BirefringencePrecisionMeasurementandReductionTutorial.docVIP

Birefringence Precision Measurement and Reduction Tutorial Darren Miller Opti 521 12/07/07 High precision optical instruments used in industrial applications require control of many variables in order to maintain precise measurements. All of the optical components in a precision system must be controlled. One of the most important parameters to control in many applications is residual linear birefringence due to a gamut of sources. An engineer must know how to measure the birefringence and techniques to use to reduce it, if it is seen as a large source of error, which it is for many precision applications. High-speed, high sensitivity and repeatable systems for measuring birefringence are used in precision applications. Applications such as lithography use the highest quality materials in the industry, and therefore need to be able to characterize the materials as completely as possible. A technique used for precision polarization measurements is to use PEMs (photo-elastic modulators) in concert with lock-in amplifiers. A system is produced by Hinds instruments, Inc. that uses this technique, the ExicorTM. Once the birefringence is measured methods of reducing it are useful to know. Birefringence reduction after the element is fabricated is difficult to accomplish, but it has been shown that annealing can accomplish it to some degree2. It is easier to reduce the birefringence of a material during the fabrication step3. Plasticizing polymers using compressed gasses can achieve reduction in stress, and therefore reduce birefringence. For course measurements, engineers can generally place the element to be measured between crossed polarizers, and rotate the element to study the changes in the illuminated sample. Just by using a trained eye, an engineer can easily determine angle and magnitude of birefringence in a sample down to a quarter-wave or better. This technique is time consuming and does not give the sensitivity required for precision optical systems. A ver