Doppler Free LASER Spectroscopy

It’s a headache for you

And a migraine for me!

LASER Spectroscopy Setup

• Tunable diode LASER

LASER Spectroscopy Setup

• Tunable diode LASER

– Diffraction grating mirror

– Piezo-electric actuator

LASER Spectroscopy Setup

• Tunable diode LASER

– Diffraction grating mirror

– Piezo-electric actuator

• Miscellaneous Optics

– Beam-splitter – splits LASER beam into two probe beams (weak) and a pump beam (strong)

LASER Spectroscopy

• Tunable diode LASER

– Diffraction grating mirror

– Piezo-electric actuator

• Miscellaneous Optics

– Beam-splitter – splits LASER beam into two probe beams (weak) and a pump beam (strong)

– Mirrors – reflect stronger pump beam to counter-propagate against probe beam

• Rubidium cell

– Probe, reference, and pump beams pass through Rubidium atoms

• Rubidium cell

– Probe, reference, and pump beams pass through Rubidium atoms

• Photodiode differential amplifier

– Subtracts the two signals from the reference and probe beams

Probe beam

Reference beam

Output to Oscilloscope

• Rubidium cell

– Probe, reference, and pump beams pass through Rubidium atoms

• Photodiode differential amplifier

– Subtracts the two signals from the reference and probe beams

• Oscilloscope

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

The Statistical Doppler Effect

• Gaussian distribution of velocities

– Fast moving atoms

The Statistical Doppler Effect

• Gaussian distribution of velocities

– Fast moving atoms

– Slow moving (stopped) atoms

The Statistical Doppler Effect

• Gaussian distribution of velocities

– Fast moving atoms

– Slow moving (stopped) atoms

• blurring of the spectral lines

How Saturation Spectroscopy Undoes the Doppler Blurring Effects

• Reference beam:

– Passes through only Rubidium

– Interacts with all the atoms in its path

How Saturation Spectroscopy Undoes the Doppler Blurring Effects

• Reference beam:

– Passes through only Rubidium

– Interacts with all the atoms

• Probe beam:

– Passes through Rubidium

How Saturation Spectroscopy Undoes the Doppler Blurring Effects

• Reference beam:

– Passes through only Rubidium

– Interacts with all the atoms

• Probe beam:

– Passes through Rubidium and “pump beam”

• Pump beam:

How Saturation Spectroscopy Undoes the Doppler Blurring Effects

• Reference beam:

– Passes through only Rubidium

– Interacts with all the atoms

• Probe beam:

– Passes through Rubidium

– Ignores “stationary” atoms at transition frequency becausethey are already excited by the pump beam

and “pump beam”

• Pump beam:– Counter-propagates through probe beam and excites the

atoms in its path

Application

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

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

The Tapered LASER Amplifier

• Requirements:

– Delicate temperature control

– Input beam

• Drives the amplifier at the given frequency

Super stable seed beam

Unthinkably powerful output beam

Amplification Process

Excited State

Gain Medium (semiconductor)

Ground State

Part

ially

ref

lect

ive

mir

ror

Mir

ror

Excited State

Gain Medium (semiconductor)

Ground State