Doppler-freeSaturated Absorption

SpectroscopyBy 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 9 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 Vz to 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

SHG(Second Harmonic

Generator)LASER

HWP(Half-Wave Plate)

1560nm 780nm

Detector

99% p

PBSNPBS

Mirror

Polarising Beam SplitterMirror

Mirror

1% s

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