1. Introduction

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 [5]. 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 [6]. Literature sources mention a number of low phonon energy glass systems: fluoride [7], tellurite [8] and HMO (Heavy Metal Oxide) glasses [9], 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 [10]. 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 [10]. 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.

2. Experimental

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 (λ_p = 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) [15] 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.

 

Fig. 1 Upconversion luminescence spectra of the Yb³⁺/Ho³⁺ co-doped antimony - germanate glass (a), Simplified diagram of the Yb³⁺/Ho³⁺ energy level with energy transfer mechanism (b)

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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 ²F_5/2 level. Firstly, the ⁵I₆ level is promoted in phonon-assisted energy transfer (PAET): ²F_5/2(Yb³⁺) → ⁵I₆(Ho³⁺). Then another excited Yb³⁺ ions promote Ho³⁺ ions in the energy transfer upconversion process (ETU): ²F_5/2(Yb³⁺) + ⁵I₆(Ho³⁺) → ²F_7/2(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 ²F_5/2 lifetime of Yb³⁺ is reduced from 781 μs (1Yb₂O₃) to 69 μs in the presence of Ho³⁺ (1Yb₂O₃/0.75Ho₂O₃).

 

Fig. 2 Luminescence decays from the ²F_5/2 state of Yb³⁺ in glasses co-doped with 1Yb₂O₃/(0, 0.1, 0.2, 0.5, 0.75)Ho₂O₃ [mol%] (a), Lifetime for ²F_5/2 state of Yb³⁺ and energy transfer efficiency as a function of Ho₂O₃ concentration (b).

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Efficiency of the Yb³⁺→Tm³⁺ energy transfer can be calculated according to the equation:

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, NA_cladding = 0.58, NA_core = 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 [17], and phosphors [14]. Additionally, relatively low phonon (600 cm⁻¹) of the glass forming compounds (antimony, germanium) minimizes non-radiative (multiphonon) transition rates.

 

Fig. 3 Cross-section and luminescence spectra of the fabricated optical fiber co-doped with 1mol% Yb₂O₃/ 0.5mol% Ho₂O₃, (inset) peak attenuation at 658 nm vs. fiber length.

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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)).

 

Fig. 4 Red/Green emission intensity ratio (a), Peaks emission wavelength (b) vs. length of the optical fiber co-doped with 1mol% Yb₂O₃/ 0.5mol% Ho₂O₃.

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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) → ⁵I_8, 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 [17].

Conclusions

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) → ⁵I_8, 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.