an optical apparatus comprises a dispersion compensating optical waveguide , and an optical pump that pumps the dispersion compensating optical waveguide with light . the light from the pump provides suitable selective gain to a fundamental mode of the dispersion compensating optical waveguide that multi - path interference is substantially reduced . a method of reducing multi - path interference comprises pumping a dispersion compensating optical waveguide with light to provide suitable selective gain to a fundamental mode of the dispersion compensating optical waveguide so that mpi from said dispersion compensating optical waveguide is significantly reduced .