Newsbreaks

June 1, 2000
VCSEL emitting at 1.6 µm is suitable for mass production; All-optical regenerator offers 160-Gbit/s transmission; Silver-halide fiber bundles transmit thermal images; and more.

VCSEL emitting at 1.6 µm is suitable for mass production

Researchers at Bandwidth9 (Fremont, CA) have developed the first monolithic long-wavelength vertical-cavity surface-emitting laser (VCSEL). The single-transverse-mode device emits at 1.6 µm and is fabricated entirely in a single epitaxy run—a process suitable for mass manufacture. Rival long-wavelength VCSELs produced so far have all been made using complex processes that are not effective for high-volume production.

The Bandwidth9 VCSEL is composed of a lattice-matched bottom distributed Bragg reflector (DBR), an indium gallium arsenide quantum-well active region, and a metamorphic top DBR. The metamorphic material allows both high reflectivity and direct electrical injection. Electrical and optical confinement is provided by selective oxidation. Continuous-wave (CW) room-temperature output power of 0.45 mW was reached for a 9-µm device, with CW operation extending up to 55°C. A 3-µm device operated at a threshold current of 0.87 mA. The VCSEL transmitted data over 50 km of single-mode fiber without optical amplification at 2.5 Gbit/s with a bit-error rate of less than 1 x 10-9 at a received optical power at the end of the fiber of -28 dBm. Contact Wupen Yuen at [email protected].

All-optical regenerator offers 160-Gbit/s transmission

In-line regeneration remains the only option for N x 40-Gbit/s wavelength-division-multiplexing (WDM) transmission over transoceanic distances of 6500-12,000 km, with all-optical regeneration based on synchronous modulation and narrowband filtering offering advantages over electronic regeneration. One reason is the simplified componentry. Another is the capability to simultaneously regenerate several WDM channels after initial resynchronization. One example is what may be the first single 160-Gbit/s all-optical regenerator, developed by researchers at Alcatel (Paris, France) and colleagues at Opto+, Groupement d'Intérêt Economique (Marcoussis, France). A central element of the device, which relies on simultaneous optical regeneration (regeneration multiplexing) of four 200-GHz-spaced 40-Gbit/s channels, is a 40-GHz indium phosphate Mach-Zehnder modulator module that is both polarization- and wavelength-independent. Also key is a dual-electrode drive that helps optimize both intensity and phase modulation. According to the scientists, the device provides regenerated loop transmission across 10,000 km with infinite-distance bit-error rate of less than 5 x 10-8.
Contact Olivier Leclerc at [email protected]r.

Silver-halide fiber bundles transmit thermal images

Researchers at Tel Aviv University (Tel Aviv, Israel) have fabricated and tested silver-halide (AgClBr) fiber bundles for transmitting infrared radiation. Unclad fibers were fabricated in the laboratory from AgClxBr1-x (value of x between 0 and 1) by extruding the material through small dies. Transmission losses were on the order of 0.2 dB/m at a wavelength of 10.6 µm. Core-clad fibers were fabricated by starting with a preform rod of this material within a tube of AgClyBr1-y where y was chosen greater than x to give a lower index of refraction in the tube. The rod-in-tube preform was then extruded through a small die.

Fiber bundles were fabricated by cutting the extruded fiber into short segments and ordering the segments side by side. Increasingly dense arrays of multifiber preforms were obtained through successive draws of bundled arrays. Fiber bundles containing 100-2500 fibers with 5-20-mm2 cross-sectional areas and 5-50-cm lengths were obtained, with individual fiber core diameters between 50 and 250 µm and resolution to 6.5 lines/mm. Thermal images of objects at temperatures between 30°C and 70°C were successfully transmitted. Contact Eran Rave at [email protected].

Nanosecond optical limiter exceeds dynamic range of 1600

Researchers at the Center for Research and Education in Optics and Lasers (Orlando, FL) have built a nanosecond optical limiter based on an f/5 cascaded-focus optical geometry with a dynamic range in excess of 1600. In the setup, a 1-cm-thick cesium sulfide cell at the first focus provided a protective buffer for a 0.1-mm reverse saturable absorber (RSA) made of lead phthalocyanine in chloroform solution at the second focus. Source light was provided by a frequency-doubled Q-switched single-longitudinal-mode Nd:YAG laser producing 7-ns pulses at a 10-Hz repetition rate. Output was measured through a 1.5-mrad-diameter focal aperture.

For input energies of 5.5 and 4 mJ, respectively, output was measured with the RSA cell positioned at the linear focus and 620 µm before the linear focus. Output was limited to 1 µJ, with the RSA at the focus and to 0.6 µJ with the RSA positioned before the focus. The researchers assert that the output should, in principle, remain clamped for much higher input energies with a dynamic range of at least 1600. They measured a 60% transmittance for the dye solution at low irradiance and a 21% total linear transmission for the entire system including Fresnel losses. Placing antireflective coatings on optical surfaces should increase transmission to almost 60%, they said. Contact Florencio Hernández at [email protected].

Multimode optical fiber transmits 10 Gbit/s over 2.8 km

By combining new high-bandwidth multimode fiber (MMF) with a single-longitudinal-mode vertical-cavity surface-emitting laser (VCSEL), researchers at Lucent Technologies' Bell

Laboratories (Holmdel, NJ) have demonstrated 10-Gbit/s transmission over a record 2.8-km length of MMF. The fiber was developed for 300-m 10-Gbit/s Ethernet applications, while the VCSEL emitted at the standard MMF transmission wavelength of 850 nm. Transmission at 10 Gbit/s over ordinary MMF is limited to lengths of a few hundred meters due to differential delay of the fiber's spatial modes; the new fiber shows a constant propagation delay for differing spatial modes and therefore minimal signal degradation due to pulse spreading.

