Modal Analysis of Fiber-optical Devices using Digital Holography

Digital holography is a device characterization technique that provides the full optical field, both phase and amplitude, as a function of input port and polarization. Further offline digital signal processing of these fields can provide meaningful insight in to the device-under-test. Here, digital holography is used to characterize a polarization-dependent loss emulation stage and a three-mode photonic lantern. Digital demultiplexing yields the full polarization-diverse transfer matrix from which important metrics such as mode-dependent loss and cross-talk deduced

Alignment of Free-Space Coupling of Few-Mode Fibre to Multi-Mode Fibre using Digital Holography

Off-axis digital holography is used to align a few-mode fiber to a multi-mode fiber in a free-space optical setup. Alignment based on power coupling measurements alone cannot guarantee low mode-dependent loss. The proposed alignment method enables reliable fiber coupling with low mode-dependent loss and crosstalk

Characterisation of Optical Fibers Using Dual-Comb Swept-Wavelength Interferometry

The growth in data traffic coupled with trends in internet use will result in a requirement for interfaces of the network to reach Tb/s data rate in the future. Considering this, novel transmission techniques that can increase the date rate with orders of magnitude must be considered. One such example is space-division multiplexing (SDM) fibers. Application of novel fibers and SDM components in communication systems is always coupled with limitations and distortions of the signal due to crosstalk, dispersion, differential mode group delay (DMGD) and other effects. They can be calculated, studied and partially mitigated if the transfer function of the fiber under test is known. Thus, it is essential to characterize the fiber\u27s and other component\u27s transfer matrix using fast and accurate measurement techniques. Moreover, these characterisation measurements can be used for building channel models, which can assist in simulations of the transmission and estimation of ultimate system performance.In this thesis various techniques for SDM devices characterisation are described and a novel method based on dual-comb spectroscopy and swept-wavelength interferometry is proposed and evaluated. The presented technique, dual-comb swept-wavelength interferometry (DC-SWI), is studied in terms of capabilities, advantages and limitations with application on different devices under test. This experimental scheme is also used for characterisation of a coupled-core fiber, where the transfer function and DMGD values were extracted.Furthermore, different channel models describing the properties of SDM fiber links are briefly reviewed and discussed

Turning Optical Complex Media into Universal Reconfigurable Linear Operators by Wavefront Shaping

Performing linear operations using optical devices is a crucial building block in many fields ranging from telecommunication to optical analogue computation and machine learning. For many of these applications, key requirements are robustness to fabrication inaccuracies and reconfigurability. Current designs of custom-tailored photonic devices or coherent photonic circuits only partially satisfy these needs. Here, we propose a way to perform linear operations by using complex optical media such as multimode fibers or thin scattering layers as a computational platform driven by wavefront shaping. Given a large random transmission matrix (TM) representing light propagation in such a medium, we can extract a desired smaller linear operator by finding suitable input and output projectors. We discuss fundamental upper bounds on the size of the linear transformations our approach can achieve and provide an experimental demonstration. For the latter, first we retrieve the complex medium's TM with a non-interferometric phase retrieval method. Then, we take advantage of the large number of degrees of freedom to find input wavefronts using a Spatial Light Modulator (SLM) that cause the system, composed of the SLM and the complex medium, to act as a desired complex-valued linear operator on the optical field. We experimentally build several $16\times16$ complex-valued operators, and are able to switch from one to another at will. Our technique offers the prospect of reconfigurable, robust and easy-to-fabricate linear optical analogue computation units

Novel Fibers and Components for Space Division Multiplexing Technologies

Passive devices and amplifiers for space division multiplexing are key components for future deployment of this technology and for the development of new applications exploring the spatial diversity of light. Some important devices include photonic lantern (PL) mode multiplexers supporting several modes, fan-in/fan-out (FIFO) devices for multicore fibers (MCFs), and multimode amplifiers capable of amplifying several modes with low differential modal gain penalty. All these components are required to overcome the capacity limit of single mode fiber (SMF) communication systems, driven by the growing data capacity demand. In this dissertation I propose and develop different passive components and amplifiers for space division multiplexing technologies, including PL mode multiplexers with low insertion loss and low mode dependent loss to excite different number of modes into few mode fibers (FMFs). I demonstrate a PL with a graded index core that better matches the mode profiles of a graded index FMF supporting six spatial modes with mode dependent loss (MDL) ranging from 2- to 3-dB over the entire C-band. Multicore fibers can alleviate the capacity limit of single mode fibers by placing multiple single mode cores within the same fiber cladding. However, interfacing single mode fibers to MCFs can be challenging due to physical limitations, in this dissertation I develop and fabricate different types of FIFO devices to couple light into MCFs with high efficiency and having up to 19 cores. I demonstrate high coupling efficiency with insertion loss below 0.5 dB per FIFO into a 4-core MCF and below 1 dB for a 19-core MCF. Multimode erbium doped fiber (EDF) amplifiers are required to amplify each mode within the few mode transmission fiber, the main challenge is to provide an amplifier with low differential modal gain, in this dissertation I present the first coupled-core amplifier concept compatible with FMFs. A 6-core coupled-core EDF can be spliced with low insertion and low MDL to a FMF supporting 6 spatial modes via a slight taper transition. The amplifier introduces 1.8 MDL with gain variation over the entire C-band below 1-dB