May 1, 2000
Ultrafast laser writes frequency-conversion crystals in glass; Hydroxyl loading boosts photosensitivity in optical fiber; Multiple-longitudinal-mode CW laser generates efficient, low-noise UV light

Ultrafast laser writes frequency-conversion crystals in glass

Researchers at Japan Science and Technology Corp. and Kyoto University (both Kyoto, Japan) have focused ultrafast pulses into glass and created microscopic embedded frequency-conversion crystals. The glass contained oxides of barium, aluminum, and boron and was used to grow beta barium borate crystals. The beam from a regeneratively amplified Ti:sapphire laser emitting 4-µJ, 130-fs pulses at 800 nm with a 200-kHz repetition rate was focused to a 10-µm spot within the glass; after irradiation for 10 min, a crystal formed at the focus. By slowly moving the focus, long, thin crystals were created continuously within the glass. Such crystals are expected to form waveguide frequency-conversion devices for the ultraviolet region.

As the beam focus irradiated one spot within the glass, the developing crystal began to generate 400-nm second-harmonic light, which greatly increased in intensity over a 40-min period. After fabrication, the researchers used the crystal to double the frequency of other lasers. In a separate experiment, they created similarly sized lithium niobate crystals in a different glass. Contact Jianrong Qiu at[email protected].

Hydroxyl loading boosts photosensitivity in optical fiber

Researchers at ACREO (Kista, Sweden) have boosted the photosensitivity of germanium-doped silicate fiber by heat-treating hydrogen-loaded fibers. Standard telecommunication fiber was subjected to hydrogen at 105-bar pressure at 50°C for five days to achieve about 0.75-mol % hydrogen in the fiber. Subsequent rapid (1 s) heat-treating at 1000°C yielded hydroxyl concentrations in the core of the fiber as high as 2.5 mol %. To measure the sensitivity of the heat-treated fibers the researchers wrote 5-mm gratings in both treated and untreated fibers with 242-nm ultraviolet radiation. Heat-treated fibers showed an increase in photosensitivity over the untreated fibers by almost an order of magnitude. The increased photosensitivity was not observed in fibers that were not hydrogen loaded. The researchers found that the increase in photosensitivity increased monotonically with thermally induced 1.4-µm hydroxyl absorption. Contact Michael Fokine at[email protected].

Multiple-longitudinal-mode CW laser generates efficient, low-noise UV light

Engineers at Spectra-Physics (Mountain View, CA) have demonstrated resonant enhancement for efficient, stable, optical second-harmonic generation from continuous-wave (CW) multiple-longitudinal-mode lasers. This result is contrary to the widely held belief that a single-frequency laser source is required for resonant enhancement and efficient second-harmonic generation. Although there are several examples in the literature of resonant second-harmonic generation for modelocked lasers, this is believed to be the first demonstration for CW multiaxial-mode systems.

In cooperation with LAS (Stahnsdorf Berlin, Germany), the approach has been refined to produce up to 1 W of stable 266-nm output. This was achieved using a 5-W solid-state, quiet multiaxial-mode green laser and a stabilized external-ring build-up cavity with a 10-mm

Brewster-cut beta barium borate crystal for frequency doubling. An experimental enhancement factor of greater than 80 was achieved with a second-harmonic conversion efficiency of approximately 20%. The UV output demonstrated low-amplitude noise and high stability over several hours. Applications that would benefit from an all-solid-state UV laser include semiconductor-wafer inspection, disk mastering, fiber-grating production, and various medical applications. Spectra-Physics will exhibit a prototype of its laser at this month's Conference on Lasers and Electro-Optics 2000 (San Francisco, CA). Contact Bruce Craig at[email protected].

Light cools thulium-doped glass

Using optical refrigeration, researchers at the University of New Mexico (UNM; Albuquerque, NM) together with a team from Los Alamos National Laboratory (LANL; Los Alamos, NM) have successfully achieved net cooling of a thulium-doped solid. Net cooling by optical refrigeration was first observed in ytterbium-doped glass by the LANL team nearly five years ago. Thulium-doped systems are expected to be more efficient than their ytterbium counterparts. The UNM team measured a net cooling efficiency (temperature change per absorbed power) of 30°C/W in a 5 x 5 x 10-mm3 sample of 1% thulium-doped ZBLANP (a fluorozirconate glass) at a wavelength of 1.9 µm. The tunable source was a 7-W continuous-wave singly resonant optical parametric oscillator constructed at UNM.

Optical refrigeration in solids is promising for developing all-solid-state cryocoolers for diverse applications such as superconductors and infrared and gamma-ray detectors. These coolers are particularly well suited for space-borne sensors. Coolers based on both ytterbium and thulium doping are expected to reach cryogenic temperatures by increasing the absorbed power through longer effective path lengths. The researchers plan to exploit laser refrigeration in various glasses and crystals, as well as in semiconductors. Contact Mansoor Sheik-Bahae at[email protected].

Chromophore shape control is key to sub-1-V polymer modulators

Electro-optic (EO) polymers have been under development for several years, with research fueled by the need for high-speed, low-drive-voltage EO modulators for fiberoptic communication links. Although half-wave voltages of 5 V are common with polymeric and lithium niobate modulators, 0.8 V at the telecommunications wavelength of 1380 nm is now possible with a device built with polymers and developed at the University of Southern California (Los Angeles, CA) and the University of Washington (Seattle, WA). The modulator also achieves a half-wave voltage-interaction-length product of 2.2 V-cm. The keys to lowering half-wave voltages are sterically modified organic chromophores in the polymeric material that reduce the attenuation of electric-field poling.

