Spectroscopic properties of a fabricated double – clad, off-set core antimony - germanate optical glass fiber co-doped with Yb3+/Ho3+ has been investigated. Upconversion luminescence of holmium ions at the wavelengths of 549 nm (5S2 (5F4) → 5I8) and 658 nm (5F5 → 5I8) was obtained as a result of the energy transfer between Yb3+ and Ho3+ ions. Glass characterized by highest intensity of upconversion luminescence (1Yb2O3/0.5Ho2O3 mol%) was used as core of double-clad optical fiber. Upconversion amplified spontaneous emission in the fabricated double – clad, off-set core optical fiber was obtained. The intensity emission ratio red (5F5→5I8)/green (5S2 (5F4)→5I8) of the fabricated optical fiber is several times higher than in bulk glass. Reabsorption of the ASE signal along the fiber resulting from Ho3+: 5S2(5F4) → 5I8, 5F5 → 5I8 transitions transition was observed.
© 2015 Optical Society of America
As a result of frequency conversion of infrared radiation into visible light the optical glasses doped with rare earth ions (RE) may be applied in numerous applications: data storage systems in HD quality, 3D displays, medical diagnostics, optical sensors, laser and ASE sources [1–4]. Universal demand for compact glass fiber sources and amplifiers necessitates the pursuit for brand new glassy materials containing RE elements. Incorporation of Yb3+ and Ho3+ ions to various hosts is important in relation to applications in modern optics . The most popular silica glass is a well-known high phonon and high bandgap-energy glass. Silica fibers are characterized by low losses at VIS spectra region. However, relatively short lifetime of excited levels related to upconversion emission, and high probability of multiphonon processes disqualifies them as a candidate for the upconversion optical fibers . Literature sources mention a number of low phonon energy glass systems: fluoride , tellurite  and HMO (Heavy Metal Oxide) glasses , which phonon vibrations are in the range of 300-750 cm−1. Unfortunately, low mechanical and thermal resistance make it difficult to use these kinds of glasses in an optical fibers manufacturing. The problem can be overcome by using antimony - based glasses having relatively low phonon energy (600 cm−1) and exhibiting high thermal stability . The Yb3+/Ho3+ systems are especially interesting because they can be pumped directly by commercial laser diode (976 nm). Spectroscopic properties of Yb3+/Ho3+ ions in glasses, glass–ceramics, crystals and phosphors were previously examined in details [11–16]. However, to the best of our knowledge, the antimony-germanate glass fibers co-doped with RE ions have not been presented and discussed. Antimony-based glasses, owing to their good capacity for dissolving lanthanides and/or transition metal ions together with their relatively low phonon energies are enable to conduct effective conversion of excited radiation within the IR to VIS spectra, thus creating an attractive alternative matrix for tellurite and heavy metal oxide glasses. RE-doped antimony-based glasses were also studied for fiber telecommunication . Their favorable material properties and high stability make it possible to form them into fiber-optic structures.
In this letter the effects of optical pumping by NIR diode-laser on upconversion luminescence in antimony-germanate glasses and double-clad optical fiber have been investigated. The results of the conducted research into analysis of difference in luminescence spectra of bulk glass and fabricated optical fiber from the SiO2 -Al2O3 - Sb2O3 - GeO2 system, are discussed.
Glasses with molar composition of 30Sb2O3 - 30GeO2 (24-x)SiO2 - 10Al2O3 −5Na2O - 1Yb2O3 - xHo2O3 (x = 0.1; 0.2; 0.5; 0.75) were prepared from spectrally pure materials (99.99%). A homogenized set was placed in a platinum crucible and melted in an electric furnace at 1350°C for 60 min in oxide atmosphere. The molten glass was poured out onto a brass plate and then exposed to the process of annealing in the temperature of 450°C for 12 hours. In order to determine spectral properties a series of samples with the dimensions of 10 x 10 x 2 mm3 were prepared. Absorption spectra of rare-earth co-doped glass samples were determined using a Stellarnet Green-Wave spectrometer in the spectral range of 280 - 1100 nm. The double-clad, off-set core optical fiber was fabricated using a modified for the purpose rod-in-tube technique. Luminescence spectra of glasses as well as optical fiber in the range of 450 – 750 nm were measured using the Stellarnet Green-Wave spectrometer and a LIMO32-F200-DL980-LM laser diode (λp = 976 nm) as a pump source. A system PTI QuantaMaster QM40 coupled with OPO, pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD) was used for luminescence decay measurements. The laser system was equipped with a double 200 mm monochromator, a multimode UVVIS PMT (R928) and Hamamatsu H10330B-75 detectors. Luminescence decay curves were recorded and stored by a PTI ASOC-10 [USB-2500] oscilloscope with an accuracy of ± 1 µs.
