A single-liquid-core/liquid-clad L2 optical waveguide/coupler is studied numerically. The device consists of a large aspect ratio-microchannel isothermally heated on its both parallel flat walls. Steady state laminar Poiseuille flow of cold water which is introduced to the heated-microchannel forms a stable thermal boundary layer adjacent to the isothermal heated walls. Thermal boundary layer development causes the lateral refractive index gradient across the channel which is required for waveguiding. At a particular mass flow rate for a given wall temperature, the waveguiding occurs. Since the thermal boundary layer, cladding region, is actively tunable through varying both surface temperature and mass flow rate, The waveguiding effect along the channel is highly configurable. We demonstrated that the excitation in the flow direction leads to defocusing of the beam at fundamental TE mode. By reversing the flow direction, the Counter-flow excitation of the waveguide also results in focusing effects. The current waveguide can be exploited as an integrated optical coupler at specific channel wall temperature and mass flow rate.

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