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 radiative 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 Laser

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.

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.