Theory of Operation 5-3
Optic s P a th
The first component in the optics path, see Figure 5-2, is the laser diode.
Light coming from the laser is polarized vertically. Using a polarization
rotator (polymer half-wave plate) the polarization of the laser is rotated by
45 degrees. After rotation, the beam passes through negative and positive
spherical lenses to focus the beam under the particle stream. A
polarization beam splitter is then used to split the vertical and horizontal
components of the beam into two separate beams spaced by 90 to
100 µm. The top beam (closest to the nozzle) is polarized horizontal and
the bottom beam is polarized vertical. Spacing is controlled by the
thickness of the splitter.
The beam pair next passes through a negative cylindrical lens. This lens
controls the width of the beams independent of the focus under the particle
stream. Two vertical knife edges clip the noisy edges of the beams to give
a clean beam under the particle stream. A window is used solely as a
sealing surface to keep the optics chamber separate from the optical
elements, and a final aperture is used to stop stray light from the far edges
of the beams before the beams reach their focal point under the particle
stream.
The beams are then passed through a large aperture into a dual
polarization beam stop. The first beam stop uses neutral density filter glass
placed at the Brewster angle for vertical polarization. This captures all of
the vertical polarization and most of the horizontal polarization. The
remaining portion of the horizontal polarization is reflected into a second
Brewster angle where it is captured. The large aperture in front of the beam
stops prevents light from the beam stops escaping and helps to keep the
beam stop glass clean.
The inset of Figure 5-2 shows that light scattered by the particle stream is
collected by an elliptical mirror and focused onto a solid-state avalanche
photodiode (APD) detector. The detector then converts the light pulses into
electrical pulses.
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