OPTICAL FIBER DESIGN FOR STRONG GRATINGS PHOTOIMPRINTING WITH RADIATION MODE SUPPRESSION

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 Δe of the permittivity, the coupling coefficient is proportionnai to the quantity ∬ E g Δ e E c * d S, where Eg is the guided mode field and Ec 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 Δe 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.