Adaptive control of modes in multi-mode fibers for imaging and communications [electronic resource]
Keywords: Electrical Engineering.

Multi-mode fibers (MMFs) can convey multiple physical variables which enables them to be used in different applications. In this dissertation, we study two specific applications of MMFs: imaging and communications. The use of MMFs for imaging or analog image transmission has long been of fundamental interest and is now being pursued vigorously for applications such as endoscopic in vivo imaging, brain studies or sugeries and cancer detection. An endoscope using one MMF would be potentially much more compact than current endoscopes, which may employ either a bundle of fibers or one fiber with a scanning probe head. The use of MMFs for space-division multiplexing (SDM) in optical communications has also gained a lot of interest recently. SDM uses different modes in MMFs as independent data channels in order to increase the link capacity and overcome the information-theoretic capacity limit of long-haul single-mode fiber systems. The properties of transmission fibers and fiber amplifiers are crucial to the ultimate feasibility of spatially multiplexed long-haul systems. In transmission fibers, low group delay spread minimizes receiver signal processing complexity, while large modal effective areas minimize nonlinear effects. In fiber amplifiers, low mode-dependent gain (MDG) minimizes the loss of capacity and the potential for outage. In the first part of this dissertation, we study imaging through a single MMF. In order to do imaging through a MMF we need to first sample an object using a set of intensity patterns at the fiber output. The image of the object can then be reconstructed using the measured optical powers that reflect from the object and couple back into the MMF. We demonstrate two methods for imaging through a MMF. In the first method a sequence of localized intensity patterns (spots) are used to sample an object at different locations and an image may be obtained using simple local reconstruction. In the second method a sequence of random intensity patterns are used to sample an object and an image may be obtained by solving a linear optimization problem. We first develop a method for synthesis of a desired intensity profile at the output of a MMF with random mode coupling by controlling the input field distribution using a spatial light modulator Abstract v (SLM). We pose the problem as optimization of an objective function, and derive a theoretical lower bound on the achievable objective function. We present an adaptive sequential coordinate ascent (SCA) algorithm for controlling the SLM, which does not require characterizing the full transfer characteristic of the MMF, and which converges to near the lower bound after one pass over the SLM blocks. We then study imaging using random patterns and optimization-based reconstruction and experimentally demonstrate endoscopic imaging through a MMF using this method. we show that when using local reconstruction, the number of independently resolvable image features is bounded by Ns, the number of spatial modes per polarization propagating in the MMF, but nonlocal reconstruction based on linear optimization can increase the number of resolvable features to 4Ns. The factor-of-four resolution enhancement is due to mixing of modes by the squaring inherent in field-to-intensity conversion. All previous methods for imaging through MMF can only resolve Ns features in the image. Most of these methods use localized intensity patterns for sampling the object and use local image reconstruction. In the second part of this dissertation, we demonstrate two adaptive methods to equalize MDG in multi-mode erbium-doped fiber amplifiers (MM-EDFAs). The first method is to place a SLM in line with the amplifier pump laser to control the modal content of the pump. The second method is to place an SLM immediately after the MMEDFA to directly control the modal gains in the signal. We compare the performance of the two methods applied to a MM-EDFA with a uniform erbium doping profile, supporting 12 signal modes in two polarizations. We show that root-mean-squared (RMS) MDGs lower than 0.5 dB and 1 dB can be achieved in systems having frequency diversity orders of 1 and 100, respectively, while causing less than a 2.6-dB loss of mode-averaged gain (MAG).