Laser Fundamentals

The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation.  Lasers are used as research aides in many departments at Princeton University.

In this document, the word laser will be limited to electromagnetic radiation-emitting devices using light amplification by stimulated emission of radiation at wavelengths from 180 nanometers to 1 millimeter.  The electromagnetic spectrum includes energy ranging from gamma rays to electricity.  Figure 1 illustrates the total electromagnetic spectrum and wavelengths of the various regions.

The primary wavelengths for lasers used at Princeton University include the ultraviolet, visible and infrared regions of the spectrum.  Ultraviolet radiation for lasers consists of wavelengths between 180 and 400 nanometers (nm).  The visible region consists of radiation with wavelengths between 400 and 700 nm. This is the portion we call visible light.  The infrared region of the spectrum consists of radiation with wavelengths between 700 nm and 1 mm.

The color or wavelength of light being emitted depends on the type of lasing material being used.  For example, if a Neodymium:Yttrium Aluminum Garnet (Nd:YAG) crystal is used as the lasing material, light with a wavelength of 1064 nm will be emitted.  Table 1 illustrates various types of material currently used for lasing and the wavelengths that are emitted by that type of laser.  Note that certain materials and gases are capable of emitting more than one wavelength. The wavelength of the light emitted in this case is dependent on the optical configuration of the laser.

A laser generates a beam of very intense light.  The major difference between laser light and light generated by white light sources (such as a light bulb) is that laser light is monochromatic, directional and coherent.  Monochromatic means that all of the light produced by the laser is of a single wavelength.  White light is a combination of all visible wavelengths (400 - 700 nm).  Directional means that the beam of light has very low divergence.  Light from a conventional sources, such as a light bulb diverges, spreading in all directions, as illustrated in Figure 2.  The intensity may be large at the source, but it decreases rapidly as an observer moves away from the source.