As the two focal volumes are thousands of times brighter than the measuring volume of a LDA of equal power, the L2F can be easily used in the back-scatter configuration, which allows placement into a single hardware of both emission and receiving optics, in this way there are no alignment problems since the vibrations are transmitted evenly to both elements.
Figure 4.28 shows a typical L2F optical head: it includes two photomultipliers (detectors 1 and 2), a beam-splitter (Rochon prism), a
1/4, the transceiver lens, and mirrors and lenses necessary to focus the laser beams and to create in the measurement of zone B the image of the two foci produced in zone A.
The laser light, monochromatic and linearly polarized, passes through a 1/4, i. e. a filter that slows and overlaps the incident waves; by the superimposition of two electromagnetic waves having the same form but out of phase by 90°, a quarter of wavelength, a circularly polarized wave is created.
The Rochon prism separates the laser beam into two beams of equal intensity: one emerges essentially not deflected, the other with a small angle with respect to the optical axis: a lens therefore focuses the two beams into two distinct focus; their distance can be changed using Rochon prisms with different angles of divergence.
The transceiver consists of two collimator lenses without spherical aberration flanked on all openings; the front collimator is interchangeable to allow for changes in working distance. The particles that pass through the focal volume scatter the laser light in all directions. The transceiver lens captures the back-scattered light and focuses it on a double pinhole that acts as a filter for the light background as in it passes only the light from the focal volume. Two broadband photomultiplier tubes convert the light pulses of start and stop into electrical pulses.
The change in the direction of measurement in the plane perpendicular to the optical axis is obtained by rotating around its axis the Rochon prism, the double pinhole optics and other stops, in this way, the off-axis focus is turned around the central focus that remains fixed in space (Figure 4.25). This fact justifies the presence of the 1/4, because the circularly polarized beam prevents variations of the light intensity of the beam when the prism rotates.