Patent ID: 7248766

Claim:
A wavelength division coupler comprising: a first waveguide having a first end and a second end; a second waveguide having a third end and a fourth end, in which a section between the third end and the fourth end is located adjacent to one end of the first waveguide to form a mode coupling section; a Bragg zone having a Bragg grating formed in portions of the first waveguide and the second waveguide of the mode coupling section, wherein, the mode coupling section further includes a non-grated zone in which the Bragg grating is not provided and the length of the non-grated zone is determined as follows: tan ⁡ ( 2 ⁢  k ab  ⁢ L l opt ) = s  k ab  ⁢ tanh ⁡ ( sL 2 ) , where L 1 indicates the length of the non-grated zone and s indicates the square root of a difference between |k| and |k ab |.