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The conventional approaches for measuring focused laser differential interferometry either use a single-point mechanism that cannot calculate velocity or a system that creates non-parallel beams in the testing zone, causing differences in time to travel between beams throughout the testing zone, adding a level of uncertainty to velocity measurements. For this technology, the inventors determined that the best approach is to use a method that ensures all laser beams propagating between the transmitter and receiver sides of the instrument are parallel to one another. This is done by crossing two orthogonally polarized beams at a Wollaston prism located just ahead of the field lens on the transmitter side of the FLDI. The polarization orientation of the two crossing beams must be at 45 degrees to one another so that the Wollaston prism can further split the beams by a small angle (this gives the instrument its sensitivity to density fluctuations at each measurement point). The use of wedge prisms (that comprise the beam crossing system) to redirect the split beams such that they cross the optical axis minimizes any distortion imparted to the beams. This is in contrast to the use of a spherical focusing lens to redirect the split beams, which can impart undesirable distortions to the beams and affect the focusing properties of the FLDI instrument between its transmitter and receiver sides.