Optical sensing apparatus and method

A method for sensing a substance in an environment, comprising the steps of:

• providing an optical waveguide having a boundary with an index of refraction that depends on said substance in the environment;
• injecting a light beam into said waveguide such that at least two modes of said beam propagate in said waveguide;
• generating an electrical signal representative of the interfering product of said at least two modes of said beam which have propagated through said waveguide;
• said signal being indicative of said substance.

Apparatus for sensing a property of an environment, comprising:

• a susbtrate having an index of refraction n.sub.s ;
• a waveguide layer having a surface adjacent said substrate, said waveguide layer having an index of refraction n.sub.f that is greater than n.sub.s ;
• a superstrate adjacent the opposing surface of said waveguide layer, said superstrate having an index of refraction n.sub.c that is less than n.sub.f, the index of refraction of said superstrate being affected by said environment;
• means for injecting an optical beam into said waveguide layer such that at least two modes of said beam propagate in said waveguide layer;
• and
• means for detecting the interfering product of said at least two modes of said beam which have propagated through said waveguide.

Apparatus for sensing the presence of a substance in an environment, comprising:

• a pair of optical waveguide sections in said environment;
• a coating on each of said waveguide sections, each of said coatings having an index of refraction that changes when it is exposed to said substance;
• a protective covering on one of said coatings such that said one coating is protected from exposure to the environment and the other coating is exposed to said environment;
• means for injecting optical beams into said optical waveguide sections;
• and
• means responsive to the combined beam outputs for sensing said substance.