The sensitivity of bimodal waveguides for integrated optical biosensors is compared to single mode waveguides and grating-assisted bimodal interferometers are proposed as improved sensor concept. Grating-assisted bimodal interferometers are an elegant and compact sensor concept, which features easy fabrication and overcomes typical weaknesses of classical Mach-Zehnder interferometers. Long period gratings for mode conversion in the proposed sensor concept have been simulated employing the FDTD method. Such gratings give full control over the power distribution in the waveguides modes, which is not possible with other methods. Designs for three typical material systems are given and fabrication tolerances were investigated.