Dual-wavelength fibre lasers based on structured-core active fibres

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 66 Number 6 2025 263 ICG 27, Kolkata, India, 20–24 January 2025 Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, 2025, 66(6), 263–265 Highly effective light sources, namely fibre lasers, represent one of mainstreams of today's optoelectronics. The laser emission wavelength depends on used rare-earth ion, on a glass matrix and on a structure (at macro/micro/nano scale) of the used material. When erbium and y�erbium ions are randomly distributed together in a silica glass matrix and pumped at absorption band of y�erbium, laser emission only at 1550 nm is obtained thanks to energy transfer from y�erbium to erbium ions. However, when erbium and y�erbium ions are specifically structured in micro or nano scale within the fibre core, controlled dual-wavelength laser operation at around 1550 nm and simultaneously at around 1040 nm can be obtained. This paper deals with such original performance of Er/Yb fibre laser. The approach of structured-core active optical fibres can be potentially applied to construction of lasers with simultaneous operation at multiple wavelengths. 1. Introduction Fibre lasers belong to the top achievements of today´s optoelectronics and photonics. They exhibit high conversion efficiency, excellent beam quality, high brightness, good thermal management and effective pumping. Specialty optical fibres doped with rare-earth ions form the heart of these lasers. From a material point of view, the most widespread matrix is silica glass co-doped with aluminium oxide.(1–3) This glass excels in high optical transparency from ultraviolet to near-infrared, thermal durability, chemical stability and mechanical strength. Usually, fibre lasers are operated at a single wavelength which depends on the rare-earth ion incorporated into a fibre core. A typical example is an erbium fibre laser emi�ing around 1550 nm or y�erbium fibre laser operating around 1060 nm. When erbium and y�erbium ions are randomly distributed together in a silica glass matrix and pumped at absorption band of y�erbium, laser emission only at 1550 nm is obtained thanks to energy transfer from y�erbium to erbium ions.(4) Such fibre lasers are commercially available. Recently, the team of R. Buczynski for the first time has demonstrated an experiment in which erbium-doped primary preforms and y�erbium-doped primary preforms were used for (nano)structuring of optical fibres(5) and controlled laser emission at 1042 nm (of Yb ions) and simultaneously at 1550 nm (of Er ions) was achieved. Process of ‘nanostructuring’ in this case meant multiple repetition of the stack-and-draw process performed with phosphate glass preforms, which led to controlled reduction of size of doped areas to nano-scale. In this paper we deal with fabrication and dual-wavelength laser performance of Yband Er-doped optical fibre based on silica glass in which doped areas were controllable structured to micro-scale. 2. Experimental Initial preforms of core composition Er3+–Al2O3– SiO2 and Yb3+–Al2O3–SiO2 were prepared by nanoparticle-doping method(6) which is a specific extension of the MCVD process.(7) ErCl3 and YbCl3 and Al2O3 nanoparticles <50 nm were used for the experiments. Initial modelling of suitable Er/Yb ratio of final fibre core and active length was performed.(8) Fabricated MCVD preforms were uniformly etched by hydrofluoric acid to achieve a suitable core-silica ratio predicted by the initial numerical model. Then the MCVD preforms were elongated at the drawing tower to rods of proper diameter. Seven rods of elongated initial MCVD preforms of diameter of 0·39 mm were assembled into hexagonal stack of five Er3+-doped rods and two Yb3+-doped rods (Er3+/Yb3+ final ratio of ~30/70). The stack was loaded into sleeving silica tube and such preform was drawn into a fibre and coated with conventional ultraviolet-curable acrylate coating DeSolite 3471-3-14. The technology concept is described at