Publications

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Sandia National Laboratories and Mytek, LLC have collaborated to develop a monolithically-integrated vertical-cavity surface-emitting laser (VCSEL) assembly with controllable polarization states suitable for use in chip-scale atomic clocks. During the course of this work, a robust technique to provide polarization control was modeled and demonstrated. The technique uses deeply-etched surface gratings oriented at several different rotational angles to provide VCSEL polarization stability. A rigorous coupled-wave analysis (RCWA) model was used to optimize the design for high polarization selectivity and fabrication tolerance. The new approach to VCSEL polarization control may be useful in a number of defense and commercial applications, including chip-scale atomic clocks and other low-power atomic sensors.

We demonstrate a robust approach to VCSEL polarization control using deeply-etched surface gratings oriented at several different rotational angles. A RCWA model is used to optimize the design for high polarization selectivity and fabrication tolerance. © 2006 Optical Society of America.

The key need that this project has addressed is a short-wave infrared light detector for ranging (LIDAR) imaging at temperatures greater than 100K, as desired by nonproliferation and work for other customers. Several novel device structures to improve avalanche photodiodes (APDs) were fabricated to achieve the desired APD performance. A primary challenge to achieving high sensitivity APDs at 1550 nm is that the small band-gap materials (e.g., InGaAs or Ge) necessary to detect low-energy photons exhibit higher dark counts and higher multiplication noise compared to materials like silicon. To overcome these historical problems APDs were designed and fabricated using separate absorption and multiplication (SAM) regions. The absorption regions used (InGaAs or Ge) to leverage these materials 1550 nm sensitivity. Geiger mode detection was chosen to circumvent gain noise issues in the III-V and Ge multiplication regions, while a novel Ge/Si device was built to examine the utility of transferring photoelectrons in a silicon multiplication region. Silicon is known to have very good analog and GM multiplication properties. The proposed devices represented a high-risk for high-reward approach. Therefore one primary goal of this work was to experimentally resolve uncertainty about the novel APD structures. This work specifically examined three different designs. An InGaAs/InAlAs Geiger mode (GM) structure was proposed for the superior multiplication properties of the InAlAs. The hypothesis to be tested in this structure was whether InAlAs really presented an advantage in GM. A Ge/Si SAM was proposed representing the best possible multiplication material (i.e., silicon), however, significant uncertainty existed about both the Ge material quality and the ability to transfer photoelectrons across the Ge/Si interface. Finally a third pure germanium GM structure was proposed because bulk germanium has been reported to have better dark count properties. However, significant uncertainty existed about the quantum efficiency at 1550 nm the necessary operating temperature. This project has resulted in several conclusions after fabrication and measurement of the proposed structures. We have successfully demonstrated the Ge/Si proof-of-concept in producing high analog gain in a silicon region while absorbing in a Ge region. This has included significant Ge processing infrastructure development at Sandia. However, sensitivity is limited at low temperatures due to high dark currents that we ascribe to tunneling. This leaves remaining uncertainty about whether this structure can achieve the desired performance with further development. GM detection in InGaAs/InAlAs, Ge/Si, Si and pure Ge devices fabricated at Sandia was shown to overcome gain noise challenges, which represents critical learning that will enable Sandia to respond to future single photon detection needs. However, challenges to the operation of these devices in GM remain. The InAlAs multiplication region was not found to be significantly superior to current InP regions for GM, however, improved multiplication region design of InGaAs/InP APDs has been highlighted. For Ge GM detectors it still remains unclear whether an optimal trade-off of parameters can achieve the necessary sensitivity at 1550 nm. To further examine these remaining questions, as well as other application spaces for these technologies, funding for an Intelligence Community post-doc was awarded this year.

