A two-point optical system installed in a larger experimental setup for Focused Laser Differential Interferometry testing. The green laser light is seen passing through the two sets of wedged windows. (Image: NASA)

NASA’s Langley Research Center has developed a Beam Crossing Optical System for use in two-point Focused Laser Differential Interferometers (FLDI) that are used in measuring density disturbances in hypersonic flows.

Conventional single-point FLDI systems 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.

Perspective CAD rendering of proposed NASA optical system with novel beam crossing adjustment (as shown by arrow) to enable a testing zone with parallel beams. (Image: NASA)

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.

The technology offers a lower-cost (half the cost) alternative to a Nomarski Prism approach and enables increased manipulation capabilities for the user, such that the separation distance between points and laser beam orientation can be manually adjusted with ease.

It has applications in differential interference contrast (DIC) microscopy for cell investigation, laser-induced thermal acoustics (LITA) for gas sensing, and supersonic wind tunnel testing for various applications in aircraft development.

NASA is actively seeking licensees to commercialize this technology. Please contact NASA’s Licensing Concierge at This email address is being protected from spambots. You need JavaScript enabled to view it. or call at 202-358-7432 to initiate licensing discussions. For more information, visit here .