An electrophotographic machine includes a mirror having a reflective surface with a fold line configured to reflect a laser beam. This optical transmission line is an optical transmission line enabling Raman amplification of an optical signal when pumping light is supplied thereto, wherein a region yielding the maximum value of Raman gain coefficient is separated from an end portion where the pumping light is supplied by a predetermined distance along a direction in which the pumping light advances. The invention also relates to the use of OLPs and oligodendrocytes thus obtained for drug screening.