Abstract
According to the analytical expression for modal gain of few-mode erbium-doped fiber amplifiers (FM-EDFAs), we propose a method of measuring the absorption loss coefficients of few-mode signals in few-mode erbium-doped fibers (FM-EDFs) by extrapolating the mode–gain curve dependent on the average population inversion. The absorption loss coefficient of an FM-EDF was measured in our experimental platform and used to estimate the effective erbium-ion doping concentration. The feasibility of the extrapolation method was verified by simulation and comparison with the transmission method. Furthermore, the FM-EDFAs with high modal gain and low differential mode gain (DMG) could be optimized by adjusting the FM-EDF’s length and pump power. The analysis process presented here is very useful for the efficient design of FM-EDFAs from a practical point of view.
Highlights
Single-mode fiber (SMF) communication systems are facing a capacity limit resulting from fiber’s nonlinear effects [1]
Jung et al first realized the simultaneous amplification of two mode groups in FM-erbium-doped fiber amplifiers (EDFAs) and investigated the influence of the refractive index and erbiumion distribution of the erbium-doped fiber (EDF) on the gain performance [6]
Before taking analysis on the practicability of the extrapolation method, we described several techniques for deriving the absorption coefficients as follows: (1) the indirect method, in which the absorption coefficients are indirectly deduced from the gain coefficients according to their relationship in terms of the known absorption and emission cross sections [17]; (2) the direct method, in which the absorption coefficients are directly calculated from the absorption cross-section and the uniformly distributed erbium-doping concentration, which can, respectively, be determined by the gain saturation measurement and the chemistry methods [18]; (3) the transmission method, which is equivalent to the case with no pump power
Summary
Single-mode fiber (SMF) communication systems are facing a capacity limit resulting from fiber’s nonlinear effects [1]. The core or mode division multiplexing technology has become an effective way to increase fiber transmission capacity [2]. Just like the commercially available erbium-doped fiber amplifiers (EDFAs) used in SMF communication systems, few-mode EDFAs (FM-EDFAs) can play an important role in mode division multiplexing (MDM) systems [3]. Jung et al first realized the simultaneous amplification of two mode groups in FM-EDFAs and investigated the influence of the refractive index and erbiumion distribution of the erbium-doped fiber (EDF) on the gain performance [6]. Bai et al studied the gain equalization of LP01 - and LP11 -mode signals under different pumping modes according to FM-EDFA theory [7].
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