Mihail Pivtoraiko Andrei Rozhkov

Applied Optics Winter 2003

A Survey of Laser Types Laser technology is available to us since 1960’s, and since then has been quite well developed. Currently, there is a great variety of lasers of different output power, operating voltages, sizes, etc. The major classes of lasers currently used are Gas, Solid, Molecular, and Free Electron lasers. Below we will cover some most popular representative types of lasers of each class and describe specific principles of operation, construction, and main highlights.

Gas Lasers Helium-Neon Laser

The most common and inexpensive gas laser, the helium-neon laser is usually constructed to operate in the red at 632.8 nm. It can also be constructed to produce laser action in the green at 543.5 nm and in the infrared at 1523 nm.

One of the excited levels of helium at 20.61 eV is very close to a level in neon at 20.66 eV, so close in fact that upon collision of a helium and a neon atom, the energy can be transferred from the helium to the neon atom.

Fig.1. The components of a Helium-Neon Laser

This process is illustrated in the Figure 2 on the next page.

Fig. 2. The lasing action of a NeHe laser

Helium-neon lasers are common in the introductory physics laboratories, but they can still be quite dangerous. An unfocused 1-mW HeNe laser has a brightness equal to sunshine on a clear day (0.1 watt/cm^2) and is just as dangerous to stare at directly.

Carbon Dioxide Laser

The carbon dioxide gas laser is capable of continuous output powers above 10 kilowatts. It is also capable of extremely high power pulse operation. It exhibits laser action at several infrared frequencies, but none in the visible spectrum. Operating in a manner similar to the helium-neon laser, it employs an electric discharge for pumping, using a percentage of nitrogen gas as a pumping gas.

The CO2 laser is the most efficient laser, capable of operating at more than 30% efficiency. That's a lot more efficient than an ordinary incandescent light bulb at producing visible light (about 90% of the output of a light bulb filament is invisible).

The carbon dioxide laser finds many applications in industry, particularly for welding and cutting.

Argon Laser

The argon ion laser can be operated as a continuous gas laser at about 25 different wavelengths in the visible between 408.9 and 686.1nm, but is best known for its most efficient transitions in the green at 488 nm and 514.5 nm. Operating at much higher powers than the helium-neon gas laser, it is not uncommon to achieve 30 to 100 watts of continuous power using several transitions. This output is produced in hot plasma and takes extremely high power, typically 9 to 12 kW, so these are large and expensive devices.

Solid Lasers

Ruby Laser

The ruby laser is the first type of laser actually constructed, first demonstrated in 1960 by T. H. Maiman. The ruby mineral (corundum) is aluminum oxide with a small amount (about 0.05%) of chromium which gives it its characteristic pink or red color by absorbing green and blue light.

The ruby laser is used as a pulsed laser, producing red light at 694.3 nm. After receiving a pumping flash from the flash tube, the laser light emerges for as long as the excited atoms persist in the ruby rod, which is typically about a millisecond.

A pulsed ruby laser was used for the famous laser ranging experiment which was conducted with a corner reflector placed on the Moon by the Apollo astronauts. This determined the distance to the Moon with an accuracy of about 15 cm.

Fig. 3. Principle of operation of a Ruby laser

Neodymium-YAG Laser

An example of a solid-state laser, the neodymium-YAG uses the Nd3+ ion to dope the yttrium-aluminum-garnet (YAG) host crystal to produce the triplet geometry which makes population inversion possible. Neodymium-YAG lasers have become very important because they can be used to produce high powers. Such lasers have been constructed to produce over a kilowatt of continuous laser power at 1065 nm and can achieve extremely high powers in a pulsed mode.

Neodymium-YAG lasers are used in pulse mode in laser oscillators for the production of a series of very short pulses for research with femtosecond time resolution.

Fig. 4. Construction of a Neodymium-YAG laser

Neodymium-Glass Lasers

Neodymium glass lasers have emerged as the design choice for research in laser-initiated thermonuclear fusion. These pulsed lasers generate pulses as short as 10-12 seconds with peak powers of 109 kilowatts.

