sFiber-feedback OPO delivers femtosecond pulses
A collaboration between Eidgenössische Technische Hochschule Zürich (Zürich, Switzerland) and the Optoelectronics Research Centre (Southampton, England) has resulted in a very compact, synchronously pumped optical parametric oscillator (OPO) that contains an optical fiber to provide feedback of the oscillating signal wave. The OPO is based on periodically poled lithium tantalate pumped by a recently developed high-power modelocked Yb:YAG thin disk laser. The first device, based on nonoptimized components, generates 900-fs pulses at 1.43 to 1.47 µm with up to 2.7 W of average signal output power when pumped with 8 W. The repetition rate is 35 MHz.
Notable is the extreme insensitivity of the OPO to intracavity losses, a result of its very high parametric gain. A deliberate insertion of 10 dB of loss into the cavity had only a small effect on efficiency. Other advantages of the fiber-feedback OPO are its compactness and the greatly reduced sensitivity to resonator-length changes compared to other synchronously-pumped OPOs. Future prospects include soliton formation and pulse compression via nonlinearity in the feedback fiber. The fiber-feedback OPO appears to be very suitable as a key element in a red-green-blue display system. Contact Rugiger Paschotta at[email protected].
Grating-waveguide structures give narrow spectral bandwidths
Investigations of resonance anomalies in grating-waveguide structures (GWSs) by researchers at the Weizmann Institute of Science (Rehovot, Israel) have led to very narrow spectral-resonance bandwidths of 0.12 nm at FWHM in the 1.55-µm range, ideal for spectral filters and modulation applications. The multilayer configuration of the GWSs was changed so that the grating layer is separated from the waveguide layer by a buffer layer that serves also as a stop layer of the etching process. By separating the waveguide and grating layers, the thickness of the layers are controlled to atomic accuracy, lowering inherent losses. The optical and geometrical parameters were predicted based on the exact eigenfunction using Maxwell's equations. To achieve the configuration, a waveguide layer of indium gallium arsenide phosphide (InGaAsP) was grown on a substrate of indium phosphide (InP) by metal-organic chemical-vapor deposition. Then a buffer layer of InP and a layer of InGaAsP were grown. A grating was formed in the upper InGaAsP layer and a cladding of InP deposited, smoothing the surface roughness at the grating/buffer interface. The resulting finesse was 13,000, and the peak reflected intensity was 86%. Improvements in fabrication may further reduce losses. Contact Guy Levy-Yurista at[email protected].
Pulsed laser deposition creates high-quality PTFE films
Researchers at the Johannes Kepler University (Linz, Austria) have used pulsed laser deposition for the fabrication of high-quality thin films of polytetrafluoroethylene (PTFE, or Teflon). Deposition was performed from sintered-powder PTFE targets by means of 248-nm krypton fluoride excimer-laser radiation. Metallic, semiconducting, and insulating substrates were used. The background atmosphere was argon. The deposited films are pure Teflon and highly crystalline and they consist of large spherulites with diameters of up to more than a millimeter. Because of the outstanding physical and chemical properties of PTFE, such films are desirable for many applications, for example as a low-dielectric-constant material in microelectronics, as an electret in electroacoustics and electromechanics, and as an inert coating in biomedical applications. Negatively charged films exhibit an exceptional charge stability with practically no decrease of the surface potential up to 225°C in open-circuit thermally stimulated discharge. Contact Dieter Bäuerle at[email protected].
Tapered standard fiber produces supercontinuum
Ultrafast pulses propagating through properly constructed photonic-crystal optical fibers can generate supercontinuum light (see Laser Focus World, July 1999, p. 17). Such an output can be useful for optical-coherence tomography, optical-frequency metrology, and pulse compression. However, photonic-crystal fibers must be custom-made and so are not widely available. Researchers at the University of Bath (Bath, England) have produced supercontinuum light from a standard single-mode telecommunications fiber, potentially easing access to such a light source.
The one added fabrication step, tapering a section of the fiber, can be done simply by heating the section in a flame and stretching it. The researchers tapered a 90-mm section of fiber to a 2-µm diameter, with the taper waist connected at both ends to untapered fiber by taper transitions. Pulses at 850 nm with energy of 3.9 nJ and 200 to 500 fs duration from a Ti:sapphire laser were launched into the fiber. The untapered fiber had a numerical aperture of 0.1 and a cutoff wavelength of 1250 nm; within the taper, the light spread to fill the entire taper diameter. The output bandwidth reached from 370 to 1545 nm at the 20-dB level. Contact Timothy Birks at [email protected].
