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Suppression mechanism of radiation-induced darkening by Ce doping in Al/Yb/Ce-doped silica glasses: Evidence from optical spectroscopy, EPR and XPS analyses
Abstract
Yb³⁺/Al³⁺ co-doped silica glasses with different Ce2O3 contents were prepared using the sol–gel method combined with high-temperature sintering. Changes in refractive index, absorption, emission and fluorescence lifetime of these glasses caused by X-ray irradiation were recorded and analyzed systematically. It is found that co-doping with certain amount of Ce could greatly improve the radiation resistance without evident negative effects on the basic optical properties of the Yb³⁺ions in the near-infrared region. The nature of the radiation-induced color centres and the mechanism by which Ce prevented the formation of these centres were studied using optical absorption, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) methods. Direct evidence confirmed that trapped electron centres (Yb²⁺/Si-E′/Al-E′) and trapped hole centres (Al-OHCs) were effectively inhibited by Ce doping, which was correlated to the coexistence of the redox couple Ce³⁺/Ce⁴⁺ in the glasses. These results are helpful to understand the micro-structural origin and the suppression mechanism by Ce co-doping of the photodarkening effect in Yb³⁺-doped silica fibers.
... It is generally accepted that the refractive index changes of Ce co-doped fibers on UV exposure, i.e., photosensitivity, originate from the valency states variation of Ce, specifically, the transformation of Ce 3+ to Ce 4+ [27]. The 4f 1 -4f 0 5d 1 transitions of Ce 3+ in silica glass have a broad UV absorption band peaking around 300 nm [28]. Under the exposure of UV light, e.g., 248 nm, the Ce 3+ ions are inclined to lose one electron (e − ) and transform into Ce 4+ . ...
... As pointed out in our previous study, the glass refractive index would increase linearly with Ce concentration [28]. Considering 3700 ppm (0.37wt.%) ...
... Both the hydrogen loaded and unloaded fibers are used for FBG inscribing. The hydrogenation mechanism for the Ge doped photosensitive As pointed out in our previous study, the glass refractive index would increase linearly 146 with Ce concentration [28]. Considering 3700 ppm (0.37wt.%) ...
Monolithic distributed Bragg reflector (DBR) cavity which directly integrates fiber Bragg gratings (FBGs) into the photosensitive RE-doped fibers is a promising configuration in constructing compact and efficient single frequency fiber lasers (SFFLs). Yet, the doping level of rare-earth (RE) ions has generally to be sacrificed in the classical Ge-photosensitized RE-doped silica fibers because of the dramatic refractive index increase caused by the introduction of Ge. Here, we demonstrate an approach to realize the trade-off between photosensitivity and RE doping concentration. We validate that the addition of a small amount of cerium (0.37wt.%) instead of Ge could photosensitize Yb³⁺-doped silica fiber (YDF), while maintaining fiber numerical aperture (NA) at 0.12 under a high 2.5-wt.% Yb doping level. Based on the short monolithic DBR cavity constructed by this germanium-free photosensitive highly YDF, a 1064 nm fiber laser with a 48.6% slope efficiency and an over 200 mW power on two orthogonally polarized modes could be realized. Further stable and linear-polarized 1064 nm SFFL is also demonstrated in a designed monolithic polarization maintaining cavity with an output power of 119 mW and an efficiency of 26.4%. Our results provide an alternative way to develop photosensitive highly RE-doped fibers towards monolithic laser cavity application.
... Recently, Ce 3+ -co-doping was shown to be a very effective method for reducing the radiation sensitivity of Er 3+ /Yb 3+ -co-doped [24] and Yb 3+ -doped fibers and amplifiers [25] without diminishing the amplification efficiency. The Ce 3+ and Ce 4+ ions coexist in these optical glasses and fibers. ...
... Table 2 summarizes the characteristic values of the color centers identified in the RIA and CW-EPR spectra. In this work, the parameters used in the RIA decomposition are quantitatively consistent with the data reported in Ref. [25], and each cumulative fitted spectrum is consistent with the experimental data. ...
... representation of the possible energy levels of dopants involved between valency and conduction bands has been detailed by Lupi [25]. The author analyzed the photodarkening process in thulium alumino-silicate fibers pumped at 1.07 μm and the quantitative effect of lanthanum, cerium, and thulium. ...
Several Ce³⁺/Tm³⁺-co-doped alumino-silicate glasses with various Ce³⁺/Tm³⁺ ratios were prepared by a sol-gel method combined with vacuum sintering. To determine their radiation resistance, the glasses were exposed to 1 and 100 kGy γ-ray. The effect of the Ce³⁺/Tm³⁺ ratio on the absorption, emission and fluorescence lifetime of the glasses under investigation, was examined before and after irradiation. The Ce⁴⁺ and Ce³⁺ cations were detected and the nature of Si-E′ and AlOHC color centers caused by irradiation was identified by radiation-induced absorption (RIA) and electron paramagnetic resonance (EPR) spectroscopy. Co-doping with Ce³⁺ ions can significantly improve the radiation resistance of the glass, although it can deteriorate the spectral properties of Tm³⁺ ions. Additionally, the related influence mechanisms are discussed. The obtained results indicate that Ce³⁺/Tm³⁺-co-doped alumino-silicate glasses with optimized Ce³⁺ ion concentrations, a Ce³⁺/Tm³⁺ molar ratio less than 0.3, can be used as an active medium for radiation-resistant materials in harsh radiation environments.
... Cerium is a rare-earth element with the [Xe] 6s 2 4f 1 5d 1 electronic configuration, that exists in different stable valence states (Ce 3+ and Ce 4+ ). These ions have attracted a lot of attention in glass and optical fibers manufacturing, due to their ability to make the glass matrix more resistant against ionizing radiation (notably γ-radiation [8][9][10], X-rays [11]). They also play an important role in the inhibition of photodarkening (PD) in multicomponent glasses [12]. ...
... It can be noticed that the sample prepared under oxidizing conditions exhibits two absorption bands centered around 265 nm and 323 nm. While the first one is assigned to charge transfer transitions from ligands to the quadruply-charged cerium cations (Ce 4+ ), the second one states for 4f→5d transition of Ce 3+ ions [11,31]. However, for the preforms obtained under inert (PHel) or reducing conditions (PNox), the spectra essentially display a Figure 2 shows Ce-doped MCVD preforms optical absorption spectra. ...
... It can be noticed that the sample prepared under oxidizing conditions exhibits two absorption bands centered around 265 nm and 323 nm. While the first one is assigned to charge transfer transitions from ligands to the quadruply-charged cerium cations (Ce 4+ ), the second one states for 4f→5d transition of Ce 3+ ions [11,31]. However, for the preforms obtained under inert (P Hel ) or reducing conditions (P Nox ), the spectra essentially display a main absorption band, peaking around 323 nm, attributed to Ce 3+ ions. ...
The incorporation of Ce3+ ions in silicate glasses is a crucial issue for luminescence-based sensing applications. In this article, we report on silica glass preforms doped with cerium ions fabricated by modified chemical vapor deposition (MCVD) under different atmospheres in order to favor the Ce3+ oxidation state. Structural analysis and photophysical investigations are performed on the obtained glass rods. The preform fabricated under reducing atmosphere presents the highest photoluminescence (PL) quantum yield (QY). This preform drawn into a 125 µm-optical fiber, with a Ce-doped core diameter of about 40 µm, is characterized to confirm the presence of Ce3+ ions inside this optical fiber core. The fiber is then tested in an all-fibered X-ray dosimeter configuration. We demonstrate that this fiber allows the remote monitoring of the X-ray dose rate (flux) through a radioluminescence (RL) signal generated around 460 nm. The response dependence of RL versus dose rate exhibits a linear behavior over five decades, at least from 330 µGy(SiO2)/s up to 22.6 Gy(SiO2)/s. These results attest the potentialities of the MCVD-made Ce-doped material, obtained under reducing atmosphere, for real-time remote ionizing radiation dosimetry.
... An interplay has been demonstrated and PD-related CCs were shown to be the same as part of those involved in RIA [6,7]. An expression of these shared origins lies in the fact that both degradation types are efficiently mitigated by cerium (Ce) co-doping [2,[8][9][10][11][12]. This empirical recipe has enabled the fabrication of commercially-available radiation-and PD-resistant fibers, but it did not put an end to the quest for the elucidation of PD and RIA mechanisms. ...
... This allowed us to gain insight into mechanisms in a systematic way, highlighting the specific role played by each newly added dopant. The way the Ce codoping hardens the glass against RIA is notably discussed, with novel conclusions compared with usual interpretations [8][9][10][11][12]. ...
... The RIA was calculated from the absorbances A before and A after measured before and after irradiation, respectively, according to RIA (dB m −1 ) = 10×(A after -A before )/e, where e is the sample thickness (in meters). Under soft X rays, radiation-induced changes in the refractive index of silica-based glasses are very small [11]. Their impact on the reflection coefficient at the sample entrance is therefore negligible. ...
