Fiber specklegram sensors (FSS) for measuring high frequency mechanical perturbations

Abstract

In this work, a technique for measuring high frequency micro-vibrations by using non holographic fiber specklegram sensor is experimentally demonstrated. In our setup, a laser source emitting at 632nm is coupled to a structure of Singlemode-Multimode-Singlemode fibers which produce a filtering effect that is used as optical transducer. Mechanical perturbations of controlled amplitude and frequency are applied to the Multimode fiber, which is fastened at the ends to induce vibrations. Perturbations higher than 3 KHz and below one micron are perfectly recovered by the system, which additionally, exhibits a linear response. Due to the low cost and the simplicity of the technique, it becomes an interesting method for the implementation of fiber sensors in a wide range of engineering applications.

Numerical modeling of fiber specklegram sensors by using finite element method (FEM)

“…These results are in agreement and are complementary to the experimental ones previously shown in [24,25,36]. From Fig.…”

“…5 and 6 represent the area subscribed by the filtering fiber, located at the end of the sensing multimode fiber, in a typical arrangement of PFSS. A strong influence between the core diameter of the filtering optical fiber d , the grain speckle size D and the performance of the sensor has been experimentally observed and reported in [24,25]. In turn, controlling the laser wavelength and the numerical aperture of the multimode fiber can modify the fiber speckle size.…”

“…Among the most interesting FSSs are those based on fiber structures (single-multi-single mode fibers), because they allow an interrogation by optical power variation. These systems are known as Power Fiber Specklegram Sensor (PFSS) [19][20][21][22][23][24][25][26]. As in any arrangement of FSS, in the case of PFSS the speckle pattern generated at the end of the SMMF is modified when an external perturbation is applied to the multi-mode fiber, therefore, when a SSMF is spliced to the multimode one, a filtering effect that translates the changes of the speckle pattern into optical power changes is reached.…”

“…This element facilitates the interrogation of PFSSs and, therefore, offers many advantages over holographic and non-holographic image-based processing FSSs [24]. The main advantages are: very low response time, high stability, low cost, and easy implementation [25]. Likewise, PFSSs have been demonstrated in the measuring of either static or high frequency mechanical perturbations [23,24], in the determination of physical liquid properties [21], and, most recently, in configurations of fiber tapers for the determination of chemical characteristics of materials [26].…”

“…on the metrological performance of the sensors [19,20]. In this sense, we experimentally demonstrated that a very important parameter in the performance of FSS is the fiber speckle grain size [24,25]. However, although experimental advances in the implementation and characterization of FSS have been reported, a suitable model to interpret the speckle-pattern variation under perturbation is desirable but very challenging to be developed due to the various factors influencing the speckle pattern [27].…”