Sapphire provides base for AlGaN solar-blind photodiodes
Researchers at the Microelectronics Research Center at the University of Texas (Austin, TX) have grown aluminum gallium arsenide (AlGaN) heteroepitaxial back-illuminated solar-blind PIN photodiodes on (0001) sapphire substrates. The AlGaN compounds work well for ultraviolet photodetectors—especially in airborne and space applications—because the materials do not require heavy and expensive filtering systems for long wavelengths. The main fabrication problem is cracking due to the substantial lattice mismatch between Al0.6Ga0.4N (window layer), Al0.4Ga0.6N (active layers), and GaN. There also can be p-type doping difficulties related to the large activation energy necessary for the ionization of magnesium acceptors with high-Al-content AlGaN alloys. To resolve these issues, the heterostructures were grown by low-pressure metal-organic chemical-vapor deposition. The scientists used a two-step growth procedure to optimize the optical and electrical characteristics of the various component materials. They eliminated cracking problems and were able to obtain activated p-type active layers. The back-illuminated devices exhibit very low dark-current densities and external quantum efficiencies up to 35% at the peak of the photoresponse, which occurs at a wavelength of 280 nm. Contact Russell Dupuis at[email protected].
Laser aids assembly of hybrid microelectromechanical devices
Hybrid microsystems must be made by combining parts produced on substrates of different materials. The relevant assembly methods normally involve manipulating individual components, which limits high-volume production. In results presented at this year's International Congress on Applications of Lasers and Electro-Optics (Oct. 2-5; Dearborn, MI), researchers from Imperial College (London, England) described a method for batch assembly of microelectromechanical devices from parts fabricated on separate substrates. The components, which are fabricated on an optically transparent carrier, are released and transferred to a target wafer by excimer-laser ablation of a polymer sacrificial layer using laser light incident through the carrier, with residual sacrificial layer material and ablation debris removed by plasma or laser-assisted cleaning. The method differs from other hybrid batch-assembly techniques because components are maintained in ordered arrays throughout the process, eliminating the need for any sorting or reordering. The process also can be applied selectively by exposing the carrier through a mask for sequential assembly. In addition, more than one type of component can be fabricated on the same carrier. Contact Andrew Holmes at[email protected].
Sum-frequency generation surpasses 200 nJ with CLBO crystals
Researchers from the Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy (Berlin, Germany) and the Kogakugiken Co. Ltd., (Kanagawa, Japan) have used a 150-µm-thick cesium lithium borate (CLBO) crystal to achieve sum-frequency generation on a femtosecond time scale. The researchers produced 100-fs pulses that were tunable between 175 and 180 nm and at energy levels from 200 to 400 nJ with a 1-kHz repetition rate.
The 100-fs pulses were supplied by a 1-kHz Ti:sapphire regenerative amplifier. Sequential fourth-harmonic generation took place in three barium borate crystals. A continuum-seeded, near-infrared, traveling-wave optical parametric amplifier was used to satisfy the phase-matching conditions for CLBO. The results achieved presented a fivefold improvement in output energy over lithium borate, due to the higher effective nonlinearity of CLBO, according to the researchers. They also expect to improve wavelength range and energy output further by cutting CLBO samples at larger angles. Contact Valentin Petrov at[email protected].
Excimer and probe beams measure UV coatings
Using a top-hat excimer-laser beam spot and a helium-neon (HeNe) laser probe beam, researchers at Friedrich-Schiller-Universität Jena (Jena, Germany) are measuring the thermoelastic and thermo-optic responses of ultraviolet (UV) dielectric coatings both below and above the laser-induced damage threshold. Such measurements are valuable for improving UV thin-film coatings.
A 475-µm-diameter spot with top-hat intensity profile from a 248-nm krypton fluoride excimer laser briefly produces a thermally or damage-induced bump on the coating's surface. A 1.7-mm-diameter Gaussian HeNe beam reflected off the same area strikes a photodiode with an aperture small enough to pass only the center of the HeNe beam. The bump alters the mode structure of the HeNe beam, causing the central intensity to drop. The signal decay reveals thermal characteristics of the coating and substrate. A permanent offset in the signal shows damage. If the area is probed in transmission, the nonlinear absorption coefficient of the coating can be measured. The technique can detect bump heights as small as 0.002 nm. Optimization of the mode mismatch between the probe beam and the bump produces the best results. Contact Bincheng Li at[email protected].
Sequentially adsorbed PPV and C60 yield photovoltaic devices
Researchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA), the University of Massachusetts (Lowell, MA) and National Taiwan University (Taipei, Taiwan) have used poly(phenylene vinylene) (PPV) and buckminster fullerene (C60) to fabricate a circuit of rectifying photovoltaic heterostructure device analogous to a capacitor and resistor in series. Layer-by-layer sequential adsorption was used to build the heterojunctions from solution. Current and voltage photo-responses were observed upon laser illumination that increased with incident laser power and photon energy to a high-end photovoltage response on the order of 800 mV.
The actual electron donor and acceptor layers consisted of PPV/poly(acrylic acid) and C60/poly(allyl amine hydrochloride), respectively. And they were built on glass slides patterned with 200-nm-thick stripes of indium tin oxide (ITO). Aluminum electrodes evaporated orthogonal to the ITO provided individually addressable cells and photostimulation was applied at 457, 488 and 514 nm. Potential short-term applications include fabrication of sensitive solar cells with low dark currents and high open circuit voltages, according to the researchers. Potential long-term applications include facilitating study of charge transfer processes between conjugated polymers and C60. Contact Hedi Mattoussi at[email protected].