Laser Basics

Laser Light

The light from a laser (Light Amplification by Stimulation Emission of Radiation) is brighter than any natural or man-made source, including the sun. The major difference between both, is that laser light is monochromatic (one wavelength or color), directional and coherent.

Laser Components

All lasers contain 3 primary components to operate: active medium, excitation mechanism and the optical resonator.

Active medium:

Is a collection of atoms, molecules, or ions which absorb energy from an outside source and generate laser light through atomic processes. Many of the laser beam’s output characteristics, including the wavelength, are determined by the active medium. Examples of media include:

  • Solid Crystalline Materials: e.g. Ruby, Neodymium: YAG
  • Semi-Conductor Materials (diode): e.g. gallium/arsenide; gallium/aluminum/arsenide
  • Liquid Dyes: utilize a flowing dye pumped by flash lamp of other lasers
  • Gaseous Materials: e.g. Helium-Neon, CO2, Argon, Krypton

Excitation Mechanism:

Is the input energy device. It could be an intense light source, an electrical current through an active gas, or in the case of dye lasers, light from another laser.

Optical Resonator:

Consists of 2 specially designed mirrors, dramatically opposed to each other. The high reflectance mirror reflects 100% of the light which strikes it, while the other mirror (called the output coupler or partially transmissive mirror) reflects less than 100% of the light which strikes it. The small fraction of light which passes through the partially transmissive mirror is the beam output.

Laser Operation

The generation of laser radiation occurs in several steps.

Step 1 Excitation mechanism or energy source causes a flash of emitted light, which is absorbed by the active medium.

Step 2 Atoms of the active medium then become excited to a higher energy level. When the number of excited atoms is greater than the number of non-excited atoms, a population inversion is created, necessary for laser action to occur.

Step 3 A small amount of energy in the form of a photon is emitted when the excited atoms return to a non-excited state. These photons, in turn, stimulate other excited atoms in the active medium to release additional photons, causing a chain reaction through the active medium, this is called stimulated emission.

Step 4 The mirrors at each end of the laser reflect emitted light back into the active medium, a process referred to as amplification. As the light inside the laser grows more and more intense, part of it escapes through the partially transmissive mirror as laser light.

Laser Categories

Lasers are classified by their by wavelength, maximum output power and by their active medium. Lasers characterized by their active medium, maybe solid, liquid or gas.

Solid State Lasers:

  • The term “solid state”, as related to lasers, refers to a group of optically clear materials such as glass or a “host” crystal with an impurity dopant.
  • Most common include the Ruby laser and the Nd: YAG (Neodymium: Yttrium/Aluminum/Garnet).
  • Solid state lasers can be operated as a continuous wave (CW) or pulse mode.

Semi Conductor (Diode) Lasers:

In terms of numbers, the diode laser is the most common laser today. The 2 common families of diode lasers are composed of:

  1. Ga AIAs (Gallium/Aluminum/Arsenide) with a wavelength output in the 750 to 950 nanometers (used in CD &CD/ROM players),
  2. InGaAsP (Indium/Gallium/Arsenide/Phosphide) with a wavelength output in the 1100 – 1650 nm range (used in optical telecommunications).

Another family of diode lasers like AlGaInP (Aluminum/Gallium/Arsenide/Phosphide) operates in the visible part of the spectrum, primarily red.

Liquid (Dye) Lasers:

The common liquid lasers utilize a flowing dye as the active medium, and are primped by a flash lamp or by another laser, such as an argon laser. They operate either as pulse or continuous wave (CW) mode.

Gas Lasers:

Gas lasers are like fluorescent light bulbs and neon signs. In a Helium-Neon (HeNe) laser, the mixture of helium and neon gas is confined to a hollow glass tube, then an electric current passes through the tube, atoms are excited and causes them to emit light. Other gas laser systems include the Carbon Dioxide (CO2) laser, and the Argon (Ar) laser. Another variation is the laser family known as the Excimer lasers (“excited dimer”), such as the Xenon-Chloride (XeCl) laser. Excimers function in the ultraviolet part of the spectrum.

Gas lasers generally operate as continuous wave (CW), with the exception of excimer lasers.

Common Laser classifications by wavelength

lase basics table

lase basics

Copyright 2005 Laser Institute of America

Laser Hazard Classification Scheme

Lasers have been classified by wavelength and maximum output power and then divided into four classes with subclasses. The classification categorizes lasers according to their ability to produce damage in exposed people, from class 1 (no hazard during normal use) to class 4 (severe hazard for eyes and skin).

The classification of a laser is based on the maximum power (in Watt) or energy (in Joule) that can be emitted in a specified wavelength range and exposure time. For infrared wavelengths above 4 microns it is specified as a maximum power density (in Watt/metre2). It is the responsibility of the manufacturer to provide the correct classification of a laser, and to equip the laser with appropriate warning labels and safety measures as prescribed by regulations. Safety measures used with the more powerful lasers include key-controlled operation, warning lights to indicate laser light emission, a beam stop or attenuator, and an electrical contact that the user can connect to an emergency stop or interlock.

Class 1 & 1M (Exempt)

  • Exempt from user control measures
  • Incapable of producing damaging radiation levels
  • Examples: Completely enclosed machine with higher powered laser inside & optical communication systems

Class 2 & 2M (Low Power)

  • Almost eye safe
  • Visible portion of the electromagnetic spectrum (0.4 to 0.7 microns)
  • Eye protection is the aversion response (blinking)
  • Continuous wave (CW) with an upper limit of 1 milliwatt (mW)
  • Examples: Supermarket or barcode scanners, low-powered laser pointers, laser printers, CD ROM, compact audio disks, leveling instruments and construction industry lasers

Class 3a & now called 3R

  • Operate between 1mW and 5mW (if visible) or up to 5 times Class 1 (in infrared and ultraviolet spectral regions)
  • Examples: Many laser pointers and construction alignment lasers

Class 3b

A Class 3b laser is hazardous if the eye is exposed directly, but diffuse reflections such as from paper or other matte surfaces are not harmful. Normally, Class 3b lasers are continuous lasers in the wavelength range of 315 nanometers to far infrared and the power is limited to 0.5 Watt. For pulsed lasers, Class 3b falls between 400 and 700 nanometers and the power limit is 30 MilliJoules. Other limits apply to other wavelengths and to ultra short pulsed lasers. Protective eyewear is typically required where direct viewing of a class 3b laser beam may occur. Class 3b lasers must be equipped with a key switch and a safety interlock.

  • Operate between 5 mW and 500 mW
  • Not a significant fire or diffuse viewing hazard
  • Hazardous under direct and specular reflection viewing
  • Examples: Some military lasers, lasers used in therapeutic medicine, some research lasers

Class 4

Class 4 lasers include all lasers with beam power greater than class 3b. Class 4 lasers must be equipped with a key switch and a safety interlock.

  • Operate above 500 mW
  • Hazardous to eye and skin from direct viewing and possibly by diffuse reflection
  • Potential fire hazard
  • May produce laser generated air contaminants
  • May produce hazardous plasma radiation
  • Examples: Laser used for cutting, drilling, marking welding materials, outdoor light shows and surgical lasers and many industrial, scientific, military, and medical lasers are found in this category

Source: Laser Safety Officer Training Course – Laser Institute of America, March 2008