Lab 4 Laser - Institutt for elektronikk og telekommunikasjon - NTNU

1.2 Laser fundamentals 7
1.2 Laser fundamentals
1.2.1 Light amplification
Atoms and molecules interact with light in three different processes:
Absorption, spontaneous emission and stimulated emission.
- Absorption: An atom or molecule can capture, ”absorb”, a photon of
light and be excited, converted to a higher energy level. The photon
must have a specific frequency n to provide the proper amount of energy
E = hν to excite the atom or molecule. This process called ”stimulated
absorption” is the atom’s response to electromagnetic stimulation by the
incoming photon. Stimulated absorption is responsible for the colors of
dyes, which absorb light of specific frequencies, while a black or gray object
absorbs all frequencies equally..
- Spontaneous emission:An excited atom or molecule with excess stored
energy E can release that energy spontaneously, by emission of a photon
with frequency ν = E/h Spontaneous emission is responsible for the light
emission by fires, fireflies, fluorescent paint, sunlight, LEDs, hot pokers,
and most types of lamps.
- Stimulated emission: An excited atom or molecule can be stimulated
to release excess energy E in response to an incident photon of the exactly
the right frequency (ν = E/h) and simultaneously release, or emit, two
photons with the same frequency, using the energy of the incident photon
plus the stored energy of the excited atom. Stimulated emission is the
process that provides optical amplification in most lasers, since one photon
in produces two photons out, an amplification factor of exactly two per
event.
Real atomic and molecular systems absorb or emit light at many discrete
frequencies, each with a range, or bandwidth D ν centered around the main
frequency ν = E/h. Optical amplification occurs when the rate of stimulated
emission exceeds the rate of (stimulated) absorption, producing gain G where
G =
light out
light in . (1.1)
Amplification (G > 1) occurs in an atomic or molecular system with a population
inversion, which means that the number of excited atoms (or molecules)
exceeds the number of unexcited atoms or molecules. This means that there
will be more stimulated emission than stimulated absorption.