Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec Hollow microsphere combined with optical harmonic Vernier effect for strain and temperature discrimination André D. Gomes a,b, ⁎ , Marta S. Ferreira a,c , Jörg Bierlich a , Jens Kobelke a , Manfred Rothhardt a , Hartmut Bartelt a , Orlando Frazão b a Leibniz Institute of Photonic Technology IPHT, Albert Einstein Str. 9, 07745 Jena, Germany b INESC TEC and Department of Physics and Astronomy, Faculty of Sciences, University of Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal c i3N and Department of Physics, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal HIGHLIGHTS • Combination of optical harmonic Vernier effect with a special strain sensor. • Complex Vernier effect case - no reference interferometer is used. • The effect allows high strain sensitivities to be achieved. • Additional demonstration of simultaneous measurement of strain and temperature. • Analysis on the performance of the optical Harmonic Vernier effect in series. ARTICLE INFO Keywords: Optical fiber sensors Hollow microsphere Harmonic Vernier effect Fabry-Perot interferometer Strain sensing ABSTRACT Achieving a new generation of enhanced sensors requires the development of structures that result from the fusion of different concepts and effects. In this paper, we combine a special strain sensing structure with an optical sensitivity magnification, through harmonics of the Vernier effect. The recently demonstrated harmonics of the Vernier effect result from increasing the optical path length (OPL) of one of two interferometers by multiple times the OPL of the other interferometer. The effect generates higher magnification factors, propor- tional to the order of the harmonics. The sensing structure is demonstrated for strain and temperature dis- crimination, allowing compensation for temperature fluctuations. We explore the complex case of the optical Vernier effect in series, where both interferometers are used as sensing interferometers (no reference inter- ferometer is used). Our results also suggest that the magnification enhancement provided by harmonics of the Vernier effect for a configuration in series is the same as for a configuration in parallel: the magnification factor increases proportionally to the order of the harmonics. 1. Introduction The catalog of interferometric optical fiber sensors is diverse, with a large variety of different configurations and new fibers coming into play. From Fabry-Perot interferometers to Michelson interferometers, each sensing structure is developed and adapted to fulfill the require- ments of a specific application. Hollow microspheres as a sensing device were first applied to strain measurement [1,2]. Although structures of this kind have been explored for many other purposes [3–9], strain sensing is still one of the most interesting applications due to the mi- crosphere properties. The sensitivity to strain changes with the di- mensions of the hollow microsphere due to mechanical effects [3,10]. Therefore, different mechanically enhanced strain sensors can be made from hollow microspheres, offering higher sensitivities than conven- tional strain sensors [3,11]. Moreover, this type of structure features a reduced temperature sensitivity, which in some cases can be negligible. Recently, researchers have been applying the Vernier effect to op- tical fiber sensors in order to enhance their sensitivity and increase the resolution of the measurements [12–16]. This effect relies on the use of two interferometers with slightly detuned interferometric frequencies, in a series configuration [17–24]. The beating generated in the inter- ference signal presents an envelope with spectral shift magnification capabilities. In the last year, the same effect was also demonstrated for a parallel configuration, with the advantage of maintaining one of the https://doi.org/10.1016/j.optlastec.2020.106198 Received 20 January 2020; Received in revised form 27 February 2020; Accepted 29 February 2020 ⁎ Corresponding author. E-mail address: andre.gomes@leibniz-ipht.de (A.D. Gomes). Optics and Laser Technology 127 (2020) 106198 Available online 07 March 2020 0030-3992/ © 2020 Elsevier Ltd. All rights reserved. T