The advent of optical communication technology in the information age has significantly increased data traffic demand. However, the current fiber communication backbone, which employs wavelength-division multiplexing (WDM) and erbium-doped fiber amplifiers (EDFAs), utilizes only the C+L bands (1520‒1620 nm) with a bandwidth of approximately 100 nm, resulting in a low efficiency of 20% in spectrum bandwidth resource utilization. To effectively use the O, E, S, and U bands other than C+L bands, communication networks must be equipped with optical fiber amplifiers to amplify the signals in the corresponding bands to compensate for transmission loss. However, to date, no fiber amplifier exists that can effectively satisfy the commercial requirements of these frequency bands. To further expand fiber amplifier gain bandwidth, germanosilicate bismuth-doped fibers have attracted attention owing to their unique luminescence characteristics, which are expected to address the commercial requirements of communication in E+S bands. However, because of the unknown formation mechanism and source of luminescent active centers (i.e., active bismuth ions) in germanosilicate bismuth-doped fibers, the mass fraction of effective active bismuth ions in the fibers is low (<10^-4). As a result, the lengths of fibers used in the E- and S-bands range from 150 m to 320 m, compared to the 5‒10 m length of commercial EDFA fibers, making the application cost of germanosilicate bismuth-doped fibers and the miniaturization difficulty of manufacturing devices significantly higher. Therefore, it is necessary to study high-absorption germanosilicate bismuth-doped fibers to expand their transmission bandwidths and shorten their lengths.

In this study, the modified chemical vapor deposition (MCVD) method combined with liquid-phase doping is used to fabricate germanosilicate bismuth-doped fibers. The small-signal absorption spectra of the germanosilicate bismuth-doped fibers are measured through the standard truncation method. The Bi and Ge doping concentrations are measured using an electron probe microanalyzer (EPMA). The unsaturated fiber loss is characterized using an unsaturable loss (UL) test system (Fig. 2). In addition, a multi-wavelength division multiplexing light source (1330‒1510 nm, interval of 20 nm) is used as the signal, and a single-stage forward pump structure (Fig. 4) is constructed to test the bismuth-doped fiber gain performance (Fig. 5) and efficiency (Fig. 6). The input pump power (wavelength of 1310 nm) and total input signal power are 367 mW and -20 dBm, respectively.

As shown in Fig. 2, the absorption peak height of the bismuth-doped fiber is higher than that of the optical fiber sold in Russia (OFSR). At a wavelength of 1310 nm, it is 1.16 dB/m, which is 3.87 times higher than that (0.3 dB/m) of the OFSR for small signals. The effective absorption of BACs-Si in small signal absorption is further determined by measuring the unsaturated absorption coefficient at 1310 nm, which is only 0.19 dB/m, accounting for 16.4% of small signal absorption (Fig. 3). The absorption attributed to the bismuth active centers (BACs-Si) is subsequently calculated by subtracting UL (0.19 dB/m) from the small signal absorption (1.16 dB/m) at 1310 nm, yielding a value of 0.97 dB/m. Figure 5 shows that the peak gain of the bismuth-doped fiber is 33 dB at a wavelength of 1450 nm when the fiber length is 65 m. As the fiber length decreases, the gain peak gradually increases at 1450 nm and subsequently slowly decreases. This is because in long fibers, amplified shortwave signals are reabsorbed, producing long-wave signals. However, as the fiber length decreases, the reabsorption of the shortwave signal weakens, resulting in a strengthened shortwave and slightly decreased long-wave signal. Consequently, the 20 dB gain bandwidth widens slightly as the fiber length decreases. The specific 20 dB gain range for fiber lengths of 70, 65, 60, 55, and 50 m are 60, 63, 63, 63, and 65 nm, respectively, as shown in Table 2. Figure 6 shows the gain and gain efficiency of the bismuth-doped fibers at a wavelength of 1450 nm for different pumping powers when the fiber length is 65 m. The gain efficiency ranges from 0.09 dB/mW to 0.23 dB/mW, and the gain coefficient per unit length reaches 0.51 dB/m.

In this study, the preparation and characteristics of a high-absorption germanosilicate bismuth-doped fiber are described. The fiber has a high concentration of BACs-Si and low UL, with small signal absorption at 1310 nm of 1.16 dB/m and UL at 1310 nm of only 0.19 dB/m, accounting for 16.4% of small signal absorption. A high-absorption germanosilicate bismuth-doped fiber is prepared based on the MCVD method and solution doping technology. When the total input power is -20 dBm and the forward input pump power is 367 mW at 1310 nm, the 50 m long optical fiber achieves a gain of over 20 dB at 1414‒1479 nm. The maximum gain of 33 dB is achieved at 1450 nm when the fiber length is 65 m, and the gain efficiency ranges from 0.09 dB/mW to 0.23 dB/mW at different pumping powers. At 65 m length, the gain reaches its maximum (33 dB) at 1450 nm, and the gain coefficient per unit length reaches 0.51 dB/m. Compared to existing reports, the fiber usage length is significantly reduced, and the gain level is further improved.