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This paper reports on transient processes and cross talk (cross modulation) in an O-band (1260 – 1360 nm) bismuth-doped phosphosilicate fibre amplifier. Times characterising dynamic parameters of the amplifier at a pump wavelength of 1.23 μm have been determined experimentally. Data have been obtained on the effect of the modulation frequency Ω of one of the channels in the range 10 Hz to 1 MHz on the cross-talk intensity in another channel. The data, in combination with numerical calculation results, show that the cross talk in the Bi-doped fibre amplifier is insignificant at modulation frequencies Ω above 100 kHz. Our results demonstrate that O-band Bi-doped fibre amplifiers are potentially attractive for practical application in multichannel high-speed data transmission systems.

In this article, we describe the technology for manufacturing of the phosphosilicate bismuth-doped optical fibers providing optical amplification near 1.32 $\mu$ m. The effect of preforming sintering process and fiber drawing on the optical properties of the fabricated fibers were studied. We showed that an increase in the vitrification temperature of the P-doped silica glass soot layers deposed on the inner surface of a fused silica tube from 1850 to $\mathbf {\sim \text{2000}}$ °C (while keeping other parameters unchanged) provides a noticeable reduction of the unsaturable losses. We also found that the fiber drawing speed significantly affects the optical properties of bismuth-doped fibers. In particular, an increase in the drawing speed from $\sim$ 1 to $\sim$ 80 m/min led to a decrease of both active absorption and unsaturable losses, while the ratio of active absorption to unsaturable losses became greater. As a result, the laser efficiency of the bismuth-doped fibers grew two-fold from 19.5% to 37%.

In this paper, we focus on the fabrication and investigation of optical properties of W-type and Graded-index single-mode Bi-doped germanosilicate fibers. The laser and gain characteristics of Bi-doped fibers of new designs were studied. It was shown that by variation of doping profile, it is possible to change characteristic parameters (active absorption, unsaturable loss level) of the active medium and, as a consequence, achieve an improvement of the performance of the optical devices based on these types of fibers. As a progress one can consider the creation of a Bi-doped fiber laser operating at 1460 nm with a record efficiency of 72% using a relatively short active fiber (L = 75 m); and a 20-dB Bi-doped fiber amplifier (L = 120 m) with a pump power of 45 mW (for the input signal powers lower than 30 µW) having a high gain efficiency of 0.52 dB/mW. We suggest that the obtained results could be a driver for further investigation in this direction.

This paper presents a study of anti-Stokes luminescence in optical fibres with a bismuth-doped ~50%GeO2 – 50%SiO2 glass core under IR excitation. The anti-Stokes luminescence intensity has been measured as a function of temperature in the range 77 – 300 K. From these data, we have evaluated the activation energy for the thermal quenching of anti-Stokes luminescence in the visible and IR spectral regions. A mechanism of anti-Stokes luminescence in this type of optical fibre has been proposed.

For the first time, we fabricated and studied optical characteristics of a bend-insensitive bismuth-doped P 2 O 5 – SiO 2 glass core fiber operating in a spectral region of 1.3 – 1.4 µm having a depressed cladding design.

The alteration of the laser properties of bismuth-doped high-GeO 2 fibers brought about by thermal treatment was investigated experimentally and simulated numerically. We have demonstrated that thermal treatment can be beneficial for the shortening of the active fiber length due to an increased concentration of the laser-active centers.

We present results on the lasing properties of the Bi-doped high-germania glass fibers thermally treated at different heating and cooling conditions. The absorption and luminescence spectra, the luminescence lifetime of bismuth-related active centers (BACs) formed in the Bi-doped fibers before and after treatment were measured. Analyzing the results, it was shown that the concentration of the BACs could be increased by approximately two times after treatment at certain conditions. A series of experiments regarding laser action at 1730 nm using pristine and treated Bi-doped fibers was performed. From the dependencies of the slope efficiencies of the Bi-doped fiber lasers on the length of the active fibers obtained at various cooling conditions, it was found that the optimal length $${L}\approx 20$$ m of the treated active fibers required for the realization of the Bi-doped fiber lasers is two times shorter than that of the pristine fibers. In addition, the efficiency of the developed lasers being $$\approx 18\, \%$$ with respect to the absorbed pump power is greater than that of the lasers based on the pristine fibers ($$\approx 10\%$$ at $${L} = 20$$ m), but it is lower than their maximum efficiency ($$\approx 25 \,\%$$ at the optimum lengths of 45 m). From the numerical simulation of the Bi-doped fiber laser, a number of parameters needed for estimation of the native and thermally induced BAC concentrations were determined.

