Light Pulses Could Improve

P urdue University researchers have developed a way to finely control ultrafast laser pulses’ spectral properties to create more powerful optical-communications technologies. The laser pulses last a trillionth to a quadrillionth of a second—a picosecond to a femtosecond. In essence, each pulse could serve as a data bit in future high-speed optical-communications systems, explained Purdue professor Andrew Weiner. Picosecond pulses could yield data rates of one terabit per second, much higher than those of today’s lightwave communications systems, he said. Each pulse’s intensity changes from start to finish. The Purdue scientists demonstrated that pulse shaping could tune the light by precisely controlling its intensity and phase to enhance signal quality and meet various applications’ needs. For example, the Purdue technique could be used for data encoding in high-speed fiber services. Weiner said his team is working on an optical version of code division multiple access technology. CDMA currently is used in cellular systems. CDMA, he noted, lets multiple users share a single data channel, thereby increasing a communications system’s capacity by encoding separate transmissions with different waveforms, which the receiver can then distinguish. In the Purdue system, pulse shaping would enable creation of the different waveforms. The technology could also be used in advanced sensors for detecting substances such as pollutants or chemical-warfare agents. Users could shape pulses so that the system would react differently to, and thereby identify, multiple substances. The Purdue research builds on the work of other scientists who have developed ways to use grooved glass, metal, or other materials to separate a light pulse into hundreds of thousands of spectral, or frequency, components. In pulse shaping, researchers modify the phase or amplitude of some or all of a light beam’s spectral components. In essence, this form of modulation places data on the various spectral components in parallel and thus at very high speeds. The Purdue scientists have learned to use optical arbitrary waveform generation—which uses waveguides to shape a pulse—to precisely control the intensity and phase of 100 spectral components within a single pulse. This is only a small percentage of the components in a laser pulse, but it represents an important start, explained Weiner. Weiner said the Purdue team is the first to shape light pulses lasting only a quadrillionth of a second and to demonstrate the control necessary to shape 100 spectral components. ■