Multicore Fiber Amplifier Research Articles

All-fiber GHz few-cycle pulse generation at 2 µm.

We demonstrate an all-fiber GHz mode-locked laser system with few-cycle duration operating at 2 µm. Based on a dispersion-managed mode-locked oscillator, a multi-stage fiber amplifier, and a nonlinear pulse compressor, the laser system can deliver watt-level few-cycle pulses at a fundamental repetition rate of 1.041 GHz. This 2-µm pulsed laser offers outstanding performance metrics, including a pulse duration of 33 fs (corresponding to ∼5 optical cycles) and an average power of 4.17 W. Moreover, the all-fiber laser system exhibits excellent power stability, and the integrated relative intensity noise (RIN) is only 0.052% (10 Hz-1 MHz). It is anticipated that this new, to the best of our knowledge, laser source is promising for frontier applications, including coherent supercontinuum generation, nonlinear frequency conversion, and laser-material interaction.

3.05 kW, 13.7 GHz linewidth fiber amplifier based on PRBS phase modulation for SBS suppression

In this paper, we establish a multi-stage fiber amplifier with pseudo-random binary sequence (PRBS) phase modulation. The stimulated Brillouin gain spectra of the main amplifier with both the unmodulated and pseudo-random binary sequence phase modulated configuration are measured (with corresponding output power), and the stimulated Brillouin scattering (SBS) threshold is investigated experimentally and theoretically. The pseudo-random binary sequence phase modulation parameters are optimized by theoretical simulation. With a two-stage preamplifier chain and a counter-pumping main amplifier stage, a maximum 3.05 kW output power with a slope efficiency of 85.9% is obtained experimentally. The central wavelength of the fiber amplifier is 1050 nm, associated with a full-width at half-maximum linewidth of 13.7 GHz. The stimulated Brillouin scattering reflectivity is below 0.01% at 3.05 kW at 13.7 GHz, which indicates that stimulated Brillouin scattering can be suppressed efficiently at this power and linewidth level.

A novel multicore Er/Yb co-doped microstructured optical fiber amplifier with peanut-shaped air holes cladding

Abstract The multicore fiber amplifier, as a key component in spatial division multiplexing (SDM) communication systems, presents higher technical difficulty compared to traditional multi-channel single core fiber amplifiers, which has sparked widespread attention. To achieve balance, efficiency, miniaturization, and cost-effectiveness in the performance of multi-core optical fiber amplifiers, we propose an innovative triple cladding 13-core Er/Yb co-doped microstructured fiber (13CEYDMOF). The proposed fiber features an outer cladding with peanut-shaped air holes, which enables uniform excitation of the 13 cores using a single multimode laser pump source within the inner cladding. This approach also prevents damage or aging of the fiber’s outer coating due to the pump laser. Furthermore, the design of Peanut-Shaped Air Holes effectively increases the numerical aperture (NA) of the inner cladding while reducing the outer diameter of the fiber, enhancing the fiber’s mechanical flexibility. To address the coupling difficulties caused by air holes, we bi-directionally pump the 13CEYDMOFA by utilizing a combined technique of the side winding and end pumping. The experimental results show that the 13CEYDMOFA can achieve an average gain of 23.8 dB, a noise figure (NF) of ∼4.6 dB, and an inter-core gain difference of less than 2 dB in the wavelength range of 1529–1565 nm. The in-line amplified transmission experiment demonstrates that the 13CEYDMOFA is well suited for the 13 spatial channels transmission. To the best of our knowledge, this is the first time to realize high performance telecommunication band amplification in a multicore microstructure fiber.

Design of gain equalization for multi-core supermode fiber amplifier (inside back cover paper)

Design of gain equalization for multi-core supermode fiber amplifier (inside back cover paper)

End-coupled cladding pumping using a dual helical lens for a 32-core doped fiber amplifier with two annular cores arrangement

We have designed a dual helical lens (DHL) to produce two rings of light intensities for end-coupled cladding pumping of a multi-core doped fiber (MCDF) amplifier. The MCDF is a 32-core doped fiber, and the cores are arranged in two annular forms of 20 and 12 cores. The DHL has been designed by combining two helical lenses with different topological charges. The angular spectrum (AS) method has been used to calculate the propagation of optical waves behind the DHL. The results show that the power efficiency and uniformity have improved compared to the direct cladding pumping conditions.

Analysis of optical core-to-core coupling: challenges and opportunities in multicore fiber amplifiers.

