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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%.

The effect of drawing conditions on the optical properties of the bismuth-doped high-germania-glass core fibers has been studied. It was found that the gain characteristics of these fibers become better with the increase of the drawing speed from 10 to 100 m/min. The small-signal absorption spectra of the Bi-doped fibers obtained at drawing speeds lower than 1 m/min are characterized by a high level of the unsaturable loss and the presence of an unstructured band situated within a wavelength region of 1100 - 1250 nm. The variation of drawing temperature ranging from 1770 to 1830 °C almost did not affect the optical properties of the fibers studied. It was shown that an optimization of the drawing parameters can significantly increase the performance of the developed bismuth-doped fiber lasers.

We have studied polarised luminescence of bismuth active centres in germanosilicate glasses. The 1.4-μm luminescence components with orthogonal polarisation states under excitation with linearly polarised light at λ = 1.26 μm have been shown to differ in intensity. The degree of polarisation reaches ∼0.12. Increasing the concentration of bismuth active centres leads to a decrease in the degree of polarisation of luminescence in the range studied.

We present a tunable, continuous-wave fiber laser with a tuning range of 187 nm (1617-1804 nm) and an fiber amplifier with an output power of more than 200 mW, both based on bismuth-doped germanosilicate fiber.

We present laser-based methane detection near 1651 nm inside an antiresonant hollow-core fiber (HCF) using photothermal spectroscopy (PTS). A bismuth-doped fiber amplifier capable of delivering up to more than 160 mW at 1651 nm is used to boost the PTS signal amplitude. The design of the system is described, and the impact of various experimental parameters (such as pump source modulation frequency, modulation amplitude, and optical power) on signal amplitude and signal-to-noise ratio is analyzed. Comparison with similar PTS/HCF-based systems is presented. With 1.3 m long HCF and a fiber amplifier for signal enhancement, this technique is capable of detecting methane at single parts-per-million levels, which makes this robust in-fiber sensing approach promising also for industrial applications such as, e.g., natural gas leak detection.

We report on, to the best of our knowledge, the first Q-switched bismuth-doped fiber laser operating at ${\sim }1.3~\mu \mathrm {m}$ . An acousto-optic modulator (AOM)-based active Q-switching was realized in a ring cavity with a bismuth-doped phosphosilicate fiber as an active medium. Stable pulsing in the range from 1310 to 1350 nm was achieved using various fiber Bragg gratings. The dependence of the laser operation on the unsaturable loss of the active fiber was studied. Optimization of the scheme and selection of an appropriate bismuth-doped fiber provided maximum pulse energy of ${\sim }11.5~\mu \mathrm {J}$ at the output of the laser. Using the data obtained, the evaluation of the extractable energy and the concentration of the bismuth active centers in the fiber samples under investigation was performed.

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.

The luminescence properties of bismuth-containing high-silica glass obtained after impregnating matrices of porous glass in nitrate solutions of bismuth nitrate pentahydrate for 24–48 h followed by heat treatment in the range from 400 to 890°C are studied. It is found that the concentration of Bi(NO3)3 in the impregnating solution affects the content of bismuth in the synthesized samples stronger than the duration of the impregnation and the temperature of the heat treatment. It is established that all the samples studied in this work have blue-green luminescence (λem = 420–520 nm at λexc = 300 nm) caused by the presence of Bi3+ ions. When the concentration of bismuth in the samples is increased a long-wavelength shift of the luminescence band maximum is observed. When the heat treatment temperature is increased a shortwave shift is observed. A yellow-orange luminescence with the maximum in the λem range of 576–582 nm (λexc = 480 nm) caused by the presence of Bi2+ ions is observed for the samples with a high bismuth content (1.17–1.18 wt % of Bi2O3) heat-treated in air at ~700 and 750°C.

