Dynamical holography is an interferometric method that allows the measurements of phase modulations in the presence of environmental low-frequency fluctuations. The technique is based on the use of a nonlinear recombining medium that performs the dynamic hologram through a beam-coupling process. In our work, as the nonlinear medium, we use an optically addressed spatial light modulator operating at 1:55 μm. The beam coupling process allows obtaining a phase modulation sensitivity of 200 μrad= √Hz at 1 kHz. The interferometer behaves as an optical high pass filter, with a cutoff frequency of approximately 10 Hz, thus, filtering slow phase disturbances, such as due to temperature variations or low frequency fluctuations, and keeping the detection linear without the need of heterodyne or active stabilization. Moreover, owing to the basic principles of holography, the technique can be used with complex wavefronts, such as the speckled field reflected by a highly scattering surface or the optical field at the output of a multimode optical fiber. We demonstrate, both theoretically and experimentally, that using a multimode optical fiber as sensing element, rather than a single mode fiber, allows improving the interferometer phase sensitivity. Finally, we present a phase-OTDR optical fiber sensor architecture using the adaptive holographic interferometer.
Photo-addressed liquid crystal media allow realizing optical phase detection based on self-adaptive holographic processes. Examples are reported of adaptive holographic systems based on optically addressed liquid crystal spatial light modulators. Owing to the physical mechanisms involved, such liquid crystal based adaptive interferometers adapt to slow phase variations, thus filtering out low frequency noise while transmitting the phase modulations at higher frequencies. This method permits measuring small phase modulations in noisy environments and with distorted and speckled wavefronts. The basic principles of the adaptive interferometer are presented, then, the association with an optical fiber is shown to realize a new class of distributed optical fiber sensors.
Adaptive holography is a promising method for high sensitivity phase modulation measurements in the presence of slow perturbations from the environment. The technique is based on the use of a nonlinear recombining medium, here an optically addressed spatial light modulator specifically realized to operate at 1.55 μm. Owing to the physical mechanisms involved, the interferometer adapts to slow phase variations within a range of 5-10 Hz, thus filtering out low frequency noise while transmitting higher frequency phase modulations. We present the basic principles of the adaptive interferometer and show that it can be used in association with a sensing fiber in order to detect phase modulations. Finally, a phase-OTDR architecture using the adaptive holographic interferometer is presented and shown to allows the detection of localized perturbations along the sensing fiber.
We report on the use of an adaptive holographic interferometer, based on a liquid crystal light valve, to achieve phase shift measurements in an optical fiber. Owing to the physical mechanisms involved, the interferometer adapts itself to slow phase variations. As a consequence, it is possible to use a multimode fiber for sensing, which improves the sensitivity. Moreover, a distributed architecture relying on phase-OTDR principle is presented and a localization experiment is performed.
Adaptive holographic interferometry allows measuring small optical phase modulations even in noisy environ- ments and with strongly distorted optical wavefronts. We report examples of adaptive holographic systems based on liquid crystals, such as optically addressed liquid crystal spatial light modulator and digital holography with an LCOS spatial light modulator.
Self-adaptive interferometry allows measuring small optical phase modulations even in noisy environments and with strongly distorted optical wavefronts. We report two examples of self-adaptive interferometers based on liquid crystals, one obtained by using an optically addressed spatial light modulator, the second one realized by adopting adopting digital holography a CCD-LCOS scheme.
A liquid crystal medium is used to perform nonlinear dynamic holography and is coupled with multimode optical fibers for optical sensing applications. Thanks to the adaptive character of the nonlinear holography, and to the sensitivity of the multimode fibers, we demonstrate that the system is able to perform efficient acoustic wave detection even with noisy signals. The detection limit is estimated and multimode versus monomode optical fiber are compared. Finally, a wavelength multiplexing protocol is implemented for the spatial localization of the acoustic disturbances.
Many sensing applications would benefit of multiplexing a maximum number of Distributed FeedBack Fiber Lasers (DFB FLs) on the same optical fiber. However, in such configurations, some physical mechanisms may impact DFB FLs stable operation, limiting, for instance, the number of DFB FLs spliced on the same fiber and the distance between them. The aim of this experimental study is to investigate the impact of optical feedback on DFB FLs stability. The results of our study are used to propose possible associated architectures.
Adaptive interferometers based on dynamic holography within a nonlinear medium allow to precisely measuring phase modulations in noisy environments. Thanks to its adaptive behavior, the hologram follows slow external perturbations cancelling the low frequency phase mismatches between the two arms of the interferometer, while it appears static at high frequencies, hence, converting phase into intensity modulation. As a holographic medium, we use a liquid crystal light valve combining a photoconductor with a liquid crystal layer. The effective refractive index and, thus, the phase shift, depend both on the incident optical intensity and the bias voltage. By characterizing the response of the light valve, we show that low frequency noise can be filtered out within a voltage-controlled frequency bandwidth. This feature can be useful for applications where the signal of interest is limited by external noise such as temperature fluctuations and/or vibrations.