Left: optical sensor developed by UPM researchers consisting of a waveguide made of tape, a light emitting diode and light detector. Because of the flexibility of the tape, it is possible to bend it and put in a liquid for its assessment (right).
The sensor consists of a waveguide using a piece of tape where light from a LED is introduced in one of its end and the light that comes out from the other end is detected through a photodiode. The light coupling to the flexible waveguide is possible thanks to a diffractive element, using a grating with aluminum lines of nano dimensions which is added to the tape through a simple process of “tear and paste”. Both ends of the waveguide can be easily adhered to the emitter (LED) and the light detector (photodiode).
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 surrounding medium, in this case the liquid in which the waveguide is introduced. Thus, it is possible to detect variations of the refractive index of the liquid by measuring with the photodiode the optical power lost during the path of light through the immersed waveguide.
The refractive index of a liquid solution is related to its both physical and chemical properties such as density and concentration. Thus, we can assess, for example, the maturation degree of the grape by measuring the refractive index of its juice or the alcoholic content of certain beverages. In this way, the developed sensor can be applied to the food sector (process control and beverage quality) and the environmental sector (water quality control).
The materials and components used to develop this sensor are common and inexpensive. Besides, the assembly of the three main components of the sensor: waveguide, LED and photodiode, is simple and there is no need of instrumentation or specialized tools, therefore the assembly can be carried out by non-qualified personnel.
Dr. Carlos Angulo Barrios, the lead researcher for this project, “these features, along with the flexibility of the tape, make this sensor very advantageous regarding other optical instruments for the detection of refractive index more complex, rigid and expensive, especially in field applications and on-site analysis of liquids in areas of difficult access”.