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Sensing & Measurement

Ferrule-top cantilevers for measurements in harsh environments

Compact, robust optical fiber sensors achieve high accuracy without alignment.
9 July 2012, SPIE Newsroom. DOI: 10.1117/2.1201206.004225

Over the last decade, optical fiber sensors (OFS) have become increasingly important measurement tools for science and industry. OFS employ an optical fiber either as a sensing element to measure temperature, strain, pressure, or other quantities, or as a means to relay a remote signal to a signal processor. OFS are small, require no electrical power at the measurement point, and can be multiplexed to record measurements over large and small distances.

Thanks to advantages such as high sensitivity and resistance to harsh environments, OFS solve many tricky metrological problems, including measurements inside oil wells and jet engines. A growing need for multiple point measurements with characteristic OFS simplicity and suitability for remote sensing networks makes OFS also very interesting for non-industrial applications. OFS are increasingly used for medical diagnoses, therapies, and health monitoring. The market for these sensors is rapidly growing, accompanied by strong support from continuous development of new light sources and measurement techniques. Now that more people rely on OFS than ever before, new ideas and optimized solutions for existing problems are needed.

Ferrule-top (FT) cantilevers1 are a new generation of all optical micromechanical sensors. They are made by carving microstructures on top of a glass sleeve, or ferrule, that terminates many optical fibers: see Figure 1(a) and (b). The structure's movement can be monitored with laser light coupled into the fiber from the opposite end. Using the same principle, it is also possible to excite vibrations in the structure using an additional light source. This unique dual-optical feature makes FT sensors ideal for applications where simplicity and compact design are important. Because of their insensitivity to high electric and magnetic fields, FT cantilevers have attracted interest as alternative tools for accelerometers and optical microphones. The geometry of the sensor can be tailored to the application: see Figure 1(c). However, in most cases the micromechanical element topping the ferrule is a simple beam clamped on one side.

Figure 1. Three-dimensional visualization of the ferrule-top cantilever sensor (a) and its optical microscope image (b). It is also possible to fabricate geometries other than a simple cantilever on the top of the ferrule. This microscope image (c) shows a torsional cantilever. Typical dimensions of the fabricated micromechanical beam are 2800×220×35μm3.

FT sensing is based on optical monitoring of the microstructure's mechanical response to stimuli. The structure is fabricated directly in front of the end-face of the fiber, and as a result, the sensor does not require any alignment, unlike most cantilever-based sensors.The bending resolution of the readout is typically subnanometer, which accommodates the demanding sensitivity requirements of applications such as atomic force microscopy (AFM) measurements.2 Ferrule-top sensors operate in two different modes: static and dynamic. One can choose the appropriate mode for the particular application or combine the two to get additional information about the measured quantity. Both the static bending and the mechanical frequency respond to changes in one or more parameters, including humidity3 (see Figure 2), velocity,4 and surface topography.2 The only limitation is that physical phenomena such as water condensation or viscosity damping can potentially influence the mechanical properties of the sensor.

Figure 2. Frequency response of ferrule-top sensor to different humidity levels. Cantilever was forced to vibrate by means of piezo excitation. au: Arbitrary units.

The FT cantilever is small, robust, and does not require any alignment, which gives the sensor an advantage in harsh environments, be they air, liquid, or vacuum. Its monolithic design and the low thermal expansion coefficient of the ferrule material minimize temperature-induced drifts of the microstructure with respect to the fiber, improving measurement accuracy in applications where temperature variations are present. This feature can be used to build transducers where high thermal stability is important. In addition, a user-friendly, handheld indenter based on FT sensors could help to improve medical diagnoses, such as rapid recognition of cancerous tissues. We are currently undertaking biomaterial indentation experiments that use FT cantilevers. Their unique combination of fully optical readout and excitation gives FT sensors the potential to become a significant sensing platform.

This work was supported by the European Research Council (ERC) under the European Commission Seventh Framework Programme (ERC grant 201739). The authors would like to thank the Dutch Foundation for Fundamental Research on Matter for its contribution to the development of ferrule-top technology.

Grzegorz Gruca, Dhwajal Chavan, Thomas van de Watering, Jan Rector, Kier Heeck, Davide Iannuzzi
Faculty of Science
VU University Amsterdam (VU)
Amsterdam, The Netherlands

Grzegorz Gruca obtained an MSc degree at the Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Poland in 2007. He has been a PhD student at VU since 2009.

Jan Hendrik Rector received his BAS from the University of Applied Sciences in Eindhoven, the Netherlands, in 1975. He joined the Philips Research Lab after graduation and researched ion implantation in semiconductor materials. He joined the Philips Telecommunication Group in 1979, where he worked on the development of optical telecommunication systems. In 1980, he joined VU, where he works on the development and research of materials.

Davide Iannuzzi is an associate professor and leads the ‘interdisciplinary engineering and applied sciences (IDEAS) at the micron scale’ group. After his PhD in physics at the University of Pavia (Italy), he worked as a postdoctoral fellow at Bell Laboratories and Harvard University, where he undertook research on the Casimir effect. He moved to Amsterdam in 2005, where he introduced ‘fiber-top technology,’ a novel approach for developing all-optical micromachined sensors fabricated on the tip of optical fibers. In 2008, he received an ERC grant for a project based on this award-winning invention. In 2011, he cofounded and became CTO of Optics11, a company that produces and distributes fiber-top probes and instruments. Iannuzzi holds five patent applications and has co-authored more than 65 publications.

1. G. Gruca, S. de Man, M. Slaman, J. H. Rector, D. Iannuzzi, Ferrule-top micromachined devices: design, fabrication, performance, Meas. Sci. Technol. 21, p. 094033, 2010. doi:10.1088/0957-0233/21/9/094033
2. D. Chavan, G. Gruca, S. de Man, M. Slaman, J. H. Rector, K. Heeck, D. Iannuzzi, Ferrule-top atomic force microscope, Rev. Sci. Instrum. 81, p. 123702, 2010. doi:10.1063/1.3516044
3. G. Gruca, J. Rector, K. Heecck, D. Iannuzzi, Optical fiber ferrule-top sensor for humidity measurements, Proc. SPIE 7753, p. 775358, 2011. doi:10.1117/12.885920
4. A. Cipullo, G. Gruca, K. Heeck, F. De Filippis, D. Iannuzzi, A. Minardo, L. Zeni, Numerical study of a ferrule-top cantilever optical fiber sensor for wind-tunnel applications and comparison with experimental results, Sens. Actuat. A: Phys. 178, p. 17-25, 2012. doi:10.1016/j.sna.2012.01.044