Wideband gain enhancement across the S, C, and L bands using TDFA–EDFA and TDFA–Raman hybrid optical amplifiers: ASE and nonlinear effects analysis

Global IP traffic growth is pushing conventional C-band erbium-doped fiber amplifiers (EDFAs) toward their practical capacity limit, making ultra-wideband amplification across the S+C+L spectrum a pressing technical need. This paper proposes two hybrid optical amplifier (HOA) architectures, TDFA–EDFA and TDFA–Raman, for continuous amplification across 1460–1625 nm (165 nm, 34 WDM channels at 5 nm spacing), and evaluates them numerically under dense WDM conditions for gain, noise figure (NF), optical signal-to-noise ratio (OSNR), amplified spontaneous emission (ASE) power spectra, and nonlinear transmission impairments including self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). Whereas the TDFA amplifies the S-band and the EDFA/Raman stage amplifies the C+L bands, this work additionally decomposes the total gain into its per-stage contributions to clarify how each stage shapes the composite spectrum. The TDFA–Raman configuration reaches a peak gain of 48.8 dB with a 3 dB ripple and NF below 5 dB across the full band, while the TDFA–EDFA configuration reaches approximately 30 dB gain with a ripple within 4 dB and NF of 3.5–5.5 dB. Bidirectional TDFA pumping is shown to suppress ASE accumulation and flatten gain relative to forward-only pumping. Fiber and pump parameters used in the model are benchmarked against previously published experimental and numerical TDFA and Raman results to confirm their physical plausibility. Parametric sweeps of TDFA length (5–20 m) and Raman pump power (500–1300 mW) expose the trade-off between peak gain and spectral uniformity, and a gain-flattening filter (GFF) is introduced between the TDFA and Raman stages; its simulated transfer function is shown to reduce gain ripple substantially at the cost of quantified peak-gain and NF penalties. Nonlinear analysis shows FWM as the dominant S-band impairment at high Raman pump power, motivating a recommended pump-power cap of 900–1000 mW, while SPM and XPM remain within acceptable bounds in both architectures. A consolidated comparison against ten recently reported HOA designs confirms the competitiveness of the proposed architectures in bandwidth coverage, gain, and noise performance.