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All-solid-state laser emits 20.5 W at 266 nm; Dopant gives silver halide film an efficiency high; Distributed-feedback laser array provides selectable WDM wavelengths; and more.

Feb 1st, 2000

All-solid-state laser emits 20.5 W at 266 nm

An all-solid-state ultraviolet (UV) laser built by researchers at Mitsubishi Electric (Hyogo, Japan), Osaka University (Osaka, Japan), and KogakuGiken (Kanagawa, Japan) produces 20.5 W of 266-nm light. Consisting of a green-emitting Q-switched diode-pumped Nd:YAG laser and a 15-mm-long cesium lithium borate (CLBO) nonlinear crystal cut at 62° for type-I fourth-harmonic generation, the laser system emits pulsed fourth-harmonic light at a repetition rate of 10 kHz. The green beam has a pulsewidth of 80 ns and a beam quality (M2) of 10 and is 0.3 mm in diameter as it enters the CLBO crystal. The crystal itself has a surface roughness of 0.59 nm rms and is heated to 140°C to prevent hygroscopic deterioration.

The UV power increases as the square of the input green power, while the UV beam quality matches the input M2 of 10. Emitting UV pulses of 46-nm duration, the laser reaches its highest UV output at a green optical input of 105.8 W—a conversion efficiency of 19.4%. Because no saturation or damage has been observed, the researchers believe that increasing the green power will boost the UV output further. Contact Tetsuo Kojima at kojima@lap.crl.melco.co.jp.

Dopant gives silver halide film an efficiency high

Conventional silver halide photographic film loses a large percentage of its photoinduced electrons to fast recombination with holes, thus reducing the film's light-capturing efficiency. Researchers at the Université Paris-Sud (Paris, France) and Agfa-Gevaert (Mortsel, Belgium) have come close to eliminating this problem by doping the silver halide with formate ions. The dopant acts to scavenge holes, preventing the recombination of electron-hole pairs. In addition, the dopant itself produces an electron that acts as a photoinduced electron, raising the theoretical yield by a factor of two. This doubling, which takes place over a span of minutes after the actual exposure, acts strictly in proportion to the light received and thus does not register as noise.

A comparison of the sensitivity of doped and undoped emulsions—after a postexposure delay of 20 minutes—showed that a factor of 10 fewer photons is required to develop half the population of silver halide grains in doped versus undoped film. Exposure efficiency, measured as the ratio of atoms in a silver halide crystal to photons received by the crystal, reached 1.5 in the doped emulsion—close to the theoretical maximum of 2.0. Contact Jacqueline Belloni at jacqueline.belloni@lpcr.u-psud.fr.

Distributed-feedback laser array provides selectable WDM wavelengths

Researchers at Fujitsu Laboratories and Fujitsu Quantum Devices (both Atsugi, Japan) have designed and fabricated a wavelength-selectable laser by monolithic integration of eight distributed-feedback (DFB) lasers into a quarter-wavelength-shifted, eight-channel DFB array. The 0.6-mm-wide, 2-mm-long device demonstrated performance characteristics comparable to those of discrete DFB lasers. Fiber-coupled output power climbed to +10 dBm with good uniformity, narrow linewidth, and a side-mode suppression ratio of up to 50 dB. Emission wavelengths for the array started at 1535 nm and increased by 3.18 nm for each successive DFB laser, with a standard deviation for wavelength spacing of 0.12 nm. The mean threshold current was 7.9 mA, with a 0.3-mA standard deviation. The DFB laser array was integrated with a compact, low-loss multimode-interference combiner circuit and a semiconductor optical amplifier. The entire device was fabricated in a high-index-contrast buried-waveguide structure to minimize the insertion loss and length of the optical combiner. The high-index contrast also allowed compact fabrication of the bent waveguides that coupled the DFB laser array to the combiner. The waveguide was fashioned out of a 200-nm-thick gallium indium arsenide phosphide layer with a 1.3-µm bandgap. Contact Martin Bouda at bouda@flab.fujitsu.co.jp.

Gallium phosphide lens attains numerical aperture of 2.0

Researchers at Yale University (New Haven, CT) and Digital Instruments (Santa Barbara, CA) have built a solid-immersion-lens (SIL) microscope that reaches a numerical aperture (NA) of 2.0 at visible wavelengths—33% higher than that of oil-immersion objectives and previously reported SILs. At the core of the instrument is a hemispherical SIL made of gallium phosphide, which has a refractive index of 3.42 at 560 nm. The lens has a 500-µm radius, is virtually achromatic, and is used in conjunction with a 0.8-NA commercial microscope objective.

