Doppler Free LASER Doppler Free LASER SpectroscopySpectroscopy

It’s a headache for youIt’s a headache for youAnd a migraine for me!And a migraine for me!

LASER Spectroscopy SetupLASER Spectroscopy Setup

• Tunable diode LASERTunable diode LASER

LASER Spectroscopy SetupLASER Spectroscopy Setup

• Tunable diode LASERTunable diode LASER– Diffraction grating mirrorDiffraction grating mirror– Piezo-electric actuatorPiezo-electric actuator

LASER Spectroscopy SetupLASER Spectroscopy Setup

• Tunable diode LASERTunable diode LASER– Diffraction grating mirrorDiffraction grating mirror– Piezo-electric actuatorPiezo-electric actuator

• Miscellaneous OpticsMiscellaneous Optics– Beam-splitter – splits LASER beam into two probe Beam-splitter – splits LASER beam into two probe

beams (weak) and a pump beam (strong)beams (weak) and a pump beam (strong)

LASER SpectroscopyLASER Spectroscopy

• Tunable diode LASERTunable diode LASER– Diffraction grating mirrorDiffraction grating mirror– Piezo-electric actuatorPiezo-electric actuator

• Miscellaneous OpticsMiscellaneous Optics– Beam-splitter – splits LASER beam into two probe Beam-splitter – splits LASER beam into two probe

beams (weak) and a pump beam (strong)beams (weak) and a pump beam (strong)– Mirrors – reflect stronger pump beam to counter-Mirrors – reflect stronger pump beam to counter-

propagate against probe beampropagate against probe beam

• Rubidium cellRubidium cell– Probe, reference, and pump beams pass through Probe, reference, and pump beams pass through

Rubidium atomsRubidium atoms

• Rubidium cellRubidium cell– Probe, reference, and pump beams pass through Probe, reference, and pump beams pass through

Rubidium atomsRubidium atoms

• Photodiode differential amplifierPhotodiode differential amplifier– Subtracts the two signals from the reference and Subtracts the two signals from the reference and

probe beamsprobe beams

Probe beam

Reference beam

Mathmagical

Manipulation!

Output to OscilloscopeOscilloscope

• Rubidium cellRubidium cell– Probe, reference, and pump beams pass through Probe, reference, and pump beams pass through

Rubidium atomsRubidium atoms

• Photodiode differential amplifierPhotodiode differential amplifier– Subtracts the two signals from the reference and Subtracts the two signals from the reference and

probe beamsprobe beams

• OscilloscopeOscilloscope– Creates a real-time plot of “absorption” vs. Creates a real-time plot of “absorption” vs.

frequencyfrequency

The Statistical Doppler EffectThe Statistical Doppler Effect

• Gaussian distribution of velocitiesGaussian distribution of velocities– Fast moving atomsFast moving atoms

The Statistical Doppler EffectThe Statistical Doppler Effect

• Gaussian distribution of velocitiesGaussian distribution of velocities– Fast moving atomsFast moving atoms– Slow moving (stopped) atomsSlow moving (stopped) atoms

The Statistical Doppler EffectThe Statistical Doppler Effect

• Gaussian distribution of velocitiesGaussian distribution of velocities– Fast moving atomsFast moving atoms– Slow moving (stopped) atomsSlow moving (stopped) atoms

• blurring of the spectral linesblurring of the spectral lines

How Saturation Spectroscopy Undoes How Saturation Spectroscopy Undoes the Doppler Blurring Effectsthe Doppler Blurring Effects

• Reference beam:Reference beam:– Passes through only RubidiumPasses through only Rubidium– Interacts with all the atoms in its pathInteracts with all the atoms in its path

How Saturation Spectroscopy Undoes How Saturation Spectroscopy Undoes the Doppler Blurring Effectsthe Doppler Blurring Effects

• Reference beam:Reference beam:– Passes through only RubidiumPasses through only Rubidium– Interacts with all the atoms Interacts with all the atoms

• Probe beam:Probe beam:– Passes through RubidiumPasses through Rubidium

How Saturation Spectroscopy Undoes How Saturation Spectroscopy Undoes the Doppler Blurring Effectsthe Doppler Blurring Effects

• Reference beam:Reference beam:– Passes through only RubidiumPasses through only Rubidium– Interacts with all the atoms Interacts with all the atoms

• Probe beam:Probe beam:– Passes through RubidiumPasses through Rubidium and “pump beam”and “pump beam”

• Pump beam:Pump beam:

How Saturation Spectroscopy Undoes How Saturation Spectroscopy Undoes the Doppler Blurring Effectsthe Doppler Blurring Effects

• Reference beam:Reference beam:– Passes through only RubidiumPasses through only Rubidium– Interacts with all the atoms Interacts with all the atoms

• Probe beam:Probe beam:– Passes through RubidiumPasses through Rubidium– Ignores “stationary” atoms at transition frequency becauseIgnores “stationary” atoms at transition frequency because they are already excited by the pump beamthey are already excited by the pump beam

and “pump beam”and “pump beam”• Pump beam:Pump beam:

– Counter-propagates through probe beam and excites theCounter-propagates through probe beam and excites the atoms in its pathatoms in its path

ApplicationApplication

• Use the LASER, which is now accurately locked Use the LASER, which is now accurately locked on a precise frequency, to trap atomson a precise frequency, to trap atoms

• Instead of using multiple LASERs, feed the Instead of using multiple LASERs, feed the locked LASER beam into a tapered amplifierlocked LASER beam into a tapered amplifier

The Tapered LASER AmplifierThe Tapered LASER Amplifier

• Requirements:Requirements:– Delicate temperature controlDelicate temperature control– Input beamInput beam• Drives the amplifier at the given frequencyDrives the amplifier at the given frequency

Super stable seed beamUnthinkably powerful

output beam

Amplification Process

Excited State

Gain Medium (semiconductor)

Ground State

Excited State

Gain Medium (semiconductor)

Ground State