23-05-2012, 01:22 PM
An Illustrated Dictionary of Optoelectronics and Photonics
[attachment=22859]
Acousto-optic modulator makes use of the photoelastic effect to modulate a light beam. Suppose
that we generate traveling acoustic or ultrasonic waves on the surface of a piezoelectric crystal
(such as LiNbO3) by attaching an interdigital electrodes onto its surface and applying a modulating
voltage at radio frequencies (RF). The piezoelectric effect is the phenomenon of generation of strain
in a crystal by the application of an external electric field. The modulating voltage V(t) at electrodes
will therefore generate a surface acoustic wave (SAW) via the piezoelectric effect. These acoustic
waves propagate by rarefactions and compressions of the crystal surface region which lead to a
periodic variation in the density and hence a periodic variation in the refractive index in
synchronization with the acoustic wave amplitude. Put differently, the periodic variation in the strain
S leads to a periodic variation in n owing to the photoelastic effect. We can simplistically view the
crystal surface region as alternations in the refractive index.
Activation energy
is the potential energy barrier that prevents a system from changing from state to
another. For example, if two atoms A and B get together to form a product AB, the activation energy
is the potential energy barrier against the formation of this product. It is the minimum energy which
the reactant atom or molecule must have to be able to reach the activated state and hence form the
product. The probability that a system has an energy equal to the activation energy is proportional to
the Boltzmann factor: exp(−EA/kT), where EA is the activation energy, k is the Boltzmann
constant and T is the temperature (Kelvins).
polarizations
incident light becomes doubly refracted because these two waves experience
different refractive indicesno and ne. There is no double refraction for light propagation along the
optic axis. If we were to cut a plate from a calcite crystal so that the optic axis (along z) would be
parallel to two opposite faces of the plate, then a ray entering at normal incidence to one of these
faces would not diverge into two separate waves. The o- and e-waves would travel in the same
direction but with different speeds. The waves emerge in the same direction as well which means
that we would see no double refraction. This optical arrangement is used in the construction of
various optical retarders and polarizers as discussed below. If we were to cut a plate so that the optic
axis was perpendicular to the plate face, then both the o and e-way would be traveling at the same
speed and along the same direction which means we would not again see any double refraction (see
also anisotropy)
[attachment=22859]
Acousto-optic modulator makes use of the photoelastic effect to modulate a light beam. Suppose
that we generate traveling acoustic or ultrasonic waves on the surface of a piezoelectric crystal
(such as LiNbO3) by attaching an interdigital electrodes onto its surface and applying a modulating
voltage at radio frequencies (RF). The piezoelectric effect is the phenomenon of generation of strain
in a crystal by the application of an external electric field. The modulating voltage V(t) at electrodes
will therefore generate a surface acoustic wave (SAW) via the piezoelectric effect. These acoustic
waves propagate by rarefactions and compressions of the crystal surface region which lead to a
periodic variation in the density and hence a periodic variation in the refractive index in
synchronization with the acoustic wave amplitude. Put differently, the periodic variation in the strain
S leads to a periodic variation in n owing to the photoelastic effect. We can simplistically view the
crystal surface region as alternations in the refractive index.
Activation energy
is the potential energy barrier that prevents a system from changing from state to
another. For example, if two atoms A and B get together to form a product AB, the activation energy
is the potential energy barrier against the formation of this product. It is the minimum energy which
the reactant atom or molecule must have to be able to reach the activated state and hence form the
product. The probability that a system has an energy equal to the activation energy is proportional to
the Boltzmann factor: exp(−EA/kT), where EA is the activation energy, k is the Boltzmann
constant and T is the temperature (Kelvins).
polarizations
incident light becomes doubly refracted because these two waves experience
different refractive indicesno and ne. There is no double refraction for light propagation along the
optic axis. If we were to cut a plate from a calcite crystal so that the optic axis (along z) would be
parallel to two opposite faces of the plate, then a ray entering at normal incidence to one of these
faces would not diverge into two separate waves. The o- and e-waves would travel in the same
direction but with different speeds. The waves emerge in the same direction as well which means
that we would see no double refraction. This optical arrangement is used in the construction of
various optical retarders and polarizers as discussed below. If we were to cut a plate so that the optic
axis was perpendicular to the plate face, then both the o and e-way would be traveling at the same
speed and along the same direction which means we would not again see any double refraction (see
also anisotropy)