Flat-gain L-band amplifier containing AlPO₄ units in aluminophosphosilicate erbium-doped fibers

We present a flat-gain single-stage L-band erbium-doped fiber amplifier (EDFA) with a gain ripple of ±0.7 dB from 1580 to 1615 nm, by using aluminophosphosilicate erbium-doped fiber (APS-EDF) with an estimated AlPO₄ composition of 13.3 mol%. A series of APS-EDFs were fabricated with increasing AlPO₄ and Er concentrations, while maintaining a low background loss of 0.031 ± 0.005 dB/m and preventing Er ion clustering. The spectroscopic study shows a slightly narrowing Er cross section and flattened emission cross-section spectrum in the L-band with more AlPO₄, thus favoring the L-band amplification with an improved gain flatness. Also, the gain coefficient, gain saturation, and temperature-dependent gain characteristics were reported. A better temperature tolerance was observed with increasing AlPO₄.