We propose an add-drop filter with an unlimited free spectral range (FSR). The device is based on grating-assisted contradirectional couplers with silicon-on-insulator ridge waveguides. The intrawaveguide coupling is significantly diminished by distancing the periodically structured corrugated waveguide away from the straight input waveguide. Based on this asymmetric structure, calculated results show that the peak reflectivity of the intrawaveguide coupling is lower than 0.1 dB, while that of the contradirectional coupling is higher than 20 dB. An experimental result of 1.8 dB for the intrawaveguide coupling is obtained. A narrow drop-port bandwidth of 0.8 nm is also achieved experimentally.