New Fibers for Opto-Mechanical Sensors

Profile of an ultrasonic wave in a coated fiber: A new approach to perform opto-mechanical sensing and analysis of media that lie outside protected fibers. (Source: Diamandi, London, Bashan, Zadok)

Optical fibers enable the internet, and they are practi­cally everywhere: under­ground and beneath the oceans. Fibers can do more than just carry information: they are also fantastic sensors. Hair-thin optical fibers support measure­ments over hundreds of kilo­meters, may be embedded in almost any structure, operate in hazardous environ­ments and withstand electro-magnetic inter­ference.

Recently a major break­through in optical fiber sensors faci­litated the mapping of liquids outside the boundary of the glass fiber, even though guided light in the fiber never reaches there directly. Such seemingly para­doxical measure­ments are based on the physical principle of opto-mechanics. The propa­gation of light, in and of itself, is sufficient to induce ultrasonic waves in the optical fiber. These ultrasound waves, in turn, can probe the sur­roundings of the fiber, similar to ultrasonic imaging that is common in medical diag­nostics. The analysis of liquids outside of fibers was now reported inde­pendently by researchers from Bar-Ilan Univer­sity, Israel and EPFL, Switzerland.

The results obtained to date all suffered, however, from one major drawback: the pro­tective polymer coating of the thin glass fiber had to be removed first. Without such protective coating bare fibers of 125 micro-meters diameter do not stand much chance. One cannot consider the appli­cation of kilo­meters-long, unpro­tected optical fibers outside the research laboratory. Unfor­tunately, the standard coating of fibers is made with an inner layer of acrylic polymer that is extremely compliant. The layer completely absorbs ultra­sonic waves coming out of the optical fiber, and keeps them from reaching any media under test. The presence of coating represents one more barrier that the sensor concept must overcome.

The solution to this challenge comes in the form of a different, suitable coating. Commer­cially-available fibers can also be protected by a jacket made of polyimide. The specific material was ori­ginally proposed for protecting the fiber at high tempera­tures. However, recent studies at Bar-Ilan and EPFL have demon­strated that the polyimide coating also provides trans­mission of ultrasound. The consequences are significant: researchers at Bar-Ilan Uni­versity report that they are now able to perform opto-mecha­nical sensing and analysis of media that lie outside protected fibers, which can be deployed in proper scenarios.

“Polyimide coating lets us enjoy the best of both worlds,” says Avi Zadok from the Faculty of Engi­neering, Bar-Ilan Univer­sity. “It gives the fiber a degree of protection, alongside mechanical connec­tivity with the outside world.” Zadok and research students Hilel Hagai Diamandi, Yosef London and Gil Bashan performed a thorough analysis of light-sound inter­actions in coated fibers. The joint structure supports a host of elastic modes, which exhibit complex coupling dynamics. “Our analysis shows that the opto-mecha­nical behavior is much more complex than that of a bare fiber,” says Zadok. “The results strongly depend on sub-micron tole­rances in the thickness and geometry of the coating layer. A proper form of calibration is mandatory.”

Despite this added diffi­culty, the mapping of liquids outside coated fibers has been demon­strated experi­mentally. The group achieved sensing over 1.6 km of polyimide-coated fiber, which was immersed in water for most of its length. A 200 meter-long section, however, was kept in ethanol instead. The measure­ments distin­guish between the two liquids, and properly locate the section placed in ethanol. The results represent a major mile­stone for this up and coming sensor concept. “One possible appli­cation,” says Zadok, “is the moni­toring of irrigation. The presence of water modifies the properties of the coating. Our measure­ments protocol is able to identify such changes.” Ongoing work is dedicated to improving the range, reso­lution and precision of the measure­ments. (Source: Bar-Ilan Univ.)

Reference: H. H. Diamandi et al.: Distributed opto-mechanical analysis of liquids outside standard fibers coated with polyimide, APL Phot. 4, 016105 (2019); DOI: 10.1063/1.5067271

Link: Institute of Nano-Technology and Advanced Materials, Bar-Ilan University, Ramat-Gan, Israel

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