  In astronomical spectroscopy, optical fibres are abundantly used for
multiplexing and decoupling the spectrograph from the telescope to provide
stability in a controlled environment. However, fibres are less than perfect
optical components and introduce complex effects that diminish the overall
throughput, efficiency, and stability of the instrument.
  We present a novel numerical field propagation model that emulates the
effects of modal noise, scrambling, and focal ratio degradation with a rigorous
treatment of wave optics. We demonstrate that the simulation of the near- and
far-field output of a fiber, injected into a ray-tracing model of the
spectrograph, allows to assess performance at the detector level.
