Biomedical Engineering Reference
In-Depth Information
3.2.2 Lenses
The lenses in an IO Bragg cell are waveguide lenses. Various types of wave-
guide lenses have been devised, four of which are illustrated in Figure 3.6
[12]. The same effect as conventional lenses is created with step increases in
thickness. A dome-shaped thickness increase is a Luneburg lens; and the
inverse, a dome-shaped depression, is basically a geodesic lens. The geodesic
lens is the most highly developed, because it is easier to grind the wave-
guide to near spherical depression than to add material to create a dome.
Holographic lenses can be produced by creating grating structures with
varying periodicity. The geodesic lens functions using the geometrically
longer paths of rays in the center portion relative to the edges. Two lenses
are used in the Bragg cell receiver—one between the light source and the cell
and the other between the cell and the photodiode array.
The beam collimator determines the interaction aperture of the acoustic
wavefront. The wider the aperture, the more frequency resolution is pos-
sible, to a point. The acoustic attenuation of various materials limits the
aperture size.
Several mechanisms are thought to be responsible for acoustic attenu-
ation. In many materials, Akheiser loss caused by thermal relaxation is
the dominant mechanism. For these materials, the attenuation (in dB per
unit length) will increase as the square of the frequency. Other acoustic
modes exhibit frequency dependencies that range from linear to square.
For these materials, other mechanisms such as scattering from lattice
imperfections are important in the attenuation process. The acoustic
attenuation measured for certain acoustic modes tends to vary widely
from sample to sample, even for pieces cut from the same crystal boule.
This indicates that lattice dislocations and foreign particle impurities are
dominant in the attenuation process. While measures of these crystal
imperfections such as etch pit density have been correlated to optical
and electrical crystal properties, these measures have not been applied
in acoustic attenuation studies. [13]
Thickness step lens
Diffraction grating lens
Dome-shaped luneburg lens
Spherical depression geodesic lens
FIGURE 3.6
Four implementations of optical waveguide lenses.
Search WWH ::




Custom Search