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A high-power, double-pass (DP), and double-clad (DC) Erbium (Er)-Ytterbium (Yb) doped optical fiber amplifier (EYDFA) was fabricated and experimentally evaluated. Because optical amplifiers for space missions must achieve higher electrical-to-optical conversion efficiency in consideration of power consumption and heat generation, we focused on a DP optical fiber amplifier that can be expected to achieve high optical-to-optical conversion efficiency. Free-space optics and integrated design were adopted for a DP module and a Faraday mirror (FM) module, so as to ensure low transmission loss (high efficiency). The DP module including a tap mirror, isolators, and a polarizing beam splitter measured 70×50×22.5 mm in size, with transmission loss of 0.65 dB (from input port to pump combiner port) and 0.52 dB (from pump combiner port to output port). In contrast, the FM module was 40×37×18.5 mm in size, with round-trip transmission loss of 0.66 dB. The fabricated DP-DC-EYDFA achieved output power of around 7 W in the wavelength range from 1540 to 1560 nm and optical-to-optical conversion efficiency exceeding 32%. To the authors’ knowledge, this is the highest output power ever recorded by a DP-DC-EYDFA. The measured polarization extinction ratio (PER) of the fabricated DP-DC-EYDFA was 23.6 dB without using PM EYDF. And in comparison with a single-pass DC EYDFA using the same EYDF, we obtained results showing that the conversion efficiency of the DP-EYDFA was at least 4.2% higher than that of the SP-EYDFA.
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