Biomedical Engineering Reference
In-Depth Information
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AODs scanners:
Additional scanner technology capable of video rates acquisition speed includes
instruments equipped with AODs. AODs utilize ultrasonic waves to generate pressure zones in
a crystal that can diffract or deflect incident laser light at an angle that varies with the acoustical
frequency (Bragg diffraction). These solid-state devices benefit from having few moving mechani-
cal parts and negligible inertia, enabling the generation of highly accurate sawtooth raster scans
having almost instantaneous flyback. Furthermore, AOD scanners can produce user-defined
deflections to generate scanned regions of interest. The disadvantages of AOD microscopes are
based on the dispersive properties of these devices, which are suited only for controlled passage of
monochromatic light. Although these problems have been solved in several experimental instru-
ment designs (Bullen et al., 1997, Sacconi et al., 2008, Reddy and Saggau, 2005), the concept of
using AOD scanners has not been popular with the microscope manufacturers. A possible reason
could be due to the fact that these systems need bright specimens to work at their best.
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Polygonal mirror scanners:
An alternative scanning solution able to reach extremely high scan-
ning speed consists in substituting the faster galvanometer scanner with a polygonal mirror scan-
ner. The polygon scanning mirror is simply a prism with polygonal basis having mirrors placed
onto the external faces. A laser beam hitting on the external face of a rotating polygonal mirror
translates into a beam deflection with a sawtooth profile. Deflection of the light beam with a
rotating polygon mirror system is a mature technology that uses optically simple, nondispersive
surfaces to create an output beam having a basic sawtooth raster similar to conventional video
scanning. However, as explained in Section 2.2.4.5, the use of polygonal mirror requires a com-
plex optical and electronic design to be implemented in an instrument.
2.2.4.2 commercial Scanning Head
A commercial scanning head is in general made by a pair of galvo-mirrors placed orthogonally to each
other, so that one is scanning along the
x
-axis, and the other along the
y
-axis. Very often, commercial
scanning heads (see Figure 2.4) are realized by placing the two galvo-mirrors very close to each other, so
that the pivot point (the conjugate point of the intersection between the optical axis and the back focal
plane of the objective) is located in between the two scanners.
This configuration can potentially create a nonuniform illumination when scanning a large field of
view, because the positioning of the two galvo-mirrors does not allow a perfect pivoting. (A perfect piv-
oting is obtained by placing both scanners in a pivot point.) However, for implementation in a commer-
cial microscope system, commercial scanning heads have the big advantage of being easily connected to
a dedicated port of the microscope stand. Moreover, commercial scanning heads are in general digitally
driven and also monitoring of the galvo-mirrors position is based on digital technology, so that it can be
easily interfaced to other electronic devices.
2.2.4.3 custom Galvo-Mirrors-Based Scanning Head
To overcome the problem of imperfect pivoting of the laser beam described above, two different solu-
tions can be used, based on lenses or spherical concave mirrors.
The first configuration (see Figure 2.5a) is based on the optical relaying between galvo-mirrors by
means of lenses. It consists in placing a telescope between the two galvo-mirrors in order to put both
of them in a pivot point. Following an optical scheme like the one shown in Figure 2.5a, the first galvo-
mirror is placed along the optical axis before the first lens, at a distance from the lens equal to its focal
length; the second galvo-mirror is placed along the optical axis, after the second lens, at a distance from
the lens equal to its focal length. This optical configuration requires an extended space, but it provides
a “perfect” optical system. The second configuration (see Figure 2.5b) is quite similar to the first, but it
uses concave spherical mirrors, instead of refractive lenses. In fact, it is known that a concave spherical
mirror works as a lens with focal length equal to one-half the mirror curvature. The two spherical mir-
rors are placed in a symmetric geometry in between the two galvo-mirrors maintaining each of them at
a focal distance from the first or the second spherical mirror.
