Analysis of the sensitivity of optical waveguide chemical sensors for TM modes by the group-index method

The sensitivity of optical waveguide chemical sensors for TM modes is investigated by use of the group-index method [J. Lightwave Technol.14, 1907 (1996)]. We compare the absorption coefficients of TM modes with those of TE modes to determine in which mode the sensor should work. For open-clad-type evanescent-wave sensors, which mode is more favorable depends on whether the refractive index of the measurand is smaller or larger than that of the substrate; for buffered-clad-type evanescent-wave sensors, the choice depends on the function of the buffer layer; for sensing-layer-type evanescent-wave sensors, the TE mode has greater sensitivity than the TM mode; and for guided-wave sensors, both TE and TM modes are suitable for use in highly sensitive sensors.