Saturated absorption spectroscopy

In saturated absorption spectroscopy, two counter-propagating, overlapped laser beams are sent through a sample of atomic gas.

This method enables precise measurements at room temperature because it is insensitive to doppler broadening.

To overcome the problem of Doppler broadening without cooling down the sample to millikelvin temperatures, a classical pump–probe scheme is used.

Although the two beams are at the same frequency, they address different atoms due to natural thermal motion.

The stronger the pump beam, the wider and deeper the dips in the Gaussian Doppler-broadened absorption feature become.

Under perfect conditions, the width of the dip can approach the natural linewidth of the transition.

[1] A consequence of this method of counter-propagating beams on a system with more than two states is the presence of crossover lines.

This is the result of moving atoms seeing the pump and probe beams resonant with two separate transitions.

More precisely, the absorption is characterized by a Lorentzian of width Γ/2 (for reference, Γ ≈ 2π × 6 MHz for common rubidium D-line transitions[2]).

The distribution of absorption as a function of the pulsation will therefore be proportional to a Gaussian with full width at half maximum For a rubidium atom at room temperature,[3] Therefore, without any special trick in the experimental setup probing the maximum of absorption of an atomic vapour, the uncertainty of the measurement will be limited by the Doppler broadening and not by the fundamental width of the resonance.

As the pump and the probe beam must have the same exact frequency, the most convenient solution is for them to come from the same laser.