Spanish Research Promises Better Optical Sensors

[pullquote]Researchers at the Universidad Politécnica de Madrid (UPM), Spain, have developed a low-cost optical sensor that can detect the optical properties of many different liquids.

As such, they say, the sensor could eventually be used in many process applications including quality control during beverage manufacturing, environmental monitoring and field applications that call for the analysis of difficult-to-access liquids.

The sensor uses the conventional, low-cost, flexible, pressure-sensitive adhesive (PSA) tape sold in stationery shops. The researchers introduced light from an LED in one end of a piece of tape and then used a photodiode to detect light coming out of the other end. The really clever bit lies in between.

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Here, they have added a diffractive element made of nano-scale lines of aluminum and known as a nano-hole array metal grating coupler (GC). This involves a series of 1.2 mm × 1.2 mm aluminum grids — with a period of 500 nm, stripe width of 270 nm and thickness of 100 nm — first being fabricated on a polycarbonate substrate and then transferred to the tape by a simple stick-and-peel procedure. The aluminum stripes lie perpendicular to the direction of the length of the tape. This is then covered by another piece of PSA tape. Finally, the tape is cut into 2-mm wide, 100-µm thick (each piece of tape is 50-µm thick) flexible waveguides, each with an integral GC.

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Figure 1. The optical sensor developed by UPM researchers (left) consists of a waveguide made of tape, a light emitting diode and light detector. Because of the tape’s flexibility, it is possible to bend it and put in the liquid to be assessed (right). Source: Universidad Politécnica de Madrid

Because of the tape’s flexibility, 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’s possible to detect variations of the liquid’s refractive index by measuring with the photodiode the optical power lost by light going through the immersed waveguide.

According to the researchers, the tape waveguide exhibits low bending loss and bend radius, making it well suited for creating sharp curves for compactness and easy accessibility purposes. The optical system exhibits a modulation largely exceeding typical frequencies required for operating existing sensing technologies based on optical modulation. In fact, early research has shown this new optical architecture performs well compared to existing optical fiber sensor-based technologies, they say.

“The refractive index of a liquid solution is related to both its 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 in process control and beverage quality, and the environmental sector for water quality control,” says project lead Carlos Angulo Barrios. He is also a researcher with the Institute of Optoelectronics Systems and Microtechnology (ISOM) and a professor at the department of photonics technology and bioengineering (TFB) of the school of telecommunications engineering, both at UPM.

“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 [for] instrumentation or specialized tools; therefore, the assembly can be carried out by non-qualified personnel. These features, along with the flexibility of the tape, make this sensor very advantageous for the detection of refractive index when compared to other more complex, rigid and expensive optical instruments — especially in field applications and on-site analysis of liquids in areas of difficult access,” he adds.

A 2015 foundation grant for researchers from the Madrid-based bank BBVA funded the project. In 2016, the ISOM also won an award from the Massachusetts Institute of Technology, Cambridge, Mass., (MIT) international science and technology initiatives (MISTI) global seed funds worth €24.7million ($26.2 million) for a two-year project to investigate both the automatic transfer of 2D materials to flexible substrates and the benchmarking of them as chemical sensors.

The MISTI global seed funds grant program promotes and supports early-stage collaborations between MIT researchers and their counterparts around the globe. It also facilitates international faculty collaborations and develops partnerships with leading companies, research institutes and universities around the world.