IBM Develops Ultracompact Photonic Modulator for On-Chip Communications
Peter Singer, Editor-in-Chief -- Semiconductor International, 12/6/2007 8:57:00 AM
IBM (Yorktown Heights, N.Y.) scientists have developed a photonic modulator that is 100-1000× smaller than previously demonstrated modulators of its kind, which could eventually enable on-chip communication with light instead of electrical signals. It is likely that hundreds, if not thousands, of such modulators will be required on a single die in the future for optical modulation — converting electrical signals to light — as well as for filtering and switching functions.
“Work is underway within IBM and in the industry to pack many more computing cores on a single chip, but today’s on-chip communications technology would overheat and be far too slow to handle that increase in workload,” said T.C. Chen, vice president of science and technology at IBM Research. “What we have done is a significant step toward building a vastly smaller and more power-efficient way to connect those cores in a way nobody has done before.”
Using silicon for such photonic functions has two main advantages. First, it is compatible with conventional CMOS processing, so monolithic integration of silicon photonic devices with advanced electronics on a single silicon substrate is possible. Second, silicon is surprisingly useful as a photonic material in that it is transparent in the range of optical telecommunication wavelengths, 1.3 and 1.55 µm. It also has a high refractive index that allows for the fabrication of high-index-contrast nanophotonic structures.
The silicon modulator performs the function of converting electrical signals into pulses of light. In recent work, reported in Optics Express, IBM researchers described several ultracompact (100-200 µm length), low RF power (5 pJ/bit), high-speed silicon on insulator (SOI) devices that are capable of operating at data rates up to 10 Gb/sec. The devices are Mach-Zehnder modulators (MZMs), which have an advantage of broadband spectral operation and improved tolerance to environmental and process fluctuations, compared with other types of silicon modulators.
>In early 2004, Intel (Santa Clara, Calif.) demonstrated a silicon-based, Mach-Zehnder device that operated at ~20 MHz. The problem was the size: Conventional silicon MZMs are based on rib waveguides, which usually need one-half to several millimeters to achieve the required phase shift.
In 2006, University of Texas at Austin (UT) researchers demonstrated a 300kHz device that got around the size problem through the use of photonic crystal waveguides with nanopores, which help disperse the light and maximize phase modulation efficiency (efficiency is a function of phase change, which is related to the change of propagation constant and waveguide length). This means the same phase change can be produced by a photonic crystal waveguide that is 100× shorter than a conventional waveguide, only 80 µm in the UT work.
The recent IBM work, which moved the technology closer to production, used 100-200 µm long p+-i-n+ diode active regions. The researchers note that “despite the conventionally held view that a non-resonant modulator, such as the MZM, must naturally be designed with a long optical interaction length, the IBM devices demonstrate that it is indeed possible to design a high-performance ultracompact MZM component with micron-scale dimensions, suitable for on-chip optical interconnect applications.”
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| IBM’s optical modulator uses 'silicon nanophotonic waveguides,' to control the flow of light on a silicon chip. The waveguides are made of tiny silicon strips (purple) in a silicon on insulator (SOI) wafer. Digital electrical signals are applied to the p+-i-n+ doped silicon nanophotonic waveguide through the electrodes (gold). Electrical charges (holes: green; electrons: red) are injected into the waveguide, changing the optical properties of silicon, which is used to perform the modulation function. |
IBM’s ultracompact MZMs were designed using nanophotonic rib waveguides with embedded p+-i-n+ diodes as charge injection-based electrooptic phase shifters. In comparison with previously demonstrated SOI MZM devices, where much larger rib waveguides having cross-sectional areas on the order of 1 µm2 or larger were employed, the ultracompact cross-section of nanophotonic rib waveguides simultaneously confines both the injected free carriers and photons to an effective area of ~0.12 µm2.
IBM researchers said localization of both photons and free carriers within such a small region produces two effects beneficial for efficient modulator performance. First, during each on/off modulation cycle, the diode injection current is forced to flow through a cross-section only 220 nm in height, producing large changes in the free carrier density within the intrinsic rib waveguide core. Second, the ultracompact modal cross-section assures nearly complete free carrier/optical mode overlap, resulting in large modulation of the modal effective index. “Taken together, these two characteristics of p+-i-n+ diode nanophotonic rib waveguides lead to highly efficient operation as electrooptic phase shifters, enabling MZM devices requiring greatly reduced active length,” the researchers not in the Optics Express article.
