A low-NA Yb-doped LMA gain fiber is fabricated to increase bend insensitivity and is presented for the first time. The design study shows that incorporating segmented-trenches decrease the low-loss bend diameter by up to 30%.

Hollow core fibers have been investigated for several use cases relating to both single mode and multi-mode operation. Single mode, low-loss operation is a desired commodity in telecommunications and high-power delivery applications. Hollow core fibers can be designed with a structure that guides multiple modes in the core at low loss while also exhibiting strong stress sensitivity. In these anti-resonant hollow core fibers, perturbations to the structure such as micro-bending can efficiently couple core guided modes in short sections of fiber. This high sensitivity to structural distortion can be exploited for higher order mode generation, sensing, and for developing multimode nonlinear light sources. This work presents an investigation on using anti-resonant hollow-core fibers as a higher-order mode converter by inducing mechanical stress on the structure of the fiber.

In this work, we demonstrate a four-core multicore fiber photonic lantern tip/tilt wavefront sensor. To diagnose the low-order Zernike aberrations, we exploit the ability of the photonic lantern to encode the characteristics of a complex incoming beam at the multimode facet of the sensor to intensity distributions at the multicore fiber output. Here, we provide a comprehensive numerical analysis capable of predicting the performance of fabricated devices and experimentally demonstrate the concept. Two receiver architectures are implemented to discern tip/tilt information by (i) imaging the four-core fiber facet on a 2D detector and (ii) direct power measurement of the single mode outputs using a multicore fiber multiplexer and photodetectors. For both receiver schemes, an angular detection window of ∼ 0.4 ∘ at 1064 nm can be achieved. Our results are expected to further facilitate the development of intensity-based fiber wavefront sensors for adaptive optics systems.

We report recent advances in the fabrication of multicore and multimode fibers, amplifiers, and devices for space division multiplexing. Low-loss and low-crosstalk fibers and devices can be achieved by carefully optimizing the fabrication processes.

We present a detailed investigation on higher order mode suppression due to differential gain in large mode area step index fiber amplifiers with confined Yb doping using spatially and spectrally resolved imaging (S2). A novel active fiber with Yb doping confined to the central 30% of the core area is fabricated and its performance is directly compared to a fiber with a conventional homogeneously doped core with almost identical parameters. At high pump rates, S2 and beam pointing stability measurements clearly demonstrate fundamental mode operation of the confined doping few mode fiber, even under imperfect launching conditions and environmental perturbations. In addition, we discuss the mode content as a function of gain in co-pumped fiber amplifiers with and without confined rare earth core doping using a power propagation model for fibers with similar parameters to those used in our experiments. Our simulation results as well as amplification experiments indicate the great potential of the confined doping concept for single mode high power operation.

High-power fiber laser systems enjoy a widespread use in manufacturing, medical, and defense applications as well as scientific research, due to their remarkable power scalability, high electrical to optical efficiency, compactness and ruggedness. However, single-mode fiber power scaling has stagnated in the past years, primarily due to the onset of nonlinear effects such as stimulated Brillouin/Raman scattering and transverse modal instabilities. This thesis addresses the analysis and mitigation of transverse modal instabilities in high-power fiber amplifiers. I describe the high-power fiber amplifier testbed that I set up to test fibers fabricated in house. I will show our results of a Yb-doped fiber amplifier with more than 2.2 kW signal power and beam quality of 1.1 M2. In consequence, I demonstrate mode-selective amplification in a large mode-area Yb-doped fiber using a 3-mode photonic lantern. All three modes were amplified to above 4 W with OSNRs higher than 16 dB. In addition, I show a novel high-speed beam analysis technique to study transverse modal instabilities. To guide fiber designs, I developed a GPU accelerated simulation suite to study the dynamics that occur in high-power fiber amplifiers. A 64× 64 spatial grid, with 6000 timeand 20000 distance-steps can be solved at 12 min ·m−1 on a GeForce GTX 1080 Ti. Based on these simulations, I will show dynamic transverse modal instability mitigation strategies that rely on mode modulation.

We present the assembly and characterization of a multimode EDFA supporting up to 45 modes using digital holography to measure the transfer matrix of the system at each step to obtain mode dependent loss and crosstalk characteristics of the amplifier.

We investigate the vibration sensitivity of spatially multiplexed links by measuring the mode-coupling dynamics of a four-core coupled-core fiber and a reference single-mode fiber. We show that the speed of dynamics increases with mode count.

We present a characterization technique for coupled space-division multiplexed (SDM) amplifiers, that can measure the complex transfer matrix over the whole wavelength range of interest of an amplifier under nominal input spectrum and power condition. The measurements are essential for the performance estimation of an SDM system.

We present latest advances in multimode fibers and components for mode-multiplexed transmission. In particular we will review large mode count mode-multiplexer and characterization techniques for multimode components and provide a summary of the latest transmission results. © 2019 The Author(s)

We show mode-multiplexed transmission over individual mode groups up to 9 groups of a 27 km long graded-index multimode fiber. We also investigate transmission distances up to 500 km using a recirculating loop arrangement for the first 6 mode groups using QPSK and 16-QAM signals.

We show combined SDM/WDM transmission over a novel 7-core coupled-core fiber over distances up to 12100 km for QPSK and 4400 km for 16QAM signals, with corresponding spectral efficiency of 21 bit/s/Hz and 42 bit/s/Hz. The measurements show that such a 7-core fiber clearly outperforms an equivalent single-mode fiber resulting in more than 7 times the capacity.

We quantify for the first time higher-order-mode content as a function of gain in large-mode-area fiber with confined Yb-doping using spatially and spectrally resolved imaging. Our results clearly indicate higher-order-mode suppression due to differential gain. © 2019 The Author(s)

We demonstrate mode-multiplexed 90×90 MIMO transmission over all nine mode groups of a 26.5-km graded-index multimode fiber span, achieving a record spectral efficiency of 202 bit/s/Hz.

We demonstrate a nonmagnetic topological insulator laser exhibiting topologically-protected transport. The topological properties of the system give rise to single mode lasing, robustness against fabrication defects, and higher slope efficiencies compared to its trivial counterpart.

At resonance, mircoring resonators tend to support two counter-propagating degenerate modes. By incorporating S-bend chiral elements in each resonator, unidirectional single mode lasing below and above PT-symmetry breaking point is experimentally demonstrated.

We report the first observation of complex lasing transitions in a topological 1D Su-Schrieffer-Heeger active array. The effect of gain saturation nonlinearities and carrier dynamics on the edge-state is investigated both theoretically and experimentally.

We describe the recent progress on topological insulator lasers, where we employed the fundamental concepts of topological insulators in the non-Hermitian laser system, to force an array of many semiconductor (InGaAs) resonators to lock together and act as one coherent laser source.

We show mode-multiplexed transmission over the 6 th mode group of a 50-μm graded-index multimode fiber using a multi-plane light conversion device, for transmission distances up to 90 km.

We report a 6-core coupled-core fiber amplifier compatible with 6-mode FMF. The coupled-core amplifier not only reduces mode dependent gain but also introduces strong random mode mixing, thus easing mode dependent impairments in FMF transmission systems.