Aug. 1, 2000
Diode-pumped Yb:YAG laser reaches 1080-W output; Ultrafast pulse mixing in glass yields colorful cascade; and VCSEL-based instrument senses oxygen reliably

Diode-pumped Yb:YAG laser reaches 1080-W output

Researchers at Lawrence Livermore National Laboratory (Livermore, CA) and Boeing Laser and Electro-Optical Systems (Canoga Park, CA) have built a diode-pumped Yb:YAG laser that emits 1080 W of continuous-wave power at an optical-optical efficiency of 27.5%. When

Q-switched, the laser produces 532 W of power with a beam quality (M2) of 2.2 and an optical-optical efficiency of 17%. The laser contains two Yb:YAG rods separated by a 90° quartz rotator for bifocusing compensation. Each rod is 50 mm long and 2 mm in diameter and includes two undoped flanged ends. The diode arrays can produce greater than 4.56 kW of pump power.

The resonator is symmetric and contains a negative lens as well as the rotator, partially compensating for thermal lensing in the laser rods. For best compensation, the negative lens must be changed for each power range. At its maximum 1080-W output, the laser has an M2 of 13.5 and requires 8780 W of electrical power—a wall-plug efficiency of 12.3%. Q-switching is achieved by inserting two acousto-optic switches into the resonator, resulting in 77-ns pulses. Higher pump powers should scale the Q-switched power to 850 W. Contact Eric Honea at [email protected].

Ultrafast pulse mixing in glass yields colorful cascade

Researchers at the Instituto Superior Técnico (Lisbon, Portugal) and the Centre Nationale de la Recherche Scientifique (Palaiseau, France) have simultaneously generated a series of broad-bandwidth light pulses extending from infrared to ultraviolet by intersecting two ultrafast light pulses at different wavelengths in a glass slide. A 618-nm pump beam with an 80-fs pulsewidth and a 516-nm signal beam with a 40-fs pulsewidth were both generated by a dye-laser-amplifier system. The two beams were directed so as to cross each other in a 150-µm glass slide, where the measured incident energy of 20 µJ in each beam yielded focused intensities of 1 TW/cm2 in the pump beam and 2.1 TW/cm2 in the signal. Output spectra were observed using a CCD camera and digitizing frame grabber to record the output of a spectrometer. Downshifted peaks from the pump and signal were observed at 690 and 750 nm, and 11 upshifted peaks were observed with the first three falling at 510, 470, and 440 nm. Peaks in the visible range could be visually observed because 5% to 10% of the total energy of the pump and signal beams was transferred to the cascade. Contact H. Crespo at [email protected].

VCSEL-based instrument senses oxygen reliably

Optical-absorption spectroscopy can replace the use of finicky electrochemical or amperometric cells for measurement of atmospheric oxygen concentrations. Although distributed-feedback laser diodes with high mode-hop stability have been demonstrated for this use, such lasers are expensive. Scientists at the Centre Suisse d'Electronique et de Microtechnique (Zurich, Switzerland) have fabricated a narrow-linewidth vertical-cavity surface-emitting laser (VCSEL) emitting at 763 nm specifically for measuring oxygen concentrations. The laser is inexpensive to make and has a wide mode-hop-free tuning range.

The laser emits in a single transverse mode—a requirement for gas sensing. Such operation was assured by incorporating a 5-µm-diameter aperture. Threshold current is typically 2.5 to 3.0 mA with a linear slope of 0.147 W/A, while the maximum single-mode output power is 1 mW. The device has a 3.4-MHz linewidth and a tuning range of 763.2 to 764.1 nm, varied by sweeping the current. Mounted in an instrument, the device reached a resolution of 0.2%/m, sufficient for medical use or industrial combustion monitoring. Contact Hans Zappe at [email protected].

Thin-film PBS has large spectral and angular ranges

Polarizing beamsplitters (PBSs) are widely used in optical systems for data storage, scientific measurement, lasers, and other applications. Conventional PBSs based on enhancement of Brewster-angle effects work well only over narrow spectral or angular ranges; although achromaticity can be gained by the use of birefringent devices such as Wollaston prisms, the property comes at the cost of a further narrowing of angular range. Researchers at the National Research Council of Canada (Ottawa, Ontario, Canada) have designed and built a PBS that operates using frustrated total internal reflection, in which internal thin films placed at greater than the critical angle transmit light as a result of evanescent coupling. Because the design is relatively insensitive to thin-film thickness variations, the PBS is easy to fabricate.

The device is hexagonal in shape to allow high internal-incidence angles. A simple prototype containing 27 thin-film layers was built to test the concept. Transmittance of s-polarization was close to 100% across much of the visible spectrum, while p-polarization transmittance reached to between 10-2 and 10-4 for an angular range of a few degrees.

Calculations show that such PBSs built with additional thin-film layers can attain either angular fields as high as ±15.8° in the visible and ±60° in the infrared or a wavelength range of 0.33 to 2.0 µm. Contact Li Li at [email protected].

Adaptive wavefront correction uses all-optical feedback

Researchers at the Mechanical Engineering Laboratory (Tsukuba, Japan), the University of Auckland (Auckland, New Zealand), and Industrial Research Limited (Lower Hutt, New Zealand) have built an adaptive wavefront-correction system based on an all-optical feedback interferometer. The two-dimensional output fringe pattern from a Mach-Zehnder interferometer with large radial shear is optically fed back to an optically addressed nonpixelized phase-only liquid-crystal spatial light modulator (SLM)—an interferometric setup that does not require a separate aberration-free reference wave.

