System for joining thermoplastic workpieces by laser transmission welding

A system for joining at least two workpieces of thermoplastic material by laser transmission welding, the system comprising at least one device for generating laser radiation and an imaging optics having a first optical axis, wherein laser radiation emitted by the device for generating laser radiation is guided into a joining zone, and wherein the imaging optics is configured for optical imaging from a joining plane in the joining zone to a detection plane of a radiation detecting device, wherein a deflection mirror is provided at a coupling point on a second optical axis, running parallel to the first optical axis, for deflecting the laser radiation from an entrance axis enclosing a nonzero angle, preferably a right angle, with the second optical axis, into a direction along the second optical axis, wherein the distance between the coupling point and the joining plane with respect to the second optical axis is smaller than the distance from the detection plane to the joining plane with respect to the first optical axis, wherein the diameter of the deflection mirror is adapted to a cross-sectional shape and cross-sectional size of the laser radiation and, when projecting the deflection mirror onto a projection plane perpendicular to the first optical axis, the projection area of the deflection mirror in the projection plane is arranged completely within a minimum aperture in the beam path of the imaging optics and only partially covers the minimum aperture.