High-sensitivity microfiber strain and force sensors
Introduction
Dynamic strain or force sensing is of most importance for applications ranging from earthquake and crack opening displacement monitoring to nondestructive evaluation of buildings, bridges and many other mechanical structures [1]. Compared to mechanical and electrical approaches, optical measurements show great advantages, including immunity to electromagnetic interference, convenient operation, compactness, distributed remote monitoring and high sensitivity, among which fiber-based or in-fiber sensing is one of the most important and successful schemes [2], [3], [4], [5], [6]. The rapid development of micro/nanotechnology, as well as the increasing demands on strain or force sensing with smaller footprints and lower detection limits, have spurred great efforts for realizing miniaturized all-optical in-fiber sensors with high sensitivity. Owing to their smaller sizes, higher sensitivity to environmental changes and higher flexibility, optical microfibers tapered from standard optical fibers have been attracting increasing attentions for optical sensing in recent years [7], [8]. While most of these sensors are demonstrated for refractive index [9], [10], [11], [12], [13], concentration [14], humidity [15] or temperature sensing [16], [17], microfiber-based strain or force sensors have not been adequately investigated. Recently, based on the multimode interference in a 8-μm-diameter microfiber, Muhammad et al. reported a strain sensor with an elongation sensitivity of 4.2 pm/μm, demonstrating the possibility of strain sensing using a single microfiber [18]. Generally, the multimode interference feature critically depends on the diameter, geometric profile and effective length of the microfiber, the sensitivity of such a sensor can be enhanced by optimizing the geometry of the microfiber. Here we experimentally demonstrate compact in-fiber strain or force sensors based on optimized biconical microfibers. Compared to the previous work [18], we use microfibers with smaller diameters and shorter tapering lengths, and obtain strain sensitivity up to 4.84 pm/με in a 2.5-μm-diameter microfiber, corresponding to an elongation sensitivity of 404 pm/μm, which is about two orders of magnitude higher than that of the previous report [18].
Section snippets
Experiments and results
Using a flame-heated fiber-tapering system, we first taper a telecom single-mode optical fiber (SMF 28, Corning Inc.) down to a microfiber with biconical geometry. With a tapering speed of 0.3 mm/s and optimized flame size, the minimum diameter at the waist of the microfiber taper is mainly determined by the tapering length, and the geometric profile of the microfibers can be obtained with good repeatability. With taper length from 10 to 16 mm used in this work, the total transmittance of the
Conclusions
In summary, we experimentally demonstrate microfiber strain and force sensors with high sensitivity and low detection limit. Typical elongation and strain sensitivity for a 2.5-μm-diameter microfiber are 4.84 pm/με and 404 pm/μm, respectively. With a 1.5-μW probing light, the sensor shows a force detecting limit down to 50 μN. To avoid the surface contamination and air disturbance influence for practical applications, certain protective structures such as glass tubes, low-index polymers [25] or
Acknowledgments
This work is supported by the National Basic Research Program of China (No. 2013CB328703), the Natural Science Foundation of Zhejiang Province, China (No.Y6110391), and Fundamental Research Funds for the Central Universities.
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