Erbium-doped waveguide amplifier (EDWA)
An optical amplifier in which erbium ions doped into an on-chip waveguide, pumped at 980 or 1480 nm, provide gain across the C and L bands. The integrated counterpart of the EDFA, revived by ultralow-loss silicon nitride and new host materials.
The erbium-doped fiber amplifier owes its success to properties of the erbium ion: a transition near 1550 nm, a metastable upper level with a lifetime of milliseconds, and therefore gain that does not follow the signal's fast fluctuations, which keeps crosstalk between channels and patterning low. An erbium-doped waveguide amplifier puts the same ions into a waveguide on a chip, pumped by a laser at 980 or 1480 nm, to bring those properties to photonic integrated circuits, where the usual on-chip gain element, the semiconductor optical amplifier, has nanosecond gain dynamics and a higher noise figure.
The difficulty is concentration. A fiber amplifier spreads its gain over meters of lightly doped glass. A chip has centimeters to spare unless the waveguide is coiled, so the erbium must be denser, and at high concentration the ions interact: energy transfer between neighboring excited ions (cooperative upconversion) and clustering waste pump power and limit gain. Early EDWAs in doped glasses and aluminum oxide worked but delivered gain and output power far below fiber amplifiers, and the approach stayed a niche for two decades.
Two developments changed that. Ultralow-loss silicon nitride waveguides, with propagation loss low enough that meter-long spirals fit on a chip, allow modest erbium concentrations spread over long lengths, as in fiber. Liu et al. implanted erbium into such waveguides and reported more than 30 dB of small-signal gain and 145 mW of output power, figures comparable with commercial fiber amplifiers. In parallel, erbium-doped thin-film lithium niobate and tellurite and other host materials have produced amplifiers and lasers integrated with the platforms' modulators.
The uses follow from what the erbium ion is good at. An EDWA can boost a microcomb or an on-chip laser without the pattern effects of an SOA, compensate the loss of a large passive circuit such as a switch or a phased array, and amplify many WDM channels at once. The trade against the SOA is pumping: an EDWA needs a separate pump laser coupled onto the chip, while an SOA is pumped electrically, so the choice depends on whether noise and channel crosstalk or simplicity and wall-plug efficiency matter more for the circuit in hand.
References: Y. Liu et al., Science 376, 1309 (2022); J. Bradley and M. Pollnau, Laser Photonics Rev. 5, 368 (2011).