When imaging 40-nm fluorescent polystyrene balls at wavelengths of 560, 645, and 720 nm, the device achieves resolutions of 145, 180, and 183 nm, respectively (defined by the full width at half maximum). Deconvolving the object diameter from the measurements results in resolutions of 139, 175, and 178 nm and effective NAs of 2.05, 1.88, and 2.06, respectively. The researchers hypothesize that a 60-nm gap between the balls and the SIL surface prevents the lens from reaching its theoretical NA of 2.50. If made of higher optical quality and used with a 0.9-NA objective, a spatial resolution of 100 nm should be possible with the lens. Contact Qiang Wu at qiang.wu@yale.edu.

Laser diodes emit at 450 nm for an estimated 200 hours

Laser diodes made of indium gallium nitride (InGaN) multiple quantum wells emitting between 390 and 420 nm have achieved continuous-wave (CW) operation at room temperature of more than 10,000 hours. While these violet laser diodes are useful for optical storage devices, applications such as laser-based full-color displays require a wavelength of 450 nm for a true blue color. The wavelength can be increased by raising the amount of indium in the InGaN well layers, but this causes the threshold current density to increase dramatically.

Shuji Nakamura and colleagues at Nichia Chemical Corp. (Kaminaka, Japan) found that they could not achieve room-temperature CW operation with two or three quantum wells, but with a single-quantum-well structure they could make diodes that emitted at 450 nm with an estimated lifetime of 200 hours. This lifetime, which is too short for commercial purposes, is probably due to poor crystal quality of the well layer, Nakamura said. If the problems of 450-nm diodes can be solved, Nakamura said it should be possible to fabricate InGaN-based laser diodes at longer (green) wavelengths, where no such devices currently exist. Shuji Nakamura has recently accepted an appointment to the faculty of the University of California at Santa Barbara (see p. 59 of this issue for details).

Orbital shaker polishes optics with beads

Researchers at the Optical Sciences Center of the University of Arizona (Tucson, AZ) are exploring an alternative to conventional optical surface-polishing techniques. Using an orbital shaker, the new process imparts an oscillating x-y motion to a slurry comprising beads, a polishing compound, and water. One benefit is that the shape of the polishing implement sets fewer limits on the shape of the surface—for example, it may be possible to polish the inner surface of a hypersphere that cannot be polished conventionally due to the difficulty of inserting and driving a polishing tool. The beads, which basically roll across the surface of the substrate to be ground or polished, can have several geometries, including spherical, irregular, or planar shapes. They also can be made from a variety of materials, with specifics depending on the optics material. For 25-mm-diameter, initially plano fused-silica samples, the researchers report the process produced average peak-to-valley nonuniformity (mostly power) on the order of 0.1 µm/day, with a material removal rate of 1.4 µm/day. The typical surface roughness was 3.5 nm after three days. Subsurface damage was removed in approximately 3.5 days without generating additional damage. Contact José Sasián at jose.sasian@optics.arizona.edu.

Long-haul 40-Gbit/s soliton transmission is without active inline control

A research group at KDD R&D Laboratories (Saitama, Japan) has demonstrated 40-Gbit/s single-channel soliton transmission across 10,200 km without any active inline transmission control. In the transmission-control scheme developed by the researchers, the cumulative chromatic dispersion is offset periodically along the system by dispersion-compensation fiber with negative (normal) dispersion. Total system dispersion is close to zero.

Good transmission performance depends on use of a dispersion map where both the Gordon-Haus timing jitter and soliton-soliton interaction are effectively suppressed. Polarization-division multiplexing is used only in the transmitter to further reduce soliton-soliton interactions. According to the researchers, the technique requires only dispersion-compensation fibers and wideband optical bandpass filters in the transmission line and not the ultrahigh-speed optical modulators and electronics required in earlier long-haul transmission experiments exceeding 10,000 km. For more information, see www.Lab.kdd.c.jp.

Nanoholes increase x-rays produced with femtosecond pulses

Short-pulse x-rays can be produced by directing femtosecond laser pulses at an alumina target to create an x-ray-generating plasma. The conversion efficiency of such a setup is limited, however, because most of the laser pulse is reflected. To increase the efficiency, researchers at NTT (Kanagawa, Japan) and Tokyo Metropolitan University (Tokyo, Japan) made an array of nanoholes in the target. They anodized the target by placing it in an oxalic-acid solution and running a constant voltage through it, creating pores approximately 40 nm in diameter. Dipping the target in phosphoric acid widened the holes by an amount depending on how long the dipping lasted. In a solid target, the energy of the laser pulses penetrates to a depth of about 50 nm. But adding cylindrical pores perpendicular to the surface increases the penetration depth, at the same time increasing the area that interacts with the laser pulses. If the walls of the pores are made thinner than the penetration depth, the whole volume of the material can be heated. The researchers enhanced soft-x-ray fluence by about 30 times in the 5-25-nm range and by more than 50 times around 6 nm. Contact Tadashi Nishikawa at nisikawa@will.brl.ntt.co.jp.

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