The sensor system for the detection of chemical substances has a spiral optical waveguide which produces an evanescent field penetrating into the adjacent medium, a light source for coupling light into the waveguide, and a detecting circuit for receiving light from the waveguide and producing an output signal reflecting the sensed chemical substance. The waveguide has the form of a bifilar, non-crossing spiral. It consists of an incoming or decreasing spiral segment and an outgoing or increasing spiral segment continuously bound by a central semi-circular segment.