one wavelength u03bb 1 onto a focal point fk which is arranged at a defined distance l from the wall w of the housing gh a connector part fa1 is secured to the wall w and a fiber f which is surrounded by a protective cladding sm is fixed in the connector part fa1 so that an end fe of the fiber f will coincide with the focal point fk it is thereby expedient to fix the end fe of the fiber f in the connector part fa1 so that at the very onset the end fe is arranged at a definite distance l from a front end surface or mounting surface ve which is secured on the wall w of the housing gh thus the end fe of the fiber f is already situated at the definite distance l from the wall w and only an adjustment in the y direction and the z direction is necessary to insure that the end fe lies on the focal point fk and the axis a of the light on the first beam path sg1 light of a second wavelength u03bb 2 is divergently emitted from the end fe of the fiber f and is emitted towards the second lens kl2 which is remote to the transmitter ld and this lens kl2 will focus it into parallel light to impinge on the light beam separating device or beam splitter lt the beam splitter lt will deflect this light by 90 u00b0 and therefore deflect it out of the beam path sg1 into a second beam path sg2 a third lens kl3 in the.