April 1, 2002

Holograms are recorded at nanosecond speeds
Researchers at the California Institute of Technology (Pasadena, CA) have developed a holographic method that records fast phenomena. Previous fast holographic methods depended on electronics or mechanical scanners and were limited in speed. The new technique captures images with a temporal resolution of a few nanoseconds, and can be extended to subpicosecond resolution.

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A frequency-doubled, Q-switched Nd:YAG laser serves as the light source. The chosen volume holographic material has a high angular selectivity to allow the capture of multiple frames recorded with adjacent pulses. A cavity with a slightly angled mirror creates a series of pulses at different angles. For holographic recording, the researchers split off a portion of the laser light and focused it in air to create optical breakdown. A plasma was created, forming an expanding shock wave. The intensity of the focused beam was 5.2 x 1012 W/cm2; the two images were separated in time by 12 ns. Contact Zhiwen Liu at [email protected].

Vacuum spectrometer pins down fluorine laser lines
The next generation of microlithography tools under development is being designed around the 157-nm output of molecular fluorine (F2) lasers. Because the calcium fluoride and barium fluoride refractive materials used to focus 157-nm light have a high chromatic dispersion, the locations of the lasers' actual spectral lines must be determined very precisely to allow proper lens design.

Physicists in the National Institute of Standards and Technology (NIST) Atomic Spectroscopy Group (Bethesda, MD) have performed measurements of six wavelengths of a F2 laser of the type to be used for production of integrated circuits, with the measurements accurate to ±0.0001 nm. Three of the spectral lines were newly observed. The NIST 10-m vacuum spectrometer was used to record the spectra, which were calibrated by spectral lines from a platinum hollow-cathode lamp. Accurate wavelengths for this lamp had been determined previously. The experiment was conducted in collaboration with Lambda Physik (Ft. Lauderdale, FL), a major producer of F2 lasers. The results are being used by the microelectronics industry. Contact Joseph Reeder at [email protected].

Ytterbium-doped laser crystal generates shorter femtosecond pulses
Researchers at Macquarie University (Sydney, Australia) and the Australian National University (ANU; Canberra, Australia) have demonstrated a passively modelocked, diode-pumped, ytterbium-doped yttrium aluminum borate (Yb:YAB) femtosecond laser. The high-quality laser crystal, partly developed at Macquarie University, offers roughly twice the bandwidth of the common Yb:YAG crystal, as well as a strong and broad pump absorption band centered at 976 nm, ideal for diode pumping and low-quantum-defect lasing. In addition, Yb:YAB is a self-doubling laser crystal, allowing for compact intracavity green generation.

The folded z-shaped cavity contained the 2.5-mm-long crystal in the confocal portion. Pumped with a 4-W, high-brightness, 970-nm laser diode, the laser produced up to 700 mW of cw radiation at 1040 nm. Stable soliton modelocking was accomplished through the use of an ion-implanted semiconductor saturable-absorber mirror developed at the ANU. The laser produced pulses as short as 198 fs at an average output power of 450 mW, comparing favorably with the 340-fs pulses at 110-mW average power achieved with Yb:YAG. The researchers believe that Yb:YAB has considerable potential for high-power diode-pumped femtosecond generation and will find applications in areas such as micromachining. Contact Max Lederer at [email protected].

Visible chemical-laser amplifier looks promising
A candidate for a practical visible-light-emitting chemical-laser amplifier is being characterized by scientists at the Georgia Institute of Technology (Atlanta, GA) and the University of Illinois (Urbana, IL). The potential amplifier is based on a chemical reaction between sodium and metastable silicon monoxide that amplifies light at 569 nm. The time constant of the decay from the excited state is 80 ns, while the saturation intensity is estimated by the researchers to be 6 W/cm2, high enough to be practical.

The researchers developed a dual-beam spectroradiometric gain-measurement technique to measure gain of the medium at zero power. The scheme corrects for variations in source intensity and spectral density, detector spectral response, and gain variations in the detector and amplification stages. The probe beam had a path length of 5 cm through the reaction energy-transfer zone. The gain coefficient was measured to be 0.8 to 1.5 x 10-3 cm-1. From this information, the cw cavity output power of a sodium-based visible-emitting laser amplifier was estimated to be between 1 and 10 W. Contact James Gole at [email protected].

Microlens forces VCSEL to lase in single mode
Getting vertical-cavity surface-emitting lasers (VCSELs) to lase in a single transverse mode has been the focus of much research. Researchers at Seoul National University (Seoul, Korea) and Samsung Advanced Institute of Technology (Suwon, Korea) have devised a technique that consists of placing a microlens on top of an ordinary VCSEL to feed a small portion of its optical output back into the cavity, forcing single-transverse-mode emission.

An indium gallium phosphide microlens was deposited onto an 850-nm-emitting oxide-confined, gallium arsenide-based VCSEL using a one-step diffusion-limited wet-etching process. The lens had a curvature of 250 μm; the microlens and oxide-aperture diameters were 30 and 15 μm, respectively. The far-field pattern of the laser consisted of a single lobe, in comparison to the double-lobed pattern produced by a similar laser without the microlens. The lensed VCSEL showed no mode-hopping up to a power level of greater than 3 mW. Calculations show that the microlens, acting as a partially reflective concave mirror, discriminates between modes and causes the stable single-mode operation. Contact Heonsu Jeon at [email protected].

Pixelless thermal imager upconverts room-temperature scenes
A pixelless thermal imaging technique being developed by a group at the National Research Council (Ottawa, Ont., Canada) has reached new heights of practicality. A near-infrared-emitting, broad-area light-emitting diode (LED) is deposited directly onto a quantum-well infrared photodetector (QWIP); when a mid-infrared image is projected onto the QWIP, a spatially varying voltage is added to the forward bias of the LED, causing an upconverted image to be emitted by the LED at its 0.82-μm wavelength. The image can be captured by an ordinary charge-coupled-device (CCD) camera. Now the researchers have constructed a transmissive QWIP-LED imager that upconverts room-temperature infrared scenes.

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The 1-cm2 device was fabricated on a gallium arsenide (GaAs) substrate. Potentially deleterious local defects were removed by femtosecond laser ablation, then the structure was bonded to a piece of sapphire and the GaAs removed. The device is cooled to 63 K during use. The lens coupling the device to the CCD could be replaced with a fiberoptic faceplate to boost efficiency, bringing imaging speed to video rates, say the researchers. Contact Emmanuel Dupont at [email protected].

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