Bismuth-related Active Centers Research Articles

Deterioration of Bismuth-Doped fiber lasers performance induced by blue and green laser light launched into cladding

For the first time, we demonstrate the photobleaching process of bismuth-related active centers (BACs) induced by blue and green laser radiation launched into the cladding of an active fiber having the low-refractive index polymer coating. The changes in small-signal loss spectra of the exposed Bi-doped fibers are presented and analyzed. Temporal evolution of luminescence intensity of BACs is measured for different powers of bleaching light. The effect of bleaching on Bi-doped fiber laser characteristics was studied. It was found that time-to-destruction of Bi-doped fiber laser varies in dependence on the power and wavelength of the used bleaching light. In particular, the blue laser radiation launched into the active fiber cladding for a timespan of almost 10 sec can completely disable the operation of a bismuth laser at 1.71 µm. We estimated the efficiency of bleaching process in Bi-doped fiber under blue laser irradiation.

Combined effect of thermal and laser treatment on the destruction of active centers in Bi-doped phosphosilicate fibers

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.

Co-doping effect of lead or erbium upon the spectroscopic properties of bismuth doped optical fibres

The effect of the lead or erbium co-doping upon the spectroscopic properties has been systematically investigated through the analysis of the absorption and emission spectra of Bi doped fibre (BDF), Bi/Pb and Bi/Er co-doped fibre (BPDF and BEDF). All these BDFs have a broadband emission from 1000 nm to 1600 nm. The Pb or Er co-doping in BDFs will evidently reduce the formation of bismuth related active centres (BACs) due to the formation of other types’ centres and then restrain the emission of BAC through their competition of the pump consumption and the cross energy transfer among these active centres. Further analysis indicates that the co-doping effect of Pb to restrain the formation of BAC and its emission is stronger than that of Er to some degree, possibly due to the analogy property between Bin+ and Pbn+ with isoelectronic configures. In addition, the analysis of the BAC concentration indicates that the atom concentration measured by energy dispersive X-ray (EDX) analyser has a large gap with the effective content reflected from the absorption and emission. All these results point out a way to design and fabricate bismuth doped/co-doped fibre with high performance in future.

Analysis of thermally activated processes in bismuth-doped GeO<i/> 2-SiO<i/> 2 glass fibers using the demarcation energy concept

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.

Effect of thermal treatment parameters on the spectral characteristics of BAC-Al in bismuth/erbium-codoped aluminosilicate fibers.

For the first time, to the best of our knowledge, the effect of thermal quenching on the spectral properties of bismuth-related active centers (BAC)-Al is investigated systematically with varying quenching conditions. It is found that the peak luminescence of BAC-Al at 1140nm is maximally enhanced by 1.6 times with a heating dwell time of 2min at 500°C and subsequent quenching, suggesting that a short dwell time with a high heating temperature (500°C) favors the activation or growth of BAC-Al, whereas the prolonged heating time with a slow cooling rate results in the deactivation of BACs and the aggregation of background loss. The underlying mechanism of activation and destruction of BAC-Al is also analyzed and discussed. The experimental results demonstrate a promising strategy for enhancing the spectral performance of BAC-Al in the range of 1.1-1.35μm.

Effect of heat treatment parameters on the optical properties of bismuth-doped GeO2:SiO2 glass fibers

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.

Formation of laser-active centers in bismuth-doped high-germania silica fibers by thermal treatment.

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.

Laser-induced bleaching and thermo-stimulated recovery of luminescent centers in bismuth-doped optical fibers

In this paper, we report the detailed study of photobleaching and recovery in bismuth-doped fibers that proved to be a unique laser media for the near IR spectral regions unreachable for rare-earth-doped lasers. The laser-induced bleaching and recovery dynamics of the characteristic absorption and emission bands assigned to different types of the bismuth-related active centers were obtained. By analyzing the Raman-scattering spectra of the bleached bismuth-doped fibers, the intensity-dependence of the photobleaching rate and the experimentally determined activation energy of the recovery process and underlying mechanisms of these phenomena are discussed. Furthermore, a bleaching–recovery cycling and “memory effect” are demonstrated.

Effect of gamma-irradiation on the optical properties of bismuth-doped germanosilicate fibers

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.

Photobleaching effect in bismuth-doped germanosilicate fibers.

Photoinduced reduction of absorption (photobleaching) in bismuth-doped germanosilicate fibers irradiated with 532-nm laser has been observed for the first time. It was demonstrated that bismuth-related active centers having the absorption bands at wavelengths of 1400 and 1700 nm degrade under photoexcitation at 532 nm. The photobleaching process rate was estimated using conventional stretched exponential technique. It was found that the photobleaching rate in bismuth-doped germanosilicate fibers does not depend on type of bismuth-related active center. The possible underlying mechanism of photobleaching process in bismuth-doped fibers is discussed.

Luminescence properties of IR-emitting bismuth centres in -based glasses in the UV to near-IR spectral region

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 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 ion, suggesting that may participate in BAC formation.

IR luminescence in bismuth-doped germanate glasses and fibres

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.

Effect of variable valence impurities on the formation of bismuth-related optical centres in a silicate glass

We have studied the effect of variable valence impurities (cerium and iron) on the formation of bismuth-related IR luminescence centres and the optical loss between 1000 and 1300 nm in amagnesium aluminosilicate glass. The results demonstrate that additional doping of the glass with ceria leads to effective bleaching in a wide spectral range, including the luminescence range of thebismuth centres. At the same time, ceria reduces the concentration of luminescence centres. Gamma irradiation of the glass bleached by cerium restores the luminescence centres but leads to a background loss in a wide spectral range. Iron is shown to be a very harmful impurity in bismuth-doped active media: even trace levels of iron prevent the formation of bismuth-related active centres in the glass and produce a strong, broad absorption band centred near 1 μm.

Bismuth-sensitized efficient near-infrared luminescence from ytterbium in zeolites

Yb–Bi co-doped zeolites are prepared by a method consisting of a simple ion-exchange process and subsequent high-temperature annealing. We observe two strong near-infrared (NIR) emission bands overlapping in the range 930–1480 nm, corresponding to the electronic transitions of bismuth-related active centres (BiRAC) and Yb3+ ions, respectively. In the obtained products, the excitation wavelength of Yb3+ is extended to the range 420–850 nm, and the lifetime reaches 665 µs. In the zeolite matrix, Bi ions exist as BiRAC and Bi compound agglomerates. The former act as a sensitizer of Yb3+ ions, and the latter act as a blockage to seal the pores of zeolites, which enables Yb3+ ions to show efficient NIR emission even if the zeolites contain a large amount of coordinated water. The excellent optical and structural properties make these NIR-emitting nanoparticles promising in applications as biological probes.