With the widespread deployment and large-scale application of 5G networks, global network data traffic is rapidly increasing. The rise of technologies such as artificial intelligence, autonomous driving, metaverse, and extended reality has presented higher requirements for the data transmission capabilities of existing fiber-optic communication networks, and upgrading them to meet future challenges is now urgent. Currently, erbium-doped fiber amplifiers (EDFAs) are widely used in fiber communication networks due to their excellent gain performance. However, the gain bandwidth of EDFAs covers only the conventional C band and a portion of the L band in the low-loss transmission band of silica fibers, which severely limits the further expansion of commercial communication bands. Utilizing communication bands other than the C+L bands is an effective means of improving the communication capacity of fiber-optic networks. Therefore, the development of high-Ge bismuth-doped fiber (BDF) amplifiers that can operate in the U band is of great significance. To date, only the Fiber Optic Research Center (FORC) of the Russian Academy of Sciences is capable of manufacturing high-Ge BDFs for U-band efficiency amplification. Therefore, developing high-gain and highly efficient BDFs for U-band amplification while overcoming foreign technological barriers and achieving the localization of related optical devices are all necessary.

In this study, a BDF is prepared using the modified chemical vapor deposition (MCVD) method combined with solution doping technology. The refractive index profile is measured using a preform analytical instrument. An optical microscope is used to observe the end face of the fiber. The mass fraction of Bi is measured using inductively coupled plasma mass spectrometry (ICP-MS) and is found to be approximately 0.02%. An electron probe micro-analyzer (EPMA) is used to test the mole fraction of the core GeO₂, which is as high as 42%, and the radial distribution of the GeO₂ in the core is measured using a line scan. The absorption spectrum of the BDF is measured using a fiber analyzer based on the truncation method. The total dispersion coefficient of the BDF in this study is 16.7?19.6 ps/(nm·km) in the range of 1600?1700 nm, as derived from a simulation conducted using COMSOL Multiphysics software. A single-stage amplification system is then constructed using a 205-m long BDF, and a 16-channel comb-shaped light source covering 1595?1670 nm with a spacing of approximately 5 nm is used to provide the input signal; the input signal power is then adjusted to -30 dBm. A 1550-nm laser diode (LD) is used to provide forward pumping, where the actual pump power that enters the BDF is ~800 mW.

Figure 4 shows the gain test results of the 205-m long BDF in a single-stage amplification system. Because the gain peak of a high-Ge BDF is typically located near 1700 nm and the long-wavelength range of the signal provided by the comb light source used in this study can reach only 1670 nm, the gain performance at 1670?1700 nm can not be directly characterized. To reasonably predict the gain performance of the BDF at 1670?1700 nm based on a test of its gain performance at 1595?1670 nm and an output spectrum at 1595?1700 nm, the input-signal and output spectra at 1590?1700 nm are shown in Fig.4(a). It shows that the amplified spontaneous emission (ASE) power increases with wavelength, and the gain is higher at wavelengths with higher ASE power in the range of 1590?1670 nm. In addition, Fig.4(b) shows that the gain increases with wavelength, and the noise figure (NF) decreases accordingly. Considering the growth trend of the ASE and gain, we can reasonably speculate that the gain between 1670 nm and 1700 nm will continue to increase, as shown by the dotted line in Fig.4(b). Figure 4(b) also shows that a measured gain of 26.3 dB is obtained at 1670 nm, and the predicted gain at 1700 nm exceeds 32 dB. The gain levels of the BDFs at different pump powers and pump conversion efficiencies (PCEs) under different input signal powers are tested, as shown in Figs.5(a) and 5(b), respectively. Figure 5(a) shows that the gain at 1670 nm increases and gradually saturates as the pump power gradually increases. A calculation of the ratio of the gain to pump power reveals that the maximum gain efficiency can reach 0.165 dB/mW.

In this study, a high-Ge BDF with a core GeO₂ mole fraction of ~42% is prepared through the MCVD method combined with solution doping technology. In a single-stage amplification system under the 1550 nm forward pump light (pump power of approximately 800 mW) and input signal power of -30 dBm, the 205-m long BDF achieves a gain of 26.3 dB at 1670 nm with a gain efficiency of 0.165 dB/mW. In addition, based on the growth trends of ASE and gain with increasing wavelength, we predict that the BDF can achieve a gain of over 32 dB at 1700 nm.