Sensor for optical characteristics of liquid using gapped waveguides: theory

摘要

A sensor for measurements of optical characteristics of liquid has been theoretically investigated. We use a gapped single mode waveguide as the sensor. A gap formed in the single mode waveguide is filled with the liquid whose optical characteristics are to be measured. A light beam gradually expands in the liquid as the beam propagates because of diffraction. An optical loss of the gapped waveguide depends on the refractive index of the liquid in the gap. That is, the optical loss becomes larger with the refractive index of the liquid as the light beam greatly expands in the media with smaller refractive index. In this work, we have derived an equation for evaluation of the optical losses by approximating the fundamental mode in the waveguide by a Gaussian beam for the two-dimensional slab waveguide. And we show the relationship between the optical loss and the refractive index of liquid filled into the gap. We have also designed an equipment to measure the refractive index of solution by using the gapped waveguide. Optimum gap width for the equipment depends on the spot size of the fundamental mode of the waveguide. For example, the optimum gap widths are evaluated as 0.5 mm for the spot sizes of 0.005 mm.