3. Results and discussion
3.1 Glasses co-doped with Yb3+/Ho3+
Compared to silica (n = 1.46) or phosphate (n = 1.53)  glasses, the fabricated antimony-germanate glasses have a high refractive index (n ~1.7). The higher value of the n index increases the emission cross-section and probability of spontaneous emission. Besides, high thermal stability of the fabricated antimony-germanate glasses determine their suitability as a core material for the construction of optical fibers. Absorption coefficient for the pumping 976 nm is 4.76 cm−1. Figure 1a presents upconversion luminescence spectra of antimony - germanate glasses co-doped with Ho3+/Yb3+ ions. As a result of exciting glasses with the 976 nm wavelength radiation, two anti-Stokes emission bands were obtained. The first band is located at 546 nm, corresponding to the 5S2(5F4)→5I8 transition, and the second one at 655.5 nm, resulting from the 5F5→5I8 transition. Holmium ions do not directly absorb a pump radiation at the wavelength of 976 nm, hence the process of their excitation occurs as a result of the phonon-assisted energy transfer from ytterbium ions.
The analysis of emission intensity at 546 nm and 655.5 nm as a function of Ho2O3 molar content, has shown that maximum of upconversion luminescence for both transitions was obtained in the glass co-doped with 1mol% Yb2O3/0.5mol% Ho2O3. The energy level scheme with mechanisms of upconversion emission from Ho3+ ions in presence of Yb3+ under 976 nm laser excitation is shown in Fig. 1b. The ground state absorption of 976 nm pump radiation leads to the high population of Yb3+ ions in the 2F5/2 level. Firstly, the 5I6 level is promoted in phonon-assisted energy transfer (PAET): 2F5/2(Yb3+) → 5I6(Ho3+). Then another excited Yb3+ ions promote Ho3+ ions in the energy transfer upconversion process (ETU): 2F5/2(Yb3+) + 5I6(Ho3+) → 2F7/2(Yb3+) + 5S2(5F4) (Ho3+). Some of the ions in 5S2(5F4) levels relax non-radiatively to the 5F5 level and enhances the intensity of red emission. In addition, 5S2(5F4) and 5F5 levels of Ho3+ ion can be also populated through excited state absorption (ESA1 and ESA2). Finally, the green emission at the wavelength of 546 nm corresponding to the 5S2(5F4) → 5I8 transition takes place. The red emission centered at the wavelength of 655.5 nm is originated from the 5F5 → 5I8 transition. Luminescence decay curves given in semi-logarithmic scale for glasses co-doped with 1Yb2O3/(0.1, 0.2, 0.5, 0.75) Ho2O3 are nearly linear, which proof single-exponential decay behavior (Fig. 2(a)). The 2F5/2 lifetime of Yb3+ is reduced from 781 μs (1Yb2O3) to 69 μs in the presence of Ho3+ (1Yb2O3/0.75Ho2O3).
Efficiency of the Yb3+→Tm3+ energy transfer can be calculated according to the equation:Fig. 2(b)). Increase in holmium content up to 1mol% results in concentration quenching of luminescence. The lifetime of Ho3+: 5S2 + 5F4 equals 4.7 μs and is constant ( ± 0.2 μs) in the investigated concentration range of RE.
