Sept. 1, 1999
An erbium-doped fiber laser emitting at 1.55 µm has been used to directly pump an optical parametric oscillator (OPO) for the first time, according to researchers at the Optoelectronics Research Centre (Southampton, England). The OPO used periodically poled lithium niobate (PPLN) as the nonlinear medium. The erbium fiber has a large effective core area of 600 µm2 but operates single-mode due to careful control of the rare-earth doping; this allows high pulse energies. The laser is Q-

Erbium-doped fiber laser directly pumps OPO

An erbium-doped fiber laser emitting at 1.55 µm has been used to directly pump an optical parametric oscillator (OPO) for the first time, according to researchers at the Optoelectronics Research Centre (Southampton, England). The OPO used periodically poled lithium niobate (PPLN) as the nonlinear medium. The erbium fiber has a large effective core area of 600 µm2 but operates single-mode due to careful control of the rare-earth doping; this allows high pulse energies. The laser is Q-switched with an acousto-optic Bragg cell. The PPLN crystal has five parallel gratings with periods of 32.4-33.2 µm and is robust enough that no photorefractive damage occurs even at low temperatures. The OPO produces 8-µJ, 3.8-µm idler pulses at a 500-Hz pulse-repetition rate when pumped with 100-µJ, 60-ns pulses at 1.55 µm. Temperature tuning of the crystal gives signal and idler output ranges of 2.55-2.7 µm and 3.65-3.96 µm, respectively. Pump tuning over a 13.5-nm range with fixed crystal temperature produces a signal-tuning range of 40 nm. Potential applications include lidar, gas sensing, and spectroscopy. Contact Paul Britton at [email protected].

Silicon on gallium arsenide wafers resist thermal stress

Creating optoelectronic devices on wafers of gallium arsenide (GaAs) on silicon poses a challenge; although lattice-constant mismatch is a problem, the large difference in thermal-expansion coefficient between these materials means that even small temperature changes can cause catastrophic stresses in many devices. Researchers at the Massachusetts Institute of Technology (Cambridge, MA) say the key to marrying III-V materials to silicon is to recognize that optoelectronic devices are typically at least 1 µm thick, while silicon metal-oxide-semiconductor (MOS) transistors need be only a few tens of nanometers thick. At such a thickness, the silicon is much less sensitive to thermal expansion. With this in mind, the researchers produced 100-nm-thick-silicon single crystal layers bonded by intervening layers on 4-in.-diameter GaAs wafers.

Starting with a specially processed silicon wafer-a layer of buried oxide and a silicon top layer-the researchers deposited a layer of silicon nitride and another of silicon oxide on top, then gave it a chemical-mechanical polish treatment. They pressed this wafer together with a GaAs wafer at room temperature, then annealed the pair. Further etching produced an oxide-silicon-oxide sandwich 1.3 µm thick on the GaAs wafer. The researchers heated the new wafer to 700°C and saw no breakage. The method, they said, could lead to monolithic integration of GaAs-based optoelectronic devices with silicon CMOS electronics. Contact Clifton Fonstad at [email protected].

Imaging technique combines ultrasound with near-infrared diffusive light

Diagnostic ultrasonic imaging cannot always differentiate between benign and malignant breast lesions, which can have overlapping characteristics. Functional imaging with diffused near-infrared (IR) light can distinguish simple structures about 1 cm in size, but sharp edges are typically blurred by a few millimeters. Now researchers at the University of Connecticut (Storrs, CT) and the University of Pennsylvania (Philadelphia, PA) have constructed a prototype device that combines complementary features of both techniques. It reportedly achieves coregistration of acoustic and optical images with accuracy of 0.27 ±0.20 cm (about half of the device's image pixel size).

The two-dimensional ultrasound array at the center of the hand-held probe comprises 20 small piezoelectric crystals distributed in a rectangular 2 x 1.6-cm matrix. Each crystal works in pulse-echo mode to provide spatial images of tissue at various depths. The optical system includes 12 light sources derived from a near-IR diode laser and four detector fibers coupled to avalanche photodiodes. Measurement can be done at multiple source-detector positions to develop image-reconstruction schemes to determine absorption and scattering coefficients of the tissue volume at various slice depths. Contact Quing Zhu at [email protected].

Single crystal yields simultaneous red, green, and blue outputs

Researchers at the Autonomous University of Madrid (Madrid, Spain) have generated red, green, and blue laser radiation from a single neodymium-doped yttrium aluminum borate (Nd:YAl3(BO3)4) crystal operating at a fundamental wavelength of 1338 nm. Red light (669 nm) was obtained from the fundamental wavelength by frequency doubling. The green (505 nm) and blue (481 nm) outputs were obtained by self-sum-frequency mixing of the fundamental wavelength with laser radiation from the tunable Ti:sapphire pump laser at 807 and 755 nm, respectively.

The 5-mm-long crystal was cut at 30.8° with respect to the optical axis and had a neodymium concentration of 5.5%. The 9.9-cm-long laser cavity was formed by a 3-m-radius-of-curvature input mirror and a 10-cm-radius-of-curvature output mirror. At 560-mW pump power, output power was almost 150 µW in the red, more than 300 µW in the green, and just over 10 µW in the blue. The researchers expect to increase blue output power by realigning the crystal within the cavity and to improve overall efficiency by simultaneous phase-matching. Contact Daniel Jaque at [email protected].

