplification desired in the laser and to select the photons which are traveling in the desired direction. As the first atom or molecule in the metastable state of the inverted population decays, it triggers via stimulated emission, the decay of another atom or molecule in the metastable state. If the photons are traveling in a direction which leads to the walls of the lasing material, which is usually in the form of a rod or tube, they are lost and the amplification process terminates. They may actually be reflected at the wall of the rod or tube, but sooner or later they will be lost in the system and will not contribute to the beam.
If, on the other hand, one of the decaying atoms or molecules releases a photon parallel to the axis of the lasing material, it can trigger the emission of another photon and both will be reflected by the mirror on the end of the lasing rod or tube. The reflected photons then pass back through the material triggering further emissions along exactly the same path which are reflected by the mirrors on the ends of the lasing material. As this amplification process continues, a portion of the radiation will always escape through the partially reflecting mirror. When the amount of amplification or gain through this process exceeds the losses in the cavity, laser oscillations said to occur. In this way, a narrow concentrated beam of coherent light is formed.
The mirrors on the laser optical cavity must be precisely aligned for light beams parallel to the axis. The optical cavity itself, i.e., the lasing medium material must not be a strong absorber of the light energy.
3. Laser Media:
Lasers are commonly designated by the type of lasing material employed. There are four types which are: solid state, gas, dye, and semiconductor. The characteristics of each type will be described. Note the wavelengths in Table II-1.
a. Solid state lasers employ a lasing material distributed in a solid matrix. One example is the Neodymium - YAG laser. The term: YAG is an abbreviation for the crystal: Yttrium Aluminum Garnet which serves as the host for the Neodymium ions. This laser emits an infrared beam at the wave length of 1.064 micrometer. Accessory devices that may be internal or external to the cavity may be used to convert the output to visible or ultraviolet wavelength.
b. Gas lasers use a gas or a mixture of gases within a tube. The most common gas laser uses a mixture of helium and neon (HeNe), with a primary output of 632.8 nm (nm = 10-9 meter) which is a visible red color. It was first developed in 1961 and has proved to be the forerunner of a whole family of gas lasers. All gas lasers are quite similar in construction and behavior. For example, the CO2 gas laser radiates at 10.6 micrometers in the far-infrared spectrum. Argon and krypton gas lasers operate with multiple frequency emissions principally in the visible spectra. The main emission wavelengths of an argon laser are 488 and 514 nm.
c. Dye Lasers use a laser medium that is usually a complex organic dye in liquid solution or suspension. The most striking feature of these lasers is their "tunability." Proper choice of the dye and its concentration allows the production of laser light over a broad range of wavelength in or near the visible spectrum. Dye lasers commonly employ optical pumping although some types have used chemical reaction pumping. The most commonly used dye is Rhodamine 6G which provides tunability over 200 nm bandwidth in the red portion (620 nm) of the spectrum.
d. Semiconductor lasers (sometimes referred to as diode lasers) are not to be confused with solid state lasers. Semiconductor devices consist of two layers of semiconductor material sandwiched together. These lasers are generally very small physically, and individually of only modest power. However, they may be built into larger arrays.
The different time modes of operation of a laser are distinguished by the rate at which energy is delivered.
a. Continuous wave (CW) lasers operate with a stable average beam power. In most higher power systems, one is able to adjust the power. In low power gas lasers, such as HeNe, the power level is fixed by design and performance usually degrades with long term use.
b. Single pulsed (normal mode) lasers generally have pulse durations of a few hundred microseconds to a few milliseconds. This mode of operation is sometimes referred to as long pulse or normal mode.
c. Single pulsed Q-Switched lasers are the result of an intra cavity delay (Q-switch cell) which allows the laser media to store a maximum of potential energy. Then, under optimum gain conditions, emission occurs in single pulses; typically of 10-8 second time domain. These pulses will have high peak powers often in the range from 106 to 109 Watts peak.
d. Repetitively pulsed or scanning lasers generally involve the operation of pulsed laser performance operating at a fixed (or variable) pulse rates which may range from a few pulses per second to as high as 20,000 pulses per second. The direction of a CW laser can be scanned rapidly using optical scanning systems to produce the equivalent of a repetitively pulsed output at a given location.
e. Mode locked lasers operate as a result of the resonant modes of the optical cavity which can effect the characteristics of the output beam. When the phases of different frequency modes are synchronized, i.e., "locked together," the different modes will interfere with one another to generate a beat effect. The result is a laser output which is observed as regularly spaced pulsations. Lasers operating in this mode-locked fashion, usually produce a train of regularly spaced pulses, each having a duration of 10-15 (femto) to 10-12 (pico) sec. A mode-locked laser can deliver extremely high peak powers than the same laser operating in the Q-switched mode. These pulses will have enormous peak powers often in the range from 1012 (tera) Watts peak.