BSA Flow Software Installation and User's Guide - CSI

In one of the beams, an acousto-optical component known as a Bragg cell is
inserted. This introduces a fixed frequency shift in the particular beam,
which as it will be explained later allow us to determine the sign of the
measured velocity. The front lens deflects the two beams so they intersect,
and in the intersecting volume, seeding particles will scatter the incoming
laser light. Part of this light is scattered backwards toward the front lens
(back scatter), and registered in the receiver (normally a photomultiplier).
Seeding particles passing the laser beams outside of the measuring volume
will of course also reflect light, but the receiving optics is focused on the
measuring volume, so this will be out of focus, and thus only increase the
background noise slightly.
Fibre optics In modern LDA equipment the light from the beamsplitter and the Bragg cell
is sent through optical fibres as is the light scattered back from seeding
particles. This reduces size and weight of the probe itself, making the
equipment flexible and easier to use in practical measurements. Laser,
beamsplitter, Bragg cell and photodetector (receiver) can be installed
stationary and out of the way, while the LDA-probe can traverse between
different measuring positions.
Beam waist The beam waist diameter df used in formulas (7-9) and (7-10) is calculated
from:
d
f
f
=
Ed
4 λ
π
I
(7-11)
-where f is the focal length of the front lens as shown in Figure 7-7, λ is the
laser wavelength, dI is the beam waist diameter of the laser beam before
passing the front lens, and E is the beam expansion factor as explained
below.
Please note that df and dI are inversely proportional, meaning that a large dI is
desirable, if you want a small df.
It is normally desired to make the measuring volume as small as possible,
which according to the formulas in (7-9) mean that df should be small. The
laser wavelength λ is a fixed parameter, and focal length f is normally
limited by the geometry of the model being investigated. Some lasers allow
for adjustment of the beam waist position, but the beam waist diameter dI is
normally fixed.
E=1 in (7-11) above correspond to no beam expander, but if the measuring
volume is too large, increasing E is the only remaining way to reduce the
size of the measuring volume. This corresponds to installing a beam
expander.
Beam expander A beam expander is a combination of lenses in front of or replacing the front
lens of a conventional LDA system. It converts the beams exiting the optical
system to beams of greater width. At the same time the spacing between the
two laser beams is increased, since the beam expander also increase the
aperture. Provided the focal length f remain unchanged, the larger beam
spacing will thus increase the angle θ between the two beams. According to
the formulas in (7-9) this will further reduce the size of the measuring
volume.
In agreement with the fundamental principles of wave theory, a larger
aperture is able to focus a beam to a smaller spot size and hence to create a
greater light intensity on the scattering particles. At the same time the greater
receiver aperture is able to pick up more of the reflected light.
As a result the benefits of the beam expander is threefold:
7-10 BSA Flow Software: Reference guide