Sensing of Multimode Optic-Fiber Dynamic Strains by Using Self-Adaptive Holographic Interferometers

Two-wave mixing (TWM) via a dynamic hologram recorded in a photorefractive crystal allows the implementation of adaptive interferometers. These interferometers can achieve high sensitivity of detection of phase shifts under unstable environmental conditions even when phase shifts are encrypted in speckled waves such as those reflected from rough surfaces or emerging from multimode optical fibers. Such abilities find potential applications in the field of vibration sensing for industrial, military and other purposes. In this thesis we present three novel configurations for adaptive interferometers which are based on the vectorial wave mixing (VWM) technique in a CdTe photorefractive crystal. The WVM is a special case of the TWM in which the mixing of light waves in the photorefractive crystal involves different polarization states of waves in order to achieve linear phase-to-intensity transformation and the highest SNR. We have developed those configurations for the detection of multimode fiber strain induced by mechanical vibrations. Two of these configurations use the reflection and orthogonal geometry of hologram recording respectively and each of them employs a single multimode optical fiber as a sensitive element. It is shown that the reflection geometry of hologram recording is characterized by a better combination of detection limit and adaptability than the transmission geometry which is more commonly used. We have found that the installation of a polarizer, which is needed in any classical configuration of TWM, results in the appearance of a strong noise. Such a noise, which we term polarization noise, relates to the polarization instability of the dynamic speckle pattern emerging from the fiber, which cannot be compensated for by the dynamic hologram. However, it has been demonstrated that this noise can be diminished by using an optical fiber with a larger core. The orthogonal geometry of hologram recording allows the linear regime of the phase-to-intensity transformation even when phase shifts are encrypted in a depolarized wave without using any polarizing element. Using a depolarized beam without a polarizing element strongly diminishes the level of the noise related to instabilities in the speckle pattern emerging from multimode optical fiber, thus increasing the sensitivity to phase transients. The third configuration represents a multiplexing scheme for sensing dynamic strains excited in different multimode optical fibers. The multiplexing of the sensors is implemented by using the vectorial wave mixing technique in the reflection geometry of hologram recording. For this configuration it is shown that we can minimize the crosstalk between the channels by choosing the appropriate crystallographic orientation that the interfering beams travels through. Moreover, we analyzed the physical mechanisms which lead to the appearance of the noise between the measuring channels in this configuration. PACS Classification: 42.25.Hz, 42.40.Pa, 42.40.Ht, 42.40.Kw, 42.65.Hw, 42.81.Pa, 42.40.Lx, 46.40.-f Universal Decimal Classification: 531.715, 534.1, 535.41, 681.7.068, 778.38 INSPEC Thesaurus: light interference; holography; interferometers; interferometry; holographic interferometry; photorefractive effect; photorefractive materials; crystals; light polarisation; multiwave mixing; speckle; optical fibres; fibre optic sensors; strain sensors; vibrations; optical noise; optical crosstalk 6