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Abstract
Nd3+ doped multi-component phosphate glass was prepared by the conventional melt-quenching method. The absorption spectrum, emission spectrum, and fluorescence lifetime of the prepared glass were measured. Based on the absorption spectrum, Judd-Ofelt (J-O) intensity parameters, spectroscopic quality factor, absorption and emission cross-sections, and figure of merit were calculated and analyzed. What is more, multi-material fibers with Nd3+ doped multi-component phosphate glass core and silicate glass cladding were successfully drawn by using the molten core method. The Nd3+ doping concentration reaches as high as 3.82 × 1020 ions/cm3 (3.86 wt.%). More importantly, 1.05 μm amplified spontaneous emission (ASE) was realized in a 4-cm-long as-drawn multi-material fiber when pumped by an 808 nm laser diode (LD). These results suggest that the Nd3+ doped multi-component phosphate glass multi-material fibers are promising gain material for 1.05 μm fiber laser applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rare earth (RE) ion doped glass fibers are the key material of fiber lasers, which have been widely used in remote sensing, laser surgery, material processing, military, and so on [1–3]. It is know that Nd3+ is one of the most efficient activators for laser operation at room temperature because it has a low threshold four-level lasing structure, which has been playing an important role in the development of fiber lasers [4,5]. In addition, a Nd3+ doped fiber laser at ∼900 nm can be achieved when the population inversion between 4F3/2 level and 4F9/2 level for the three-level laser is obtained over the entire gain fiber [6–9]. Meanwhile, multi-component phosphate glass has excellent network-forming glass and high RE ion solubility, making it an ideal host to fabricate active fiber with a high gain per unit length [10–12]. The RE doping levels of phosphate glass are, on average, an order of magnitude higher that achievable in Al-doped and P-doped silica glass, which is usually used as a core material of silica fibers. However, the poor chemical stability and mechanical strength of multi-component phosphate glass fiber degenerates its performance [12,13].
Recently, multi-material fibers with heavily RE ions doped multi-component phosphate glass core cladded by silica or silicate glass, which combine the advantages of high RE ion solubility of the multi-component phosphate glass and good chemical stability and mechanical strength of silica/silicate glass, have been developed for high gain fibers [12–17]. Compared with silicate glass, the advantage of using silica glass as a cladding for drawing multi-material fiber with multi-component phosphate glass core is that it is easier to be spliced with commercial silica-based fiber devices. However, the silica glass cladding is usually drawn at about 2000 °C, which is much higher than the softening point of multi-component phosphate glass (∼516 °C), causing serious element volatilization in the phosphate core and mutual diffusion between the phosphate core and silica cladding during the drawing process [17,18]. In addition, the deformation of fiber core is appeared and the core/clad structure of the fiber is not preserved completely due to the large difference in softening temperature between silica glass (∼1730 °C) and multi-component phosphate glass (∼516 °C) [14,16]. In contrast, the softening temperature of commercially available silicate glass is 500-800 °C which matches the drawing and even the softening temperature of the multi-component phosphate glass. The silicate-clad multi-material fiber can be fabricated in a shorter time and at a much lower temperature to minimize the element volatilization in the core and mutual diffusion between the core and cladding, in which the initial optical properties and spectra properties of the multi-component phosphate glass can be maintained to facilitate the fiber design [12,13,19]. Martin et al., have reported single-clad multi-material fibers with Nd3+ phosphate glass core and silica clad, but the fiber core is elliptical and the core/clad structure is not preserved completely [14]. To the best of our knowledge, Nd3+ doped multi-component phosphate glass single-clad multi-material fiber with silicate cladding was not reported.
In this work, Nd3+ doped multi-component phosphate core glass was prepared by the conventional melt-quenching method. Spectroscopic properties of the core glass were systematically investigated. Then, silicate-clad Nd3+ doped multi-component phosphate core multi-material fibers were successfully drawn by a molten core method, whose core/clad structure was preserved completely. The Nd3+ doping concentration in core reaches as high as 3.82 × 1020 ions/cm3 (3.86 wt.%). Furthermore, 1.05 μm ASE was obtained in a 4-cm-long as-drawn multi-material fiber when pumped by an 808 nm LD.
2. Experimental
Nd3+ doped multi-component phosphate glass (core glass) was prepared with molar composition of 63P2O5-7K2O-14BaO-10Al2O3-4La2O3-2Nd2O3 by using conventional melting-quenching technique. Well-mixed raw materials (500 g) were melted in a covered alumina crucible at 1250 °C with optimized reaction atmosphere procedure dehydration process. Then, the melt was stirred and clarified to remove bubbles and stripes. Finally, the melt was cast into a preheated steel mold and annealed before they were cooled to room temperature. Part of the annealed glass was cut and polished for physical and optical measurements. Another part of the core glass was cold worked into cylindrical rod with diameter of 20 mm in lathe. Small core glass rods with diameter ∼2 mm were first drawn from the polished cylindrical core glass in a commercial drawing tower. Subsequently, the core glass rod was inserted into a commercial silicate glass cylindrical tube with inner diameter of 2 mm and external diameter of 20 mm. The bottom of the tube was sealed to form a preform. Finally, continuously silicate-clad Nd3+ doped multi-component phosphate core multi-material fibers were drawn around 860 °C inside the drawing tower under N2 controlled atmosphere.
