Richard F. Carson

Multimode phenomena in semiconductor-clad dielectric optical waveguide structures.

Semiconductor-clad optical waveguides possess unique properties which arise from the periodic coupling effect between the lossless modes of the dielectric waveguide and the lossy modes which are supported by the thin semiconductor layer. This paper presents a detailed analysis of this mode structure showing mode conversion as a function of cladding thickness. The analysis predicts that several types of mode may exist for certain guide thicknesses, depending on the magnitude of the absorption in the semiconductor. The combined effects of waveguide coupling and loss are determined and illustrated in devices using silicon claddings.

Damage from proton irradiation of vertical-cavity surface-emitting lasers

Damage resulting from irradiating oxide-confined vertical-cavity surface-emitting lasers became significant (threshold shift /spl ap/20%, peak power degradation /spl ap/20%) at fluence levels approaching 1/spl times/10/sup 13/ protons/cm/sup 2/. The threshold current shifted to higher values, and the peak light output power decreased. Forward-current annealing led to partial recovery of the performance of two of the three lasers for which annealing was attempted. Recent results on proton-implanted devices are summarized in a table.

Neutron effects in high-power GaAs laser diodes

The radiation response of broad-area GaAs quantum-well laser diodes and arrays that emit up to 2 W from a 100- mu m to 150- mu m region at the front facet was tested. Results at neutron fluence levels up to 10/sup 15/ n/cm/sup 2/ are reported. The lasing threshold currents of these high-power GaAs laser diodes and arrays increased less than 40% after exposure to 10/sup 14/ n/cm/sup 2/. Threshold current rose by 100% to 250%, and a possible increased susceptibility to facet damage was observed at 10/sup 15/ n/cm/sup 2/. The results indicate that the mechanisms for degradation are the same as those observed for the older semiconductor laser technologies. >

Surface emitting laser technology and its application to the space radiation environment

Present and future space-based applications such as sensors, low-weight and low-power data links for satellites, communication between electromagnetically-shielded modules, and short-distance cross-links within satellite constellations may benefit from the inclusion of small, low-power, and high-efficiency lasers such as the recently-developed Vertical Cavity Surface-Emitting Laser (VCSEL). Many factors influence the application of these devices to space. Temperature response, operational lifetime and reliability, and power consumption are all important considerations for space applications. In addition, the space radiation environments must be considered. In this work, the effects of ionizing radiation on VCSELs are studied with an emphasis on proton damage, and with comparisons to related neutron and gamma-induced phenomena. The influence of proton irradiation is studied in-depth for selected VCSEL structures by the use of an ion microbeam. The experiments indicate that VCSELs exhibit much less threshold current shift for a given radiation dose, compared to the more traditional edge-emitting semiconductor lasers, but that self-heating is a more important consideration for VCSELs. The high current densities associated with VCSELs also lead to a strong influence from forward-bias annealing. These effects are common to various VCSEL types (780 nm and 850 nm) and their magnitude at a given dose is strongly dependent on device size. This indicates that, while VCSELs appear to be very insensitive to ionizing radiation when compared with alternative technologies, there are a number of factors that must be taken into account when optimizing for the space environment.

Low-power modular parallel photonic data links

Many of the potential applications for parallel photonic data links could benefit from a bi-directional Optoelectronic Multi-Chip Module (OEMCM), where the optical transmitter, receiver, and first-level interface electronics are combined into a single package. It would be desirable for such a module to exhibit low power consumption, have a simple electronic interface that can operate at a variety of speeds and possess a capability to use interchangeable optics for a variety of external connections. Here, we describe initial results for a parallel photonic link technology that exhibits those properties. This link uses high-efficiency, back-emitting, two-dimensional Vertical Cavity Surface-Emitting Laser (VCSEL) arrays operating at 980 nm. The lasers are matched, via integrated microlenses, to corresponding monolithically-integrated photoreceiver arrays that are constructed in a InGaAs/InP Heterojunction Bipolar Transistor (HBT) technology. In initial breadboard-level tests, the photonic data channels built with these devices have been demonstrated with direct (3.3 V) CMOS drive of the VCSELs and a corresponding CMOS interface at the photoreceiver outputs. These links have shown electrical power consumption as low as 42 mW per channel for a 50% average duty cycle while operating at 100 Mb/s.

In vacuo responses of an AlGaAs vertical cavity surface emitting laser irradiated by 4.5 MeV protons

Vertical cavity surface emitting lasers (VCSELs) have high potential for space applications, yet little is known of their sensitivity to radiation under vacuum conditions. The first observations of a commercially available proton implanted quantum well AlGaAs VCSEL operating at 850 nm in vacuo and irradiated by 4.5 MeV protons by a scanning ion microbeam is presented. Degradation of L-I-V responses at a proton dose of 1.19 MGy are discussed with particular attention drawn to heating arising from increased nonradiative carrier recombination and that resulting from the vacuum environment.

Waveguide-to-fiber coupling using a second-order grating and an anamorphic binary optic

Historically, obtaining efficient coupling from single-mode waveguides in high performance GaAs modulator devices to single-mode fiber has been difficult. The reasons are; (1) the large modal mismatch between the elliptical waveguide output and the gaussian profile of the optical fiber; and (2) the large NA difference (0.9 for the waveguide in one direction) and 0.16 for fiber. Despite this difficulty, there exists a need for packaging devices with multiple fiber outputs, that have been gang-aligned, efficiently coupled, and hermetically sealed. (The latter item will be very important in automotive or aerospace applications.) Instead of trying to have fiber penetrate the package wall, the SNL approach to efficient coupling and hermeticity has been to allow light to penetrate the package wall. This has been accomplished by sending out the light normal to the waveguides and collecting it with a binary optic that focuses it on to a fiber outside the package. The optical design of this system requires that the beam be nearly collimated as it leaves the surface of the device. To accomplish this, a second-order grating was etched into a 200 /spl mu/m long section of an adiabatically expanded single-mode waveguide.<<ETX>>

Integration of diffractive lenses with addressable vertical-cavity laser arrays

An optical interconnection system is being developed to provide vertical, digital data channels for stacked multichip modules. A key component of the system is an array of individually addressable vertical-cavity surface-emitting lasers with diffractive lenses integrated into the substrate to control beam divergence and direction. The lenses were fabricated by direct-write e-beam lithography and reactive ion beam etching into the GaAs substrate. Preliminary device performance data and the design and fabrication issues are discussed.

VCSEL applications in sensors and microsystems

Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited to miniaturized free-space optical systems in which surface-mounting and hybrid assembly techniques can be used to combine different technologies together. Two examples are described of such microsystems that are being developed for sensing applications. The first example is an optical position sensing system for rotating parts. Progress on fabricating similar system by flip-chip bonding techniques is then discussed. The second examples is a chemical sensing/analysis system which uses a miniature fluorescence detection model that is based on surface- mounted VCSELs and diffractive optical elements. The detection modules is integrated with a capillary electrochromatography separation system and uses substrate- mode light propagation to focus the VCSEL beam on the capillary channel.

Monolithic integration of waveguide structures with surface-micromachined polysilicon actuators

The integration of optical components with polysilicon surface micromechanical actuation mechanisms shows significant promise for signal switching, fiber alignment, and optical sensing applications. Monolithically integrating the manufacturing process for waveguide structures with the processing of polysilicon actuators allows actuated waveguides to take advantage of the economy of silicon manufacturing. The optical and stress properties of the oxides and nitrides considered for the waveguide design along with design, fabrication, and testing details for the polysilicon actuators are presented.