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 {HEAD ~ at least two gratings} are arranged for {TAIL ~ co directional coupling} 
are arranged for
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 { HEAD ~ at least two gratings } are arranged for { TAIL ~ co directional coupling }
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