Laser Characteristics

The term LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation.  The figure below is a simplified diagram of a laser:

Lasers operate by the excitation of a medium through the introduction of energy.  As electrons in this medium return to their ground state, they stimulate the release of light of a certain wavelength.  This chain reaction continues until a certain number of photons reach the totally reflective mirror.  This reverses the direction of the beam and the beam continues to intensify until it passes through the partially transmissive mirror, constituting the laser beam.  Laser radiation will continue to be produced as long as energy is applied to the lasing medium.  Laser radiation differs from normal light in that it is coherent, electromagnetic radiation characterized by one or more specific wavelength(s).  The wavelengths are determined primarily by the composition of the lasing medium, which can be a solid, liquid, or gas.  Laser radiation may be emitted in the visible portion of the electromagnetic spectrum (wavelengths of 400 – 700 nm) or in the invisible infrared (700-3x106 nm) and ultraviolet (180-400 nm) regions.  

Laser radiation transmits energy which, when a laser beam strikes matter, can be transmitted, absorbed, or reflected.  If a material transmits a laser beam it is said to be transparent.  If the beam is not transmitted, the material is said to be opaque and the incident radiation is absorbed or reflected.

Absorption

Absorbed laser energy appears in the target material as heat.  Absorption and transmission are functions of the chemical and physical characteristics of the target material and the wavelength of the incident radiation.  At visible wavelengths, laser radiation impinging on the eye is focused on the retina and, if sufficient energy is absorbed, can cause cell destruction.  At longer and shorter wavelengths, such as the far infrared and the ultraviolet, radiation striking the eye is absorbed in the cornea and the lens rather than being focused on the retina.  Although these structures are less easily damaged than the retina, excessive energy absorption can cause cell damage and impairment of vision.

Reflection

Reflection is a function of the physical character of the surface of the target material.  A smooth polished surface is generally a good, or specular, reflector; a rough uneven surface usually is a poor reflector producing a diffuse reflection.  A reflector such as a flat mirror changes the direction of an incident beam with little or no absorption.  A curved mirror or surface will change the divergence angle of the impinging laser beam as well as its direction.

For a diffuse reflection, the reflected energy is scattered in all directions thereby reducing the energy or power density.  Generally, diffusely reflecting surfaces are favored when designing a laser experiment since their use reduces the likelihood of a specular reflection and hence enhances the safety of the experiment.

Transmission

Most visible light laser beams, such as those generated by HeNe, Nd:Yag, and Krypton lasers are transmitted through clear objects, such as a room window or water.  Use of these types of lasers often requires the use of window coverings that absorbs the beam and prevents the laser hazard from existing outside of the immediate work area.  It is important to note that these coverings will need to be fire resistant for use with higher- powered lasers.  Some lasers, such as CO2, are not transmitted through glass, and therefore do not require the use of window coverings.