G. A. Vawter

Traveling-wave photodetectors for high-power, large-bandwidth applications

The traveling-wave photodetectors (TWPD) discussed here offer theoretical quantum efficiencies approaching 100% while maintaining a very large electrical bandwidth. Additionally, they are capable of dissipating the high-power levels required for large dynamic range applications. In this paper, the power dissipation limit of the TWPD is explored. A small-signal steady-state model is developed that includes the effects of electrical propagation losses along the detector. Fabrication details are presented and experimental data shows a 3/spl times/1250 /spl mu/m/sup 2/ detector with a 4.8-GHz bandwidth. >

Optical properties of two‐dimensional photonic lattices fabricated as honeycomb nanostructures in compound semiconductors

We have experimentally studied two‐dimensional photonic lattices, honeycomb nanostructures, fabricated by electron beam lithography with (Al,Ga)As materials. Surface normal optical properties were investigated by measuring reflectance to determine the effective index of refraction and lattice stability against degradation. Also, continuous wave and time‐resolved luminescence spectroscopy was used to assess electron‐hole recombination. Finally, light scattering was employed to study photon coupling and propagation through the lattice. These measurements show that the structures are stable, that nonradiative surface recombination is present, and that resonant coupling of light into/out of the lattice occurs at selected wavelengths satisfying a Bragg condition.

Coherent beams from high efficiency two‐dimensional surface‐emitting semiconductor laser arrays

We have fabricated and operated large two‐dimensional (2D) arrays of phase‐locked surface‐emitting semiconductor lasers. The arrays were fabricated by reactive ion beam etching of epitaxial Fabry–Perot resonators comprising GaAs/AlGaAs quantum wells surrounded by AlAs‐AlGaAs quarter‐wave mirrors. Different arrays corresponding to different pixel size (2–5 μm) and spacing (1–2 μm) were produced to investigate evanescent coupling between pixels. The arrays were photopumped so that the array size could be conveniently varied from 1×1, 2×2,... up to 20×20. Except for the 1×1 which emits a circular pattern, all arrays exhibit a well‐defined four‐lobed far‐field pattern in agreement with our theoretical analysis of the optical modes which predicts domination by the 2D out‐of‐phase eigenmode. As a consequence this pattern can be understood with simple Fraunhofer diffraction theory. The angular spread of the lobes, determined by the periodicity of the array elements, is 10° for the array with element size/spacing...

Nanofabrication of photonic lattice structures in GaAs/AlGaAs

The nanofabrication of two‐dimensional photonic lattice structures in GaAs/AlGaAs is reported. The nanofabrication procedure combines direct‐write electron‐beam lithography and reactive‐ion‐beam etching to achieve etched features as small as 50 nm. The lattice comprises a hexagonal array of air cylinders etched into a semiconductor surface with a refractive index contrast of 3.54. A range of air volume fractions from 14% to 84% was investigated. The lithographic, masking, and etching processes necessary to fabricate the lattice are described along with practical limitations to achieving a lattice of arbitrary air volume fraction. Initial results from optical characterization of the lattice are also presented.

On-axis far-field emission from two-dimensional phase-locked vertical cavity surface-emitting laser arrays with an integrated phase-corrector

We have fabricated large, two‐dimensional (2D) arrays of optically pumped, phase‐locked vertical cavity surface‐emitting lasers that emit more than 50% of their light in a central on‐axis lobe. The emission of the arrays was modified from the usual four‐lobed far‐field of 2D coupled arrays by incorporation of a binary phase‐shift mask on the surface of the array. The array consists of Fabry–Perot resonators comprising GaAs/AlGaAs quantum wells surrounded by AlAs/AlGaAs quarterwave mirrors with a multiple order AlGaAs phase‐delay layer on the top mirror stack. The phase‐shift layer was etched away on alternating elements of the array. The resulting on‐axis emission had an angular width of 2° for an array of approximately 100 elements.

Single‐frequency continuous‐wave operation of ring resonator diode lasers

We report continuous‐wave room‐temperature operation of a semiconductor ring resonator diode laser with a single Y‐junction outcoupling waveguide. This device with a 150 μm radius and 8‐μm‐wide etched‐rib waveguide has a threshold current of 72 mA and emits up to 1 mW of single‐frequency output. The side‐mode‐rejection ratio of this laser exceeds 22 dBm over a ≳50 mA current range corresponding to a stable operating mode of the laser. Studies of the emission behavior in this and other similar ring lasers suggest that the ring is lasing in ‘‘whispering gallery’’ modes, and also that the Y junction is affecting mode selection in the ring.

Unidirectional operation in a semiconductor ring diode laser

We report the first demonstration of a unidirectional ring diode laser based on the use of an active crossover waveguide. The crossover waveguide introduces both a nonreciprocal loss and gain into the ring cavity to promote unidirectional lasing in the clockwise ring direction. Up to 95% of the continuous‐wave lasing output of 3.5 mW (at 150 mA ring current) is emitted from this preferred lasing direction. Unidirectional operation of the ring diode laser is also shown to increase the linearity of the light‐vs‐current curves, eliminating kinks otherwise occurring from gain competition in the ring cavity.

Polarization-sensitive subwavelength antireflection surfaces on a semiconductor for 975 nm.

We present the results of subwavelength antireflection surfaces etched into GaAs for use at 975 nm. These surfaces comprise linear gratings with periods less than the wavelength of light in GaAs. The structure appears as a homogeneous birefringent film. For one of the two polarizations, the film is directly analogous to the well-known quarter-wavelength antireflection coating. For the other polarization there is little effect on the surface reflectivity.

Surface-emitting laser-based smart pixels for two-dimensional optical logic and reconfigurable optical interconnections

A monolithic smart pixel technology based on the integration of two-dimensional arrays of cascadable optical switches, optical logic gates, and optical switching nodes consisting of vertical-cavity surface-emitting lasers and heterojunction phototransistors or photothyristors is described. Different combinations of these components perform optical switching, logic, routing, memory, and regeneration. Latching, nonlatching, and bistable switches with high optical gain and contrast are demonstrated, along with all the simple single-stage Boolean optical logic functions. A 2*2 optical bypass-exchange node and a reconfigurable, multistage, two-dimensional optical switching network architecture are also described. >

Passive mode locking of monolithic semiconductor ring lasers at 86 GHz

We report the development of monolithic passively mode‐locked semiconductor ring lasers. These direct‐waveguide‐coupled lasers with a ring radius of 150 μm emit a continuous train of 1.3 ps transform‐limited lasing pulses at 86 GHz repetition rate with a peak power of 10 mW/pulse at the laser output facet. We also observe, under certain conditions, the presence of a second pulse in the ring cavity doubling the pulse repetition rate to 172 GHz.