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Techniques for increasing output power from mode-locked semiconductor lasers

Description: Mode-locked semiconductor lasers have drawn considerable attention as compact, reliable, and relatively inexpensive sources of short optical pulses. Advances in the design of such lasers have resulted in vast improvements in pulsewidth and noise performance, at a very wide range of repetition rates. An attractive application for these lasers would be to serve as alternatives for large benchtop laser systems such as dye lasers and solid-state lasers. However, mode-locked semiconductor lasers have not yet approached the performance of such systems in terms of output power. Different techniques for overcoming the problem of low output power from mode-locked semiconductor lasers will be discussed. Flared and arrayed lasers have been used successfully to increase the pulse saturation energy limit by increasing the gain cross section. Further improvements have been achieved by use of the MOPA configuration, which utilizes a flared semiconductor amplifier s amplify pulses to energies of 120 pJ and peak powers of nearly 30W.
Date: February 1, 1996
Creator: Mar, A. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

RF and mm-Wave Photonics at Sandia National Laboratories

Description: RF and mm-wave photonic devices and circuits have been developed at Sandia National Laboratories for applications ranging from RF optical data links to optical generation of mm-wave frequencies. This talk will explore recent high-speed photonics technology developments at Sandia including: (1) A monolithic optical integrated circuit for all-optical generation of mm-waves. Using integrated mode-locked diode lasers, amplifiers, and detectors, frequencies between 30 GHz and 90 GHz are generated by a single monolithic (Al,Ga)As optical circuit less than 2mm in its largest dimension. (2) Development of polarization-maintaining, low-insertion-loss, low v-pi, Mach-Zehnder interferometer (MZI) modulators with DC-to-potentially-K-band modulation bandwidth. New low-loss polarization-maintaining waveguide designs using binary alloys have been shown to reduce polarization crosstalk in undoped (Al,Ga)As waveguides, yielding high extinction ratio (>40dB) and low on-chip loss (<6dB) in Mach-Zehnder interferometers. RF drive voltage is reduced through use of 45rnrn-active length devices with modulator sensitivity, v-pi, less than 3V.
Date: July 8, 1999
Creator: Vawter, G.A. & Sullivan, C.
Partner: UNT Libraries Government Documents Department

NIR high-efficiency subwavelength diffractive structures in semiconductors

Description: We have fabricated sub-wavelength diffractive optical elements with binary phase profiles for operation at 975 nm. Blazed transmission gratings with minimum features 63 nm wide were designed by using rigorous coupled-wave analysis and fabricated by direct-write e-beam lithography and reactive ion beam etching in gallium arsenide. Transmission measurements show 85% diffraction efficiency into the first order. Anti-reflection surfaces, with features 42 nm wide were also designed and fabricated.
Date: March 1, 1995
Creator: Smith, R.E.; Warren, M.E.; Wendt, J.R. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

Monolithically integrated active waveguides and lasers using rare-earth doped spin-on glass

Description: This LDRD program No. 3505.230 explored a new approach to monolithic integration of active waveguides and rare-earth solid state lasers directly onto III-V substrates. It involved selectively incorporating rare-earth ions into spin-on glasses (SOGs) that could be solvent cast and then patterned with conventional microelectronic processing. The patterned, rare-earth spin-on glasses (RESOGs) were to be photopumped by laser diodes prefabricated on the wafer and would serve as directly integrated active waveguides and/or rare-earth solid state lasers.
Date: September 1, 1996
Creator: Ashby, C.I.H.; Sullivan, C.T. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

A complete monolithically-integrated circuit for all-optical generation of millimeter-wave frequencies

Description: An optoelectronic integrated circuit for generating mm-wave frequencies is demonstrated and design issues detailed. A monolithically integrated ring laser, optical amplifier, and photodiode generate electrical signals up to 85.2 GHz.
Date: February 1, 1997
Creator: Vawter, G.A.; Mar, A.; Hietala, V. & Zolper, J.
Partner: UNT Libraries Government Documents Department

