Device Icons:

Many types of sources and detectors exist. Icons for some of the common devices which emit or detect light are shown in Figure 1. 

Spectral Characteristics:

The term “laser” is an acronym for light amplification by stimulated emission of radiation. Laser operation requires three things: an active material, a resonant cavity, and a pumping mechanism. The great spectral purity of laser light is a key property. See Figure 1.

Laser Operation

Active Material - The active material is one in which energy from another source can be converted to light. Lasers are based on light-emitting quantum transitions of ions, electrons, or molecules. These transitions produce light over a narrow spectrum. 

Resonant Cavity - A resonant cavity is used to produce optical feedback. The light is amplified through stimulated emission in which transitions are induced by the presence of the light and the resulting emissions are similar in phase and direction. (The cavity can be spectrally and polarization sensitive in order to further narrow the spectrum or to produce a specific polarization state.) By contrast, spontaneous emission occurs without respect to the presence of optical radiation and does not produce light with any particular phase or direction. The laser output is the loss out of one end of the cavity.

Pumping Mechanism - A pumping mechanism must exist to cause more excited ions, electrons, or molecules at the upper state of the light-emitting quantum transition verses the lower state. With this population inversion, light will be preferentially generated rather than being absorbed. This is rarely a natural condition.

Beam Properties:

Laser light is characterized by a number of properties. A measurement of these properties are often given in the specifications of laser systems.

Laser Light Properties

Narrow Spectrum – the range of wavelengths is very small. Lasers can approach the monochromatic ideal of a single wavelength with a infinitely narrow spectral width and still provide usable power or energy.

High Coherence – the degree of phase correlation between different parts of the beam is high. Temporal coherence refers to phase correlation at a point as a function of time. Spatial coherence refers to phase correlation at a time as a function of spatial coordinates. For perfect coherence, the phase at any location or time is known if the phase is known at one location and time. 

High Directionality – the directional spread in propagation vectors is small. The collimation is often measured in terms of the divergence angle, i.e twice the angle that the extremes of the beam make with the center propagation direction. The wavefront is close to planar. A finite-sized beam increases in spot size slowly. Typical divergence angles are on the order of milliradians.

Other Beam Properties

Power or Energy – the total power for a continuous-wave beam or energy in a pulsed beam will vary greatly depending on the specific system. However, due to the spectral, coherence, and directional properties of the light, the beam power or energy is very concentrated.

Polarization - directional dependence of vibration for electromagnetic radiation (the polarization direction is regarded as the direction of the electric field). A laser may have a well-defined polarization direction or random polarization

Plane waves are mathematical representations of monochromatic waves with perfect coherence. Their usefulness as a model for laser light is dependent on the degree of spectral purity and coherence.

Applications:

Lasers are tools found in commercial, industrial and scientific environments. Their applications are varied. The special properties of laser light have led to the many developments in metrology, communication, power delivery, etc.

Metrology, Pointing, and Sensing – Laser systems exploit the various properties to measure large distances, to provide a line-of-sight reference, and to sense small physical variations. Examples include remote sensing of the orbital position of satellites, alignment of surveying instruments, and reading the tracks in a compact disk.

Communication – Laser systems are used to transmit information in noisy environments, over long distances, or for high bandwidth situations. The immunity of light to electromagnetic noise and the capability to guide light in glass fibers are among the communication advantages.

Power – Laser systems can delivery high power to small areas. Examples include materials processing such as welding, cutting, and heating of metals and fine tissue removal in surgical operations.

Optical Eye Safety:

The safe operation of lasers require a knowledge of their power or energy, wavelength, etc.  the electromagnetic radiation from some lasers are incapable of causing eye and skin injury; however, others are capable of causing blindness and severe burns.  The greatest danger is to the eye due to its inherent sensitivity to light.  Injuries to the eye can be extremely painful and can result in permanent loss of vision.  The hazards are spectrally dependent and require appropriate control measures.  Safety signs are required to identify the degree of hazard and their instructions should be followed explicitly.  General safety practice for lasers includes the avoidance of direct viewing the laser beam or its specular reflection, especially with collecting optics.  Also, the beam path should be known and limited to safe working areas.at all times.  Note that laser light exiting the end of an optical fiber may be as hazardous as the original beam.

Optical Hazards

The eye responds differently to radiation in different parts of the optical spectrum. The eye consists several components including the cornea, the lens, the vitreous humor, and the retina as shown in Figure 2. The cornea is the transparent outer layer that has protective functions and provides part of the focusing power of the eye.  The lens is located behind the iris and provides variable focusing power, i.e. accommodation.  The vitreous humor is the fluid within the eye. The retina is located on the back interior surface of the eye and contains the light sensitive elements which are known as rods and cones.

Optical radiation or light is subdivided into ultraviolet, visible, and infrared portions of the electromagnetic spectrum.  It has smaller wavelengths, or larger frequencies, than radio waves and longer wavelengths, or smaller energies, than X-rays.  The wavelength l is associated with color in the visible portion of the spectrum and is conventionally the preferred designation rather than frequency f or energy.  Note that wavelength is typically defined as the wavelength in vacuum and that wavelength l is related to frequency f by l f =c where c is the speed of light.

Visible radiation with wavelengths between 0.4 µm and 0.7 µm and near-infrared (invisible) radiation with wavelengths between 0.7 µm and 1.4 µm pass through the cornea and lens and are focused on the retina.  This light is concentrated as much as 10,000 times by the eye!  Retinal burns and other damage caused by excessive exposure cannot regenerate and may result permanent vision loss or blindness. 

Mid- to far-infrared radiation with wavelengths greater than 1.4 µm and ultraviolet radiation with wavelengths less than 0.4 µm are absorbed in the cornea and the lens.  Excessive exposure can lead to cataract formation in the lens and to burns and inflammation in the cornea.  Permanent vision impairment is possible. 

General Safety Measures

Lasers are powerful sources of electromagnetic radiation.  They are particularly dangerous for the eye since their high collimation and coherence allow high concentrations of energy on the retina.  Safe operation depends on wavelength, output power or energy, and continuous or pulsed operation.  According to the American National Standards Institute’s (ANSI) Z136.1 Safe Use of Lasers (1993) standard, lasers are grouped according to hazard classifications.  The higher the classification number, the more safety measures are required.  The risk associated with each class are given below.

“Class 1 denotes lasers or laser systems that do not, under normal operating conditions, pose a hazard.  Direct viewing of the beam or its specular (mirror-like) reflections is included which is known as intrabeam viewing.

Class 2a denotes low-power visible lasers or laser systems that are not intended for prolonged viewing, and under normal operating conditions will not produce a hazard if the beam is viewed directly for periods not exceeding 1000 seconds.

Class 2b denotes low-power visible or laser systems which, because of the normal human aversion response (i.e. blinking, eye movement, etc.), do not normally present a hazard, but may present some potential for hazard if viewed directly for extended periods of time (like many conventional light sources).  The aversion response time is conservatively estimated as 0.25 seconds.

Class 3a denotes two groupings of lasers and laser systems.  The first grouping includes lasers that normally would not injure the eye if viewed for only momentary periods (within the aversion response period) with the unaided eye, but may present a greater hazard if viewed using collecting optics (lens systems). A CAUTION label is required for this grouping.  Another group of Class 3a lasers are capable of exceeding permissible exposure levels for the eye in 0.25 s and still pose a low risk of injury.  DANGER labels are required for these lasers.

Class 3b denotes lasers or laser systems that can produce a hazard if viewed directly. This includes intrabeam viewing of specular reflections.  Normally, Class 3b lasers will not produce a