March 03, 2010

Microrings for 60 Gigahertz Wireless


The wireless house of the future might use a system being developed at Purdue University that could eliminate wires for communications in homes, businesses and cars. The researchers designed and built a miniature device capable of converting ultra fast laser pulses into bursts of radio-frequency signals using innovative "microring resonators." Such an advance could enable all communications, from high-definition television broadcasts to secure computer connections, to be transmitted from a single base station. (Purdue University, Michael Esposito)

Purdue University researchers have developed a miniature device capable of converting ultrafast laser pulses into bursts of radio-frequency signals, a step toward making wires obsolete for communications in the homes and offices of the future. The Purdue researchers have miniaturized the technology small enough to fit on a computer chip. They shrank the size of the bulk optical setup by thousands of times. The researchers fabricated tiny silicon "microring resonators," devices that filter out certain frequencies and allow others to pass. A series of the microrings were combined in a programmable "spectral shaper" 100 microns wide, or about the width of a human hair. Each of the microrings is about 10 microns in diameter. Others working on 60 Ghz communication have talked about 15 gigabit per second communication speed.


A key factor making the advance potentially useful is that the pulses transmit radio frequencies of up to 60 gigahertz, a frequency included in the window of the radio spectrum not reserved for military communications.

The Federal Communications Commission does not require a license to transmit signals from 57-64 gigahertz. This unlicensed band also is permitted globally, meaning systems using 60 gigahertz could be compatible worldwide.


Ordinarily, the continuous waves of conventional radio-frequency transmissions encounter interference from stray signals reflecting off of the walls and objects inside a house or office. However, the pulsing nature of the signals produced by the new "chip-based spectral shaper" reduces the interference that normally plagues radio frequency communications, said Andrew Weiner, Purdue's Scifres Family Distinguished Professor of Electrical and Computer Engineering.

Each laser pulse lasts about 100 femtoseconds, or one-tenth of a trillionth of a second. These pulses are processed using "optical arbitrary waveform technology" pioneered by Purdue researchers led by Weiner

"What enables this technology is that our devices generate ultrabroad bandwidth radio frequencies needed to transmit the high data rates required for high resolution displays," Weiner said.

Such a technology might eventually be developed to both receive and transmit signals.

"But initially, industry will commercialize devices that only receive signals, for 'one-way' traffic, such as television sets, projectors, monitors and printers," Qi said. "This is because the sending unit for transmitting data is currently still a little bulky. Later, if the sending unit can be integrated into the devices, we could enjoy full two-way traffic, enabling the wireless operation of things like hard-disc drives and computers."

The approach also might be used for transmitting wireless signals inside cars.

The researchers first create laser pulses with specific "shapes" that characterize the changing intensity of light from the beginning to end of each pulse. The pulses are then converted into radio frequency signals.



This diagram shows the design for new silicon "microring resonators," miniature devices used in a system that converts ultra fast laser pulses into bursts of radio-frequency signals. The innovation is a step toward making wires obsolete for communications in homes and offices. Such an advance could enable all communications, from high-definition television broadcasts to secure computer connections, to be transmitted from a single base station. The microring filter can be tuned by heating the rings. (Purdue University, Minghao Qi)

Nature Photonics - Ultrabroad-bandwidth arbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper

Ultrabroad-bandwidth radiofrequency pulses offer significant applications potential, such as increased data transmission rate and multipath tolerance in wireless communications. Such ultrabroad-bandwidth pulses are inherently difficult to generate with chip-based electronics due to limits in digital-to-analog converter technology and high timing jitter. Photonic means of radiofrequency waveform generation, for example, by spectral shaping and frequency–time mapping, can overcome the bandwidth limit in electronic generation. However, previous bulk optic systems for radiofrequency arbitrary waveform generation do not offer the integration advantage of electronics. Here, we report a chip-scale, fully programmable spectral shaper consisting of cascaded multiple-channel microring resonators, on a silicon photonics platform that is compatible with electronic integrated circuit technology. Using such a spectral shaper, we demonstrate the generation of burst radiofrequency waveforms with programmable time-dependent amplitude, frequency and phase profiles, for frequencies up to 60 GHz. Our demonstration suggests potential for chip-scale photonic generation of ultrabroad-bandwidth arbitrary radiofrequency waveforms.

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