3.2 Double – clad off-set core optical fiber co-doped with Yb3+/Ho3+
Figure 3 presents cross-section and luminescence spectra of the fabricated double – clad optical fiber co - doped with 1mol% Yb2O3/ 0.5mol% Ho2O3. Basic parameters of the manufactured optical fiber (Fig. 3) were as follows: outer cladding diameter = 400 µm, core diameter = 15µm, NAcladding = 0.58, NAcore = 0.4. In conventional double-clad optical fibers, only the meridional rays can be absorbed in inner cladding. Off-set the core from the center of the inner cladding enables to improve efficiency of optical pumping of the double-clad optical fiber. Measured attenuation of the optical fiber at 658 nm corresponding to the 5I8→5F5 transition is 3.68 dB/m. It was determined that total integrated upconversion intensity luminescence bands in fabricated optical fiber is comparable to the total integrated intensity of the downconversion luminescence bands. This indicates high quantum yield of upconversion in the antimony-germanate optical fiber, which is comparable with results obtained in silicate glass ceramics , and phosphors . Additionally, relatively low phonon (600 cm−1) of the glass forming compounds (antimony, germanium) minimizes non-radiative (multiphonon) transition rates.
Comparing the results in optical fiber with results in bulk glass it can be seen that shape of the upconversion emission spectra of the optical fiber is significantly different from the luminescence spectra of bulk glass. The intensity emission ratio (I658/I546) of the fabricated optical fiber is several times higher than in bulk glass (Fig. 4(a)).
Similar intensity of upconversion emission bands at 546, 655.5 nm were found in the sample of the glass. In the examined optical fiber with a length of 50 cm the ratio of red/green emission is 5.25 and increases with the increase of the length of the optical fiber. For a fiber length of 180 cm it is 8.9. With the increase of the length of the optical fiber there is a decrease in the intensity of the pumping radiation, and thus pumping speed of the fiber in places increasingly remote from the source of pumping. As a result, participation of the ESA2 phenomenon in filling the 5S2(5F4) level is lower. In the optical fiber, due to a different geometry of the impact of the pump radiation, with the increase of its length dominant phenomena resulting in population the 5F5 level, which is responsible for red emission, are ETU and ESA1. Moreover, peaks emission at 546 nm and 655.5 nm are shifted by 2.5 nm towards longer wavelengths (Fig. 4(b)). This phenomenon is related to reabsorption of the ASE signal resulting from Ho3+: 5S2(5F4) → 5I8, 5F5 → 5I8 transitions respectively. During the reabsorption process, the radiation emitted by the ions located at the beginning of the optical fiber is absorbed by ions located towards the end of the fiber .
In summary, NIR to visible upconversion luminescence in antimony – germanate glasses and the double – clad, off-set core optical fiber co-doped with Yb3+/Ho3+ was studied. The optimal concentration of lanthanides is 1Yb2O3/0.5Ho2O3 (mol.%), which shows the highest emission intensity at 546 nm (green: 5S2(5F4) → 5I8) and 655.5 nm (red: 5F5→5I8). Performed analyses show that changes of geometry of the RE-doped material play a crucial role in shaping of upconversion emission spectra. The intensity emission red/green ratio of the fabricated optical fiber is several times higher than in bulk glass. Reabsorption of the ASE signal resulting from the Ho3+: 5S2(5F4) → 5I8, 5F5 → 5I8 transitions caused a shift of peaks emission at 546 nm and 655.5 nm by 2.5 nm towards longer wavelengths. To summarize, constructional parameters as well as length of the optical fiber Yb3+/Ho3+ ions strongly influence on their spectroscopic properties.
The project was funded by the National Science Centre (Poland) granted on the basis of the decision No. DEC-2012/07/B/ST8/04019.
