A waveguide includes a first layer and a second layer. The first layer comprises a material of a first refractive index. The second layer is surrounded by the first layer and comprises a material of a second refractive index greater than the first refractive index. The second layer comprises a main body, a first fork and a second fork. The main body has a first substantially constant thickness. The first fork is extended from the main body and has a first tapering end exposed by the first layer. The first fork has the first substantially constant thickness. The second fork is extended from the main body and has a second tapering end exposed by the first layer. The second fork has the first substantially constant thickness.