In this paper, we performed numerical and experimental study of the stability of bismuth-doped high-GeO2 glass core fiber used as an active medium in lasers operating in the wavelength region 1600 - 1800 nm. Mainly, we focus on the investigation of the joint effects of temperature and pumping radiation on the spectroscopic and laser characteristics of the fibers. Temporal evolution of the degradation of bismuth-related active centers (BACs) under pumping at 1550 nm, as well as the annealing of the fibers at temperature ranging from 300 to 550 °C was experimentally revealed and studied. A model describing the photochemical processes of the transformation of the BACs at different ambient conditions was proposed and used to make a long-term prediction of the dynamics of the process. The ability to simulate the long-term behavior of the medium might be instrumental since direct measurements are time consuming and therefore impractical. In addition, we performed numerical simulation to find out how the effect of photoinduced degradation of BACs affects the performance of a laser based on this type of fibers.

During last decades there has been considerable interest in developing a fiber amplifier for the 1.3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu $$\end{document}μm spectral region that is comparable in performance to the Er-doped fiber amplifier operating near 1.55 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu $$\end{document}μm. It is due to the fact that most of the existing fiber-optic communication systems that dominate terrestrial networks could be used for the data transmission in O-band (1260–1360 nm), where dispersion compensation is not required, providing a low-cost increase of the capacity. In this regard, significant efforts of the research laboratories were initially directed towards the study of the praseodymium-doped fluoride fiber amplifier having high gain and output powers at the desired wavelengths. However, despite the fact that this type of amplifiers had rapidly appeared as a commercial amplifier prototype it did not receive widespread demand in the telecom industry because of its low efficiency. It stimulated the search of novel optical materials for this purpose. About 10 years ago, a new type of bismuth-doped active fibers was developed, which turned out to be a promising medium for amplification at 1.3 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu $$\end{document}μm. Here, we report on the development of a compact and efficient 20-dB (achieved for signal powers between \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-40$$\end{document}-40 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-10$$\end{document}-10 dBm) bismuth-doped fiber amplifier for a wavelength region of 1300–1350 nm in the forward, backward and bi-directional configurations, which can be pumped by a commercially available laser diode at 1230 nm with an output power of 250 mW. The compactness of the tested amplifier was provided by using a depressed cladding active fiber with low bending loss, which was coiled on a reel with a radius of 1.5 cm. We studied the gain and noise figure characteristics at different pump and signal powers. A record gain coefficient of 0.18 dB/mW (at the pump-to-signal power conversion efficiency of above 27\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document}%) has been achieved.

We investigated the optical properties of a depressed-cladding bismuth-doped phosphosilicate fiber. The performance characteristics of CW laser and optical amplifier operating within the O-band based on this fiber were studied.

Photobleaching of IR luminescence in bismuth-doped fibers was studied using laser radiation with different wavelengths: 407, 532, 639, 975, 1460 nm. For each wavelength the bleaching time was determined by fitting a stretched-exponential function to the data measured. It was revealed that the bleaching time increases with the decrease of the photon energy. The measured dependence of the bleaching rate on the launched power at 532 nm is also presented. The effect of temperature on the bleaching rate was also investigated. The role of the oxygen-deficient centers in the bleaching process is discussed.

We report a recently discovered effect of enhancement of the luminescence intensity in bismuth-doped high-germania-core glass fibers as a consequence of heat treatment. Analyzing the experimental results, it was shown that an increase of the luminescence intensity is caused by a thermally induced growth of the bismuth-related active centers (BACs) concentration. The activation energies of formation of the thermally induced bismuth-related active centers and other complexes responsible for unsaturable loss were estimated. It was found that there are major parameters, such as total Bi content, heating temperature, which affect significantly the increment of the BACs concentration. In addition, the gain and lasing characteristics of the bismuth-doped GeO 2 -SiO 2 glass fibers in which a part of the BACs was induced by thermal treatment were studied.

