Photonics researchers at the Singapore University of Technology and Design (SUTD) have created a high-gain optical amplifier tiny enough to fit on a chip.
Used in an optical transceiver or fiberoptic network, for instance, this telecommunications optical amplifier would increase the power of the transmitted light before it is completely depleted through optical losses and, thus, could strengthen the integrity of the transmitted data.
“This amplifier has been demonstrated to amplify light by as much as 17000 times,” confirms Assistant Professor Dawn Tan of SUTD’s Photonics Devises and Systems Group, who led the development of the amplifier. The amplifier uses a CMOS-compatible, ultra-silicon-rich nitride (USRN) platform, which possesses a large optical nonlinearity and negligible nonlinear losses at the telecommunications wavelength. “These desirable characteristics lead to a very high nonlinear figure of merit, which is a highly sought-after quality for high-efficiency, low-loss generation of light using low powers,” says the photonics expert. She further elaborates that the amplification mechanism relies on the Kerr nonlinearity of the ultra-silicon-rich nitride platform to generate an efficient transfer of photons from an optical pump to amplify an optical signal.
Strengthening integrity of transmitted data
The professor explains that the propagation of light through a guided medium, such as a fiber or optical waveguide, is “an inherently lossy process”. Optical signals, such as those transmitted through terrestrial fiberoptic networks or short-haul communications in active optical cables to data centers, get attenuated because of optical absorption. Therefore, “The restoration of the optical signals needs to be performed periodically, to ensure that optical signals have a sufficiently high power to be detected at the receiver end,” says Tan. “This new technology has very large gain and therefore allows for the efficient regeneration of optical data while reducing the number of amplifier stages needed along the transmission path.”
The telecommunications optical amplifier is also compatible with CMOS electronics, so it is well-suited for such regeneration in silicon photonics-based transceivers serving the 500 m to 2 km range.
“In CMOS-based optical parametric amplifiers demonstrated to date at the telecommunications wavelength, the amplification achieved was much lower than what we achieved here,” notes Tan, imputing the throttled performance of previous CMOS optical platforms to either low nonlinearity or high nonlinear losses. The researcher, whose work focuses on quantum physics, optics and photonics, explains that low nonlinearities limit the amount of pump photons that can be transferred to the optical signal thus limiting the gain. And nonlinear losses effectively limit the amount of achievable amplification because the pump intensity is severely attenuated when it is increased. “We managed to design and engineer an optical platform, using some specific design rules outlined in the paper, such that the USRN platform simultaneously possesses a very large optical nonlinearity and negligible nonlinear losses,” she says. “Coupled with optical design of the nanoscale waveguide device, these two important characteristics allowed us to fully engineer a waveguide device capable of very large optical amplification.”
The before-mentioned research paper, which Tan co-authored, is titled “Pushing the limits of CMOS optical parametric amplifiers with USRN:Si7N3 above the two-photon absorption edge,” published in Nature Communications.
Performance and impact on future of photonic data transmission
“Our technology was demonstrated in our experiments to amplify light by up to 17000 times,” Tan once more notes. “This is the largest gain we know of on a CMOS chip at the telecommunications wavelength.” In terms of absolute gain, Tan and her colleagues believe this performance is sufficiently high for it to be applicable for use even today. “We have demonstrated the conversion of light from the E-band to the U-band,” she reports. “We would like to be able to look into conversion covering the O-band as well as [whether] there exists a class of transceivers which operate in this telecommunications band.”
The innovation out of Singapore could eventually enable high-speed regeneration of photonic data — such as that used in internet data transmission — at higher efficiencies and lower costs than available today.
Applications for the new optical amplifier
There are several key applications for this new kind of high-gain optical amplifier that is compact enough to fit on a chip: “Firstly, a conventional optical parametric amplifier costs several hundred thousand dollars and occupies an entire optical table, while the newly developed amplifier is much smaller than a paper clip and costs a fraction of the former,” says Tan. “This technology could bring high-gain amplifiers to laboratories studying ultrafast optics and attosecond science at a fraction of the cost today.”
Furthermore, the professor projects that providing high gain on such a small footprint could also enable new opportunities in low-cost broadband spectroscopy, precision manufacturing and hyperspectral imaging. “The device’s efficiency is also revealed through cascaded four-wave mixing, which is a higher order mixing of the amplified and converted photons,” she adds. “This phenomenon also allows the amplifier to operate as a tunable broadband light source, enabling cheaper and more efficient spectroscopic sensing and molecular fingerprinting than what is available today.”
The expert concludes that when used within an optical interconnect, such as a transceiver or fiber optic network, their amplifier would help to regenerate and restore optical data by efficiently increasing the power of the transmitted light before it is completely depleted through optical losses.
Next steps at SUTD
Tan notes that this work serves as the start in a study of rich nonlinear optical phenomena availed through this new optical platform: “We hope to continue pushing the limits of the platform by designing and studying nonlinear optical devices leveraging the high nonlinear figure of merit in the USRN platform.”
Written by Sandra Henderson, research editor Novus Light Technologies Today