For testing purposes, the researchers sent a plane wavefront from a 19.5-mW HeNe laser through a plate with several waves of aberration to produce a distorted input wavefront. To increase feedback-loop gain to the point where the system worked, the researchers adjusted the SLM for a response time of 60 ms and a resolution of 60 line pairs/mm. Without feedback, the wavefront was highly distorted—a focused spot exhibited a Strehl ratio of 0.22. When the loop was closed, the Strehl ratio reached 0.92. Contact Tomohiro Shirai at [email protected].

Liquid-crystal electro-optic switch achieves nanosecond speeds

Researchers at Cambridge University (Cambridge, England) have obtained a contrast ratio of 6.5:1 and an electro-optical response time of 162 ns in a single-pixel liquid-crystal (LC) device filled with an M68 electroclinic mixture based on the Bahr and Heppke A series of chloroester homologs. The researchers increased the switching speed of the homolog above A8 (the fastest in the A series) by mixing A8 with A6 (an analog of A8 with a C6 alkyl chain substituted for C8). This mixture provided the LC modulating layer that was sandwiched between an indium tin oxide glass layer and a reflective mirror. The device was tested at a temperature of 75.6°C with a 10-V operating voltage and yielded contrast ratios greater than five for twist angles in excess of 2.5°. The researchers expect device performance to improve with development of a device-fabrication procedure and precise alignment of the LC modulator material. Thickness variation in the LC material must also be controlled to a tolerance on the order of 0.01 µm to maintain the desired resonant behavior. The researchers expect this work to facilitate fabrication of fast-switching multipixel active-silicon-backplane spatial light modulators. Contact Huan Xu at [email protected].

Hard-x-ray microscope fits into laboratories

Microscopes that operate in the kiloelectronvolt (hard) x-ray region can be used for microtomography of copper integrated-circuit interconnects and other difficult-to-study objects; however, synchrotron-based x-ray microscopes, while powerful, are far too large for the average laboratory. Researchers at Purdue University (West Lafayette, IN), the National

Institute of Standards and Technology (Gaithersburg, MD), and J. Pedulla Associates (Silver Spring, MD) have developed a small hard-x-ray microscope that uses a copper-anode x-ray tube as a light source and a charge-coupled-device (CCD) camera for fast imaging.

Two spherical mirrors in a Kirkpatrick-Baez (K-B) grazing-incidence configuration image 8-keV x-rays onto a phosphor-coated CCD at 4-µm resolution. Multilayer thin-film coatings on the mirrors permit large grazing angles, reducing mirror diameters to 2.54 and 5.08 cm. The K-B optics unavoidably produce magnification that differs in orthogonal directions, resulting in an image aspect ratio of 3.25. For adequate resolution and reflection, the mirrors were superpolished to a 1-Å surface roughness and made spherical to Λ/40 at 630 nm. Image exposure times were on the order of 20 s. Future versions of x-ray CCD cameras that have smaller pixels will further increase resolution, say the researchers. Contact Stephen Durbin at [email protected].

Stimulated auroras aid in mapping lower ionosphere

Researchers from Cornell University (Ithaca, NY), the Naval Research Laboratory (Washington, DC), Geospace Research Inc. (El Segundo, CA), and the Arecibo Observatory (Arecibo, Puerto Rico) have taken detailed observations of the E-layer of the ionosphere (below 200 km) from the Arecibo Observatory by irradiating a passing group of low-lying metallic-ion clouds using a 3.175-MHz radio-wave source with an effective radiating power of about 80 MW. An artificial 557-nm airglow of up to 55 rayleighs was induced allowing visualization of the structures of metallic-ion clouds. The 10-day observation period was primarily focused on observing structures in the F-layer ionosphere (between 200 and 500 km), but a series of metallic-ion clouds at about 90-km altitude arrived unexpectedly and provided an unusually detailed view of the lower ionosphere. The stimulated emissions at primarily 630 nm from the F-layer and 557.7 nm from the E-layer were recorded using photometers and CCD cameras. Mapping ion clouds in the E-layer can provide useful planning information for radio communications and satellite-based navigation. The researchers have proposed repeating the observation during seasons when the formation of such ion clouds increases. Contact Michael Kelley at [email protected].

Eigensystem calculation simplifies dispersion compensation

Researchers at the University of Valencia (Valencia, Spain) have developed a procedure for designing photonic crystal fibers with nearly zero ultraflattened group-velocity dispersion. The researchers approached the problem of calculating dispersion of guided modes in a fiber by writing the vector equations for electromagnetic propagation as a two-dimensional eigensystem for the operators that act upon the magnetic and electric transverse-field components. This approach simplified the task of solving a system of differential equations into a standard matrix diagonalization procedure. It also provided a highly accurate vector calculation that allowed for an explicit inclusion of chromatic dispersion in the calculation. The design method also provided flexibility in manipulation system parameters, such as increasing flatness of the group velocity dispersion, while decreasing the wavelength window. The researchers hope that the design flexibility provided by this method will help to broaden the range of fiber designs with dispersion properties tailored to various optoelectronics and communications applications. Contact Pedro Andrés at [email protected].

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