Lasers (in general)
HeNe laser
Laser Diodes
Optical spectrum

Operation of Laser

A laser (light amplification by stimulated emission of radiation) is a device which uses a quantum mechanical effect, stimulated emission, to generate a coherent beam of light from a lasing medium of controlled purity, size, and shape. The output of a laser may be a continuous, constant-amplitude output (known as CW or continuous wave), or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved. A laser medium can also function as an optical amplifier when seeded with light from another source. The amplified signal can be very similar to the input signal in terms of wavelength, phase, and polarisation; this is particularly important in optical communications. The verb "to lase" means to give off coherent light or possibly to cut or otherwise treat with coherent light, and is a back-formation of the term laser.

The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, and Schawlow at Bell laboratories. Maiman used a solid-state flashlamp-pumped ruby crystal to produce red laser light at 694-nanometres wavelength. In the same year the Iranian physicist Ali Javan invented the gas laser. He later received the Albert Einstein Award.

Common light sources, such as the electric light bulb, emit photons in almost all directions, usually over a wide spectrum of wavelengths. Most light sources are also incoherent; i.e., there is no fixed phase relationship between the photons emitted by the light source. By contrast, a laser generally emits photons in a narrow, well-defined, polarised, coherent beam of near-monochromatic light, consisting of a single wavelength or hue. Some types of laser, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds). A great deal of quantum mechanics and thermodynamics theory can be applied to laser action, though in fact many laser types were discovered by trial and error.

The basic physics of lasers centres around the idea of producing a population inversion in a laser medium by "pumping" the medium; i.e., by supplying energy in the form of light or electricity, for example. The medium may then amplify light by the process of stimulated emission. If the light is circulating through the medium by means of a cavity resonator, and the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. Eventually it will get so strong that the gain is saturated (reduced). In continuous operation, the intracavity laser power finds an equilibrium value which is saturating the gain exactly to the level of the cavity losses. If the pump power is chosen too small (below the "laser threshold"), the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers.

Population inversion is also the concept behind the maser, which is similar in principle to a laser but works with microwaves. The first maser was built by Charles H. Townes and graduate students J. P. Gordon, and H. J. Zeiger in 1953. Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical maser as it was then known. The word laser was coined in 1957 by Gordon Gould. Gordon also coined the words iraser, intending "aser" as the suffix and the spectra of light emitted at as the prefix (examples: X-ray laser = xaser, UltraViolet laser = uvaser) but these terms never became popular. Gordon was also credited with lucrative patent rights for a gas-discharge laser in 1987, following a protracted 30 year legal battle.

The first maser, developed by Townes, was incapable of continuous output. Nikolai Basov and Alexander Prokhorov of the USSR worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. In 1964, Charles Townes, Nikolai Basov and Alexandr Prokhorov shared a Nobel Prize in Physics "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."

Laser light can be highly intense-able to cut steel and other metals. The beam emitted by a laser often has a very small divergence (highly collimated) though a perfectly collimated beam cannot be created, due to the effect of diffraction, a laser beam will spread much less than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. Some lasers, especially semiconductor lasers due to their small size, produce very divergent beams. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well or much. Using a waveguide such as an optical fibre though, diffraction laws governing divergence no longer apply. Other interesting effects happen in nonlinear optics.

An unforseen discovery counter to expected and long-held laser properties, lasing without maintaining the medium excited into a population inversion, was discovered in sodium gas in 1992 and again in 1995 each in sodium and rubidium gas by various international teams. Normally, electrons in the ground state absorb the pumping and emitted radiation, thwarting the laser gain by heating up the medium. So media with electron levels and transitions amenable to the driving current are desired, and generally those which involve three or four energy levels rather than two make better lasers because the electrons are kept above the ground state, excited, and optically-transparent so as not to heat up, but such media are prone to noisy beams. By using an external maser to induce "optical transparency" in the media by introducing and destructively interfering the ground electron transitions between two paths, the likelihood for the ground electrons to absorb any energy has been cancelled. Now that less energy is needed to drive the lasing process, lasers are expected to run more efficiently than the .01 to .3 for typical media and wavelengths.

A HeNe laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The glowing ray in the middle is a electric discharge producing light in much the same way as a neon light; though it is the gain medium through which the laser passes, it is not the laser beam itself which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.

Uses of lasers

At the time of their invention in 1960, lasers were called a "a solution looking for a problem". Since then, they have become virtually ubiquitous, finding utility in thousands of highly varied applications in every section of modern society from vision correction to guidance for transportation and spacecraft to thermonuclear fusion. They have been widely regarded as one of the most influential technological achievements of the 20th century.

The exceptional utility which lasers have found in scientific, industrial and commercial applications stems from their coherency, high monochromaticity, capability for reaching extremely high powers, or a confluence of these factors. For instance, a laser beam's coherence potentially allows it to be focused down to its diffraction limit, which at visible wavelengths corresponds to only a few hundred nanometers. This property is what allows a laser to record gigabytes of information in the microscopic pits of a DVD. It is also what allows a laser of modest power to be focused to very high intensities and used for cutting, burning or even vaporizing materials. For example, a frequency doubled neodymium yttrium aluminum garnet (Nd:YAG) laser emitting 532 nanometer (green) light at 10 watts output power is theoretically capable of achieving an intensity of megawatts per square centimeter. In reality however, perfect focusing of a beam to its diffraction limit is very difficult.

