Lasers

TOPIC AREA

What Are Lasers?

Lasers are devices that produce coherent, monochromatic, and highly directional beams of electromagnetic radiation through the process of stimulated emission of radiation. The acronym itself describes the operating principle: Light Amplification by Stimulated Emission of Radiation. A laser requires three components: a gain medium that amplifies light, an energy pump that excites the medium to a population inversion, and an optical resonator, typically two mirrors, that provides feedback to sustain and direct the amplified emission. The result is a beam with spatial and temporal coherence properties that distinguish it from any thermal or broadband light source.

Lasers span an enormous range of wavelengths, from X-ray free-electron lasers operating below 1 nm to far-infrared molecular lasers operating beyond 1 mm, and an equally broad range of power levels, from microwatt laser pointers to petawatt pulsed systems used in fusion research. Their classification by gain medium defines both their emission wavelength and their operating characteristics.

Gas Lasers

Gas lasers use an ionized or neutral gas as the gain medium, typically excited by an electrical discharge passing through the gas. The helium-neon (He-Ne) laser, emitting at 632.8 nm, was one of the first continuous-wave lasers demonstrated and remains a laboratory and alignment standard. Carbon dioxide (CO2) lasers, emitting at 10.6 micrometers in the mid-infrared, are among the highest-power continuous-wave lasers available and are central to industrial cutting and welding. Excimer lasers, which use a reactive gas mixture such as ArF or KrF to produce pulsed UV output at 193 nm or 248 nm, are the light source of choice for photolithography nodes from 90 nm down to the current 7 nm generation. Laserax's overview of laser types provides a practical comparison of gas laser variants alongside solid-state and fiber systems.

Solid-state lasers use a crystalline or glass host doped with a rare-earth or transition-metal ion as the gain medium. The neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, emitting at 1064 nm in the near-infrared, is the most widely deployed solid-state laser. It can be operated in continuous-wave or pulsed mode, and its output is frequently frequency-doubled to 532 nm (green) or tripled to 355 nm (UV) using nonlinear crystals. ScienceDirect's overview of the neodymium-YAG laser covers its use in ophthalmology, dermatology, and industrial material processing, where its pulse energy and peak power are well matched to ablation and marking applications. Diode-pumped solid-state (DPSS) variants replace flashlamp pumping with high-efficiency laser diodes, achieving wall-plug efficiencies in the 30 to 50 percent range.

Semiconductor and Diode Lasers

Semiconductor lasers, also called laser diodes, achieve stimulated emission at the junction of a forward-biased p-n structure formed from direct-bandgap semiconductors such as GaAs, InP, or GaN. The bandgap of the material determines the emission wavelength, which can range from the near-ultraviolet (GaN, around 405 nm) through the near-infrared (InP-based structures, 1300 to 1550 nm). Their small size, high wall-plug efficiency, direct electrical modulation at gigahertz rates, and manufacturability in large volumes make diode lasers the dominant laser type by production volume, appearing in optical fiber transceivers, optical disc drives, and laser printing. RP Photonics' resource on diode-pumped lasers discusses how diode lasers also serve as pump sources for fiber and solid-state lasers.

Fiber Lasers

Fiber lasers guide light through a rare-earth-doped optical fiber, most often erbium- or ytterbium-doped silica, which serves simultaneously as the gain medium and the waveguide. The distributed gain along the fiber length and the waveguiding geometry produce excellent beam quality, high efficiency, and compact footprint. High-power ytterbium fiber lasers, operating near 1070 nm, have largely displaced CO2 lasers in sheet metal cutting because of their higher wall-plug efficiency and ability to couple output into a delivery fiber for flexible beam routing.

Applications

Lasers have applications across a wide range of scientific, industrial, and medical domains, including:

Industrial material processing including cutting, welding, marking, and additive manufacturing
Optical fiber communications, where semiconductor lasers carry data at terabit-per-second aggregate rates
Medical procedures including ophthalmologic surgery, dermatology, and photodynamic therapy
Scientific instrumentation including spectroscopy, LIDAR, and laser cooling of atomic systems
Defense systems including range finding, target designation, and directed-energy applications