Detection of light backscattered from liquids by means of integrated optical waveguides

A silicon nitride based waveguide device is presented for the detection of light backscattered from liquids. The device is composed of a source waveguide and a series of collector waveguides displaced relatively to the source. The experimental result shows that detection of photons backscattered from the interior of a droplet in direct surface contact with source and detector waveguides is feasible, although the amount of light captured by the latter is extremely small. The intensity of the backscattered light measured from each of the collector waveguides is found to be in good agreement with simulation results based on a Monte-Carlo approach. Introduction We present a device for the detection of light backscattered from liquids, which is realized in silicon nitride technology. The device, shown in Fig. 1, is composed of a source waveguide and a number of collector waveguides. The collector waveguides are placed directly in contact with the sample’s surface and are used to measure the backscattered light intensity as a function of distance from the source. The spatial information retrieved through this measurement is then used to validate our numerical model based on a Monte-Carlo approach. Several devices were designed by varying parameters such as waveguide width and spacing. In this paper we present measurements carried out on one of these devices in which the distance between adjacent waveguides at the device/sample interface is approximately 2.6 μm. The waveguides, which are multimode at the excitation wavelength of 633 nm, have a width of 1.45 μm and core layer thickness of 45 nm, as can be seen in Fig. 2. Fig. 1 Multi-waveguide based collector device for backscattered light detection from liquids. The reference waveguide is not used in the measurement technique discussed in this paper. Fig. 2 – Cross-section of Si3N4 waveguides. Refractive index of thermal oxide and PECVD oxide is approximately nclad = 1.4579, while index of core is ncore= 2.0124. collector waveguides