The destruction of laser-active centers in bismuth-doped phosphosilicate fibers exposed to laser light at 532 nm and 1240 nm during annealing at 300 – 600 °C was demonstrated. Both laser radiations were shown to facilitate bleaching of the bismuth-related active centers (BACs), although the rate of this process for 1240 nm is noticeably slower than that of the 532 nm. A phenomenological model including optical excitation processes and thermally activated conversion through a first-order reaction is presented to describe the joint impact of thermal and laser-light treatment. The proposed model is consistent with our experimental data on bleaching and allows us to determine some parameters inherent to the studied process. We demonstrate that the time behavior of the BACs destruction induced with different laser light is characterized by different activation energies. The possible underlying mechanism of the bleaching is discussed taking into account the structural features of phosphosilicate glass.

We have studied the luminescence properties of optical fibres with a bismuth-doped alumino- and phosphosilicate glass core under two-step excitation and obtained new experimental data on the properties of luminescence centres in such fibres.

For the first time, we report on the fabrication of a bend-insensitive single-mode bismuth (Bi)-doped P 2 O 5 − S i O 2 fiber having a depressed cladding design and study its gain characteristics at a spectral region of 1.3–1.4 µm. It was shown that the obtained Bi-doped fiber can efficiently operate in the spectral band even at a bend radius of 1.5 cm. In addition, it was shown that this type of fiber has a smaller mode-field diameter in comparison with a step-index single-mode Bi-doped P 2 O 5 − S i O 2 fiber with Δ n ≈ 0.006 that resulted in a decrease of saturation power and, as a consequence, in a reduction of the total pump power required to a high-level-gain operation. The laser and gain experiments show the possibility of the construction of a compact high-performance optical amplifier for O-band based on the depressed-cladding Bi-doped fiber.

We have studied UV excitation spectra of IR luminescence in bismuth-doped glasses of various compositions and obtained energy level diagrams of IR-emitting bismuth-related active centres (BACs) associated with silicon and germanium atoms up to ~5.2 eV over the ground level. A possible energy level diagram of the BACs in phosphosilicate glass has been proposed. The UV excitation peaks for the IR luminescence of the BACs in the glasses have been shown to considerably overlap with absorption bands of the Bi3+ ion, suggesting that Bi3+ may participate in BAC formation.

We report experimental measurements and numerical calculations regarding the photostability of laser-active centers associated with bismuth (BACs) in Bi-doped GeO2-SiO2 glass fibers under pumping at 1550 nm at different temperatures. It was discovered that BACs are unstable under 1550-nm pumping when the temperature is elevated to hundreds of degrees centigrade. A simple numerical model was proposed to account for the discovered instability which turned out to be in good agreement with the experimental data.

We report a study of the thermally activated processes occurring in Bi-doped GeO2-SiO2 fibers using the demarcation energy concept based on a distribution of the activation energies of a first order physico-chemical reaction. Our attention was paid to the processes of formation of the bismuth-related active centers (BACs) and Bi-centers responsible for the unsaturable loss, as well as to the restoration of the BACs in laser-bleached fibers. The activation energies of these processes were determined analyzing the thermal kinetics of these processes. The formation energies of new BACs and Bi-centers responsible for the unsaturable loss were determined to be around 1.1 eV and higher than 2 eV, respectively. The activation energy of thermal restoration process of the bleached BACs is ≈ 0.4 eV. The possible underlying mechanisms of these processes are discussed.

We present experimental results on the effects of isothermal and isochronal heat treatments, cooling and heating rates in the temperature range 400–600 °C on the optical properties of the bismuth-related active centers (BACs) formed in glass with high GeO2 content. The results reported here provide evidence that a luminescence intensity increment is caused by an increase in the amount of the BACs, but is not related to the modification of the structure of the BACs itself. Analyzing the experimental data, it was determined that the cooling rate has no noticeable effect on the ratio between the number of the BACs and that of the non-laser-active ones responsible for the unsaturable loss. Finally, some discussion of the annealing behavior of the BACs is provided.

The effect of thermal annealing on the luminescent and laser properties of high-germania-core silicate fibers doped with bismuth was investigated. We studied the behavior of optical absorption assigned to the bismuth-related active centers associated with germanium as well as the behavior of unsaturable absorption in annealed fibers with respect to the Bi content. The dependence of the increment of the active center content on the Bi concentration in the annealed fibers was obtained. We achieved laser oscillations near a wavelength of 1700 nm with a slope efficiency of 18% using a 8.5 m long Bi-doped fiber. The comparison of the output parameters of the laser based on an annealed Bi-doped fiber with the ones of a pristine Bi-doped fiber laser is given. The performance of the obtained bismuth-doped fiber lasers was modeled using the propagation and rate equations of a homogeneous quasi-two-level laser medium. Theoretical results are compared with experimental ones.

