【Domestic Papers】Professor Wang Rongping Research Group of Ningbo University: Erbium-doped Ga₂O₃ Waveguide for Light Amplification

Introduction to Research

      In recent years, with the development of integrated optics, the integration scale is getting larger and larger, the performance of devices is becoming more and more complex, and the energy loss of optical signals in the transmission process of such large-scale integrated optical chips is inevitable. Therefore, using on-chip optical amplifier to compensate the attenuated optical signal has become an essential component in the optical integrated chip. At present, an Erbium-doped Waveguide Amplifier (EDWA) is mainly used to amplify the on-chip waveguide in the communication band. Its working principle is to be doped Er³⁺ in the compact waveguide structure, and pumped light excites Er³⁺ to emit light near the wavelength of 1.55μm. So as to realize the function of signal optical amplification. The fundamental problem restricting the performance of Erbium-doped waveguide amplifiers is the small optical emission cross section of Er³⁺ in most solid substrates and the low solubility of rare earth, all of which require increasing the interaction path between light and rare earth to achieve efficient optical amplification capability, however, this contradicts the compact size of the optical waveguide. In addition, the choice of substrate material is not arbitrary, requiring a high Er³⁺ solubility, a long ⁴I₁₃/₂ radiation life, a low phonon energy, and a high purity. Up to now, Er³⁺ doped Al₂O₃ was considered to be the most successful rare earth magnifying materials in communication band, but Er³⁺ doped Al₂O₃ waveguide magnifying was not enough for large-scale commercial use.

      The research idea of this topic is that Ga and Al belong to the same family in the periodic table, so their oxides can be expected to have similar physical and chemical properties to Al₂O₃. At the same time, Ga has a unique advantage in dissolving rare earths. For example, adding Ga to chalcogenide glass can promote the solubility of Er and avoid photoluminescence due to the formation of Er clusters in the glass. It is natural to expect that Ga₂O₃ is a better carrier for erbium-doping. On the other hand, erbium-doped Ga₂O₃ has a wider photoluminuminization spectrum at 1550 nm with a material refractive index of 1.9, which is greater than Al₂O₃. This is expected to further compress the size of the chip, and there are almost no reports of Ga₂O₃ as a planar waveguide optical material. Based on the above reasons, we propose to make erbium-doped Ga₂O₃ waveguide amplifiers. Erbium-doped Gallium Oxide amorphous films were prepared by RF magnetron sputtering, and waveguides were prepared by UV lithography and inductively coupled plasma etching (as shown in FIG. 1). Then, the optical amplification performance of waveguides was studied. In a 7 mm long waveguide, we achieved a net on-chip gain of 4.7 dB at 48.6 mW of pump power and 57.5 nW of signal power (FIG. 2). This is the first time in the world to realize on-chip optical amplification by using rare-earth doped Ga₂O₃, which proves the potential of Ga₂O₃ thin film as a photonic material, further reduces the waveguide loss, improves the waveguide preparation process, and is expected to achieve high gain on-chip optical amplification.

FIG. 1 Erbium-Doped Ga₂ ₃ waveguide preparation process

FIG. 2 The relationship between the internal gain and the pump power in an Erbium-doped Ga₂O₃ waveguide with a fixed signal power of 57.5nW (-42.4dBm)

Paper Link：https://doi.org/10.1063/5.0168092

Team introduction

      The project was completed under the guidance of Professor Wang Rongping, Institute of Advanced Technology, Ningbo University. Liu Ruixue was engaged in the film preparation, waveguide etching and magnification performance measurement work involved in the project, and also got the help of Zhang Zheng, Yang Zhen, Wang Wei, Song Maozhuang and others. At the same time, Dr. Yan Kunlun from the Laser Center of Australian National University also provided many useful suggestions during the research and implementation process. This project is also supported by the key research and development program of the Ministry of Science and Technology, Ningbo 3315 Project.