NEWSBREAKS
Intracavity frequency doubling produces yellow-red tunable light
A tunable yellow-red-emitting solid-state laser developed by researchers at the University of Otago (Dunedin, New Zealand) achieves an operating wavelength range of 587-654 nm with a pulse energy of 0.75 mJ at a peak wavelength of 620 nm. The researchers combine a room-temperature gain-switched chromium-doped forsterite laser tunable in the 1150-1350-nm range with a type-II intracavity potassium titanyl phosphate (KTP) crystal for second-harmonic generation. The intracavity configuration converts the fundamental wavelength to its second harmonic with 28% efficiency.
The forsterite rod is pumped longitudinally at 1064 nm by a lamp-pumped Q-switched Nd:YAG oscillator-amplifier emitting 55-ns pulses at a repetition rate of 20 Hz. A lens with a 500-mm focal length focuses the pump beam to a waist at the cavity high reflector, creating a 1-2-mm pump spot size in the forsterite. Versions include a 162-mm-long free-running plane-plane resonator and a 284-mm Brewster-prism-tuned resonator. Acceptance angle of the KTP was measured at 65 mrad. Applications include remote sensing, biomedicine, and laser isotope separation. Contact Iain McKinnie at [email protected].
Fourier-synthesized laser pulse train exceeds 250-GHz repetition rate
Researchers from the Kansai Advanced Research Center (Kobe, Japan) have Fourier-synthesized an optical pulse train with repetition rate of 257 GHz. A frequency-modulated sideband heterodyne technique was used to construct a stable homodyne optical phaselocked loop. Fourier synthesis of a laser pulse train-unlike modelocking-is not restrained by the cavity resonance frequency, external modulation sources, or the gain bandwidth of the laser medium. Such pulse trains might, say the researchers, be useful for time-resolved measurements and optical communications.
A semiconductor optical amplifier (SOA) provided four-wave mixing (FWM), which the researchers said simplified the experimental setup for stable operation by reducing phase-matching restrictions. Three independent continuous-wave pump diode lasers emitting near 1.53 µm were used, each with a 12-cm-long, 1200-line/mm grating-extended external cavity. Acoustic vibrations broadened the 100-kHz spectral linewidth of each laser to several hundred kilohertz. The FWM signals were generated by fiber coupling outputs of two of the lasers into the SOA. After the output pulse trains were amplified by an erbium-doped fiber amplifier, correlation of temporal waveforms of the second-harmonic generation indicated good agreement with theory. The researchers expect their method to exceed a 1-THz repetition rate using ordinary single-mode lasers once a more efficient SOA is used. Contact M. Hyodo at [email protected].
Melting mirror makes broadband passive Q-switch
Researchers at the University of Southampton (Southampton, England) are using a liquefying gallium mirror to passively Q-switch fiber lasers. They have discovered that the reflectivity of a gallium-glass interface becomes strongly nonlinear when its temperature is held just below the melting point of the metal (29.6°C). Optical intensities of 5-10 kW/cm2 incident on the gallium cause up to a 40% change in reflected light intensity. As an example of the broadband nature of the effect, the researchers have used the device to create a passively Q-switched erbium-doped fiber laser emitting at 1550 nm and a ytterbium-doped fiber laser emitting at 1030 nm.
The gallium mirror is formed by pressing a glass slide onto an initially molten gallium bead that is in direct thermal contact with a temperature-control unit. The laser cavity is defined at one end by Fresnel reflection from the fiber end and from the other by a coupling lens and the liquefying mirror. Pulse durations as short as 50 ns were achieved by careful cavity adjustment. Contact Periklis Petropoulos at [email protected].
Diode-laser array produces 240 W of continuous-wave output at 930 nm
A single monolithic diode-laser array emits the highest power ever achieved at 930 nm, according to Opto Power Corp. (Tucson, AZ). The 1-cm-wide array had a total emitting aperture of 4800 µm with a 70% fill factor and delivered 240 W of CW optical power at a drive current of 240 A. The laser array was mounted on a multilayer diffusion-bonded copper heat sink and operated with a cooling water temperature of 20°C.
The company also achieved what it said was the highest power density ever from a similar array at 808 nm. The device emitted 115 W of CW power at a drive current of 125 A. The device had an emitting aperture of 2600 µm with a fill factor of 26% and was operated with a coolant temperature of 20°C. Researcher Jim Harrison noted that the company does not have any reliability data yet and that these are not standard products. But such arrays could have uses as pump modules for high-power solid-state lasers, in materials processing, and in medical applications including hair removal. Contact Mike Atchley at [email protected].
Acute-angled fiber tip couples well with sphere
Because optical microsphere cavities confine light by total internal reflection, coupling light into and out of the cavities without ruining the properties of the cavity can only be done by evanescent coupling of light from an adjacent optical interface-an exacting process. Now, researchers at California Institute of Technology (Pasadena, CA) have developed a fiber coupler that excites high-Q whispering-gallery modes in microspheres, yet is simple, compact, and inexpensive.
