Introduction
Laser Theory and Operation
Components of a Laser
Types of Lasers
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Introduction (top)
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.
Laser Theory And Operation (top)
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.
In contrast, the output of a laser, as shown in Figure 3, has a very small divergence and can maintain high beam intensities over long ranges. Thus, relatively low power lasers are able to project more energy at a single wavelength within a narrow beam than can be obtained from much more powerful conventional light sources.
Coherent means that the waves of light are in phase with each other. A light bulb produces many wavelengths, making it incoherent.
Components of a Laser (top)
Figure 5 illustrates the basic components of the laser including the lasing material, pump source or excitation medium, optical cavity and output coupler.
The lasing material can be a solid, liquid, gas or semiconductor, and can emit light in all directions. The pump source is typically electricity from a power supply, lamp or flashtube, but may also be another laser. It is very common in Princeton University laboratories to use one laser to pump another.
The excitation medium is used to excite the lasing material, causing it to emit light. The optical cavity contains mirrors at each end that reflect this light and cause it to bounce between the mirrors. As a result, the energy from the excitation medium is amplified in the form of light. Some of the light passes through the output coupler, usually a semi-transparent mirror at one end of the cavity. The resulting beam is then ready to use for any of hundreds of applications.
The laser output may be steady, as in continuous wave (CW) lasers, or pulsed. A Q-switch in the optical path is a method of providing laser pulses of an extremely short time duration. The Q-switch may use a rotating prism, a pockels cell or a shutter device to create the pulse. Q-switched lasers may produce a high-peak-power laser pulse of a few nanoseconds duration.
A continuous wave laser has a steady power output, measured in watts (W). For pulsed lasers, the output generally refers to energy, rather than power. The radiant energy is a function of time and is measured in joules (J). Two terms are often used to when measuring or calculating exposure to laser radiation. Radiant Exposure is the radiant energy divided by the area of the surface the beam strikes. It is expressed in J/cm2. Irradiance is the radiant power striking a surface divided by the area of the surface over which the radiant power is distributed. It is expressed in W/cm2. For repetitively pulsed lasers, the pulse repetition factor (prf) and pulse width are important in evaluating biological effects.
Types of Lasers (top)
The laser diode is a light emitting diode that uses an optical cavity to amplify the light emitted from the energy band gap that exists in semiconductors. (See Figure 6.) They can be tuned to different wavelengths by varying the applied current, temperature or magnetic field.
Gas lasers consist of a gas filled tube placed in the laser cavity as shown in Figure 7. A voltage (the external pump source) is applied to the tube to excite the atoms in the gas to a population inversion. The light emitted from this type of laser is normally continuous wave (CW). One should note that if Brewster angle windows are attached to the gas discharge tube, some laser radiation may be reflected out the side of the laser cavity. Large gas lasers known as gas dynamic lasers use a combustion chamber and supersonic nozzle for population inversion.
Dye lasers employ an active material in a liquid suspension. The dye cell contains the lasing medium. These lasers are popular because they may be tuned to several wavelengths by changing the chemical composition of the dye. Many of the commonly used dyes or liquid suspensions are toxic.
Free electron lasers such as in Figure 8 have the ability to generate wavelengths from the microwave to the X-ray region. They operate by having an electron beam in an optical cavity pass through a wiggler magnetic field. The change in direction exerted by the magnetic field on the electrons causes them to emit photons.
