Sun 22 October, 2017

Laser Stabilisation

Lock-in set-up
Figure 1: The saturated absorption set-up.

Locking the laser

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.

Frequency Locking Using an Absorption Cell

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.

Helium discharge cell
Figure 2: The absorption cell used to lock the laser showing the He discharge and the Helmholtz coils used to provide detuning.
By scanning the frequency of the laser around the transition frequency a Doppler broadened absorption signal can be detected by a photodetector and viewed on an oscilloscope (figure 3). A narrower feature is generated by passing the laser twice through the absorption cell, the first pass acting as a pump beam, and the second as a probe beam. Due to the Doppler effect, only atoms moving perpendicular to the laser will interact with both beams. Excitation of these atoms to the 23P2 transition is saturated by the pump beam so that no further absorption of the probe beam occurs. This accounts for the sharp peak seen in the centre of the absorption signal which provides a clear feature to lock the laser to.

Saturated Absorption Signal
Figure 3: A saturated absorption signal.

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.

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