The invention relates to a device (17) for treatment of body tissue, in particular for the permanent occlusion of varicose veins, preferably in the lower limbs, of varicocele and/or of vascular malformations and/or for the use in aesthetic surgeries, preferably laser assisted lipolysis, and/or for tumor treatment by means of laser induced thermotherapy and/or photodynamic therapy, by means of a light diffuser (13) circumferentially and endoluminally irradiating said tissue by laser light energy, said diffuser (13) being connected at its proximal end to a source (10) of laser light energy via a flexible wave guide (12) comprising a fiber optic core (1) covered by an optical cladding (2) having a refractive index smaller than that of the core (1), wherein in the cladding (2) and/or in the core (1) imperfections (18) are provided, designed as recesses and adapted to direct the light, preferably to refract and/or reflect the light propagating within the core (1) and/or its optical cladding (2) in generally radial directions, wherein a cap (7) transparent to the laser light enclosing the distal end of the core (1) and its optical cladding (2) in a fluid tight and/or liquid tight manner is provided. According to the invention the device (17) is characterized in that the outer surface (19) of said optical cladding (2) is fused in the region (A) between said imperfections (18) to the inner surface (21), preferably the inner diameter, of the cap (7) and/or in that the outer surface (19) of said optical cladding (2) extending over a distance in front and/or behind the region (A) provided with the imperfections (18) is fused to the inner surface (21), preferably the inner diameter, of the cap (7).