Newsbreaks

Oct. 1, 2000
Electroholographic optical switch works at 30-ns speed; Lead sulfide passively Q-switches erbium:glass laser; Polymer waveguide amplifier achieves 8 dB of optical gain ...

Electroholographic optical switch works at 30-ns speed
In results announced at the National Fiber Optic Engineers Conference (August 2000; Denver, CO), researchers at Trellis Photonics (Columbia, MD) showed they have achieved submicrosecond switching speeds in an optical switch designed for high-speed optical cross-connects. The switch is based on potassium lithium tantalate niobate (KLTN), a photorefractive crystal in which a wavelength-specific hologram can be written and then electrically switched on and off. By assembling KLTN crystals into a matrix of rows and columns with columns representing incoming wavelengths and rows representing exiting fibers, any wavelength can be routed to any fiber. Signals also can be dynamically tested, measured, and attenuated.

The KLTN crystal is para-electric and shows a quadratic electro-optic effect. An applied electric field transforms a space charge into birefringence, causing diffraction to occur. A 4 x 4 switch configuration made of three crystals with four holograms per crystal exhibited a switching speed of 30 ns and an overall attenuation of 3.5 dB. Bit-error-rate measurements carried out at 1.25 Gbit/s show that the power penalty induced by the electroholographic mechanism is less than 1 dB. The device should enable optical packet switching, say the researchers. Contact Aharon Agranat at[email protected].

Lead sulfide passively Q-switches erbium:glass laser
Using a phosphate glass doped with lead sulfide (PbS) quantum dots as an intracavity saturable absorber, scientists at the International Laser Center (Minsk, Belarus) and the St. Petersburg State Technical University (St. Petersburg, Russia) have achieved passive Q switching of a 1.54-µm flashlamp-pumped erbium-doped glass laser. With saturation of the lowest-energy quantum-confined transition of 6-nm PbS quantum dots, the research team produced Q-switched pulses with 110-ns duration and 0.2-mJ energy without intracavity focusing elements. The 0.8-mm-thick glass Q switch had a small-signal transmission near 81% at 1.54 µm without antireflection coating. The 170-mm-long cavity of the laser consisted of a concave high-reflector with a 200-mm radius of curvature and a flat output coupler with 78% reflectivity at the emitting wavelength. The laser rod was erbium-doped ytterbium:chromium:phosphate glass with an erbium ion concentration of 1.6 x 1019 /cm3 and an emission cross section of 7 x 10-21 cm2 at 1.54 µm. The PbS saturable absorber was placed between the gain medium and the concave mirror so that no intracavity focusing was necessary to produce Q-switched pulses. Contact Alexander Malyarevich at[email protected].

Polymer waveguide amplifier achieves 8 dB of optical gain
Researchers at the University of Texas (Austin, TX) demonstrated about 8 dB of optical gain at 1.06 µm in a rare-earth-doped polymer waveguide. In their experimental setup, the researchers used neodymium-chloride-hexahydrate dopant molecules to provide photoluminescence in a 5-cm-long, partially fluorinated, multimode channel waveguide made of polyimide. The typical density of neodymium ions within the doped layers of the polymer was 1019/cm3, and the refractive index of the doped polymer was 1.57 at 633 nm.

An 800-nm continuous-wave Ti:sapphire laser provided the pump source for the experimental setup and the 1.06-µm output of a single-mode continuous-wave Nd:YAG laser provided the signal. The 8 dB of optical gain was obtained at about 110 mW pump power and 90 mW signal power. A gain peak was observed at the 800-nm absorption peak of the neodymium (Nd3+) dopant ions, and the researchers expect to increase optical gain by optimizing dopant density, pump power, and signal power. Contact Ray Chen at[email protected].

Solid-state laser crystal measures force
Precise measurements of forces and force-related magnitudes (pressure, mass, torque, acceleration) are frequent tasks in technology and science. New research results at the University of Kassel (Kassel, Germany) show that mechanical forces can be converted with high precision into electrical signal frequencies by means of the photoelastic effect in Nd:YAG laser crystals. The magnitude and direction of the force vector can be simultaneously detected. The researchers have shown that both static forces and time-dependent forces with rise times and periods up to the microsecond range are detectable at high resolution and broad measurement range. The monolithic laser crystal, mirrored at its end faces and pumped with a low-power laser diode, detects forces over nine orders of magnitude. The conversion sensitivity of the photoelastic effect strongly increases if laser crystals of smaller dimensions (for example, microlasers with resonator lengths in the millimeter and submillimeter range) are used. By varying the crystal dimensions within the technically possible limits, therefore, a measurement range of 15 orders of magnitude (nanonewton to meganewton) can be achieved. The force sensors are potentially useful for high-precision weighing and pressure measurement, as well as in vibration detection and inertial navigation. Contact Wolfgang Holzapfel at[email protected].

Fast, accurate prism-based tuning system exceeds 10-GHz range
Researchers at the National Center of Scientific Research (Orsay, France) have designed an extended laser cavity that provides modehop-free tuning over more than 10 GHz with high linearity and reproducibility. The researchers used an electro-optic prism to synchronously tune both the cavity-mode frequency and the grating feedback frequency in a Littrow-type, extended-cavity laser-diode system. Unlike tuning systems based on motors that provide broad range at low speeds, fast and broadband tuning systems based on piezoelectric mounts that lack reproducibility; or tuning systems based on traditional electro-optic crystal designs that provide high speed tuning over mode-hop limited ranges, the prism system provides the combination of broad bandwidth, high speed, and accurate performance needed for increasingly important applications such as photon-echo-based optical signal processing.

