Editorial Feature

What are Excimer, Solid-State, Fiber-Hosted, Semiconductor, Dye and Free Electron Lasers?

Article Updated on 11th March 2019

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Lasers have a multitude of applications across industries and professions and are used in microelectronic device production, micromachining, and even eye surgery. A laser is made in three parts: an energy source, a gain medium or laser medium, and two or more mirrors that make an optical resonator. The type of laser is governed by the laser or gain medium, which can be either solid, liquid, or gas. It can also be a semiconductor.

Excimer Lasers

Excimer lasers are pulsed gas discharge lasers that produce ultraviolet (UV) light. A chemical reaction with an excimer or an excited dimer is used to power the excimer laser. An excimer can be defined as a heterodimeric or a dimeric molecule that is short-lived, formed from two atom species. At least one of the atom species is in the excited electronic state.

There are four commonly used excimer molecules for the excimer laser that are derived from fluorine and chlorine gas. Argon fluoride (ArF) operates in the wavelength of 193 nm; Krypton fluoride (KrF) operates in the wavelength of 248 nm; Xenon fluoride (XeF) operates in the wavelength of 351 nm, and; Xenon Chloride (XeCl) operates in the wavelength of 308 nm.

The wavelength output of an excimer laser can be changed simply by changing the gas mixture. However, the laser mirrors may have to be exchanged to obtain maximum output.

Excimer lasers are well suited to precision micromachining organic material, as well as for delicate eye surgery and in semiconductor photolithography.

Solid-State Lasers

Solid-state lasers or SSLs employ a glass or a crystalline material doped with an ion. This serves as the lasing species. A pump source (an external source of energy that is transferred to the gain medium) such as diodes or flashlamps is required to pump the ions to excited levels. This then leads to the ions to give off radiation.

There are only a few solid-state media that are widely used, despite there being hundreds of solid-state media with which laser action has been achieved. The most common type of solid-state laser is neodymium (Nd) doped into crystals like Yttrium aluminum garnet (YAG). Nd:YAG lasers give off excellent wavelength, which is used for transmission through the atmosphere.

Nd:YAG lasers are also used in the medical field, in area  such as ophthalmology, cosmetic surgery including laser hair removal, and also in dentistry for soft tissue surgeries. Additionally, it is used in the manufacturing industry for applications such as engraving and etching with a variety of plastics and metals.

Neodymium is also doped with yttrium orthovanadate to form Nd:YVO4, and with yttrium lithium fluoride to form Nd:YLF. They are commonly used as lasing medium for diode-pumped solid-state lasers.

Other common dopants used in solid-state lasers are ytterbium, holmium, thulium, and erbium. Another type of solid-state laser is titanium doped into sapphire (Ti:sapphire). Ti:sapphire produces highly tunable infrared lasers that are commonly used in spectroscopy.

Fiber-Hosted Lasers

Glass fiber or optical fiber hosted lasers are also classed as solid-state lasers. Due to the high surface area to volume ratio of the fiber, they can support very high power output and allow for sufficient cooling. In addition, there is a reduction in the beam's thermal distortion due to the fiber's waveguiding properties.

Erbium-doped fiber lasers are one of the most common types of fiber laser. Erbium is used because erbium particles possess an energy level capable of absorbing photons with a wavelength of 980 nm, which decays to a meta-stable state equivalent to 1550 nm, meaning cost-effective pump sources can be used to achieve high quality, high energy beams.

The benefits of fiber-hosted lasers lie in their stability. Unlike other types of laser that require very fine tuning to align them correctly. However, as fiber lasers generate beams within the fiber itself, sensitive optics and fine-tuning aren't necessary.

Semiconductor Lasers

Semiconductor lasers are the most basic of the existing laser types. They are also commonly referred to as laser diodes. Semiconductor lasers or laser diodes are diodes that, by stimulated emission, give off coherent light. The structure of a laser diode is similar to the light-emitted diode where it consists of a p-n junction inside a semiconductor slab. The applications of low power laser diodes can be found in laser printers and CD/DVD players. More powerful laser diodes are frequently used to optically pump other lasers with high efficiency.

Another type of semiconductor laser is the vertical cavity surface-emitting lasers or VCSELs. Their direction of emission is perpendicular to the wafer surface.

Dye Lasers

Dye lasers are lasers that use an organic dye as the laser medium. The organic dye laser medium is normally in liquid form and is continuously circulated through the laser chamber. The dye may be pumped by flash lamps or by another laser such as an argon ion laser.

Rhodamine 6G, commonly referred to as Rh6G, is a widely used dye. Dye lasers are commonly used in dermatology but are also used in astronomy, manufacturing, medicine, and spectroscopy.

Free Electron Lasers

Free electron lasers, or FELs, are lasers that share the same optical properties as conventional lasers. FELs consist of a coherent high power electromagnetic radiation. The lasing medium for the FEL is relativistic electrons beam, hence its title. This makes the laser very tunable, and currently, the wavelengths range from microwaves to terahertz radiation, infrared, visible spectrum, ultraviolet, and X-ray.

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

Summary

The application for lasers is widespread, and the mechanisms by which lasers are built and operate vary accordingly. Name the industries that benefit from lasers. It is thought that future applications of lasers may include developments in nanotechnology, and even to supply energy to spacecraft.

Sources and Further Reading

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