The microstructure of porous glass has been studied by using electron microscopy and X-ray microanalysis. Porous glass has an average pore size of about 4 nm in its interior, while larger pores and channels, up to 40 nm in size, exist in a surface layer, which is about 200 nm in thickness. Transparent consolidated samples with a bismuth concentration in the range ≤ 0.01–0.8 at. % were prepared by impregnation of porous glass with an aqueous solution of bismuth nitrate followed by thermal treatment. The addition of aluminum nitrates to the soaking solution results in the reduction of the bismuth content in the samples. The composition inhomogeneity of the porous glass and nanocrystalline inclusions were investigated on the nanolevel by means of transmission electron microscopy. The luminescence spectra of the samples were typical for aluminosilicate host glasses. The luminescence bands had the same shape in the entire bismuth concentration range from ≤0.01 to 0.8 at.%. The band intensities depend on the bismuth concentration and reach their maxima at 0.1–0.5 at.%.

The spectral region between 1600 and 1800 nm is still a band where optical amplifiers struggle to achieve satisfactory gain and output power levels. The output power typically does not exceed few tens of milliwatts, what severely limits some applications. In this paper, the bismuth-doped fiber amplifier (BDFA) operating beyond 1600 nm is presented. We demonstrate the in-house developed BDFA capable of providing output powers that exceed 200 mW for wavelengths near 1700 nm. The performance of the amplifier is discussed and various properties of the device are presented such as gain characteristics and noise figure.

We present highly efficient b ismuth-doped fi ber am plifier cov ering almost all O- and E-bands. Using 250 mW single wavelength pumping at 1256 nm and low OH bismuth-doped fiber, we managed to achieve 26dB peak gain with -3dB bandwidth of 116 nm.

We present a wideband fiber amplifier with a flat gain spectrum ranging from 1306 to 1441 nm at a 3 dB level from maximum of 26 dB based on bismuth-doped active fibers with phosphosilicate and germanosilicate glass core. Combination of pumping at 1180 nm and gain clamping at 1280 nm was used to achieve a wider gain spectrum.

We report experimental results and numerical calculations regarding the stability of laser-active bismuth-related centers in Bi-doped GeO 2 –SiO 2 glass fibers to laser radiation at 1550 nm (at different temperatures).

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.

A bismuth-doped fiber amplifier (BDFA) operating between 1650 nm and 1700 nm will be presented. This wavelength region is particularly interesting due to potential application is laser-based methane detection. However, typical output power from laser diodes operating in this spectral region is only between 5 and 15 mW which may limit sensitivity and/or detection range in some spectroscopic systems. Application of fiber amplifiers could help to overcome these limitations. BDFA presented in this paper provides output powers up to 80 mW at 1651 nm and 100 mW at 1687 nm. We analyze the noise at the output of the amplifier and demonstrate its application to photothermal spectroscopy of methane near 1651 nm.

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.

We present the absorption spectra of bismuth active centers associated with phosphorus obtained experimentally. The results showed that the peak values are ~2.5 ±0.4 pm2 and weakly depend on the active centers concentration.

Using a bismuth-doped aluminosilicate fibre laser tunable in the range 1130–1210 nm, we have examined the effect of pump wavelength on the gain spectrum of bismuth-doped phosphosilicate fibres in the range 1220–1400 nm. At a pump wavelength of 1195 nm, we obtained a peak gain wavelength of 1300 nm, which is near the centre of the standard range for O-band transmission (1270–1320 nm) in fibre-optic communication links, in particular between data centres. Using a bismuth-doped phosphosilicate fibre amplifier pumped at 1195 nm, we have demonstrated that the transmission distance for a signal from a semiconductor laser diode directly modulated at 25 Gb s−1 [on – off keying (OOK) modulation] can be increased for G.652 fibre from the standard 10 to 80 km or even more.

Short-lived radiation-induced absorption (RIA) of light peaking at wavelengths >1700 nm has been revealed in a γ-irradiated (≤1 kGy) heavily (50 mol% GeO2) Ge-doped silica optical fiber (Ge-doped fiber) at low temperatures (≤−30 °C). This RIA is found to closely resemble in properties the known RIA due to inherent self-trapped holes (STHs) in undoped-silica-core optical fibers (same spectral range, low thermostability, non-monotonic evolution with dose, shift of the peak wavelength with temperature, Arrhenius dependence on temperature, growth of the activation energy in the course of irradiation and recovery). The STH-like RIA is found not to occur in more lightly Ge-doped fibers and in a Bi-codoped Ge-doped fiber with the same (50 mol%) GeO2 concentration. A possible microscopic structure of the STH in heavily Ge-doped fibers is discussed.

