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Research on photo-radiation darkening performance of ytterbium-doped silica fibers for space applications
摘要： Yb3+/Al3+-doped and Yb3+/Al3+/P5+-doped silica fibers with almost identical Yb3+ doping concentrations were prepared using modified chemical vapor deposition methods. A harsh experimental environment for simultaneous photo-darkening (PD) and radio-darkening (RD) of an ytterbium-doped silica fiber (YDF) was established. The RD, PD, and photo–radio-darkening (PRD) of the YDF were characterized at two dose rates of 0.1 rad(Si)/s and 1 rad(Si)/s. The laser performances before and after the darkening process were tested under 974-nm pumping. The results demonstrated that the PD largely dominated the PRD kinetics of the YDF at a low dose rate (e.g., that in space) and that the PRD was not a superposition of RD and PD owing to the pump light bleaching if the induced loss was larger than the PD equilibrium level. The induced loss and defect types were investigated through absorption spectra and electron paramagnetic resonance. The results revealed that the P doping could enhance the PD, RD, and PRD resistances by inhibiting the formation of aluminum–oxygen hole centers.
关键词： photo-radio darkening,radio-darkening,photodarkening,defects in silica glass,ytterbium doped silica fiber
更新于2025-09-23 15:23:52
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Radiation Effects in Materials || Effects of Electron Irradiation Upon Absorptive and Fluorescent Properties of Some Doped Optical Fibers
摘要： A review of the recent studies of the effect of irradiating silica-based fibers doped with rare earths and metals by a beam of high-energy (β) electrons is presented. Of the review’s main scope are the attenuation spectra’ transformations occurring in optical fiber of such types under electron irradiation, allowing, from one side, to recover some general essence of the phenomena involved and, from the other side, to draw the features that would make such fibers useful for applications, for example, in dosimetry and space technologies. Among the fibers of the current review’s choice, exemplifying the effect of electron irradiation most brightly, are ytterbium (Yb) and cerium (Ce) (the rare earths’ representatives) and bismuth (Bi) (the post-transitional metals representative) doped fibers, where a diversity of the electron-irradiation-related effects is encouraged.
关键词： optical bleaching,photodarkening,cerium- and bismuth-doped silica fibers,electron irradiation,ytterbium-
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Photo Darkening Suppression in Highly Yb-Doped Aluminophosphosilicate Fiber by Addition of Cerium
摘要： Photodarkening (PD) suppressed low numerical aperture (NA), high absorption, low loss, large mode area (LMA) fiber is an important requirement for high power fiber lasers, in both pulsed and continuous wave operation regimes. We have reported the fabrication of high absorption (~1200 dB/m core absorption) Yb doped aluminophosphosilicate fiber with raised Ge cladding to realize low NA step index fiber [1]. The presence of Aluminium (Al) and phosphorus (P) in the core helps in lowering the PD loss by a significant margin, but some PD loss is still observed. In this work we intend to further lower the PD loss by adding Cerium (Ce) to this equimolar P:Al core. Previous reports suggests that addition of Ce to Yb:Al core significantly reduce the PD loss when the ratio of Ce oxide to Yb oxide ratio is ~0.5-0.7 [2-4]. However, like Yb, Ce has a high molar refractivity, and all dopants (Yb, Ce and Al) push up the core NA. In our work, equimolar P:Al does not contribute to index of the core, and only the Ce and Yb raise the core NA. Furthermore, the reported raised Ge cladding technique can compensate the core NA to a desired level [1]. We here present three fibers with high Yb concentration of ~0.5mol% and with equimolar Al and P composition, all fabricated by modified chemical vapor deposition and solution doping technique. Fiber #1 contains no Ce while Fiber #2 and Fiber #3 contain Ce with different Ce/Yb oxide ratio of ~0.24 and ~0.625, respectively. The index profiles of the three fibers are shown in Fig. 1(a). As expected, the core NA increases with Ce concentration. Compositional concentration profile in Fiber #3 measured by energy dispersive X-ray analysis (EDX) is shown in Fig. 1(b). P2O5, Al2O3, Yb2O3 and CeO2 were found to be around 5 mol%, 4.6 mol%, 0.48 mol% and 0.3 mol% respectively, in the core. Hence the Ce/ Yb oxide ratio was observed to be ~0.625, which according to the reported works is supposed to yield quite good PD suppression while creating no additional thermal issues. All the fibers have similar Yb, Al and P contents with only difference in the Ce concentration. Fiber#2 contains ~0.12mol% of Ce2O3. The background losses in the fibers were found to be <25dB/Km after a cut-back method measurement. This is considered low for fibers with such high Yb concentration. Finally, the fibers were tested for PD loss using core pumped configuration with 634 nm as a probe wavelength and 976 nm as pump wavelength. Population inversion levels of ~45% was maintained in all the fibers. The PD losses for the fibers are shown in Fig. 1(c). Fiber#1, Fiber #2 and Fiber#3 has PD losses of 70 dB/m, 30 dB/m and 7 dB/m respectively. Thus, it can be clearly seen that addition of Ce to Yb doped aluminophosphosilicate fiber reduces PD significantly, with the ratio of Ce:Yb playing a major role as well. In conclusion, we report the first fabrication of Ce and Yb doped aluminophosphosilicate fiber with very high absorption, and extremely low PD.
