Optical Pumping

1
Optical Pumping
Intense light source at h? (e.g. flash
lamp) Excites to a metastable state to achieve
population inversion With fast flashing, initial
photons start chain reaction
Eugene Hecht, Optics, Addison-Wesley, Reading,
MA, 1998.
2
Electrical Discharge
Accelerated e- and ions excite atoms/molecules
into higher energy states Common in gas lasers
Ingle and Crouch, Spectrochemical Analysis
3
Three - Level System
No saturation Not very efficient Better for
pulsed mode operation
Ingle and Crouch, Spectrochemical Analysis
4
The ruby laser is a three level laser
Commercial ruby laser operates with efficiency
1
Eugene Hecht, Optics, Addison-Wesley, Reading,
MA, 1998.
5
Four - Level System
More efficient than 3-level Laser transition does
not involve ground state or most highly excited
state Easier to achieve population inversion
Ingle and Crouch, Spectrochemical Analysis
6
The He Ne laser is a four level laser
He Ne ? He Ne ?E
Ingle and Crouch, Spectrochemical Analysis
7
Resonance Cavity and Gain
Gain degree of amplification based on positive
feedback
Ingle and Crouch, Spectrochemical Analysis
8
Gain
Gain (G) es(nj-ni)b s transition
cross-section b length of active
medium Oscillation begins when gain in medium
losses of system r1r2G2 1 Threshold population
inversion
Ingle and Crouch, Spectrochemical Analysis
9
Light Amplification in Resonance Cavity
Highly collimated beam Typically mm beam width,
mrad divergence A typical photon travels about
50 times forward and backward within the cavity
Eugene Hecht, Optics, Addison-Wesley, Reading,
MA, 1998.
10
Mirror Arrangements
Eugene Hecht, Optics, Addison-Wesley, Reading,
MA, 1998.
11
Are you getting the concept?
Knowing that the purpose of the resonance cavity
is to direct the majority of the photons back
through the active medium, what cavity
characteristics will be most important?
Eugene Hecht, Optics, Addison-Wesley, Reading,
MA, 1998.
12
Achieving Resonance
Stimulated emission is coherent (all light waves
in phase) If the cavity is an integer multiple of
the wavelength, each wave will be at the same
phase when it reflects from one of the cavity
mirrors (recall that a photon make many round
trips in a laser cavity before it is
emitted). This allows constructive interference
between all photons. Want ml 2nL Other
wavelengths will not be strongly amplified, and
thus, will die out. In practice, laser
transitions have gain over a range of wavelengths
the gain bandwidth so that resonance cavity
lengths are not impossible to achieve.
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Achieving Resonance
Goal Laser cavity where L ml/2
Estimate amplification factor
Amp (1Gain)L

• This condition is not as strict as it sounds
  because
• Laser transitions have gain over a range of
  wavelengths
• Any integer multiple (longitudinal mode) of l
  will work

http//micro.magnet.fsu.edu/primer/java/lasers/gai
nbandwidth/index.html
14
Longitudinal Modes
Actual ? is the convolution of the transition
bandwidth and the ? of the longitudinal modes.
Eugene Hecht, Optics, Addison-Wesley, Reading,
MA, 1998.
15
Transverse Modes
Transverse modes determine the pattern of
intensity distribution across the width of the
beam. TEM00 has a Gaussian distribution and is
the most commonly used. The resonator geometry
of many commercial lasers is designed to obtain
single transverse mode operation.
www.wikipedia.org and www.lexellaser.com