P.J.M. van der Slot, K.-J. Boller, and A. Strooisma,
   "A Mirror-Less, Multi-Beam Photonic Free-Electron Laser Oscillator Pumped Far Beyond Threshold",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper TUP005, pp. 334-337,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-TUP005,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/tup005.pdf, 2015.