Abstract: 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