Ricoh coretech appendix5 laser

http://www.ee.washington.edu/class/ConsElec/Chapter6.html

Chapter 6 - A Brief Introduction to Laser
Diodes

6.1 A very brief introduction to lasers

6.1.1 What is a laser?

The word LASER is an acronym for Light Amplification by Stimulated Emission of
Radiation. It is one of the few acronyms which is now accepted as a word. Thus, the verb
forms "to lase" and "lasing" are accepted as real words (check a dictionary!).

But, what does "Light Amplification by Stimulated Emission of Radiation" mean?
Consider how photons are created in an atom. If an electron spontaneously decays from

one state to another, it emits a photon of the energy . This process is called
"spontaneous emission".

However, it is also possible to force a transition from one state to another by means of a
photon. In other words, a photon of the energy h can force an electron to transfer between
states 2 and 1 yielding another photon of the energy . This "stimulated emission"
process results in two photons of the energy . Furthermore, these two photons will be in

phase.

Thus, ideal laser light is formed of groups of photons where all the photons are at exactly
the same frequency (wavelength) and all the photons are in phase.

6.1.2 Monochromaticity and Coherency

The property of having the same frequency is referred to as monochromaticity and the
property of having the same phase is referred to as coherency. Thus, lasers are often
termed monochromatic and coherent sources of light. (Notice that this is a redundant
definition for an ideal laser, because a perfectly coherent source of light must be
monochromatic. However, notice that a perfectly monochromatic source of light, such as
spontaneous emission, need not be coherent.)

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Now, real lasers are neither perfectly monochromatic nor perfectly coherent. However,
when characterizing a real laser system, it is generally assumed that the laser beam was
initially in phase and the incoherence of the laser arises only from the lack of
monochromaticity of the source. (This is a reasonable assumption for conventional lasers
with feedback -- but may not be accurate for unusual laser systems.) Thus, coherency and
monochromaticity are generally assumed to measure the same parameter.

6.1.3 Typical laser construction

Most lasers are constructed of three important elements, a gain media, a pumping source
and a resonant cavity.

The gain media is the location of the energy states which participate in stimulated
emission, the pumping source provides the energy to set the states up so stimulated
emission can occur, and the resonant cavity provides the path for the photons.

In many lasers, the resonant cavity is not even necessary. The functions of the resonant
cavity are simply to 1) physically shorten the laser and 2) tailor the profile of the
electromagnetic mode. In essence, the laser is simply "folded up" between the two cavity
mirrors.

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Now, it is certainly possible to make a laser long enough so that a reasonable intensity of
light beam emerges from the cavity (most x-ray lasers are made this way). However, such
lasers have poor output beam quality.

6.1.4 Basic Laser Concepts

When the laser beam exits the laser, it is consider collimated if the beam size remains
constant as the beam propagates. In other words, if the laser is collimated, the beam size
at location 2 will be the same as the beam size at location 1.

The output "spot" of the laser beam is termed the transverse electro-magnetic mode
(TEM). In most commercial lasers the transverse mode is TEMoo, which is a round mode
with a Gaussian profile in cross-section.

A laser can only lase at those wavelengths for which an integral multiple of wavelengths fit
into the cavity. The set of possible integral multiples of the cavity length is termed the set of
longitudinal modes of the cavity.

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