However, these require very high voltages (e.g. When faster control is necessary electro-optic modulators are used. This is fast enough to create active modelocking in an ultrafast laser. The time it takes an AOM to shift the exiting beam in is roughly limited to the transit time of the sound wave across the beam (typically 5 to 100 ns). Modelocking Īcousto-optic modulators are much faster than typical mechanical devices such as tiltable mirrors. The acousto-optic tunable filter, especially the dazzler, which can generate variable pulse shapes, is based on this principle. The acoustic waves induce a birefringent phase-shift, much like in a Pockels cell. The phase can be changed by an arbitrary amount.Ĭollinear transverse acoustic waves or perpendicular longitudinal waves can change the polarization. In addition, the phase of the diffracted beam will also be shifted by the phase of the sound wave. In this case the spectrum of the diffracted beam contains multiple frequency shifts, in any case integer multiples of the frequency of the sound wave. In some AOMs, two acoustic waves travel in opposite directions in the material, creating a standing wave. A typical frequency shift varies from 27 MHz, for a less-expensive AOM, to 1 GHz, for a state-of-the-art commercial device. This frequency shift can be also understood by the fact that energy and momentum (of the photons and phonons) are conserved in the scattering process. The interaction can be thought of as a three-wave mixing process resulting in Sum-frequency generation or Difference-frequency generation between phonons and photons.Ī typical AOM operates under Bragg Condition, where the incident light comes at Bragg angle θ B ≈ sin θ B = λ 2 Λ Incoming light scatters (see Brillouin scattering) off the resulting periodic index modulation and interference occurs similar to Bragg diffraction. These can be thought of as moving periodic planes of expansion and compression that change the index of refraction. An oscillating electric signal drives the transducer to vibrate, which creates sound waves in the material. A piezoelectric transducer is attached to a material such as glass. They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. By vibrating the material with a pure sinusoid and tilting the AOM so the light is reflected from the flat sound waves into the first diffraction order, up to 90% deflection efficiency can be achieved.Īn acousto-optic modulator (AOM), also called a Bragg cell or an acousto-optic deflector (AOD), uses the acousto-optic effect to diffract and shift the frequency of light using sound waves (usually at radio-frequency). Most of the published work in the area of acoustic diffraction relies on the vast amount of research done in the fields of optics and microwaves, where understanding the effects of diffraction is essential.
A light beam is diffracted into several orders. Diffraction has been the subject of a great deal of discussion in the acoustics literature 7-11. An acousto-optic modulator consists of a piezoelectric transducer which creates sound waves in a material like glass or quartz.
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