Publications

Abstract not provided.

We report a fully integrated high-Q factor micro-ring resonator using silicon nitride/dioxide on a silicon wafer. The micro-ring resonator is critically coupled to a low loss straight waveguide. An intrinsic quality factor of 2.4 x 10{sup 5} has been measured.

At Sandia National Laboratories, miniaturization dominates future hardware designs, and technologies that address the manufacture of micro-scale to nano-scale features are in demand. Currently, Sandia is developing technologies such as photolithography/etching (e.g. silicon MEMS), LIGA, micro-electro-discharge machining (micro-EDM), and focused ion beam (FIB) machining to fulfill some of the component design requirements. Some processes are more encompassing than others, but each process has its niche, where all performance characteristics cannot be met by one technology. For example, micro-EDM creates highly accurate micro-scale features but the choice of materials is limited to conductive materials. With silicon-based MEMS technology, highly accurate nano-scale integrated devices are fabricated but the mechanical performance may not meet the requirements. Femtosecond laser processing has the potential to fulfill a broad range of design demands, both in terms of feature resolution and material choices, thereby improving fabrication of micro-components. One of the unique features of femtosecond lasers is the ability to ablate nearly all materials with little heat transfer, and therefore melting or damage, to the surrounding material, resulting in highly accurate micro-scale features. Another unique aspect to femtosecond radiation is the ability to create localized structural changes thought nonlinear absorption processes. By scanning the focal point within transparent material, we can create three-dimensional waveguides for biological sensors and optical components. In this report, we utilized the special characteristics of femtosecond laser processing for microfabrication. Special emphasis was placed on the laser-material interactions to gain a science-based understanding of the process and to determine the process parameter space for laser processing of metals and glasses. Two areas were investigated, including laser ablation of ferrous alloys and direct-write optical waveguides and integrated optics in bulk glass. The effects of laser and environmental parameters on such aspects as removal rate, feature size, feature definition, and ablation angle during the ablation process of metals were studied. In addition, the manufacturing requirements for component fabrication including precision and reproducibility were investigated. The effect of laser processing conditions on the optical properties of direct-written waveguides and an unusual laser-induced birefringence in an optically isotropic glass are reported. Several integrated optical devices, including a Y coupler, directional coupler, and Mach-Zehnder interferometer, were made to demonstrate the simplicity and flexibility of this technique in comparison to the conventional waveguide fabrication processes.

The optical birefringence in an isotropic glass medium was created by using a regeneratively amplified Ti:sapphire femtosecond laser. The regions between two crossed polarizers modified by the femtosecond laser shows bright transmission with respect to the dark background of the isotropic glass. It was found that the angular dependence of transmission through the laser-modified region was consistent with that of an optically birefringent material. It was also observed that the optical axes of laser-induced birefringence can be controlled by the polarization direction of the femtosecond laser.

With the build-out of large transport networks utilizing optical technologies, more and more capacity is being made available. Innovations in Dense Wave Division Multiplexing (DWDM) and the elimination of optical-electrical-optical conversions have brought on advances in communication speeds as we move into 10 Gigabit Ethernet and above. Of course, there is a need to encrypt data on these optical links as the data traverses public and private network backbones. Unfortunately, as the communications infrastructure becomes increasingly optical, advances in encryption (done electronically) have failed to keep up. This project examines the use of optical logic for implementing encryption in the photonic domain to achieve the requisite encryption rates. In order to realize photonic encryption designs, technology developed for electrical logic circuits must be translated to the photonic regime. This paper examines two classes of all optical logic (SEED, gain competition) and how each discrete logic element can be interconnected and cascaded to form an optical circuit. Because there is no known software that can model these devices at a circuit level, the functionality of the SEED and gain competition devices in an optical circuit were modeled in PSpice. PSpice allows modeling of the macro characteristics of the devices in context of a logic element as opposed to device level computational modeling. By representing light intensity as voltage, 'black box' models are generated that accurately represent the intensity response and logic levels in both technologies. By modeling the behavior at the systems level, one can incorporate systems design tools and a simulation environment to aid in the overall functional design. Each black box model of the SEED or gain competition device takes certain parameters (reflectance, intensity, input response), and models the optical ripple and time delay characteristics. These 'black box' models are interconnected and cascaded in an encrypting/scrambling algorithm based on a study of candidate encryption algorithms. We found that a low gate count, cascadable encryption algorithm is most feasible given device and processing constraints. The modeling and simulation of optical designs using these components is proceeding in parallel with efforts to perfect the physical devices and their interconnect. We have applied these techniques to the development of a 'toy' algorithm that may pave the way for more robust optical algorithms. These design/modeling/simulation techniques are now ready to be applied to larger optical designs in advance of our ability to implement such systems in hardware.

