Flexible optical sensor to monitor liquid properties

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An optical sensor using a low cost and flexible conventional tape has been developed by researchers from Universidad Politécnica de Madrid. The sensor can detect variations of the optical properties of a liquid and could be used for environmental monitoring and to control the quality of beverages.

“Its simplicity, along with the flexibility of the tape, make this sensor advantageous regarding other more complex, rigid and expensive optical instruments for the detection of refractive index, especially in field applications,” said researcher Dr Carlos Angulo Barrios.

The sensor consists of a waveguide where light from a LED is introduced in one end and the light coming out the other is detected through a photodiode. The light coupling to the flexible waveguide is possible thanks to a diffractive element, using a grating with aluminium lines of nanodimensions. The ends of the waveguide can be adhered to the LED emitter and the photodiode light detector.

Because of the flexibility of the tape, the waveguide can bend and is partially immersed in the liquid under examination. Due to the waveguide bending, part of the propagated light is lost by radiation. This curvature loss depends on the optical properties, in particular the refractive index, of the liquid. It is therefore possible to detect variations of the refractive index of the liquid by measuring the optical power lost during the path of light through the immersed waveguide.