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

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.

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 0.25 dB 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.

The Performance of Orbital Angular Momentum Mode (|l| = 1~3) Amplification Based on Ring-Core Erbium-Doped Fibers

We demonstrated that a ring-core erbium-doped fiber amplifier (RC-EDFA) can support orbital angular momentum (OAM) modes with topological charges (|l| = 1~3). The dependence of the characteristics on the length of the RC-EDF was investigated experimentally, including an investigation of gain and 3 dB gain bandwidth over the whole C band (i.e., 1530~1565 nm). The 3 dB gain bandwidth was improved to 21 nm. At a signal wavelength of 1550 nm, the maximum gain of all signal modes was up to 30.1 dB. Differential modal gain was maintained below approximately 1.3 dB.

A Multi-Layer Erbium-Doped Air-Hole-Assisted Few-Mode Fiber with Ultra-Low Differential Modal Gain

The air-hole assisted few-mode fiber (AH-FMF) enables modal intensity balance and offers a profound prospect in gain equalization with the combination of adaptive ion doping. In this paper, we proposed an AH-FM-EDF with a multi-layered erbium doping profile. In AH-FM-EDF, due to the central air hole, only the first radial order modes (LP01, LP11, LP21, and LP31) are supported, and all the modes are confined in the same high refractive index core region. The differential modal gain (DMG) is highly reduced by optimizing the erbium doping proportion in each layer. Compared with uniform doping, the DMG is reduced from 4 dB to 0.14 dB as triple-layer doping is deployed. Additionally, the proposed erbium-doped fiber performs well in gain flattening and fabrication tolerance over the whole C-band.

Electro-optic mode-selective switch based on cascaded three-dimensional lithium-niobate waveguide directional couplers

We propose an electro-optic mode-selective switch based on cascaded three-dimensional lithium-niobate waveguide directional couplers fabricated with a single-step annealed proton-exchange process. To compensate for discrepancies due to uncertainties in the fabrication process, we develop a post-tuning technique to improve the performance of the coupler by means of depositing a layer of titanium oxide (TiO₂) onto one of the waveguides of the coupler. By integrating two cascaded dissimilar directional couplers, we experimentally demonstrate switchable (de)multiplexing of the LP₀₁, LP_11a, and LP_11b modes, where the LP_11a mode can be switched at an efficiency over 75% from 1530 nm to 1612 nm with an applied voltage varying between -9 V and +30 V, and the LP_11b mode can be switched at an efficiency higher than 90% from 1534 nm to 1577 nm with an applied voltage varying between -21 V to 0 V. The switching times are 230-300 ns. Our proposed waveguide platform could be employed to develop advanced switches for applications in areas where high-speed switching of spatial modes is required, such as reconfigurable mode-division-multiplexing communication.

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.

Applicability of a model with average inversion level to a cladding-pumped multicore erbium-doped fiber amplifier

This paper investigates the applicability of a model of a cladding-pumped multicore erbium-doped fiber amplifier (CP-MC-EDFA) that employs the average inversion level of an erbium ion. The model is modified to include the effects of multiple cores for modifications of the background loss coefficient of the pump light. The model is validated experimentally by measurements of the pump power, gain transient, and intercore cross-gain modulation of a cladding-pumped 12-core EDFA in which the pump light is uniformly distributed in the inner cladding. The calculated and measured pump power, gain transient response, and gain change caused by the intercore cross-gain modulation agree well, suggesting the modified model is useful for characterizing a CP-MC-EDFA.