Mode size converter for a planar waveguide

A mode size converter, comprising:

• a first layer of a dielectric material with a first refractive index deposited over a substrate;
• a rare earth-doped aluminosilicate layer of a second refractive index deposited over the first layer,
• wherein, the rare earth-doped aluminosilicate layer includes a vertical taper, and the vertical taper has an as-deposited average surface roughness of about 1.5 nm or less;
• wherein
• the resulting average surface roughness is substantially as-deposited, and further having a rate of taper of about 0.14 parts per thousand.