Subpicosecond jitter in picosecond electron bunches

摘要

We have simulated and measured 60-120 fs time jitter of photoelectron pulses emitted by a nitride photocathode at 100 GHz rate as in order to evaluate the resolution performance of a previously proposed photonic analog to digital converter. Recently, there has been an increasing demand for high speed analog-to-digital converters (ADCs) for microwave bandwidth signals. State of the art electronic ADCs have reached 10 Gigasamples/second (GS/s), 6-12 bit performance [P. W. Juodawlkis, J. C. Twichell, G. E. Betts, J. J. Hargreaves, R. D. Younger, J. L. Wasserman, F. J. O'Donnell, K. G. Ray, and R. C. Williamson, IEEE Microwave Theory Tech. 49, 1840 (2001)]. We have previously introduced a photoelectronic ADC implementation with measured performance of 3 bits at 100 GS/s [K. Ioakeimidi, R. Leheny, S. Gradinaru, K. Ma, R. Aldana, J. Clendenin, J. S Harris, R. F. W. Pease, IEEE Trans. Microwave Theory Tech. (to be published); Conference on Lasers and Electro-Optics, Baltimore MD, 1-6 June 2003]. The basic operating principle of the ADC is based on a miniaturized cathode ray tube where a bunch of photoemitted electrons passing through an electric deflection system is directed to a specific detector whence a digital code word emanates. The electron bunch samples the analog deflecting voltage that is then quantized according to the position of the detector receiving the bunch. The fundamental limit of the number of distinguishable voltage levels is the ratio of the deflecting voltage to the energy spread due to diffraction of the electron beam, This allows for up to 12 bits at 100 GS/s [R. F. Pease, K. loakeimidi, R. Aldana, and R. Leheny, J. Vac. Sci. Technol. B 21, 2826 (2003)]. At a more practical level, the bit resolution is primarily limited by the uncertainty in the emission of each electron bunch (temporal jitter). For 100 fs time uncertainty 5 bits of resolution are attainable with the nitride cathode for a 50 GHz bandwidth analog signal.