Zhang, H. Aryshev, A. Doucas, G. Harrison, H. Konoplev, I.V. Lancaster, A.J. Shevelev, M. Terunuma, N. Urakawa, J. JACoW Geneva, Switzerland 978-3-95450-182-3 10.18429/JACoW-IPAC2017-THPVA141 English 4798-4800 THPVA141 Contribution to a conference proceedings 2017 2017-05 http://dx.doi.org/10.18429/JACoW-IPAC2017-THPVA141 http://jacow.org/ipac2017/papers/thpva141.pdf With the development of femtosecond lasers, the generation of micro-bunched beams directly from a photocathode becomes routine; however, the monitoring of the separation is still a challenge. We present the results of proof-of-principle experiments measuring the distance between two bunches via the amplitude modulation analysis of a monochromatic radiation signal. Good agreement with theoretical prediction is shown.