We present an original experimental approach to the investigation of radiation-induced attenuation (RIA) mechanisms in optical fiber preforms. This protocol combines thermally stimulated luminescence (TSL) measurements with the characterization of RIA annealing during TSL readouts. It is systematically applied to compositions of increasing complexity to resolve the specific role played by each dopant. Silicate, aluminosilicate, Yb-doped silicate, Yb-doped and Yb,Ce-codoped aluminosilicate preform samples are examined. Annealing processes are described in detail as a function of temperature throughout TSL readouts. The protocol reveals the temperature ranges at which trapped-carrier states forming intrinsic or dopant-related color centers are released, thus enabling the assessment of their activation energies. Metastable Ce²⁺ ions are proved to be formed by electron trapping under irradiation. Along with the formation of Ce³⁺⁺, they play a crucial role in the RIA mitigation.
... When YDF is used in space missions, excess optical losses can be induced by external ionizing radiations, resulting in a drastic decrease of laser slope efficiency. Co-doping with Ce 3+ is one of the most effective methods to improve the radiation resistance of rare earth (RE) doped silica-based optical fibers [2][3] . Al 3+ is generally adopted in silica glass to increase RE ions solubility and maintain laser efficiency. ...
... For making comparison, the RIA of Al 3+ single-doped and Yb 3+ /Al 3+ -co-doped samples as well as the absorption spectrum of Yb 2+ ions are depicted in Fig. 4a as well. It is obvious that the RIA intensity of Yb 3+ /Al 3+ /Ce 3+ -co-doped sample (YACF0) was significantly lower than that of Al 3+ single-doped and Yb 3+ /Al 3+ -co-doped samples, in which the associated suppression mechanism by Ce co-doping in Yb 3+ /Al 3+ /Ce 3+ -co-doped silica fibers has been systematically studied [3] . Furthermore, the RIA intensity of Yb 3+ /Al 3+ /Ce 3+ /F --co-doped sample further decreased with the increase of F content. ...
... As shown in the inset of Fig. 4a, a "hole" at Yb 3+ absorption wavelength of 975 nm in the Yb-containing samples can be observed, whereas it is absent in the Al 3+ single-doped sample. This phenomenon suggests that a minor part of the Yb 3+ ions can be reduced into Yb 2+ ions under irradiation, and this result has been further supported by the low-temperature of EPR in our previous researches [3,22] . ...
Yb3+/Al3+/Ce3+/F--co-doped silica glasses with different fluorine contents were prepared by a sol-gel method combined with high-temperature sintering. Changes in refractive index, spectroscopic properties and radiation resistance of these glasses caused by fluorine doping have been correlated with their microscopic structure information, obtained via several structural methods. The fictive temperature (Tf) as a proper indicator for structural disorder was determined by Fourier transform infrared spectroscopy (FTIR). The glass network structure was characterized by nuclear magnetic resonance (NMR) and Raman scattering. The local coordination atom structures of Yb3+ions in pristine glasses as a function of fluorine content were analyzed by advanced pulse electron paramagnetic resonance (EPR). The radiation-induced Si-(Si-E', NBOHC), Al-(Al-E', Al-ODC, AlOHC) and Yb-(Yb2+) related color centers were determined by optical absorption and continuous wave-EPR spectroscopies. The results show that fluorine can effectively adjust the refractive index and significantly improve the radiation resistance of glasses, but cannot evidently deteriorate the spectroscopic properties of Yb3+ ions. FTIR confirms that the fictive temperature as well as structural disorder is greatly reduced by fluorine doping. The three- and four-membered ring structures decrease and the six-coordinated Al increases with increasing fluorine content as detected by Raman and NMR, respectively. Pulse EPR confirms that fluorine is located at the local coordination sphere of Yb3+ ions. This structure identification could favor to explain the composition-dependent macroscopic properties of Yb3+/Al3+/Ce3+/F--doped silica glasses, besides, this work provides an available solution to obtain the radiation-resistive Yb3+-doped silica fibers with a low core numerical aperture as well.
... To investigate the radiation induced color centers in the studied silica glasses, the room-temperature CW-EPR spectra were measured as shown in Figure 7. The RIA spectra of DAP0 and DAP1.61 samples were accordingly fitted by Gaussian functions (shown in Figure 8) with corresponding absorption peak positions and FWHM of the point defects listed in Table 3. From Figure 7A, the EPR signal of the irradiated samples declined obviously with increasing P/Al ratio, indicating the descended concentration of defects, which is consistent with the result in Figure 2. From Figure 7B, the EPR signal of Si electron center (Si-E′), 28,29 and Al-OHC 30,31 are observed in all irradiated samples. With respect to that the trapped hole centers and electron centers are generated in pairs due to valance balance, 30 the possible formation mechanisms of these defects are as follows: ...
... The RIA spectra of DAP0 and DAP1.61 samples were accordingly fitted by Gaussian functions (shown in Figure 8) with corresponding absorption peak positions and FWHM of the point defects listed in Table 3. From Figure 7A, the EPR signal of the irradiated samples declined obviously with increasing P/Al ratio, indicating the descended concentration of defects, which is consistent with the result in Figure 2. From Figure 7B, the EPR signal of Si electron center (Si-E′), 28,29 and Al-OHC 30,31 are observed in all irradiated samples. With respect to that the trapped hole centers and electron centers are generated in pairs due to valance balance, 30 the possible formation mechanisms of these defects are as follows: ...
In this work, we study the mechanisms of X‐ray induced darkening in the Dy/Al/P co‐doped silica glasses at visible wavelengths and explore the role of P/Al ratio played in the enhancement of irradiation resistance. The absorption, emission spectra and lifetime, Raman, and electron paramagnetic resonance spectra of pristine and irradiated Dy‐silica glasses as a function of P/Al ratio were measured and analyzed. The Si‐related defects (Si‐E′ and non‐bridge oxygen hole center), Al‐related defects (Al‐E′ and Al‐related oxygen hole center), P‐related defects (P‐OHC and P2), and Dy²⁺ ions were studied and analyzed. Results demonstrate that increasing P/Al is efficient to suppress the photodarkening in Dy‐silica glasses at visible wavelengths. Results imply that Dy‐silica glasses with P/Al > 1 may be a promising candidate for efficient visible fiber lasers operated at around 575 nm wavelength.
... The fitted two peaks (880.58 eV, 884.58 eV) are from Ce 3+ 3d 5/2 , and two other peaks (897.48 eV and 902.98 eV) are originated from Ce 3+ 3d 3/2 , respectively. The Ce 4+ 3d 5/2 peak is located at 888.68 eV, and 3d 3/2 peaks are located at 900.18 eV and 917.08 eV [15,[20][21][22]. The results indicate that Ce 3+ is the predominant state in Tb 2.8 Ce 0.2 Fe 5 O 12 , with small proportions of Ce 4+ . ...
... The results indicate that Ce 3+ is the predominant state in Tb 2.8 Ce 0.2 Fe 5 O 12 , with small proportions of Ce 4+ . In Fig. 4c, the main peaks of the Fe 3+ 2p 3/2 [20]. Since the reaction occurred in the air during the preparation, small amount of Fe 3+ and Ce 3+ may undergo redox reaction to form Fe 2+ and Ce 4+ [24,25]. ...
Bi³⁺ and Ce³⁺ substituted rare-earth iron garnet is famous magneto-optical material for the application in integrated optical system and microwave devices. Nevertheless, the modulation of Bi³⁺ and Ce³⁺ on the structure and magnetic behavior of terbium iron garnet (TIG) is not very clear, which limits its practical applications. Herein, a series of Tb3−xBixFe5O12 (Bi:TIG, x = 0–0.9) and Tb3−xCexFe5O12 (Ce:TIG, x = 0–0.3) have been readily prepared by the sol-gel method. Both of pure phase TIG and Bi:TIG was obtained when calcined at 800 °C, while Ce:TIG has been prepared at 900 °C. Cubic garnet structure for both systems has been maintained, while the solubility limit of Ce in TIG was around x = 0.2. With larger ionic radius, the introduction of Bi³⁺ and Ce³⁺ effectively induces enlarged lattice parameters and modification of magnetic behavior. When Bi³⁺ was increased from x = 0 to 0.9, the saturated magnetization (Ms) was moderately enhanced and the coercive field (Hc) was effectively reduced. However, compared with TIG, Ms of Ce:TIG was slightly decreased, and Hc is mildly increased. The modulation in the magnetic behaviors is systematically discussed, regarding the variation in the super-exchange interaction, particle morphology and the magnetic anisotropy. These results are beneficial for the design of novel iron garnets for the application of switching, microwave and non-reciprocal related devices.
The introduction of diamagnetic Bi and paramagnetic Ce effectively induces structure distortion and magnetic modulation.
... The excited state 5d can be split into different sub-energy levels under the different crystal fields. Therefore, the 5d energy vary significantly depending on the host lattice incorporating Ce 3+ [29][30][31]. For A2 sample, the energy difference between the emission bands at 3.49ev and 3.19ev was 0.3ev (2396 cm −1 ), which was consistent with the L-S splitting ( 2 F J = 5/2,7/ 2) of the Ce 3+ ion in the ground state. ...
... As shown in Fig. 11, the PL decay curves of the prepared aluminosilicate glasses showed a characteristic of non-exponential and faster decay. These profiles were totally different than that of Ce 3+ ions (nearly single exponential curve with a decay lifetime of nanoseconds) [31,39]. This result revealed the presence of more than one type luminous centers in the aluminosilicate glasses apart from Ce 3+ ions. ...
