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Pulses Shaped at Fs Frequency

Photonics.com
Aug 2007
WEST LAFAYETTE, Ind., Aug. 1, 2007 -- Engineers have finely controlled the spectral properties of ultrafast light pulses, a step toward creating advanced sensors, more powerful communications technologies and more precise laboratory instruments.

The laser pulses could be likened to strobes used in high-speed photography to freeze fast-moving objects such as bullets or flying insects. These laser pulses, however, are millions of times faster than such strobes, with flashes lasting a trillionth or quadrillionth of a second -- a picosecond or femtosecond (fs), respectively.
PurdueMcKinney.jpg
Jason McKinney, a former visiting assistant professor in Purdue University's School of Electrical and Computer Engineering, works with equipment that produces pulsing laser light in the university's Ultrafast Optics and Optical Fiber Communications Laboratory. Researchers in the lab recently have shown how to finely control the spectral properties of ultrafast light pulses, a step toward creating advanced sensors, more powerful communications technologies and more precise laboratory instruments. McKinney is now an engineer at the Naval Research Laboratory. (Purdue Engineering Communications Office photo/Vincent Walter)
The properties of the pulses, when represented on a graph, take on specific shapes that characterize the changing light intensity from the beginning to end of each pulse. Precisely controlling this intensity, which is called "pulse shaping," will enable researchers to tune the laser pulses to suit specific applications, said Andrew Weiner, a professor of electrical and computer engineering at Purdue University.

Researchers at other institutions have developed ultrafast lasers producing trains of pulses that are split into hundreds of thousands of segments, with each segment representing a different portion of the spectrum making up a pulse. The segments are called "comb lines" because they resemble teeth on a comb when represented on a graph, and the entire pulse train is called a "femtosecond frequency comb." The 2005 Nobel Prize in physics was awarded to researchers who precisely controlled the frequencies of these comb lines and demonstrated applications related to advanced optical clocks, which could improve communications, enhance navigation systems and enable new experiments to test physics theory, among other possible uses.

In the new research, based at Purdue's Ultrafast Optics and Optical Fiber Communications Laboratory, the engineers precisely "shaped" 100 comb lines from such a frequency comb in a single pulse.

"There are still huge technological challenges ahead, but we really see 100 comb lines as a milestone, a significant step," Weiner said. 

The pulse-shaping technique, called "optical arbitrary waveform generation," is not new. However, the Purdue team is the first to accomplish shaping of light pulses from a fs frequency comb and to demonstrate the technique on such a fine scale by controlling the properties of 100 spectral comb lines within each pulse.

By precisely controlling this "fine frequency structure" of laser pulses, researchers hope to create advanced optical sensors 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 the bandwidth. However, fully realizing these goals will require controlling 100,000 to 1 million comb lines in each pulse, Weiner said.

The advancement enables the researchers to control the amplitude and "phase" of individual comb lines, or the high and low points of each spectral line, representing a step toward applying the technique for advanced technologies.

The findings are detailed in a paper appearing online this week in the journal Nature Photonics written by postdoctoral research associate Zhi Jiang, doctoral student Chen-Bin Huang, senior research scientist Daniel E. Leaird and Weiner, all in the School of Electrical and Computer Engineering.

The research is funded by the National Science Foundation and DARPA.

For more information, visit: http://news.uns.purdue.edu

GLOSSARY
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
phase
In a periodic function or wave, the segment of the period that has elapsed, measured from some fixed origin. If the time for one period is expressed as 360° along a time axis, the phase position is called the phase angle.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
spectral
Pertaining to or as a function of wavelength. Spectral quantities are evaluated at a single wavelength.
waveform
The graph of the oscillating variations making up a wave, relative to time.
bandwidthBiophotonicscomb linesCommunicationsfiber opticslaser pulsenanoNews & Featuresoptical arbitrary waveform generationoptical fiberoptical sensorsopticsphasephotographyphotonicspulsePurdueSensors & Detectorsspectralspectroscopyultrafast laserswaveform

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