Sub-wavelength diffractive optics

Description: This report represents the completion of a three-year Laboratory-Directed Research and Development (LDRD) program to investigate sub-wavelength surface relief structures fabricated by direct-write e-beam technology as unique and very high-efficiency optical elements. A semiconductor layer with sub-wavelength sized etched openings or features can be considered as a layer with an effective index of refraction determined by the fraction of the surface filled with semiconductor relative to the fraction filled with air or other material. Such as a layer can be used to implement planar gradient-index lenses on a surface. Additionally, the nanometer-scale surface structures have diffractive properties that allow the direct manipulation of polarization and altering of the reflective properties of surfaces. With this technology a single direct-write mask and etch can be used to integrate a wide variety of optical functions into a device surface with high efficiencies; allowing for example, direct integration of polarizing optics into the surface with high efficiencies; allowing for example, direct integration of polarizing optics into the surfaces of devices, forming anti-reflection surfaces or fabricating high-efficiency, high-numerical aperture lenses, including integration inside vertical semiconductor laser cavities.
Date: March 1, 1998
Creator: Warren, M.E.; Wendt, J.R. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

Vertical cavity surface emitting lasers emitting near 1.5 {mu}m with Sb-based reflectors

Description: We describe use of AlAsSb/AlGaAsSb lattice matched to InP for distributed Bragg reflectors. These structures are integral to several surface normal devices, in particular vertical cavity surface emitting lasers. The high refractive index ratio of these materials allows formation of a highly reflective mirror with relatively few mirror pairs. As a result, we have been able to show for the first time the 77K CW operation of an optically pumped, monolithic, all-epitaxial vertical cavity laser, emitting at 1.56 {mu}m.
Date: April 1, 1998
Creator: Blum, O.; Klem, J.F. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

Subwavelength diffractive elements fabricated in semiconductor for 975 nm

Description: This paper presents a high-efficiency, dielectric, subwavelength surface relief ``blazed grating`` and reports recent results on a subwavelength ``anti-reflection`` surface. These structures were designed for use at 975 nm, probably the shortest wavelength for which semiconductor structures of these types have been successfully demonstrated. They were fabricated in GaAs substrates.
Date: January 1, 1996
Creator: Smith, R.E.; Warren, M.E.; Wendt, J.R. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

High-Aspect-Ratio Nanophotonic Components Fabricated by Cl(2) RIBE

Description: We describe highly anisotropic reactive ion beam etching of nanophotonic structures in AlGaAs based on the ion beam divergence angle and chamber pressure. The divergence angle is shown to influence the shape of the upper portion of the etch while the chamber pressure controls the shape of the lower portion. This predictable region of parameter space resulted in highly anisotropic nanostructures. Deeply etched distributed Bragg reflectors are etched to an aspect ratio of 8:1 with 100 nm trench widths. The profile of the grating etch is straight with smooth sidewalls, flat bottoms, and squared corners. Two-dimensional photonic crystal post arrays are fabricated with smooth and vertical sidewalls, with structures as small as 180 nm in diameter and 2.0 {micro}m in height.
Date: July 8, 1999
Creator: Zubrzycki, W.J.; Vawter, G.A. & Wendt, J.R.
Partner: UNT Libraries Government Documents Department

Highly-Efficient Laser with Self-Aligned Waveguide and Current Confinement by Selective Oxidation

Description: We report highly efficient, low-threshold-current edge-emitting lasers where both the optical waveguide and lateral current confinement are achieved by lateral selective oxidation of AlGaAs. External differential quantum efficiency in excess of 95% and 40% wall-plug efficiency are demonstrated in 600 {micro}m-long devices without facet coatings. Shorter, 300-{micro}m-long, uncoated devices have <6 mA threshold currents. This high-performance is a combined result of placement of the oxide layers so as to achieve the minimum optical mode volume and bi-parabolic grading of the Al{sub x}Ga{sub 1{minus}x}As heteroepitaxy for minimum height/potential barriers, less than 15 meV, created by the wide-energy-gap layers required for selective wet oxidation. Since the initial development of wet AlGaAs oxidation methods, a number of oxidized edge-emitting laser concepts have been tried. The most successful of these have used lateral selective oxidation of AlGaAs layers between 100 and 300 nm thickness. These layers have been used as current restricting apertures or for both current restriction and lateral waveguiding. Use of an oxide layer above and below the laser active region offers the ability to create a self-aligned waveguide with current apertures on both sides of the pn-junction in a process requiring only one epitaxial growth step. Previous use apertures for these dual purposes resulted multi-moded lasers with reduced efficiency and elevated threshold current density due to non-ideal formation of the waveguide and possibly excess stress caused by the thick (300 nm) oxide layer.
Date: July 27, 1999
Creator: Allerman, A.; Blum, O.; Gao, Y. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

Phased-array antenna control by a monolithic photonic integrated circuit, COMPASS

