Integrated optical sensor for liquid control in micro-fluidic channel systems

A robust optical sensor for liquid control in fluidic channels is reported. The sensor operates on light intensity modulation resulting from alteration of total internal reflection into partial reflection. When a liquid guided in a channel covers an integrated prism, the total internal reflection is changed into a partial reflection, resulting in an intensity modulation of the reflected light. The set-up comprises a fibre which is built in a coupler unit with integrated LED and photodiode as well as a prism micro-machined directly into a micro-fluidic polymeric channel by laser ablation. The Prism is of 45-90-45 degrees type with a dimension of 0.5 mm x 1 mm x 2 mm. In this design the radiation of the LED light source is transmitted and collected from the prism by a 50:50 fibre coupler by means of total or partial internal reflection. The sensor was characterised by filling alternately the channel with water and air. The signal level for the liquid in contact with the prism was determined to be 222 mV while the signal level of the air filled channel was 336 mV. The influence of stray light onto the sensor signal was tested by applying a strong uncollimated illumination of the channel. Only a small increase in the output signal level in the presence of air but a strong increase in case of the presence of water could be detected. However, the discrimination between air and liquid was still possible sufficiently (290 mV for liquid, 340 mV for air). The sensor was also demonstrated to be operated as a micro-refractometer.