The photoluminescence and decay properties of Yb³⁺/Ce³⁺ co-doped aluminosilicate glasses were investigated under ultraviolet irradiation. A strong excitation band was ascribed to 4f → 5d transitions of Ce³⁺ ions. A broad ultraviolet-visible emission band belonged to 5d → 4f transitions of Ce³⁺ ions and oxygen-deficient centers (ODCs) in the host matrix, and the visible-near infrared emission band was attributed to non-bridging oxygen hole centers. Comparative tests of the core composition were performed. The results showed Yb2O3 and the ODCs played a crucial role on ultraviolet-visible emission band. ODCs rather than Ce³⁺ ions were responsible for the 1–35 μs lifetime decay of the ultraviolet-visible emission, but had no contribution to the longer decay lifetimes (77.58 μs) for Ce-free Yb³⁺-doped glasses. A complex schematic energy-level diagram including the electron transfer mechanism such as Ce³⁺-Yb³⁺ → Ce⁴⁺-Yb²⁺, the cooperative up- and down-conversion processes was proposed to describe different luminescence mechanisms UV-expose.
... However, excessive Al 3+ increases the number of Al-OHC [26], which is another source of the PD of YDF [17], and co-doped P reduces the absorption and emission cross sections of Yb 3+ ions [27]. Additionally, Ce doping can improve the PD resistance [28,29] but is not convenient for the regulation of the numerical aperture (NA). Hence, tradeoffs are often required for improving the PD resistance of the laser fiber. ...
... The photochemical reaction of the PD process can be described as follows: (2) Al-OHC is an aluminum-oxide trapped hole center, although the Si-O trapped hole center (Si-OHC) is expected to form in the PD process. However, neither emission signals nor EPR signals of Si-OHC were detected in our previous works [29,49]. This might be because the negatively charged [AlO 4/2 ]tetrahedral has the priority to trap the positively charged holes. ...
Yb³⁺/Al³⁺ co-doped silica fibers (YDFs) with almost identical core glass compositions were prepared using the sol-gel and modified chemical vapor deposition (MCVD) methods. The photodarkening (PD) and laser performance before and after the PD process were tested under 974 nm pumping. The doping homogeneity of Yb³⁺ ions and clusters of Yb³⁺ ions in preform slices of these two fibers were investigated via optical absorption spectroscopy, photoluminescence emission spectra, electron probe microanalysis (EPMA), and low-temperature (4 K) electron paramagnetic resonance (EPR). It is known that the PD resistance of YDFs prepared via the sol-gel method is significantly better than that of YDFs prepared via MCVD under the same test conditions. EPMA mapping reveals that the doping homogeneity of Yb³⁺ ions in the sol-gel fiber core glass is better than that in the MCVD fiber. The low-temperature (4 K) EPR and cooperative luminescence spectra of Yb³⁺ ions indicate that the clustering degree of Yb³⁺ ions in the sol-gel fiber is lower than that in the MCVD fiber. In the absorption and emission spectra, small amounts of Yb²⁺ ions are observed in the preform slice from the sol-gel method. A model of the color-center generation in the PD process was proposed to explain the mechanism of PD resistance improvement for the YDFs fabricated via the sol-gel method.
... Figure 7(b) clearly shows that the concentration of the new photodarkening center is increased from DAC25-R to DAC50-R, with increasing Ce-codoping. A higher Ce provides a higher possibility of trapping holes produced by X-ray irradiation via Ce 3+ + h → Ce 4+ , effectively suppressing [50] the Al-OHC (a hole trapped in an oxygen bonded to Al), as demonstrated in Section 3.4. The single Ce 3+ doped silica [47] irradiated with UV laser exhibits a decreased photoluminescence of trivalent Ce 3+ , implying the decrease in the concentration of trivalent Ce 3+ . ...
To develop Dy doped silica glass with a higher irradiation resistance, which can be adapted to high power violet or blue LD pumped yellow laser fibers, the designed Dy-Al-Ce codoped silica glasses, 0.05Dy2O3-1.5Al2O3-xCe2O3-(98.45-x)SiO2 (x = 0, 0.05, 0.1, 0.25, 0.5), were prepared by the sol-gel method. Their excitation spectra, emission spectra and emission decay curves associated with the yellow emission from ⁴F9/2 to ⁶H13/2 of Dy were determined before and after the X-ray irradiation of 1000 Gy. The relation between these spectra and Ce-codoping concentrations is discussed, including the sensitization from Ce to Dy, the reverse energy transfer from Dy to Ce, and especially the X-ray-induced photodarkening, which is detrimental to the 576 nm yellow emission of Dy. The centers that cause the photodarkening are analyzed by electron paramagnetic resonance and radiation induced absorption spectra. It is found that Ce can effectively suppress the Al-oxygen hole center induced by the X-ray across the entire concentration range of Ce-codoping, but a new photodarkening center is generated at higher concentrations of Ce-codoping. Finally, the optimized Ce-codoping concentration of ∼0.1 mol% is used to achieve a promising yellow laser glass of Dy-Al-Ce codoped silica with enhanced irradiation resistance, resulting in its X-ray-induced photodarkening that is only 6% - 14% of that in the Ce-undoped.
... As the active fiber is the most radiation sensitive, the first and most widely investigated solution involves enhancing the radiation tolerance of the active fiber to decrease the radiation induced attenuation (RIA) levels at the pump and signal wavelengths. And, a plenty of methods have been investigated, including Ce-doping [5,[16][17][18][19][20], F-doping [20], H 2 /D 2 loading [9,10,[21][22][23], proportion optimization of co-doping elements [24][25][26], and pre-irradiation [27,28]. ...
Radiation hardening should be considered for various active fiber systems operated in adverse environments to reduce their sensitivity to complex ionizing radiations. And, architecture optimization through numerical simulation provides an efficient choice. Here, introducing radiation effects into the conventional fiber laser model, a radiation model concerning the design optimization of low- and moderate-power Yb-doped fiber lasers is developed. And, experiments at different radiation levels up to 750 Gy are carried out for validation, demonstrating the ability of this model to correctly simulate the performance of the Yb-doped fiber laser in harsh environments. Then, with this model, impacts of active fiber length, pump scheme, and pump allocation on the output characteristics of Yb-doped fiber lasers are analyzed numerically. And, optimization of Yb-doped fiber lasers are conducted through architecture design. Simulations show that a proper design with relatively short active fiber and dynamic pump allocation can remarkably improve the radiation tolerance of Yb-doped fiber lasers.
... This may be primarily due to changes in the valence state of rare-earth ions. For example, the coexistence of the Ce 3+/ Ce 4+ redox couple in glasses provides pathways for the capture of both hole and electron color centers [18,19]. Cerium oxide and gadolinium oxide powders are successfully used as additives for making radiation-resistant glasses [20][21][22][23]. ...
The radiation resistance and optical properties of micron-sized mCaSiO3 wollastonite powders (initial and modified by gadolinium oxide nGd2O3 nanoparticles) were studied. For this purpose, the powders were irradiated with accelerated electrons, and diffuse reflection spectra (ρλ) were recorded in high vacuum (in situ). The mCaSiO3 powder features a high reflectance throughout the measured spectrum range from 200 to 2,500 nm. The obtained value of the solar absorptance (αs = 0.117) is less compared to known powders used as pigments for reflective coatings, such as ZnO and TiO2. Depending on the concentration of nanoparticles in the range of 1–10 wt%, the radiation resistance of the modified mCaSiO3/nGd2O3 powder increases up to 1.7 times compared to the unmodified powder. The achieved increase is of great practical importance for various applications, which require high radiation resistance, such as nuclear industry (cement and ceramics fillers, dosimetry materials) and aerospace industry (pigment for thermal control coatings).
... To overcome this issue, a hole-assisted carbon-coated EDF was developed, but its complex process is not conducive to industrialization [15]. Another approach is to dope heavy ions, such as cerium (Ce) or lead (Pb) ions, to alter the composition of the optical fiber and enhance its radiation resistance [16]. During the irradiation process, heavy ions can capture holes and electrons, inhibit the formation of corresponding hole-type and electron-type defect centers, and provide irradiation buffer space for active ions in the fiber [17,18]. ...
Three Er-doped fibers (EDFs) with different concentrations of Bi ions doping were fabricated by atomic layer deposition combined with modified chemical vapor deposition. The radiation-induced absorption (RIA) could be dramatically weakened by co-doping Bi. Especially, the RIA of Bi/Er co-doped fiber (BEDF) at 1300 nm was 56.0% lower than that of EDF after a 1500 Gy irradiation treatment. With the increase of the irradiation dose, the fluorescence intensity and lifetime of EDF decreased continuously, while BEDF showed a trend, increasing first and then decreasing, and changed little before and after irradiation. The gain characteristics and laser threshold power of BEDF are less varied than those of EDF before and after irradiation. In addition, an irradiation simulation model of EDF and BEDF fiber was established through GEANT4 simulation toolkit and found that Bi ions are more likely to absorb gamma rays, thereby reducing the impact of irradiation on Er ions in BEDF. These results indicate that Bi co-doped EDF has significant performance improvements in radiation resistance, making it ideal for applications in harsh radiation environments.
... Core composition optimization is the most fundamental method. Ce co-doping has been used to suppress the formation of aluminum-oxygen hole center (Al-OHC) defects [17,18], and the valence variation of Ce 4+ →Ce 3+ reduces the generation of Yb 2+ ions and further improves the radiation resistance. However, this presents the problem that Ce co-doping seriously impacts the spectral properties of Yb 3+ ions [19]. ...
