Few-mode Erbium-doped Fiber Amplifier Research Articles

Low-modal-crosstalk doped-fiber amplifiers in few-mode-fiber-based systems

Independent light propagation through one or multiple modes is commonly considered as a basic demand for mode manipulation in few-mode fiber (FMF)- or multimode fiber (MMF)-based optical systems such as transmission links, optical fiber lasers, or distributed optical fiber sensors. However, the insertion of doped-fiber amplifiers always kills the entire effort by inducing significant modal crosstalk. In this paper, we propose the design of doped-fiber amplifiers in FMF-based systems adopting identical multiple-ring-core (MRC) index profiles for both passive and doped fibers to achieve low modal crosstalk. We develop the direct-glass-transition (DGT) modified chemical vapor deposition (MCVD) processing for precise fabrication of few-mode erbium-doped fibers (FM-EDFs) with MRC profiles of both refractive index and erbium-ion doping distribution. Then, a few-mode erbium-doped-fiber amplifier (FM-EDFA) with a maximum gain of 26.08 dB and differential modal gain (DMG) of 2.3 dB is realized based on fabricated FM-EDF matched with a transmission FMF supporting four linearly polarized (LP) modes. With the insertion of the FM-EDFA, 60 + 60 km simultaneous LP01/LP11/LP21/LP02 transmission without inter-modal multiple-input multiple-output digital signal processing (MIMO-DSP) is successfully demonstrated. The proposed design of low-modal-crosstalk doped-fiber amplifiers provides, to our knowledge, new insights into mode manipulation methods in various applications.

Simultaneous realization of high gain and low DMG of four-mode EDFA under bidirectional hybrid-mode pump.

We theoretically and experimentally verify that, the bidirectional hybrid-mode pumping scheme can address the optimization problem of trade-off between high gain and low differential modal gain (DMG) of four-mode erbium-doped fiber amplifier (4M-EDFA), in comparison with traditional both forward and backward hybrid-mode pumping scheme. It is noticed that, when the total pump power is fixed, the bidirectional hybrid-mode pumping scheme can not only achieve higher gain, but also suppress DMG due to different overlap integrals for the forward and backward pumping schemes. The bidirectional hybrid-mode pumped 4M-EDFA is developed with the forward pumping at LP02 mode and the backward pumping at LP21 mode, under a pump power ratio of 30%:70%. Thus, we can achieve an average gain of up to 21.16 dB and a low DMG of 0.43 dB at 1550 nm, and an average gain of up to 20.64 dB with a DMG of less than 1.6 dB over the C-band. In particular, the bidirectional hybrid-mode pumping scheme allows us to tailor the gain characteristics of the few-mode erbium-doped fiber amplifiers (FM-EDFAs), by adjusting the power ratio between forward and backward pumps.

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 (LP01) 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.

A practical in-line FM-EDFA based on cladding pumping for MIMO-free MDM amplification and transmission

The amplification of the long-haul mode division multiplexing (MDM) transmission systems is currently mainly implemented by three steps due to the lack of high-performance in-line few-mode erbium-doped fiber amplifiers (FM-EDFA): firstly, the multiplexed signals need to be demultiplexed. Secondly, multiple single-mode EDFAs are used to amplify the modes separately. Finally, the signals are re-multiplexed into the few-mode fiber for transmission. The multi-stage multiplexing and demultiplexing bring significant complexity and additional modal crosstalk, resulting in degraded transmission performance. To address this issue, an in-line FM-EDFA based on cladding pumping is proposed for simultaneous amplification and transmission in MDM systems. The direct connection of the in-line FM-EDFA to the transmission FMFs reduces the need for mode multiplexing/demultiplexing devices, which provides excellent stability and applicability for MDM transmission systems. The experimental results show that the modal gains of in-line FM-EDFA can reach 21 dB with a differential modal gain (DMG) of less than 0.9 dB. Moreover, the in-line FM-EDFA is applied for simultaneous amplification and transmission in a weakly-coupled MDM system with MIMO-free direct detection. The modal crosstalk of the in-line FM-EDFA is less than −11 dB. Therefore, the in-line cladding-pumped FM-EDFA provides good integration to the fiber links, showing much potential in practical long-haul transmission systems.

