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Electro-optic modulators vs. acousto-optic modulators Electro-optic modulators (EOM) and acousto-optic modulators (AOM) are optical devices that can be used for controlling the intensity of a laser beam. Being driven by an electrical signal and working without moving parts makes them faster and more precise than mechanical control devices. However, their principle of operation is based on completely different physical effects, which again explains their different optical properties. EOM EOMs use the linear electro-optic or Pockels effect, which describes the variation of the refractive index of an optical medium under the influence of an external electrical field. In this case certain crystals become birefringent in the direction of the optical axis, which is isotropic when no voltage is applied. Thus, the state of linear polarization of light can be manipulated: When voltage is applied to the crystal the Pockels effect introduces a phase shift between two polarization components of the propagating light, whereas the polarization remains unchanged when no voltage is applied. This way the Pockels effect makes the crystal an electrically switchable and adjustable wave plate. The magnitude of this phase shift scales linearly with the applied voltage. Positioned between two crossed Polarizers, the Pockels cell forms an electro-optical modulator which allows for continuous intensity adjustment of a laser beam. Switching speed and modulation frequency are limited by the high voltage driver electronics and can be faster than 5ns and several hundred kilo Hertz. The aperture size is only restricted by the electro-optical crystals available and has no influence on the switching speed of the modulator. High voltage driver Polarizer Polarizer Pockels cell Pockels cell No voltage applied λ/2 - voltage applied 1 AOM Acousto-optic modulators (AOM) introduce a periodic modulation of the refractive index in a transparent medium, of which light is scattered similar to the Bragg diffraction. The periodic index modulation is generated by sound waves which form a periodic density grating when propagating through the medium. The sound waves are created by a Piezo electric transducer which in turn is driven by a radio frequency signal. An acoustic absorber on the other end of the crystal prevents the acoustic wave from travelling back to the transducer. Due to the Bragg diffraction the laser beam changes its direction slightly. Therefore, one has to distinguish between the “transmission” in the original beam direction and the “efficiency” which gives the fraction of the original beam diffracted into the first order beam. The intensity of the sound wave determines the efficiency of the AOM and is therefore used to modulate the light intensity. The switching speed of an AOM is limited by the time the sound wave needs to cross the beam diameter. So in order to achieve fast modulation, the beam diameter has to be small which on the other hand generates a conflict between the light intensity and the laser induced damage threshold of the modulator. Unlike to ordinary Bragg diffraction, the light is scattered from a moving refractive index grating, which generates a slight frequency shift of the diffracted light, equal to the frequency of the sound wave. Piezo electric transducer Radio frequency generator Crystal or Glass Diffracted first order beam Blocked beam Absorber 2 Comparison of EOM and AOM in terms of application / properties Application / properties EOM AOM Q-switching Yes, possible Yes, possible Intensity modulation Yes, possible Yes, possible Phase modulation Yes, possible Not possible Transmission 98% transmission: 98% efficiency: 90% Extinction ratio 3000:1 2000:1 Rise / fall time of applied modulation driver limited (approx. 5ns) beam diameter limited -> conflict with laser damage induced threshold spectral filtering up to 500kHz – 1MHz (Pockels Cells) up to 20 MHz (Modulators) Not possible diffraction Not possible Yes, possible spectral range 250nm - 5µm 250nm-1600nm 9.3µm, 11µm Modulation Bandwidth 3 Up to 50 – 100 MHz Yes, possible