Toward a10 dB net-gain waveguide amplifier in an Er³⁺-Yb³⁺ co-doped phosphate glass

High-gain materials and high-quality structures are the two main conditions that determine the amplification performance of optical waveguides. However, it has been hard to balance each other, to date. In this work, we demonstrate breakthroughs in both glass optical gain and optical waveguide structures. We propose a secondary melting dehydration technique that prepares high-quality Er³⁺-Yb³⁺ co-doped phosphate glass with low absorption loss. Additionally, we propose a femtosecond laser direct-writing technique that allows controlling the cross section, size, and mode field of waveguides written in glass with high accuracy, leveraging submicron-resolution multi-scan direct-writing optical waveguide technology, which is beneficial for reducing insertion loss. As a proof of concept demonstration, we designed and fabricated two kinds of waveguides, namely, LP₀₁- and LP₁₁-mode waveguides in the Er³⁺-Yb³⁺ co-doped phosphate glass, enabling insertion loss as low as 0.9 dB for a waveguide length of 2 mm. Remarkably, we successfully achieved an optical amplification for both the waveguides with a net gain of >7 dB and a net-gain coefficient of >3.5 dB/mm, which is approximately one order of magnitude larger than that in the Er³⁺-Yb³⁺ co-doped phosphate glass fabricated by the traditional melt-quenching method. This will open new avenues toward the development of integrated photonic chips.