Long period gratings formed in depressed cladding fibres

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 and therefore where packaging is easier