Device and method for controlling focus of a laser beam

A device for projection of a laser beam onto a surface, the device being adapted for controlling focus of the laser beam during a micro sweep of the laser beam along a path on the surface to be exposed as a deflection angle of the laser beam varies by varying acoustic waves in frequency, the device comprising:

• an acousto-optic deflector arranged to receive the laser beam and to cause the laser beam to pass through the acousto-optic deflector;
• an acoustic transducer arranged to provide acoustic waves to the acousto-optic deflector;
• and
• a control unit arranged to vary the acoustic waves in frequency over time through the acousto-optic deflector to vary the deflection angle of the laser beam over time thereby achieving the micro sweep of the laser beam, the micro sweep being a sweep of the laser beam along the path based on the variation of the deflection angle of the laser beam,
• wherein the control unit is further arranged to adapt a rate of variation of frequency of the acoustic waves to vary over a time of the micro sweep in such a way that adapted differences in frequencies over the time of the micro sweep of the acoustic waves in the acousto-optic deflector are caused over a width of the laser beam in a direction parallel with the micro sweep when the laser beam is passing through the acousto-optic deflector, where the adapted differences in frequencies over the time of the micro sweep are such that they cause a desired focus of the laser beam in a the direction parallel with the micro sweep over the micro sweep.