A series of barium gallium-germanate glasses (84-х) GeO2–14BaO–2Ga2O3–x Bi2O3 (х = 0; 0,1; 1; 3; 5 mol%) was synthesized by crucible technology. It was revealed that the appearance of a BaGe4O9 crystalline phase took place after additional thermal treatment of the glass samples. The structural and optical properties of the glasses and glass-ceramics were studied. It was shown that the spectral shapes and the peak positions of the absorption, luminescence and excitation bands strongly depend on the type of materials. In particular, the absorption band peaked at 480 and 660 nm is characteristic for the glass samples whereas in the absorption spectrum of the glass-ceramic two bands at 480 nm and 560 nm were observed. Moreover, with the appearance of a crystalline phase, the luminescence peak shifts towards shorter wavelengths, while its intensity increases. The possible states of Bi ions in the BaGe4O9 crystalline phase were calculated using a molecular cluster model. The obtained results indicate that an incorporated in the crystalline phase Bi+ ion could be considered as a possible near IR luminescence source.

Performance characteristics of superluminescent fiber source (SFS) operating at 1.73 μ m under different temperature conditions and gamma-irradiation of active fiber were studied. The SFS in a single-pass and backward pumping configuration was constructed using a bismuth-doped high-germania silicate fiber. The output parameters of the SFS including output power, spectral shape, mean wavelength, full width at half maximum, and peak position were measured in the temperature range from –60 to +60 °C. The simulation of the bismuth SFS at room temperature was performed for the first time to our knowledge. The simulation was also used for theoretical study of the SFS performance under gamma-irradiation. The obtained results of calculation are in good agreement with the experimental data. The effect of gamma-radiation exposure (60Co source) of active fiber on the SFS output power was investigated at different temperatures. The output power as a function of radiation dose is presented. No photobleaching of radiation induced absorption in bismuth doped fibers by pump light at 1550 nm was observed.

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.

A comprehensive theoretical investigation of a dual wavelength, cascade cavity bismuth-doped fiber (BDF) laser operating in the wavelength range of 1.7μm is presented. The fiber laser model is based on parameters extracted from experimental characterization of the BDF. The BDF serves as an active medium with optical gain in the wavelength region from 1.65μm to 1.8μm. The laser cavity is defined by a 90% mirror on one end of the BDF and two fiber Bragg gratings (FBG1 and FBG2) separated with the BDF on the other end of the laser cavity. One of the gratings FBG1 with the peak reflectivity 95% is centered at 1.725μm. The second one FBG2 with peak reflectivity of 90% is centered at 1.729μm. Both FBGs have a 3-dB bandwidth of ~0.5nm. It is shown that the cascade laser can operate at two 1.725μm and 1.729μm wavelengths with different powers depending on the parameters of the structure.

We developed a simple silica-based BDFA with 80nm 6-dB gain-bandwidth flexibly centred within 1305–1325nm, and parameters comparable to EDFAs. The amplifier can extend 400GBASE-LR8 transmission (8×26.6 Gbaud/s PAM-4 channels) beyond 50 km of G.652 fiber.

A single-mode fibre based on bismuth-doped germanosilicate glass with a high germanium concentration is considered as an active medium of a laser continuously tunable within the range 1.65 – 1.8 μm. The laser scheme included a broadband ring cavity, and the laser wavelength was controlled by an external plane diffraction grating in the Littrow configuration. For a pump power of 300 mW at a wavelength of 1.564 μm, the maximum output laser power was 15 mW in the zero diffraction order and 6 mW at the fibre output.

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.

