April 1, 1999
At the Optoelectronics Division of the National Institute of Standards and Technology (NIST; Boulder, CO) and Schott Glass Technologies Inc. (Duryea, PA), researchers have demonstrated an array of high-power, robustly single-frequency, integrated distributed-Bragg-reflector (DBR) waveguide lasers operating near 1536 nm. The single-transverse-mode waveguides were fabricated in a commercially available ytterbium/erbium (Yb/Er) codoped phosphate alka* glass by ion exchange. A single-pitch DBR surfa

Distributed-Bragg-reflector waveguide laser arrays emit near 1536 nm

At the Optoelectronics Division of the National Institute of Standards and Technology (NIST; Boulder, CO) and Schott Glass Technologies Inc. (Duryea, PA), researchers have demonstrated an array of high-power, robustly single-frequency, integrated distributed-Bragg-reflector (DBR) waveguide lasers operating near 1536 nm. The single-transverse-mode waveguides were fabricated in a commercially available ytterbium/erbium (Yb/Er) codoped phosphate alka* glass by ion exchange. A single-pitch DBR surface-relief grating was fabricated holographically across the waveguides. The slope efficiency of the lasers as a function of launched 977-nm pump power was 26%, and the threshold was less than 50 mW. An output power of 80 mW was produced with 350 mW of coupled pump power. The researchers expect both output and efficiency will increase and threshold power will decrease when the grating reflectance is improved. By writing several gratings of varying period, the researchers believe they have demonstrated that lasers of several output wavelengths can be integrated onto a single glass substrate as a source for wavelength-division multiplexing. Contact David Veasey at [email protected].

Zernike coefficients guide lithographic lens to proper setup

As part of a program to shrink the size of features printed by a semiconductor-wafer stepper down to less than one-half the wavelength of the exposure light, engineers at Canon Inc. (Utsunomiya, Japan) have developed a method for "tuning" lithographic projection lenses that is based on the analysis of Zernike wavefront coefficients. Lens tuning involves not only design, but also assembly and measurement; the lens criterion chosen to be measured and optimized affects how fast and well a manufacturer can achieve the proper lens configuration. Traditional criteria include modulation transfer function and root-mean-square wavefront error.

The Zernike analysis allowed the engineers to reduce residual wavefront aberration by a factor of four compared to what they were getting at the start of the program. "The improvement achieved by these new tuning methods . . . also allows future lenses to be designed for dramatically lower aberration in the first place," says Nobuyoshi Tanaka, director and chief executive of Canon Optical Products Operations. Canon researchers predict that this technique will allow a 0.7-NA, 193-nm wafer stepper to print features in the 80-nm region. See

Second market seminar to be held in Europe

The Laser Marketplace `99 Seminar sponsored by Laser Focus World in conjunction with Strategies Unlimited (Mountain View, CA) will be held in Munich, Germany, at Laser 99 on June 16. The seminar provides a unique forum for industry leaders to review key business and technology trends for the laser and optoelectronics industry. "We held the seminar for the first time in Europe at Laser 97, where it was well received," notes Jeff Bairstow, group editorial director at Laser Focus World. "That and the ongoing success of the Laser Marketplace Seminar series at Photonics West (San Jose, CA) has convinced us we should continue to provide the material to a broader audience," he says.

Presentations based on the Annual Review and Forecast of Laser Markets (see Laser Focus World, Jan. 1999, p. 80, and Feb. 1999, p. 52) will include detailed analysis of the worldwide markets for both diode and nondiode lasers. "The seminar provides attendees with much more background and market analysis than is available in the printed material in Laser Focus World and Optoelectronics Report," explains Bairstow. "The opportunity to interact with other attendees and to question the presenters about the markets is a key aspect of the seminar," he said. Contact Carole Root at [email protected].

Two-photon-absorbing polymerization initiators have use in optical data storage

Laser-induced polymerization holds promise for such applications as three-dimensional (3-D) optical data storage. Such an approach, however, has not been considered commercially viable because of the high-power sources required to excite polymerization. Now scientists at the University of Arizona (Tucson, AZ), with colleagues from the California Institute of Technology (Pasadena, CA), have demonstrated materials with enhanced two-photon sensitivity and have shown the potential of the material for 3-D optical storage and the creation of miniature devices.

Unlike previous materials, the resin developed by these researchers starts to polymerize with 730-nm, 5-ns pulses at energies as low as 0.2 mJ. This system can write information with densities of 1012 bit/cm3 because the excitation light is absorbed only at the focal region, so it penetrates deeply into the material, and because the longer wavelengths used reduce Rayleigh scattering. The same process can produce tiny 3-D structures, such as those needed for photonic bandgap materials. Microelectromechanical systems, usually produced through two-dimensional lithography, can be made more quickly with a single 3-D step. Contact Joseph Perry at [email protected].

Efficient tripling of 1057-nm pulses produces intense UV with 40% efficiency

Extending ultrashort-pulse capability to near-ultraviolet (UV) wavelengths with microjoule to milliwatt pulse energies is important for applications such as photophysics. But there are problems, particularly if energetic pulses are sought, because of lack of a direct laser broadband UV source with high saturation fluence or to bandwidth limitation in the frequency-conversion process. Now, researchers at the Centre d`Études de Limeil-Valenton (St. Georges, France) and the Laboratoire pour l`Utilisation des Lasers Intenses (Palaiseau, France) have shown that by controlling the phase chirp to maintain phase-matching, it is possible to frequency-triple a broadband infrared ultrashort pulse at a conversion efficiency as high as 40%. Their system is based on a Ti:sapphire regenerative amplifier and a Nd:phosphate glass head delivering 1-ns chirped pulses with 100-mJ maximum energy that can be compressed to a duration on the order of 250 fs with a spectrum bandwidth of 10-nm maximum near 1057 nm.