Description: Phased-array antenna systems are well known for rapid beam steering and their ability to bring high power to the target. Such systems are also quite complex and heavy, which have limited their usefulness. The issues of weight, size, power use, and complexity have been addressed through a system named COMPASS (Coherent Optical Monolithic Phased Array Steering System). All phased-array antenna systems need: (1) small size; (2) low power use; (3) high-speed beam steering; and (4) digitally-controlled phase shifting. COMPASS meets these basic requirements, and provides some very desirable additional features. These are: (1) phase control separate from the transmit/receive module; (2) simple expansion to large arrays; (3) fiber optic interconnect for reduced sensitivity to EMI; (4) an intrinsically radiation-hard GaAs chip; and (5) optical power provided by a commercially available continuous wave (CW) laser. 4 refs., 8 figs.
Date: January 1, 1991
Creator: Kravitz, S.H.; Hietala, V.M.; Vawter, G.A. & Meyer, W.J.
Partner: UNT Libraries Government Documents Department

Characterization of semiconductor surface-emitting laser wafers

Description: The development of epitaxial semiconductor surface-emitting lasers has begun in recent years. These lasers are ultra-short (few {mu}m) Fabry-Perot resonators comprising epitaxial multilayer semiconductor mirrors and quantum well active regions. The resonators are single crystals grown along the lasing axis by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD). They offer significant advances over conventional cleaved, edge-emitting lasers for creating lasers with single elements of 2 dimensional arrays, low beam divergence, engineered active regions, single longitudinal modes, and improved temperature characteristics. To realize the high potential of these new laser structures, techniques for characterizing the laser wafer after growth and between fabrication steps must be developed. In this paper we discuss several optical techniques that we have developed for this emerging surface-emitting laser technology.
Date: January 1, 1990
Creator: Gourley, P.L.; Vawter, G.A.; Brennan, T.M. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

III-V compound semiconductor strip-loaded waveguide devices for PICs: Design for minimum crosstalk and high density

Description: Compact, low-cost photonic integrated circuits (PICs) have long been a desire of systems engineers. Unfortunately, the majority of PICs in use today use regrown buried heterostructure waveguides to achieve low crosstalk at reasonable packing density. These regrown structures are very expensive and limit PIC applications to high performance niches. The alternative low-cost approach is to use etched-rib, or strip-loaded, waveguides. Strip-loaded waveguides are simple to manufacture but may have guided slab-modes carrying unwanted light between devices within the PIC. These slab modes can result in very high crosstalk or low device density. This paper addresses techniques for control of stray light in strip-loaded PICs. Methods include mesa isolation of waveguides and ion implantation outside the waveguide rib. In addition, some devices such as Mach-Zehnder interferometers and waveguide power combiners generate radiation and slab modes as a fundamental means of operation. Improved designs for both of these structures with proper removal of both radiated and slab-mode light and high contrast-ratio operation will be covered.
Date: December 31, 1993
Creator: Vawter, G. A. & Hadley, G. R.
Partner: UNT Libraries Government Documents Department

High-performance GaAs/AlGaAs optical modulators: Their performance and packaging for microwave photonic integrated circuits

Description: The goal of this effort is to build and package photonic integrated circuits (PICs). This infers that compact device design is very important, with all building blocks of the circuit aimed toward integration, low voltage operation, and manufacturability. With such a device, it is important that optical packaging by considered in the initial design. To this end, an advanced photonic packaging concept was designed. This concept employs vertical coupling of light both in and out of the package. This package concept is aimed at hermeticity, with no fiber penetrations through the walls of the package. This paper will describe the building blocks of this package, including output gratings, binary optics, and an automatic fiber capture device, called CLASP.
Date: December 31, 1994
Creator: Kravitz, S. H.; Hietala, V. M. & Vawter, G. A.
Partner: UNT Libraries Government Documents Department

Digitally controlled distributed phase shifter

Description: A digitally controlled distributed phase shifter is comprised of N phase shifters. Digital control is achieved by using N binary length-weighted electrodes located on the top surface of a waveguide. A control terminal is attached to each electrode thereby allowing the application of a control signal. The control signal is either one of two discrete bias voltages. The application of the discrete bias voltages change the modal index of a portion of the waveguide that corresponds to a length of the electrode to which the bias voltage is applied, thereby causing the phase to change through the underlying portion of the waveguide. The digitally controlled distributed phase shift network has a total phase shift comprised of the sum of the individual phase shifters.
Date: December 31, 1992
Creator: Hietala, V. M.; Kravitz, S. H. & Vawter, G. A.
Partner: UNT Libraries Government Documents Department

