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Broadband semiconductor laser shows potential to rival X-rays

Boon S. Ooi

Along the spectrum of visible and invisible lightwaves, scientists seeking to develop optical technologies are often guided by a critical factor that also animates real-estate agents—location.

This is especially true with lasers, which can gain or lose the ability to perform optical telecommunications and other functions if the wavelength at which they are emitted shifts by a few tens of nanometers. One nm is equal to one one-billionth of a meter.

Conventional lasers emit light along a single wavelength, creating a coherent electromagnetic field in which all the lightwaves contain the same frequency and are aligned in phase.

Broadband semiconductor lasers achieve greater spectral range by emitting light along multiple wavelengths at the same time.

Boon S. Ooi and his students in Lehigh’s Center for Optical Technologies have developed a new type of broadband semiconductor laser that emits light over an 85-nm span of the infrared region of the spectrum.

Ooi, an associate professor of electrical and computer engineering, says his group’s broadband laser can be generated at a cost of a few hundred dollars from a device measuring just a few hundred micrometers in size. (A micrometer, or micron, is one one-millionth of a meter.)

By contrast, conventional broadband lasers, which are generated with a short-pulse crystal laser technique, require equipment that costs several hundreds of thousands of dollars and must be housed on a large table.

The small size and low cost of Ooi’s laser, coupled with an exceptional power of more than 500 milliwatts, give the laser potential applications not only in optical telecommunications but also in biosensing and biomedical imaging and diagnosis, Ooi says.

The new laser might also find use in wavelength demultiplexing, a process by which multiple signals or data streams are transmitted on a single channel, thus saving bandwidth. And the short pulses of the laser, says Ooi, could boost the speed and capacity of data transmission by enabling more information to be compressed into a greater number of pulses.

Ooi and his group reported the results of their research last summer in an invited paper in the IEEE Journal of Selected Topics in Quantum Electronics. The paper was titled “Quantum Dashes on InP Substrate for Broadband Emitter Applications.”

The group’s work was also featured in an article in the November 2008 issue of Compound Semiconductor magazine titled “Dashes deliver broad spectral bandwidth.”

Members of the group include James Hwang, professor of electrical and computer engineering at Lehigh, and several Lehigh alumni, as well as the U.S. Army Research Laboratory (ARL) and IQE Inc., an international supplier of advanced semiconductor wafers based in Bethlehem. The group’s work is supported by ARL, the National Science Foundation and the Pennsylvania Department of Community and Economic Development.

Beyond dots to dashes

The success of Ooi’s laser rests on an understanding not only of the behavior of lightwaves, but also of the physical, mechanical and optical properties of semiconducting materials at the nanoscale.

Ooi’s laser device contains an ensemble of light-emitting quantum dashes arrayed on an indium-phosphide substrate at a density of 1 billion per square centimeter. A quantum dash is an elongated version of a quantum dot, a nanosized semiconductor that spatially confines electrons and hole pairs.

Ooi’s team uses quantum dashes made of two semiconducting materials and assembled into a laser structure measuring half a millimeter long and 300 microns wide. The structure’s diode contains four sheets, each with five quantum-dash monolayers, including embedding quantum-well and barrier layers. The dimensions of all these features measure in the tens of nanometers or smaller.

The laser’s inhomogeneous structure, says Chee-Loon Tan, a Ph.D. student in electrical engineering, enables it to emit light along a relatively wide range of the spectrum.

“Each of the dashes emits light,” Tan says. “Because the dashes have different sizes, heights, compositions and geometries, they generate different wavelengths.”

After the laser structure has been assembled, Ooi’s team uses an intermixing technique called impurity-free vacancy disordering (IFVD) to enhance bandwidth and achieve the desired wavelength for the laser.

A potential rival for X-rays

Before the laser can realize its potential applications, says Ooi, its diode must be mounted on a package and its temperature stabilized.

The researchers hope to increase the bandwidth emission of the single laser to 160 nm.

“Our immediate goal,” Ooi told Compound Semiconductor, “is to integrate multiple diodes along a single cavity using a spatial bandgap selective intermixing technique. This should produce a supercontinuum covering 1460 to 1620 nm from a single laser.”

One potential application of the new broadband laser will be to improve optical coherence tomography (OCT), a noninvasive imaging technique developed in the 1990s that obtains high-resolution images of subsurface tissue. The new laser will be able to achieve superior resolution at deeper penetrations, says Ooi, enhancing the accuracy of diagnostic techniques.

The small size of Ooi’s broadband laser also makes it more practical for medical applications than larger, conventional OCT devices. Its power – 600-700 mW – gives it more potential capability than the superluminescence diodes typically used to do OCT.

“This new technology is aimed in part at doctors,” Ooi says. “But for doctors to access it, it has to be comparable in price with the cost of X-rays. We’re getting close to the point where this will be possible.

“Also, because a high-powered broadband laser can branch out to many wavelengths and feed an array of optical fibers, it will enable medical personnel to map or scan entire regions of the body at one time. Current technology, which is based on a single source of light, is limited to mapping a region one dot at a time.”

--Kurt Pfitzer


Posted on Monday, December 29, 2008

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