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Researchers create capsule to protect ultra-thin optical fibers

Device is designed for several types of sensors, such as those used to detect gas leaks

A housing made with polymeric material on a 3D Printer

Léo Ramos Chaves / Revista Pesquisa FAPESP

Ultra-thin optical fibers, measuring around 1 to 3 micrometers (μm) in diameter, are increasingly being used to build advanced sensors for detecting leaked gases such as acetylene and methane in industrial facilities and around gas pipelines. These compact structures are exceptionally sensitive to external environmental conditions and perform reliably even in the presence of electromagnetic disturbances, making them well-suited for sensing applications. However, handling the delicate fibers is challenging, as any moisture and dust can affect the optical signal.

A team of researchers from the Laboratory of Special Fibers & Optical Sensors (LaFE) at the University of Campinas (UNICAMP) has developed a plastic device that effectively encases and protects these ultra-thin optical fibers. “We have come up with a practical solution to a problem that research centers dealing with optical fibers have been attempting to solve for years. Our approach is simple yet highly effective,” says physicist Cristiano Monteiro de Barros Cordeiro, coordinator at LaFE.

Developed with funding from FAPESP, the device is described in detail in an article published in 2020 in Photonic Sensors. That same year, the researchers filed a patent application for the housing technology with the National Institute of Industrial Property (INPI). In 2021, the invention was included in the Patents and Software Portfolio managed by UNICAMP’s innovation agency, INOVA, making it available for commercial licensing or scientific applications.

Optical fibers are flexible filaments made of transparent materials such as glass or plastic, and are known for their remarkable capability to transmit light. Optical fiber sensors utilize variations in light propagation to measure various parameters of interest, including gas leakage. The sensing mechanism uses an optical light source, such as a laser or a light-emitting diode (LED), that injects light pulses into an optical fiber cable. On the other end, the cable is connected to a measurement device, such as a photodetector. Any detected changes in the electromagnetic wave properties of the light transmitted through the fiber indicate an anomaly.

Conventional optical fibers used in telecommunications have a diameter of 125 μm. In gas sensing applications, a section of about 1 to 2 centimeters in length is tapered to a diameter of less than 3 μm, creating a light funnel of sorts. In the narrowest region of the taper, a significant portion of the light energy propagates outside the fiber, making it sensitive to the external environment. In a sensing network, tapers are placed only at specific points that require monitoring, while regular optical fiber covers the remaining path between the light source and the photodetector.

The housing developed for the tapers is 3D printed with polymeric material (see Pesquisa FAPESP issue nº 276). The taper is encased within the housing, which is built around the fiber. “The user doesn’t come in direct contact with the ultra-thin fiber; they only handle the housing. This avoids the need for specialized training to handle the sensors,” explains physicist Jonas Henrique Osório, who was part of the team that developed the device. The housing has minute pores that allow the optical fiber to interact with the target substance while preventing the ingress of moisture or dust.

The device was specifically designed for detecting acetylene, a colorless, unstable, and highly combustible gas. The taper enables effective interaction between the waveguide and its surroundings, supporting gas detection via optical signal absorption. The researchers are currently testing the device with other gases like methane, ammonia, and carbon dioxide. They are also exploring potential applications in biological sensing, including the detection of protein molecules, DNA, and bacteria.

According to Eric Fujiwara, a professor of control and automation engineering at the UNICAMP School of Mechanical Engineering who was not involved in the research, the new device will enable fiber taper sensors — which are currently used only for gas sensing in laboratory settings — to be used in industrial applications. “The housing gives the sensor the robust construction required for industrial settings,” he says.

Sensors based on plasmon resonance and microstructured optical fibers (nº 14/50632-6); Grant Mechanism Regular Research Grant; Principal Investigator Cristiano Monteiro de Barros Cordeiro (UNICAMP); Investment R$339,961.75.
Devices and sensors based on hollow-core optical fibers (nº 21/13097-9); Grant Mechanism Postdoctoral Fellowship; Supervisor Cristiano Monteiro de Barros Cordeiro (UNICAMP); Beneficiary Jonas Henrique Osório; Investment R$223,850.88.
Anti-resonant optical fibers (nº 17/06411-3); Grant Mechanism Undergraduate Research Grant; Supervisor Cristiano Monteiro de Barros Cordeiro (UNICAMP); Beneficiary Kaleb Roncatti de Souza; Investment R$8,433.80.

Scientific article
SOUZA, K. R. et al. 3D printing technology for tapered optical fiber protection. Photonic Sensors. june 16, 2020.