'Microrings' could lead to new era in wireless communications

In a step that could see communication wires banished from homes and offices researchers have developed a miniature device capable of converting ultra-fast laser pulses into bursts of radio-frequency signals. The advance could enable all communications, from HDTV broadcasts to secure computer connections, to be transmitted from a single base station.

"This base station would be sort of a computer by itself, perhaps a card inserted into one of the expansion slots in a central computer. The central computer would take charge of all the information processing, a single point of contact that interacts with the external world in receiving and sending information,” said Minghao Qi, an assistant professor of electrical and computer engineering at Purdue University.

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.

Although the technology might eventually be developed to both receive and transmit signals, initially it is likely to be commercialized in devices that only receive signals, for "one-way" traffic, such as television sets, projectors, monitors and printers, say the researchers. This is because the sending unit for transmitting data is still a little bulky. If its size can be reduced enough to allow it to be integrated into the devices, it would enable full two-way traffic, making possible the wireless operation of things like hard-disc drives and computers. The approach could also 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.

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

"There is only a very limited window for civil operations, and 60 gigahertz falls within this window," Qi said.

Ordinary computer chips have difficulty transmitting electronic signals at such a rapid frequency because of "timing jitter," or the uneven timing with which transistors open and close to process information.

This uneven "clock" timing, or synchronization, of transistors does not hinder ordinary computer chips, which have a speed of about 3 gigahertz. However, for devices switching on and off at 60 gigahertz, this jitter prevents proper signal processing.

Another complication is that the digital-to-analog converters needed to convert pulsing laser light into radio frequency signals will not work at such high frequencies.

To sidestep these limitations, researchers have previously created "bulk optics" systems, which use mirrors, lenses and other optical components arranged on a vibration-dampened table several feet long to convert and transmit the pulsed signals. However, these systems are far too large to be practical. Now, the Purdue researchers have miniaturized the bulk optical setup by thousands of times and made the technology small enough to fit on a computer chip.

The system is programmable so that it could be instructed to produce and transmit only certain frequencies and the researchers have 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 and the microring filter can be tuned by heating the rings, which causes them to change so that they filter different frequencies.

Purdue filed a provisional patent in January for the technology, which is at least five years away from being ready for commercialization, Qi said