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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 demonstrated a modified chemical vapor deposition (MCVD) process for the fabrication of multilayer photonic bandgap fiber based on high-purity silica glass. Sequential growth of layers differing in melting point has been shown to lead to distortion of the layers in the resulting photonic bandgap structure and a sharp rise in optical loss as a result of deviations from Bragg’s reflection conditions. By optimizing the chemical composition of the layers in the photonic bandgap structure, we were able to suppress excessive optical losses and reach the optical loss limit, which is only determined by the guidance properties of the photonic bandgap structure itself.

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.

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.

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.

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.

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.

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.

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.

Radiation-induced absorption spectra are analyzed in germanosilicate fibers with different GeO2-content (3.5–50 mol%) under gamma-irradiation at different temperatures. GeY-center peaking at ~1.38 eV is resolved for the first time. GeY- and GeX-center properties are compared.

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.

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.

Using bismuth/erbium-doped optical fibre, we have fabricated the first optical amplifier with a gain of in a wide spectral range , which covers the C-, L- and U-bands.

We have demonstrated the first superluminescent IR source based on bismuth-doped high-germania fibre. Its main output characteristics are as follows: centre wavelength, ; emission bandwidth, at an output power of ; optical efficiency, .

We report the first bismuth-doped fibre amplifier operating between 1600 and 1800 nm, which utilises bidirectional pumping (co-propagating and counter-propagating pump beams) by laser diodes at a wavelength of 1550 nm. The largest gain coefficient of the amplifier is 23 dB, at a wavelength of 1710 nm. It has a noise figure of 7 dB, 3-dB gain bandwidth of 40 nm and gain efficiency of 0.1 dB mW-1.

The temperature dependencies of the characteristics of a bismuth-doped fiber amplifier (BDFA) for a 1725-nm band have been studied. The amplifier was backward-pumped with an Er-Yb-codoped fiber laser at 1568 nm. A peak gain of 24 dB, a 3-dB gain bandwidth of 55 nm, a noise figure of 5 dB and slope gain efficiency of 0.17 dB/mW at room temperature have been obtained. It has been shown that peak gain of the BDFA varies by 25 % in the temperature range from −60 to +60 °C while the noise figure remains almost unchanged. Some possible mechanisms responsible for the thermal behavior of the characteristics of the BDFA are considered.

Superfluorescent fiber sources (SFS) find wide application in many areas of science and technology: in fiber-optic gyroscopes, ophthalmology, optical time-domain reflectometry, optical telecommunications and devices that need a signal with low temporal and high spatial coherence [1]. There is interest in development of new fiber sources operating in spectral IR regions, where rear-earth-doped fibers do not provide gain. Previously, we presented SFS based on bismuth-doped fibers at 1.35 [2] and 1.45 pm [3]. In this letter we report a new broadband SFS operating at a wavelength of 1.73 pm using bismuth-doped germanosilicate fiber.

Mode-locked fibre laser as a dissipative system is characterized by rich forms of soliton interaction, which take place via internal energy exchange through noisy background in the presence of dispersion and nonlinearity. The result of soliton interaction was either stationary-localized or chaotically-oscillated soliton complexes, which have been shown before as stand-alone in the cavity. Here we report on a new form of solitons complex observed in Bi-doped mode-locked fibre laser operated at 1450 nm. The solitons are arranged in two different group types contemporizing in the cavity: one pulse group propagates as bound solitons with fixed phase relation and interpulse position eventuated in 30 dB spectrum modulation depth; while the other pulses form a bunch with continuously and chaotically moving solitons. The article describes both experimental and theoretical considerations of this effect.

An all-fiber ultrafast dissipative soliton laser at 1.3 microns based on phoshosilicate fiber doped with bismuth is presented. A nonlinear optical loop mirror containing high-germanium fiber with high nonlinearity and large positive dispersion was used. The scheme yields 11.3 ps pulses with energy of 1.7 nJ at repetition rate of 3.5 MHz. By means of bismuth-doped fiber amplifier and diffraction gratings compressor, the pulses were amplified up to 8.5nJ and compressed down to 530 fs. To achieve best results the optimal bismuth active fiber was chosen according to the investigated dependence of the gain coefficient on bismuth active centers concentration in phosphosilicate fibers.

The use of a bismuth-doped germanosilicate fiber as a saturable absorber for mode-locking of a thulium/holmium-codoped fiber laser is experimentally demonstrated. It is shown that stable mode-locked pulses of ~3.9 ns can be obtained at 1.9 μm.

We present figure-of-eight picosecond bismuth laser operating at 1.7 μm. Stable pulses as short as 21 ps were obtained. The MOPA setup yielded pulses with average power of 20 mW and pulse energy of 5.7 nJ.

The current state of the art in infrared Bi-doped fiber laser research is reviewed. The relevant fiber glass compositions and fiber technologies are introduced. Lasers operating on transitions ranging from 1.15 to 1.55 μm occurring in the bismuth active centers and their energy level schemes are discussed on the basis of the spectroscopic properties of these centers. Continuous-wave fundamental-mode power levels ranging from a few mW near 1.55 μm up to 16 W near 1.16 μm and 22 W near 1.46 μm have been demonstrated in recent years.

