A polarimeter for measurement of transient retardation changes

An electronic polarimeter is described in which a signal-sampling technique and feedback are used together to measure periodic retardation changes. A Faraday cell is arranged to feed back a rotation just sufficient to cancel that caused by the unknown retardation, so that the Faraday cell current provides the quantitative measure desired. The photomultiplier output is fed to a sampling oscilloscope in the feedback loop. The instrument is best suited for the measurement of small retardation changes, of the order of 50 mrad or less. The 6-nsec time resolution obtained was limited by the photomultiplier response; a faster detector could improve it considerably. The repeatability obtained of 10(-4) rad was determined by the usual compromise between noise, primarily photon noise, and averaging time. However, other factors limit the accuracy attainable to, at best, +/-3%, for a retardation change of ~20 mrad, while a more typical accuracy was +/-5%. Either Faraday rotation, or small rotations of the optical ellipsoid of aspecimen, could also be observed.