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

Gain equalization for a few-mode erbium-doped fiber amplifier supporting eight spatial modes

A trench-assisted ring few-mode erbium-doped fiber amplifier (FM-EDFA) supporting eight spatial modes is designed and proposed in this work. The gain equalization for the FM-EDFA is achieved by selecting the appropriate doping radius and concentration using a particle swarm optimization (PSO) algorithm when only the pump in the fundamental mode (L P 01) is applied. When the signals in the eight spatial modes are simultaneously amplified, the average modal gain is about 20 dB, and the DMG is less than 0.3 dB for a signal at 1550 nm. Considering the gain competition of six wavelength signals, the modal gain and DMG are more than 20 and 1 dB, respectively. In addition, the tolerance analysis for manufacturing with this design is also discussed. For a fluctuation in the refractive index, the average modal gain is about 19.5 dB, and the DMG is 0.77 dB, indicating that the structure has good fabrication tolerance.

Optimization criteria and design of few-mode Erbium-doped fibers for cladding-pumped amplifiers

We propose a novel optimization method that combines two design criteria to reduce the differential modal gain (DMG) in few-mode cladding-pumped erbium-doped fiber amplifiers (FM-EDFAs). In addition to the standard criterion that considers the mode intensity and dopant profile overlap, we introduce a second criterion that ensures that all doped regions have the same saturation behavior. With these two criteria, we define a figure-of-merit (FOM) that allows the design of FM-EDFAs with low DMG without high computational cost. We illustrate this method with the design of six-mode erbium-doped fibers (EDFs) for amplification over the C-Band targeting designs that are compatible with standard fabrication processes. The fibers have either a step-index or a staircase refractive index profile (RIP), with two ring-shaped erbium-doped regions in the core. With a staircase RIP, a fiber length of 29 m and 20 W of pump power injected in the cladding, our best design leads to a minimum gain of 22.6 dB while maintaining a DMG_max under 0.18 dB. We further show that the FOM optimization achieves a robust design with low DMG over a wide range of variations in signal power, pump power and fiber length.

Low-noise-figure and high-purity 10 vortex modes amplifier based on configurable pump modes

We have explored an orbital angular momentum (OAM) amplifier of 10 vortex modes under different-order OAM pump modes, i.e. OAM₀, OAM₁, and OAM₂. The all-fiber amplification system consists of an active few-mode erbium-doped fiber (FM-EDF), a mode selective pump (MSP), and a mode selective signal (MSS). These mode selective components are based on fused-taper mode selective couplers (MSC) under different wavelengths fabricated by a passive ring-core fiber (RCF). Under different-order mode pumps, the OAM amplifier experimentally exhibits mode gains (MGs) above 15 dB for 10 vortex modes with the mode purities only 89%, essentially in line with the simulation results. Especially when the signal-mode profiles are better matched to the pump-mode profiles, i.e. the OAM pumps with the same order as signals, the obtained MGs are all over 20.2 dB and the amplified OAM mode purity is up to 97%; the acquired noise figures (NFs) are <4.9 dB and even the minimum NF is 3.2 dB. The results reveal that the OAM amplifier shows low-NF and high-purity characteristics under configurable pump modes in C-band. The amplified high-order OAM mode could be promising for uses in the long-distance mode division multiplexing (MDM) and in mitigation of the upcoming capacity crunch in optical fiber communication.

Amplifying Orbital Angular Momentum Modes in Ring-Core Erbium-Doped Fiber

Lots of research efforts have been devoted to increase the transmission capacity in optical communications using orbital angular momentum (OAM) multiplexing. To enable long-haul OAM mode transmission, an in-line OAM fiber amplifier is desired. A ring-core fiber (RCF) is considered to be a preferable design for stable OAM mode propagation in the fiber. Here, we demonstrate an OAM fiber amplifier based on a fabricated ring-core erbium-doped fiber (RC-EDF). We characterize the performance of the RC-EDF-assisted OAM fiber amplifier and demonstrate its use in OAM multiplexing communications with OAM modes carrying quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM) signals. The amplification of two OAM modes over four wavelengths is demonstrated in a data-carrying OAM-division multiplexing and wavelength-division multiplexing system. The obtained results show favorable performance of the RC-EDF-assisted OAM fiber amplifier. These demonstrations may open up new perspectives for long-haul transmission in capacity scaling fiber-optic communications employing OAM modes.

