IMPROVED DETECTION METHOD FOR EVANESCENT-WAVE INTERFEROMETRIC CHEMICAL SENSING

A phase modulation system for a waveguide Mach-Zehnder interferometer sensor system has been developed which is capable of direct detection of optical phase changes smaller than 2.2 pi milliradians. When implem ented in a chemical sensor interferometer system this corresponds to a sensitivity for refractive index changes of the analyte of better tha n 10(-6), provided temperature variations are kept below 0.1 degrees C during the measuring period. The sensitivity of the instrument is dem onstrated by a stepwise addition of low concentrations of glucose in w ater. The possibility to reset the instrument by a whole number of fri nges makes it feasible to combine the high sensitivity with a virtuall y unlimited range. The accuracy of the detection system is independent of interferometer visibility changes during the measurement. This mak es it possible to perform accurate measurements in solutions with vary ing degrees of opaqueness, such as milk, blood, or wastewater. These f eatures are demonstrated with adsorption experiments of proteins to th e underlying waveguide substrate. Also, the possibility to have the vi sibility as an extra parameter during the measurements can be of impor tance for further investigation of protein adsorption processes. (C) 1 997 American Institute of Physics.