With differential mode delay eliminated, chromatic dispersion due to the finite spectral bandwidth of an ordinary multimode VCSEL becomes the most important source of signal degradation—thus the choice of a single-longitudinal-mode light source. The researchers used gallium arsenide quantum-well oxide-confined VCSELs that remained single-mode at 1.7-V peak-to-peak modulation voltage. The resulting bit-rate-length product is the highest ever for any MMF transmission, say the researchers. The remaining barrier to transmission over even longer distances is simply the fiber's high attenuation at short wavelengths, they note. Contact Giorgio Giaretta at [email protected].

Crystal resonator stores hard x-ray photons

A team of researchers at the European Synchrotron Radiation Facility (ESRF; Grenoble, France), the Lehrstuhl fur Kristallographie und Strukturphysik (Erlangen, Germany), and the Motoren- und Turbinen-Union (Munich, Germany) has demonstrated the storage of hard x-ray photons in a resonator formed by two plates of crystalline silicon. The vertical plates were cut from a monolithic silicon crystal and spaced 150 mm apart. A source x-ray beam of 15.817 keV was obtained from an undulator at the ESRF using the silicon 888 reflection at a Bragg angle of 89.865°. The researchers observed a maximum of 14 1-ns reflection cycles within the resonator, dependent on the crystal thickness. A wedged crystal profile enabled the researchers to vary effective crystal thickness between 50 and 500 µm by shifting the beam horizontally and perpendicular to its axis. Increasing crystal thickness diminished transmission intensity, while decreasing thickness diminished reflectivity. Intensity of transmitted light was measured using a fast avalanche photodiode with a response time that was matched to the synchrotron x-ray beam. Contact Klaus-Dieter Liss at [email protected].

SBR-based modelocked laser emits more than 50 W

Engineers at Spectra-Physics (Mountain View, CA) have demonstrated more than 50 W of quasicontinuous-wave (quasi-CW) output from a laser system incorporating saturable-Bragg-reflector (SBR) technology. The output consists of an 80-MHz stream of ultrafast pulses with a pulse duration of approximately 10 ps. Spectra-Physics obtained a license from Lucent Technologies (Murray Hill, NJ) in 1997 to commercialize the technology; the first commercial product—a compact modelocked Ti:sapphire laser with an output pulse duration of less than 30 fs—was introduced at the Conference on Lasers and Electro-Optics 2000 (San Francisco, CA) in May of this year.

The SBR is a specially fabricated cavity mirror that incorporates a quantum-well absorber. At low laser fluence, the mirror has a reduced reflectivity, whereas at higher fluences the absorption saturates and the SBR mirror becomes a high reflector. This saturable reflector results in reliable, self-starting, passive modelocking. The researchers anticipate that this development will lead to a number of commercial quasi-CW Nd:YVO4 (vanadate) lasers. Applications will include precision materials processing and pumping optical parametric oscillators. Contact Bruce Craig at [email protected].

Multicore fiber laser emits a high-brightness beam

A high-brightness beam has been successfully obtained from a multicore fiber-laser array comprised of seven ytterbium-doped single-mode cores embedded in a common inner cladding. Developed at PC Photonics Corp. (Waterford, CT), the array has six cores in a ring around one central core. By using a large inner cladding, it is possible to couple high power from a diode laser into the cladding to pump the cores. Although such a laser has five supermodes, the very strong model competition means that only the in-phase lowest-order supermode (M2 = 1) and the out-phase highest-order supermode will persist in a very-long-evanescent-wave-coupled and cladding-pumped fiber laser. The PC Photonics researchers thus found it easy to select the in-phase phase-locked supermode that yields an extremely stable, very-high-brightness Gaussian beam with a magnitude 30 dB above the out-phase donut supermode. They believe a high-brightness beam with continuous-wave power exceeding 1 kW will be possible using the multicore phase-locked fiber laser array. Contact Peter Cheo at [email protected].

Hydrogen gas efficiently produces Raman downconverted light

Scientists at Montana State University (Bozeman, MT) are using hydrogen (H2) gas in a high-finesse cavity (HFC) to generate infrared light through Raman downconversion, producing wavelengths ranging from 1180 nm to 4 µm. Pumped by a laser diode and operating at thresholds of less than 0.5 mW, the setup produces 14 mW of Stokes power at 1180 nm with a 61% photon-conversion efficiency—a 25-fold increase in power and fourfold increase in efficiency over earlier diode-pumped systems. Changing the pump diode wavelength changes the output wavelength. Output linewidth is less than 10 kHz.

Resonant optical feedback locks a 100-mW, 792-nm-emitting free-running laser diode to the H2-containing HFC. The cavity-mirror reflectivities are made nonidentical so that the pump field decreases with incident pump power above the Stokes laser threshold. The resulting interference effect allows more pump power to enter the HFC for Stokes conversion, raising efficiency. With greater pump power, the researchers predict the device will reach greater than 80% efficiency. Applications include spectroscopy, atomic physics, and remote sensing. Contact John Carlsten at [email protected].

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