Figure 1. Basic characteristics of preforms and the fibre were determined. Preforms were characterised by refractive index profile and by local chemical composition by electron microprobe analysis. Fabricated fibre was characterised by refractive index profile, spectral * Corresponding author. Email [email protected] DOI: 10.13036/17533562.66.6.A10 Dual-wavelength fibre lasers based on structured-core active fibres M. Kamradek, a, * I. Kasik, a I. Barton, a O. Podrazky, a P. Varak, a J. Aubrecht, a P. Peterka, a P. Honzatko, a M. Franczyk, b D. Pysz, b A. Filipkowski b & R. Buczynski b a Institute of Photonics and Electronics of the Czech Academy of Sciences, Chaberská 1014/57, Praha 8, 182 51, Czech Republic b Lukasiewicz Research Network – Institute of Microelectronics and Photonics Al. lotnikow 32/46, 02-668 Warsaw, Poland 264 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 66 Number 6 2025 attenuation by cut-back method, optical microscopy and scanning electron microscopy. The lasing characteristics of the fabricated fibre was determined in Fabry–Perot configuration with a pumping source operating at 974 nm (Lumics) with a maximum output power of 450 mW. The laser cavities for both erbium and ytterbium laser were formed by two single high resolution FBG (HRFBGs reflecting at 1064 and 1561 nm) or by one single HRFBG (reflecting at 1042 and 1550 nm). The active fibre which was perpendicularly cleaved at the output end to get a low-reflectivity mirror through Fresnel reflection. The optical filters (Thorlabs, FELH1000 or FELH1150) with absorption edges at 1000 and 1150 nm were gradually placed in a forward direction before the thermopile power detector (Gentec, XLP12-3S-H2-D0) to separate the pump and individual signal beams. (9) 3. Results Preforms of diameter of around 10 mm were prepared without visible inhomogeneities, phase separation or clusters. Maximum refractive index differences of cores of 0·023 and 0·025 were observed at refractive-index profiles; core diameters of preforms (FWHM) were around 1·34 mm. No central dip was observed. Corresponding values of maximum content were around 10 mol% Al2O3, 3500 mol. ppm of Er3+ and 5000 mol. ppm of Yb3+ were determined in these preforms. Fibre of total diameter of 125 µm and core diameter of 6 µm was drawn by the stack-and-draw technique. N o holes or cavities were observed in the fibre cross-section. Seven white spots (corresponding to doped areas) within the core diameter (6 µm) were observed by optical microscopy. The maximum absorption band of Yb 3+ at 978 nm was about 70 dB/m and the maximum of absorption band of Er 3+ at 1531 nm was about 12 dB/m, these were determined by spectral attenuation measurements. Minimum background optical losses at 1220 nm and at 1310 nm of ~0·15 dB/m were achieved. Two distinct lasing peaks at the emission spectrum were observed. Emission at the shorter wavelength of 1042 nm can be attributed to Yb 3+ ions, emission at the longer wavelength of 1550 nm can be attributed to Er 3+ ions. Slope efficiencies (SE) at ytterbium emission wavelength (1042 nm) of 53·43% (versus pump power) and for the erbium emission wavelength (1550 nm) of 16·43% were observed (Figure 2). The values of SE depended on the length of the structured-core fibre. So, the fibre length can be considered as one of the parameters for modelling fibre laser properties. This behaviour can be considered as proof of controlled dual-wavelength performance of the fibre laser based on Yb 3+ and Er 3+ -doped structured core silica fibre. This behaviour contrasts, thanks just to fibre core structuring, with performance of conventional fibre lasers based on Er/Yb co-doped fibres emitting at single wavelength of around 1550 nm. (4,10) 4. Conclusions A Er3+ and Yb3+-doped structured-core optical fibre was fabricated, characterised and employed for fibre laser operation. Dual-wavelength operation with controlled output at 1042 nm and simultaneously at 1550 nm was observed with this fibre. This behaviour contrasts with the performance