Seven F-doped- and undoped-silica-core optical fibers were produced by the MCVD-method, the ratio of the O2 and SiCl4 molar flow rates in the vapor-gas mixture during the fabrication of the perform core being varied among the performs. The fibers were γ-irradiated to 8.1 kGy (0.75 Gy/s), radiation-induced absorption (RIA) being measured during and after the irradiation in the spectral range 1.1-1.7 μm. The fiber optical loss spectra were also measured after γ-irradiation to 1.31 MGy. Three RIA mechanisms affecting the near-IR region have been revealed, namely, short-wavelength RIA tails due to the Cl0-center and to the self-trapped holes of the second type (STH2) and a long-wavelength RIA tail due to the self-trapped holes of the first type (STH1). STH2 are argued to result from the strain frozen-in in the fiber glass in the fiber drawing process. The RIA due to STH 1 is shown to anticorrelate with the RIA due to STH2. All the RIA mechanisms revealed are shown to be strongly suppressed by providing a high O2 excess in the vapor-gas mixture during the perform core synthesis. The O2-excess technique is therefore proposed as a very promising one for the development of radiation-resistant fibers.

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.

It is now almost twenty-five years since the first Erbium-Doped Fiber Amplifier (EDFA) was demonstrated. Currently, the EDFA is one of the most important elements widely used in different kinds of fiber-optic communication systems. However, driven by a constantly increasing demand, the network traffic, growing exponentially over decades, will lead to the overload of these systems (“capacity crunch”) because the operation of the EDFA is limited to a spectral region of 1530–1610 nm. It will require a search for new technologies and, in this respect, the development of optical amplifiers for new spectral regions can be a promising approach. Most of fiber-optic amplifiers are created using rare-earth-doped materials. As a result, wide bands in shorter (1150–1530 nm) and longer wavelength (1600–1750 nm) regions with respect to the gain band of Er-doped fibers are still uncovered. Here we report on the development of a novel fiber amplifier operating in a spectral region of 1640–1770 nm pumped by commercially available laser diodes at 1550 nm. This amplifier was realized using bismuth-doped high-germania silicate fibers fabricated by MCVD technique.

For the near IR spectral region from 1150 to 1800 nm, including the ranges from 1250 to 1500 nm and 1600 to 1800 nm where efficient rare-earth-doped fiber lasers don’t exist, bismuth-doped optical fibers are promising active materials. The last two spectral ranges are of great interest for some applications, in particular for optical fiber communication. Earlier, we developed Bi-doped fiber lasers and optical amplifiers operating in the first of these spectral ranges. Here, we report new results on the development of bismuth-doped optical fibers and fiber lasers for a spectral range of 1600-1800 nm.

The first bismuth-doped fiber amplifier (BDFA) for 1640–1770-nm region was developed. The BDFA diode-pumped at 1550 nm is characterized by 23-dB peak gain at 1710 nm, noise figure∼7 dB, Δλ1/2=40 nm and g=0.1 dB/mW.

A Bismuth-doped fiber laser tunable in the new wavelength range 1360-1510 nm with the output power 50 mW has been developed for the first time. The tuning of the laser was carried out by rotating diffraction grating.

The visible and near-IR absorption and luminescence bands of bismuth-doped silica and germanosilicate fibres have been measured for the first time as a function of temperature. The temperature-dependent IR luminescence lifetime of a bismuth-related active centre associated with silicon in the germanosilicate fibre has been determined. The Bi3+ profile across the silica fibre preform is shown to differ markedly from the distribution of IR-emitting bismuth centres associated with silicon. The present results strongly suggest that the IR-emitting bismuth centre comprises a lowvalence bismuth ion and an oxygen-deficient glass network defect.

The first fibre preforms with a Pb-doped silica core, free of other dopants, have been produced using chemical vapour deposi- tion. The preforms have been used to fabricate holey optical fibres. The spectroscopic properties of the fibres have been studied in detail in the range 400 - 1700 nm: we have measured the optical loss, constructed a three-dimensional luminescence excita- tion - emission graph and determined the decay time for the major luminescence peaks.

We have studied the properties of silica-core photonic bandgap fibres with a small diameter to pitch ratio (no greater than 0.4) of high-index rods and small refractive-index difference (no greater than 0.03) in the cladding. Theoretical analysis and experimental data demonstrate that the number of core modes depends only on the number of pure-silica elements that form the core and is independent of cladding parameters. If there is one such element, only one dispersion curve of a core mode falls in the fundamental bandgap and the fibre is single-mode. The fibres having seven such elements are multimode.

This paper describes an experimental setup for precision measurements of the angular distribution of light scattered by optical fibres in a wide angular range and demonstrates that the models of anomalous scattering proposed to date need to be refined. We have found and interpreted a discrepancy between the Rayleigh scattering coefficients measured by different techniques.