Experimental evaluation on OSNR and DMG performance of cascade FM-EDFA systems by equivalent input spectrum method

An all-fiber few-mode erbium-doped fiber amplifier (FM-EDFA) is constructed by the integration of few-mode isolator wavelength division multiplexers (FM-IWDMs), which act as the multiplexing or demultiplexing function between signal and pump light. The gain performance of the FM-EDFAs under forward, bidirectional and backward pumping cases is investigated by experiment. For LP01, LP11e and LP11o-mode signals, the minimum differential mode gain (DMG) of 0.14 dB can be achieved by forward LP11e and LP11o-mode pumping. We propose the equivalent input spectrum method to experimentally simulate the cascade FM-EDFA systems. For the three-mode amplification, the optical signal-to-noise ratio (OSNR) and overall DMG after 8-stage transmission are respectively 13.89 dB and 1.59 dB, which may theoretically support 1000 km-long transmission of 100 Gb/s DP-QPSK signal. A cascade amplification system composed of six-mode EDFA with the DMG of 1.35 dB is also considered to verify the feasibility of the equivalent input spectrum method.

Hybrid-Pumped Few-Mode Erbium-Doped Fiber Amplifier for Gain Equalization

Hybrid-Pumped Few-Mode Erbium-Doped Fiber Amplifier for Gain Equalization

High-power, low noise, high gain few-mode fiber amplifier

The few-mode erbium-doped fiber amplifier (FM-EDFA) is a necessary component for high-capacity long-haul mode-division multiplexing (MDM) fiber optic communication systems. In this paper, an all-fiber few-mode amplifier based on erbium-ytterbium co-doped double-clad fiber is designed and demonstrated. We have developed a fiber-optic pump beam combiner and a few-mode isolator. Combined with the two-stage cascade method, an all-fiber few-mode fiber amplifier with high-power, high-gain and low-noise is realized. Simultaneous amplification of 6 modes (LP01, LP11a, LP11b LP21a, LP21b and LP02) are achieved in the 1535–1565 nm range with average modal gain greater than 40 dB, maximum saturated output power up to 29.5 dBm, differential modal gain (DMG) of ∼3 dB and minimum noise figure (NF) of 4.69 dB.

Experimental investigation on transmission system of dual-polarization mode-division-multiplexing signals with few-mode erbium-doped fiber amplifiers

The transmission capacity of optical fiber communication can be effectively improved by the combination of dual-polarization (DP) high-order modulation format with mode division multiplexing (MDM) technology. We build up an experimental system for amplification and transmission of DP-MDM signals with orthogonal mode combination (OMC) and identical mode combination (IMC). The MDM transceiver units are composed of mode-selective photonic lanterns (MSPLs) and few-mode polarization controllers (FMPCs). The few-mode erbium-doped fiber amplifier (FM-EDFA) used here is constructed by homemade isolation and wavelength division multiplexers (IWDMs). The amplification and transmission experiments for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) MDM signals are carried out by the commercial optical transport network (OTN) platform. An effective method of optimally receiving DP-MDM signals are presented by appropriately controlling the FMPC in the MDM receiver unit. The experimental results show that, (1) OMC DP-MDM signals have a better performance than IMC cases in differential modal gain (DMG), which provides a new means to reduce the DMG; and (2) under the optimization conditions, OMC and IMC DP-MDM systems possess a close receiver sensitivity, but the former has a larger optical power margin than the latter by 2.2 dB.

Few-mode erbium-doped fiber with trench and weak coupling for mode gain equalization based on layered-doping technology.

A few-mode erbium-doped fiber (FM-EDF) with a step refractive index and trench structure is designed and proposed to realize the modal gain equalization of a few-mode erbium-doped fiber amplifier (FM-EDFA). The layered-doping technology is used to reduce the mode gain difference (DMG). The doping radius and doping concentration are adjusted to obtain the optimum FM-EDF structure. When the designed FM-EDF is applied to the FM-EDFA, the DMG of the whole C-band is less than 0.15dB and the DMG is less than 0.12dB at 1550nm. The minimum refractive index difference (Δ n eff) between modes can be calculated according to the refractive index and radius of the fiber core; i.e.,1.35×10-3, which will greatly reduce the coupling between modes in a practical application. Tolerances in the fiber manufacturing process are also considered for reliable FM-EDFA performance. When the doping radius and concentration of each doping layer fluctuate by ±15% based on the precise value, the maximum DMG increases to 1.8dB. In general, DMG can maintain a small value, which is beneficial for application in optical communications systems.

Static analysis of gain control for a few-mode erbium-doped fiber amplifier employing pump power adjustment.

