Laser irradiation has been previously investigated for achieving uniform heating of polyethylene terephthalate (PET) fibres in the hot-drawing stage of the production process, so as to obtain better fibre mechanical properties. The optical properties and dye uptake of PET fibres also depend on the polymer chain orientation and crystallinity within the fibre structure. This paper reports an investigation of a concept whereby laser irradiation and interferometry could be used to modify and trace a small change in the optical properties of a PET monofilament fibre, but the corresponding change in the dye uptake would not be detected visually. A copper vapour laser (550–580nm wavelengths) was used to expose consecutive 4mm lengths along a running length of monofilament to 39.8Wcm−2, at a pulse rate of 9.89kHz in order to modify, in a controlled way, the polymer crystallinity and orientation. A 3D finite element simulation, based on uncoupled heat-transfer analysis, indicated that rapid heating and cooling could be obtained with the laser to give the small changes required. Irradiated and untreated samples were analysed by interferometry and a 0.16% change was detected in the birefringence profiles, corresponding to a small reduction in the degree of orientation and crystallinity of the irradiated samples. Density measurements and wide-angle X-ray scattering (WAXS) analysis confirmed the change in crystallinity. Tests conducted for dye adsorption and tensile strength showed a small increase in the former and only a very small decrease in the latter. It was concluded that these changes in property provide the opportunity for a laser-irradiated PET monofilament fibre to be used as a subtle tracer element in brand labels for textile garments as an anti-counterfeit measure.