The tripling scheme relies on a potassium dihydrogen phosphate (KDP) type I-II collinear crystal configuration. Two pulses are created with an adapted chirp. One is frequency-doubled, then the pulses are combined for tripling in the KDP II crystal and the resulting pulse compressed. This method is compatible with a high-energy laser chain and should allow kilojoule petawatt laser pulses in the UV range. Contact Arnold Migus at [email protected].

Self-organized GaAs quantum-wire structures lase at room temperature

Quantum-wire structures have been studied as an alternative to quantum-well lasers because of their theoretically narrower linewidths and higher gain; however, horizontal and stacking densities have limited their development. Now, at Osaka University (Osaka, Japan) and Kubota Corp. (Amagasaki, Japan), researchers have shown how self-organized gallium arsenide (GaAs) quantum wires can be created with high uniformity, high density, high optical quality, and a simple fabrication process-molecular-beam epitaxy. The researchers say they have achieved the first laser application of such quantum wires in the form of stripe-geometry quantum-wire lasers fabricated on (775) B GaAs substrates with a graded-refractive-index separate-confinement heterostructure. The densities of the quantum wires were as high as 8 x 106/cm. At 20°C, the lasing wavelengths were between 761 and 846 nm. Threshold currents were lower than those of simultaneously grown (100) quantum-well lasers. Contact Masataka Higashiwaki at [email protected].

Laser diodes pump flat-gain Raman amplifiers to 100-nm bandwidth

Researchers from the Opto-Technology Laboratory at Furukawa Electric (Chiba, Japan) have produced Raman amplifiers with flat gain over a 100-nm bandwidth, according to a postdeadline paper presented in February at the Optical Fiber Communication conference (San Diego, CA). The amplifiers were pumped and gain equalized by high-power laser diodes with 12 wavelength-division multiplexing (WDM) channels. The wavelength range of the WDM laser-diode unit extended from 1405 to 1510 nm, with eight channels between 1405 and 1457.5 nm and four between 1465 and 1510 nm. The unit was constructed using planar-lightwave-circuit (PLC) technology and strained-layer multiple-quantum-well laser-diode modules. A fiber Bragg grating at the output of each laser diode provided a narrow and stable spectrum for low insertion loss through the WDM coupler, which was a silica-based PLC consisting of 11 Mach-Zehnder interferometers. A maximum output power of more than 2.2 W was achieved, with a channel spacing of about 7.5 nm in the 1405- to 1457.5-nm wavelength section and a channel spacing of about 15 nm between 1465 and 1510 nm. The system was tested in 25 km of single-mode fiber (SMF), 25 km of dispersion-shifted fiber (DSF), and 20 km of reverse dispersion fiber (RDF). A 100-nm bandwidth with ±0.5-dB variation was obtained at 2-dB gain for SMF and 6.5 dB for DSF and RDF. Contact Yoshihiro Emori at [email protected].

Timing jitter falls below 10 fs in 10-GHz modelocked fiber laser

Researchers at the Optical Sciences Division of the Naval Research Laboratory (Washington, DC) have achieved a timing jitter of less than 10 fs from 100 Hz to 1 MHz in a harmonically modelocked fiber laser. The results were reported in February during a postdeadline session at the Optical Fiber Communication conference (San Diego, CA). The laser used in the experiment was an actively modelocked erbium fiber laser in a sigma configuration. Intracavity soliton pulse compression generated approximately 1-ps pulses at repetition rates in excess of 10 GHz. An external, low-noise microwave synthesizer was used to drive a Mach-Zehnder amplitude modulator at 11.4 GHz in the laser cavity and enable synchronization to a master clock. A very small net anomalous dispersion on the order of 0.4 ps/(nm-km) was obtained by adding dispersion-compensating fiber to the laser cavity and helped to control the spontaneous emission contribution to the timing jitter. The researchers expect that the low noise performance of the laser system, limited by the noise of the driving synthesizer, will qualify it for use in communication systems, optical analog-to-digital conversion, and other photonic applications. Contact Thomas Clark at [email protected].

Transfer standard is insensitive to input beam position

Researchers at the National Institute of Standards and Technology (NIST; Boulder, CO) have built and tested an improved transfer standard for the calibration of fiberoptic power meters for wavelengths between 700 and 1800 nm. A transfer standard is a detector assembly used to carry NIST standards to "NIST-traceable" metrology laboratories. With two germanium photodiodes and a spherical mirror arranged so that light strikes each photodiode twice, the assembly has a sensitivity as a function of beam position of less than ±0.15% over a 5 x 5-mm area—a factor of ten better than existing transfer standards.

Suitable for optical power measurement of either a single-mode fiber or monochromator with divergence of roughly 15°, the device functioned to an uncertainty of less than 1% in laser-based testing. To ensure thermal stability, the researchers mount the photodiodes on aluminum blocks attached to thermistors and cool each block with a thermoelectrically cooled, passive air heat exchanger. This arrangement maintains temperature to within ±0.1°C. The researchers aim to verify the device`s field of view and evaluate aging of the mirror and photodiodes. Contact John Lehman at [email protected].

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