Transcript Signal-to-Noise Optimization

Quasi-Continuum Sources
Emit a spectral continuum over a relatively
narrow spectral interval.
FWHM ≈ 0.1 nm
“LASER”
Light Amplification by Stimulated Emission
of Radiation
Radiative Processes in a 2-level
System
Spontaneous
Emission
Energy
Absorption
Stimulated
Emission
4 Processes of LASER Action
1. Pumping
2. Spontaneous Emission
3. Stimulated Emission
4. Absorption
Pumping
Spontaneous Emission
Random process of the excited atom or
molecule losing energy by releasing a photon.
Each photon has wavelength corresponding
to a specific transition. They travel in different
directions, and with different phase
(Incoherent).
Spontaneous Emission
Stimulated Emission
When the atom or molecule in the excited
state encounters a photon of the appropriate
energy, stimulated emission may occur.
The excited species may emit a photon in the
same direction and with the same phase as
the incoming photon (Coherent).
Stimulated Emission
Absorption
If a ground state species encounters a photon
of the appropriate energy, that energy may be
absorbed, thereby promoting the species to
the excited state.
This process competes with stimulated
emission.
Absorption
Population Inversion
For stimulated emission to become more
probable than absorption, a population
inversion must occur.
More species must be in the excited state
than in the ground state.
This is an uncommon occurrence, and a
favorable energy level system must be
employed.
LASER Energy Levels
The Ruby LASER
LASER Cavity
In order to define the wavelength and
direction of the LASER beam, the active
medium (group of absorbers/emitters) is
placed between 2 mirrors.
LASER Cavity
The length of the cavity (L) defines the
wavelengths of light that may be resonant.
L = mλ/2
where m is an integer
L
LASER Cavity
The Gain Curve of the LASER medium
determines which of those possible
wavelengths may sustain the LASER.
LASER Action
LASER Media
1. Atomic and Ionic Gases
a) He-Ne
b) Ar ion
Cu vapor
2. Molecular Gases
a) CO2
b) N2
c) HF
d) Excimers (exciplexes)
LASER Media
3. Liquid Solutions (Dyes)
4. Solids
a) Rupy
b) Nd: YAG
c) Semiconductors
d) OPO crystals
LASER Modes of Operation
1.
2.
3.
4.
5.
6.
Continuous Wave (CW)
Pulsed
Cavity Dumped
Q-Switched
Mode locked
Frequency Doubled