Biomedical Engineering Reference
In-Depth Information
Type of Cell
Characteristics
Diffraction
cell
his device merely difracts an input beam. he characteristics of the output
beams are not relevant except in the cases of imaging devices. Examples include
modelockers and simple modulators.
Beam
modulator
his device turns an input beam “ON” and “OFF” with possible intermediate
states, but it maintains beam quality under all conditions down to the “OFF”
state. The primary examples would be an RF communications device.
Beam
defelector
This device defelects an input beam in a controllable fashion. Beam characteris-
tics and quality are maintained under all conditions. Examples include defelec-
tors for laserwriting applications and cavity dumpers.
Phase
modulator
This device shifts the phase of an input beam by an amount, possible variable in
the “ON” state. One example is an acousto-optic delay line.
Focuser/
defocuser
This device focuses or defocuses a beam while maintaining other beam charac-
teristics. In effect, this device uses cylindrical acoustic waves as a lens.
Frequency
shifter
This device shifts the frequency of an input light beam up or down while main-
taining other beam characteristics. Examples include an optical spectrum
analyzer and devices for producing beat frequencies for optical information
processing.
Image
projector
Such a device will “break up” or transmit without distortion in a controllable
fashion. As such, it can deflect or project an optical image. An example would
be the projection of largescreen television images.
Information
processor
Here, a variable phase grating generated by acoustic wave in a transparent
medium replicates an input very-high-frequency signal. This is used to spa-
tially modulate a light beam. Thus, an input electrical signal can be processed
by spatial correlation techniques applied to a light beam transmitted through
the medium.
Convert
light into
sound
This can happen in two related ways and is the inverse of the Debye-Sears effect.
In one case, high-energy light beams create sound waves in a transparent mate-
rial at their difference (beat) frequency. In other cases, pulsed high-energy light
beams generate sound in a material at the pulse frequency-thermally produced
sound. he sound originates from thermal expansions or contractions of the
material resulting from deposition or of beam energy in the material.
FIGURE 3.4 (continued)
(b) Types of AO devices.
value of 1.2-2.0. If the bandwidth is read on the vertical axis, and the time delay
on the horizontal axis, the maximum time bandwidth product of the Bragg cell
can be determined. The low frequency time bandwidth product is limited by
the size of the crystal that governs the acoustic wave's transit time.
The high frequency time bandwidth product is limited by the acoustic loss
of the crystal. There are other parameters that govern the choice of materials,
and they will be introduced later.
Some of the uses of the Bragg cell are shown in Figure 3.4b [8]. We will
concentrate here on the Bragg cell receiver, especially the integrated optic
Bragg cell receiver, where the beam collimator, the Bragg cell, and the Fourier
Transform lens are formed on the same substrate. Figure 3.5 shows the basic
IO Bragg Cell receiver [9].
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