Velocity matched electrode structures on doped semiconductors for large bandwidth optoelectronic modulators

Description: High-speed optoelectronic modulators are becoming increasingly important in microwave applications. These devices are necessarily electrically large and hence require velocity matching of the microwave signal to the light. A design methodology for velocity matched electrodes on doped semiconductor devices will be presented. As an example of a successful device design, experimental results on a >10 bandwidth high-efficiency (>15{degrees}/V/mm) Mach Zehnder interferometer will be presented.
Date: December 1995
Creator: Hietala, V. M.; Vawter, G. A.; Fuchs, B.; Armendariz, M. G. & Sullivan, C. T.
Partner: UNT Libraries Government Documents Department

Expanded-mode semiconductor laser with tapered-rib adiabatic-following fiber coupler

Description: A new diode laser using a Tapered-Rib Adiabatic-Following Fiber Coupler to achieve 2D mode expansion and narrow, symmetric far-field emission without epitaxial regrowth or sharply-defined tips on tapered waveguides is presented.
Date: February 1, 1997
Creator: Vawter, G.A.; Smith, R.E.; Hou, H. & Wendt, J.R.
Partner: UNT Libraries Government Documents Department

A study of p-type ohmic contacts to InAlAs/InGaAs heterostructures

Description: Optical modulators operating at near-infrared wavelengths are of interest for a variety of applications including bidirectional communications and optical interconnects. The fabrication of 1.06 {micro}m and 1.32 {micro}m operating wavelength strained-layer-superlattice vertical-cavity optoelectronic modulators requires the formation of a p-type ohmic contact to the InAlAs/InGaAs quarter-wave bottom mirror stack. In this study, BeAu and TiPtAu p-type ohmic contact metallization schemes were evaluated for use on molecular beam epitaxy (MBE) grown In{sub .10}Al{sub .90}As/In{sub .12}Ga{sub .88}As and In{sub .32}Al{sub .68}As/In{sub .33}Ga{sub .67}As device heterostructures. Recessed and nonrecessed transmission line measurement (TLM) structures were fabricated and evaluated as a function of rapid thermal anneal (RTA) temperatures over the range of 360 C--420 C. Atomic force microscopy (AFM) was used to determine the surface morphology of each sample for evidence of metal or material degradation. For contacts directly on InGaAs layers, TiPtAu contacts had relatively high specific contact resistance values of {rho}{sub c} {approximately} 3 {times} 10{sup {minus}4} {Omega}cm{sup 2} and displayed no dependence on the anneal. The BeAu contacts had minimum specific contact resistance values of {rho}{sub c} {approximately} 5 {times} 10{sup {minus}7} {Omega}cm{sup 2} but showed evidence of degradation at higher temperatures. Contacts directly made to InAlAs layers had minimum specific contact resistances of {rho}{sub c} {approximately} 4 {times} 10{sup {minus}5} {Omega}cm{sup 2} and were improved slightly with the addition of a thin GaAs layer.
Date: December 31, 1995
Creator: Briggs, R.D.; Howard, A.J.; Baca, A.G.; Hafich, M.J. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

Multiple wavelength vertical-cavity surface-emitting laser arrays using surface-controlled MOCVD growth rate enhancement and reduction

Description: Multiple-wavelength VCSEL and photodetector arrays are useful for wavelength-multiplexed fiberoptic networks, and for optical crosstalk isolation in parallel, free-space interconnects. Multiple wavelength VCSEL arrays have been obtained by varying the growth rate using thermal gradients caused by a backside-patterned substrate, by growth enhancement on a patterned substrate, and by varying the cavity length through anodic oxidation and selective etching of the wafer. We show here for the first time both the enhancement and the reduction of the growth rate of the entire VCSEL structure on a topographically patterned substrate, and demonstrate the controlled variation of the lasing wavelengths of a VCSEL array over an extended spectral range.
Date: April 1, 1997
Creator: Ortiz, G.G.; Hains, C.P.; Luong, S.; Cheng, J.; Hou, H.Q. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