The experimental device consisted of a 25° prism cut from a 5.2-mm thick by 25-mm wide lithium niobate crystal, an 800-nm laser diode with an antireflection coating, an aspheric lens for beam collimation, and a 2400-line/mm grating. With design improvements, the researchers expect to achieve scan ranges of several hundred gigahertz within time periods on the order of microseconds. Contact Ivan Lorgeré at[email protected].

Photodetector combines linewidth narrowing and high quantum efficiency
While resonant cavity-enhanced photodetectors have been the focus of recent research related to wavelength-division multiplexing (WDM), they have some limitations. Although there is no trade-off between response speed and quantum efficiency, as with conventional photodetectors, there is another trade-off between the spectral response linewidth and the quantum efficiency. A device with a quantum efficiency exceeding 50% often has a spectral response linewidth wider than 20 nm, making the photodetector unacceptable for WDM applications. Now researchers at the Beijing University of Posts and Telecommunications (Beijing, China) have fabricated a wavelength-selective photodetector with filtering, spacer, and absorption subcavities that offers simultaneous linewidth narrowing and quantum efficiency. With ideal conditions, the scientists believe spectral response linewidth could reach less than 1 nm and quantum efficiency could go as high as 90%. Under practical conditions, the photodetector has a tolerable mode mismatch range of 3 nm, and an acceptable range in variation of approximately 100 nm from the optimal thickness of its spacer cavity. In addition, its filtering cavity loss can be compensated for, to some extent. During experiments, scientists obtained a spectral response linewidth of approximately 0.97 nm and an external quantum efficiency exceeding 50%. Contact Yongqing Huang at[email protected].

Metal and PTFE form multicolor pixelated array
Researchers at the Hochschule Zürich (Zürich, Switzerland) have fabricated pixelated multicolor polarizing films on glass, a step toward their use in flat-panel displays. The films consist of silver or gold coated onto a poly (tetrafluoroethylene) (PTFE) layer on glass. The PTFE layer consists of submicron-wide strips laid down by sliding a PTFE rod across the glass at a temperature of 350°C and a speed of 100 mm/s. When gold or silver is coated on top, the metal forms into 20- to 40-nm particles in the case of silver, and strips mixed with 10- to 20-nm particles in the case of gold.

The silver coated samples appear blue in transmitted light with polarization parallel to the strips and red for perpendicular polarization. Similarly, the gold coated samples appeared yellow for parallel and green for perpendicular polarization. The coating process allow for easy fabrication of pixels with the help of masks. The researchers created structures containing some gold and some silver pixels (all of 250-µm size), resulting in a multicolor array.

Substrates of alternating polar and nonpolar lamellar structures could also be used as a base for metal coating, say the researchers. Contact Walter Caseri at[email protected].

Carbon aerogel makes first-rate infrared absorber
Highly absorbent materials find use in optical systems as baffles, light traps, and blackbody absorbers. Materials that absorb in the near to mid-infrared are useful in thermal imaging systems. Researchers at the Naval Research Laboratory (Washington, DC) have characterized an infrared absorber with qualities that compare favorably with those of existing absorbers. The material, a carbon aerogel, is made by drying a gel including formaldehyde and resorcinol under supercritical carbon dioxide, then pyrolyzing the resulting substance by slow heating to 1288 K under flowing argon.

The aerogel's directional hemispherical reflectance (DHR)—a measure that includes both diffuse and specular reflectance—was measured over the 2.5 to 14.3-µm wavelength region. The measured DHR was 1.0% to 1.2% ± 0.2%; when the angle from the normal is increased from 8° to 30°, the DHR increases by a mere 0.2%. Another reflectance measure, the bidirectional reflectance distribution function (BRDF), provides information on how close the material is to a Lambertian reflector. Results showed a quasi-Lambertian behavior, with a value in the forward-scatter direction of 4 x 10-3 sr-1 and half that in the backscatter direction. Contact Celia Merzbacher at[email protected].

Resonant-cavity polymer LEDs achieve narrow linewidths
Researchers at the University of Sheffield (Sheffield, England) and Dow Chemical Co., (Midland, MI) have constructed narrow-bandwidth, polymer-based, resonant-cavity light-emitting diodes (RCLEDs) using a polymeric charge-transporting dielectric mirror on top of a commercial indium tin oxide film to improve device performance. Device performance varied with cathode material from a maximum luminance of 15 cd/m2 at a 50-V bias and a 0.03 cd/A electro-luminescent efficiency for aluminum, to 350 cd/m2 at 28 V and 0.40 cd/A for calcium, and 300 cd/m2 at 35 V and 0.95 cd/A for composite calcium aluminum. Linewidths on the order of 12 nm also were achieved. In addition, the researchers observed that reductions in linewidth were accompanied by corresponding reductions in brightness, which indicates that RCLED design will require optimization of both electrical qualities and material reflectivity to simultaneously minimize emission linewidth and optical losses. Contact David Lidzey at[email protected].

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