In this work we study in detail core-to-core coupling effects in multicore fibers (MCFs) using a simulation tool based on supermodal interference. We pay particular attention to the impact of core area scaling, which plays an important role in prospective amplifier systems. We consider geometrical and optical properties of the MCF structure, including the ability for dense packaging of the cores but also the influence on the core guidance (V-parameter). In general, this study is important to unlock the power and energy scaling potential of the next-generation MCF amplifiers.

Cable Capacity and Cost/Bit Modeling of Submarine MCF Systems With MC-EDFA Alternatives

We model and examine various amplification options for trans-oceanic repeatered submarine systems using multicore fiber (MCF) transmission in the context of relative cable capacity and relative system cost/bit. First, we compare the optical performance of various configurations of parallel standard single-core erbium-doped fiber amplifiers (SC-EDFAs) and multicore erbium-doped fiber amplifiers (MC-EDFAs), including both the C/L-bands and core/cladding pump designs. Key amplifier parameters governing expected capacity and cost/bit performance include noise figure, amplifier bandwidth, relative electrical-to-optical conversion efficiency, and relative amplifier cost. We make our best estimates of these parameters through modeling, analysis, and component cost estimates and assess the sensitivity of the results to the nominal values. We find that MC-EDFAs using either core- or cladding-pumping might offer lower cost/bit than parallel SC-EDFAs but cladding-pumping may reduce cable capacities due to the higher noise figure and potentially smaller amplifier bandwidth.

1.2-kW all-fiber Yb-doped multicore fiber amplifier.

We have demonstrated a record-high 1.2 kW, all-fiber multicore amplifier using a six-core single-mode Yb-doped fiber and a multicore pump-signal combiner (PSC). The output power is limited by the pump power of 1.9 kW. We have developed double-clad six-core fibers and PSCs for this demonstration. Each of the six Yb-doped cores has a 17-µm mode-field diameter (MFD) with a trench index profile and is capable of kW-class operation. The potential power scaling to the 10-kW level in a single amplifier with high brightness should be feasible with advanced thermal management and coherent beam combination.

Controlled generation of picosecond-pulsed higher-order Poincaré sphere beams from an ytterbium-doped multicore fiber amplifier

Higher-order Poincaré sphere (HOPS) beams with spatially variable polarization and phase distributions are opening up a host of unique applications in areas ranging from optical communication to microscopy. However, the flexible generation of these beams with high peak power from compact laser systems remains a challenge. Here, we demonstrate the controlled generation of HOPS beams based on coherent beam combination from an Yb-doped multicore fiber (MCF) amplifier. Using a spatial light modulator to adaptively adjust the wavefront and polarization of the signals seeded into the individual cores of the MCF various structured beams (including cylindrical vector beams and first- and second-order vortex beams) were generated with peak powers up to 14 kW for ∼ 92 ps pulses.

Large area Bragg grating for pump recycling in cladding-pumped multicore erbium-doped fiber amplifiers.

We demonstrate for the first time that a Bragg grating can be written over a large area inside the cladding of a multicore erbium-doped fiber amplifier to increase the power conversion efficiency (PCE) by recycling the output pump power. Our results indicate that a Bragg grating covering ∼25% of the cladding area allows us to recycle 19% of the output pump power which leads to a relative increase of the PCE by 16% for an input pump power of 10.6 W in the specific case of an eight-core erbium-doped fiber with a length of 20.3 m and one core loaded with an input signal power of 1.5 dBm.

Mitigation of thermally-induced performance limitations in coherently-combined multicore fiber amplifiers.

Multicore fiber (MCF) amplifiers have gained increasing interest over the past years and shown their huge potential in first experiments. However, high thermal loads can be expected when operating such an amplifier at its limit. Especially in short MCF amplifiers that are pumped in counter-propagation, this leads to non-uniform mode-shrinking in the cores and, consequently, to a degradation of the system performance. In this work we show different ways to counteract the performance limitations induced by thermal effects in coherently-combined, multicore fiber amplifiers. First, we will show that pumping MCFs in co-propagation will significantly improve the combinable average power since the thermal load at the fiber end is reduced. However, this approach might not be favorable for high energy extraction. Therefore, we will introduce a new MCF design pumped in counter-propagation that leads to a reduction of the thermal load at the fiber end, which will allow for both high combined output power and pulse energy.

