Continuous-Wave OPA Supports Noise-Reduced Chip-Scale Optical Signal Processing

An optical amplifier developed at Chalmers University of Technology is poised to radically improve optical communications performance. The compact amplifier is designed to fit on a chip and amplify light without generating excess noise.

Light-based communications make it possible to send information around the world and into space. However, when it travels long distances, the light loses power. Without amplifiers, up to 99% of the signal in an optical fiber cable would disappear within 100 km.

As a result, multiple amplifiers are needed to support optical transmission, and each added amplifier increases the noise level. This diminishes signal quality.

The compact optical amplifier developed at Chalmers is thousands of times smaller than its precursor, its developers said. The component is made of silicon nitride and consists of nine separate waveguides. Each waveguide is composed of 22 spirals and can amplify light by about 10 times with a noise figure of only 1.2 decibels. Each waveguide on the 20-mm chip outperforms a single bulky amplifier. Courtesy of Ping Zhao, Zhichao Ye, and Yen Strandqvist/Chalmers University of Technology.

Optical parametric amplifiers (OPAs), which use a nonlinear optical material to create amplification, can amplify signals without generating excess noise. The monolithic OPA developed by the Chalmers team based on the Kerr effect demonstrates a noise figure that is well below the conventional quantum limit when operated in phase-sensitive mode.

Although other amplifiers have used the Kerr effect, none have demonstrated the ability to do so in a format as compact as the Chalmers device — which is small enough to fit on a computer chip just several mm in size.

The amplifier operates in a continuous wave (CW). The researchers said that until now, parametric amplifiers have operated with a pulsed pump only, limiting their use in real applications.

The Chalmers team used a low-loss (1.4 dB/m) silicon nitride waveguide within a chip area of 23 mm², to demonstrate CW parametric amplification of 9.5 dB with a noise figure of 1.2 dB — considerably below the conventional 3-dB quantum limit and, according to the team, the lowest loss ever achieved in a dispersion-engineered, integrated waveguide, silicon-nitride material platform. Because silicon nitride is transparent from the visible to the mid-infrared wavelength range, its use makes the amplifier scalable to different wavelengths.

With very low noise and a small monolithic footprint, the CW-pumped, silicon nitride-based amplifier could be a milestone for optical communications, ultrafast spectroscopy, and quantum optics and metrology. The researchers believe that an even lower noise figure and an even higher gain in amplification are possible by further reducing the waveguide losses, increasing the waveguide length, and reducing the crosstalk between the fundamental and higher-order optical modes.

The new component is made of silicon nitride and consists of nine separate waveguides (left). Each waveguide is composed of 22 spirals and can amplify light by about 10 times with a noise figure of only 1.2 dB. Each spiral (right) has an area of 1 mm² and the shape enables the compact design of the amplifier. Courtesy of Zhichao Ye and Yen Strandqvist/Chalmers University of Technology.

“Since it’s possible to integrate the amplifier into very small modules, you can get cheaper solutions with much better performance, making this very interesting for commercial players in the long run,” said professor Peter Andrekson, who led the research.

The amplifier’s strong performance also means that fewer amplifiers would be needed for long-distance data transmission, making it a cost-effective option for boosting optical signals.

“We consider this to be an important step toward practical use, not only in communication, but in areas including quantum computers, various sensor systems, and in metrology when making atmospheric measurements from satellites for Earth monitoring,” Andrekson said. “This could be compared to switching from older, dial-up internet to modern broadband, with high speed and quality.”

The research was published in Science Direct (www.science.org/doi/10.1126/sciadv.abi8150).