Birefringence measurement at deep-ultraviolet wavelengths

A method of measuring birefringence properties of a sample, comprising the steps of:

• separately directing through the sample at least three light beams comprising a first beam of polarization-modulated light having a first wavelength, a second beam of polarization-modulated light having a second wavelength, and a third beam of polarization-modulated light having a third wavelength, the first, second, and third wavelengths being different from one another;
• modulating the polarization of the first, second, and third beams after those beams pass through the sample;
• analyzing the first, second, and third beams;
• detecting the intensity of the first, second, and third beams;
• and
• calculating an actual birefringence property of the sample based on the detected intensities.

A system for measuring birefringence properties in a sample, comprising:

• a source of two or more beams of light having wavelengths that are different from one another;
• means for modulating the polarization of the light beams;
• means for separately directing the beams through the sample;
• means for analyzing the beams after the beams pass through the sample;
• and
• detection means for detecting the intensity of the beams, thereby to provide information suitable for calculating a birefringence property of the sample based on the detected intensities.

28 A method of aligning optical elements of a system for measuring birefringence properties in a sample, wherein the system includes a first and second photoelastic modulator (PEM), each PEM having an optical axis and operable for modulating the polarity of a light beam that passes though the sample, the system also including a polarizer associated with the first PEM, an analyzer associated with the second PEM, and a detector for measuring the intensity of the light beam after the beam passes through the PEMs, polarizer, and analyzer, the method comprising the steps of:

• fixing the optic axis of the first PEM;
• operating the first PEM while the second PEM is off;
• rotating the polarizer while monitoring the measured intensity of the light beam;
• noting the rotational position of the polarizer when the measured intensity corresponding to a first harmonic frequency of the light beam is at a minimal value;
• rotating the polarizer by 45 degrees beyond the noted rotational position;
• rotating the analyzer until a second harmonic frequency of the modulated light is at a minimal value;
• and
• rotating the second PEM until a second harmonic frequency of the modulated light beam is at a minimal value.