Chinese scientists increase optical output power of photonic integrated chips by more than 100 times

Recently, the team of Prof. Tobias J. Kippenberg of the Swiss Federal Institute of Technology Lausanne (EPFL) demonstrated an erbium-doped waveguide amplifier based on a photonic integrated circuit with an output power of 145 mW and a small-signal gain of over 30 dB, which is equivalent to continuous operation It can amplify the signal by more than 1000 times in the telecommunications frequency band of 1,000, which is comparable to the performance of commercial fiber amplifiers and surpasses the state-of-the-art III-V hetero-integrated semiconductor amplifiers.

On June 16, the related results were published in the journal Science [1] under the title of "Erbium-Doped Amplifiers Based on Photonic Integrated Circuits".

The authors of the paper include a number of Chinese people. Among them, the team's postdoctoral fellow Dr. Liu Yang, doctoral student Qiu Zheru, and doctoral student Ji Xinru are the main authors of the paper; two former EPFL colleagues also participated in the work, Dr. He Jijun of Nanjing University of Aeronautics and Astronautics, Shenzhen International Dr. Junqiu Liu from the Institute of Quantum Research.

An erbium-doped waveguide amplifier on a photonic integrated chip, measuring 1X1 cm 2 , excites erbium ions to emit green light.

An erbium-doped fiber amplifier (EDFA) is a device that can provide gain to the optical signal power in an optical fiber. They are commonly used in long-distance communication cables and fiber lasers. Invented in the 1980s, EDFA has had a profound impact on our information society, enabling signals to travel across oceans and replacing electronic repeaters.

The interesting thing about erbium ions in optical communications is that they can amplify light in the 1.55 mm wavelength range, which results in the lowest transmission loss in silicon-based fibers. The unique electronic intra-4-f shell structure of erbium and rare earth ions in general enables long-lived excited states when doped in host materials such as glass. Erbium ions provide an ideal gain medium for simultaneously amplifying multiple information transmission channels, with negligible crosstalk, high temperature stability, and low noise.

Optical amplification is used in nearly every laser application, from fiber-optic sensing and frequency metrology, to industrial applications including laser processing and lidar. Today, optical amplifiers based on rare earth ions are widely used in optical frequency combs and atomic clocks.

Achieving optical amplification with rare-earth ions in photonic integrated circuits could bring about a shift in integrated photonics. In the 1990s, Bell Labs started working on Erbium-Doped Waveguide Amplifiers (EDWAs), but eventually abandoned them because their gain and output power were not comparable to fiber-based amplifiers, and their fabrication could not be compared to contemporary photonic integrated fabrication Technology combination.

Even with the recent rise of integrated photonics, new efforts on EDWA can only achieve output powers of less than 1 mW, which is insufficient for many practical applications. The issues here are high waveguide background losses, high cooperative upconversion (a gain limiting factor at high erbium concentrations), or the longstanding challenge of achieving meter-scale waveguide lengths in compact photonic chips.

Now, an EPFL team led by Prof. Tobias J. Kippenberg has built an EDWA based on silicon nitride (Si3N4) photonic integrated circuits at millimeter-scale dimensions up to half a meter in length, producing a record output power of over 145 mW , and provides a small-signal net gain of more than 30 dB, which is equivalent to amplifying the signal by a factor of more than 1000 in continuously operating telecommunications frequency bands. Its performance is comparable to commercial high-end EDFAs as well as state-of-the-art hetero-integrated III-V semiconductor amplifiers in silicon photonics.

Dr. Yang Liu, a researcher in Kippenberg's lab and lead scientist on the study, said: "We have overcome longstanding challenges by applying ion implantation to ultra-low loss silicon nitride integrated photonic circuits. Ion implantation is a kind of Wafer-level processes, even at high ion concentrations, benefit from very low collaborative up-conversion."

"This approach allows us to achieve low loss, high erbium concentration, and large mode ion overlap factor in compact waveguides with meter-scale lengths, which have not been solved in previous decades," said Zheru Qiu, PhD student and co-author of the study. ."

"Operating at high output power and high gain is not just an academic achievement; in fact, it is critical to the practical operation of any amplifier because it means that any input signal can reach high speeds sufficient for long distances," Kippenberg said. power levels for data transmission and shot-noise-limited detection; it also marks the ultimate possibility of on-chip high-pulse-energy femtosecond lasers using this approach.”

This important breakthrough marks the renaissance of rare-earth ions as viable gain media in integrated photonics, as the applications of EDWA are nearly limitless - from optical communications and lidar for autonomous driving, to quantum teleportation for large-scale quantum networks sense and memory. In addition, it is expected to spark follow-up studies covering more rare earth ions, offering optical gains from the visible to mid-infrared part of the spectrum, and even higher output powers.

Reference link:

[1] https://www.science.org/doi/10.1126/science.abo2631

[2] https://phys.org/news/2022-06-photonic-erbium-doped-amplifiers-commercial.html