In this study, Yb/Al/Ge co-doped silica fiber core glasses with different GeO2 contents (0–6.03 mol%) were prepared using the sol–gel method combined with high-temperature sintering. The absorption, fluorescence, radiation-induced absorption, continuous-wave electron paramagnetic resonance spectra, and fluorescence decay curves were recorded and analyzed systematically before and after X-ray irradiation. The effects of GeO2 content on the valence variations of Yb3+/Yb2+ ions, spectral properties of Yb3+ ions, and radiation resistance of Yb/Al/Ge co-doped silica glasses were systematically studied. The results show that even if the GeO2 content of the sample is relatively low (0.62 mol%), it can inhibit the generation of Yb2+ ions with slight improvement in the spectral properties of Yb3+ ions in the pristine samples and effectively improve its radiation resistance. Direct evidence confirms that the generation of trapped-electron centers (Yb2+/Si-E’/Al-E’) and trapped-hole centers (Al-OHC) was effectively inhibited by Ge co-doping. This study provides a theoretical reference for the development of high-performance, radiation-r esistant Yb-doped silica fibers.
... It is well known that co-doping with Ce can improve the anti-darkening performance of bulk glass and active fiber. 29,30 Our research 21 shows that co-doping with a certain amount of Ce (Ce 2 O 3 < 0.125 mol%) does not significantly deteriorate the spectral properties of Yb 3+ ions (not shown), and it can significantly improve the radiation resistance of YDGs, as shown in Figure 13A. In Figure 13B, the CW-EPR test shows that co-doping with Ce can effectively inhibit the generation of electron-type (e.g., Si-E') and hole-type (e.g., Al-OHC) color centers simultaneously, which is correlated with the coexistence of the redox couple Ce 3+ /Ce 4+ . ...
Yb³⁺‐doped silica fiber (YDF) laser has important applications in space. However, the darkening effect caused by space ionizing radiation (called RD effect), which is due to the generation of color centers at the microscopic scale, can significantly deteriorate the performance of the YDF laser. This review mainly includes three aspects. First, we briefly introduce space radiation environment and challenges that optical fibers encounter in space. Second, we review the nature and generation mechanism of color centers responsible for RD in YDFs. Finally, we systematically introduce five methods to inhibit RD and their inhibition mechanisms.
... In silicate-based glasses, the coexistence of several oxidation states has been demonstrated for a few RE 3+ , such as Ce 4+/3+ , Sm 3+/2+ , Eu 3+/2+ , and Yb 3+/2+ redox pairs. [70][71][72][73] The reduction of Nd 3+ to Nd 2+ has also been previously observed in phosphate glasses irradiated with g rays, 74 but not in silica-based glasses. Based on the literature 6, 7, 27, 75 on Nd2O3-doped silicate glasses and our own experience, 15 the majority of Nd in the investigated glasses are expected to be in the 3+ state, though the presence of a minor fraction of Nd 2+ in the glasses cannot be excluded. ...
... This suggests that some Yb 3+ ions are reduced to Yb 2+ under the 193 nm laser irradiation. Under X-ray irradiation, the reduction of Yb 3+ to Yb 2+ has been proven using a low-temperature (10 K) CW-EPR method in our previous works [23,24]. ...
... The broad band over the entire range (0-300 mT) in CW-EPR spectra correlates with the Yb 3+ ions in glass. 34,35 In particular, a minor shoulder at 75 mT (g ≈ 9.8) is attributed to Yb 3+ clusters in silica glass. 36 It is evident that the intensity of this shoulder decreases from Yb-APS1 to Yb-APS5, as shown in Fig. 4(b). ...
A series of Yb-doped silica glasses containing equimolar amounts of P2O5 and Al2O3 were prepared using a sol–gel method combined with high-temperature sintering. Raman spectroscopy confirmed the formation of AlPO4 join with a P/Al ratio of 1. However, for a P/Al ratio slightly larger than 1, both the AlPO4 join and P=O bond are formed. Based on the absorption, emission spectra, and electron paramagnetic resonance measurements, it was determined that the increase of the AlPO4 join concentration in Yb-doped silica glass leads to (i) a decrease in the refractive index with a factor of 1.1005 × 10⁻⁴ (which allows for a low numerical aperture facilitating single-mode laser), (ii) the sustained spectroscopic properties of Yb³⁺, (iii) a decrease in Yb cluster concentration, and (iv) an improved radiation hardening performance (beneficial to high-power space lasers). These results suggest that Yb-doped Al2O3-P2O5-SiO2 glass with an appropriate concentration of AlPO4 join has potential applications in high-power space fiber lasers.
... [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Specifically, PD is enhanced via a high Yb 3+ excited state population (inversion level) [6,8] and significantly reduced via photo-or thermal-bleaching, [4,10] loading of H 2 or O 2 , [17] and co-doping with other ions (P, Ce/Al, P/Al, Na, Mg, Ca etc.). [11][12][13][14][15][16]19] However, the underlying mechanisms responsible for the PD are not very clear to date. [17,20,21] The nearly identical optical losses in near-infrared (NIR) pump source and ultraviolet (UV) laser irradiated fibers indicate that the same type of color center is generated in the PD and UV-radiation processes. ...
Yb ³⁺ , Yb ³⁺ /Al ³⁺ , Yb ³⁺ /P ⁵⁺ , and Yb ³⁺ /Al ³⁺ /P ⁵⁺ doped silica glasses were prepared via the sol-gel method combined with high-temperature sintering. The amorphous nature of as-prepared glasses was confirmed by the X-ray diffraction analysis. The composition dependent ultraviolet (UV) absorption changes in the glasses were correlated with the local structure of Yb ³⁺ ions that was obtained by pulsed electron paramagnetic resonance (EPR). In situ photoluminescence evolution and continuous wave EPR spectrum of color centers induced by a 193-nm UV laser were recorded, and the role of UV absorption bands in the photodarkening process was discussed. The results indicated that the strong UV absorption bands in the range of 190–290 nm were primarily attributed to the charge-transfer (CT) transition from O 2– to Yb ³⁺ and the position of CT bands shifted to a higher energy level with increases in the electronegativity of next nearest neighbor atoms (Al/Si/P) of Yb ³⁺ ions. Furthermore, the results revealed that excitation into the CT bands leads to the generation of oxygen hole centers and Yb ²⁺ ions. The study is useful for understanding the underlying mechanism of photodarkening in Yb ³⁺ -doped high-power laser fibers.
... For the sample with P/Al = 0 (YAP0), the secondneighboring shell of Yb 3+ is primarily a mixture of Al and Si atoms (Figure 5a). Our earlier study shows that trapped electron centers (TECs) and trapped hole centers (THCs) are always generated in pairs: 31 In Al 3+ single-doped silica glasses, TECs (Si-E′/Al-E′) and THCs (AlOHCs) are observed, as shown in Figure 3a; and in Yb 3+ /Al 3+ -co-doped glasses (YAP0), Yb 2+ ions, which are formed by electron trapping in Yb 3+ ions, dominate the TECs. Compared to that in the Al 3+ single-doped sample, the clear decline in the absorption and EPR intensities of Si-E′/Al-E′ is observed in the Yb 3+ /Al 3+ -co-doped sample (YAP0); however, there is little change in the intensities of AlOHCs (Figure 3a,d). ...
Yb3+/Al3+/P5+ co-doped silica glasses with different P/Al ratios were prepared using the sol–gel method combined with high-temperature sintering. The evolution of composition-dependent color centers caused by X-ray irradiation in these glasses were correlated with their structural changes, which is controlled by the P/Al ratio. Nuclear magnetic resonance (NMR) and Raman spectra have been used to characterize the glass network structure, and advanced pulse electron paramagnetic resonance (EPR) has been employed to study the local coordination atom structures of Yb3+ ions in pristine glasses as a function of P/Al radio. Si-related (Si-E', Si-ODC), Al-related (Al-E', AlOHC), P-related (P1, P2, POHC) and Yb-related (Yb2+) color centers in irradiated glasses have been observed and explained by optical absorption and continuous wave (CW) EPR spectroscopies. The formation mechanisms of these centers, structural model of glasses, and the relationship between them were proposed. Direct evidence confirms that the formation of Yb2+ ions induced by radiation is highly dependent on the coordination environment of Yb3+ ions in glasses. In addition, the glass network structure significantly affects the generation of oxygen hole color centers (AlOHCs/POHCs) caused by radiation. These results are useful in understanding the micro-structural origin and the suppression mechanism by phosphorus co-doping of the radio-darkening effect in Yb3+-doped silica fibers.
The increasing development of radiotherapy technology and treatment modalities has put forward new and higher requirements for dose monitoring. In this paper, the Ce/Tb co-doped silica fiber was employed to implement the dose detection on a Varian EDGE linear accelerator. The stability of the system was assessed by measuring 10 repeated irradiations with 6 MV X-ray beams, the maximum percentage deviations of these exposures was 0.38%. Dose linearity was studied by measuring the accumulated optical intensity signal versus incident irradiation dose, the experimental results indicated that the signal increases with the increasing delivered dose, and exhibits apparent linear dependence on the dose (R
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=0.99998). The dose rate independence was also verified by the test of fixed doses with increased dose rates. Moreover, the sensor gantry rotated from 180° to 180°E, and the detected values for all the angles are almost identical with a maximum difference of 1.26%. Excellent radiation resistance has also been observed by repeated measurement with the sensor pre-irradiated at different doses. All the tests indicate that the novel sensor can meet the basic requirements for clinical dose detection. Furthermore, it provides a new way to solve some challenges in clinic dosimetry, such as the validation of small-field dosimetry and surface dose assessment.
