Nonperiodic Oscillation With Wavelength of Mode Guided in a Special Ti-Diffused $\hbox {LiNbO}_{3}$ 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 4and 6-μm-wide strip waveguides is embedded in a planar waveguide, nonperiodically oscillates with the wavelength along the waveguide width direction. The oscillation amplitude reaches 2 μm. The oscillation results in a comb-like transmission spectrum at 1.5-μm 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-μm 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 singlemode pattern is because the higher order mode, which takes part in the MMI, is close to the cutoff.