We present a measuring method for any wave plate retardation with fairly high precision that utilizes a laser frequency splitting technique. To avoid strong mode competition in measuring half and full wave plates, we use two separate methods: comparing adjacent longitudinal mode spacing, and phase offset with an additional quarter wave plate. Therefore any wave plate can be characterized by a single instrument, and no complicated experimental arrangement or data analysis is required. The performance of the system is demonstrated by determining the phase retardation of several samples to a precision and repeatability better than lambda/10~(4); moreover, an error analysis is proposed.
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