An optical mode size converter 300, comprising a: guide portion 350; first strip 330 with first refractive index; and second strip 340 with second refractive index. The first and second strips have indexes higher than that of the guide and overlap to form an evanescent coupling region (260, Fig. 2). An evanescent coupling layer 350 may be disposed between the strips. The strip’s indexes may be identical. The strips may comprise hydrogenated, or hydrogen-free, silicon oxynitride. The coupling layer’s refractive index may be larger than the guide portion’s and smaller than the strip’s. The second strip’s cross-section may be identical to an output waveguide core’s (121, Fig. 1) cross-section. The refractive indexes of the second strip and guide portion may be identical to an output waveguide core’s and output waveguide cladding’s (122, Fig. 1) respectively. The strip’s cross-section areas may vary; the first strip’s area may be smaller than the second strip’s. The first strip and guide portion may operate below a wavelength cut-off. The converter may be supported on a waveguide chip connected (butt-coupled) to a cleaved optical fibre. The guide portion may comprise: a trench delimited by two slabs of lower refractive index than a core.
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