The refractive index was measured on a prim coupling apparatus (Metricon 2010). Differential scanning calorimetry (DSC) (Netzsch SAT449C3 Jupiter) was carried out at a heating rate of 10 K/min under Ar atmosphere. The absorption spectra were measured on a Perkin-Elmer Lambda 900 UV-Vis-NIR double beam spectrophotometer (Waltham, MA). The fluorescence spectrum of the core glass was obtained by a computer controlled Triax 320 type spectro-fluorimeter (Jobin-Yvon Crop.) under excitation of an 808 nm LD. The lifetime was obtained from the first e-folding time of emission intensity in the decay curve recorded with a digitizing oscilloscope. The electron micrograph of the as-drawn multi-material fiber in cross section and the elemental distribution of the fiber cross section were observed by an electron probe X-ray micro-analyzer (EPMA-1600, Shimadzu, Japan). The ASE spectra were measured under excitation of an 808 nm LD. All the measurements were carried out at room temperature.
3. Results and discussions
Figure 1 shows the absorption spectrum of the core glass in the wavelength range from 300 to 1050 nm. The characteristic absorption bands corresponding to the transitions from the ground state to excited states of Nd3+ are labeled in the spectrum. The absorption bands at 330, 351, 430, 461, 478, 511, 524, 583, 627, 683, 745, 802, 875 nm are assigned to the transitions from 4I9/2 to 4D7/2, 4D3/2+4D5/2, 2P1/2, 2G9/2+2D3/2+4G11/2, 4G9/2, 4G7/2, 4G5/2+2G7/2, 2H11/2, 4F9/2, 4F7/2+4S3/2, 4F5/2+2H9/2, 4F3/2, respectively [20]. Based on the measured absorption spectrum, Judd-Ofelt (J-O) theory was used to calculate the spectroscopic parameters of Nd3+ in the prepared multi-component phosphate glass [21,22]. The experimental oscillator strength (fexp) of an absorption band can be determined by the following expression (1) [4]:
The calculated J-O intensity parameters of Nd3+ in the prepared multi-component phosphate glass are given in Table 1, which are compared to the values from other host glasses. The root-mean square deviation (δrms) between the fexp and fcal of Nd3+ is 4 × 10−8, indicating the validity of the calculations. In the case of Nd3+ ions, only Ω4 and Ω6 are effective for evaluating the stimulated radiative parameters because the value of reduced matrix element $\left\langle {{}^4{F_{3/2}}||{{U^2}} ||{}^4{I_J}} \right\rangle $ of 4F3/2 level is zero, indicating that Ω2 has no effect on the subsequent calculation [4,23]. It is known that the spectroscopic quality factor, χ=Ω4/Ω6, is an important parameter to predict the stimulated emission in a laser-active host [30]. Larger χ is beneficial to realize intense laser transition [31]. In the case of Nd3+ ions, the larger the χ value is, the stronger the 1.05 μm emission corresponding to 4F3/2→4I11/2 transition is. It is noted that χ in the prepared glass is the largest, suggesting that the multi-component phosphate glass is an appropriate host material to obtain intense 1.05 μm laser.
Table 1. Comparison of J-O intensity parameters and the values of Ω4/Ω6 of Nd3+ ions in various glass hosts
According to the J-O intensity parameters, the spontaneous emission probability (Arad) can be calculated using the Eq. (3) [32]:
Figure 2(a) presents near-infrared emission spectrum of the core glass pump by an 808 nm LD. Intense 1055 nm emission from the transition of 4F3/2→4I11/2 of Nd3+ can be obtained. Figure 2(b) shows the decay curve of the 4F3/2→4I11/2 transition from the core glass, which was fitted in good agreement with single exponential function. It was measured to be 406.7 μs, which is larger than that in Nd3+ doped silicate (263 μs) [24], germanate (218 μs) [25], tellurite (195.9 μs) [26], fluoro-phsophate (180 μs) [27], and other phosphate glasses (220 μs) [29]. The higher lifetime (τexp) is favorable to realize laser action. In addition, the quantum efficiency (η=(τexp/τrad)×β×100%) of the 4F3/2→4I11/2 transition from the core glass was calculated to be 50.3%, which is equivalent to the values from other glasses [4]. In this case, β is the fluorescence branching ratio of the 4F3/2→4I11/2 transition from the core glass, which was calculated to be 0.46 based on the obtained J-O intensity parameters.
Fig. 2. (a) Near-infrared emission spectrum of the core glass. (b) Fluorescence decay curve of the core glass.
