# Yb3+ Doped Crystal-in-Glass Optical Fibers

Natalia Vakula, Matiss Bardins, Khaldoon Nasser, Catherine Boussard-Plédel, Johann Troles, Wilfried Blanc, Laeticia Petit

► To cite this version:

Natalia Vakula, Matiss Bardins, Khaldoon Nasser, Catherine Boussard-Plédel, Johann Troles, et al. Yb3+ Doped Crystal-in-Glass Optical Fibers. EPJ Web of Conferences, 2025, 335, pp.07020. 10.1051/epjconf/202533507020 . hal-05494965

HAL Id: hal-05494965

https://hal.science/hal-05494965v1

Submitted on 5 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# Yb$^{3+}$ Doped Crystal-in-Glass Optical Fibers

Natalia Vakula<fnref n="1" />* Matiss Bardins<fnref n="1" />, Khaldoon Nasser<fnref n="1" />, Catherine Boussard-Plédel<fnref n="2" />, Johann Troles<fnref n="2" />, Wilfried Blanc<fnref n="3" />, Laeticia Petit<fnref n="1" />

**Abstract.** Composite optical fibers with Yb$^{3+}$ doped crystals embedded in glass matrices combine the benefits of crystalline and glassy phases for photonic applications. This work demonstrates the feasibility of preparing composite fibers within glass systems. YbPO$_4$ crystals were incorporated into silica via solution doping of the soot layer during MCVD, followed by collapse and fiber drawing. Electron and Raman microscopy confirmed the survival of ~100 nm crystals after processing up to 2100 °C. Another fiber was prepared by embedding LiNbO$_3$:Yb$^{3+}$ crystals in phosphate glass using direct doping method. Both fibers demonstrate the potential of post-treatment-free, crystal-in-glass fibers for laser applications.

## 1 Introduction

Nanoparticle-infused optical fibers are of growing interest for applications such as lasers, amplifiers, and sensors [1]. In particular, embedding rare-earth (RE) doped crystals in glass allows better control of RE ion properties, regardless of the glass composition, leading to improved optical performance. An example is the use of Al$_2$O$_3$:Er nanoparticles in silica fibers, which has led to more uniform erbium distribution and better performance at typical doping concentrations [2].

Yb$^{3+}$ is widely used in high-power laser systems due to its simple energy level structure, allowing efficient excitation of the $^2$F$_{5/2}$ level. Both phosphate and silica glasses doped with Yb$^{3+}$ have gained attention, phosphate for its high RE solubility and silica for its compatibility with fiber fabrication processes. These hosts offer high emission cross-sections, broad spectral bands, and long fluorescence lifetimes [3,4].

## 2 Experimental methods

YbPO$_4$ crystals were synthesized by aqueous precipitation from YbCl$_3$·6H$_2$O and (NH$_4$)$_2$HPO$_4$, followed by washing, drying at 80 °C, and calcination at 200 °C for 2 h. Silica-based composite preforms were fabricated by the MCVD process. A GeO$_2$ doped porous core layer was deposited, and YbPO$_4$ suspended in ethanol (4.6 g/L) were introduced via two sequential 10 mL injections. After solvent evaporation, the layer was sintered at up to

<footnote n="1">¹Tampere University, Korkeakoulunkatu 3, 33720 Tampere, Finland</footnote>

<footnote n="2">²Univ Rennes, CNRS, ISCR [(Institut des Sciences Chimiques de Rennes)] – UMR 6226, F-35000 Rennes, France</footnote>

<footnote n="3">³Université Côte d'Azur, Institut de Physique de Nice, CNRS UMR7010, Nice, France</footnote>

<footnote>* Corresponding author: nataklia.vakula@tuni.fi</footnote>
1800 °C, followed by tube collapse at 2100 °C to form the preform. Fibers were drawn at 1950 °C (diameter ~120 µm) without coating using a conventional fiber drawing tower.

LiNbO₃:Yb³⁺ crystals were synthesized by solid-state reaction at 1200 °C for 6 h in an alumina crucible. 1 wt% of the crystals were added to a molten glass (50P₂O₅–40SrO–10Na₂O, mol%) at 1050 °C, stirred manually, and cast into a preheated graphite mold (300 °C). Preforms were annealed at 400 °C for 17 h, polished, and drawn into fibers at 580 °C under 1.5 L/min helium flow.

SEM analysis was performed using a Zeiss Crossbeam 540 with an X-MaxN 80 EDS detector. All samples were carbon-coated to reduce charging.

## 3 Results and Discussions

YbPO₄ crystals (~100 nm) were successfully incorporated into silica glass using the MCVD process with solution doping of the porous layer. Scanning and transmission electron microscopy confirmed the presence and crystalline nature of YbPO₄ after both the collapse and fiber drawing stages (Figure 1a), indicating thermal stability up to 2100 °C. The crystals remained well-dispersed, and a partial alignment of their c-axis along the fiber axis was observed, likely due to the drawing-induced stress. Confocal Raman spectroscopy further verified the structural integrity of the YbPO₄ phase within the fiber core.

<figure><img src="image_2.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_5.png" /></figure>

Figure 1 - SEM and elemental mapping of (a) the center of the silica preform, (b) LiNbO₃:Yb³⁺ crystal in the phosphate fiber.

For phosphate-based fibers, LiNbO₃:Yb³⁺ crystals were embedded into a phosphate glass matrix. Raman spectroscopy confirmed the presence of crystal in the fiber, while SEM revealed partial elemental diffusion from the crystal surface into the glass already occurring during the composite preparation, leading to the precipitation of Sr-rich phosphate phases at the crystal–glass interface (Figure 1b). Crack-free preforms with good transparency were obtained after optimizing the amount of crystals and drawn into fibers. The measurement of the emission spectrum of the fiber exhibits an emission around 1 µm under 980 nm excitation, confirming the presence of the crystals

## 4 Conclusion

Yb³⁺ doped crystal-in-glass composite fibers were fabricated using both silica and phosphate glass matrices. The embedded crystals survived high temperature composite preparation and fiber drawing, confirming the potential of these materials for photonic applications.

## References

1. W. Blanc, D. Tosi, A. Leal-Junior, M. Ferrari, J. Ballato, Are Low- and High-Loss Glass–Ceramic Optical Fibers Possible Game Changers? Opt. Com. **575**, 131300 (2025)

2. A. Pastouret, C. Gonnet, C. Collet, O. Cavani, E. Burov, C. Chaneac, A. Carton, and J.-P. Jolivet, Nanoparticle doping process for improved fibre amplifiers and lasers, Fiber Lasers VI: Technology, Systems, and Applications, Proc. SPIE, 7195 71951X (2009)

3. R. Gumenyuk, A. Poudel, T. Jouan, C. Boussard-Plédel, T. Niemi, L. Petit, Super-luminescence and spectral hole burning effect in ultra-short length Er/Yb-doped phosphate fiber, Opt. Mat. Exp. **7**(12), 4358 (2017)

4. H. Pask, R. Carman, D. Hanna, A. Tropper, C. Mackechnie, P. Barber, J. Dawes, Ytterbium-doped silica fiber lasers: versatile sources for the 1-1.2 µm region, IEEE Journal of Selected Topics in Quantum Electronics, **1**, 2-13 (1995)