CLEO '99 tackles the science and technology of light

From optical networks to quantum dots and diode lasers, CLEO '99 highlights the growing diversity of laser and optoelectronics R&D.

From optical networks to quantum dots and diode' lasers, CLEO `99 highlights the growing diversity of laser and optoelectronics R&D.

This year`s annual Conference on Lasers and Electro-Optics (CLEO `99) will be held in Baltimore, MD, May 23-28, and attendees can expect to hear about many "firsts" in lasers and optoelectronics R&D. Lucent Technologies (Murray Hill, NJ), for example, has developed the first quantum-cascade lasers operating at very long wavelengths (1-17 µm). Out of Germany comes a report of the first singly resonant CW optical parametric oscillator (OPO) pumped directly with semiconductor lasers.

Speed records will be another topic of discussion. The Massachusetts Institute of Technology (Cambridge, MA) and another research organization have independently clocked record-breaking ultrafast pulses directly from a near-IR laser. Japanese manufacturer Hamamatsu and the University of Tokyo have produced ultrafast pulses in the visible region from an optical parametric amplifier-tunable sub-10-fs pulses.

In the plenary sessions, Gerald Butters, president of the optical networking group at Lucent Technologies (Somerset, NJ), will discuss the explosive growth of optical networking. Donald Scifres, CEO of SDL Inc. (San Jose, CA), will review recent advances and future prospects in semiconductor lasers. Addressing quantum communications and computing will be Peter Knight of Imperial College in England, who will describe developments and prospects for quantum mechanics and cryptography.

Collocated with CLEO will be the Quantum Electronics and Laser Science Conference (QELS), which is reportedly the largest North American conference focusing on research in lasers, nonlinear optics, and the fundamental laser spectroscopy of atoms and condensed matter. Heavy emphasis here will be on ultrafast-laser sources. Almost a third of conference sessions will focus on it, according to one of the combined events` cosponsors, the Optical Society of America (OSA; Washington, DC). Other popular topics will include Bose-Einstein condensation and quantum dots, which will be examined in two conference sessions.

In addition, the Lasers and Electro-Optics Program (LEAP) will discuss diverse topics, from applications of lasers in biological sciences to high-power industrial-laser processing. Conference attendees will also have access to some 32 CLEO short courses on topics ranging from fundamentals of laser diodes and rare-earth-doped fiberoptic amplifiers to optical-fiber communications systems and holographic storage.

Boosting solid-state efficiency
Solid-state, semiconductor, high-field, or fiber-laser-technology advances are not slowing as we approach the 40th anniversary of the laser. In areas such as chip marking and graphics applications such as exposure of thermal media, this leads to some interesting competition among laser sources-specifically fiber lasers and certain solid-state lasers.

It is hard to beat the remarkable quantum efficiencies reported for fiber lasers, but William Nighan and colleagues at Spectra-Physics Lasers OEM Business Unit (Mountain View, CA) claim to have done just that with a diode-bar-pumped neodymium-doped yttrium vanadate (Nd:YVO4) laser (see Fig. 1). According to Nighan, the unit generates up to 35 W of average power in a polarized TEM00 mode with M2 roughly equal to 1.1. This performance is achieved with 62% optical-to-optical conversion efficiency from the fiber-coupled diode-bar pump power to the 1064-nm output of the laser. The resulting quantum efficiency approaches 94%-the percent of 1064-nm photons generated per quantity of 809-nm pump photons absorbed in the Nd:YVO4 laser crystals.

"To our knowledge," says Nighan, "these are the highest efficiencies reported to date for a diode-pumped solid-state TEM00 laser, exceeding even the remarkable quantum efficiencies reported for fiber lasers. Through the years, it has been said that for anything greater than 2 W it would be difficult to reach this level of efficiency, yet each year research advanced a little further to the point that we are right near the quantum limit."

The Nd:YVO4 laser configuration is based on four fiber-coupled 20-W diode bars, each with output reduced to extend lifetime in industrial applications. Per diode bar, the output bundle is 1.1-mm diameter, and the numerical aperture is less than 0.10. The four-bundle outputs are imaged into opposing faces of the two Nd:YVO4 active media through four dichroic fold mirrors. Although the researchers` prior work in this area used a dual-end-pumped Z resonator, the system developed for operation beyond 30 W is a period resonator with two of the Z building blocks placed end to end (see paper C-304).