The spectral range between 1650 and 1700 nm is an interesting region due to its potential applications in optical telecommunication and optical-based methane sensing. Unfortunately, the availability of compact and simple optical amplifiers with output powers exceeding tens of milliwatts in this spectral region is still limited. In this paper, a single-frequency continuous-wave bismuth-doped fiber amplifier (BDFA) operating at 1651 and 1687 nm is presented. With the improved signal/pump coupling and modified pump source design, the output powers of 163 mW (at 1651 nm) and 197 mW (at 1687 nm) were obtained. Application of the BDFA to the optical spectroscopy of methane near 1651 nm is also described. We demonstrate that the BDFA can be effectively used for signal amplitude enhancement in photothermal interferometry.

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 present a comprehensive theoretical and experimental investigation of a dual-wavelength, bismuth-doped fiber (BDF) laser operating near 1700 nm based on cascaded cavities. The BDF provides optical gain from 1650 nm to 1800 nm when pumped at 1550 nm. The linear laser cavity is defined by a 90% fiber mirror on one end and two fiber Bragg gratings (FBGs) with two lengths of BDF in cascade on the other. The laser can operate at either wavelength alone, or both wavelengths simultaneously by simple adjustment of the pump power. We develop a model based on rate equations and radiation transport equations for two level BDF lasers which we then extend to the case for cascaded cavities. Experimental results agree with simulations.

In this paper, we report the performance of a bismuth-doped fiber amplifier at 1687 nm. This wavelength region is particularly interesting for laser-based spectroscopy and trace gas detection. The active bismuth-doped fiber is pumped at 1550 nm. With less than 10 mW of the seed power, more than 100 mW is obtained at the amplifier’s output. We also investigate the signal at the output when a wavelength-modulated seed source is used, and present wavelength modulation spectroscopy of methane transition near 1687 nm. A significant baseline is observed in the spectra recorded when the fiber amplifier is used. The origin of this unwanted background signal is discussed and methods for its suppression are demonstrated.

Bismuth-doped fiber is a promising active media for pulsed lasers operating in various spectral regions. In this paper, we report on a picosecond mode-locked laser at a wavelength of 1.32 μm, based on a phosphosilicate fiber doped with bismuth. Stable self-starting generation of dissipative solitons, using single-walled carbon nanotubes (SWCNT) as a saturable absorber, was achieved. Evolution of the pulsed regime, depending on pump power, and stability of the pulsing were investigated.

The effect of laser and thermal annealing on the gain characteristics of bismuth-doped optical fibers was studied. Net gain enhancement in a wavelength region of $1.6-1.8\ \mu m$ in Bi-doped germanosilicate fibers after laser or/and thermal treatment was revealed.

We report the results on lasing features in annealed bismuth-doped optical fibers. It was found that the annealed fibers present better lasing properties than the pristine ones which contain similar amount of the bismuth active centers associated with germanium.

Stable self-starting bismuth-doped fiber laser mode-locked by polymer-free singlewalled carbon nanotubes is described. Dissipative solitons as short as 7.8 ps and energy of ~110 pJ was obtained. Evolution of pulsed regime on pump power is presented.

The laser performance at the wavelength 1700 nm with a slope efficiency of 18% has been demonstrated using a 8.5-m long bismuth-doped fiber containing thermally induced active centers.

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.

The spectral and luminescent properties of bismuth-containing germanate glass (0.1 and 1.0 mol % Bi2O3) were studied as a function of redox synthesis conditions. CeO2 and CaF2 were introduced into the glass charge as oxidants. The temperature of glass synthesis (1500–1600°C) was a factor that stimulated the reduction processes in the melt. It was shown that the absorption coefficient of glass in the spectral visible region decreased with an increased concentration of oxidants in the charge; and the narrowing of the luminescence band in the region of 1100 nm at excitation at 532 nm was observed. Similar effects were observed at a decrease of the temperature of the glass synthesis to 1500°C. It is assumed that the optically active centers in the resulting glass are the bismuth ions in the oxidation state below 3+. The concentration of these centers in the glass can be controlled by changing the redox balance of the synthesis conditions.