Recent advances in optical waveguides have brought long-awaited technologies closer to practical realization. Although the concept of a single-mode (SM) waveguide has been around for a while, SM condition usually posed very stringent conditions in fabrication for small waveguides. Researchers have developed low loss silicon nitride (Si3N4) at 1550nm wavelength, the developments in specific application have down converted to 1310nm (O-band) so they do not have to compete with internet data for bandwidth and could share the existing optical fiber infrastructure. However, wavelengthdemultiplexer technology at this band is not readily commercial available. Custom-made O-band optical devices for wavelength-demultiplexing have typical losses. Such high losses deplete more than 75% of the already-scarce photons. We studied Si3N4 channel waveguide with ultra-thin slab for (SM) condition at 1310nm wavelength using finite element method (FEM) and 3-D imaginary beam propagation method (IDBPM). We have shown that SM condition is possible for ultra-thin slab with wide waveguide width; such condition can ease the constraint of photolithography, allowing deposition of thin Si3N4 layer to be accomplished in minutes. Studies show that for ultra-thin layer, for example, at 60nm, we can achieve a wide range of widths that fulfilled the SM condition, ranging from 2μm to 5μm. SM condition becomes more stringent when the Si3N4 layer increases. Substrate losses are estimated at 0.001 dB/cm, 0.003 dB/cm, and 0.1 dB/cm for slab height at 100nm, 80nm, and 60nm respectively.