Optical lime-domain reflectometry (OTDR) is an effeclive technique for locating faults in fiber communication links. The fact that most OTDR measurements are performed manually is a significant drawback, because it makes them too costly for use in many short-distance networks and too slow for use in military avionic platforms. Here we describe and demonstrate an automated, low-cost, real-time approach to fault monitoring that can be achieved by integrating OTDR functionality directly into VCSEL-based transceivers. This built-in test capability is straightforward to implement and relevant to both multimode and single mode networks. In-situ OTDR uses the transmitter VCSEL already present in data transceivers. Fault monitoring is performed by emitting a brief optical pulse into the fiber and then turning the VCSEL off. If a fault exists, a portion of the optical pulse returns to the transceiver after a time equal to the round-trip delay through the fiber. In multimode OTDR, the signal is detected by an integrated photodetector, while in single mode OTDR the VCSEL itself can be used as a detector. Modified driver electronics perform the measurement and analysis. We demonstrate that VCSEL-based OTDR has sufficient sensitivity to determine the location of most faults commonly seen in short-haul networks (i.e., the Fresnel reflections from improperly terminated fibers and scattering from raggedly-broken fibers). Results are described for single mode and multimode experiments, at both 850 nm and 1.3 μm. We discuss the resolution and sensitivity that have been achieved, as well as expected limitations for this novel approach to network monitoring.

We report high differential slope of the light versus current (L-I) characteristic in excess of 400% external quantum efficiency from a monolithic dual resonator vertical-cavity surface-emitting laser. The additional optical cavity of the composite resonator can provide gain or loss to the distributed laser mode, depending on the bias conditions. We describe the factors contributing to the internal optical loss, and present a qualitative model for the L-I characteristic. With sufficient current injected into the top cavity, the composite-resonator vertical-cavity laser achieves over 6 W/A of differential slope efficiency from threshold to greater than 1 mW of output power, which may be applicable for analog optical data links. © 2006 IEEE.

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This report describes the research accomplishments achieved under the LDRD Project ''Leaky-mode VCSELs for photonic logic circuits''. Leaky-mode vertical-cavity surface-emitting lasers (VCSELs) offer new possibilities for integration of microcavity lasers to create optical microsystems. A leaky-mode VCSEL output-couples light laterally, in the plane of the semiconductor wafer, which allows the light to interact with adjacent lasers, modulators, and detectors on the same wafer. The fabrication of leaky-mode VCSELs based on effective index modification was proposed and demonstrated at Sandia in 1999 but was not adequately developed for use in applications. The aim of this LDRD has been to advance the design and fabrication of leaky-mode VCSELs to the point where initial applications can be attempted. In the first and second years of this LDRD we concentrated on overcoming previous difficulties in the epitaxial growth and fabrication of these advanced VCSELs. In the third year, we focused on applications of leaky-mode VCSELs, such as all-optical processing circuits based on gain quenching.

Vertical-external-cavity surface-emitting lasers (VECSELs) combine high optical power and good beam quality in a device with surface-normal output. In this paper, we describe the design and operating characteristics of an electrically-pumped VECSEL that employs a wafer-scale fabrication process and operates at 850 nm. A curved micromirror output coupler is heterogeneously integrated with AlGaAs-based semiconductor material to form a compact and robust device. The structure relies on flip-chip bonding the processed epitaxial material to an aluminum nitride mount; this heatsink both dissipates thermal energy and permits high frequency modulation using coplanar traces that lead to the VECSEL mesa. Backside emission is employed, and laser operation at 850 nm is made possible by removing the entire GaAs substrate through selective wet etching. While substrate removal eliminates absorptive losses, it simultaneously compromises laser performance by increasing series resistance and degrading the spatial uniformity of current injection. Several aspects of the VECSEL design help to mitigate these issues, including the use of a novel current-spreading n-type distributed Bragg reflector (DBR). Additionally, VECSEL performance is improved through the use of a p-type DBR that is modified for low thermal resistance.