References and links
3. D. M. Costantini, H. G. Limberger, T. Lasser, C. A. P. Muller, H. Zellmer, P. Riedel, and A. Tünnermann, “Actively mode-locked visible upconversion fiber laser,” Opt. Lett. 25(19), 1445–1447 (2000). [CrossRef] [PubMed]
4. A. S. Gouveia-Neto, L. A. Bueno, R. F. do Nascimento, E. A. da Silva Jr, E. B. da Costa, and V. B. do Nascimento, “White light generation by frequency upconversion in Tm3+/Ho3+/Yb3+-codoped fluorolead germanate glass,” Appl. Phys. Lett. 91(9), 091114 (2007). [CrossRef]
5. J. Ho Chung, J. Ho Ryu, S. Yeop Lee, J. Hoon Lee, B. Geun Choi, and K. Bo Shim, “Yellow lighting upconversion from Yb3+/Ho3+ co-doped CaMoO4,” Mater. Res. Bull. 47(8), 1991–1995 (2012). [CrossRef]
6. Ch. Nan-Kuang, K. Pei-Wen, Z. Junjie, Z. Liyan, H. Lili, L. Chinlon, and T. Limin, “Multicolor upconversion emissions in Tm3+/Er3+ codoped tellurite photonic microwire between silica fiber tapers,” Opt. Express 25(18), 2561 (2010).
7. J. Lucas, “Review Fluoride glasses,” J. Mater. Sci. 24(1), 1–13 (1989). [CrossRef]
8. N. Kumar Giri, S. B. Rai, and A. Rai, “Intense green and red upconversion emissions from Ho3+ in presence of Yb3+ in Li:TeO2 glass,” Spectrochimica Acta Part A 74(5), 1115–1119 (2009). [CrossRef]
9. K. J. Pluciński, W. Gruhn, J. Wasylak, J. Ebothe, D. Dorosz, J. Kucharski, and I. V. Kityk, “Luminescence of the Yb-doped PbO-Bi2O3-Ga2O3-BaO glasses,” Opt. Mater. 22(1), 13–19 (2003). [CrossRef]
10. S. Tanabe, “Optical properties and local structure of rare-earth-doped amplifier for broadband telecommunication,” J. Alloys Compd. 408–412, 675–679 (2006). [CrossRef]
11. S. Balaji, A. K. Mandal, and K. Annapurna, “Energy transfer based NIR to visible upconversion: Enhanced red luminescence from Yb3+/Ho3+ co-doped tellurite glass,” Opt. Mater. 34(11), 1930–1934 (2012). [CrossRef]
12. W. Xu, X. Gao, L. Zheng, Z. Zhang, and W. Cao, “Short-Wavelength Upconversion Emissions in Ho3+/Yb3+ Codoped Glass Ceramic and the Optical Thermometry Behavior,” Opt. Express 20(16), 18127–18137 (2012). [CrossRef] [PubMed]
13. Y. Jiang, R. Shen, X. Li, J. Zhang, H. Zhong, Y. Tian, J. Sun, L. Cheng, H. Zhong, and B. Chen, “Concentration effects on the upconversion luminescence in Ho3+/Yb3+ co-doped NaGdTiO4 phosphor,” Ceram. Int. 38(6), 5045–5051 (2012).
14. I. Iparraguirre, J. Azkargorta, R. Balda, K. Venkata Krishnaiah, C. K. Jayasankar, M. Al-Saleh, and J. Fernández, “Spontaneous and stimulated emission spectroscopy of a Nd3+-doped phosphate glass under wavelength selective pumping,” Opt. Express 19(20), 19440–19453 (2011). [PubMed]
15. X. Wang, H. Lin, D. Yang, L. Lin, and E. Y. B. Pun, “Optical transitions and upconversion fluorescence in Ho3+/Yb3+ doped bismuth tellurite glasses,” J. Appl. Phys. 101(11), 113535 (2007). [CrossRef]
16. V. D. Rodríguez, V. K. Tikhomirov, J. Méndez-Ramos, J. del-Castillo, and C. Görller-Walrand, “Measurement of Quantum Yield of Up-conversion Luminescence in Er3+-Doped Nano-Glass-Ceramics,” J. Nanosci. Nanotechnol. 9(3), 2072–2075 (2009). [CrossRef] [PubMed]
17. V. Scarnera, B. Richards, A. Jha, G. Jose, and C. Stacey, “Green up-conversion in Yb3+–Tb3+ and Yb3+–Tm3+–Tb3+ doped fluoro-germanate bulk glass and fibre,” Opt. Mater. 33(2), 159–163 (2010). [CrossRef]