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Scientists Upgrade EDFAs to Boost Optical Signal Power

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Jun 17 2022Reviewed by Bethan Davies

Erbium-doped fiber amplifiers (EDFAs) are devices that boost the optical signal power in optical fibers. They are commonly employed in long-distance communication fiber optic cables and fiber-based lasers.

Scientists Upgrade EDFAs to Boost Optical Signal Power. — An erbium-doped waveguide amplifier on a photonic integrated chip in 1X1 cm² size, with green emission from excited erbium ions. Image Credit: EPFL Laboratory of Photonics and Quantum Measurements/Niels Ackermann.

EDFAs, which were invented in the 1980s and have had a significant influence on our information society by allowing signals to be sent over the Atlantic and replacing electrical repeaters, are perhaps one of the most important technologies.

The ability of erbium ions to magnify light in the 1.55 mm wavelength region, which is where silica-based optical fibers have the lowest transmission loss, is intriguing in optical communications.

When doped inside host materials like glass, the distinctive electronic intra-4-f shell structure of erbium — and rare-earth ions in general — allows for long-lived excited states. With little cross-talk, good temperature stability, and low noise figure, this is a perfect gain medium for simultaneous amplification of several information-carrying channels.

Optical amplification is employed in almost every laser application, from fiber sensing and frequency metrology to laser machining and LiDAR. Optical frequency combs (2005 Nobel Prize in Physics) are used to make the world’s most precise atomic clocks, and optical amplifiers reliant on rare-earth ions have become their workhorse for them.

Using rare-earth ions in photonic integrated circuits to provide light amplification can revolutionize integrated photonics.

Bell Laboratories looked at erbium-doped waveguide amplifiers (EDWAs) in the 1990s, but they were eventually abandoned since their gain and output power could not compete with fiber-based amplifiers, and their manufacture did not fit with modern photonic integration manufacturing processes.

EDWAs have only been able to reach less than 1 mW output power, which is insufficient for many practical applications, despite the recent advent of integrated photonics.

The problem here is associated with 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.

EPFL researchers, led by Professor Tobias J. Kippenberg, have constructed an EDWA based on silicon nitride (Si₃N₄) photonic integrated circuits with lengths up to half a meter on a millimeter-scale footprint, generating a record output power of more than 145 mW and delivering a small-signal net gain above 30 dB, equating to over 1000-fold amplification in the telecommunication band.

This performance is comparable to commercially available high-end EDFAs and cutting-edge 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.

Dr. Yang Liu, Study Lead Scientist, EPFL

Dr. Yang Liu is a researcher in Kippenberg’s laboratory.

Zheru Qiu, Study Co-Author and Ph.D. student at EPFL adds, “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.”

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.

Tobias J. Kippenberg, Professor, Laboratory of Photonics and Quantum Measurements, EPFL

The accomplishment heralds a new era for rare-earth ions as viable gain media in integrated photonics, with applications ranging from optical communications and LiDAR for self-driving cars to quantum sensing and memory for huge quantum networks.

It is likely to spur greater research into rare-earth ions, with optical gain ranging from visible to mid-infrared and higher output power.