关键词： Aluminophosphosilicate fiber,Fiber lasers,Yb-doped,Photodarkening,Cerium
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Pulsed Yb-Doped Phospho-Silicate Fiber MOPA Source with 25kW Peak Power and Excellent Beam Quality
摘要： With ytterbium doped fiber (YDF) lasers and amplifiers have reached continuous wave output power of multi-kW with direct diode pumping [1] and 10kW with tandem pumping [2] in a good beam quality, today fiber lasers are becoming the laser choice for many industrial applications and processes, defense, and scientific research. One of the challenges of high average power fiber laser is to maintain a long-term stability of the output power. In particular, photodarkening (PD) is seen as a power loss in YDF gain medium that can significantly influence the operation lifetime of the device. Moreover, it was observed that the PD is related to the transverse mode instability (TMI) that can limit the output power of the laser [3]. In silica host, co-dopant phosphorous (P) is more effective than aluminium (Al) to suppress the PD effect in YDF. In addition, Yb-doped phosphosilicate (Yb-PS) fiber is found suitable for high power laser operation below 1020nm [4]. Such lasers are used as pumps in tandem pumping to reduce the heat load associated with the quantum defects in YDF lasers operating at 10kW level [4]. However, Yb-PS fiber are generally considered difficult to fabricate due to evaporation of P2O5 during the preform fabrication process, resulting in a central dip in the core refractive index profile that has detrimental effect on the output beam quality of fiber lasers. Other disadvantage of P co-doping is its smaller Yb - absorption and emission cross-sections compared to Al counterpart. To compensate for the cross-sections, higher concentrations of Yb and P (to prevent Yb ions from clustering) are needed in fiber. This contributes to a high core NA and poses a challenge to fabricate a large mode area Yb-PS fiber. Here we report an efficient Yb-PS high power laser fiber fabricated using an optimized MCVD (modified chemical vapor deposition) and all-vapor-phase chelate precursor doping technique. Double-clad fiber with a 150μm quasi-octagonal inner cladding and a 12μm core diameter was drawn with a low index polymer outer cladding. The core NA was 0.1, Fig.1 (a). The small signal absorption at the pump wavelength of ~976nm was measured as 2.5 dB/m. Initially the fiber was tested in a 4%-4% laser cavity. An output power of >100W (limited by the available pump power) and the slope efficiency of >85% were obtained. The laser emission was centered at 1066nm. We then tested the same fiber in a pico-second MOPA configuration. Fig. 1(b) shows the experimental schematic. A 4m long Yb-PS fiber was used in the final stage of the MOPA. A gain-switched diode operating at ~1035nm with 180ps pulse width and 2.95MHz repetition rate was used as a seed laser. Fig. 1(c) shows the average signal output power with respect to the absorbed pump power in the final stage of the amplifier. The output power reached 13.3W and the slope efficiency was 74%. The corresponding pulse energy and peak power was 4.5μJ and 25kW respectively. The measured beam quality (M2) was ~ 1.13, as shown in inset of Fig. 1(c). The spectra of the seed laser and at the maximum signal output power are shown in Fig. 1(d). The 3dB spectral bandwidth at the maximum output power was measured as 0.29nm compared to 0.03nm of the seed laser. The magnitude of the stimulated Raman scattering (SRS) appeared at a wavelength of ~1080nm was 30dB lower than the output signal – see Fig. 1(d). In conclusion, we have demonstrated that an all-vapor-phase chelate doping technique holds great potential to fabricate LMA Yb-doped phosphosilicate fibers with diffraction limited beam for high power lasers.
关键词： high power laser,Yb-doped phosphosilicate fiber,photodarkening,all-vapor-phase chelate precursor doping,transverse mode instability,MCVD
更新于2025-09-11 14:15:04