of conventional fibre lasers based on Er/Yb optical fibres. In general, the proposed approach enables fabrication of active fibres and fibre lasers emi�ing simultaneously and controllably at more wavelengths. Acknowledegments This work was supported by the Grant Agency of CR (contract 21-45431L), Narodowe Centrum Nauki (OPUS LAP 2020/33/IST7/02143), the M. KAMRADEK ET AL: DUAL-WAVELENGTH FIBRE LASERS BASED ON STRUCTURED-CORE ACTIVE FIBRES Figure 1. Scheme of fabrication of structured-core Er 3+ and Yb 3+ -doped silica fibre Figure 2. Performance of dual-wavelength fibre laser based on structured-core Yb 3+ - and Er 3+ - doped silica optical fibre Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 66 Number 6 2025 265 Czech Academy of Sciences (contract of internal support CSIR-25-03), and co-funded by the EU project under the project LasApp CZ.02.01.01./00./ 22_008/0004573. Data availability statement The data supporting the results of this study are available in Kamradek.(11) References 1. Kamradek, M., Kasik, I., Aubrecht, J., Mrazek, J., Podrazky, O., Cajzl, J., Varak, P., Kubecek, V. Ceramic nanoparticle-doping implementation into MCVD method for fabrication of holmiumdoped fibers for fiber lasers. I E E E Photon. J., 2019, 11, 1–10. https://doi.org/10.1109/ JPHOT.2019.2940747 2. Varak, P., Mrazek, J., Jasim, A. A., Bysakh, S., Dhar, A., Kamradek, M., Podrazky, O., Kasik, I., Barton, I. & Nekvindova, P. Thermal stability and photoluminescence properties of RE-doped (RE=Ho, Er, Tm) alumina nanoparticles in bulk and fiber-optic silica glass. Opt. Mater., 2021, 118, 111239, https://doi.org/10.1016/j.optmat.2021.111239 3. Kamrádek, M., Kasik, I., Aubrecht, J., Varak, P., Podrazky, O., Barton, I., Pokorny, J., Peterka, P. & Honzatko, P. Nanoparticle doping as a way to enhance holmium fiber lasers efficiency, Opt. Commun., 2025, 575, 131290. https://doi.org/10.1016/j.optcom.2024.131290 4. Townsend, J. E., Barnes, W. I., Jedrzejewski, K. P. & Grubb, S. G. Yb3+ sensitized Er 3+ doped silica optical fiber with ultrahigh transfer efficiency and gain. Electron. Lett., 1991, 27, 1958–9. https://doi.org/10.1049/ EL%3A19911214 5. Franczyk, M., Pysz, D., Stepien, R., Cimek, J., Kasztelanic, R., Chen, F. L., Klimczak, M., Zhao, L., Kasik, I., Peterka, P. & Buczynski, R.Dual band active nanostructured core fiber for two-color fiber laser operation. J. L ightwave Technol., 2022, 40, 7180–90. https://doi.org/10.1109/ JLT.2022.3199581 6. Podrazky, O., Kasik, I., Pospisilova, M. & Matejec, V. Use of alumina nanoparticles for preparation of erbium-doped fibers. IEEE LEOS Ann. M eeting C onf. P roc., 2007, 1–2, 246–7. https://doi.org/10.1109/ LEOS.2007.4382369 7. Nagel, S. R. & MacChesney, J. B., Walker, K. L. An overview of the modified chemical vapor deposition (MCVD) process and performance, IEEE J. Quant. Electron., 1982, 18 (4), 459–76. https://doi.org/10.1109/ TMTT.1982.1131071 8. Barton, I., Franczyk, M., Peterka, P., Aubrecht, J., Varak, P., Kamradek, M., Podrazky, O., Kasztelanic, R., Buczynski, R. & Kasik, I. Optimization of erbium and ytterbium concentration in nanostructured core fiber for dual-wavelength fiber lasers. SPIE Proc. Specialty Optical Fibers, 2023, 12573, 1257311. https://www.doi.org/10.1117/12.2666703 9. Kasik, I., Barton, I., Kamradek, M., Podrazky, O., Aubrecht, J., Varak, P., Peterka, P., Honzatko, P., Pysz, D., Franczyk, M. & Buczynski, R. Doped and structured silica optical fibers for fiber laser sources. Opt. Commun., 2025, 557, 131437. https://doi.org/10.1016/j.optcom.2024.131437 10. Vienne, G. G., Caplen, J. E., Dong, L., Minelly, J. D., Nilsson, J. & Payne, D. N. Fabrication and characterization of Yb 3+ :Er 3+ phosphosilicate fibers for lasers. J . L ightwave Technol., 1998, 16 , 1990–2001. http://dx.doi.org/10.1109/50.730360 11. Kamradek, M. Data for Dual-wavelength fiber lasers based on structured-core active fibers. 2025. https://zenodo.org/records/15739212 M. KAMRADEK ET AL: DUAL-WAVELENGTH FIBRE LASERS BASED ON STRUCTURED-CORE ACTIVE FIBRES