Amplification of 18 OAM modes in a ring-core erbium-doped fiber with low differential modal gain

We theoretically propose two orbital angular momentum (OAM) erbium-doped ring-core fiber (RCF) amplifiers capable of providing relatively uniform gain for 22 modes with 18 OAM ones over the C-band. Two schemes of doping profile are discussed, one with single layer erbium doping and the other with double layer erbium doping. Theoretical analyses and numerical simulations suggest that the proposed first OAM erbium-doped fiber amplifier (OAM-EDFA) can obtain a gain larger than 20 dB for all 22 modes with differential modal gain (DMG) lower than 0.71 dB. The second OAM-EDFA performs better and can provide a larger gain over 21.5 dB for all 22 modes, with a smaller DMG below 0.27 dB and the noise figure (NF) lower than 3.9 dB over the whole C band. The presented OAM-EDFA may open up new perspectives for long-distance transmission in capacity scaling fiber-optic communications using OAM modes.

Non-destructive characterization of rare-earth-doped optical fiber preforms

We present a non-destructive optical technique for rare-earth-doped optical fiber preform inspection, which combines luminescence spectroscopy measurements, analyzed through an optical tomography technique, and ray-deflection measurements for calculating the refractive-index profile (RIP) of the sample. We demonstrate the technique on an optical fiber preform sample with a Yb³⁺-doped aluminosilicate core. The spatial distribution of the photoluminescence signals originating from Yb³⁺-single ions and from Yb³⁺-Yb³⁺ cluster sites were obtained inside the core. By modifying the characterization system, we were able to concurrently evaluate the RIP of the core and, thus, establish with good accuracy the dopant distribution within the core region. This technique will be useful for quality evaluation and optimization of optical fiber preforms.

Demonstration of an erbium-doped fiber with annular doping for low gain compression in cladding-pumped amplifiers

We present the design and characterization of a cladding-pumped amplifier with erbium doping located in an annular region near the core. This erbium-doped fiber is proposed to reduce gain saturation, leading to smaller gain compression when compared to uniform core doping. Through numerical simulations, we first compare the performance of three fibers with different erbium doping profiles in the core or the cladding. When the doped fibers are operated at the optimum length, results show that the smaller overlap of the signal mode field with the annular erbium doping region leads to higher gain and lower saturation of the amplifier. A single-core erbium-doped fiber with an annular doping and a D-shaped cladding was fabricated. Measurements demonstrate less than 4 dB of gain compression over the C-band for input power ranging from -40 dBm to 3 dBm. Small gain compression EDFAs are of interest for applications that require input channel reconfiguration. Higher gain and saturation output power are also key issues in cladding-pumped multi-core amplifiers.

Broadband and low-loss mode scramblers using CO2-laser inscribed long-period gratings

We demonstrate broadband and low-loss three-mode and six-mode scramblers employing CO₂-laser inscribed long-period gratings (LPGs) for space-division multiplexing. Step-index (SI) few-mode fibers are used to avoid mode coupling to the cladding modes. We characterize the mode scramblers using a swept-wavelength interferometer. Mode-dependent loss (MDL) and modal transfer matrices over the C+L band are presented. Demonstrated LPGs with negligible MDL and low insertion loss contributed to high-performance CO₂-laser inscription. The total MDLs induced by the SI fiber with LPGs in three-mode and six-mode scramblers are measured to be 2 and 4 dB, respectively.

21 spatial mode erbium-doped fiber amplifier for mode division multiplexing transmission

We experimentally demonstrate a 12-mode group (21 spatial mode) cladding-pumped few-mode erbium-doped fiber amplifier. The differential modal gain (DMG) is dramatically reduced by using the double cladding erbium-doped fiber (DC-EDF) with a refractive index trench structure, which helps to tightly confine all 21 spatial modes in the core and to mitigate the bending loss. Our experimental results show that the DMG gain is about 3 dB when the average signal modal gain for all 12 mode groups is up to 15 dB across the C-band. Our method of using a DC-EDF with a properly designed trench structure can be used to develop gain equalized few-mode amplifiers supporting more spatial modes.

