11-10 :: October 2011

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that detect and measure hazardous materials or pollutants, ultrasensitive spectroscopy for laboratory research, and optics-based communications systems that transmit greater volumes of information with better quality while increasing bandwidth. The comb technology also has potential for a generation of high-bandwidth electrical signals with possible appli- cations in wireless communications and radar.

The light originates from a continuous-wave laser, also called a single-frequency laser.

“This is a very common type of laser,” Weiner said. “The intensity of this type of laser is constant, not pulsed. But in the microring the light is converted into a comb consisting of many frequencies with very nice equal spacing. The microring comb generator may serve as a competing technology to a special type of laser called a mode-locked laser, which gene- rates many frequencies and short pulses. One advan- tage of the microrings is that they can be very small.”

The laser light undergoes „nonlinear interaction“ while inside the microring, generating a comb of new frequencies that is emitted out of the device through another optical fiber.

“The nonlinearity is critical to the generation of the comb,” said doctoral student Fahmida Ferdous. “With the nonlinearity we obtain a comb of many frequencies, including the original one, and the rest are new ones generated in the microring.”

Although other researchers previously have demons- trated the comb-generation technique, the team is the first to process the frequencies using „opti- cal arbitrary waveform technology,“ pioneered by

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Purdue researchers led by Weiner. The researchers were able to control the amplitude and phase of each spectral line, learning that there are two types of combs - „highly coherent“ and „partially coherent“ - opening up new avenues to study the physics of the process.

“In future investigations, the ability to extract the phase of individual comb lines may furnish clues into the physics of the comb-generation process,” Fer- dous said. “Future work will include efforts to create devices that have the proper frequency for commer- cial applications.”

The silicon-nitride device was fabricated by a team led by Houxun Miao, a researcher at NIST‘s Cen- ter for Nanoscale Science and Technology and the Maryland Nanocenter at the University of Maryland. Some of the work was performed at the Birck Nano- technology Center in Purdue‘s Discovery Park, and experiments demonstrating short-pulse generation were performed in Purdue‘s School of Electrical and Computer Engineering.

Fahmida Ferdous, Houxun Miao, Daniel E. Leaird, Kartik Srinivasan, Jian Wang, Lei Chen, Leo Tom Varghese, and Andrew M. Weiner: Spectral Line-by-Line Pulse Shaping of an On-Chip Microresonator Frequency Comb, In: Nature Photonics AOP, October 09, 2011, DOI:10.1038/ nphoton.2011.255 2011: http://dx.doi.org/10.1038/nphoton.2011.255 2011