An optical apparatus 20 for evanescently coupling an optical signal across an interface 30 is described. The optical apparatus 20 comprises a first substrate 22 and a second substrate 24. The optical signal is evanescently coupled between a first waveguide 26 formed by laser inscription of the first substrate 22 and a second waveguide 28 of the second substrate 22. The first waveguide 26 comprises a curved section 34 configured to provide evanescent coupling of the optical signal between the first and second waveguides 26, 28 via the interface 30.