In a singlemode fibre of a depressed cladding design, the LP11 mode is leaky but can propagate over a short length of fibre. This leaky LP11 mode can be used instead of the conventional cladding modes in long period gratings, with two extra benefits: a larger overlap, giving stronger coupling, and coupling which is insensitive to the glass-air interface, therefore packaging is easier.
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Dong et al. (1997) studied this question.
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