Gain Lever, an effect for enhancing amplitude modulation (AM) efficiency in multisection laser diodes1, has been characterized in InGaAs DQW edge emitting lasers that are integrated with passive waveguides. Specifically designed structures which give a range of split ratios from 1:1 to 9:1 have been fabricated and measured to fully characterize the parameter space for operation in the gain lever mode. Additionally the experimental results are compared to a hybrid 3-D simulation involving effective index method (EIM) reduction to 2-D. Gains greater than 6 dB in the AM efficiency can be achieved within the appropriate operating range, but this gain drops rapidly as the parameter range is exceeded. High speed RF modulation with significant gain is, in principle, possible if proper biasing and modulation conditions are used. This phenomenon can also be useful for high-speed digital information transmission.
Photonic integrated circuits require the ability to integrate both lasers and waveguides with low absorption and coupling loss. This technology is being developed at LLNL for digital logic gates for optical key generation circuits to facilitate secure communications. Here, we demonstrate an approach of integrating InGaAs DQW edge emitting lasers (EEL) with electron beam evaporated dielectric waveguides. The EELs are defined by electron cyclotron resonance etching (ECR). This approach results in highly anisotropic etched mirrors with smooth etched features (sidewall rms roughness = 28 Å, surface rms roughness = 10 Å). The mirror is etched to form both the laser cavity and define the waveguide mesa, which accommodates a dielectric stack, where the core is aligned with the active region of the laser to achieve maximum vertical mode overlapping. The waveguides are based on SiO2/Ta2O5/SiO2 which yields a high index contrast of 0.6, resulting in low loss guides (~2-3dB/cm). The design of the interface has taken into account the waveguide transmission loss, air gap spacing and tilt between the laser and waveguide. The critical feature for this deposition technique is its required high directionality or minimal sidewall deposition and corner effects. In the butt coupled EEL/waveguide system we have measured a slope efficiency to be as high as 0.45 W/A. We have in conclusion demonstrated a technology that allows direct coupling of a dielectric optical interconnect to a semiconductor laser monolithically fabricated on the semiconductor substrate.
Michael Pocha, Henry Garrett, Rajesh Patel, Leslie Jones, Michael Larson, Mark Emanuel, Steven Bond, Robert Deri, R. Drayton, Holly Petersen, Mark Lowry
At Lawrence Livermore National Laboratory, we have extensive experience with the design and development of miniature photonic systems which require novel packaging schemes. Over the years we have developed silicon micro-optical benches to serve as a stable platform for precision mounting of optical and electronic components. We have developed glass ball lenses that can be fabricated in-situ on the microbench substrate. We have modified commercially available molded plastic fiber ribbon connectors (MT) and added thin film multilayer semiconductor coatings to create potentially low-cost wavelength combiners and wavelength selective filters. We have fabricated both vertical-cavity and in-plane semiconductor lasers and amplifiers, and have packaged these and other components into several miniature photonics systems. For example, we have combined the silicon optical bench with standard electronic packaging techniques and our custom-made wavelength-selective filters to develop a four-wavelength wavelength-division-multiplexing transmitter module mounted in a standard 120-pin ceramic PGA package that couples light from several vertical-cavity-surface-emitting-laser arrays into one multimode fiber-ribbon array. The coupling loss can be as low as 2 dB, and the transmitters can be operated at over 1.25 GHz. While these systems were not designed for biomedical or environmental applications, the concepts and techniques are general and widely applicable.
Examples are presented of the application of Lawrence Livermore National Laboratory's expertise in photonics packaging. Several examples of packaged devices are described. Particular attention is given to silicon microbenches incorporating heaters and their use in semiconductor optical amplifier fiber pigtailing and packaging.
