Fiber mode coupling in transmissive and reflective tilted fiber gratings

Whereas core-mode reflection and core-mode-to-radiation-mode coupling in tilted fiber Bragg gratings is well understood, as is coupling between a core mode and higher-order core and cladding modes in untilted gratings, here we analyze in detail the coupling among core modes and cladding modes in reflective and transmissive tilted fiber gratings. We show that strong coupling between an LP01 core mode and the exact (1m) cladding modes occurs in a transmissive tilted grating for nearly any tilt angle except angles close to 90°, whereas the LP01-to-(lm) cladding mode coupling (l ≠ 1) is appreciable only for tilt angles just below 90° (∼88°). In a reflective grating, strong coupling between the LP01 core mode and the exact (1m) cladding modes occurs only for angles less than ∼5°, whereas coupling to (1m) cladding modes for m > 1 occurs only for angles greater than ∼5°. Coupling among bound core modes exhibits a similar behavior, except that in general the coupling is stronger. Experimentally we show coupling to both higher-order bound core modes and cladding modes in a transmissive tilted grating at visible and near-infrared wavelengths.