Egyptian blue and green are among the oldest synthetic pigments produced by humanity. The two pigments are complex multi-phase copper-based systems synthesised with different protocols from the same raw materials. Since the 2000s, synchrotron X-ray techniques have provided significant new insights on their chemistry and microstructure. However, the potential impact of high flux irradiation of these pigments has not yet been studied despite the fact that it can lead to visual discoloration and less readily observable alterations such as defects formation or redox changes. In this work, we investigate the effects of synchrotron X-ray irradiation on Egyptian blue and green samples. Radiation-induced effects are monitored after irradiation at increasing doses using electron paramagnetic resonance (EPR) spectroscopy at temperatures of 290 K and 30 K. The cupric ion (Cu2+) is in D4h axial geometry in Egyptian blue and in disordered geometry in Egyptian green, which makes the two pigments very easily identifiable by EPR. Egyptian green samples are found to be much more sensitive to X-rays than Egyptian blue. In particular, a browning of the green samples is observed from the lowest doses tested while no color change is detected for the blue ones. Three types of radiation-induced defects are detected after irradiation: E′, non-bonding oxygen hole and aluminum hole centers. Correlations between defect intensity and dose are calculated. Archaeological and modern pigments (whether blue or green) do not show the same reactivity to X-rays, which opens the prospect of using radiation-induced defects as a marker of their history.
Yb³⁺-doped silica fiber (YDF) lasers have important application prospects in space. However, YDFs face various types of harsh ionizing radiation (such as protons, electrons, and γ-rays) during space missions. Radiation-induced darkening (RD), primarily because of the generation of color centers at a microscopic scale, can seriously deteriorate the laser performance of the YDFs. In this review, the nature, formation mechanism, and suppression methods of color centers in YDFs are introduced.
Yb-doped fiber laser is a very appealing technology to implement space communication, laser radar and nuclear facilities due to its reduced weight, size, high efficiency, high peak power combined with good beam quality. However, Yb-doped fiber materials will suffer a harsh ionizing radiation (such as neutron, proton and electron) under the condition of irradiation. The radiation-induced darkening effect can lead the fiber loss to increase obviously, the laser slope efficiency to decrease drastically, and even no laser output to be observed in severe cases. Therefore, it is necessary to conduct in-depth research on the performance changes of Yb-doped fiber materials subjected to the irradiation. In this paper, a series of Yb-doped optical fibers and optical fibers is prepared by the improved chemical vapor deposition method combined with rare-earth chelate-doped. And the optical properties of the optical fiber before and after being irradiated and annealed are tested. We mainly investigate the absorption spectrum of Yb-doped fiber material. The results show that the concentration of Al-related defects in the Yb-doped fiber material increases after neutron irradiation, and the absorption loss in the visible region increases. And the color center defects produced by the irradiation will significantly reduce the Yb ion fluorescence lifetime. The doping of Ce ions can reduce the Al-oxygen hole center (Al-OHC) color center defects, and can suppresse the radiation-induced darkening effect of Yb-doped fiber to a certain extent. Thermal annealing can reduce the absorption of fiber material by reducing the concentration of neutron radiation-induced color center defects, and thus eliminating the darkening effect to a certain extent. Finally, with our previous research, we find that neutron irradiation and ray irradiation have similar effects on the optical properties of Yb-doped fiber materials. The main reason is that the electron ionization effects occur due to both ray irradiation and neutron irradiation to generate free electron and hole pairs, which then combine with the original defects in the material to turn into color center defects. However, the color center defects caused by neutron irradiation are more stable and require thermal annealing to be eliminated. And the results obtained in this study provide theoretical basis and solution for developing the Yb-doped fibers with high laser performance and high radiation resistance.
Rare Earth Elements-doped glasses have applications in many fields, and for example, Al and Ce codoping in silica optical fibers is usually done to attenuate the optical degradation (photodarkening) of, e.g. Yb and Tm doped silica optical fibers. To shed some light on the effect of codoping on the optical properties and the glass structure of SiO2 fibers, we investigated a SiO2 fiber preform having a gradual increase of the CeO2 + Al2O3 content from 1.5 to 6.3 mol% and a CeO2/Al2O3 molar ratio varying from 0.04 to 1.4. Raman and photoluminescence spectroscopies have been used to evaluate changes in the short- and medium-range order both in the cladding and in the core portions, and the evolution of the glass optical properties, respectively. The variations of the excitation and emission bands have been linked to modifications in the vibration bands of the Raman spectra, and in turn, the relationship between optical properties and glass connectivity has been carefully considered. Our results show that there is a strong positive linear relationship between the increase of the Raman bands in the high-frequency range and the red shift of the excitation maxima. Therefore, the change of <Ce–O> distances and bond covalency character influences, in the same way, both the photoluminescence behavior of the core and the connectivity of the amorphous network.
Humic acid (HA) can be rapidly decomposed in single ozonation (SOZ); however, it undergoes incomplete degradation and inefficiency in abatement of the total organic carbon (TOC) is observed. In this study, titanium silicalite-1 grafted with carbon nitride was used as a carrier for cerium oxide (Ce-O/C3N4@TS-1), and this catalyst led to more rapid and thorough decomposition of HA in water. The TOC removal efficiencies of HA solution were 20% in SOZ against 62% in catalytic ozonation (COZ) after 60 min of reaction. Furthermore, in order to investigate the reasons for increasing degree of mineralization, in this study, the catalytic effect of the catalyst toward ozonation of oxalic acid (OA) was also investigated. The results showed that Ce-O/C3N4@TS-1 not only presented a good catalytic activity, but also exhibited an excellent stability. The good activity was attributed to the presence of Ce redox pair (Ce3+/Ce4+) on its surface and generation of hydroxyl radicals which has a rapid reaction rate with OA. C3N4 provide a large scaffold for anchoring Ce-O, which was responsible for the favorable stability.
Optical, structural properties and redox were studied for Ce-bearing silicate and aluminosilicate glasses and Ce-activated silica (SiO2) fiber preforms, by X-ray Absorption, Raman and Photoluminescence Spectroscopy.
The introduction of Al2O3 in Ce-bearing silicate glass strongly stabilizes reduced species in agreement with the optical basicity concept and additionally induces modifications in the Ce3 + surrounding. In Al-free silica preforms, depending on the collapsing conditions, the Ce3 +/ΣCe ratio has the highest variability (Ce3 +/ΣCe = 0.7–0.85 ± 0.07), whereas in Al-bearing fiber preforms, Al codoping induces a strong stabilization of Ce3 + species, despite the collapsing atmosphere.
Al presence induces modifications on Ce local environment, and thus variations of the distance between Ce3 + and its ligands, and/or to variations of the covalency of the bonds. Absorption bands in the UV region, ascribed to Charge-transfer (CT) bands, in our samples, occur only for low amount of Ce4 + (Ce3 +/ΣCe > 0.85 ± 0.07). Indeed, in silicate and aluminosilicate glasses the higher amount of Ce4 + does not give rise to UV absorption bands ascribed to CT processes, whereas, in fiber preforms, Ce3 + species can induce the trapping of hole centers, and enhance the photoluminescence efficiency. More generally, these results enhance our understanding of REE structural and chemical roles in amorphous materials and increase our knowledge for technical applications.
We report on detailed investigations of ytterbium (Yb) and aluminum (Al) doped silica fiber and preform samples co-doped with cerium (Ce). The prevention of pump-induced photodarkening (PD) by temporary oxidation of Ce³⁺ to Ce⁴⁺ (or rather Ce³⁺⁺) was proved by observed modifications in the ultraviolet (UV) spectra of transient absorption during near-infrared (NIR) pumping of thin preform slices. Only a small part of available Ce³⁺ ions (< 4%) was found to be involved in this process despite Yb inversions of up to 0.28. The modifications in the UV absorption spectra disappeared completely when the pump power was switched-off. From these observations we conclude that the recombination to Ce³⁺ takes place very fast thereby enabling these ions to capture liberated holes h⁺ perpetually during further pumping. We found a concentration ratio of Ce/Yb ≈0.5 to be sufficient to reduce PD loss to 10% in comparison to Ce-free fibers. Thus, the thermal load caused by absorption of PD color centers at pump (and laser) wavelength is expected to be also reduced. Unfortunately, new heat sources arise with the presence of Ce which cannot be explained by the absorption of Ce ions at the pump wavelength but must be attributed to the interaction with excited Yb ions. Fiber temperature increase of more than 200 K was observed if both, Yb2O3 and Ce2O3 concentration exceed 0.4 mol%.
