Nonperiodic oscillation with wavelength of mode guided in a special Ti-diffused LiNbO₃ waveguide structure

We report an abnormal waveguiding phenomenon that the fundamental mode guided in strip waveguides of a special Ti-diffused LiNbO₃ waveguide structure, in which an array of 4- and 6-\mu\hbox{m}-wide strip waveguides is embedded in a planar waveguide, nonperiodically oscillates with the wavelength along the waveguide width direction. The oscillation amplitude reaches \sim2 \mu\hbox{m}. The oscillation results in a comb-like transmission spectrum at 1.5\mu\hbox{m} wavelength region. The phenomenon is abnormal as it displays the features of multimode interference (MMI), but the single-mode pattern was captured from the strip waveguide at the 1.5\mu\hbox{m} wavelength. Further numerical analysis for the waveguiding characteristics shows that the strip waveguide also supports a nearly cutoff higher order mode, in addition to the fundamental mode, allowing to explain the abnormal phenomenon: the MMI may take place in some regions of the strip waveguide and the observed single-mode pattern is because the higher order mode, which takes part in the MMI, is close to the cutoff.