Luminosity is one of the main characteristics of an accelerator. It determines the number of colliding particle (e.g. protons) interactions in a bunch crossing. The task of luminosity measurements in two-beam experiments is challenging. It becomes harder with the LHC upgrade – High-Luminosity LHC – with even higher energies, luminosity and a complex time structure of the beam. The proposed luminometer is being designed to perform online bunch-by-bunch luminosity measurements with the frequency of 25 ns, the accuracy of 1% and the ability to withstand high radiation. The proposed for this task radiation hard quartz fibers are simulated using Geant4: the physics, geometry and optical parameters are varied to optimize the accuracy of the processes of emission, capturing and transportation of Cherenkov light.