Abstract

The performances of fiber gratings can be impaired by coupling to cladding modes, especially for applications in multiplexed systems. This coupling is due to a non-zero weighted overlap integral between the guided mode and the cladding modes. It can generate transmission losses larger than 3 dB over more than 20 nm on the blue side of the resonance wavelength and are commonly observed in high reflectivity gratings (> 50%) (see for example transmission curves of figures 2 and 7 of reference [1]). From a perturbation theory, it comes that, for a tensorial perturbation Δε of the permittivity, the coupling coefficient is proportionnai to the quantity ${\displaystyle \iint E_g\Delta \epsilon E_c^{*}dS}$, where E_g is the guided mode field and E_c a cladding mode field. In the case of a circularly symmetrical perturbation (generally assumed in photowritten gratings), the coupling is limited to cladding modes of LP₀ symmetry. The above integral can be nulled if Δε is constant over the area where the guided mode field is confined [2]. This is not usely the case of the refractive index difference associated to gratings, since the cladding is far less photorefractive than the germanium-doped core. One way to solve the problem is to obtain the same refractivity in a region of a few core diameters and to ensure a constant UV illumination intensity in the same volume. This implies to modify the dopant concentration profiles of the fiber.

© 1995 Optical Society of America