The development of active media with unique gain characteristics in a wavelength range of 1-2 mu m is highly attractive for realizing ultrabroadband amplifiers for the deployment of multi-band wavelength platform to maximize the transmission capacity in existing optical communication and next-generation systems exploiting a spectral region from T- to U-bands. In this regard, fibers with bismuth active centers (BACs), which possess incredible optical properties, are suitable candidates for creating high-gain amplifiers for telecom bands. In this paper, the discovery of a novel BAC, which is formed in Bi-doped fibers with AlPO4 structural units, is reported. This BAC has a hybrid "nature" as it exhibits features of two types of BACs, specifically BAC associated with Al and P atoms. The discovered BAC offers a gain band peaked at 1240 nm, which bridges the gap between the ones of BAC-Al and BAC-P. The excellent performance characteristics of hybrid BACs are confirmed by the development of the first (to our knowledge) watt-level Bi-doped laser at a wavelength of 1256 nm and a 30-dB Bi-doped fiber amplifier for T+O-band. This study provides a better understanding of the structural features of BACs and offers an effective pathway for developing new fibers with advanced characteristics.

Tremendous advances in high-speed communications technologies have enabled modern services,including live 4K video streaming,real-time remote surgery,artificial intelligence(AI),the Internet of Things(IoT),virtual reality(VR),cloud storage,and social media.The further development of these digital platforms will inevitably require increased data transmission rate,which significantly exceed the current capabilities of existing high-speed communications systems.To meet the ever-growing data traffic,it is necessary to develop and implement new advanced solutions.Multi-wavelength transmission technology is considered one of the most promising approaches,which can potentially increase a bandwidth of transmission data over optical fiber systems by utilizing an extended range of wavelengths(from O-to U-band),where the optical loss of conventional single-mode fiber is below 0.2-0.3 dB/km.However,the success of this approach depends on the development of new amplification technologies,as traditional optical amplifiers based on fibers doped with rare earth ions,especially Er3+ions,are inherently incapable of providing effective amplification beyond the C+L telecom bands.This has spurred research into promising amplification media,which began more than 20 years ago. Bismuth(Bi)-doped fibers(BDFs)are a unique active medium suitable for optical amplifiers and lasers operating in a spectral range of 1.15-1.78 µm.The progress achieved in the development of BDFs and optical devices based on them gives hope that multi-band technologies capable of operating over the entire available spectral range can be successfully implemented in the near future.This is confirmed by the presence of commercially available devices developed by a number of telecom companies,as well as the start of implementation of bismuth-doped fiber amplifiers(BDFAs)for O-,E-,and S-band data transmission over optical communication systems.However,the progress achieved in the development of BDFA and BDF lasers was due not only to the solution of applied problems,but also to a deeper understanding of the fundamental principles of formation of bismuth active centers(BACs)and their physical nature.This review presents the main achievements in terms of optical characteristics of Bi-doped materials(crystals,ceramics,bulk glasses and optical fibers)and devices developed using these materials.This highlights that the structure and chemical composition of the glass matrix strongly influence the resulting optical properties of these media.Some fabrication strategies such as the modulation of topological order,coordination engineering,smart confined doping,and direct cluster control,and novel approaches for performance analysis(for example,"hidden potential")of BDFs are emphasized and discussed.The peculiar properties of bismuth active centers(BACs),in particular,optical anisotropy and"dark precursors",characterizing their structure and possible process leading to their formation are considered.Also,this review evaluates novel designs of BDFs,especially,heterogeneous glass-core fibers,which can be used for solution of the practical problems.For instance,such designs can be useful for developing a broadband flattop optical amplifier with adopted characteristics.In addition to bismuth-doped materials,this review includes the mainstream results in BDFAs for advanced optical technologies,summarizing the obtained results over two decades.Despite the significant progress the prospects for commercial production of BDFs remain uncertain that primarily due to difficulties in the reproducibility of Bi-doped fiber parameters and the high level of unsaturable loss in highly Bi-concentrated fibers.Addressing these challenges is essential to advancing commercialization and ensuring rapid deployment of this technology. Summary and Prospects Bismuth-doped fibers(BDFs)have already proven themselves as active materials that can be used to develop optical devices with unique characteristics in previously inaccessible spectral ranges.Optical amplifiers based on these active fibers exhibit high gain and low noise across all telecommunication spectral bands(from O-to U-band),while BDF-based lasers offer the benefits of high efficiency and wide wavelength tunability.However,existing research still faces significant challenges in achieving a reliable technology for reproducing the parameters of BDFs,as well as in fabricating optical fibers with increased Bi concentrations and low unsaturable losses.Developing a high-gain,ultra-wideband amplifier that can be effectively integrated into existing communication systems remains a challenge.Future research should focus on balancing cost,energy efficiency,and device performance,which can be partially addressed by optimizing the BDF design and the device itself.All of this is necessary to meet the growing demand for high-speed data transmission over fiber-optic communication systems,which is crucial in the context of rapidly evolving artificial intelligence technologies.In this regard,the ability to utilize all available telecommunications bands appears very promising.We believe that progress in this area will undoubtedly lead to the development of optical communication systems with significantly increased bandwidth,where bismuth-doped optical amplifiers are key components.Moreover,thanks to ongoing advances in optical materials and process technology,bismuth-doped fiber technology can pave the way for efficient,reliable,and scalable solutions for next-generation fiber-optic systems.

