Lasers

Lasers
William T. Silfvast
School of Optics/CREOL
University of Central Florida
Orlando, Florida
Lasers are devices that amplify or increase the intensity of light to produce a highly directional,
high-intensity beam that typically has a very pure frequency or wavelength. They come in sizes
ranging from approximately one-tenth the diameter of a human hair to that of a very large
building. Lasers produce powers ranging from nanowatts to a billion trillion watts (1021 W) for
very short bursts. They produce wavelengths or frequencies ranging from the microwave region
and infrared to the visible, ultraviolet, vacuum ultraviolet, and into the soft-X-ray spectral
regions. They generate the shortest bursts of light that man has yet produced, or approximately
five million-billionths of a second (5 × 10–15 sec).
Lasers are a primary component of some of our most modern communication systems and are
the probes that generate the audio signals from our compact disk players. They are used for
cutting, heat treating, cleaning, and removing materials in both the industrial and medical
worlds. They are the targeting element of laser-guided bombs and are the optical source in both
supermarket checkout scanners and tools (steppers) that print our microchips.
Because of the special stimulated nature of the laser light source, and the apparatus needed to
produce laser light, laser photons are generally not as cheap to produce or to operate as are other
light sources of comparable power. We presently do not use them to light our rooms, as lamp
bulbs for our flashlights, as headlights for our automobiles, or as street lamps. Lasers also don’t
generally provide “white light” but instead produce a specific “color” or wavelength, depending
upon the laser used.
The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
Stimulated emission of radiation is a natural process first identified by Einstein. It occurs when
a beam of light passes through a specially prepared medium and initiates or stimulates the atoms
within that medium to emit light in exactly the same direction and exactly at the same
wavelength as that of the original beam. A typical laser device (Figure 5-1) consists of an
amplifying or gain medium, a pumping source to input energy into the device, and an optical
cavity or mirror arrangement that reflects the beam of light back and forth through the gain
Downloaded from SPIE Digital Library on 18 Jan 2011 to 109.224.28.235. Terms of Use: http://spiedl.org/terms
F U N D AM E N T AL S O F P H O T O N I C S
2
medium for further amplification. A useful laser beam is obtained by allowing a small portion of
the light to escape by passing through one of the mirrors that is partially transmitting.
Figure 5-1 Basic laser components including gain medium, pumping source, and mirror cavity
Prerequisites
Before you begin working with this module, you should have completed Modules 1-1, Nature
and Properties of Light; 1-2, Light Sources and Laser Safety; 1-3, Basic Geometrical Optics;
and 1-4, Basic Physical Optics. In addition you will need a working knowledge of algebra,
exponents, and logarithms.
Objectives
When you finish this module you will:
• understand how lasers operate
• understand how gain or amplification is produced
• know how various beam characteristics occur
• know about longitudinal and transverse modes
• design laser amplifiers
• design laser cavities or resonators
• understand unstable resonators
• be familiar with Q-switching
• understand mode locking
• be familiar with how a variety of laser types work and be familiar with their
wavelengths, power capabilities, and beam properties
• know about the laser’s unique properties (different from other light sources), which are
essential in a variety of applications
Downloaded from SPIE Digital Library on 18 Jan 2011 to 109.224.28.235. Terms of Use: http://spiedl.org/terms
L AS E R S
3
Scenarios
Three types of job functions involving lasers are those in laser manufacturing
relating to designing, assembling, and testing of lasers; those relating to using
lasers in various types of applications; and those associated with field servicing of
lasers.
Assembling and testing lasers—John is involved in designing, assembling, and
installing a laser amplifier, cavity mirrors, and the associated optical elements into
the laser assembly. He is also challenged by carrying out critical functions such as
mirror alignment, using a reference laser to obtain a course alignment, and then
doing a fine alignment by observing the beam quality and the output power. John
might have to determine the optimum transmission of the laser output mirror to
match the laser gain, and test it to obtain the maximum power from the laser. In this
case, Equation 5-10 of this module might be a useful start to the optimization.
Designing procedures for testing the quality and cleanliness of the optics as well as
checking the beam quality with a commercial mode analyzer would also be
important job functions.
Using lasers in various applications—Rod had a large number of opportunities
when he sought a job in the area of laser