Side-pumped fiber lasers
Even with advances such as these, there is no question that fiber-laser technology has been somewhat more of an R&D hotspot in the last few years. Developments have been fast and dramatic, as evidenced by research by Lew Goldberg and Jeffrey Koplow at the Naval Research Laboratory (Washington, DC), working with Dahv Kliner at Sandia National Laboratories, Combustion Research Facility (Livermore, CA).

Several implementations of 1-µm high-power double-cladding fiber amplifiers and lasers have been demonstrated recently. Wide gain bandwidth and efficiency make these devices useful in spectroscopy, remote sensing, materials processing, frequency conversion, and more. Goldberg and colleagues will report on an ytterbium-doped double-clad fiber amplifier that generates 3.3 W and exhibits a maximum electrical-to-optical conversion efficiency of 34% when v-groove side-pumped by a

100-µm-wide broad stripe laser diode (see Fig. 2). "Prior to this," they said, "output powers of 1 W have been reported for ytterbium-doped amplifiers pumped with a single broad-stripe laser diode."

The new amplifier is characterized in a single-pass counterpropagating arrangement. As part of a tunable fiber laser, its cavity is defined by a diffraction grating at one end of the amplifier and a 90° cleaved fiber face on the other. The device generates 3 W as a tunable fiber laser.

The 15-m-long fiber had a 5.3 µm-diameter ytterbium-doped core, a 130-µm nearly hexagonal inner cladding, and a polymer outer cladding. The 100-µm-wide, 1-mm-long broad-stripe laser diode (from SDL) was operated to a current of 6 A, where it emitted 4.5 W at 975 nm. According to the researchers, the diode-to-fiber coupling efficiency was 84%. The electrical-to-optical conversion efficiency-the ratio between the extracted and electrical power-reached a maximum 34% at 5 A, where the extracted power was 2.9 W. The maximum optical conversion efficiency approached 76%, and the amplifier optical slope efficiency was about 88% (see paper C-281).

Circuit microfabrication
Also benefiting from rapid advances in laser technology are microelectronics-processing applications. Michael Renn and Robert Pastel at Michigan Technological University (Houghton, MI) will present a discussion on direct writing of materials by laser guidance (see Fig. 3). The basic premise of their research is to use optical forces to guide aerosol particles through hollow-core optical fibers and deposit them on a substrate placed near the fiber output tip.

According to Renn, laser light
(300 mW at 532 nm) is focused at low numerical aperture into the hollow region of a 19-µm-inner-diameter fused-silica capillary fiber. Small particles in the laser beam are attracted to the beam center by optical gradient forces and propelled along the fiber by optical scattering and absorption forces. An aerosol generator introduces the particles into the laser beam near the fiber input tip.

By translating the substrate during material deposition, the researchers can build arbitrary structures of lines and dots. The deposition conforms to the substrate curvature, and the structures can be defined with micron resolution. The laser beam is intense enough to melt many types of particles during delivery to the substrate, and the resulting structures are fully dense and well adhered. So far, the process has written lines of silver, glass, and various salts on substrates of glass, plastic, and semiconductors. Line width is adjustable from 3 to 10 µm with a tolerance of

1 µm. Line thickness can vary from 0.5 to 10 µm (see paper C-362).

QELS for quantum dots
For R&D in photophysics, nonlinear optical phenomena, atom optics, and quantum dots, the QELS presentations are the place to be. Consider semiconductor quantum dots. Efforts to understand these nanometer-size structures-in which the charge carriers are confined in all three directions to characteristic lengths smaller than their De Broglie wavelengths-are driven by their potential device applications, as well as how they can aid experimental studies of basic quantum-mechanical principles. Erez Dekel, David Gershoni, and Eitan Ehrnfreund of the Israel Institute of Technology (Haifa, Israel), along with Pierre Petroff at the University of California (Santa Barbara, CA), will present a discussion on optical spectroscopy of single semiconductor quantum dots.

One promising build technique is the self-assembled quantum-dots method. This involves self-formation of small islands that reduce the elastic energy associated with the lattice mismatch strain between epitaxially deposited layers of different lattice constants. By capping these islands with an additional layer of higher-energy band gap, one can produce high-quality quantum dots. The size distribution of the dots (typically about 10%) and their resulting inhomogeneous broadening have so far limited scientists` ability to clearly understand and unambiguously interpret the experimental results. Dekel and colleagues reportedly have overcome this by applying diffraction-limited confocal optical microscopy to spectroscopically study a single indium arsenide quantum dot embedded within an aluminum gallium arsenide host (see QELS presentation I6A).

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