July 1, 2000
Saturable absorber reduces gain error in EDFA; Waveguide avalanche photodiode shows promise for 1550 nm; Output of angled-grating distributed-feedback laser is nearly diffraction-limited; and more.

Saturable absorber reduces gain error in EDFA

Optical gain control (OGC) in erbium-doped fiber amplifiers (EDFAs), in which a self-oscillating laser is introduced into the system, is one method for maintaining optical gain in the surviving channels of a wavelength add/drop multiplexing system as channel loading changes. However, EDFAs can exhibit spectral hole burning at the OGC laser wavelength, causing gain error in surviving channels. Researchers at Corning Inc. (Corning, NY) have introduced a saturable absorber into the OGC laser cavity to reduce such errors.

The EDFA itself is 14 m long. A pair of wavelength-selective couplers forms a ring OGC cavity with a 1527-nm laser wavelength, while an isolator selects a path. A variable optical attenuator sets the cavity loss and thus the amplifier inversion. The saturable absorber dynamically adjusts cavity loss as a function of power. The gain spectrum was measured with eight saturable channels spaced in the 1530-1560-nm range, both with and without the saturable absorber. With seven channels dropped, the gain error was 1.3 dB for a fixed loss and 0.4 dB with the saturable absorber included. The saturable absorber has been used with a remote-control laser in the cavity for all-optical switching. Contact Chia-Chi Wang at [email protected].

Waveguide avalanche photodiode shows promise for 1550 nm

Researchers at the University of Texas (Austin, TX) and Lucent Technologies (Holmdel, NJ) have used a side-illuminated waveguide structure to fabricate a waveguide-confined indium gallium arsenide/indium aluminum arsenide avalanche photodiode (APD). They hope that the device structure, which combines the attributes of waveguides with APD design, will provide fast, sensitive photodiodes for the 1.55-µm telecommunications wavelength range. The device was constructed based on a separate absorption, charge, and multiplication (SACM) scheme and achieved a unity-gain bandwidth of 27 GHz and a gain-bandwidth product of 120 GHz.

Indium aluminum arsenide was chosen for the multiplication region and cladding material of the device because of its transparency at 1.55 µm and low excess-noise characteristics.

The device layers were grown on an indium phosphide substrate and buffer layer by molecular-beam epitaxy. Dark current in the completed waveguide SACM APD remained below 50 nA at 90% of breakdown, and the RC (resistance and capacitance) bandwidth was estimated to be greater than 40 GHz. Bandwidth also appeared to be transit-time limited, because the maximum bandwidth of 27 GHz was independent of area for devices less than 100 µm2.

Contact Geoffrey Kinsey at [email protected].

Output of angled-grating distributed-feedback laser is nearly diffraction-limited

High-power continuous-wave semiconductor lasers traditionally have not produced diffraction-limited beams. One way to approach single-mode operation involves an angled-grating distributed-feedback (a-DFB) laser, in which a diffraction grating is etched into the laser cavity at an angle to the facets. Along this line, researchers at the Naval Research Laboratory (Washington, DC) have produced nearly diffraction-limited output from a a-DFB laser emitting near 3.4 µm. For pulsed optical pumping of a 50-µm-wide stripe at 78 K, the far-field beam divergence angle was only 1.4°. The slope efficiency was 64% of that for a conventional Fabry-Perot laser on the same bar. Beam quality was also substantially better out to stripe widths of at least 800 µm. In contrast to shorter-wavelength a-DFB lasers, however, the spectral linewidth decreased by only a factor of two under the same pumping conditions. The researchers believe this is related to the presence of much-higher background internal losses relative to the diffraction losses at longer wavelengths. Contact Robert Bartolo at [email protected].

Sub-ten-femtosecond pulses approach relativistic intensity

Researchers at the University of Michigan (Ann Arbor, MI) and the Ecole Polytechnique (Palaiseau, France) have focused millijoule-level, sub-ten-femtosecond pulses into a spatial region on the order of a wavelength cubed to achieve a focal-point intensity of 1018 W/cm2, approaching the level of relativistic interaction. To achieve this, the researchers constructed a system based on a high-numerical-aperture, f/1 off-axis paraboloid to focus ultrashort pulses and on adaptive optics to correct wavefront distortion. Seed pulses were generated by a modelocked Ti:sapphire laser pumped by a Nd:YVO4 laser. An all-reflective grating stretcher expanded the pulses to 40 ps. A diode-pumped Nd:YLF laser was used to pump the system amplifier, yielding 1-mJ pulses that were then compressed to 21 fs with 700-µJ energy.

An output spectrum of 700-900 nm was obtained by focusing 420-µJ pulses into a hollow, 85-cm-long, 320-µm-core-diameter fiber at 9-10 Torr of argon gas pressure. The spectrum was compressed using chirped mirrors, producing 8-fs FWHM pulses of 0.17-mJ energy. Using a wedge, 0.5% of the beam was reflected and then expanded using a telescope to the 50-mm aperture of a deformable mirror that directed the beam to the paraboloid. A thin BBO crystal provided second-harmonic generation for small spot size. Contact Gerard Mourou at [email protected].

