Bandwidth Analysis of Waveguide Mode Converters Based on Optical Analogy of Stimulated Raman Adiabatic Passage in Engineered Multimode Waveguides

Mode conversion can be achieved in multimode waveguides using optical analogy of stimulated Raman adiabatic passage. The coupled mode equations (CMEs) for adiabatic mode conversion via an intermediate mode with wavelength detunings are derived for an engineered step-index multimode waveguide. It is shown that the detunings cause diabatic coupling of adiabatic modes and result in lowered conversion efficiency. The Landau-Zener (LZ) model of nonadiabatic crossing is used to analyze the spectral response of the mode converters and shown to be in good agreement with numerical solutions of the CMEs and beam propagation simulations. The analytical expressions from the LZ analysis offer useful insight into the spectral properties of the mode converters and suggest bandwidth tunability of these devices. We demonstrate bandwidth tunability of the mode converter by a numerical example.