Biomedical Engineering Reference
In-Depth Information
Acoustic absorber
Incident
beam
Acousto-optic
material
Deflected
beam
θ
φ
Undeflected
beam
Sound
Waves
Transducer
FIgurE 2.6
Diagram showing the principle of operation of an acousto-optic light-beam deflector. The diagram
defines the deflection angle ϕ used in this chapter.
The AOD makes use of the acoustic frequency
f
dependent diffraction angle, where a change in the
angle Δϕ can be expressed as a function of the change in frequency Δ
f
:
=
v
∆
φ
∆
f
(2.4)
where λ and
ν
are the acoustic wavelength and velocity of the acoustic wave, respectively.
The first implementation of a high-speed, random-access, laser-scanning fluorescence microscope
configured to record fast physiological signals from small neuronal structures with high spatiotempo-
ral resolution has been presented by Bullen et al. (1997). Recently, Sacconi et al. (2008) combined the
advantages of SHG with an AOD-based random-access (RA) laser excitation scheme to produce a new
microscope (RASH) capable of optically recording fast (~1 ms) membrane potential (Vm) events. The
RASH is based on a custom-made upright scanning microscope. The spatial distortion of the laser pulse
in the AODs is known to affect the radial and axial resolutions of the microscope.
To compensate for the larger dispersion due to two crossed AODs, an acousto-optical modulator
(AOM) placed at 45° with respect to the two axes of the AODs (see Figure 2.7) can be used. To compen-
sate for the spatial distortion at the center of the field of view (F0), the AOM frequency can be fixed at
a value given by F0 ⋅ √2 (Reddy et al., 2005). Clearly, the propagation direction of the ultrasonic wave
Y-axis
AOD
Laser
beam
X-axis
AOD
Compensating
AOM
Acoustic waves
FIgurE 2.7
Scheme showing how the AOM is inserted at 45° before the two AODs. The propagation direction of
the ultrasonic waves in each AOD is indicated by arrows.