We compare three all-fiber amplifier schemes for highly-chirped pulses near 1.3 micron: one based on stimulated Raman scattering in a standard passive fiber, another utilizing the active centers of a Bi-doped fiber, and a third combining both approaches in a hybrid configuration. Amplification in the Bi-doped fiber provides a higher net gain than Raman amplification but introduces spectral distortions of the pulses. We have shown that spectrally uniform amplification simultaneously with high gain can only be achieved in the hybrid scheme, resulting in a 19-fold net gain, a 25 nJ output pulse energy, and a compressed pulse duration of 600 fs. The resulting source holds great potential for application in deep-tissue multiphoton microscopy.

Pulsed laser sources operating in the spectral range of 1.8 to 2.1 μm draw considerable attention due to their wide range of practical applications in many areas, including medicine, sensing, materials processing, etc. In this work, we propose a pulsed Tm-doped fiber laser scheme operating in the 2 μm spectral region with pulsed pumping at 1.57 μm. The pump source consisted of a series-connected Er-Yb pulsed master oscillator and an EDFA emitting ∼400 μs pulses with an energy of 9.3 mJ. Using this setup, we made a Tm-doped fiber laser that provided 2 mJ pulses in the 2 μm spectral region with a slope efficiency of 28% from pulses at 1.57 μm.

Application of multi-phonon-assisted energy relaxation for adjustment properties of laser media is demonstrated. A novel approach of laser active media luminescence properties improvement by co-doping of germanosilicate bismuth-doped optical fiber with boron is suggested. Influence of co-doping with boron on quantum yield, branching ratio and lifetime of luminescence of bismuth active center associated with silicon (BAC-Si) in Bi-doped borosilicate fibers were investigated. The multi-phonon decay rates of electron transitions in BAC-Si were obtained for germanosilicate fibers doped with different concentrations of boron.

In this paper, we propose a novel design of bismuth-doped fibers (BDFs) for wideband amplification within telecom wavelength range based on a multi-layered heterogeneous glass core structure, where different types of bismuth active centers (BACs) can be formed. This concept offers potential benefits on "smart" control of formation and spatial selection of different BACs by variation of chemical composition of the core glass layers. According to the proposed approach, step-index single-mode active fibers with a core consisting of bismuth-doped layers made of P$_{2}$O$_{5}$-SiO$_{2}$ and GeO$_{2}$-SiO$_{2}$ glass have been fabricated by MCVD technique. We have performed a comprehensive study of optical properties of the preforms and developed BDFs, their chemical composition analysis, including the radial distribution of various types of BACs. As a result, it was experimentally found that the developed fiber design can successfully be used to obtain optical amplification in an extra wide wavelength region from 1250 to 1500 nm ($\approx$ 250 nm bandwidth) due to overlapping the gain bands of the BACs associated with P and Si atoms. In addition, we demonstrate lasing in various spectral bands in the studied BDFs under cladding-pumping with commercially available multimode diodes at 793 and 808 nm. The presented results may be useful for further advance in development of key optical devices for wideband communication systems.

We report on the development of a cladding-pumped ultra-broadband optical amplifier for a spectral region of 1260–1480 nm using a Bi-doped fiber (BDF) with heterogeneous core formed by combination of PO- and GeO-containing glass layers. This BDF design ensures a favorable local environment for the effective formation process of BACs associated with P and Si atoms. The BDF amplifier (BDFA) pumped into the cladding by multi-mode laser diodes at 793 and 808 nm is characterized by a peak gain of 24 dB, a minimum noise figure of 6 dB, and a bandwidth of 160 nm at a gain level of 20 dB. The effect of the parameters of the fiber design on the output characteristics of BDFA was analyzed by numerical simulation. The calculated data were in good agreement with the experimental ones, which allowed us to outline possible ways to improve the BDFA.

The quantum yield (QY) of luminescence of bismuth active centers associated with silica (BAC-Si) in bismuthdoped optical fibers was measured. The luminescence bands peaked at 830 nm and 1400 nm were excited at $800 \mathrm{~nm}, 1240 \mathrm{~nm}$ and 1310 nm. The branching ratio (BR) of radiative transitions at 830 nm and 1400 nm excited at 800 nm was defined. The influence of co-doping with boron on QY and BR is examined.