We report the characteristics of 46.9, 69.8 and 72.6 nm lasers of Ne-like Ar in a plasma column pumped by capillary discharge. A main current of 12 kA with rise time of 40 ns was used to generate the plasma in a 35-cm-long capillary. The temporal and spatial characteristics of 46.9 and 69.8 nm laser pulses were measured. The time of lasing onset for the two lasers is about 35 ns. The pulse widths are about 1.3 ns for 46.9 nm laser and 1.2 ns for 69.8 nm laser. The FWHM beam divergences of the 46.9 and 69.8 nm lasers are approximately 0.5 and 0.4 mrad, respectively. The initial pressure ranges for 46.9, 69.8 and 72.6 nm lasers correspond to 13–24, 12.5–20 and 14–20 Pa, respectively. A gain coefficient of 0.58 cm−1 was measured and gain saturation behavior was observed at 46.9 nm, conditioning the initial pressure of 19 Pa. At the same discharge current, a 69.8 nm laser with gain coefficient of 0.41 cm−1 and a 72.6 nm laser with gain coefficient of 0.22 cm−1 were achieved by changing the initial pressure to 16 Pa.
This Letter relates the clear evidence of Yb 2 + formation under 2.5 MeV electron irradiation in optical fiber preforms showing a darkening of the core. We thus detected by in situ photoluminescence measurements the green emission of divalent Yb 2 + under the 355 nm excitation. Moreover, we showed the existence of two types of Yb 2 + ion species with different stabilities. We demonstrated that the radiodarkening mechanism is based on a pair association of Yb 2 + with aluminum oxygen hole center point defects.
This paper reports, for the first time to our best knowledge, a nearly diffraction-limited output in a Yb 3 + / Al 3 + / F − codoped double cladding silica fiber with a 50 μm core diameter and 0.02 core NA. The core glass with a diameter > 6 mm was fabricated through solgel process combined with high temperature sintering. Laser performances of this fiber at different bend diameters were studied. The mean M 2 < 1.1 in this 50 μm core diameter fiber was achieved at a bend diameter of 0.35 m. The core glass with the refractive index nearly equal to pure silica glass is suitable for the fiber design, such as large-mode-area photonic crystal fiber.
Photo-bleaching (PB) is considered as a convenient way to mitigate the photo-darkening (PD) in Yb-doped fibers. In this work, Ce doping into the fiber was proposed to improve the PD resistance and the efficiency of PB. Compared to Yb/Al-co-doped fiber, the PD-induced excess loss in Yb/Al/Ce-co-doped fiber was more than 50 % suppressed. The measurements also showed that more than 30 % PD loss in Yb/Al/Ce-co-doped fiber was bleached after 180-min pumping of 793-nm LD, while the percentage in Yb/Al-co-doped fiber was less than 20 %. Besides, a self-bleaching phenomenon was observed in Yb/Al/Ce-co-doped fiber. The PD was effectively relaxed after the 915-nm pump power was turned off for 16 h. This work shows a capable and convenient approach for maintenance of fiber lasers and amplifiers.
A large-size (∅5 mm) Yb³⁺-doped silica fiber core-glass rod with an Al-P co-doped composition: 0.035Yb2O3-1.0Al2O3-1.0P2O5-97.965SiO2 (mol%) was prepared by the sol-gel method combined with high-temperature melting technology. We successfully solved the doping homogeneity problem caused by the volatility of P2O5. The doping homogeneity of the glass rod was very high with the maximum refractive index fluctuation Δn < 2 × 10⁻⁴. The refractive index of the glass rod was very low because of the equimolar amounts of Al and P co-doping, which yielded an AlPO4 structure. A large-mode-area single cladding fiber (LMA SCF) and photonic crystal fiber (LMA PCF) with core diameters of 35 and 50 µm, respectively, were drawn from this glass rod. Owing to the low refractive index of the core-glass rod and the corresponding low numerical aperture (NA, 0.033) of the core, the LMA SCF exhibited a high laser beam brightness with M² = 1.3-1.4. The LMA PCF exhibited the maximum output power, which was limited by the available pumping power, of 46 W and the slope efficiency of 61%.
A method for optimizing the spectral distortion of an ultrafast pulse in a polarization-maintaining picosecond linear-cavity fiber laser with a one-stage fiber amplifier is proposed and demonstrated. The mechanism of control of the spectral distortion in the fiber system has been investigated. The experimental and theoretical results illustrate that the filtering effect of a fiber Bragg grating can effectively decrease the spectral oscillatory distortion accumulated by self-phase modulation. Injected into a Nd:YAG regenerative amplifier, the ultrafast pulse could produce high pulse energy of 1.2 mJ at a repetition rate of 1 kHz.
Rare earth doped active glasses and fibers can be exposed to ionizing radiations in space and nuclear applications. In this work, we analyze the evolution of ²F5/2 excited state lifetime in Yb³⁺ ions in irradiated aluminosilicate glasses by electrons and γ rays. It is found that the variation of lifetimes depends on the Yb³⁺ clusters content of the glasses for irradiation doses in the 10² – 1.5∙10⁹ Gy range. In particular, glasses with high clustering show a smaller decrease in lifetime with increasing radiation dose. This behavior is well correlated to the variation in paramagnetic defects concentration determined by electron paramagnetic resonance. This effect is also observed in Yb³⁺ doped phosphate and Er³⁺ doped aluminosilicate glasses, inferring that clustering plays an important role in irradiation induced quenching.
The gain of erbium-doped fiber amplifiers is damaged by irradiation partly because of creation of color centers responsible of excess absorption at pump and signal wavelengths. Based on the combination of thermally stimulated luminescence and spectrophotometry, this Letter demonstrates that a part of the gain loss should be associated with the reduction of the density of Er 3 + ions by irradiation.
Three series of Yb3+-doped silica glasses containing different amounts of Al2O3 and P2O5 were prepared successfully by using a sol–gel method. Absorption, excitation and fluorescence spectra of Yb2+ ions in these silica glasses as well as X-ray photoelectron spectra (XPS) of Yb4d were recorded and analysed systematically. It is found out that the addition of Al3+ or P5+ ions has great influence on the redox state of ytterbium ions. With increasing Al3+ ion contents in these silica glasses, more trivalent Yb3+ ions are reduced to divalent Yb2+. In contrast, the increase of P5+ ion contents greatly promotes divalent Yb2+ ions to be oxidized to trivalent Yb3+. The possible redox mechanisms have been explored and discussed in detail. The influence of Al3+ and P5+ ion contents on the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs was also discussed. Both the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs decrease gradually with increasing Al3+ and P5+ ion contents. The decrease of cooperative luminescence of Yb3+ ion pairs indicates a good dispersion effect of Al3+ and P5+ ions on Yb3+ ions in Yb3+-doped silica glass. The results are useful for optimization of fabrication of the high quality Yb3+-doped silica fiber by the composite design of Yb–Al–P co-doped silica glass.
We have studied the photodarkening effect in fiber preforms with an ytterbium-doped aluminosilicate glass core. The room-temperature stable Yb2+ ions formation in the glass matrix under both UV- and NIR-pumping irradiation was revealed by the method of absorption spectra analysis and the fluorescence spectroscopy technique. Comparative studies of preforms and crystals samples luminescence spectra, obtained under UV-excitation, were performed. A general mechanism of Yb2+ ions and aluminium oxygen-hole centers (Al-OHC) formation as a result of photoinduced process of Yb3+ ions excitation to "charge-transfer state" (CTS) was found for both Yb:YAG crystal and aluminosilicate glass.
By using a modal interference method, photodarkening-induced refractive index change in ytterbium doped fibers is measured with high accuracy. This refractive index change is positive at the ytterbium lasing wavelengths near 1080nm, and it gradually approaches a saturated level over time. It is found that the refractive index change is linearly proportional to the photodarkening-induced excess loss at an arbitrary probe wavelength in the visible band. The maximum refractive index change in our experiment is close to 1×10-5. The origin and influence of the photodarkening-induced refractive index change on fiber lasers is discussed.
In this work we present experimental evidence that the valence instability of the ytterbium ion play a key role for the observed photodarkening mechanism in Yb-doped fiber lasers. Luminescence and excitation spectroscopy performed on UV irradiated Yb/Al doped silica glass preforms and near-infrared diode pumped photodarkened fibers show a concentration increase of Yb²⁺ ions. A concentration decrease in Yb³⁺ could also be observed for the UV irradiated preform. The findings contribute to an increased understanding of the kinetic processes related to photodarkening in Yb-doped high power fiber lasers.
We investigated the photodarkening (PD) kinetics of two fiber series with variation of the Yb content for constant Al concentration or constant ratio of Al/Yb, respectively. The results show the outstanding importance of the absolute value of Al concentration also in the case of fibers with strongly reduced Yb content. An Al/Yb ratio of 5 to 6 is not sufficient to mitigate PD loss. Moreover, a model to describe PD loss and rate constant as functions of Yb concentration and excitation is suggested that links measurements of PD in single fibers of the same type (variation of Yb inversion) and in fiber series (constant Yb inversion).
High-accuracy measurements and analysis of refractive index change induced by photodarkening and thermal bleaching in ytterbium-doped fibers are presented, based on a modal interference method. Photodarkening-induced refractive index change is positive at the ytterbium lasing wavelengths near 1080 nm, and it approaches a saturated level, which is in the order of 10(-6)∼10(-5) for the tested fiber samples. It is found that the value of this refractive index change is linearly proportional to the photodarkening-induced excess loss at an arbitrary probe wavelength in the visible band. Thermal bleaching can only partially erase the photodarkening-induced refractive index change, leaving a residual index change of (0.2∼0.3)×10(-5). The influence of the photodarkening-induced refractive index change on fiber lasers is discussed.