Multicore Cladding-Pumped Fiber Amplifier With Annular Erbium Doping for Low Gain Compression

We report the characterization of a cladding-pumped multicore fiber with annular erbium doping for low gain compression over the C-band (1528.8 nm to 1563.9 nm). The fiber aims to minimize saturation effects by placing the erbium doping in the cladding where the signal intensity is lower. The challenge of ensuring adequate erbium ion solubility in the cladding, without unduly raising its refractive index, was answered by using aluminophosphosilicate for the doped region. Before assembling the eight-core amplifier with fan-in and fan-out, we needed to determine the optimum fiber length to meet the gain compression target when the eight cores are loaded, for a given pump power injected in the cladding. We thus performed a thorough experimental characterization of a single core. This allowed the determination of all the relevant fiber parameters needed to do numerical simulations and predict the multicore fiber amplifier performance under the fully loaded scenario. For the first time, these numerical results are compared to the experimental results of a fully loaded multicore amplifier with cladding pumping and erbium doping in the cladding. We discuss the amplifier performance in terms of gain compression when the input power signal is varied, as would be the case in dynamic optical networks. We also examine the core-to-core gain variations and the sensitivity of the design to fabrication variations. Results show that significant reduction in gain compression of multicore cladding-pump amplifiers can be achieved with the proposed annular doping design in the cladding, although improvement in the fabrication uniformity of the core will be required for practical applications. Lastly, we discuss how this erbium doped fiber design can also lead to improved power conversion efficiency (PCE) in cladding-pumped amplifiers.

Effect of pitch on gain-noise figure and crosstalk performance of different core structure Multicore Erbium-Doped Fiber Amplifier: A comparative analysis

A detailed analysis of 7-cores Erbium-Doped Fiber Amplifier has been done to assess the effect of different core and pitch arrangement on the fiber amplifier's performance. The investigation is performed within the C-band communication wavelengths to compare the performance of Y-structure, T-structure, circular and linear structure of the core. The comparison is also extended further by including the performance of a widely used hexagonal structure. A thorough analysis is conducted by examining the profiles of gain, noise figure, and mode of transmission. The inter-core crosstalk (IC-XT) is also calculated for all the core structures and pitch, making this research effort conclusive. Maximum gain has been observed for the Y-shape/structure core arrangement, whereas minimum IC-XT has been observed for the hexagonal structure. However, an encouraging observation is that the percentage decrease in crosstalk due to an increase in pitch is significantly higher for Y-structure than hexagonal. Therefore, the authors believe that this detailed investigation gives significant insight to design efficient multicore EDFA in current research and development.

Impact of thermo-optical effects in coherently combined multicore fiber amplifiers.

In this work we analyze the power scaling potential of amplifying multicore fibers (MCFs) used in coherently combined systems. In particular, in this study we exemplarily consider rod-type MCFs with 2 × 2 up to 10 × 10 ytterbium-doped cores arranged in a squared pattern. We will show that, even though increasing the number of active cores will lead to higher output powers, particular attention has to be paid to arising thermal effects, which potentially degrade the performance of these systems. Additionally, we analyze the influence of the core dimensions on the extractable and combinable output power and pulse energy. This includes a detailed study on the thermal effects that influence the propagating transverse modes and, in turn, the amplification efficiency, the combining efficiency, the onset of nonlinear effect, as well as differences in the optical path lengths between the cores. Considering all these effects under rather extreme conditions, the study predicts that average output powers higher than 10 kW from a single 1 m long ytterbium-doped MCF are feasible and femtosecond pulses with energies higher than 400 mJ can be extracted and efficiently recombined in a filled-aperture scheme.

Reconfigurable structured light generation in a multicore fibre amplifier

Structured light, with spatially varying phase or polarization distributions, has given rise to many novel applications in fields ranging from optical communication to laser-based material processing. However the efficient and flexible generation of such beams from a compact laser source at practical output powers still remains a great challenge. Here we describe an approach capable of addressing this need based on the coherent combination of multiple tailored Gaussian beams emitted from a multicore fibre (MCF) amplifier. We report a proof-of-concept structured light generation experiment, using a cladding-pumped 7-core MCF amplifier as an integrated parallel amplifier array and a spatial light modulator (SLM) to actively control the amplitude, polarization and phase of the signal light input to each fibre core. We report the successful generation of various structured light beams including high-order linearly polarized spatial fibre modes, cylindrical vector (CV) beams and helical phase front optical vortex (OV) beams.

Simplified design of optical elements for filled-aperture coherent beam combination.

