Phase retardation measurement of an arbitrary wave plate based on magneto-optical modulating and residue detecting of the base frequency component of the signal

Wave plates (WPs) are key components in optical polarization systems whose phase retardations should be measured accurately. The key point of most light extinction methods focuses on how to precisely judge the system’s extinction state and then confirm corresponding angle positions of the optical components. Usually the extinction state of a measurement system is judged by detecting residue of outgoing light intensity. In this Letter, a method of measuring the phase retardation of an arbitrary WP based on magneto-optical modulation is proposed. A magneto-optical modulator is placed between the standard quarter-WP and the second polarizer on the basis of the traditional optical path. The measurement signal is obtained after the outgoing light passing through a band-pass filter whose center frequency is the same as the frequency of the magneto-optic modulator’s driving voltage. This method detects the base frequency component instead of the whole intensity of the outgoing light, so as to get better judgement accuracy of the system’s extinction state. The arbitrary WP’s phase retardation can be accurately derived by detecting the residue of the base frequency component of the outgoing light signal and then judging the angle position of the second polarizer when the system is in the state of extinction. The corresponding theoretical formulas are deduced by the Jones matrix, and the WP measuring system is established. The error analysis shows that the system measurement uncertainty is about 4.5′ if the experiment temperature varies in the range of 0.1°C. Experiment results on half-WPs and quarter-WPs show that the presented method is in good agreement with other methods. The measurement repeatability is also good, with the standard deviation about 2′.