Multi-wavelength-band transmission technology based on the exploitation of the extended spectral region is considered as a potential approach to increase the transmission capacity in the deployed fiber-optic communication infrastructure.The development of optical amplifiers operating in the O-,E-,S-,and U-telecom bands is an extremely important challenge for the successful implementation of this technology.Bismuth-doped fibers are of increasing interest as gain materials,which can be used to provide broadband amplification in the mentioned telecom bands.This is due to the ability of Bi ions incorporated into glass network to form bismuth active centers（BACs） with specific optical properties,which are primarily determined by the glass modifiers.In this work,the impact of the doping profiles of both Ge atoms as glass modifiers and Bi ions on the BACs formation is studied using a series of bismuth-doped fibers fabricated by the modified chemical vapor deposition （MCVD） technique.The Bi-to-BACs conversion efficiency in various spatial regions of the studied samples is presented.It is turned out that for high-Bi concentration regions,the conversion efficiency is very low （less than 10%）.In addition,the relationship of the conversion efficiency to the distribution of Bi ions and/or Ge atoms is discussed.Finally,a continuous-wave laser at 1.46μm with a record slope efficiency of 80%is demonstrated using a Bi-doped fiber with confined doping profile,where the Bi-to-BACs conversion efficiency is 35%.This paper provides new information which might help to facilitate understanding of the features of Bi-doped fibers and their potentially achievable characteristics.

Multicore fibers are promising structures with specific light propagation properties, which can be managed to benefit several applications in optical communications, fiber lasers and amplifiers, high-resolution imaging, and fiber-based sensors. The current use of multicore fibers in laser technology is mainly focused on in-phase coherent beam combining in far-field regions (out-cavity) using bulk optical elements. However, this approach is challenging in terms of the power scalability of all-fiber lasers (intra-cavity), particularly with using low-gain media, where it is needed to provide mode-coupling (supermode propagation) stability along relatively long lengths. Here, we report a conceptual design and fabrication of a multicore bismuth-doped fiber that is capable of achieving light amplification and stable lasing in the E-telecom band inside the cavity using the supermode selection approach. By analysis of experimental and simulation data, it was found that the employment of the proposed design of a Bi-doped fiber provides a considerable advantage over the single-core fiber in terms of laser performance (output power, slope efficiency) in the cladding-pumped configuration. These results open up new opportunities for further advancement of the optical fiber technology towards efficient bismuth-doped fiber-integrated amplifiers and lasers for the O+E+S+C+L+U-telecom bands, which may find important applications, especially for the development of next-generation multiband optical transmission systems.

We report results on cladding-pumped laser based on two low-Ge silica fibers with various bismuth concentration. The maximum efficiency and output power were 3.8% and $\sim 0.38\mathrm{W}$ , respectively. Lasing for >100 hours led to >10% of power degradation.

In this Letter, we investigated the potential scalability of output power of a cladding-pumped laser and a power amplifier (booster) based on a multimode Bi-doped fiber (BDF) using the mode-selection approach. We fabricated the multimode double-clad graded-index (GRIN) fiber with a confined Bi-doped germanosilicate glass core with a diameter of ≈30 and ≈60 µm. Using femtosecond (fs) inscription technology with high spatial resolution, Bragg gratings of a special transverse structure allowing the selection of low-order modes were written into the core of BDFs. The operation features of the cladding-pumped multimode bismuth-doped GRIN fiber lasers with the inscribed Bragg gratings with various reflection coefficients were investigated. In addition, the behavior of the output power and the beam quality (M 2 parameter) of the optical radiation of the developed devices was studied. The CW laser and booster operating at nearly 1.45 µm with maximum output powers of ≈0.8 and ≈1 W, respectively, based on the 60-µm-core BDF under pumping by multimode laser diodes at 808 nm were developed, which are, to the best of our knowledge, the most powerful cladding-pumped BDF devices to date. Near single-mode lasing (M 2 <1.3) is demonstrated for a 30-µm-core fiber. The experimental data open new possibilities to achieve higher powers in cladding-pumped BDF sources, which are more cost-effective compared to core-pumped counterparts.

We report the amplification of highly-chirped dissipative solitons around $1.3 \mu \mathrm{m}$ in an all fiber configuration by two methods: Stimulated Raman Scattering in a standard passive fiber and using active centers in a Bi-doped fiber pumped by CW radiation. The former provides spectrally uniform amplification resulting in good quality of compressed pulse, and the latter provides a 16 -fold net gain.