Large-bandwidth, low-loss, efficient mode mixing using long-period mechanical gratings

We propose a new architecture for using long-period fiber gratings (LPGs) to induce strong mode mixing with low loss for space-division multiplexing. In this architecture, LPGs are installed in step-index (SI) few-mode fibers that support more modes than the transmission fiber. Such a design could significantly reduce losses due to coupling from the highest-order mode group to cladding modes. In our experiment, efficient mixing of three spatial modes over a broad bandwidth was achieved by a mechanical long-period grating on a SI fiber that supports eight spatial modes. The insertion loss, including two splice losses, is less than 0.5 dB, and the coupling matrix and mode-dependent loss (MDL) are characterized experimentally for the first time, to the best of our knowledge. Strong mixing between LP₀₁ and LP₁₁ for a whole C band is demonstrated, and MDL introduced to the system is negligible.

Few-mode erbium-doped fiber amplifier with photonic lantern for pump spatial mode control

We demonstrate a few-mode erbium-doped fiber amplifier employing a mode-selective photonic lantern for controlling the modal content of the pump light. Amplification of six spatial modes in a 5 m long erbium-doped fiber to ∼6.2  dBm average power is obtained while maintaining high modal fidelity. Through mode-selective forward pumping of the two degenerate LP₂₁ modes operating at 976 nm, differential modal gains of <1  dB between all modes and signal gains of ∼16  dB at 1550 nm are achieved. In addition, low differential modal gain for near-full C-band operation is demonstrated.

2-LP mode few-mode fiber amplifier employing ring-core erbium-doped fiber

A fiber amplifier supporting 2 LP modes that employs a ring-core erbium-doped fiber (RC-EDF) is investigated to reduce differential modal gain (DMG). The inner and outer radii of the ring-core of the RC-EDF are clarified for 2-LP mode operation of the amplifier, and are optimized to reduce the DMG. It is shown that using the overlap integral between the erbium-doped core area and the signal power mode distribution is a good way to optimize the inner and outer radii of the ring-core of the RC-EDF and thus minimize the DMG. A fabricated RC-EDF and a constructed 2-LP mode EDFA are described and a small DMG of around 1 dB is realized for LP01, LP11 and LP21 pumping.

Variable optical attenuator and dynamic mode group equalizer for few mode fibers

Variable optical attenuation (VOA) for three-mode fiber is experimentally presented, utilizing an amplitude spatial light modulator (SLM), achieving up to -28dB uniform attenuation for all modes. Using the ability to spatially vary the attenuation distribution with the SLM, we also achieve up to 10dB differential attenuation between the fiber's two supported mode group (LP₀₁ and LP₁₁). The spatially selective attenuation serves as the basis of a dynamic mode-group equalizer (DME), potentially gain-balancing mode dependent optical amplification. We extend the experimental three mode DME functionality with a performance analysis of a fiber supporting 6 spatial modes in four mode groups. The spatial modes' distribution and overlap limit the available dynamic range and performance of the DME in the higher mode count case.

Cladding pumped few-mode EDFA for mode division multiplexed transmission

We experimentally demonstrate a few-mode erbium doped fiber amplifier (FM-EDFA) supporting 6 spatial modes with a cladding pumped architecture. Average modal gains are measured to be >20dB between 1534nm-1565nm with a differential modal gain of ~3dB among the mode groups and noise figures of 6-7dB. The cladding pumped FM-EDFA offers a cost effective alternative to core-pumped variant as low cost, high power multimode pumps can be used, and offers performance, scalability and simplicity to FM-EDFA design.

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.

Few mode Er(3+)-doped fiber with micro-structured core for mode division multiplexing in the C-band

Design and experimental characterization of Er(3+)-doped fiber amplifiers supporting 6 spatial modes in wavelength division multiplexing regime are reported. The study is first focused on Er(3+)-doped circular ring-structured profiles accessible with conventional fiber manufacturing techniques. However, these fiber designs, optimized for gain equalization, prove to be difficult to obtain experimentally. So as to go beyond these limits, an alternative approach based on a "pixelated" Er(3+)-doped core is proposed. Several possible designs are theoretically investigated and a first fabrication of micro-structured fiber is presented.

Three mode Er3+ ring-doped fiber amplifier for mode-division multiplexed transmission

We successfully fabricate three-mode erbium doped fiber with a confined Er(3+) doped ring structure and experimentally characterize the amplifier performance with a view to mode-division multiplexed (MDM) transmission. The differential modal gain was effectively mitigated by controlling the relative thickness of the ring-doped layer in the active fiber and pump launch conditions. A detailed study of the modal gain properties, amplifier performance in a MDM transmission system and inter-modal cross-gain modulation and associated transient effects is presented.