handbook of modern sensors

4.10 Electro-optic and Acousto-optic Modulators 147
Fig. 4.21. Electro-optic modulator consists of two polarizing filters and a crystal.
In some crystals, the refractive index can be linked to an applied electric field
[11]. The effect is characterized in the context of the propagation of a light beam
through a crystal. For an arbitrary propagation direction, light maintains constant
linear polarization through a crystal for only those polarization directions allowed by
the crystal symmetry. An external electric field applied to a crystal may change that
symmetry, thus modulating the light intensity. Lithium niobate (LiNbO 3 ) is one of the
most widely used materials for electro-optic devices. A crystal is positioned between
two polarizing filters which are oriented at 90 ◦ with respect to one another (Fig.
4.21). The input polarizer is oriented at 45 ◦ to the axis of the modulating crystal [12].
The crystal modulator has two electrodes attached to its surface. By changing the
modulator voltage, the polarization of the light incident on the output polarizer is
varied, which, in turn. leads to the intensity modulation.
A similar effect can be observed when the crystal is subjected to mechanical
effects—specifically, to acoustic waves [11,13]. However, acousto-optic devices are
used most often in fiber-optic applications as optical frequency shifters, and only to
a lesser extent as intensity modulators. In the modulator, the light beam propagating
through a crystal interacts with a traveling-wave index perturbation generated by an
acoustic wave. The perturbation results from a photoelastic effect, whereby a mechanical
strain produces a linear variation in refractive index. This resembles a travelingwave
diffraction grating, which, under certain conditions, can effectively deflect an
optical beam (Fig. 4.22). Acousto-optic devices are often fabricated from lithium
niobate and quartz, because acoustic waves can effectively propagate through these
crystal over a frequency range from tens of megahertz to several gigahertz. The acoustic
velocity in lithium niobate is about 6 × 10 3 m/s; thus a 1-GHz acoustic wave has a
wavelength of about 6 µm, which is comparable to light in the infrared spectral range.