26-11-2012, 12:47 PM
Acousto-optic modulator
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An acousto-optic modulator (AOM), also called a
Bragg cell, uses the acousto-optic effect to diffract
and shift the frequency of light using sound waves
(usually at radio-frequency). They are used in
lasers for Q-switching, telecommunications for
signal modulation, and in spectroscopy for
frequency control. A piezoelectric transducer is
attached to a material such as glass. An oscillating
electric signal drives the transducer to vibrate,
which creates sound waves in the glass. These can
be thought of as moving periodic planes of
expansion and compression that change the index
of refraction. Incoming light scatters (see Brillouin
scattering) off the resulting periodic index
modulation and interference occurs similar to in
Bragg diffraction. The interaction can be thought
of as four-wave mixing between phonons and
photons.
Phase
In addition, the phase of the diffracted beam will also be shifted by the phase of the sound wave. The
phase can be changed by an arbitrary amount.
Polarization
Collinear transverse acoustic waves or perpendicular longitudinal waves can change the polarization. The
acoustic waves induce a birefringent phase-shift, much like in a Pockels cell. The acousto-optic tunable
filter, especially the dazzler, which can generate variable pulse shapes, is based on this principle.
Acousto-optic modulators are much faster than typical mechanical devices such as tiltable mirrors. 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 nanoseconds). This is fast enough to create active modelocking in an ultrafast laser.
When faster control is necessary electro-optic modulators are used. However, these require very high voltages
(e.g. 10 kilovolts), whereas AOMs offer more deflection range, simple design, and low power consumption (less
than 3 watts)
The Acousto-Optic Modulator.
The principal optical component in this experiment is the acousto-optic modulator (AOM). Because
of the central importance of the acousto-optic modulator to this experiment and to numerous
technical applications such as laser printers and optical spectrum analyzers, the acousto-optic
modulator will be described in some detail. Although some details of the physical principles
underlying the operation of the acousto-optic modulator will be given here, the major emphasis will
be on the useful role the acousto-optic modulator can play in the optics laboratory.
The acousto-optic effect occurs when a light beam passes through a transparent material, such as
glass, in which travelling acoustic waves are also present, as depicted in Fig. 1.2 Acoustic waves are
generated in the glass by a piezoelectric transducer that is driven by a RF signal source. The
spatially periodic density variations in the glass corresponding to compressions and rarefactions of
the travelling acoustic wave are accompanied by corresponding changes in the index of refraction
for propagation of light in the medium. These travelling waves of index of refraction variation
diffract the incident light much as the atomic planes of a crystal diffract x-rays in Bragg scattering.2
For acoustic waves of sufficiently high power, most of the light incident on the acousto-optic
modulator can be diffracted and therefore deflected from its incident direction.