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Single-mode Er–Yb fibers with the core based on a phosphorosilicate glass matrix (up to 6.5 mol.% P2O5) highly doped with fluorine (up to 0.9 wt.%) were fabricated using an all-gas-phase modified chemical vapor deposition method. The core numerical aperture was in the range of 0.07–0.08 relative to the pure silica, which allowed us to increase the single-mode core diameter up to 20 μm. The slope efficiency in lasers based on the fabricated fibers reached 34% relative to the launched pump power.

During last decades there has been considerable interest in developing a fiber amplifier for the 1.3-$$\upmu $$μm spectral region that is comparable in performance to the Er-doped fiber amplifier operating near 1.55 $$\upmu $$μm. It is due to the fact that most of the existing fiber-optic communication systems that dominate terrestrial networks could be used for the data transmission in O-band (1260–1360 nm), where dispersion compensation is not required, providing a low-cost increase of the capacity. In this regard, significant efforts of the research laboratories were initially directed towards the study of the praseodymium-doped fluoride fiber amplifier having high gain and output powers at the desired wavelengths. However, despite the fact that this type of amplifiers had rapidly appeared as a commercial amplifier prototype it did not receive widespread demand in the telecom industry because of its low efficiency. It stimulated the search of novel optical materials for this purpose. About 10 years ago, a new type of bismuth-doped active fibers was developed, which turned out to be a promising medium for amplification at 1.3 $$\upmu $$μm. Here, we report on the development of a compact and efficient 20-dB (achieved for signal powers between $$-40$$-40 and $$-10$$-10 dBm) bismuth-doped fiber amplifier for a wavelength region of 1300–1350 nm in the forward, backward and bi-directional configurations, which can be pumped by a commercially available laser diode at 1230 nm with an output power of 250 mW. The compactness of the tested amplifier was provided by using a depressed cladding active fiber with low bending loss, which was coiled on a reel with a radius of 1.5 cm. We studied the gain and noise figure characteristics at different pump and signal powers. A record gain coefficient of 0.18 dB/mW (at the pump-to-signal power conversion efficiency of above 27$$\%$$%) has been achieved.

We report on a new record in the field of broadband BDFA in the O+E spectral range. Using only 250 mW single wavelength pump at 1256 nm and a low-OH bismuth-doped fiber containing two types of bismuth active centers, we achieved >23 dB gain from 1325 nm to 1441 nm with flatness <3 dB and a NF of <6.8 dB. Three different pump wavelengths were considered: 1230, 1256, 1270 nm. Pumping at the shortest wavelength enabled wider gain bandwidth – 125 nm but required pump power up to ∼620 mW, whereas the longest one provided higher pump efficiency but narrower gain bandwidth. We also studied multiple wavelength amplification effects in the BDFA to reveal potential problems with simultaneous broadband signal amplification in such amplifiers.

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