The authors fabricated and tested a compact optical sensor with an integrated waveguide and a microchannel in spiral geometry. The multimode waveguide, consisting of an SU-8 core of 40μm thick and 50μm wide and a fluid cladding layer of 60μm thick, realizes a light-fluid interaction length of 110mm within a device area of 4cm2. The waveguide sensor measures changes in liquid sample concentration and is sensitive to changes in liquid refractive index. Absorbance measurements using the spiral waveguide sensor demonstrate increased sensitivity compared with a linear geometry sensor.