Absorption and emission cross sections are very important parameters to obtain efficient laser output. Larger values are favorable for the generation of laser. Based on the absorption spectrum, the absorption cross section (σa) can be calculated by the Beer-Lamber theory (4) [30]:
where I0 and I are the intensities of incident and transmitted light, respectively. In addition, the emission cross section (σe) can be calculated by the Fuchtbauer-Ladenburg theory (5) [33]:The calculated absorption and emission cross sections of the core glass are shown in Fig. 3. It can be found that the maximum values of σa and σe are 2.37×10−20 cm2 at 802 nm and 2.92×10−20 cm2 at 1055 nm, respectively. The core glass has higher emission cross section than that of Nd3+ doped fluoro-sulfo-phosphate glass (2.26×10−20 cm2) [4], silicate glass (2.33×10−20 cm2) [24], and germanate glass (2.09×10−20 cm2) [25]. But it is lower than that of Nd3+ doped fluoro-phosphate glass (5.43×10−20 cm2) [27]. In addition, the product of emission cross section and lifetime can be defined as a figure of merit (FOM=σe(λ)×τexp). It is noting that the FOM of the core glass for 1055 nm emission was calculated to be 11.87×10−24 cm2 s, which is larger than that of Nd3+ doped fluoro-sulfo-phosphate glass (9.51×10−24 cm2 s) [4], silicate glass (6.13×10−24 cm2 s) [24], germanate glass (4.56×10−24 cm2 s) [26] and fluro-phosphate glass (8.10×10−24 cm2 s) [27]. The larger FOM means a lower operation threshold and higher laser efficiency [4,34].
The coefficient of thermal expansion (CTE) of the core glass (multi-component phosphate) and the cladding glass (silicate) were measured to be 4.62 × 10−6/°C and 4.22 × 10−6/°C in the 25-400 °C, respectively. The heating temperature of the furnace in the fiber drawing tower was kept gradual and slow proceeded [17]. It is noted that the softening temperature of the cladding glass is ∼120 °C higher than that of the core glass. At the draw temperature (∼860 °C), the core glass has been tuned into melt while the cladding glass became softened. Continuously silicate-clad Nd3+ doped multi-component phosphate core multi-material fibers were successfully drawn by using the molten core method.
Figure 4(a) presents the electron micrograph of the as-drawn fiber in cross section. The core/clad structure of the as-drawn fiber is preserved completely and it has an outer diameter of about 125 μm and core diameter of about 12.5 μm. Compared with the fiber cladding, the fiber core shows a brighter color in the backscattered electron image. According to the refractive indexes of the core glass (1.5317) and cladding glass (1.5066), the numerical aperture (NA) of the fiber was calculated to be 0.276 at 1.0 μm. Then the normalized frequency V of the fiber was calculated to be 10.833 when the λ is 1.0 μm. Therefore, the as-drawn fiber is a multimode fiber. The two-dimensional energy-dispersive X-ray mapping distributions of P, Si, and Nd are illustrated in Figs. 4(b)-(d). It can be observed that the distribution boundary of each element forms a circle, and the P and Nd are mainly distributed in the core region, while the Si is mainly distributed in the cladding region. All the EPMA images show that silicate-clad Nd3+ doped multi-component phosphate glass core multimode fibers with complete core/clad structure were successfully fabricated. The propagation loss of the as-drawn fibers at 1310 nm was measured to be 8.5 dB/m. The scattering at the interface between core and cladding may be one of the main sources of the relatively large propagation loss.
Fig. 4. (a) Electron micrograph of the as-drawn fiber in cross section. (b)-(d) EPMA images of the marked area in (a).
A 4-cm-long as-drawn multi-material fiber was pumped by an 808 nm LD. An optical spectrum analyzer (AQ6370B, YOKOGAWA, Japan) with a resolution of 0.02 nm was employed to monitor the output characteristic. The measured ASE spectra of the multi-material fiber were shown in Fig. 5. An intense emission at 1.05 μm originated from Nd3+: 4F3/2→4I11/2 transition is obtained in the as-drawn multi-material fiber. In addition, as the pump power increases, the intensity of the 1.05 μm ASE spectrum increases gradually. Based on the above analysis, the 1.05 μm fiber lasers will be possibly realized by using the as-drawn multi-material fibers.
4. Conclusion
In conclusion, spectroscopic properties of the highly Nd3+ (3.82 × 1020 ions/cm3) doped multi-component phosphate glass were systematically investigated. The prepared multi-component phosphate glass has moderate emission cross (2.92×10−20 cm2), long lifetime (406.7 μs), and thus relatively large figure of merit (11.87×10−24 cm2 s). Then silicate-clad highly Nd3+ doped multi-component phosphate glass core multi-material fibers with complete core/clad structure were successfully drawn. What is more, an intense ASE at 1.05 μm was obtained in a 4-cm-long as-drawn multi-material fiber excited by an 808 nm LD. These results show that the highly Nd3+ doped multi-component phosphate glass multi-material fibers are promising candidate for 1.05 μm fiber laser.
Funding
National Natural Science Foundation of China (11774071, 62005080); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X137); Guangdong Basic and Applied Basic Research Foundation (2019A1515110765, 2021A1515012049); the Key R&D Program of Guangzhou (202007020003).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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