Transcript Saturated Absorption Spectroscopy

Doppler-free
Saturated Absorption
Spectroscopy
By Priyanka Nandanwar
What I’m going to talk about
• Theory
• Doppler-free spectrum
• Role in the Laser Stabilisation
History
• Arthur Leonard Shawlow
• Nobel Prize 1981
• Beforehand:
– Doppler widths of 500 MHz in optical spectra
– Limits on resolution
Lasers
• Light Amplification by
Stimulated Emission
of Radiation
• Monochromatic
• Directional
• Coherent
Interaction with atomic vapour
Spectral broadening
Maxwell Boltzmann-distribution
Spectral broadening
• As the laser is finely tuned, atoms with a
variety of velocities about 0msˉ¹ absorb
radiation
• Absorption peak broadened
• Blurs transition energy
Doppler Shifts
• Random thermal motion of atoms
and molecules creates shift in the
absorbed radiation
• Blue or red-shifted
• Laser frequency is increased (blue
shifted) from frame of atoms
approaching it, so it needs to be less
than ν ̥ for transition to occur
Doppler broadened spectrum of
rubidium sample
Δν½
The Solution
•Doppler-broadened
spectral lines shown
beforehand
obtained from
second probe
beam.
•←Doppler
broadened spectral
lines with hyperfine
structure.
•Subtracting the two
gives…
• Subtraction of the two
spectral yields
hyperfine structure
• Bottom waveform:
– Piezo driver increases
and then decreases
frequency
– Partial mirror image
First probe and pump beams
• Pump changes population of atomic states
• Probe detects changes
• Doppler effect casues atoms only with
certain velocity Vzto be in resonance with
left-moving probe beam
Laser frequency correctly tuned
• Atoms moving 0msˉ¹ (stationary) relative to beams
absorb both probe and pump energy
• Pump depletes unexcited population
• First probe reduced absorption
• Second probe has no reduced absorption
•Frequency of
laser<Resonance
frequency
•Pump photons
absorbed if, in atoms’
frame, laser frequency
shifted to resonance
•Probe beam excites left-moving atoms with
same speed as right-moving atoms
•Subtract detected signals from each probe
beam
LASER
Mirror
1560nm
SHG
(Second Harmonic
Generator)
780nm
HWP
(Half-Wave Plate)
Polarising
Beam Splitter
1% s
Mirror
99% p
Detector
Mirror
NPBS
PBS
How we will use this…
• Rubidium vapour cell
• ~780 nm frequency standard
• At PBS, light split into opposite
polarisations that progress to probe and
pump beams
• NPBS and PBS cause two probes to pass
through the cell
• Two detectors
Conclusion
• Doppler effect:
– Broadens the atomic absorption spectrum
• Saturated absorption spectroscopy:
– Doppler broadened peak is resolved
– Exact energy of hyperfine transitions can be
found
Crossover peaks
• Appears midway between any two
transitions that have the same lower level
and two different excited levels.
• Laser tuned midway between two
transitions
• Atoms moving relative to beams resonate
with pump and probe