Silicon photonics advances
BY ED KORCZYNSKI, Senior Technical Editor
With rapidly increasing use of “cloud” client:server computing there is motivation to find cost-savings in the cloud hardware, which leads to R&D of improved photonics chips. Silicon photonics chips could reduce hardware costs compared to existing solutions based on indium-phosphide (InP) compound semiconductors, but only with improved devices and integration schemes. Now MIT researchers working within the US AIM Photonics program have shown important new silicon photonics properties. Meanwhile, GlobalFoundries has found a way to allow for automated passive alignment of optical fibers to silicon chips, and makes chips on 300-mm silicon wafers for improved performance at lower cost.
In a recent issue of Nature Photonics, MIT researchers present “Electric field-induced second-order nonlinear optical effects in silicon waveguides.” They also report prototypes of two different silicon devices that exploit those nonlinearities: a modulator, which encodes data onto an optical beam, and a frequency doubler, a component vital to the development of lasers that can be precisely tuned to a range of different frequencies.
This work happened within the American Institute for Manufacturing Integrated Photonics (AIM Photonics) program, which brought government, industry, and academia together in R&D of photonics to better position the U.S. relative to global competition. Federal funding of $110 million was combined with some $500 million from AIM Photonics’ consortium of state and local governments, manufacturing firms, universities, community colleges, and nonprofit organizations across the country. Michael Watts, an associate professor of electrical engineering and computer science at MIT, has led the technological innovation in silicon photonics.
“Now you can build a phase modulator that is not dependent on the free-carrier effect in silicon,” says Michael Watts in an online interview. “The benefit there is that the free-carrier effect in silicon always has a phase and amplitude coupling. So whenever you change the carrier concentration, you’re changing both the phase and the amplitude of the wave that’s passing through it. With second-order nonlinearity, you break that coupling, so you can have a pure phase modulator. That’s important for a lot of applications.”
The frequency doubler uses regions of p- and n-doped silicon arranged in regularly spaced bands perpendicular to an undoped silicon waveguide. The space between bands is tuned to a specific wavelength of light, such that a voltage across them doubles the frequency of the optical signal passing. Frequency doublers can be used as precise on-chip optical clocks and amplifiers, and as terahertz radiation sources for security applications.
At the start of the AIM Photonics program in 2015, MIT researchers had demonstrated light detectors built from efficient ring resonators that they could reduce the energy cost of transmitting a bit of information down to about a picojoule, or one-tenth of what all-electronic chips require. Jagdeep Shah, a researcher at the U.S. Department of Defense’s Institute for Defense Analyses who initiated the program that sponsored the work said, “I think that the GlobalFoundries process was an industry-standard 45-nanometer design-rule process.”
GlobalFoundries has found a way to automatically align twelve optical fibers to a silicon chip while the fibers are dark. Because the micron-scale fibers must be aligned with nanometer precision, default industry standard has been to expen- sively align actively lit fibers. Leveraging the company’s work for Micro-Electro-Mechanical Sensors (MEMS) customers, GlobalFoundries uses an automated pick-and-place tool to push ribbons of multiple fibers into MEMS groves for the alignment. Ted Letavic, Global Foundries’ senior fellow, said the edge coupling process was in production for a telecommu- nications application. Silicon photonics may find first appli- cations for very high bandwidth, mid- to long-distance trans- mission (30 meters to 80 kilometers), where spectral efficiency is the key driver according to Letavic.
GobalFoundries has now transferred its monolithic process from 200mm to 300mm-diameter silicon wafers, to achieve both cost-reduction and improved device performance. The 300mm fab lines feature higher-N.A. immersion lithog- raphy tools which provide better overlay and line width roughness (LWR). Because the of the extreme sensitivity of optical coupling to the physical geometry of light-guides, improving the patterning fidelity by nanometers can reduce transmission losses by 3X.