{TAIL ~ The waveguides} in one section are arranged in the proximity of each other, such that an optical field in the first waveguide can overlap a second waveguide and vice versa and that in said section at least two gratings are arranged for {HEAD ~ co directional coupling} 
{ TAIL ~ The waveguides } in one section are arranged in the proximity of each other , such that an optical field in the first waveguide can overlap a second waveguide and vice versa and that in said section at least two gratings are arranged for { HEAD ~ co directional coupling }
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