Integrated optical device regenerates signal and converts wavelength

At the University of Bristol (Bristol, England) researchers have achieved simultaneous, all-optical signal regeneration and wavelength conversion using an integrated, semiconductor multiple-quantum-well device consisting of a semiconductor optical amplifier (SOA) and a distributed-feedback (DFB) laser. The SOA-DFB laser combination was fabricated in indium phosphide/indium gallium arsenide phosphide (InP/InGaAsP) with an 800-µm DFB laser section and a 500-µm SOA section. At 20°C the laser threshold current was 45 mA with a single-mode emission at 1553.5 nm. The device, which was fabricated at Nortel Networks (Harlow, England), was tested in an optical circuit illuminated by a 1548-nm continuous-wave laser with its output modulated into a 2.488-Gbit/s signal prior to launching into a 93-km span of standard single-mode optical fiber at an average power of 6.25 dBm. Upon emerging from the fiber span, the optical signal had acquired an error floor at a bit-error rate of 5 x 10-6. After signal regeneration and wavelength conversion from 1548 to 1553.5 nm, the error floor was removed and the regenerated signal exhibited a negative sensitivity penalty of 1.3 dB over the original input signal. Contact Marc Stephens at [email protected].

All-fiber instrument measures Fourier-transform spectra

Researchers from the NASA Langley Research Center (Hampton, VA) and the University of Rochester (Rochester, NY) have fabricated an all-fiber Fourier-transform spectrometer. The device measures the wavelength of signals passing through it based on the angle of outcoupled radiation from its fiber phase grating fabricated by ultraviolet (UV) irradiation. An in-line reflector produces a counterpropagating beam to interfere with the propagating beam in the fiber grating and project an interference pattern onto the fiber cladding, where it can be read by a charge-coupled device (CCD) or diode array. The researchers tested the concept using a fiber Bragg grating with a period of 533 nm and a peak UV-induced index change of 1.4 x 10-3. A tunable 780-nm external-cavity semiconductor laser provided the propagating signal, which was reflected by an uncoated cleave. The outcoupled light (779.87 nm normal to the axis of the fiber) was measured using a 16-bit CCD chip with 9-µm spacing. Proposed applications for the device include a compact wavelength meter or spectrum analyzer for source stabilization or network monitoring and for use as an optical-period monitoring tool during the fabrication of fiber Bragg gratings. Contact Mark Froggatt at [email protected].

Researchers see single photon

Measuring photons is difficult; light detectors annihilate them by converting them to electrical signals, and the Heisenberg Uncertainty Principle says that their position and momentum cannot be known simultaneously. Physicists studying the nature of light sometimes use optical quantum nondemolition, in which a signal laser beam is coupled to a meter beam in a nonlinear medium with a refractive index that depends on light intensity. Nonresonant couplings of the beams cause a phase shift, which is measured by optical interferometry.

Researchers at the Ecole Normale Supérieure (Paris, France) have adapted this method to measure a single photon repeatedly by replacing the meter beam with an atom. They used a niobium cavity cooled with liquid helium to make it superconducting so a photon would remain within it for up to a millisecond. The meter was a stream of rubidium atoms. If a photon was present in the cavity, it was absorbed by the atom and re-emitted, with a net energy change of zero. Though the photon was unchanged, the atom underwent a phase shift that could be measured through atomic interferometry. The researchers say this technique could lead to quantum logic gates, which would be important in designing quantum computers. Contact Serge Haroche at [email protected].

Aided by liquid crystal, light focuses light

Researchers at Nagaoka Institute of Technology (Nagaoka, Japan) have demonstrated an all-optical focal-length converter based on dye dissolved in a nematic liquid crystal (NLC). In effect a variable focal-length lens, the device absorbs polarized laser light of one wavelength, causing a positive focusing effect for laser light of orthogonal polarization and a different wavelength. The dye selected by the researchers absorbs 633-nm light from a helium-neon (HeNe) laser and moves the focus of a transmitted converging beam of 788-nm light emitted by a diode laser. The NLC is sandwiched and oriented between two parallel glass plates. A polarizing beam combiner sends both beams into the NLC, and a filter after the device passes only the 788-nm light.

By varying the intensity of the HeNe laser from 0 to 10 mW, the researchers can shift the focus position of the beam along the optical axis by 3 cm. They envision replacing the HeNe laser with a second diode laser and postulate using the device in active optical-fiber couplers, laser microscopes, and optical heads for data-storage systems. Contact Hiroshi Ono at [email protected].

Gallium arsenide modulates fiber transmission

Gallium arsenide/gallium aluminum arsenide (GaAs/AlGaAs) forms the basis for a multiple-quantum-well (MQW) in-line fiber intensity modulator fabricated by researchers at Stanford University (Palo Alto, CA) and the University of California (Davis, CA). The use of GaAs/AlGaAs is significant in that the material can be monolithically integrated with various optoelectronic components. The modulator-which includes an AlAs/AlGaAs distributed Bragg mirror-is attached to a side-polished single-mode fiber, with phase-matching achieved through high reflection of the mirror for a specific mode angle. By applying an electric field perpendicular to the MQW layers, their complex index of refraction is changed, varying intensity transmission through the fiber. Depending on the applied field, transmission ranges from 14% to 30%. According to the researchers, reduction of defects in the MQW should permit the application of higher fields and allow close to 100% modulation contrast. In addition, the 3-dB insertion loss can be eliminated by improved fiber splicing. The 1-nm bandwidth of the modulator can be enlarged by tapering the semiconductor waveguide, with calculations showing 20 nm possible. Contact Erji Mao at [email protected].

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