Optoelectronic component costs are often dominated by the costs of attaching fiber optic pigtails-especially for the case of single transverse mode devices. We present early results of our program in low-cost packaging. We are employing machine vision controlled automated positioning and silicon microbench technology to reduce the costs of optoelectronic components. Our machine vision approach to automated positioning has already attained a positional accuracy of less that 5 microns in less than 5 minutes; accuracies and times are expected to improve significantly as the development progresses. Complementing the machine vision assembly is our manufacturable approach to silicon microbench technology. We will describe our silicon microbench optoelectronic device packages that incorporate built-in heaters for solder bonding reflow.
We will describe research being conducted in the following areas: high-speed, 50 ohm, phased-matched modulators and their applications to digital links; promising new research on flat-panel displays that will be full color, fast response, very thin, and have a very high resolution; all optical switches that are extremely fast, integrable and do not have the latency problems that exist with current optical switches; semiconductor optical amplifiers that are monolithically integrable, more flexible and less expensive than existing fiber amplifiers; novel, semiconductor waveguide devices; and automated packaging techniques that will lower the cost of photonics components.
Silicon positive-intrinsic-negative (p-i-n) diodes have been used in plasma diagnostics by the Los Alamos and Lawrence Livermore National Laboratories (LANL and LLNL) since the early seventies. Since the response bandwidth of these detectors is relatively poor (typically, approximately 5 ns FWHM for 1 cm2 sensitive area and 250 micrometers depletion depth), they are too slow for high-speed applications. GaAs photoconductive detectors (PCD) have been developed since the early eighties at LANL and later at LLNL, and can be tailored by judicious neutron damage to provide the required high-speed bandwidth. Unfortunately, for surface absorbed or non-penetrating radiation, we have discovered that the PCD sensitivity is not flat across its gap, where the incident radiation is perpendicular to the bias electric field. This response non-uniformity can lead to erroneous measurements in cases where the radiation is spatially varying. To overcome this problem, we reoriented the GaAs chip to allow the radiation to be incident through the electrode and parallel to the bias electric field. Then to increase bandwidth, we doped the GaAs crystal with chromium to create trapping sites and provide large resistivity (approximately 109 (Omega) cm), thus creating a semi-insulator detector (SID). We present and discuss the merits of the SID versus PCD and p-i-n diode by showing pulse response data of each detector characterized with 16 MeV endpoint gamma and electron radiation created by the EG&G/EM linear accelerator (Linac) and 5 to 16.5 MeV proton radiation produced by the LLNL Tandem Van de Graaff (TVDG). Application of the SID in Compton electron spectrometry also is discussed.
The ionization coefficient of deep traps in GaAs is determined from a gas breakdown model together with the recent experimental data obtained at LLNL (Lawrence Livermore National Laboratory) and Boeing. Using this coefficient in our nonlinear device transport code, we have investigated theoretically the nonlinear switching phenomena in GaAs devices. The results obtained from our investigations show that if we take into consideration the effect of the field ionization of the deep traps, we can show how the "Lock-On" phenomena could occur in the device.
We are conducting research on the switching properties of photoconductive materials to explore their potential for generating highpower microwaves (HPM) and for high reprate switching. We have investigated the performance of Gallium Arsenide (GaAs) in linear mode (the conductivity of the device follows the optical pulse) as well as an avalanchelike mode (the optical pulse only controls switch closing) . Operating in the unear mode we have observed switch closing times of less than 200 Ps with a 100 ps duration laser pulse and opening times of less than 400 ps at several kV/cm fields using neutron irradiated GaAs. In avalanche and lockon modes high fields are switched with lower laser pulse energies resulting in higher efficiencies but with measurable switching delay and jitter. We are currently investigating both large area (1 cm2) and small area 1 mm2) switches illuminated by AlGaAs laser diodes at 900 nm and Nd:YAG lasers at 1. 06 tim.
Photoconductive switching is a technology that is being increasingly applied to generation of high power microwaves. Two primary semiconductors used for these devices are silicon and gallium arsenide. Diamond is a promising future candidate material. This paper discusses the important material parameters and switching modes critical issues for microwave generation and future directions for this high power photoconductive switching technology.
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