Flow Diagnostics by Laser Interferometry

Abstract

Abstra ctA survey is made of interferometric techniques receiving the optical path 4 = fnds in real time. Using differential interferometry or a new phase coupling technique, I(Dt) can be obtained in a wide range of conditions. A variety of applications to gasdynamic problems is described.A laser beam traversing a flow gives information on the optical path 1 = f nds in the flow when it is brought to interference with a suitable reference beam. As in most cases the refractive index n can be related to one or more of the interesting flow parameters: density p, pressure p, temperature T, or electron density ne; laser interferometry is a suitable means for flow diagnostics.As is already described in [1 ] , it is the Wollaston prism differential interferometer that is especially well suited and offers great advantages in laser interferometry. The essential features of this technique will be described and demonstrated with a number of different applications to gasdynamic investigations.To overcome the limitations inherent in differential interferometry, a new phase coupling technique is now being developed in our laboratory. This leads to a new generation of laser interferometers having a wide range of applications especially in gasdynamics and related fields.Laser Differential Interferometer PrincipleThe optical system is based on a differential interferometer using Wollaston prisms as beam splitting elements. A continuous gas laser (He-Ne laser) is used as light source and photodiodes are used for registration. A schematic diagram of the arrangement is shown in Fig. 1.The linear polarization of the original laser beam is transformed into circular polarization using a A/4 plate. The laser beam is then split under a small angle c by means of a Wollaston prism. e is given by c = 2 tanalne-n ex or(1) where a is the prism angle and Inex -nor the difference between ordinary and extraordinary refractive index of the crystal. Quartz and calcite are used, the Inex -norI of which is 9.05 X 10-3 and 0.169, respectively. The partial beams leaving the Woilaston prism are polarized perpendicularly to each other. Their phase relation depends on the point at which the laser beam traverses the Wollaston prism so that it can be influenced by shifting the Wollaston prism. A lens of focal length f is placed at a distance f from the Wollaston prism. The partial beams run then parallel to each other in the test section. Their spacing a is a = 2f tan nex -nor -(

Observations of hypervelocity boundary-layer instability

“…Another very promising optical method is the two-beam laser differential interferometry (LDI) technique that was used to experimentally characterize gasdynamic flows by Smeets and George at the French-German Research Institute of Saint-Louis (ISL) in the 1970s (Smeets 1972(Smeets , 1973(Smeets , 1974(Smeets , 1977Smeets & George 1973). The technique is sensitive to changes in optical path length that are related to density by the Gladstone-Dale relation.…”

Reflected Shock Tunnel Noise Measurement by Focused Differential Interferometry

“…Smeets developed the technique, along with a number of similar configurations, at ISL (Saint Louis, France) for use in gas dynamic studies in the 1970's. [18][19][20][21] Following the optical path in Fig. 2, starting from the laser (200 mW Spectra Physics Excelsior-532-200-CDRH), the beam is turned by a periscope arrangement for precise directional control.…”

Interferometric Measurement in Shock Tube Experiments

“…Figure 20 shows the laser differential interferometer used in this experiment. The laser differential interfeormeter is the polarization phase difference interferometer by using three Wollaston prisms, developed by Smeets (Smeets, 1972;Smeets, 1977). The bright and dark, observed in the interferometric fringe, is measured by photo detectors.…”