Project head Larry Dalton reports that a single 1-µm-wide polymeric modulator can provide more than 300 GHz of bandwidth. In early tests, scientists at Tacan Corp. (Carlsbad, CA) and Lockheed Martin Corp. (Palo Alto, CA) used the devices, which were driven at less than 1 V, for high-speed electronic-to-optical signal conversion. The devices show promise for use in optical gyroscopes and radar-detection systems, as well as optical switches in fiberoptic communications networks. Contact Larry Dalton at[email protected].

Annealing improves Ge/Si heterojunction photodetectors

Epitaxial germanium (Ge) is often the material of choice for photodetectors due to its compatibility with silicon (Si) technology and its high absorption in the near-infrared up to 1.55 µm. One potential problem is the 4% lattice mismatch between the two materials, which is often resolved with the insertion of a superlattice buffer layer. Now, researchers at Terza University of Rome (Rome, Italy) and the Massachusetts Institute of Technology (Cambridge, MA) have used low-temperature-deposited Ge as a simple buffer layer for the growth of high-quality Ge on Si. They also have shown that a simple postgrowth cyclic thermal-annealing treatment can improve the characteristics of the resulting Ge/Si heterojunction photodetectors. Without annealing, the metal-semiconductor-metal photodetectors exhibited a responsivity as high as 240 mA/W at 1.32 µm. When simple cyclic thermal annealing was added after the ultrahigh-vacuum/chemical-vapor deposition process, responsivity reached 550 mA/W at 1.32 µm and 250 mA/W at 1.55 µm, with response time shorter than 850 ps. According to the researchers, the beneficial effect of the postgrowth thermal annealing on the electrical properties of Ge epilayers is due to the reduction of threading-dislocation densities. Contact Lionel Kimerling at[email protected].

Three-cascaded 1407-nm Raman laser fuels optical communications

A Raman fiber laser based on a single phosphorus pentoxide (P2O5) doped silicon fiber has been developed that provides successive generation of Stokes components associated with both constituents of the core glass (P2O5 and silicon oxide); frequency shifts are 1330 and 490 cm-1, respectively. According to researchers at the Fiber Optics Research Center, General Physics Institute of the Russian Academy of Sciences (Moscow, Russia), the laser can generate almost any wavelength of light up to 1.6 µm using no more than three conversion cascades. Output power of the diode-laser-pumped Raman fiber laser is 1 W, and slope efficiency is 35%, which may be adequate when the sharp dependence of the Raman fiber-laser efficiency on the loss level in the laser cavity is taken into account. Because of the interest in exploiting the 1450- to 1530-nm spectral region for optical communications, the laser is designed to serve as a pump unit for a 1500-nm Raman fiber amplifier. To obtain the necessary wavelength of 1407 nm in an efficient cascade laser based on a single p-doped silica fiber, the researchers used one P2O5 related Stokes shift and two silicon oxide ones. Contact Sergei Vasiliev at[email protected].

Continuous dynamic tuning achieved in high-power CGSELs

Researchers at the University of Arizona (Tucson, AZ) have achieved a continuous tuning range of 0.5 nm in a high-power circular grating surface-emitting laser (CGSEL) by injecting charge carriers into the second-order grating section. A transparent indium tin oxide (ITO) electrode was used to inject the current into the devices, which were fabricated in a strained triple-quantum-well, graded-index separate-confinement heterostructure. Epitaxial layers were grown on a gallium arsenide (GaAs) substrate by molecular-beam epitaxy, and a GaAs etch stop layer was incorporated 0.3 µm above the quantum wells to accurately position the gratings for optimal coupling. Radio-frequency sputtering was used to deposit ITO film onto the sample.

Threshold current for the devices was 28.5 mA, and wavelength tuning was performed at 350 mA. Output powers in excess of 220 mW were obtained, and the initial output wavelength was 982 nm. ITO current densities on the order of 25 A/cm2 were applied, and a wavelength shift of 1.0 nm was obtained over an increase in injection current from zero to 30 mA with the device operating at 110-mW output power. The tuning range had a discontinuity in the center, however, due to mode hopping as ITO current increased from 15 to 20 mA. Contact R. Scott Penner at[email protected].

Neodymium-doped lithium niobate yields multifrequency conversion

Researchers at the Universidad Autónoma de Madrid (Cantoblanco, Spain) have used neodymium-doped aperiodically poled bulk lithium niobate to obtain participation of a fundamental laser wavelength in self-frequency doubling and self-sum-frequency mixing, simultaneously. The researchers had originally intended to grow an unchirped bulk PPLN: Nd3+ crystal, but accidental chirping of the domain lengths during the growth process between 3 µm in the external part of the crystal and 6.75 µm in the middle part led to the aperiodic poling.

The crystal was placed in a 9.9-cm-long quasi-hemispherical laser cavity with a nearly flat dichroic input mirror and a spherical output coupler. A continuous-wave Ti:sapphire laser pumped the cavity at 812 nm in a TEM00 mode. Fundamental output was obtained at 1084 nm, and self-frequency mixing and self-sum-frequency mixing yielded 542-nm green light at 1.5 mW along with approximately 440 nm of blue light at 0.5 mW, respectively. Total fundamental-plus-visible output power was 30 mW. Contact Juan Capmany at[email protected].

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