Advanced photonic measurement techniques for fiber optic characterization, sensing and imaging
AffiliationElectrical and Electronic Engineering
MetadataShow full item record
Document TypePhD thesis
Access StatusOpen Access
© 2017 Dr. Yifei Wang
In the past few years, coherent detection has achieved dramatic success due to adoption of powerful electronic digital signal processing (DSP). In the meantime, direct detection (DD) based optical communication systems have also attracted attentions because of its low cost and simple implementation. The directly-detected optical channel, however, is nonlinear due to the square-law detection of the photo-diodes. In order to remove the nonlinearity, it is necessary to calibrate the response for each optoelectronic component. In the meantime, the rapid growth of the bandwidth-rich internet applications has driven the research in maximizing the capacity of optical transport. As the spectral efficiency (SE) in single-mode fiber (SMF) is ultimately limited by the fiber nonlinearity, the natural solution is to use large effective-area SMF, or even multi-mode fiber (MMF) with center launch technique to excite only the fundamental (LP01) mode to mitigate the nonlinearity penalty. Most importantly, MMF or few-mode fiber (FMF) supports many spatial modes, and therefore the fiber capacity can be increased in theory by taking advantage of this additional degree of freedom in space aided by multiple-input multiple-output (MIMO) transmission. In this thesis, we propose and demonstrate a novel digital signal processing (DSP) algorithm for photodiode characterization probed by three wavelengths. The DSP algorithm has low computational complexity and scales only with logarithmic of number of the spectral points. We verify our proposed algorithm by measuring the phase response of an electrical filter. We also characterize the frequency response for a high-speed optical receiver with 30-GHz bandwidth photodiode, and a low-speed optical receiver with 14-GHz bandwidth photodiode, respectively. For the investigation of few-mode fibers (FMFs), we present novel design and fabrication of few-mode fiber based optical sensors. Particularly, we report two FMF based discrete sensors, one is a fiber Sangac loop sensor based on polarimetric interference, and the other is a few-mode interferometer sensor based on intermodal interference. We also continue to introduce the concept of FMF based distributed sensors. We demonstrate characterization of SBS in a FMF based on Brillouin optical time domain analysis (BOTDA) technique and propose two BOTDA methods for the FMF. The proposed setup can also be used as a Brillouin distributed sensor. For the characterization of the modal propagation in few-mode fibers (FMFs), we propose and implement a novel method called complex imaging to recover both amplitude and phase images and coherently detect the in-phase and quadrature components of the heterodyne signal simultaneously with a pair of charge-coupled device (CCD) cameras. In this way, both amplitude mode pattern and phase mode pattern (or mode image) for modes transmitted in optical fibers could be characterized. In this thesis, we characterize mode patterns of pure fundamental mode and higher-order modes in FMFs as a validation of our imaging method. Then our method is also applied to mode decomposition and modal content analysis of an arbitrary mixed mode pattern. Finally, we also sweep the input wavelength and measure differential mode delay (DMD) in FMFs.
Keywordsfiber characterization; fiber optics sensors; fiber optics imaging
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