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Rare-Earth Ion Shows Promise as Gain Media for Integrated Photonics

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Researchers at École polytechnique fédérale de Lausanne (EPFL), led by professor Tobias J. Kippenberg, fabricated an erbium-doped waveguide amplifier on a compact photonic chip using erbium ion implantation and a silicon nitride (Si3N4) photonic integrated circuit (PIC). The waveguide length of the amplifier is up to 0.5 m on a millimeter-scale footprint. The amplifier operates in the continuous-wave regime and provides large optical gain in the telecommunication bands.

Erbium ions that display such output power could provide the basis for efficient optical amplification in PICs.

The advancement supports the possibility of a shift from electronics to faster, photonics-based chip technologies.

By applying erbium ion implantation to ultralow-loss Si3N4 PICs, the researchers increased the soliton microcomb output power by 100× and achieved the power required for low-noise photonic microwave generation and wavelength-division, multiplexing optical communications. The amplifier demonstrated the capability to generate an output power of more than 145 mW and a small-signal net gain of more than 30 dB, which is more than a thousand-fold amplification in the telecommunication band in continuous operation.

According to the researchers, the performance level shown by the amplifier is comparable to commercial, high-end, erbium-doped fiber amplifiers and surpasses state-of-the-art, III-V, heterogeneously integrated semiconductor amplifiers.

“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,” researcher Zheru Qiu said.
An erbium-doped waveguide amplifier on a photonic integrated chip in 1X1 cm<sup>2</sup> size, with green emission from excited erbium ions. Courtesy of EPFL Laboratory of Photonics and Quantum Measurements (LPQM)/Niels Ackermann.
An erbium-doped waveguide amplifier on a photonic integrated chip with green emission from excited erbium ions. Courtesy of EPFL Laboratory of Photonics and Quantum Measurements (LPQM)/Niels Ackermann.
Until now, erbium ions have not demonstrated sufficient output power to be used for optical amplification in photonic integrated circuits. In the 1990s, Bell Laboratories investigated the use of erbium-doped waveguide amplifiers and concluded that their gain and output power could not match fiber-based amplifiers, nor could they be fabricated using existing manufacturing techniques for PICs. Renewed efforts to develop Er-doped waveguide amplifiers have achieved less than 1 mW output power, which is not enough for many applications. Performance has been limited by high waveguide background loss and high cooperative upconversion — a gain-limiting factor at high erbium concentration. Realizing meter-scale waveguide lengths in compact photonic chips has long been a challenge.

“We overcame the long-standing 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,” researcher Yang Liu said.

Erbium ions amplify light in the 1.55-mm wavelength region, where silica-based optical fibers have the lowest transmission loss. The electronic intra-4-f shell structure of erbium and many other rare-earth ions enables long-lived excited states when doped inside host materials like glass. This provides a gain medium for simultaneous amplification of multiple information-carrying channels with minimal cross-talk, high temperature stability, and low noise.

The results could ignite interest in rare-earth ions as viable gain media in integrated photonics. Optical amplification is used in virtually all laser applications, from fiber sensing and frequency metrology to industrial applications like laser-machining and lidar. Optical amplifiers based on rare-earth ions enable the optical frequency combs that are used to create the world’s most precise atomic clocks.

The researchers expect their work will encourage follow-up studies to investigate rare-earth ions that provide optical gain from the visible to the mid-infrared, and even higher output power than erbium ions.

An amplifier that provides Si3N4 photonic integrated circuits with gain could also enable the miniaturization of fiber-based devices. “It also signals that high-pulse-energy femtosecond-lasers on a chip can finally become possible using this approach,” Kippenberg said.

The research was published in Science (www.science.org/doi/10.1126/science.abo2631).

Photonics Handbook
GLOSSARY
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.
chip
1. A localized fracture at the end of a cleaved optical fiber or on a glass surface. 2. An integrated circuit.
metrology
The science of measurement, particularly of lengths and angles.
integrated photonicsPICPICsmaterialsgain mediagain mediumrare earthrare earth ionsErbiumoptical amplificationoptical fiberEuropeEPFLResearch & TechnologyeducationWaveguidetelecommunicationsemiconductorschipmetrology

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