Minimizing differential modal gain in cladding-pumped EDFAs supporting four and six mode groups.

We employ a Genetic Algorithm for the purpose of minimization of the maximum differential modal gain (DMG) over all the supported signal modes (at the same wavelength) of cladding-pumped four-mode and six-mode-group EDFAs. The optimal EDFA designs found through the algorithm provide less than 1 dB DMG across the C-band (1530-1565 nm) whilst achieving more than 20 dB gain per mode. We then analyze the sensitivity of the DMG to small variations from the optimal value of the erbium doping concentration and the structural parameters, and estimate the fabrication tolerance for reliable amplifier performance.

Bending performance of large mode area multi-trench fibers.

Bending performance of the Multi-trench Fibers (MTFs) has been investigated using the Finite Element Method. Numerical investigations show that MTFs can provide low-loss effective single mode operation under bent configuration, thanks to the resonant coupling of the Higher order Modes (HOMs). Large ratio between the HOMs and the Fundamental Mode (FM) losses can be ensured, although the ratio drops with increasing Effective Area (A(eff)) of the FM. MTFs provide better losses ratio between the HOMs and the FM in comparison with other fibers like step-index, W-type, and parabolic fibers.

Mode area scaling with multi-trench rod-type fibers.

We propose a novel all-solid rod-type fiber structure that presents a cylindrical symmetry and low refractive-index contrasts. Effectively single-mode propagation for the fundamental mode is ensured thanks to resonant couplings between Higher Order Modes (HOMs) and cladding modes. Numerical simulations demonstrate the possibility of achieving a fundamental mode effective area as large as 5000 µm² at a wavelength of 1.06 µm in fibers ensuring a high leakage loss ratio (>100) between the HOMs and the fundamental mode while keeping the fundamental mode leakage losses at a level lower than 0.2dB/m. Further scaling to an effective area of 12,200 µm2 at 1.06 µm in an effectively single-mode fiber is also presented by exploiting the power delocalization of several HOMs on top of the high-leakage loss filtering.

Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping and a simple LP01 pump configuration.

We experimentally validate a numerical model to study multimode erbium-doped fiber amplifiers (MM-EDFAs). Using this model, we demonstrate the improved performance achievable in a step index MM-EDFA incorporating a localized erbium doped ring and its potential for Space Division Multiplexed (SDM) transmission. Using a pure LP01 pump beam, which greatly simplifies amplifier construction, accurate modal gain control can be achieved by carefully tuning the thickness of the ring-doped layer in the active fiber and the pump power. In particular, by optimizing the erbium-ring-doped structure and the length of active fiber used, over 20dB gain for both LP01 and LP11 signals with a maximum gain difference of around 2 dB across the C band are predicted for a pure LP01 pump beam delivering 250 mW power at 980 nm.

Microbending behavior of large-effective-area Bragg fibers.

We use a phase-sensitive optical low-coherence interferometry technique and camera imaging to identify microbend-induced mode couplings in Bragg fibers. Results show the importance of the quality of fabrication of Bragg fibers on their modal content when they are subject to microbends. Design strategies to improve the microbending robustness are identified.