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A simple III-V semiconductor dual-slab waveguide interferometer is demonstrated which can detect picometer level changes in the C-band laser wavelength. Optical path length imbalance between the two single-slab waveguide modes provides the primary mechanism for detection. The dual-mode output field phase change differences due to input wavelength changes are encoded in shifts in the device output far-field interference pattern. The device detects path length dependent wavelength changes with a phase change difference sensitivity of around 660 mrad/nmldrmm at 1.55 mum and a phase detection floor of plusmn1 mrad. The transverse electric and transverse magnetic polarization responses are approximately equivalent.