When a single-mode fiber with its tip polished at a steep angle is positioned so that its core nearly touches the microsphere, an efficient energy exchange occurs between the waveguide mode of the fiber and the whispering-gallery mode of the microsphere. The researchers select the polish angle precisely to maximize phase-matching-and therefore coupling efficiency-with a typical example being a 77.5° angle and a coupling efficiency of more than 60% at 1300 nm. Input and output fiber couplers were used to create a microsphere narrowband filter with a free spectral range of 2.6 GHz at 1550 nm. The researchers say that customization of the fiber core cross section will further improve coupling efficiency. Contact Vladimir Ilchenko at [email protected].
Hot rubidium slows light speed to 90 m/s
Texas A&M University (College Station, TX) researchers have achieved light speeds as low as 90 m/s without the need for a Bose-Einstein condensate (BEC). The team created a BEC-like electronically induced transparency (EIT) using an optically dense environment of rubidium gas heated to about 360 K. An EIT is produced when a laser manipulates quantum states in an opaque cloud of gas, causing transparency for a narrow wavelength band. The index of refraction in the band depends strongly on wavelength and can slow the group velocity of light.
During the experiment a pulse of light traversed a 1-in. gas cell in 0.25 ms instead of the normal fraction of a nanosecond. Potential research applications for such lengthened interaction times between photons and atoms include performing sensitive nonlinear optics experiments without the need for high-intensity lasers or the expensive equipment required for a BEC. Potential device applications include the facilitation of quantum computers and all-optical communication systems. While a BEC has the advantage of producing much slower light (about 17 m/s), an EIT made from rubidium gas currently provides a larger volume and more interaction time. Contact Michael Kash at [email protected].
All-optical switching toggles semiconductor optical-amplifier-based flip-flop
Researchers at the Universities of Tokyo (Japan) and Rochester (NY) have demonstrated all-optical setting and resetting of bistable memory in a semiconductor optical amplifier (SOA) based flip-flop switch. The method is based on varying the hysteresis between stable states while maintaining fixed beam power in the state-holding laser. The researchers changed the hysteresis by varying the gain in the amplifier. Gain was decreased optically through gain saturation using a signal wavelength within the amplifier gain spectrum. Gain was increased optically by pumping with a signal wavelength outside of the amplifier gain spectrum.
When gain was increased, refractive indices decreased and moved the amplifier Bragg resonances away from the optical signal. The increased separation from resonance also increased the input power threshold for bistable switching. Similarly, when gain was decreased, the input power threshold for bistable switching decreased. Switching was accomplished whenever the switching threshold traversed the constant power level of the continuous-wave-state holding laser. A 1.48-µm diode laser provided the required optical power of about 100 µW. According to the researchers, low pulse-set powers (about 10 µW) over a wide wavelength range (more than 40 nm) indicate that optical signals in wavelength-division-multiplexed lightwave systems can set the memory. Contact Drew Maywar at [email protected].
Optically induced thermal hologram produces self-starting self-conjugating loop oscillator
Self-conjugating loop oscillators can be used for self-adaptive correction of distortions within a laser system because self-interaction of a single beam in a nonlinear medium forms a diffractive volume hologram. Researchers have used several mechanisms to produce nonlinear holographic elements, including saturable gain media such as Nd:YAG amplifier rods, saturable absorbers such as Cr:YAG crystals, and optically induced thermal gratings in nonsaturable absorbers. Santiago Camacho-López and Michael J. Damzen at Blackett Laboratory (Imperial College; London, England) have used a thermal nonlinearity to produce the holographic element in a self-starting holographic laser oscillator, an achievement they say may be a first.
The loop cavity includes a lamp-pumped, dual-rod Nd:YAG amplifier and an absorbing cell made with copper nitrate diluted in acetone. With the lamp switched on, partially coherent radiation is absorbed by the copper nitrate, which heats the acetone following the intensity of the interference pattern, changing the refractive index and forming diffraction gratings. With each round-trip, the radiation flux increases and the gratings are enhanced until the gain is saturated and a strong output signal is produced. The researchers say their system has high extraction efficiency and the ability to correct for intracavity phase distortions. Contact Santiago Camacho-López at [email protected].
Gamma rays confirm that speed of light is independent of frequency
Measurement of gamma-ray bursts from the edge of the known universe support Einstein's theory of relativity-and its insistence on a universal light speed-with a degree of precision never before seen. Basing his conclusions on the observed arrival of light from gamma-ray bursts that last only milliseconds, Bradley Schaefer at Yale University (New Haven, CT) estimated that the speed of light is independent of frequency to within a factor of 6 x 10-21. The best previous effort to locate a frequency dependency for light speed was at the 5 x 10-17 level and was deduced by observing light coming from the Crab pulsar, which, with its Milky Way galaxy location, is significantly closer to the Earth than the gamma-ray bursts.
According to Schaefer, one future observation that could suggest the opposite would involve proof that photons do in fact have a measurable mass, as was recently found to be the case for neutrinos. For now, though, he places an upper limit of 10-44 g on any putative photon mass. Contact Bradley Schaefer at [email protected].