With the rapid application of 5G, big data, cloud computing, internet of things (IoT), and other technologies, the demand for data traffic has greatly increased in recent decades. Current optical fiber amplifiers are no longer able to cope with the capacity crunch in communication systems, and extending the gain bandwidth of the amplifier is considered an economical and feasible solution. However, commercially available erbium-doped fiber amplifiers (EDFA) can only achieve optical amplification in the C-band and the L-band, and 1260‒1500 nm band is under-exploited. Recently, bismuth-co-doped glasses and fibers have attracted attention because of their various luminescence characteristics based on different host materials, which cover the wavelength range of most communication transmission windows. Currently, bismuth co-doped phosphosilicate fibers have great potential in the second transmission window (1260‒1360 nm) owing to their excellent compatibility with silica communication fibers. Therefore, bandwidth extension using Bi-doped phosphosilicate fibers is an effective solution for increasing transmission capacity.

Because of the characteristics of bismuth ions, such as unstable chemical valence and sensitivity to the glass matrix, it is difficult to prepare Bi-doped silica-based fibers. In this study, we demonstrate a Bi-doped phosphosilicate fiber fabricated using modified chemical vapor deposition (MCVD) technology. The refractive index profile of the preform is measured. The preform is then drawn to a fiber with core diameter/cladding diameter of 9 μm/120 μm. Optical parameters, such as background loss and absorption spectra, are recorded. Moreover, the ratio of the unsaturated loss to small-signal absorption indicates the extrinsic loss level of the Bi-doped fibers, which is measured by testing the output power variation with increasing pump power. Finally, an all-fiber experimental configuration of a Bi-doped fiber amplifier is constructed to evaluate the amplification properties of the fiber based on the single stage with forward-pumping scheme.

To ensure adequate optical properties, the refractive index profile of the fiber is measured, and a cutoff wavelength of 1000 nm is calculated with a refractive index difference between the core layer and cladding layer of 0.0045. The absorption coefficient of 0.55 dB/m at 1240 nm and the background loss of 21 dB/km at 1500 nm are measured using the standard cutback method, and no significant water peak is observed. In addition, the variation in loss with increasing pump power is measured to estimate the unsaturated loss and the ratio of the unsaturated loss to small-signal absorption, which are 0.079 dB/m and 13.6%, respectively. The results indicate that only a small fraction of bismuth ions form inactive centers to induce loss, whereas most of them form bismuth active centers associated with phosphorus(BACs-P). Finally, the amplification characteristics of the Bi-doped fibers are measured using a single-stage amplifier configuration with a forward-pumping scheme. With the signal power of -15 dBm and the pump power of 460 mW at 1240 nm, the maximum gain of 21.2 dB is achieved using a fiber length of 140 m. A net gain with a bandwidth (1270‒1480 nm) covering the O-band and E-band is obtained, and the 3 dB bandwidth from 1310 nm to 1365 nm is also achieved. It can be observed that the gain in the O-band is significantly greater than that in the E-band; thus, we believe the difference is attributable to the higher concentration of BACs-P than that of bismuth active centers associated with silicon (BACs-Si).

We report a bismuth co-doped phosphosilicate fiber fabricated using MCVD technology. The maximum gain of 21.2 dB is achieved at 1340 nm for the 460 mW pump power of a 1240 nm laser diode and the signal power of -15 dBm in the single-stage and forward-pumping amplifier configuration. Meanwhile, a net gain bandwidth from 1270 nm to 1480 nm covering the O+E band is achieved, and the 3 dB bandwidth is approximately 55 nm.