Types of Lasers: Gas, Solid-State, Fiber, Semiconductor and More

Scope

This article surveys the kinds of laser in use, grouped by the gain medium that provides the light, and then by how they operate in time. For each it gives the characteristic wavelengths and the typical applications. Every laser shares the same three elements: a gain medium in which a population inversion allows stimulated emission to amplify light, a pump that supplies the energy, and a resonator, usually a pair of mirrors forming a Fabry-Perot resonator, that feeds light back through the gain. The gain medium sets the wavelengths a laser can reach; the resonator and pump set its power, linewidth and time behavior.

At a glance

Gas lasers

Helium-neon (HeNe). The classic red laser at 632.8 nm, also made at green, yellow and infrared lines. Low power (milliwatts) with excellent beam quality and a stable wavelength, which kept it in alignment, interferometry and metrology long after cheaper diodes arrived.

Carbon dioxide (CO₂). Emits in the mid-infrared around 10.6 μm (with lines near 9.4 μm), at powers from watts to many kilowatts, with high efficiency for a gas laser. The standard source for cutting and engraving non-metals, wood, plastics and glass, and for sheet-metal cutting before fiber lasers took much of that market.

Argon-ion. Blue-green lines at 488 nm and 514.5 nm, historically used for pumping other lasers, for spectroscopy and for displays; now largely replaced by diode-pumped solid-state and semiconductor sources.

Excimer. Pulsed ultraviolet lasers using rare-gas halide molecules: ArF at 193 nm, KrF at 248 nm, XeCl at 308 nm. Used for semiconductor lithography, eye surgery and fine micromachining, where the short wavelength and the strong absorption of UV by most materials matter.

Solid-state lasers

Solid-state lasers use ions doped into a crystal or glass, pumped by flashlamps or, now more often, by diode lasers.

Nd:YAG and Nd:YVO₄. Neodymium emits at 1064 nm; frequency doubling gives the familiar 532 nm green, and further harmonics give 355 nm and 266 nm. Widely used Q-switched for marking, ranging and materials processing.

Yb:YAG and other ytterbium hosts. Emit near 1030 nm with a small energy gap between pump and laser photons, which keeps heating low and suits high-power thin-disk and ultrafast lasers.

Ti:sapphire. Tunable from about 650 nm to 1100 nm, with gain bandwidth broad enough for pulses of a few femtoseconds. The standard source for ultrafast science, usually mode-locked and pumped by a green laser.

Others. Ruby (694.3 nm), historically the first laser; erbium glass near 1.54 μm for eye-safer ranging; holmium and thulium near 2 μm for medicine.

Fiber lasers

A fiber laser uses a doped optical fiber as the gain medium, usually a double-clad fiber whose inner cladding guides the light of multimode pump diodes into a single-mode doped core. The long, thin gain medium dissipates heat easily and keeps the beam quality high at high power.

• Ytterbium (about 1030 to 1080 nm) dominates industrial cutting and welding, at powers from watts to tens of kilowatts; see ytterbium-doped fiber.
• Erbium (about 1530 to 1610 nm) matches the telecom band and is also the basis of the erbium-doped fiber amplifier.
• Thulium (about 1.9 to 2.1 μm) serves medical and mid-infrared applications.

Semiconductor lasers

A laser diode is pumped directly by electrical current, which makes it the smallest, most efficient and cheapest laser, and the pump source for most solid-state and fiber lasers. The semiconductor sets the wavelength: GaN-based materials for violet to green, AlGaAs and InGaAs on GaAs for red to about 1100 nm, and InGaAsP on InP for the 1.3 and 1.55 μm telecom bands. The resonator sets the rest.

High-power diode bars and stacks, with many emitters side by side, reach hundreds of watts but with poor beam quality in the slow axis; they are used to pump other lasers and, directly, for some welding and heat treatment. Tunable lasers of several of these kinds are covered in their own entry.

Dye lasers and parametric sources

Dye lasers use organic dyes in solution, pumped by another laser or a flashlamp. Each dye covers a band of a few tens of nanometres, and changing dyes spans the visible. They were the tunable source of choice for spectroscopy for decades and are now largely replaced by Ti:sapphire and parametric sources.

Optical parametric oscillators are not lasers in the strict sense, since they have no population inversion: a pump laser drives a nonlinear crystal that splits each pump photon into two longer-wavelength photons. An OPO is tunable over wide ranges from the visible to the mid-infrared by changing the crystal's phase matching.

Operation in time

The same gain medium can be run in different regimes:

Measuring the shortest of these pulses needs the methods in How to Measure Ultrashort Pulse Duration.

Choosing a laser

The wavelength is usually decided first, by the material to be processed, the fiber to be used, or the transition to be probed; it narrows the families to one or two. Power, then beam quality (M²), then the time regime, then linewidth and tunability select among them. Cost and size favor semiconductor lasers wherever they reach the wavelength and power; fiber lasers where high power with good beam quality is needed near 1 μm; and solid-state and gas lasers where their particular wavelengths, pulse energies or linewidths cannot be matched otherwise.

References: A. E. Siegman, Lasers (University Science Books, 1986); O. Svelto, Principles of Lasers (5th ed., Springer, 2010); W. Koechner, Solid-State Laser Engineering (6th ed., Springer, 2006); L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits (2nd ed., Wiley, 2012).