A system for automatic null ellipsometric measurement, which uses a new kind of achromatic rhomb-type λ/4 retarder, is designed and constructed. By use of terbium glass as the modulator’s material, a magneto-optical modulator and its driving circuit are constructed. The polarized light is modulated by dc and ac. Combined with an ultra-low noise frequency-selective amplifier and a lock-in amplifier, the null time of the mechanical and photoelectric automatic null system can be less than 15 s. After the error of the mechanical parts is excluded, the repeatability standard deviation S 4 of nulling angles can be less than 0.00025 deg.
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