Long Period Gratings Formed in Depressed Cladding Fibres

Abstract

There has been a significant interest in long period gratings, primarily due to their applications in gain-flattened erbium doped fibre amplifiers [1]. Up till now, cladding modes supported by the glass-air interface of an optical fibre have been used in a forward mode coupling scheme involving also the guided fundamental mode of the optical fibre. Here we propose an alternative technique. In a single mode depressed cladding fibre of an appropriate design (an example is given in fig. 1), the higher order LP11 mode can be made to be a leaky mode in such a structure, i.e. it can propagate for a short length before being stripped off the high index region beyond the depressed cladding. This enables coupling between the guided fundamental LP01 and leaky LP11 modes, despite the fact that the optical fibre is single-moded. This coupling was first observed in [2], where a Bragg grating caused strong coupling into the backward-propagating leaky LP11 mode and much weaker coupling into a series of cladding modes. In a forward coupling scheme with a long period gratings (several hundred micrometres pitch), the LP01 mode can be coupled into the forward propagating leaky LP11 mode, therefore creating a loss peak in the transmission in the same way as when the LP01 mode is coupled into a cladding mode supported by the glass-air interface of the optical fibre which is subsequently stripped off over the coated section of the optical fibre. Two advantages are anticipated, I) potentially much stronger coupling due to the much large modal overlap possible and II), insensitivity to the glass-air interface as the LP11 mode is supported mainly by the core. It must be stressed that as the LP11 mode is an asymmetrical mode, the coupling from LP01 to LP11 will not occur if a circularly symmetrical grating is written over the core of the fibre, but this is not usually a problem when H2 or D2 loading is used, because of the asymmetry of the index change in such gratings due to strong absorption induced at the writing wavelength.