A refractive index grating according to this invention is a tilted grating in a single mode optical waveguide, with a photosensitivity profile that includes at least one "tuning region" in the waveguide core. Appropriate choice of the photosensitivity profile of the waveguide can result in a "supernull" in the LP.sub.01,f to LP.sub.01,b coupling. That is to say, the angular range of the tilt angle .theta. over which the core mode coupling is essentially zero (i.e., <-30 dB) can be substantially increased, to more than 0.1.degree., or even 0.2 or 0.5.degree., as compared to the angular range obtainable without a "tuning region" in the waveguide core. The increased angular range provides for improved manufacturability of the grating. In preferred embodiments the grating has a large cladding loss (>20 dB) and bandwidth (>20 nm). A method of trimming a grating is also disclosed.