Effects of the speckle size on non-holographic fiber specklegram sensors Nelson Darío Gómez a , Jorge A. Gómez b,n a Grupo de Investigación en Máquinas Inteligentes y Reconocimiento de Patrones—MIRP, Centro de Investigación, Instituto Tecnológico Metropolitano, Medellín, Colombia b Grupo de Física Básica y Aplicada, Politécnico Colombiano Jaime Isaza Cadavid, Medellín, Colombia article info Article history: Received 14 October 2012 Received in revised form 13 April 2013 Accepted 8 May 2013 Available online 29 May 2013 Keywords: Sensing Speckle Optical fiber Mechanical perturbations. abstract In this work, a technique to improve the dynamical behavior of a non holographic fiber specklegram sensor for measuring high frequency mechanical perturbations is experimentally demonstrated. In these arrangements, the speckle pattern produced by a multimode optical fiber is coupled to an optical fiber of low numerical aperture, which produces a filtering effect that can be used as optical transduction mechanism. In our proposal, the influence of the speckle size on the sensor performance is evaluated by changes in the numerical aperture of the multimode fiber. A strong effect of the near field fiber speckle size on the reproducibility of the mechanical perturbation is found. The behavior of the reproducibility is evaluated by the calculation of the mean of the variance and the correlation between the mechanical and optical normalized signals. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction. Between the most known optical metrology methodologies are those based on “speckle” patterns [1–3]. A “speckle” distribution appears when laser radiation interacts with surfaces whose rough- ness can be compared with the wavelength of light and the effect is due to interference of several waves with statistical phase distribution. In the early years of the laser as an instrumental tool, the “speckle” was considered as an undesired optical noise, especially in holographic applications. However, since the 70's, many experimentalists have used this phenomenon to determinate a wide variety of physical parameters by interferometric measures. On the other hand, when laser radiation is launched in a multimode optical fiber, at the output end of the fiber appears a complex speckle pattern which is known as modal noise in optical communication systems and, it is an undesired effect in data transmission [5]. Fiber speckle patterns contain information of the spatial state of the optical fiber, which can be used in metrological applications. In multimode optical fibers, the speckle phenomenon can be explained by the coupled mode theory in terms of the relative phases of the optical propagation modes traveling throughout the fiber. Small perturbations on the multimode optical fiber will produce changes in the spatial distribution of the speckle pattern which can be detected by optical correlation techniques or by intensity changes [6–7]. Sensing systems based on fiber speckle are known as Fiber specklegram Sensors (FSS). Holographic and non holographic fiber specklegram sensors for measuring several engineering parameters have been reported formerly [8–17]. Holographic techniques have usually been imple- mented with photorefractive crystals by the implementation of optical correlation operations [8–11]. Recently, one of us reported detection systems developed on BSO photorefractive crystals based on fringe analysis and self correlation functions and demon- strated the possibility of tuning the dynamic range of this kind of systems simply changing geometrical characteristics of the speckle grain [18–20]. In non holographic techniques two possibilities have been reported. In the first case, image processing is used to calculate the normalized inner product (NIP) of speckle patterns captured by a CCD camera at the output end of the multimode fiber [12–14]. In the second case, experimental arrangements of multimode–singlemode fibers have been used to generate and filtering a fiber speckle pattern. In this way, it is possible to have configurations of FSS interrogated by optical power variation. FSS interrogated by optical power variation, exhibit many advantages over holographic FSS and non holographic based on image processing FSS [15–17]. Between the main advantages of non holographic FSS based on optical power detection are: the very low response time, the stability and the low cost and easy implementation [21]. In this work, we explore a FSS based on optical power detection for monitoring high frequency mechanical disturbances and, unlike previous reports, we demonstrate that Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.05.007 n Corresponding author. Tel.: +57 3148145516; +57 4 3197900 x 439; fax: +57 4 4118779. E-mail addresses: jagomez@elpoli.edu.co, jogolo@une.net.co (J.A. Gómez). Optics and Lasers in Engineering 51 (2013) 1291–1295