In mode-division multiplexing (MDM) optical transmission systems and MDM networks, the gain must be kept nearly constant even when the input signal power of the few-mode (FM), erbium-doped fiber amplifier (EDFA) changes. This paper investigates controllability of the gain in a 4-LP mode EDFA, whose EDF has signal modes of L P 01, L P 11, L P 21, and L P 02 or L P 01, L P 11, L P 21, and L P 31, by using analytical and full models. The results of calculation using the analytical model and a gain simulation using the full model show that the gain can be kept almost constant with an error less than 0.25dB simply by adjusting the L P 01 pump power of the EDFA with signal modes of L P 01, L P 11, L P 21, and L P 31. This result suggests that it is possible to control the gain simply by adjusting the pump power in FM-EDFAs where all signal modes have a radial mode number of one.

Long-distance modal power equalization with hybrid pumped FM-EDFA

In a few-mode fiber-based mode division multiplexing system, the different modes as distinct channels suffer from discrepant mode-dependent loss, leading to an intensive channel power imbalance and large bit error. The few-mode erbium-doped fiber amplifier (FM-EDFA) enables mode power regulation to ensure high-quality long-distance transmission. In this paper, we propose an FM-EDFA for long-haul equalization with a core and cladding hybrid pump structure. The particle swarm optimization is applied to determine the multilayered erbium ions doping profile. Based on the profile, targeted compensation for high-order modes with high loss is realized to mitigate the power difference in long-distance transmission. Compared with the single pump selection, the differential modal gain reduces from 6.553 dB to 1.239 dB in the hybrid pump with a 1000 km multistage amplification and transmission. We provide new ideas for loss compensation and power balance in long-distance transmission. This method has the ability of dynamic power adjustment while keeping the design simple.

Design of few-mode erbium-doped fiber with a low differential modal gain and weak coupling based on layered doping.

A step index few-mode erbium-doped fiber (FM-EDF) for mode gain equalization is designed and proposed in this paper, which uses the layered-doping method to reduce the differential mode gain (DMG). The optimum structure of FM-EDF is obtained by adjusting the doping radius and doping concentration. When this structure is applied to a few-mode erbium-doped fiber amplifier (FM-EDFA), the DMG in the range of 1550-1565nm is ∼0.28d B, and the DMG of the whole C-band is usually less than 0.5dB. At the same time, the gain of each mode in 1530-1555nm is ∼20d B, while the gain decreases gradually in the 1555-1565nm due to the absorption characteristics of erbium ions. In addition, the minimum refractive index difference (Δ n eff) between modes is 1.29∗10-3 due to the selection of the refractive index and radius of the fiber core, which will greatly reduce the coupling between modes in practical application. Tolerances in the fiber manufacturing process are also considered for reliable FM-EDFA performance. When the doping concentration or the doping radius changes based on the precise value, the DMG will increase to a certain extent. In general, the DMG can maintain a small value, which is beneficial to applications in optical communication systems.

Few-mode erbium-doped fiber amplifier based on a multi-inclusions core optical fiber.

In this work, we demonstrate and evaluate a new design of micro-structured core erbium-doped few-mode fiber to be used as optical amplifier in the context of mode-division multiplexing. This concept is proposed so as to better control the distribution of the Er3+ ions in the core area, thus permitting to adjust the overall differential modal gains between the different signal modes. The design presented here consists of 19 erbium-doped inclusions embedded in a pedestal geometry guiding 10 modes in the C-band. It has been optimized numerically so as to reach the equalized amplification of all the signal modes. The fiber has been realized and combined with custom-made dual-wavelength mode multiplexers based on multi-plane light conversion to shape the signal and pump beams. Amplification properties have finally been evaluated experimentally.

Transmission Experiment of High-Speed DP-QPSK Mode Division Multiplexing Signals with Few-Mode Erbium-Doped Fiber Amplifier

Transmission Experiment of High-Speed DP-QPSK Mode Division Multiplexing Signals with Few-Mode Erbium-Doped Fiber Amplifier

Experimental Investigation on Transmission System of Dual-Polarization Mode-Division-Multiplexing Signals with Few-Mode Erbium-Doped Fiber Amplifiers

Experimental Investigation on Transmission System of Dual-Polarization Mode-Division-Multiplexing Signals with Few-Mode Erbium-Doped Fiber Amplifiers

High-gain integrated in-line few-mode amplifier enabling 3840-km long-haul transmission

An integrated few-mode erbium-doped fiber amplifier (FM-EDFA) with high modal gain is suitable for the in-line amplification in mode-division multiplexing transmission (MDM) systems. We first experimentally demonstrate a dual-stage integrated FM-EDFA supporting three linear polarization modes. Consisting of integrated passive components with low insertion losses, the FM-EDFA has a similar structure and performance to widely used commercial single-mode EDFAs. The averaged modal gain of 25 dB, the differential modal gain (DMG) of < 1.1 dB , and noise figures of 5–7 dB are simultaneously achieved. In addition, the DMG of the 3M-EDF itself is ∼ 0.3 dB . Moreover, an MDM transmission experiment with the in-line few-mode amplification by our proposed FM-EDFA over a 3840-km few-mode fiber link for a 28-Gbaud quadrature phase-shift keying (QPSK) signal is demonstrated.

