Provided are methods of forming an optical coupler. The method includes forming a first waveguide and an in-plane tapered layer on a silicon layer, forming a mask with first and second openings. The first opening is formed between the in-plane tapered layer and the second opening, and the second opening extends from the first opening with a gradually narrowing width. Thereafter, a planar waveguide and a three-dimensional tapered layer are simultaneously formed in the first and second openings, respectively. The planar waveguide has a substantially uniform thickness, and the three-dimensional tapered layer has a thickness gradually increasing with a decrease of the width thereof.