A waveguide is provided including first and second diffraction gratings and a phase-matching region conterminous with the first and second diffraction gratings and disposed in an optical path between the gratings. For an optical beam propagating along the optical path, the first grating adds a first phase shift to the optical beam reflecting from the first grating, the second grating adds a second phase shift to the optical beam reflecting from the second grating, and the phase-matching region adds a matching phase shift to the optical beam reflecting from the phase-matching region. The matching phase shift is between minimum and maximum values of the first and second phase shifts.