Laser Diodes Laser action (with the resultant monochromatic and coherent light output) can be achieved in a p-n junction formed by two doped gallium arsenide layers. The two ends of the structure need to be optically flat and parallel with one end mirrored and one partially reflective. The length of the junction must be precisely related to the wavelength of the light to be emitted. The junction is forward biased and the recombination process produces light as in the LED (incoherent). Above a certain current threshold the photons moving parallel to the junction can stimulate emission and initiate laser action.

Type Peak Power Wavelength Application

GaAs 5 mW 840 nm CD Players

AlGaAs 50 mW 760 nm Laser printers

GaInAsP 20 mW 1300 nm Fiber communications

Fig. 5. Construction of a laser diode

Molecular Lasers Eximer Lasers

Eximer is a shortened form of "excited dimer", denoting the fact that the lasing medium in this type of laser is an excited diatomic molecule. These lasers typically produce ultraviolet pulses. They are under investigation for use in communicating with submarines by conversion to blue-green light and pulsing from overhead satellites through sea water to submarines below.

The eximers used are typically those formed by rare gases and halogens in electron-excited gas discharges. Molecules like XeF are stable only in their excited states and quickly dissociate when they make the transition to their ground state. This makes possible large population inversions because the ground state is depleted by this dissociation. However, the excited states are very short-lived compared to other laser metastable states, and lasers like the XeF eximer laser require high pumping rates. Eximer lasers typically produce high power pulse outputs in the blue or ultraviolet after excitation by fast electron-beam discharges

XeF Eximer Laser

The rare-gas xenon and the highly active fluorine seem unlikely to form a molecule, but they do in the hot plasma environment of an electron-beam initiated gas discharge. They are only stable in their excited states, if "stable" can be used for molecules which undergo radioactive decay in 1 to 10 nanoseconds. This is long enough to achieve pulsed laser action in the bluegreen over a band from 450 to 510 nm, peaking at 486 nm. Very high power pulses can be achieved because the stimulated emission cross-sections of the laser transitions are relatively low, allowing a large population inversion to build up. The power is also enhanced by the fact that the ground state of XeF quickly dissociates, so that there is little absorption to quench the laser pulse action.

Free-Electron Lasers

The radiation from a free-electron laser is produced from free electrons which are forced to oscillate in a regular fashion by an applied field. They are therefore more like synchrotron light sources or microwave tubes than like other lasers. They are able to produce highly coherent, collimated radiation over a wide range of frequencies. The magnetic field arrangement which produces the alternating field is commonly called a "wiggler" magnet.

Fig. 6. Principle of Operation of Free-Electron laser

The free-electron laser is a highly tunable device which has been used to generate coherent radiation from 10-5 to 1 cm in wavelength. In some parts of this range, they are

the highest power source. Particularly in the mm wave range, the FELs exceed all other sources in coherent power. FELs involve relativistic electron beams propagating in a vacuum and can be tuned continuously, filling in frequency ranges which are not reachable by other coherent sources.

Applications of free-electron lasers are envisioned in isotope separation, plasma heating for nuclear fusion, long-range, high resolution radar, and particle acceleration in accelerators.

Dye Lasers

Tunable laser operation over a nearly continuous range of frequencies has been attained with the molecules of certain organic dyes. The molecules of these dyes have a large number of spectral lines and each of them has a characteristic spread of frequencies which is large compared to the spread of gaseous atomic spectral lines. With the overlap of these lines in the dyes, the dye laser can be tuned to produce laser action for laser spectroscopy.

A widely used dye is rhodamine 6G, commonly referred to as Rh6G. It is one of the most highly fluorescing materials known and was used by early astronauts to mark the position of their capsules when landing in the ocean. The unique properties which have made it useful in such exotic applications have also made it popular as a laser medium. Another dye used for spectroscopy is known as "ring dye" and is capable of essentially continuous tuning.

The dye laser medium is typically in liquid form and the dye is circulated continuously through the laser chamber to keep it from being limited by saturation effects. The dye may be pumped by flash lamps or by another laser such as an argon ion laser.