Compensation of Multimode Fiber Dispersion Using Adaptive Optics via Convex Optimization

Abstract

In this paper, we propose a provably optimal technique for minimizing intersymbol interference (ISI) in multimode fiber (MMF) systems using adaptive optics via convex optimization. We use a spatial light modulator (SLM) to shape the spatial profile of light launched into an MMF. We derive an expression for the system impulse response in terms of the SLM reflectance and the field patterns of the MMF principal modes (PMs). Finding optimal SLM settings to minimize ISI, subject to physical constraints, is posed as an optimization problem. We observe that our problem can be cast as a second-order cone program, which is a convex optimization problem. Its global solution can, therefore, be found with minimal computational complexity, and can be implemented using fast, low-complexity adaptive algorithms. We include simulation results, which show that this technique opens up an eye pattern originally closed due to ISI. We also see that, contrary to what one might expect, the optimal SLM settings do not completely suppress higher order PMs.

Analysis of Laser and Detector Placement in Incoherent MIMO Multimode Fiber Systems

“…For instance, a weakly guided MMF with a diameter of 50 μm, nominal refractive index of 1.45, and core-cladding refractive index difference of 0.01 supports 55 guided spatial modes at 1550 nm. We restrict our consideration to linearly polarized (LP) Hermite-Gauss modes, since these modes effectively approximate propagation in weakly guiding MMFs under the infinite-core approximation [39]. The electric field is polarized in the cross section of the fiber and contains x and y components for each of the guided modes of the fiber.…”

Vector Intensity-Modulation and Channel State Feedback for Multimode Fiber Optic Links

“…Although we have conducted these experiments with DFB lasers, MIMO techniques can be applied effectively over MMFs using VCSELs, indicating that the approach is broadly applicable [50]. Mach-Zehnder modulators at C-band were used for measuring data rates achievable in these links since this provide a convenient benchmark to evaluate the utility of the techniques discussed in this paper, as in references [4], [5], [24], [51]. The maximum output power of the laser was 13 dBm, and subsequent to splitting and the modulator, the maximum output power from each modulator was about 8 dBm in the cases where the laser signal was split by a coupler (2 × 1 and 2 × 2), and about 11.5 dBm from a single modulator for the cases where a single-transmitter was used (1 × 2 and 1 × 1), since the power launched from the laser was varied over the same range in all cases to provide a fair comparison.…”

Coherent Multi Channel Transmission over Multimode-Fiber and Related Signal Processing

“…Nevertheless, selective and low-loss excitation of discrete fiber modes by a transmitter via lense coupling with phase holograms appears to be possible [12,13]. A remaining challenge is the mode separation at the receiver.…”