Even low-power lasers with only a few milliwatts of output power can be hazardous to a person's eyesight. At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localised burning and permanent damage in seconds or even faster. Lasers are classified into safety classes numbered I, inherently safe, to IV, even scattered light can cause eye and/or skin damage. Laser products available for consumers, such as CD players and laser pointers are usually in class I or II.

Common laser types

Laser safety

Helium-neon laser

A helium-neon laser, usually called a HeNe laser, is a small gas laser. Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum.

Schematic diagram of a helium-neon laser

The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, approximately in the ratio 5:1, contained at low pressure (typically ~300 Pa) in a glass envelope. The energy or pump source of the laser is provided by an electrical discharge of around 1000 V through an anode and cathode at each end of the glass tube. The cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end. HeNe lasers are typically small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW.

The laser process in a HeNe laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting them into the 2s and 3s states. This is due to a coincidence of energy levels between the helium and neon atoms.

This process is given by the reaction equation:

He* + Ne → He + Ne* + ΔE

where (*) represents an excited state, and ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV.

Energy level diagram of a HeNe laser

The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion between the neon 3s and 2s, and 3p and 2p states. Spontaneous emission between the 3s and 2p states results in emission of 632.8 nm wavelength light, the typical operating wavelength of a HeNe laser.

After this, fast radiative decay occurs from the 2p to the 1s energy levels, which then decay to the ground state via collisions of the neon atoms with the container walls. Because of this last required step, the bore size of the laser cannot be made very large and the HeNe laser is limited in size and power.

With the correct selection of cavity mirrors, other wavelengths of laser emission of the HeNe laser are possible. The 3s→3p and 2s→2p transitions give infrared operation at 3.39 μm and 1.15 μm wavelengths, and a variety of 2s→1s transitions are possible in the green (543.5 nm, the so-called GreeNe laser), the yellow (594 nm) and the orange (612 nm).

Laser diode

More about Diode Lasers PDF text

A laser diode is a laser where the active medium is a semiconductor p-n junction similar to that found in a light-emitting diode. Laser diodes are sometimes referred to (somewhat redundantly) as injection laser diodes or by the acronyms LD or ILD.

Principle of operation

When a diode is forward biased, holes from the p-region are injected into the n-region, and electrons from the n-region are injected into the p-region. If electrons and holes are present in the same region, they may radiatively recombine-that is, the electron "falls into" the hole and emits a photon with the energy of the bandgap. This is called spontaneous emission, and is the main source of light in a light-emitting diode.

Under suitable conditions, the electron and the hole may coexist in the same area for quite some time (on the order of microseconds) before they recombine. If a photon of exactly the right frequency happens along within this time period, recombination may be stimulated by the photon. This causes another photon of the same frequency to be emitted, with exactly the same direction, polarization and phase as the first photon.

In a laser diode, the semiconductor crystal is fashioned into a shape somewhat like a piece of paper-very thin in one direction and rectangular in the other two. The top of the crystal is n-doped, and the bottom is p-doped, resulting in a large, flat p-n junction. The two ends of the crystal are cleaved so as to form perfectly smooth, parallel edges; two reflective parallel edges are called a Fabry-Perot cavity. Photons emitted in precisely the right direction will be reflected several times from each end face before they are emitted. Each time they pass through the cavity, the light is amplified by stimulated emission. Hence, if there is more amplification than loss, the diode begins to "lase".

The wavelength emitted is a function of the band-gap between p and n region energy levels. No photons with higher energy than the band-gap will be emitted.

Due to ubiquitous diffraction, the laser light leaving the diode from the thin active region will undergo a Fourier transform of intensity very quickly, and will need a collimating lens to mae the light a beam. Just like a wave packet spreading, the beam divergence away from the plane of the active region will by far be the highest, and thus the lenses most often used are cylindrical.

Because the device is, after all, based on a tiny p-n junction, the diode will be extremely sensitive to fluctuations in power, often changing wavelengths. This most simple diode described above has been heavily modified in recent years to accommodate modern technology.

Types of laser diode

The type of laser diode just described is called a homojunction laser diode, for reasons which should soon become clear. Unfortunately, they are extremely inefficient. They require so much power that they can only be operated in short "pulses;" otherwise the semiconductor would melt. Although historically important and easy to explain, such devices are not practical.

Double heterostructure lasers

In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. One commonly-used pair of materials is GaAs with AlGaAs. Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser. The kind of laser diode described in the first part of the article is referred to as a "homojunction" laser, for contrast with these more popular devices.

The advantage of a DH laser is that the region where free electrons and holes exist simultaneously-the "active" region-is confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification-not so many are left out in the poorly amplifying periphery. In addition, light is reflected from the heterojunction; hence, the light is confined to the region where the amplification takes place.

Quantum well lasers

If the middle layer is made thin enough, it starts acting like a quantum well. This means that in the vertical direction, electron energy is quantised. The difference between quantum well energy levels can be used for the laser action instead of the bandgap. This is very useful since the wavelength of light emitted can be tuned simply by altering the thickness of the layer. The efficiency of a quantum well laser is greater than that of a bulk laser due to a tailoring of the distrubution of electrons and holes that are involved in the stimulated emission (light producing) process.

The problem with these devices is that the thin layer is simply too small to effectively confine the light. To compensate, another two layers are added on, outside the first three. These layers have a lower refractive index than the centre layers, and hence confine the light effectively. Such a design is called a separate confinement heterostructure (SCH) laser diode.

Almost all commercial laser diodes since the 1990s have been SCH quantum well diodes.

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