A new design of fiber optic chemical sensor is proposed and demonstrated as a working pH sensor. The sensor is based on the evanescent coupling between a side polished single mode optical fiber and a single mode overlay planar waveguide. The stringent tolerances placed on the planar waveguide thickness are met by depositing the overlay, one molecular layer at a time, by Langmuir-Blodgett (LB) deposition. The advantages of being able to design the optical properties of the organic dye material is demonstrated by comparing two different overlay material and their responses over different wavelength ranges. Finally, a Kramers-Kronig based model, relating the absorption spectrum of the overlay material to its material dispersion, is shown to be able to predict the sensor response.