By examining the spectral and amplification characteristics of Er-doped silica fiber (EDSF) and Bi/Er codoped silica fiber (BEDSF), an active doped silica fiber with broadband-and flat-gain is investigated. Results show that the bandwidth of BEDSF with a gain greater than 20 dB is approximately 50 nm. Moreover, in the entire C-band, the gain of BEDSF exceeds 26 dB with a fluctuation range of approximately 2 dB. The excellent performance may mainly come from the role of bismuth ions. Further analysis demonstrates that the emission and gain cross-sections are enhanced, the bandwidth is broadened, and the flatness is also optimized by co-doping Bi ions. The fluorescence lifetime of Er 3+ is lengthened by 1.82 ms, illustrating that there may exist energy transfer from Bi-related active centers to Er 3+ . The results indicate that the proposed BEDSF has significant potential application in optical fiber amplifiers, lasers, sensors, and so on Index Terms-Bi/Er co-doped silica fiber, broadband-and flat-gain, gain cross-sections, energy transfer * This paper has been supported by National Key Research and Development Projects (2020YFB1805800).

We investigate the spectral and amplification characteristics of Bi/Er co-doped fiber (BEDF) and Er-doped fiber (EDF), prepared by atomic layer deposition (ALD). BEDF features a noise figure (NF) below 5.2 dB, the bandwidth with NF less than 4 dB is approximately 23 nm (1560-1583 nm), and that with a gain exceeding 25 dB is approximately 42 nm (1525-1567 nm). The quantum conversion efficiency (QCE) of BEDF at 1550 nm is 72.1% at a pump power of 300 mW. The saturated output power of BEDF at 1550 nm is 21.4 dBm. Raman spectra indicate that the maximum phonon energy of BEDF is lower than that of EDF. The fluorescence lifetime of the BEDF is 11.64 ms, which is longer than that of the EDF. These results show that co-doping with Bi ions could increase the fluorescence lifetime of Er 3+ , reduce the NF, and improve the QCE and saturated output power. Therefore, this type of BEDF is a promising gain medium for optical amplifiers and lasers.

We report a novel PbS/Er co-doped silica fiber (PEDF) with improved spectral properties in O-band (1260-1360 nm). The results show that the gain of the PEDF exceeds 15 dB in the range of 1150-1360 nm. Compared to PbS-doped silica fiber, the PEDF has a gain improvement of 6.4-10.9 dB in O-band. In addition, the concentration of Pb and S in the PEDF core is significantly higher, and the size distribution of PbS nanoparticles is more concentrated in a narrow range. These factors may account for the enhanced gain characteristics of the PEDF. This work reveals the positive effect of co-doping Er 3+ ions on the gain characteristics of the PEDF in O-band, and suggests an important research direction for the design of broadband amplifiers and light sources for optical communication with zero dispersion. Index Terms-Broadband amplifiers, gain characteristics, PbS nanoparticles, PbS/Er co-doped silica fiber. I. INTRODUCTIONE XPANDING the bandwidth of optical fiber amplifiers and enhancing the gain characteristics can improve the efficacy of communication systems [1], [2]. Erbium-doped fiber amplifiers and other rare-earth-doped fiber amplifiers can achieve high gain in specific wavelength bands. However, the gain bandwidths are usually limited because of the inherent energy-level structure of rare earth (RE) ions [3], [4]. Bi-doped fiber amplifiers exhibit broadband luminescence and amplification. However, the nature of Bi active centers remains controversial and it is difficult to control during the fabrication process. Hence, it is crucial to identify new materials for improving the optical properties of actively doped fibers in the near-infrared band.Recently, nano-semiconductor materials with high quantum efficiency, bandgap tunability, and broad spectral properties have received widespread attention [5], [6]. Compared to other nano-semiconductors, those in the IV-VI group have significant Manuscript

Soliton microcombs, offering large mode spacing and broad bandwidth, have enabled a variety of advanced applications, particularly for telecommunications, photonic data center, and optical computation. Yet, the absolute power of microcombs remains insufficient, such that optical power amplification is always required. Here, we demonstrate a combined technique to access power-sufficient optical microcombs, with a photonic-integrated soliton microcomb and home-developed erbium-doped gain fiber. The soliton microcomb is generated in an integrated Si3N4 microresonator chip, which serves as a full-wave probing signal for power amplification. After the amplification, more than 40 comb modes, with 115-GHz spacing, reach the onset power level of >−10 dBm, which is readily available for parallel telecommunications , among other applications.

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