Optimized few-mode EDFA of applicability for variable mode groups with an extended doping region and a PSO algorithm

We employ particle swarm optimization to present a cladding-pumped few-mode erbium-doped fiber amplifier (FM-EDFA) with an extended erbium-doped distribution over the core. By enlarging the radius of the doping region to improve the gain of high-order modes, the differential modal gain (DMG) can be minimized. The proposed FM-EDFA can provide average gain of 21.7 dB and DMG of less than 0.61 dB across the C-band (1530–1565 nm) for seven-mode groups. In addition, the value of the optimal doping radius has been investigated for different numbers of mode groups, which will gradually increase with the number of mode groups. Finally, simulation results indicate acceptable applicability for four- to six-mode groups even using the proposed design.

Demonstration of an all-fiber cladding-pumped FM-EDFA with low differential modal gain

An all-fiber few mode erbium doped fiber amplifier (FM-EDFA) with low differential modal gain (DMG) is theoretically and experimentally demonstrated. Firstly, a double-cladding six-mode EDF (6M-EDF) with triple-layer erbium ion doping is designed and fabricated. Then, based on the homemade 6M-EDF, an all-fiber cladding-pumped FM-EDFA is implemented with high modal fidelity. Our experimental results show that the DMG is less than 2.65 dB in the wavelength range of 1535–1570 nm, the modal gains for the 6 modes are higher than 19 dB, and the noise figures are about 4.61–7.66 dB. The all-fiber FM-EDFA with low DMG offers a cost-effective and stable alternative for high-integrated all-fiber long-haul spatial division multiplexing applications.

Demonstration of a ring-core few-mode erbium-doped fiber for mode gain equalization based on layered doping

A ring-core few-mode erbium-doped fiber (FM-EDF) with step refractive index is designed and proposed to realize the modal gain equalization of the amplifier. The layered-doping method is used to reduce the mode gain difference (DMG), and the doping radius and doping concentration are adjusted to obtain the best FM-EDF structure. When it is applied to the few-mode erbium-doped fiber amplifier (FM-EDFA), the DMG at 1550 nm is less than 0.44 dB. The gains of each mode in the entire C-band are greater than 16 dB. To further improve the gain equalization characteristics of the designed fiber, the DMG can be reduced to 0.03 dB by adding trenches at appropriate positions on the basis of the same core structure. The tolerances in the optical fiber manufacturing process are considered to verify the reliability of the FM-EDFA performance. The doping radius and doping concentration are changed by ± 15 % based on the precise value to consider the tolerances in the fiber manufacturing process to verify the reliability of the FM-EDFA performance. Generally speaking, doping radius changes have a greater impact on DMG, so higher production accuracy is required. To explore the experimental structure and procedures of FM-EDFA, a preliminary experiment is carried out. The experimental results obtained have great reference significance for subsequent experiments. The characteristics of the FM-EDFA, such as all-fiber and low DMG, are difficult to implement by other methods.

Modal superposition and gain competition in clad-pump few-mode EDFA

The clad-pump few-mode erbium-doped fiber amplifier (FM-EDFA) is independent to the pump mode selection and it shows much potential to construct an all-fiber scheme due to the natural all-fiber interconnect. In the case of cladding-pump, we utilize the mode superposition theory to expound the modal gain competition under different modes multiplexing in the FM-EDFA. It is found that, at the same multiplexed signal mode number, both the modal gain and the differential modal gain (DMG) are relatively immune to the different degenerate signal mode selection. With the growing number of multiplexing modes, there is a fiercer modal gain competition and thus, compared with four modes multiplexing, the highest gain is reduced by 1.594 dB in the case of six modes multiplexing. The DMG is reduced by 0.45 dB due to the competitive balance. The modal superposition and gain competition theory specialized for clad-pump are helpful for erbium-doped fiber design and FM-EDFA scheme optimization.