Projects

Acousto-optic modulators (AOMs) are used, for example, as amplitude and frequency modulators for laser beams, or as Q-switches and cavity dumpers in laser oscillators. In these modulators, an ultrasonic transducer is used to couple a sound wave with a frequency of some 10 to 100 MHz into an optical crystal. The density variations accompanying the sound wave result in a periodic spatial modulation of the refractive index. This creates an optical grating on which the incident light is diffracted into one or more diffraction orders.

When used as an amplitude modulator, the first diffraction order is generally used, and the intensity in this diffraction order can be varied by changing the ultrasonic power.

Acousto-optic amplitude modulators usually require an electrical excitation power of a few watts, which leads to heating of the modulator. In many modulators, this heating leads to minimal deformations in the optical crystal and its mounting and thus to a drifting of the exit angle of the laser beam. This has negative effects, e.g. on the accuracy of direct-writing laser lithography systems, where drifting of the beam angle leads directly to errors in the microstructures to be fabricated.

As part of this project, driver electronics and a suitable control system were developed that effectively reduce thermal drift in acousto-optical modulators. The concept is based on coupling two carrier frequencies into the AOM. One carrier serves to modulate the laser beam. The second carrier is controlled in such a way that the total power coupled into the crystal is constant.

The driver was realized on the basis of a two-channel direct digital synthesizer (DDS). This allows the generation of two programmable carrier frequencies with known relative phase angles. This property enables the systematic manipulation of the intermodulation effects in the modulator, which is essential to ensure high modulation contrast and high stability of the diffraction efficiency.

With the help of this driver, the drift in the modulator used could be reduced from 183 µrad to 39 µrad.