Polarization-dependent gain (PDG) effect was studied in a conventional core-pumping configuration of bismuth-doped fiber amplifiers (BDFAs) based on PANDA-type phospho- and germanosilicate core fibers. The PDG value was determined as the gain difference between the orthogonal signal polarizations, which was found to be in range of 2.5-3 dB at total gain of >20 dB in such BDFAs. This effect is more pronounced for BDFA with a germanosilicate fiber. The experimental results are in a good agreement with the calculated data derived from the simulation model, relying on the anisotropy parameter of bismuth active centers (BACs) obtained from the degree of luminescence polarization, which ranges between 9 and 12% for BACs-Si and 18 and 22% for BACs-P. The obtained data can be useful for the optimization of performance of BDFAs and studying structural peculiarities of BACs.

In this article, output power characteristics of cladding-pumped lasers based on bismuth-doped phospho- and germanosilicate core glass fibers were thoroughly studied and analyzed. The developed lasers are capable of providing the output power of several hundred milliwatts with the slope efficiency close to 1%. The output power saturation for the Bi-doped germanosilicate fiber laser and absence of the saturation regime for the Bi-doped phosphosilicate fiber one was found. This effect is explained by a relatively long relaxation time of bismuth active centers (BACs), namely a non-radiative transition from the higher-lying pump level to the upper laser (metastable) level in the corresponding energy levels diagram. The experimentally obtained data are in good agreement with the results of numerical simulation. In addition, it was investigated that the relaxation time of the BACs from the higher-lying level can have a significant impact on the laser efficiency and the achieved output power. The consequences caused by the saturation effect is possible to overcome with dual-wavelength pumping (combining cladding-pumping at 808-nm wavelength and core-pumping at a wavelength in the range of 1930 – 2040 nm), which provides both the BACs excitation and the stimulated relaxation of BACs from the excited (pump) state to the upper laser level. Using this approach the output power of the Bi-doped fiber laser was increased by almost two times.

For the first time, the paper reports on the development of a bismuth-doped fiber laser with a generation wavelength of 1.3–1.4 μm pumped into the inner cladding of the active optical fiber by radiation of multimode semiconductor diodes at λ = 0.79 μm. The experimental data concerning the generation modes of various configurations of such laser obtained at variation in the reflectivity and spectral selectivity of used mirrors, as well as the length of active optical fiber are presented. The slope efficiency of the bismuth-doped fiber laser operating at wavelength of 1360 nm achieves 1.4% with respect to the pumping power launched into the optical fiber cladding, and the maximum output power increases 300 mW.

Bismuth-doped fibers (BDFs) are considered nowadays as an essential part of the development of novel optical amplifiers, which can provide a significant upgrade to existing fiber optic telecommunication systems, securing multiband data transmission. In this paper, a series of BDF amplifiers (BDFAs) for O-, E-, and S-telecom bands based on a cladding pumping scheme using low-cost multimode semiconductor laser diodes at a wavelength of 0.7-0.8 µm were demonstrated for, it is understood, the first time. The developed BDFAs are characterized by a high peak gain of >25-30 dB in the corresponding telecom bands and a relatively low noise figure of 5-6 dB. Comparative analysis shows that most of the parameters of cladding pumped BDFAs are close to those of the best core pumped ones. This research opens up new opportunities for utilizing Bi-doped fibers as a key element of cost-effective and ready-to-work BDFAs for various practical applications.

The polarization properties of bismuth active centers (BACs) are important for many applications of bismuth-doped fibers, but they are still lacking in study. In this paper, we present the measurements of polarized luminescence (PL) of the BACs formed in a Bi-doped phosphosilicate glass matrix. This research was performed on phosphosilicate fiber preforms used for the drawing of active bismuth-doped fibers for efficient optical amplifiers and lasers. The degree of polarization (DOP) of luminescence of the BACs associated with phosphorus and silica (BAC-P and BAC-Si) is provided and discussed. The DOP of luminescence at the 1320 nm wavelength appeared to be around 19%, 0% and 7.5% for pumps at 1240 nm, 762 nm and 425 nm, respectively. The DOP of PL caused by resonant excitation may be described in terms of a model of a partially anisotropic oscillator with the parameters represented by the principal axes of an ellipsoid. For the resonant excitation at 1240 nm, the ratio of the major principal axis to the minor one turned out to be 5.1 and 3.0 for BAC-P and BAC-Si, respectively.

The results in creation of continuous-wave bismuth-doped fiber lasers operating in a wavelength range of 1.3–1.5 μ m with diode pumping (using multimode semiconductor laser diodes) are presented. The concept of development of such devices is considered by an example of a laser scheme. The problems concerning the peculiarities of operation of cladding-pumped bismuth-doped fiber lasers, their main parameters (efficiency, generation power, and stability) are discussed. In addition, the possibilities of optimization of the parameters of such devices by changing the geometry of the inner cladding and using dual-wavelength pumping are considered.