Measurement of the two-photon correlation of synchrotron radiation in the VUV region by a delay-time modulation technique

The two-photon correlation (second-order coherence) of synchrotron radiation in the VUV region (hnu = 55 eV) has been measured using a novel photon-counting method. A new technique has been developed to measure a small bunching effect by using a coincidence unit composed of a constant fraction discriminator, a time-to-amplitude converter (TAC), a single-channel analyzer (SCA) and two solid-state switches. The path of the circuit through which the stop signal for the TAC passes can be changed by a control voltage generated by a function generator, and the relative arrival time of two photons on condition that the output signal from the SCA appears is consequently changed. By modulating the arrival time and measuring the output rate from the SCA with a digital lock-in amplifier, an apparent bunching effect has been observed which is characteristic of the chaotic light. The electron-beam emittance in the horizontal direction was estimated as 39(-8)(+13) nm rad by this experiment, and the value was consistent with the designed value of 36 nm rad.