Artificially structured photonic lattice materials are commonly investigated for their unique ability to block and guide light. However, an exciting aspect of photonic lattices which has received relatively little attention is the extremely high refractive index dispersion within the range of frequencies capable of propagating within the photonic lattice material. In fact, it has been proposed that a negative refractive index may be realized with the correct photonic lattice configuration. This report summarizes our investigation, both numerically and experimentally, into the design and performance of such photonic lattice materials intended to optimize the dispersion of refractive index in order to realize new classes of photonic devices.

Abstract not provided.

Rotation sensors (gyros) and accelerometers are essential components for all precision-guided weapons and autonomous mobile surveillance platforms. MEMS gyro development has been based primarily on the properties of moving mass to sense rotation and has failed to keep pace with the concurrent development of MEMS accelerometers because the reduction of size and therefore mass is substantially more detrimental to the performance of gyros than to accelerometers. A small ({approx}0.2 cu in), robust ({approx}20,000g), inexpensive ({approx}$500), tactical grade performance ({approx}10-20 deg/hr.) gyro is vital for the successful implementation of the next generation of ''smart'' weapons and surveillance apparatus. The range of applications (relevant to Sandia's mission) that are substantially enhanced in capability or enabled by the availability of a gyro possessing the above attributes includes nuclear weapon guidance, fuzing, and safing; synthetic aperture radar (SAR) motion compensation; autonomous air and ground vehicles; gun-launched munitions; satellite control; and personnel tracking. For example, a gyro of this capability would open for consideration more fuzing options for earth-penetration weapons. The MEMS gyros currently available are lacking in one or more of the aforementioned attributes. An integrated optical gyro, however, possesses the potential of achieving all desired attributes. Optical gyros use the properties of light to sense rotation and require no moving mass. Only the individual optical elements required for the generation, detection, and control of light are susceptible to shock. Integrating these elements immensely enhances the gyro's robustness while achieving size and cost reduction. This project's goal, a joint effort between organizations 2300 and 1700, was to demonstrate an RMOG produced from a monolithic photonic integrated circuit coupled with a SiON waveguide resonator. During this LDRD program, we have developed the photonic elements necessary for a resonant micro-optical gyro. We individually designed an AlGaAs distributed Bragg reflector laser; GaAs phase modulator and GaAs photodiode detector. Furthermore, we have fabricated a breadboard gyroscope, which was used to confirm modeling and evaluate signal processing and control circuits.

We have used selective AlGaAs oxidation, dry-etching, and high-gain semiconductor laser simulation to create new in-plane lasers with interconnecting passive waveguides for use in high-density photonic circuits and future integration of photonics with electronics. Selective oxidation and doping of semiconductor heterostructures have made vertical cavity surface emitting lasers (VCSELs) into the world's most efficient low-power lasers. We apply oxidation technology to improve edge-emitting lasers and photonic-crystal waveguides, making them suitable for monolithic integrated microsystems. Two types of lasers are investigated: (1) a ridge laser with resonant coupling to an output waveguide; (2) a selectively-oxidized laser with a low active volume and potentially sub-milliAmp threshold current. Emphasis is on development of high-performance lasers suited for monolithic integration with photonic circuit elements.

Abstract not provided.

Abstract not provided.

The authors describe the microfabrication of a multi-level diffractive optical element (DOE) onto a micro-electromechanical system (MEMS) as a key element in an integrated compact optical-MEMS laser scanner. The DOE is a four-level off-axis microlens fabricated onto a movable polysilicon shuttle. The microlens is patterned by electron beam lithography and etched by reactive ion beam etching. The DOE was fabricated on two generations of MEMS components. The first generation design uses a shuttle suspended on springs and displaced by a linear rack. The second generation design uses a shuttle guided by roller bearings and driven by a single reciprocating gear. Both the linear rack and the reciprocating gear are driven by a microengine assembly. The compact design is based on mounting the MEMS module and a vertical cavity surface emitting laser (VCSEL) onto a fused silica substrate that contains the rest of the optical system. The estimated scan range of the system is {+-}4{degree} with a spot size of 0.5 mm.

A new type of GaAs laser is based on the electron-hole plasma in a current filament and is not limited in size by p-n junctions. High energy, electrically controlled, compact, short-pulse lasers are useful for: active optical sensors (LADAR, range imaging, imaging through clouds, dust, smoke, or turbid water), direct optical ignition of fuels and explosives, optical recording, and micro-machining. The authors present a new class of semiconductor laser that can potentially produce much more short pulse energy than conventional (injection-pumped) semiconductor lasers (CSL) because this new laser is not limited in volume or aspect ratio by the depth of a p-n junction. They have tested current filament semiconductor lasers (CFSL) that have produced 75nJ of 890nm radiation in 1.5ns (50W peak), approximately ten times more energy than ISL. These lasers are created from current filaments in semi-insulating GaAs and, in contrast to CSL, are not based on current injection. Instead, low-field avalanche carrier generation produces a high-density, charge-neutral plasma channel with the required carrier density distribution for lasing. They have observed filaments as long as 3.4cm and several hundred microns in diameter in the high gain GaAs photoconductive switches. Their smallest dimension can be more than 100 times the carrier diffusion length in GaAs. This paper will report spectral narrowing, lasing thresholds, beam divergence, temporal narrowing, and energies which imply lasing for several configurations of CFSL. It will also discuss active volume scaling based on recent high current tests.

