Near Infrared Absorption Spectroscopy in Microfluidic Devices With Selectable Pathlength

In this work we present a smart micro-opto-fluidic platform based on rectangular-section glass micro-capillaries with integrated reflectors. The micro-opto-fluidic setup was exploited for absorption spectroscopy measurements in the near infrared range from 1.0 to 1.7 μm. The external flat sides of the micro-capillaries are coated with thin Aluminum layers to create multipath micro-devices in which light can bounce multiple times. Broadband light provided by a Tungsten lamp is coupled into a multimode optical fiber and shone onto the flat surface of the micro-capillary. After travelling inside the micro-channel filled with the sample, radiation is directed towards an optical spectrum analyzer. The line-shape of the output light spectrum depends on the absorption features of the sample and, in particular, on the concentration of water in alcoholic solutions. Experimental results are in good agreement with the prediction given by the developed theoretical model that describes light transmission through the fluidic micro-channel. Moreover, a responsivity parameter, defined as the output power ratio at two wavelengths, is retrieved: water sensitivity is greatly enhanced by the multipath effect since it is proved to be directly proportional to the light bounces in the micro-channel.