The photonic integrated circuits used in this investigation
The ability to amplify optical signals at their quantum limit is a vital technological advancement that underpins our modern information society. The 1550 nm wavelength band is used in optical telecommunications because it not only has low loss in silica optical fibers (for which the 2008 Nobel Prize in Physics was awarded), but also because it allows for trans-oceanic fiber optical communication.
Optical amplification plays a significant role in virtually all laser-based technologies, from data centers to data exchange between servers and between continents via trans-oceanic fiber connections, 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.
Amplification by optical transitions is one of the mainstays of optical signal amplification. Traveling-wave parametric amplifiers are capable of accomplishing signal amplification by altering a small system "parameter," such as the capacitance or the nonlinearity of a transmission line.
Since the 1980s, it has been known that optical fibers' intrinsic nonlinearity may be utilized to create traveling-wave optical parametric amplifiers, which generate a gain independent of atomic or semiconductor transitions, enabling them to be broadband and practically covered at any wavelength.
Parametric amplifiers do not require a minimum input signal, thus they may be used to amplify both the faintest signals and significant input power in a single setting. Waveguide geometry optimization and dispersion engineering also allow enormous design flexibility for desired wavelengths and applications.
Parametric gain may be obtained in unusual wavelength bands that are beyond the reach of conventional semiconductors or rare-earth-doped fibers. It is inherently quantum-limited, and can even be noiseless.
Despite their attractiveness, optical parametric amplifiers in fibers are exacerbated by their extremely high pump power requirements due to the weak Kerr nonlinearity of silica. Over the past two decades, integrated photonic platforms have enabled significantly improved effective Kerr nonlinearity, but has not developed continuous-wave-operated amplifiers.
Professor Tobias Kippenberg, head of EPFL's Laboratory of Photonics and Quantum Measurements, says that operating in a continuous-wave environment is not a mere academic achievement. "It is essential to the practical operation of any amplifier," says the researcher. "Time- and spectrum-continuous, traveling-wave amplification is critical for successful application of amplifier technologies in modern optical communication systems and emerging applications for optical sensing and ranging."
According to Riemensberger's latest study, a traveling-wave amplifier based on a photonic integrated circuit operating in the continuous regime has now addressed the challenge.
The Chalmers University researchers used an ultralow-loss silicon nitride photonic integrated circuit over two meters long to construct the first traveling-wave amplifier on a photonic chip 35 mm2 in size. The chip provides 7 dB net gain on-chip and 2 dB net gain fiber-to-fiber.
Further fabrication improvements might increase the chip's maximum parametric gain by more than 70 dB with only 750 mW of pump power, surpassing the best fiber-based amplifiers.
"These amplifiers have a myriad of uses," says Kippenberg. "From optical communications where one might broaden signals beyond the typical telecommunication bands, through mid-infrared or visible laser signal amplification, to LiDAR or other applications where lasers are used to probe, sense, and interrogate classical or quantum signals,"
Johann Riemensberger, Nikolai Kuznetsov, Junqiu Liu, Rui Ning Wang, and Tobias J. Kippenberg, Nature, DOI: 10.1038/s41586-022-05329-1