This report represents the completion of a three-year Laboratory-Directed Research and Development (LDRD) program to investigate combining microelectromechanical systems (MEMS) with optoelectronic components as a means of realizing compact optomechanical subsystems. Some examples of possible applications are laser beam scanning, switching and routing and active focusing, spectral filtering or shattering of optical sources. The two technologies use dissimilar materials with significant compatibility problems for a common process line. This project emphasized a hybrid approach to integrating optoelectronics and MEMS. Significant progress was made in developing processing capabilities for adding optical function to MEMS components, such as metal mirror coatings and through-vias in the substrate. These processes were used to demonstrate two integration examples, a MEMS discriminator driven by laser illuminated photovoltaic cells and a MEMS shutter or chopper. Another major difficulty with direct integration is providing the optical path for the MEMS components to interact with the light. The authors explored using folded optical paths in a transparent substrate to provide the interconnection route between the components of the system. The components can be surface-mounted by flip-chip bonding to the substrate. Micro-optics can be fabricated into the substrate to reflect and refocus the light so that it can propagate from one device to another and them be directed out of the substrate into free space. The MEMS components do not require the development of transparent optics and can be completely compatible with the current 5-level polysilicon process. They report progress on a MEMS-based laser scanner using these concepts.

An edge-emitting buried-oxide waveguide (BOW) laser structure employing lateral selective oxidation of AlGaAs layers above and below the active region for waveguiding and current confinement is presented. This laser configuration has the potential for very small lateral optical mode size and high current confinement and is well suited for integrated optics applications where threshold current and overall efficiency are paramount. Optimization of the waveguide design, oxide layer placement, and bi-parabolic grading of the heterointerfaces on both sides of the AlGaAs oxidation layers has yielded 95% external differential quantum efficiency and 40% wall-plug efficiency from a laser that is very simple to fabricate and does not require epitaxial regrowth of any kind.

A two-dimensional (2D) photonic crystal is an attractive alternative and complimentary to its 3D counterpart, due to fabrication simplicity. A 2D crystal, however, confines light only in the 2D plane, but not in the third direction, the z-direction. Earlier experiments show that such a 2D system can exist, providing that the boundary effect in z-direction is negligible and that light is collimated in the 2D plane. Nonetheless, the usefulness of such 2D crystals is limited because they are incapable of guiding light in z-direction, which leads to diffraction loss. This drawback presents a major obstacle for realizing low-loss 2D crystal waveguides, bends and thresholdless lasers. A recent theoretical calculation, though, suggests a novel way to eliminate such a loss with a 2D photonic crystal slab. The concept of a lightcone is introduced as a criterion for fully guiding and controlling light. Although the leaky modes of a crystal slab have been studied, there have until now no experimental reports on probing its guided modes and band gaps. In this paper, a waveguide-coupled 2D photonic crystal slab is successfully fabricated from a GaAs/Al{sub x}O{sub y} material system and its intrinsic transmission properties are studied. The crystal slab is shown to have a strong 2D band gap at {lambda} {approximately} 1.5 {micro}m. Light attenuates as much as {approximately}5dB per period in the gap, the strongest ever reported for any 2D photonic crystal in optical {lambda}. More importantly, for the first time, the crystal slab is shown to be capable of controlling light fully in all three-dimensions. The lightcone criterion is also experimentally confirmed.

In-situ optical diagnostics and ion beam diagnostics for plasma-etch and reactive-ion-beam etch (RIBE) tools have been developed and implemented on etch tools in the Compound Semiconductor Research Laboratory (CSRL). The optical diagnostics provide real-time end-point detection during plasma etching of complex thin-film layered structures that require precision etching to stop on a particular layer in the structure. The Monoetch real-time display and analysis program developed with this LDRD displays raw and filtered reflectance signals that enable an etch system operator to stop an etch at the desired depth within the desired layer. The ion beam diagnostics developed with this LDRD will permit routine analysis of critical ion-beam profile characteristics that determine etch uniformity and reproducibility on the RIBE tool.

The authors have developed diode lasers for short pulse duration and high peak pulse power in the 0.01--100.0 m pulsewidth regime. A primary goal of the program was producing up to 10 W while maintaining good far-field beam quality and ease of manufacturability for low cost. High peak power, 17 W, picosecond pulses have been achieved by gain switching of flared geometry waveguide lasers and amplifiers. Such high powers area world record for this type of diode laser. The light emission pattern from diode lasers is of critical importance for sensing systems such as range finding and chemical detection. They have developed a new integrated optical beam transformer producing rib-waveguide diode lasers with a symmetric, low divergence, output beam and increased upper power limits for irreversible facet damage.