An optical device for sensing properties in an environment such as the presence of a substance or chemical in the zone to be monitored using optical components integrated on a microchip base or substrate. A preferred embodiment introduces a method for fabricating a miniature microchip chemical sensor by integrating a GaAs LED 14 with a polyimide waveguide 48 and a silicon photosensor 16 on the same chip. Light 18 is emitted at the edge of the GaAs LED 14. A portion of the light propagates is detected by a PIN diode 16. A chemical sensitive material 50 is coated on top of a polyimide waveguide 48. When the gas or chemical to which the material is sensitive appears, the light transmitted from the polyimide to air increases, thus the total signal sensed by the photodetector decreases, whereby the change in light signal indicates detection.