A simplification strategy for segmented mirror splitters (SMS) used as beam combiners is presented. These devices are useful for compact beam division and the combination of linear and 2-D arrays. However, the standard design requires unique thin-film coating sections for each input beam; thus, potential for scaling to high beam-counts is limited due to manufacturing complexity. Taking advantage of the relative insensitivity of the beam combination process to amplitude variations, numerical techniques are used to optimize highly simplified designs with only one, two or three unique coatings. It is demonstrated that with correctly chosen coating reflectivities, the simplified optics are capable of high combination efficiency for several tens of beams. The performance of these optics as beam splitters in multicore fiber amplifier systems is analyzed, and inhomogeneous power distribution of the simplified designs is noted as a potential source of combining loss in such systems. These simplified designs may facilitate further scaling of filled-aperture coherently combined systems in linear array or 2-D array formats.

Modeling and Characterization of Cladding-Pumped Erbium-Ytterbium Co-Doped Fibers for Amplification in Communication Systems

Cladding-pumped optical fiber amplifiers are of increased interest in the context of space-division multiplexing but are known to suffer from low power efficiency. In this context, ytterbium (Yb) co-doping can be an attractive solution to improve the performance of erbium (Er) doped fiber amplifiers. We present a detailed direct comparison between Er/Yb-co-doping and Er-doping using numerical simulations validated by experimental results. Two double-cladding fibers, one doped with Er only and the other one co-doped with Er and Yb, are designed, fabricated and characterized. Using the experimentally extracted parameters, we simulate multi-core fiber amplifiers and investigate the interest of Er/Yb-co-doping. We calculate the minimum gain of the amplifiers over a 35-nm spectral window considering various scenarios.

High Spatial Density 6-Mode 7-Core Fiber Amplifier for L-Band Operation

We present a high spatial density space division multiplexed fiber amplifier (i.e., 6-mode 7-core multicore fiber amplifier) supporting 42 spatial channels. A high average modal gain of 23.1 dB is obtained in the L-band with an average differential modal gain of 3.2 dB and a minimum output power of 17.5 dBm per core per mode. The total power consumption of the amplifier is improved by a power reduction factor of 0.36 and a fully fiberized optical amplifier is realized in a cladding-pumped configuration. The assembled amplifier is then tested in a few-mode multicore fiber transmission link and simultaneously amplified 42 spatial channel fiber transmission is demonstrated with the highest spatial density and energy efficiency to date.

A parametric study to create an optimal design of seven-cores hexagonal structured erbium-doped fiber amplifier

Abstract Numerical analysis for calculating the gain and noise figure (NF) of 7-cores hexagonal structured Multicore Erbium-Doped Fiber Amplifier (MC-EDFA) is carried out. In addition to this, a mode analysis has been carried out for the same geometric structure of the Erbium-Doped Fiber (EDF). The effect of different core radius, signal wavelengths are analyzed and the results are laid down in the form of gain and NF of the MC-EDFA, propagating mode, normalized frequency, maximum and minimum modal intensity in order to analyze the modal field confinement in the MC-EDFA. Along with this, the inter-core crosstalk has also been investigated for 7-cores Erbium-Doped Fiber Amplifier (EDFA). Results other than gain and NF, calculated with respect to the operating wavelength range from 1525 nm to 1565 nm are compared with the calculated gain for each core radius of the MC-EDFA so as to obtain an optimum set of conditions for designing a superior MC-EDFA. Thus, it is ascertained that the performance of a 7-cores MC-EDFA can be significantly improved by designing it with reference to optimum value of core radius and signal wavelength.

Gain Control in Multi-Core Erbium-Doped Fiber Amplifier With Cladding and Core Hybrid Pumping

This paper investigates gain controllability of cladding- and core-pumping hybrid-pumped single-stage and double-stage multi-core (MC) erbium-doped amplifiers (EDFAs) for changes in the input signal power per channel and number of channels. During the gain control, the core-pumping power is adjusted, whereas the cladding-pumping power is kept constant. Static characteristics are experimentally measured, and both kinds of hybrid-pumped MC-EDFAs are gain controlled with a small gain change for both the input signal changes. Dynamic characteristics are investigated by calculating the gain transient response of a surviving channel of the gain-controlled hybrid-pumped MC-EDFAs for signal channel dropping, and the response characteristics are compared with those of reference EDFAs whose erbium-doped fiber (EDF) has the same core parameters as those of EDF used in a conventional practical EDFA. It is shown that there is no significant difference in dynamic gain controllability between the hybrid-pumped MC-EDFAs and reference EDFAs even though the cladding-pumping power is not adjusted and kept constant. The results for the static and dynamic gain control characteristics suggest that constant cladding-pump power has little impact on gain controllability of the hybrid-pumped MC-EDFAs.