A directional coupler (DC) architecture consisting of curved single-mode waveguides with a structured depressed cladding was proposed. The DCs were written in the bulk of industrial borosilicate K8 glass using a femtosecond laser beam. The dependence of the optical power fraction transferred between the DC arms in the telecommunication wavelength range on the length of the coupling section was experimentally and theoretically studied. The maximum fraction of transferred power was 56% and was found to be limited by the asymmetry of the DC arms. The modal propagation loss was as low as 0.8 dB/cm.
Likhov et al. (Mon,) studied this question.