Erbium-doped fiber amplifiers (EDFAs) can provide gain to the optical signal power in optical fibers, often used in long-distance communi­cation fiber optic cables and fiber-based lasers. Invented in the 1980s, EDFAs are arguably one of the most important inventions, and have profoundly impacted our infor­mation society enabling signals to be routed across the Atlantic and replacing elec­trical repeaters.

What is interesting about erbium ions in optical communi­cations is that they can amplify light in the 1.55 mm wavelength region, which is where silica-based optical fibers have the lowest transmission loss. The unique electronic intra-4-f shell structure of erbium – and rare-earth ions in general – enables long-lived excited states when doped inside host materials such as glass. This provides an ideal gain medium for simul­taneous ampli­fication of multiple infor­mation-carrying channels, with negligible cross-talk, high tempera­ture stability and low noise figure. 

Optical amplification is also used in virtually all laser appli­cations, from fiber sensing and frequency metro­logy, to industrial applications including laser-machining and Lidar. Today, optical amplifiers based on rare-earth ions have become the workhorse for optical frequency combs, which are used to create the world’s most precise atomic clocks. Achieving light amplifi­cation with rare-earth ions in photonic integrated circuit can transform integrated photonics. Already in the 1990s, Bell Laboratories were looking into erbium-doped waveguide amplifiers (EDWAs), but ultimately abandoned them because their gain and output power could not match fiber-based amplifiers, while their fabrication doesn’t work with con­temporary photonic inte­gration manu­facturing techniques.

Even with the recent rise of integrated photonics, renewed efforts on EDWAs have only been able to achieve less than 1 mW output power, which is not enough for many practical appli­cations. The problem here has been high waveguide background loss, high cooperative upconversion – a gain-limiting factor at high erbium concentration, or the long-standing challenge in achieving meter-scale waveguide lengths in compact photonic chips.

Now, researchers at EPFL, led by Tobias J. Kippenberg, have built an EDWA based on silicon nitride (Si₃N₄) photonic integrated circuits of a length up to half meter on a millimeter-scale footprint, generating a record output power of more than 145 mW and providing a small-signal net gain above 30 dB, which translates to over 1000-fold amplifi­cation in the tele­communication band in continuous operation. This performance matches the commercial, high-end EDFAs, as well as state-of-the-art hetero­geneously integrated III-V semi­conductor amplifiers in silicon photonics.

“We overcame the longstanding challenge by applying ion implantation – a wafer-scale process that benefits from very low coopera­tive upcon­version even at a very high ion concen­tration – to the ultralow-loss silicon nitride integrated photonic circuits,” says Yang Liu, a researcher in Kippenberg’s lab. “This approach allows us to achieve low loss, high erbium concen­tration, and a large mode-ion overlap factor in compact waveguides with meter-scale lengths, which have previously remained unsolved for decades,” says PhD student Zheru Qiu.

“Operating with high output power and high gain is not a mere academic achieve­ment; in fact, it is crucial to the practical operation of any amplifier, as it implies that any input signals can reach the power levels that are sufficient for long-distance high-speed data trans­mission and shot-noise limited detection; it also signals that high-pulse-energy femto­second-lasers on a chip can finally become possible using this approach,” says Kippenberg.

The breakthrough signals a renais­sance of rare-earth ions as viable gain media in integrated photonics, as appli­cations of EDWAs are virtually unlimited, from optical communi­cations and LiDAR for autonomous driving, to quantum sensing and memories for large quantum networks. It is expected to trigger follow-up studies that cover even more rare-earth ions, offering optical gain from the visible up to the mid-infrared part of the spectrum and even higher output power. (Source: EPFL)

Reference: Y. Liu et al.: A photonic integrated circuit–based erbium-doped amplifier, Science 376, 1309 (2022); DOI: 10.1126/science.abo2631

Link: Center for Quantum Science and Engineering, Swiss Federal Institute of Technology Lausanne EPFL, Lausanne, Switzerland