We have studied the optical properties of lightly bismuth doped (≤0.002 mol %) germanate glasses prepared in an alumina crucible. The glasses are shown to contain bismuth-related active centres that have been identified previously only in bismuth-doped fibres produced by MCVD. With increasing bismuth concentration in the glasses, their luminescence spectra change markedly, which is attributable to interaction between individual bismuth centres.

Luminescence excitation spectra of active centers in bismuth-doped vitreous SiO(2) and vitreous GeO(2) optical fibers under the two-step excitation have been obtained for the first time. The results revealed only one bismuth-related IR active center formed in each of these fibers. The observed IR luminescence bands at 1430 nm (1650 nm) and 830 nm (950 nm), yellow-orange (red) band at 580 nm (655 nm), violet (blue) band at 420 nm (480 nm) belong to this bismuth-related active center in the vitreous SiO(2) (vitreous GeO(2)), correspondingly.

In this paper we present the results on the effect of γ-irradiation on the optical properties of bismuth-doped germanosilicate fibers. It has been revealed that the absorption and emission bands of the bismuth-related active centers are mildly affected by γ-radiation with a dose ranging from 1 to 8 kGy. Experimental data on the radiation-induced absorption (RIA) dynamics of Bi-doped fibers under irradiation and post-irradiation recovery have been obtained. The RIA spectra of the Bi-doped germanosilicate fibers have been measured. The effect of Bi concentration on the RIA level is discussed.

Structural and spectroscopic properties of the pristine and γ-irradiated Bi-doped KCl and SrF2 crystals grown by the Bridgman technique were studied. New emission bands in the visible and near IR regions from the irradiated crystals were observed. An origin of optical centers responsible for near IR luminescence is discussed.

The effect of annealing temperature on the luminescent properties of bismuth-doped fibers bleached by 532 nm laser radiation was investigated. The photoluminescence (PL) measurements were performed in pristine and photobleached samples which were thermally annealed at various temperatures ranging from 100 to 900 °C and slowly cooled. We observed that the intensity of the PL band at 1700 nm in the photobleached fibers recovered its pre-bleached level. Moreover, it was shown that a significant increase of the PL level could be achieved using the special annealing regime. Thereby, we obtained the experimental evidence of a thermally activated recovery process of the PL intensity showing that photoinduced changes of PL in bismuth-doped fibers are completely reversible. The mechanism of the thermal recovery of the PL is discussed.

For the first time, 3-dimensional luminescence spectra (luminescence intensity as a function of the excitation and emission wavelengths) have been obtained for bismuth-doped optical fibers of various compositions in a wide spectral range (450-1700 nm). The bismuth-doped fibers investigated have the following core compositions: SiO(2), GeO(2), Al-doped SiO(2), and P-doped SiO(2). The measurements are performed at room and liquid nitrogen temperatures. Based on the experimental results, the positions of the low-lying energy-levels of the IR bismuth active centers in SiO(2)- and GeO(2)-core fibers have been determined. Similarity of the energy-level schemes for the two core compositions has been revealed.

Luminescence emission and excitation spectra of bismuth-doped silica optical fibers free of other dopants have been obtained to construct an emission-excitation map in a wide wavelength range of 400-1600 nm. The main low-lying energy levels of the bismuth active centers in such fibers have been determined. For the first time (to our knowledge), optical gain and lasing have been obtained in such fibers. A gain of 8 dB has been achieved with a pump power of 340 mW, and a cw fiber laser emitting at 1460 nm with an output power of 40 mW and an efficiency of ≈3% has been created.

The effect of drawing conditions on the laser properties of Bi-doped high-GeO2 fibers was investigated. The drawing speed was established to have a significant effect on the lasing efficiency of the fibers.

We present 30 dB Bi-P-doped fiber amplifier from 1287 to 1354 nm. The wider bandwidth was achieved using inhomogeneous broadening of bismuth active centers (BAC-P). Blue shifted BAC-P were pumped at 1178 nm and generated laser radiation at 1276 nm which serves as a pump source for red shifted BAC-P.