Laser cooling can be applied to any atom with a two-level transition accessible by available lasers. At a wavelength of 1083nm (near-IR) the 23S1-23P2 transition of helium can be excited using relatively inexpensive diode lasers. The linewidth of the 23S1-23P2 transition used for laser cooling is 1.6 MHz compared to a linewidth of ~1 MHz for the laser used to drive it. Due to thermal drifts of the diode which can be anything up to 100MHz/K it is important to lock the laser to the transition frequency. This is done using saturated absorption spectroscopy.
The set-up depicted in figure 1 is used to generate the reference signal needed to stabilise the laser. A discharge is struck in an absorption cell filled with helium to a pressure of ~1 mbar. Electron collisions excite the helium atoms into the 23S1 metastable state and these are then pumped into the 23P2 state by passing a laser through anti-reflection coated windows in the cell.
The saturated absorption signal is input into a lock-in regulator. This generates an error signal that is then locked onto using PID circuitry. A phase-locked feedback loop is created which corrects any deviation from the transition frequency.
To compensate for the Doppler shift of the He* atoms in the atomic beam and maximise the laser light force it is desirable to detune the laser frequency below the transition frequency (by ~40MHz). Detuning is applied by Zeeman shifting the 23S1-23P2 transition using a magnetic field generated by passing a DC current through coils set-up in the Helmholtz configuration (figure 2). AC coils can also be used to scan the transition frequency relative to the laser frequency and provide the reference absorption signal for stabilisation.