Tech Briefs

Miniature Birefringence-Measuring System

Advantages over older systems include compactness, light weight, low power consumption, and greater sensitivity.

A miniature birefringence-measuring system has been developed for use in investigating extensional flows of non-Newtonian polymers in microgravity. The system could also be used on Earth to perform general optical-retardation and dichroism measurements, to measure stresses with high sensitivity, to characterize polymers, and to measure orientation angles of molecular chains. The system includes a dual-crystal transverse electro-optical phase modulator that makes it possible to measure small (of the order of 10 -9) changes in the birefringence of a material under test as the material is subjected to extensional deformation or shear stress.

alt Moreover, because the modulation frequency can be > 100 MHz, it may be possible to use the system to study chaotic flow birefringence.

Previously developed phase-modulation birefringence-measuring systems have traditionally been built around single-crystal photoelastic modulators (PEMs). One representative fused-silica PEM assembly for visible light weighs 9.1 kg and has a 27-mm useful aperture. A typical previously developed system can occupy 2 m of lineal space. Given these typical weight and size parameters, previously developed systems are too heavy and large for many applications. In contrast, the present miniature birefringence-measuring system weighs only about 2.5 kg and occupies only 28 cm of lineal space. In addition to smaller size and weight, the advantages of this system over previously developed birefringence-measuring systems include lower power consumption, greater robustness, shorter warmup time, and greater accuracy and sensitivity in the birefringence measurement.

The modulator in this system is a commercially available one that contains two LiNbO3 crystals oriented with their principal axes orthogonal to each other and at ±45° with respect to the polarization of incident laser light. The figure depicts the basic optical configuration. The second crystal is needed to counteract effects of thermal instability because the static birefringence of LiNbO3 is highly temperature-dependent. In addition, when the modulation voltage applied to the second crystal is 180° out of phase with that applied to the first crystal, the effective modulation depth is doubled.

The system has been calibrated by incrementing the modulator driver voltage and mapping the appropriate Bessel functions. Calibration data were taken at 500 points, and the experimentally generated calibration curves were found to agree with the expected curves at every data point with a worst correlation error of less than 0.5 percent. This is an order of magnitude better than the accepted calibration standard, for which the worst correlation error ranges from 2 to 5 percent. This system has also been tested against a PEM-based birefringence-measuring system, with favorable results.

This work was done by Jeffrey R. Mackey of Analex Corp. for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4 —8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16705.

This Brief includes a Technical Support Package (TSP).

Miniature Birefringence-Measuring System (reference LEW-16705) is currently available for download from the TSP library.

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