Device characteristics of photonic crystal lasers formed in InGaAsP membranes bonded to a sapphire substrate are discussed. Also discussed are waveguide loss mechanisms in type-A and type-B photonic crystal waveguides and the transmission properties of photonic crystal waveguide bends.
Photonic crystal microcavity lasers are potentially attractive optical sources for future communication systems. They operate at lithographically defined wavelengths and because of their small volumes they are expected to exhibit low operating powers. Much work remains to be done, however, in order for these sources to find mainstream applications. In this presentation we will report on our work on optically pumped photonic crystal lasers. Finite-difference time-domain and finite element simulations will be presented as part of a discussion of the resonant cavity design. The trade-offs in the design of photonic lattice hole radius and membrane thickness will also be included, and we will discuss strategies for minimizing the optical loss in these cavities. The photonic crystal laser cavities reported here are defined by electron beam lithography in pmma. The pmma is subsequently used as a mask to transfer the pattern into a Cr/Au layer in an ion beam milling step. This patterned metal layer is then used as a mask for a reactive ion etch that patterns a silicon nitride layer. Finally this layer is used as a mask to transfer the lattice into the InGaAsP semiconductor using an ECR etching step. Suspended membranes are formed by chemically undercutting the lattice. This provides strong optical confinement at the semiconductor/air interfaces at the top and bottom of the cavity.
We have demonstrated pulsed, optically pumped lasing at and above room temperature in these resonant cavities using a semiconductor diode laser as the pump. The resonant cavity in our demonstration is formed by removing 19 holes from a triangular lattice and is about 2.6 mm across. Incident threshold pump powers for this cavity size as low as 0.5 mW have been demonstrated at room temperature. The peak output power collected through an optical fiber is approximately 2 mW. Lasing is seen for pump pulses as long as 200 ns. We have also demonstrated lasing in these cavities at elevated substrate temperatures. This demonstration was done using an 860 nm top emitting VCSEL as the pumping source because we expect it to provide a direction towards monolithic, electrically addressable lasers. Input power versus output power lasing characteristics for substrate temperatures up to 50 °C have been obtained. We will also report on our work on lithographic fine-tuning of the lasing wavelength. This wavelength can be defined through the lattice constant or the hole radius. This feature of photonic crystal lasers allows the definition of multiwavelength arrays. We have built and characterized arrays in which the lattice constant varies 2 nm steps across the array. The lasing wavelength redshifts with increasing lattice constant with an average separation between adjacent lasing wavelengths of 4.6 nm. The lasing wavelength tunes through the gain spectrum before the laser mode hops. Finally, we will present data on the optical loss in these cavities obtained by varying the number of lattice periods. We observed a reduction in incident threshold pump powers with increasing number of lattice periods at least through 11 periods.
The development of a truly smart camera, with inherent capability for low latency semi-autonomous object recognition, tracking, and optimal image capture, has remained an elusive goal notwithstanding tremendous advances in the processing power afforded by VLSI technologies. These features are essential for a number of emerging multimedia- based applications, including enhanced augmented reality systems. Recent advances in understanding of the mechanisms of biological vision systems, together with similar advances in hybrid electronic/photonic packaging technology, offer the possibility of artificial biologically-inspired vision systems with significantly different, yet complementary, strengths and weaknesses. We describe herein several system implementation architectures based on spatial and temporal integration techniques within a multilayered structure, as well as the corresponding hardware implementation of these architectures based on the hybrid vertical integration of multiple silicon VLSI vision chips by means of dense 3D photonic interconnections.
We have developed a new method for the fabrication of monolithic AlGaAs microlenses on the surface of GaAs/AlGaAs light emitting diodes by combing crystal growth, ion etching and steam oxidation with wet chemical removal of the oxide. Control over the precise processing parameters has resulted in the precise control over the shape, radius, position and smoothness of the microfabricated hemispheres. These microlenses can readily be used for the fabrication of highly efficient light-emitting diodes.
Ridge waveguide, edge-emitting single quantum well GaAs lasers with an integrated gating electrode have been fabricated. These devices integrate a MESFET structure with the laser PN junction so that the SBD (Schottky barrier diode) depletion layer can be used for transverse current confinement in the laser. Device fabrication was very simple requiring only an anisotropic etch for waveguide definition followed by a single self-aligned contact deposition step. The Schottky barrier depletion layers on either side of the ridge waveguide act to confine free carriers. This structure allows for separation of the optical and electrical confinement in the transverse direction without requiring complex fabrication. The device demonstrated modulation of the pulsed lasing threshold with gate control voltage on a 30 micron wide ridge. Above threshold, increasing power output with increasing gate voltage was demonstrated with negligible gate current. The multimode lasing spectrum showed that the increased power output occurred for all modes with no shift in the mode wavelengths to within the resolution of the measurement system.
This paper discusses the vibration isolation problem as it applies to spaceborne interferometers, and presents evidence that vibration isolation will be a required technology for these instruments. A hardware solution to the spaceborne interferometer isolation problem is offered with experimental evidence of its effectiveness.
This paper presents initial results that demonstrate the end-to-end operation of the Micro- Precision Interferometer (MPI) testbed. The testbed is a full-scale model of a future space- based interferometer, containing all the spacecraft and support systems necessary to perform an astrometric measurement. The primary objective of the testbed is to provide an end-to-end problem to evaluate and integrate new interferometer technologies, such as vibration isolation, structural quieting, active optics, and metrology systems. This paper shows initial testbed functionality in terms of the ultimate performance metric: stabilization of stellar fringes (from a pseudo star). The present incarnation of the evolving testbed uses a fringe tracker and pointing control subsystem to stabilize the fringe position to the 72 nm (RMS) level in the presence of the ambient laboratory seismic noise environment which is a factor of 10 higher than that expected on-orbit. These encouraging preliminary results confirm that the MPI testbed provides an essential link between the extensive ongoing ground-based interferometer technology development activities and the technology needs of future spaceborne interferometers.
This paper describes the validation of an integrated modeling tool using data collected from a laboratory set-up. The modeling package, Integrated Modeling of Optical Systems (IMOS), combines structural modeling, optical modeling and control system simulation into a single environment. The Micro-Precision Interferometer Testbed, a ground-based version of a full- scale spaceborne interferometer, provides the opto-mechanical problem for this investigation. The objective of the effort is twofold: (1) validate the predictive capabilities of IMOS; (2) initiate the controller design for the subsystem under investigation. Ground-based validation of this modeling tool will provide a crucial step towards the ultimate goal of accurately predicting on-orbit behavior of future precision optical instruments.
The experimental research at the Jet Propulsion Laboratory (JPL) aimed at developing and validating new design methodology arising out of NASA's Control Structure Interaction and Orbital Stellar Interferometer programs is presented. Structural and direct optical pathlength controls are combined to maintain the pathlength variation below 10 nanometer rms. The bandwidth of the controller is 500 Hz with the disturbance rejection of over 70 dB at frequencies below 10 Hz and over 20 dB at frequencies near 100 Hz.
This paper presents analytical and experimental results in actively damping flexible structures with reaction mass actuators. A two degree of freedom spring-mass model of a flexible structure is analyzed and the key parameters of actuator mass participation and pole-zero separation are related to the maximum damping achievable from rate feedback control. The main conclusion of the paper is that the larger the pole-zero separation the larger the amount of damping that can be imparted to a structural mode. Laboratory experiments conducted on an 8-foot truss structure support the analytical predictions.