The tiny fiber optic force sensor can measure extremely slight forces exerted by small objects.

A tiny fiber-optic, light-based force center can measure miniscule forces to provide new insights for scientists, medical device manufacturers, and engineers who need insight into nearly imperceptible measurements.

Overcoming limitations of micro-electro-mechanical sensors (MEMS)-based force sensors could be useful for various applications, including medical systems and manufacturing.

“Applications for force sensing are numerous, but there is a lack of thoroughly miniature and versatile force sensors that can perform force measurements on small objects,” says research team leader Denis Donlagic from Slovenia’s University of Maribor. “Our sensor helps meet this need as one of the smallest and most versatile optical-fiber force sensors designed thus far.”

All-glass sensor

MEMS-based sensors can provide miniature force sensing capabilities, but they require application-specific protective packaging and multiple electrical connections, limiting applications. Without proper packaging, MEMS devices aren’t biocompatible and can’t be immersed in water.

To develop a more versatile miniature force sensor, the researchers created an all-optical fiber optic sensor made of silica glass formed into a cylinder 800µm long and 100µm in diameter – roughly the same diameter as a human hair. A special etching process the researchers had previously developed to create complicated all-fiber microstructures enabled the project. The micromachining process created a sensor based on a Fabry-Perot interferometer – an optical cavity made from two parallel reflecting surfaces.

The end of the sensor’s lead-in fiber and a thin flexible silica diaphragm were used to create the tiny interferometer. When external force is exerted onto a silica post with a round or cylindrical force sensing probe on the end, it changes the length of the interferometer in a way that can be measured with subnanometer resolution.

Researchers used the new sensor to measure the surface tension of a liquid by measuring the retraction force when a miniature cylinder was removed from water (left) and isopropyl alcohol (right).

Researchers designed and fabricated the sensor’s structures to create an air-sealed cavity, protected from contamination and amenable for biochemical environments. It can be immersed in various liquids and can measure positive and negative forces, without additional packaging for most applications.

After evaluating and calibrating the sensor, the researchers used it to measure Young modulus – a measure of stiffness – of a human hair and a common dandelion seed. They also measured surface tension of a liquid by calculating the retraction force when a miniature cylinder was removed from a liquid. The researchers were able to measure force with a resolution of about 0.6µN and a force range of about 0.6mN.

“The force sensing tip can be made substantially smaller – down to about 10µm in diameter – and can be adapted to perform various force sensing tasks,” Donlagic says.

The team says the current version of the sensor is ready for use. However, improving the overload robustness, producing probe tips with other shapes, or adding miniaturized packaging could expand potential applications. The researchers are also working to automate the processes used to fabricate the sensor to make it more practical.

University of Maribor