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By using Fabry-Perot laser diode (FPLD) as a resonant ultranarrow bandpass filter in an Erbium-doped fiber amplifier or laser (EDFA or EDFL), the theory and experiment for side-mode suppression and linewidth reduction of mutually injection-locked EDFL-FPLD and EDFA-FPLD links are demonstrated. Based on the amplified feedback injection loop, the 3-dB linewidth of 3.4 MHz for the EDFA-FPLD link is determined by using self-heterodyne interferometric spectral analysis. The EDFA-FPLD link exhibits a nearly mode-beating noise-free performance as compared to the EDFL-FPLD link. This is due to the release of the resonant cavity configuration in the EDFL-FPLD link at a cost of slightly lower side-mode suppression ratio (∼42 dB). The maximum output power of the EFDA-FPLD link is 20 mW under an FPLD input power of 0.1 mW.