**4.10 Anti Helmholtz coils: magneto optical trap**

The force acting on the atoms in the magneto optical trap is position space dependant being larger for atoms that are more distant from the center of the trap. The MOT coils are two copper solenoids with same dimensions and number of windings. The coils are disposed in anti Helmholtz configuration one over the other. Fig. 21 shows a diagram of the coils. The current in one coil flows in opposite direction with respect to the other coil. It is recommended (Wieman, 1995) to have a variable magnetic field gradient with a maximum of 0.2 T/m. We used normally between 0.10 and 0.15 T/m. We used a 1.15 mm diameter (AWG 17), enameled copper wire. Each coil has 196 windings ordered in 14 sheets with 14 windings per sheet. To drive the coils we used two 5A variable current supplies.

<sup>30</sup> CVI - Melles Griot, Model QWPM-780-10-2

<sup>31</sup> Thorlabs, Model RSP1

Cold Atoms Experiments: Influence of Laser Intensity Imbalance on Cloud Formation 177

3

85Rb (F = 2 F')

F' = 1

85Rb (F = 3 F')

F' = 2

saturated absorption (a.u.)

frequencies to be locked.

interferometer signal (a.u.)

0 50 100 150 200 250

frequency (MHz)

Fig. 23. Doppler free spectra of a) repumping and b) cooling lasers. The arrows indicate the

Fig.24 shows a typical measured modulated laser spectra and Fig.25 shows the error function obtained experimentally. In both cases, the interferometer cavity length was held fixed and the laser was scanned continuously. The alignment procedure of the light reflected from the interferometer into the fast photodiode can be best done using a surveillance camera an trying to group the multiple reflections on a single point at the photodiode.

> -40 -20 0 20 40 relative frequency (MHz)

Fig. 24. Laser modulated profile recorded with interferometer.

3

**7. Signals needed to stabilize the repumping and cooling laser** 

2

b

a

4

Fig. 21. Construction of anti Helmholtz coils.
