Page 149 - Photonics Essentials an introduction with experiments

Source: Photonics Essentials

Chapter
7

Lasers

The word laser is an acronym for Light Amplification by Stimulated
Emission of Radiation. Laser action is most commonly used to gener-
ate light. However, a laser can also be used to amplify light generated
by an external source. An excellent example of this application is the
erbium-doped fiber amplifier that is used in optical fiber communica-
tions to amplify light signals at 1550 nm. There is no battery hooked
up to an erbium-doped fiber amplifier. It gets its power from an exci-
tation light beam at one wavelength, and it uses this power to amplify
light at another wavelength. Laser action is a general principal of the
behavior of light absorption and emission by matter. As a result, las-
ing has been observed in a wide range of conditions and materials
where luminescence is generated, including chemical reactions, an-
tifreeze, gases, solids, liquids, and semiconductor p-n junctions. Even
water can be made to support lasing in the far infrared. It is probably
true that any material that can be made to emit light can also be
made to lase under some conditions. It is thus not a surprise that
these conditions are more easily achieved for some materials than for
others. Semiconductor p-n junctions are among the materials in
which we can achieve laser action most easily.
Semiconductor lasers cover a very wide range of optical wave-
lengths. Lasers can be built that span a range from less than 400 nm
to more than 10,000 nm. No other materials system has this flexibili-
ty. Semiconductor lasers are relatively inexpensive. The cheapest ex-
amples sell for less than a dollar, and the most expensive for less than
$10,000. This is a lot less than you would pay for a TiAlO 3 (Ti-sap-
phire) laser that sells for $60,000 or a tunable dye laser that sells for
$150,000. As a result of these and other considerations, the semicon-

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