An ultra-high-precision clock system for long time delay has been developed for picosecond time-resolved x-ray diffraction measurements using synchrotron radiation (SR) pulses and synchronized femtosecond laser pulses. The time delay control between pump laser pulse and the probe SR pulse was achieved by combining an in-phase quadrature modulator and a synchronous counter. This method allowed us to change the delay time by a nearly infinite amount while maintaining the precision of ±8.40 ps. Time-resolved diffraction measurements using the delay control system were demonstrated for precise measurement of an acoustic velocity in a single crystal of gallium arsenide.
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