This SBIR Phase I effort proposes the development of a potassium titanyl phosphate (KTP) waveguide phase modulator for future integration into a Planar Lightwave Circuit (PLC) used for high spectral resolution aerosol and cloud lidar applications. The PLC is a monolithic substrate that integrates key optical and electro-optical components. These individual components are currently used to lock a seed laser and high power host laser to an iodine absorption line. This technology is important for lidar systems requiring high frequency stability and accuracy. The PLC concept advances NASA's lidar systems due to its compact, efficient, and reliable design, thus enabling use on small aircraft and satellites. The key objective in this SBIR proposal is to establish the feasibility of fabricating a low-voltage, 250MHz waveguide phase modulator in KTP that can support moderate optical powers at 532nm. A baseline PLC design will be presented that will incorporate the phase modulator. The focus of the Phase II effort will be to fabricate a PLC with integrated phase modulator based on this Phase I study.