Diffraction gratings for coupling an electromagnetic wave into a planar waveguide are disclosed. The diffraction grating may include a first diffraction grating and a second diffraction grating slanted relative to one another. Alternatively, the diffraction grating may include a first diffraction grating and a second diffraction grating spaced apart to form a gap therebetween.

 
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> Low loss spectrally periodic all-fiber filters

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