Optical amplification plays a key role in virtually all laser-based technologies such as optical communication, used for instance in data-centers to communicate between servers and between continents through trans-oceanic fiber links, to ranging appli­cations like coherent Frequency Modulated Continuous Wave (FMCW) Lidar – an emerging technology that can detect and track objects farther, faster, and with greater precision than ever before. Today, optical amplifiers based on rare-earth ions like erbium, as well as III-V semiconductors, are widely used in real-world appli­cations. These two approaches are based on amplifi­cation by optical transi­tions. But there is another paradigm of optical signal amplifi­cation: traveling-wave parametric amplifiers, which achieve signal amplification by varying a small system parameter, such as the capacitance or the non­linearity of a transmission line.

It has been known since the 80’s that the intrinsic nonlinearity of optical fibers can also be harnessed to create traveling-wave optical parametric amplifiers, whose gain is independent of atomic or semi­conductor transitions, which means that it can be broad-band and virtually cover any wavelength. Parametric amplifiers also do not suffer from a minimum input signal, which means that they can be used to amplify both the faintest signals and large input power in a single setting. And finally, the gain spectrum can be tailored by waveguide geometry optimi­zation and dispersion engi­neering, which offers enormous design flexi­bility for target wavelengths and applications.

Most intriguingly, parametric gain can be derived in unusual wavelength bands that are out of reach of conventional semi­conductors or rare-earth-doped fibers. Parametric amplifi­cation is inherently quantum-limited, and can even achieve noiseless amplification. Despite their attractive features, optical parametric amplifiers in fibers are compounded by their very high pump power requirements resulting from the weak Kerr non­linearity of silica. Over the past two decades, the advances in integrated photonic platforms have enabled signi­ficantly enhanced effective Kerr nonlinearity that cannot be achieved in silica fibers, but have not achieved continuous-wave-operated amplifiers.

“Operating in the continuous-wave regime is not a mere academic achieve­ment,” says Tobias Kippenberg, head of EPFL’s Laboratory of Photonics and Quantum Measure­ments at EPFL. “In fact, it is crucial to the practical operation of any amplifier, as it implies that any input signals can be amplified – for example, optically encoded information, signals from Lidar, sensors, etc. Time- and spectrum-continuous, travelling-wave ampli­fication is pivotal for successful implemen­tation of amplifier technologies in modern optical communication systems and emerging applications for optical sensing and ranging.” A new study led by Johann Riemens­berger in Kippenberg’s group has now addressed the challenge by developing a traveling-wave amplifier based on a photonic integrated circuit operating in the continuous regime. “Our results are a culmination of more than a decade of research effort in integrated nonlinear photonics and the pursuit of ever lower waveguide losses,” says Riemens­berger.

The researchers used an ultralow-loss silicon nitride photonic integrated circuit more than two meters long to build the first traveling-wave amplifier on a photonic chip 3x5 square millimeter in size. The chip operates in the continuous regime and provides 7 dB net gain on-chip and 2 dB net gain fiber-to-fiber in the tele­communication bands. On-chip net-gain parametric amplifi­cation in silicon nitride was also recently achieved by the groups of Victor Torres-Company and Peter Andrekson at Chalmers University.

In the future, the team can use precise lithographic control to optimize the waveguide dispersion for parametric gain bandwidth of more than 200 nm. And since the funda­mental absorption loss of silicon nitride is very low (around 0.15 dB/meter), further fabrication optimi­zations can push the chip’s maximum parametric gain beyond 70 dB with only 750 mW of pump power, exceeding the performance of the best fiber-based amplifiers. “The appli­cation areas of such amplifiers are unlimited,” says Kippenberg. “From optical communi­cations where one could extend signals beyond the typical tele­communication bands, to mid-infrared or visible laser and signal amplifi­cation, to Lidar or other applications where lasers are used to probe, sense and inter­rogate classical or quantum signals.” (Source: EPFL)

Reference: J. Riemensberger et al.: A photonic integrated continuous-travelling-wave parametric amplifier, Nature 612, 56 (2022); DOI: 10.1038/s41586-022-05329-1

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