The light sources operating in the spectral range 1600-1800 nm are of great interest because of a number of potential scientific and practical applications. This stimulates a search for effective laser media for this wavelength region. Recent progress in the studying of bismuth (Bi)-doped fibers is related to the development of Bi-doped high-germania-core fibers for the mentioned wavelength region. This paper is concerned with the current state of the art in the research field of this kind of the laser-active fibers. The fabrication and spectroscopic properties of the Bi-doped high-germania-core fibers are described. The photo-induced bleaching and recovery stimulated by thermal treatment of luminescent centers in these fibers are presented. The review also includes the description of the basic parameters of the optical devices (continuous wave (CW) and pulsed lasers, amplifiers, superluminescent sources) developed using Bi-doped high-germania-core fibers.

Excited-state absorption (ESA) in various bismuth-doped fibers (BDFs) was investigated. No significant ESA in IR emission bands of Bi-doped germanosilicate and phosphosilicate fibers was found. Considerable ESA was observed in Bi-doped aluminosilicate fibers at 800–1700 nm. The ESA spectra of the aluminosilicate BDFs with different bismuth concentration were measured at room and 77 K temperature. Significant dependence of the ESA on the bismuth concentration and the fiber temperature was found.

The first bismuth-doped superfluorescent fiber source (SFS) operating at 1.44 μm was developed. An SFS maximum output power in the double-pass backward configuration reached 82 mW with a fairly high efficiency of 31% at a pump wavelength of 1310 nm. The output spectrum is close to Gaussian shape with FWHM of $\sim 25\mathrm{nm}$.

In this paper, 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.

We report on the first demonstration of the Tm-doped all-PM mode-locked fiber laser with CNT used as a saturable absorber and two fiber Lyot filters embedded into the ring resonator. Using these two filters, we were able to adjust the central wavelength of laser generation to ~1855 nm as well as to obtain the required spectral width and necessary pulse duration that are required for the proposed applications. Possible issues with the method used are discussed.

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.