A self-coherent optically-sampled analog-to-digital converter using a 90¡ã optical hybrid coupler is experimentally demonstrated. It leverages practical, commercially available components and is used for high-resolution undersampling at 50 Msps of up to 20 GHz carrier frequencies. The measured ADC resolution is 9 effective bits for a 10 GHz carrier frequency and shows little carrier frequency dependence from 6 to 20 GHz.