Photonic integrated circuit for all-optical millimeter-wave signal generation

Description: Generation of millimeter-wave electronic signals and power is required for high-frequency communication links, RADAR, remote sensing and other applications. However, in the 30 to 300 GHz mm-wave regime, signal sources are bulky and inefficient. All-optical generation of mm-wave signals promises to improve efficiency to as much as 30 to 50 percent with output power as high as 100 mW. All of this may be achieved while taking advantage of the benefits of monolithic integration to reduce the overall size to that of a single semiconductor chip only a fraction of a square centimeter in size. This report summarizes the development of the first monolithically integrated all-optical mm-wave signal generator ever built. The design integrates a mode-locked semiconductor ring diode laser with an optical amplifier and high-speed photodetector into a single optical integrated circuit. Frequency generation is demonstrated at 30, 60 and 90 Ghz.
Date: March 1, 1997
Creator: Vawter, G.A.; Mar, A.; Zolper, J. & Hietala, V.
Partner: UNT Libraries Government Documents Department

Millimeter-wave signal generation using an integrated mode-locked semiconductor laser and photodiode

Description: A compact optoelectronic integrated circuit for generation of mm-wave frequencies is demonstrated. A monolithically integrated semiconductor ring laser, optical amplifier and waveguide photodiode are used to generate electrical signals up to 85.2 GHz.
Date: February 1, 1997
Creator: Vawter, G.A.; Mar, A.; Hietala, V. & Zolper, J.
Partner: UNT Libraries Government Documents Department

Mode-expanded semiconductor laser with tapered-rib adiabatic-following fiber coupler

Description: Expanded-mode semiconductor lasers are of great interest due to the benefits of reduced far-field divergence and improved coupling efficiency to optical fiber. The authors present a new diode laser using a Tapered-Rib Adiabatic-Following Fiber Coupler (TRAFFiC) to achieve 2D mode expansion without epitaxial regrowth or sharply-defined tips on tapered waveguides. The expanded mode size would allow 0.25 to 1 dB coupling loss to standard telecommunications fiber making smaller-core specialty fibers unnecessary, increasing misalignment tolerance, and eliminating the need for coupling optics.
Date: December 1, 1996
Creator: Vawter, G.A.; Smith, R.E.; Hou, H. & Wendt, J.R.
Partner: UNT Libraries Government Documents Department

Images and spectra of inhibited light propagation in a 2-dimensional photonic lattice at 1.5 {micro}m

Description: Using infrared light scattering microscopy, the authors have directly observed the inhibition of photon propagation in a 2-dimensional photonic lattice fabricated as a hexagonal array of AlGaAs posts. The lattice was formed by reactive ion etching of {approximately}400 nm diameter posts defined by electron beam lithography. The lattice design parameters correspond to a photonic bandgap near 1.5 {micro}m as calculated by Meade et al. This hexagonal array of posts is an improvement over early honeycomb lattices because it is easier to fabricate. The photonic lattice of 1.4 {micro}m high posts was incorporated into waveguide designed for single mode at 1.5 {micro}m. Several waveguide/lattice combinations were fabricated, including M-bar and K-bar lattice orientations aligned parallel to the waveguide and different numbers of lattice periods. The waveguide/lattice structures were fabricated on GaAs substrates that were subsequently thinned and cleaved to couple light into the waveguide facets. Using a specially designed triple infrared microscope system, they simultaneously imaged the input and output facets and the top surface of the waveguide as laser light was focused onto the input facet. Because of internal scattering in the waveguide, light is scattered upward outward and can be imaged with an infrared camera. Images for reflected input, waveguide scattered light, and transmitted output light for the waveguide with (left images) and without the photonic lattice (right images) are shown. The lefthand image shows how the lattice interrupts the transport of light through the waveguide.
Date: June 1, 1996
Creator: Gourley, P.L.; Wendt, J.R.; Vawter, G.A.; McDonald, A.E. & Bieber, A.E.
Partner: UNT Libraries Government Documents Department

Optical generation of radio-frequency power

Description: An optical technique for high-power radio-frequency (RF) signal generation is described. The technique uses a unique photodetector based on a traveling-wave design driven by an appropriately modulated light source. The traveling-wave photodetector (TWPD) exhibits simultaneously a theoretical quantum efficiency approaching 100 % and a very large electrical bandwidth. Additionally, it is capable of dissipating the high-power levels required for the RF generation technique. The modulated light source is formed by either the beating together of two lasers or by the direct modulation of a light source. A system example is given which predicts RF power levels of 100`s of mW`s at millimeter wave frequencies with a theoretical ``wall-plug`` efficiency approaching 34%.
Date: November 1, 1994
Creator: Hietala, V. M.; Vawter, G. A.; Brennan, T. M.; Hammons, B. E. & Meyer, W. J.
Partner: UNT Libraries Government Documents Department