Third-harmonic 351-nm radiation is emitted in a tight cone of coherent light

When a powerful infrared laser pulse hits a plasma of ionized hydrogen, it generates a tightly focused ring of coherent ultraviolet light. A similar process could lead to a compact source of coherent x-rays. In a step toward that goal, a research team at the University of Michigan (Ann Arbor, MI) has detected high-frequency coherent light generated by a new technique. The scientists sent two 400-fs-long, 1053-nm pulses (with intensity 1017 W/cm2) through a gas of hydrogen or helium—the first pulse to ionize the atoms, the second to generate the harmonics. They detected a phase-matched third harmonic at 351 nm, which was emitted with high efficiency in a tight cone of coherent light. Their theory proposes that the third-harmonic light propagates slightly slower through the gas than the incident light, so with an angle of 6° between them, the two light waves are in phase, and the emission from each electron in the plasma is synchronized. The scientists believe much higher frequencies should be possible in the future. Contact Donald Umstadter at [email protected].

Optical fiber tip excites surface plasmons on thick metal film

Scientists from the University of Munich (Munich, Germany) report that two-dimensional surface plasmons on a thick metal film can be locally excited using a near-field optical-fiber tip. Because the plasmons are longitudinal waves, light polarization controls the direction in which they are launched on the metal surface. This phenomenon has allowed the researchers to, in a sense, "play golf" with surface plasmons on a metal film perforated with nanometer-sized holes. When the near-field optical fiber tip is located next to a nanohole, the surface plasmons reach the nanohole only when the electric-field vector points to the hole. The arrival of the surface plasmon in the hole is detected by sensing its reradiation on the other side of the metal film with a photodiode. By placing several nanoholes in a circle, it may be possible to optically address specific holes just by changing the light polarization, the researchers believe. Their experiments show that surface plasmons on specifically nanostructured metal films might scale down optical devices to the nanometer world. Contact Jochen Feldmann at [email protected].

Notch increases absorption length in photodiode

Integrating an angled total-internal-reflection mirror into a high-speed unitraveling-carrier photodiode (UTC-PD) has enabled researchers at NTT Photonics Laboratories (Kanagawa, Japan) to raise the efficiency of the device by 50%. The mirror consists of an air-filled notch in the back surface of the indium phosphide (InP) substrate; the photodiode itself is fabricated on the substrate's back surface off to one side of the notch. Light passes through the InP, bounces off the notch, and strikes the photodiode at an angle of 56° to the normal. The photodiode itself has an absorption layer 4700 Å thick for normal-incidence light. The oblique incidence angle results in an absorption length increased by a factor of 1.8.

The fabricated device simultaneously exhibits a responsivity of 0.65 A/W, a 3-dB bandwidth of up to 58 GHz, and an output voltage of 5 V at a 1.55-µm wavelength. External quantum efficiency tops 50%. Useful for telecommunications and ultrafast measurement, the device has a high enough output voltage that it can directly drive lithium niobate and electroabsorption modulators, say the researchers. Contact Hiroshi Ito at [email protected].

Superluminal microwave transmission length approaches one meter

Three researchers at the Istituto di Ricerca sulle Onde Eletromagnetiche Nello Carrara (Firenze, Italy) have used methods already developed for optics to enable propagation of localized microwave packets over distances of tens of wavelengths at velocities in excess of the speed of light in a vacuum. Superluminal effects for evanescent waves have been previously demonstrated in both optical and microwave tunneling experiments over distances on the order of one wavelength. The optical distance has been further extended to the centimeter range by using X-shaped Bessel beams; Daniela Mugnai, Anedio Ranfagni, and Rocco Ruggeri report in that they have achieved a similar extension for microwaves [see Physical Review Letters 84, 4830 (2000)].

In scaling the optical experiment into the microwave wavelength range of about 3.5 cm, the researchers modulated an 8.6-GHz microwave carrier to provide rectangular pulses with rise- and falltimes of a few nanoseconds. The input signal was fed via a horn-antenna launcher through a circular slit with a mean diameter of 7-10 cm and into the focal plane of a spherical mirror with a 50-cm diameter and 12-cm focal length. A second horn antenna, placed 30-130 cm from the mirror focal plane, received both launched and reflected signals for detection by a two-channel digital real-time oscilloscope. For receiver-mirror distances on the order of tens of centimeters, transmission velocities were achieved that exceeded the speed of light in a vacuum by more than 5%. Contact Rocco Ruggeri at [email protected].

Silica is template for 3-D silicon photonic crystal

Researchers at the University of Toronto (Toronto, Ontario, Canada), the Universidad Politécnica (Valencia, Spain), and the Instituto de Ciencia de Materiales de Madrid (Madrid, Spain) have fabricated large-scale silicon (Si) photonic crystals that have a complete three-dimensional (3-D) bandgap with a center wavelength of 1.46 µm. To make the crystals, the researchers first created a 3-D lattice of silica spheres by suspending the spheres in a solution of water and ethylene glycol and allowing them to settle. The diameter of the spheres can be chosen to be between 600 and 1000 nm. The resulting lattice has a typical single-domain size of 100 µm. The lattice is then sintered to connect the spheres.

By means of chemical vapor deposition, Si was deposited in the voids within the lattice. The silica spheres were then dissolved. The researchers used Raman spectroscopy to confirm the presence of crystalline silicon. By varying the Si infiltration level from 88% to 100%, the center of the stopband can be changed from 1.46 to 1.55 µm. The structure exhibits an additional stopband centered at 2.5 µm. The researchers envision infiltrating the Si lattice with light-emitting molecules to create lasers and other devices. Contact Sajeev John at [email protected].

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