Variation of emission spectra of Er3+-doped YAG-based solid solution

doi:10.1016/j.jallcom.2004.12.074

Journal of Alloys and Compounds

Volumes 408–412, 9 February 2006, Pages 788-790

https://doi.org/10.1016/j.jallcom.2004.12.074 Get rights and content

Abstract

Er³⁺-doped YAG crystal has intense emission bands in the U-band region (1625–1675 nm), which is uncommon to the ⁴I_13/2–⁴I_15/2 transition of Er³⁺ in many hosts except for garnet type crystals. In order to seek the possibility to broaden the spectrum composed of many sharp bands, solid solutions based on Y₃Al₅O₁₂ (YAG) and Ca₃Al₂(SiO₄)₃ (CASG) crystals doped with Er³⁺ ions were prepared by solid-state reactions and their emission spectra in optical-telecommunication band were examined. Both of the YAG and CASG crystals have garnet type structures with similar lattice constants. The spectral linewidth dramatically increased with increasing CASG content, which promises to overcome the difficulty in the gain flatness for optical amplification in the wavelength-division-multiplexing (WDM) system. The line broadening can be attributed to the inhomogeneous distribution of the Er³⁺ sites achieved by the double substitutions of Si⁴⁺ for Al³⁺ in the tetrahedral site and of Ca²⁺ for the Y³⁺.

Introduction

Er³⁺-doped fiber amplifiers (EDFAs) are used in the wavelength-division-multiplexing (WDM)-based optical telecommunication for the following reasons: the wavelength of the ⁴I_13/2–⁴I_15/2 transition of Er³⁺ matches the minimum loss region of the silica-based optical telecommunication fiber and the quantum efficiency of the transition is usually very high even in oxide materials with high phonon energy. The EDFAs are generally used for the C-band (1530–1565 nm) and the L-band (1565–1625 nm) owing to their emission spectra in glass hosts [1], [2], [3], [4], [5]. In order to meet the growing demand for data capacity, the extension of the telecommunication band will be required [6], [7]. The Er³⁺-doped YAG crystal has the emission spectrum extending to the U-band (1625–1675 nm), owing to the characteristic large Stark splitting of both the initial ⁴I_13/2 and terminal ⁴I_15/2 levels [8], [9]. An Er³⁺-doped YAG (Er:YAG) precipitated glass–ceramic material can be an excellent candidate for optical amplifiers, especially in the U-band, since they would have optical properties of the Er:YAG and fiberizabilty of the glass materials; they can be transparent to the incoming signal light if the crystal size is much smaller than the wavelength [10], [11], [12]. Although the Er:YAG crystal itself can also be a candidate for optical amplifiers as planer wave-guide forms, the spectrum consists of many sharp bands, which causes the difficulty in gain flatness for amplification of signal lights with different wavelengths in the WDM system. In this report, solid solutions between Y₃Al₅O₁₂ (YAG) and Ca₃Al₂(SiO₄)₃ (CASG) were prepared in various molar ratios and the emission spectra were measured in order to seek the possibility to broaden the sharp bands. Relationship between the emission bandwidth and CASG content was investigated. CASG is chosen since both YAG and CASG have the garnet structure and the difference in lattice constant is ∼1%: the partial substitutions of Si⁴⁺ for Al³⁺ and Ca²⁺ for Y³⁺ in YAG will induce structural inhomogeneity around Er³⁺ so that the emission spectrum will be homogeneously broadened.

Section snippets

Experimental

The Er³⁺-doped polycrystalline pellets were prepared from regent grade Y₂O₃, Al₂O₃, CaCO₃, SiO₂ and Er₂O₃. The raw materials were mixed thoroughly in the molar ratios of xYAG-(10 − x)CASG (x = 10, 9, 8, 7 and 5) doped with 0.375 mol% of Er₂O₃ substituting Y₂O₃. The mixtures were pressed into pellets. The pellets were heated in an electrical furnace at a suitable temperature (=1350–1600 °C) for 6 h. The heat-treated pellets were crushed into powders and pressed into pellets, which were sintered again

Results

The XRD patterns of the prepared crystals are shown in Fig. 1. All of the prepared crystals show the XRD patterns attributed to the YAG (JCPDS card No. 33-0040)-based garnet crystal. It was found that the diffraction peaks are shifted to the higher angle as the CASG content increases. Fig. 2 shows the compositional dependence of the lattice constants of the prepared garnet crystals. The solid line in the figure indicates the lattice constant expected by the Vegard's rule for ideal solid

Discussion

The lattice constants reported in JSPDS cards are 12.0089 Å for YAG and 11.8493 Å for CASG, respectively. The almost linear decrease in the lattice constants with increasing CASG content indicates that the solid solutions between YAG and CASG are successfully prepared. Since the Vegard's rule is only applied to the ideal homogeneous solid solutions, the deviation from the rule can occur even for the homogeneous solid solution and can also occur for the heterogeneous solid solution and for the

Conclusions

In the solid solutions between the YAG and CASG crystals the spectral linewidth of the ⁴I_13/2–⁴I_15/2 transition dramatically increased as the CASG content increases. This line broadening is mainly attributable to the inhomogeneous broadening derived from increase in variation of Er³⁺ sites with different ligand field by substitutions of Si⁴⁺ for Al³⁺(4) and Ca²⁺ for Y³⁺(8). The line broadening in the solid solutions between YAG and CASG crystals can overcome the difficulty in the gain flatness

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Variation of emission spectra of Er3+-doped YAG-based solid solution

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

J. Non-Cryst. Solids

Electron. Lett.

IEEE Photonics Technol. Lett.

IEEE Photonics Technol. Lett.

OFC’97

Variation of emission spectra of Er³⁺-doped YAG-based solid solution