Laser Safety Program: Laser Fundamentals

• LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
• A laser is a device that produces an intense, coherent directional beam of light by stimulating electronic or molecular transitions to higher energy levels.
• A laser system is an assembly of electrical, mechanical, and optical components that includes a laser.

Laser light is uniquely different from more common light sources due to 3 important properties:

1. Monochromaticity – The color of light is dependent upon the wavelength used. Laser light occurs at a single wavelength, thus it consists of a single color. White light, in comparison, is the combination of all visible colors or wavelengths.

2. Directionality (collimation) – Light from an ordinary source radiates in an omnidirectional manner. This divergence makes ordinary light useful for lighting homes and workplaces. Laser light, however, diverges very slowly as it radiates away from the source, and is concentrated into a narrow cone that propagates outward from the source in a single direction.

3. Coherence – The coherence of laser light contributes to most aspects of monochromaticity and directionality and is based on the wave properties of light. Light waves generated by an ordinary light source, such as a fluorescent bulb, are incoherent — a mixture of different frequencies and wavelengths of light, not in phase with one another. Light waves generated by a laser are all of the same frequency and are in phase with one another.
   
   The relationships between coherence, monochromaticity, and directionality are listed below:

   • Coherent light waves are monochromatic.
   • Since coherent light waves are all in phase with one another, the beam is highly directional and has the ability to travel great distances with little divergence.
   • Radiance is very high since power is concentrated within this narrow cone of divergence.

All lasers are composed of 4 basic elements:

1. Active medium: The collection of atoms or material that can be excited to a state of population inversion.
2. Excitation medium: The source of energy to move atoms from ground state to an excited state to create the population inversion.
3. Feedback mechanism: The system that returns a fraction of coherent laser light produced in the active medium back to an active medium.
4. Output coupler: By making one of the mirrors partially transmitting, a portion of the coherent light is allowed to escape.

Laser sources operate within 4 primary regions of the electromagnetic spectrum:

1. Ultraviolet (UV) wavelengths, 100 to 400 nanometers (nm), are shorter than the visible portion of the spectrum.
2. Visible wavelengths, 400 to 700 nm, range from the blue to the red spectrum respectively.
3. Near-infrared wavelengths, 0.7 to 1.4 µm, are longer than the visible portion of the spectrum.
4. Far infrared wavelengths, 1.4 to 10³ µm, where heat is predominately radiated by material objects and terminates where the microwave portion of the spectrum begins.

There are 4 important types of lasers with differentiation based on active medium type, pumping method, and the character of the output beam.

1. Solid-state lasers

   • Solid crystal lasers employ a solid crystalline material as an active medium such as ruby (crystalline aluminum oxide doped with chromium) or neodymium: YAG (triply ionized neodymium-doped with yttrium aluminum garnet). The active medium is a cylindrical rod with ends cut plane-parallel to each other, then polished. The pumping method is usually a tungsten filament lamp coupled with an AC power supply (for ruby).

   • Semiconductor lasers, also called laser diodes or injection lasers, employ an active medium that is a junction between slabs of semiconductor material such as gallium/ arsenide. The feedback mechanism is provided by cleaving sides of slab along crystal planes to form parallel mirror surfaces. The pumping method is an application of power supply across p-n junction where the intensity of light is controlled by varying the power applied. The output is generally in the infrared end of the electromagnetic spectrum.

2. Gas lasers use gas or a gas mixture such as Argon or helium-neon as an active medium and are contained within a sealed glass tube called a plasma tube. Mirrors are either attached to the ends of the plasma tube or are mounted externally. The pumping method is usually DC discharge within the plasma tube and may be either continuous or pulsed.

   • Excimer lasers, an abbreviation for Excited Dimer, operate using reactive gases such as chlorine and fluorine mixed with inert gases, such as argon, krypton, or xenon. The combinations, when electrically excited, produce a pseudo molecule or "dimer," with an energy level configuration that allows the generation of a specific laser wavelength in the UV spectra.

3. Liquid lasers employ liquid as an active medium such as complex organic dyes in alcohol solutions (rhodamine 6G). Dye solutions circulate through the glass tube. Different chemicals are fed into the reaction chamber. The feedback mechanisms are mirrors mounted externally to the glass tube or reaction chamber. The output wavelength can be varied by changing the dye's concentrations. The pumping method could be a high-intensity flashlamp or a second laser. Liquid lasers can be pulsed or continuous wave, ultraviolet, infrared, or visible.

4. Free-electron lasers are similar in many respects to a microwave oscillator tube. The photon emission occurs between continuum states of free electrons. The transition wavelength is determined by momentum conservation in the interaction with an "undulator" magnet. The undulator magnets consist of a transverse magnetic field which varies sinusoidally along the electronic beam trajectory. An electron transversing such a magnet is free to scatter a virtual photon from the magnet into a real proton.

In brief, the lasing process occurs when:

1. The excitation mechanism supplies sufficient energy to create a population inversion.
2. Excitation atoms in the active medium emit the laser wavelength in all directions by spontaneous emission. The resulting incoherent light is called fluorescence.
3. Photons are emitted traveling perpendicular to mirrors at the ends of the active medium and cause stimulated emission as photons are reflected back and forth through the active medium.
4. The reflection of photons continues and builds up optical standing waves inside the active medium that are composed of photons of the same wavelength, direction of travel, and coherence. Some photons escape through the output coupler to form the laser beam.
5. Laser emissions may be either continuous wave (cw) or pulsed, with pulsed repetition frequencies ranging from 1 to 10¹⁰ pulse per second. The pulse duration will typically range from a few milliseconds (10^-3 seconds) to several femtoseconds (10^-15 seconds).

Learn more at Laser Safety Program: Overview.