We show that the photodarkening resistivity of ytterbium-doped fiber lasers can be greatly improved by cerium codoping. It is suggested that the coexistence of the redox couple Ce(3+)/Ce(4+) in the glass provides means for trapping both hole- and electron-related color centers that are responsible for the induced optical losses in Yb-doped fiber lasers.
A broad visible luminescence band and characteristic IR luminescence of Yb(3+) ions are observed under UV excitation in ytterbium-doped aluminosilicate glass. Samples made under both oxidizing and reducing conditions are analyzed. A strong charge-transfer absorption band in the UV range is observed for glass samples containing ytterbium. Additional absorption bands are observed for the sample made under reducing conditions, which are associated with f-d transitions of divalent ytterbium. The visible luminescence band is attributed to 5d-4f emission from Yb(2+) ions, and the IR luminescence is concluded to originate from a relaxed charge-transfer transition. The findings are important to explain induced optical losses (photodarkening) in high-power fiber lasers.
Photodarkening experiments are performed on ytterbium-doped silicate glass samples. A strong charge-transfer (CT) absorption band near 230nm in aluminosilicate glass is found to be correlated to the mechanism of induced color center formation. Excitation into the CT-absorption band generates similar color centers as observed in ytterbium-doped fiber lasers under 915nm high power diode pumping. The position of the CT-absorption band is compositional dependent and is shifted to shorter wavelengths in ytterbium doped phosphosilicate glass. Very low levels of photodarkening is observed for the ytterbium doped phosphosilicate glass composition under 915nm high power diode pumping. Possible excitation routes to reach the CT-absorption band under 915nm pumping are discussed.
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X-ray photoelectron spectroscopy (XPS) was used to investigate the valence state of cerium in unsupported composite CexMn1−xO2−y catalysts. Manganese–cerium composite oxides are being widely used in sub- and supercritical catalytic wet oxidation for the treatment of wastewater containing toxic organic pollutants. The performance of such catalysts depends, among others, on the redox reactions involving CeO2−x suboxides and manganese oxides. In order to investigate the surface chemistry of such environmental catalysts, 10 samples with Mn/Ce ratios ranging from 0/10 to 9/1—molar basis—were synthesized by co-precipitation and characterized as-prepared and after gold coating. Principal component analysis and curve fitting of Ce 3d and O 1s core-level spectra were used to determine the types of valence states and their proportions on the catalysts’ surface. The study revealed valence state modifications in cerium and changes in the amount of lattice oxygen depending on the Mn/Ce ratio and on whether or not the as-prepared catalysts were subjected to gold deposition. The proportion of surface Ce3+ species in the non-stoichiometric CeO2−x oxides was successfully quantified through linear correlation between v2′ peak area and integral of the Ce4+ 3d spectrum.
Two series of silica glasses [(95.9−x)SiO2–4Al2O3–xP2O5–0.1Yb2O3 (in mol%, x=0–10) and (92−y) SiO2–4Al2O3–4P2O5–yYb2O3 (in mol%, y=0–0.2)] were prepared by sol–gel method combined with high-temperature sintering. The relationship between the glass structure and spectroscopic properties is investigated. Significant alterations in density, refractive index, absorption and emission cross sections, fluorescence lifetime, and scalar crystal field NJ are observed at a molar ratio of P5+/Al3+=1 due to the transformation of P5+ structure type from AlPO4-like to PO4 units. This structural change is clearly observed in Raman spectra. In addition, NMR experimental results suggest that Yb3+ is mainly located in AlPO4-like but not SiO4 units at a molar ratio of P5+/Al3+=1, and also Raman spectra reveals the presence of P=O linkage in samples with molar ratio of P5+/Al3+>1. This further demonstrates that it is due to the formation of P=O site but not the simple addition of P2O5 in Yb3+/Al3+/P5+ co-doped silica glass to significantly decrease the absorption and emission cross sections of Yb3+. It reflects in the change of Yb3+ Stark splitting, and is revealed by the decline of Yb3+ asymmetry degree.
A model of steady-state rate equations including amplified spontaneous emission for long-wavelength ytterbium-doped fiber laser is set up, which provides design principle for a practical laser system. We demonstrate a diode-pumped all-fiber Yb-doped fiber laser at 1150 nm with an output power of 33.6 W, the optical efficiency is 60%.
A set of Ce-/Yb-co-doped silica optical fiber preform cores, differing in terms of dopant concentrations are studied by Electron Paramagnetic Resonance (EPR) spectroscopy before and after irradiation of the samples with excimer UV laser light and γ-rays. Evidence of Yb3 + clustering in the case of high Yb content samples is observed on as prepared samples regardless of the presence of Ce3 + ions. Si-E′ and Al-OHC centers were identified upon photon irradiation. The results are correlated to the micro-structural origin of the photodarkening process occurring in Ce–Yb doped glass fibers.
Electron paramagnetic resonance (EPR) measurements have been carried out on various types of natural fused silica (I301, Q310 and H102) irradiated with gamma-ray doses up to 14 MGy. Paramagnetic defects E′γ as well as the non-bridging oxygen-hole centre (NBOHC), the peroxide radical (POR) and the aluminium oxygen-hole centre (AlOHC) account for all lines observed in the EPR spectra of irradiated samples. The growth kinetics of AlOHC has been followed and analysed. It is found that AlOHC defects are produced by two processes. One of them, called rapid process, saturates at low doses and has been related to the precursor {AlO4−/H+}, H+ acting as charge compensator. The second one, called slow process, takes place up to the highest dose and has been related to the precursor {AlO4−/M+}, the compensator M+ being an alkaline-earth ion Li+, Na+ or K+.
Two series of Yb3+-doped silica glasses (YA and YAP) are fabricated using the sol-gel method, and their spectroscopic properties are investigated. The longest fluorescence lifetime of 0.96 ms is obtained in YAP2 and YAP3 glasses. The emission cross-section (sigma(emi)) and sigma(emi) x tau of Yb3+ in the YAP1 glass are 1.00 pm(2) and 0.94 pm(2).ms, respectively. Co-doping with Al and P decreases the hydroxyl group content in the glasses because of the reduction in non-bridging oxygen content. The lowest OH content in the YAP3 glass is 9.6 ppm.
Optical absorption bands and paramagnetic centers induced with γ-rays or excimer laser lights were examined in high purity synthetic SiO2:Al glasses prepared by VAD method. Gamma-induced absorptions ranging 1.5–6.2 eV were deconvoluted into 5-Gaussian components peaking at 2.3, 3.2, 4.1, 4.9 and 5.8 eV. Two paramagnetic EPR centers associated with an Al ion were observed, Al-OHC and Al E' center and both centers are close to each other. These results indicate that a hole and an electron generated by irradiation are predominantly trapped at an oxygen bridging an Al and a Si and an Al coordinated with only three oxygens, respectively. A close correlation between 5-optical bands except the 5.8-eV band and Al-OHC/Al E′-center was obtained. It was assigned from a consideration of the nature of these Al-related centers that a band peaking at 2.3 eV are due to Al-OHC and Al E' centers give a band peaking at 4.1 eV. Oscillator strength was evaluated to be 0.06 for the 2.3 eV-band or 0.2 for the 4.1 eV-band. Although no coloring and paramagnetic centers were noted for XeCl laser light (304 nm), the results for ArF laser light (193 nm) were almost the same as the case of γ-irradiation. This difference suggests that a pair generation of Al-OHC and Al E'-center occurs via two photon process.
Photodarkening phenomenon in ytterbium (Yb)-doped silica glasses was experimentally investigated by measurements such as electron spin resonance (ESR), X-ray absorption fine-structure (XAFS), and optical transmittance. A predominant increase of Al-oxygen hole center (OHC) was observed for Al-Yb co-doped silica glasses both by the incidence of pump light and by gamma-ray irradiation. It was also recognized that the optical transmission loss similarly increased in both cases. These results indicate that the formation of Al-OHC is the prime cause of the excess loss induced by photodarkening. XAFS measurement indicated that the second-nearest-neighbor atoms around Yb may be related to the mechanism of photodarkening phenomenon.
Rare earth doped silica based fiber lasers and amplifiers with very high output power and excellent beam quality are efficient devices for a variety of applications in industry, science and medicine. During the last years, important progress was possible by new design concepts but also by carefully tailoring the material properties. Aspects of material and technology development concerning the interaction of different dopants and co-dopants will be discussed in the following. Our discussion concentrates on the optical properties of the laser fiber as refractive index, absorption and emission of the rare earths and especially on phenomena concerning the background loss of the fibers. We have found a strong rare earth specific loss component which remarkably depends on the kind and the ratio of the co-dopants. The relations of this background loss to material composition and fabrication technology are demonstrated and discussed on an empirical base.
Electron-spin-resonance studies of a series of air-annealed samples of glassy SiO2 having various degrees of enrichment (or depletion) in the 29Si isotope have confirmed that a γ-ray-induced doublet of 420-G splitting is the 29Si hyperfine structure of the well-known E′ center. This finding validates the widely accepted model of the E′ center as an unpaired electron spin in a dangling sp3 hybrid orbital of a silicon bonded to three oxygens in the glass structure and eliminates a recently proposed alternative model. Continuous-wave microwave saturation measurements at ∼9.2 GHz were carried out in order to establish spin-lattice relaxation behavior and to determine absolute line intensities in the low-power limit. The spin-lattice relaxation process for the 29Si E′ center is shown to be dominated by a hyperfine mechanism. Spin-lattice relaxation times T1 could be extracted from the cw saturation data only by means of a semiempirical formulation differing from the usual approaches found in the literature.
