Patent GB-1428813-A (GB)
Title: Polarization interferometer with beam polarizing and retarding mea ns
IPC: G01J4/04
Filed: 1973-05-15
Abstract: 1428813 Dichroism measurement VARIAN ASSOCIATES 15 May 1973 [15 May 1972] 22962/73 Heading G1A Circular or linear dichroism is measured using interferometric techniques. The apparatus comprises a broadband source 10 of visible light and/or ultraviolet radiation, or alternatively infrared radiation linear polarizer 26, stop 110 and collimator 11, feeding an interferometer. This is arranged to provide a beam whose ellipticity alternates between left and right circular polarization, becoming linear in one direction as the ellipticity alternates from left to right circular polarization, and linear in another - direction as the ellipticity alternates from right to left circular polarization, the characteristic frequency #a of such alternation varying as a function of wavelength: The output of the interferometer is passed via lens 16a through the sample under test, located at 12, and the transmitted radiation focused by lens 16 on to a detector 15. The detector output varies in intensity with the variation in ellipticity at frequency Na, and represents an interferogram function F(Na) which is processed by computer circuitry 18 to produce dichroism spectra. Interferometer details. In the arrangement of Fig. 1, the interferometer includes a beamsplitter 30, e.g. a thin metal film on a glass support, and reflection polarizers 35, 38. Reflector 38 is mounted on a movable carriage 36 driven by actuator 37. Alternatively, reflector 35 is mounted on movable carriage 36a. In the arrangement of Fig. 6 (not shown), the reflection polarizers comprise mirrors (140, 141) with transmission polarizers (142, 143), mounted at +45 degrees and -45 degrees to the incident beam linear polarization direction. In the arrangement of Fig. 7 (not shown), the reflection polarizers comprise retroreflectors, each comprising a pair of 90 degrees angled mirrors (146, 147) with linear transmission polarizers (150, 151). In the arrangement of Fig. 8 (not shown), the reflectors comprise mirrors (160, 161) and a beam retarding means, e.g. a quarter wave plate, is located in the path of at least one of the beams. In Fig. 9 (not shown) retroreflectors as in Fig. 7 are used, together with retarders, e.g. half wave plates, mounted between the mirrors of each retroreflector. Processing circuit. The detector may be a pyroelectric device. The output is amplified at 19, digitized at 20, recorded at 24 and fed to computer 23 for deriving the Fourier transmittance dichroism spectrum. This is divided, in computer component 23a, by the modulation function to derive the absorption spectrum at 90 for circular or linear dichroism, for display at 91. The modulation function is derived by sensing the position of carriage 36 using an interferometer comprising broadband source 81, beam-splitter 30 and detector 83, feeding computer component 23b, to define the zero position, and an interferometer comprising laser source 86, beam-splitter 30, and detector 88, to provide a fringe counting system for accurate position determination and for providing by means of 23b the demodulation cosine or sine function, used for determining linear or sine function, used for determining linear dichroism and circular dichroism, respectively.