A new instrument for measuring both the magnitude and angle of low level linear birefringence

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Abstract
This article describes a sensitive method for measuring both the magnitude and the angle of the fast axis of low level linear birefringence in optical materials. Several different approaches have been investigated and one of them has been chosen as the basis for a linear birefringence measurement instrument. The instrument employs a low birefringence photoelastic modulator for modulating the polarization states of a He–Ne laser beam. After the modulated laser beam passes through the sample, two detecting channels analyze the polarization change caused by the sample. An algorithm has been developed to calculate the magnitude and angle of the retardance in a sample. A computer program implements the algorithm and displays the calculated values. Using this instrument, selected samples with different levels of linear birefringence have been studied. The resulting instrument achieves high precision and sensitivity for the final measurements. The sensitivity of the magnitude of linear birefringence is better than 0.005 nm (∼0.003 ° with a He–Ne laser at 632.8 nm), and the sensitivity of the fast axis angle of the sample is <1 ° (for retardance ⩾0.5 nm).