A new series of 27Al hyperfine spectra have been discovered by EPR studies in irradiated type-I and -II vitreous silica. Our analysis of these spectra indicate that under ionizing irradiation at low temperatures (T≲260 K), pre-existent, diamagnetic network defects of the form AlO3/2 trap an electron to form paramagnetic AlO3/2-. Upon annealing or irradiation at 300 K, AlO3/2- traps a compensator such as Na+, Li+, or Hn+. With the aid of high-temperature electrolysis, specific spectra are associated with specific compensators. In one spectrum a superhy-perfine interaction with an isotope having I=3/2 and attributed to 23Na is observed. We observe that the role of charge compensators with respect to the charge (magnetic) state of these Al defects in SiO2 gives a consistent indication of the aluminum-oxygen coordination. We also demonstrate a correlation between the strength of the 27Al contact hyperfine interaction, the presence of charge compensators, and the thermal stability of the trapped electron. An analysis of the 27Al and 29Si hyperfine interactions arising from paramagnetic AlO3/2- and SiO3/2 (E1′) centers indicate that these two centers are isoelectronic. Each of the four 27Al hyperfine spectra are analyzed in terms of coaxial Zeeman and hyperfine interactions which include broadening effects due to variations in bond angles and lengths. Ideas about how Al and Si E1′ centers might be incorporated in the v-SiO4/2 network are also presented. In particular, a model for an extrinsic Si E1′ center is proposed which may explain various effects observed with EPR in different kinds of v-SiO2 60Co γ irradiated with fluences ≤107 R.
The formation and thermal bleaching of radiation‐produced color centers in cerium‐containing soda—silica glass are studied to determine the effect of cerium on color‐center kinetics. The optical absorption changes occurring during and after irradiation with gamma and x rays are measured. The data are fit by equations obtained by integrating a set of reaction‐rate equations. These equations are an approximate description of the following three processes that account qualitatively for the effects of cerium concentration and oxidation state: Ce3+ ions, by capturing radiation‐produced holes to form Ce3+ + centers, inhibit formation of all other kinds of centers due to trapped holes; Ce4+ ions, by capturing electrons to form (Ce4+ + electron) centers, inhibit formation of all other kinds of centers due to trapped electrons and inhibit recombination of electrons with trapped holes; after cessation of irradiation, holes are transferred from their traps to Ce3+ ions.
This article presents an XPS study of Ce 3d emission spectra dominated by atomic multiplet effects in core level spectroscopy of rare earth compounds (Ce oxides). Core level spectroscopy has been used to study the electronic states of Ce 3d5/2 and Ce 3d3/2 levels in Ce4+ and Ce3+ states. The well-resolved components of Ce 3d5/2 and Ce 3d3/2 spin-orbit components, due to various final states (4f0, 4f1, 4f2 configurations), were determined on 3d XPS spectra from commercial powders (CeO2, CePO4).
These results were used to study the 3d spin-orbit component of mixed cerium-titanium oxide.
This compound was prepared by co-melting commercial powders of CeO2 and TiO2 at 1800 K under air using a solar furnace with a flux density of 16 MW.m−2 at the focal point of the parabolic concentrator. The mixed oxide Ce2Ti2O7 was produced and contained Ce(III) species which may be reactive with water to give back the initial metal oxides and generate hydrogen, a valuable product considered as a promising energy carrier in the future in replacement of oil.
The 3d photoemission spectra revealed the presence of mixed components attributed to mainly Ce(III) and Ce(IV) species. Copyright
Yb3+ reduction under β and γ irradiation has been studied in aluminoborosilicate glasses by EPR spectroscopy. From the Yb3+ EPR line variation, we demonstrate that more than one Yb3+ sites coexist in the pristine Yb-doped glasses. Reduction of Yb3+ into Yb2+ is observed for all integrated doses and Yb doping contents. For doses higher than 108 Gy, an Yb3+ ion environment change occurs, this change is correlated with a stabilization of the reduced Yb2+ state. The paramagnetic defect concentration displays a linear variation as a function of the logarithm of the dose. The glass doping with Yb2O3 leads to a substantial decrease of the defect concentration as well as a modification in the relative proportion of the defects produced. In particular, Yb doping tends to increase the relative content of Oxy defects.
The present work describes photodarkening from the viewpoint of cooperative luminescence. The temporal evolution of both effects was measured simultaneously by means of ytterbium doped aluminosilicate fibers for concentrations up to 1.8 wt% Yb3+. The quadratic dependence of photodarkening and cooperative luminescence versus dopant concentration was observed. The change in the photodarkening and cooperative luminescence mutual dynamics for highly and low doped fibers is ascribed to a different ion number which forms the cluster. Cooperative luminescence is proved to be a natural probe for photodarkening since it provides new pieces of information and contributes to the photodarkening mechanism description.
Using a combination of pulse electron paramagnetic resonance and photoluminescence spectroscopy, we demonstrate the major role of phosphorous rather than aluminium in the rare-earth dissolution process, an essential advance in telecommunication and solid laser fields. Our results also provide new insight into the micro-structural origin of the photodarkening process occurring in Yb doped fiber.
The description of the photodarkening (PD) kinetics of Yb-doped silica fibers with a stretched exponential function is analyzed in more detail, revealing a dependence of the determined PD parameter values on the measurement time. Thereby, serious uncertainties may occur in advanced interpretations of the results, e.g., the discussion of the rate constant as a function of the Yb inversion, if the measurement time is too short. A method to circumvent the problems of the fitting procedure is suggested, taking advantage of the self-similarity of the PD curves for different inversions. An exponent of 4.3 +/- 0.5 in the power-law dependence of the PD rate constant tau(-1) on the Yb inversion was found for a typical aluminium-codoped fiber.
We report on photodarkening (PD) investigations at Yb doped fibers with specific variation of the concentrations of the codopants aluminum and phosphorus, measured during cladding pumping at 915 nm. A core composition with equal content of Al and P is most promising to achieve Yb fibers with low PD, high laser efficiency and low numerical aperture of the laser core despite of high codoping. A laser output power of more than 100 W was demonstrated on such a fiber with a slope efficiency of 72%. The correlation of the PD loss with the NIR-excited cooperative luminescence encourages the supposition that cooperative energy transfer from excited Yb(3+) ions to the atomic defect precursors in the core glass enables the formation of color centers in the pump-induced PD process.
The effects of ArF excimer laser irradiation on dehydrated high-purity silica glass were investigated on both the optical-absorption bands due to oxygen-deficient centers (ODC) and the formation of E' centers. With an intense uv flux from an excimer laser, an E'-center density of the order of 1015 cm-3 was created. The 7.6-eV absorption band remains at the original level, while the 5.0-eV absorption band having the 4.3-eV emission band decreased. Both bands were reduced by heat treatment in an O2 atmosphere. These results suggest that there exist two types of ODC:ODC(I), which is responsible for the 7.6 eV band, and ODC(II), for the 5.0-eV band. The concentrations of ODC(I) and ODC(II) were evaluated to be 1×1018 cm-3 through an analysis of the gas treatment data and of the order of 1014 cm-3 through the growth curve of the E' centers, respectively. The structural origin of ODC(I) has been attributed to the Si-Si homopolar bond, judging from the fact that the peak energy and cross section of the absorption were in close agreement with those of the Si2H6 molecule. Theoretical calculations on defect energy levels by O'Relly and Robertson supported the structural model for ODC(I) and suggested an unrelaxed oxygen vacancy for ODC(II). These assignments were also consistent with the results of a quenching experiment in which the fictive temperature of samples was changed; the equilibrium between concentrations of ODC(I) and ODC(II) shifted in a reasonable manner.
A custom-developed high-power laser based on a Yb-doped amplifier is characterized as a transmitter for deep-space optical communications. The key requirements are high peak power at moderate data rates with good beam quality. The transient pulse dynamics are modeled via a simple rate equation approach and reveal a qualitative agreement with the pulsed performance. Peak powers were obtained up to 8 kW for a 22-ns pulsewidth at a 3.5-kHz repetition rates. The peak power dropped significantly as the repetition rate was increased to 50 kHz.
The Laser Megajoule project is a major component of the French simulation program to study nuclear fusion by inertial confinement. The future Laser Megajoule facility requires control-command systems that will operate in a harsh radiative environment. Commercial off-the-shelf optical fiber data links are envisaged as a radiation tolerant solution for this application. In this paper, we present our preliminary study of their vulnerability. For this, we firstly have used an original method consisting of ultraviolet (∼5 eV) exposures of the fibers to identify the different germanosilicate optical fibers containing phosphorus, which leads them unacceptable for both steady state γ-rays and successive pulsed X-ray irradiations. We have demonstrated the validity of the γ-UV comparison by spectroscopic measurements. After this first selection, we have tested under pulsed X-rays (dose rate >10 MGy/s dose <0.5 kGy) the resistance of the P-free optical fibers at 1310 nm for the shortest times after an ionization pulse (10<sup>-9</sup> to 10<sup>-1</sup> s). Based on these results, we discuss the validity of the optical fiber data links for the control-command applications in LMJ facility.
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