This paper describes the photonic component development, which exploits pioneering work and unique expertise at Sandia National Laboratories, ARDEC and the Army Research Laboratory by combining key optoelectronic technologies to design and demonstrate components for this fuzing application. The technologies under investigation for the optical fuze design covered in this paper are vertical cavity surface emitting lasers (VECSELs), integrated resonant cavity photodetectors (RCPD), and diffractive micro-optics. The culmination of this work will be low cost, robust, fully integrated, g-hardened components designed suitable for proximity fuzing applications. The use of advanced photonic components will enable replacement of costly assemblies that employ discrete lasers, photodetectors, and bulk optics. The integrated devices will be mass produced and impart huge savings for a variety of Army applications.

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A new approach to optical time-domain reflectometry (OTDR) is proposed that will enable distributed fault monitoring in singlemode VCSEL-based networks. In situ OTDR uses the transmitter VCSEL already resident in data transceivers as both emitter and resonant-cavity photodiode for fault location measurements. Also valuable at longer wavelengths, the concept is demonstrated here using an 850 nm oxide-confined VCSEL and simple electronics. The dead times and sensitivity obtained are adequate to detect the majority of faults anticipated in local- and metropolitan-area networks.

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This report describes the research accomplishments achieved under the LDRD Project 'Radiation Hardened Optoelectronic Components for Space-Based Applications.' The aim of this LDRD has been to investigate the radiation hardness of vertical-cavity surface-emitting lasers (VCSELs) and photodiodes by looking at both the effects of total dose and of single-event upsets on the electrical and optical characteristics of VCSELs and photodiodes. These investigations were intended to provide guidance for the eventual integration of radiation hardened VCSELs and photodiodes with rad-hard driver and receiver electronics from an external vendor for space applications. During this one-year project, we have fabricated GaAs-based VCSELs and photodiodes, investigated ionization-induced transient effects due to high-energy protons, and measured the degradation of performance from both high-energy protons and neutrons.

The wavelength splitting between the LP01 and LP11 modes of selectively oxidized, ion implanted, and hybrid ion implanted/selectively oxidized vertical-cavity surface-emitting lasers is studied by experiment and theory. Measured splittings at threshold show marked differences between the different laser structures due to the effects of index guiding and thermal lensing. Theoretical results were obtained using a vector optical mode solver and show good agreement with experimental results. The hybrid lasers exhibited behavior intermediate between the ion implanted and selectively oxidized lasers and could be optimized for high power single transverse mode emission.

This report describes the research accomplishments achieved under the LDRD Project ''High-Bandwidth Optical Data Interconnects for Satellite Applications.'' The goal of this LDRD has been to address the future needs of focal-plane-array (FPA) sensors by exploring the use of high-bandwidth fiber-optic interconnects to transmit FPA signals within a satellite. We have focused primarily on vertical-cavity surface-emitting laser (VCSEL) based transmitters, due to the previously demonstrated immunity of VCSELs to total radiation doses up to 1 Mrad. In addition, VCSELs offer high modulation bandwidth (roughly 10 GHz), low power consumption (roughly 5 mW), and high coupling efficiency (greater than -3dB) to optical fibers. In the first year of this LDRD, we concentrated on the task of transmitting analog signals from a cryogenic FPA to a remote analog-to-digital converter. In the second year, we considered the transmission of digital signals produced by the analog-to-digital converter to a remote computer on the satellite. Specifically, we considered the situation in which the FPA, analog-to-digital converter, and VCSEL-based transmitter were all cooled to cryogenic temperatures. This situation requires VCSELs that operate at cryogenic temperature, dissipate minimal heat, and meet the electrical drive requirements in terms of voltage, current, and bandwidth.

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Selectively oxidized vertical cavity surface emitting lasers (VCSEL) typically operate in multiple transverse optical modes. High power VCSEL operation is desirable for many applications such as optical storage and printing, modulation spectroscopy, bar code scanning, and data communication over single mode optical fiber. The modal discrimination can be augmented by creating a central region of gain surrounded by a region of optical loss. A VCSEL fabricated via hybrid ion implantation and selective oxidation device structure is designed to demonstrate a single mode output of more than 5 mW for 850 nm.

A record high fundamental-mode power of 5.1 mW was achieved from coupled-resonator vertical-cavity lasers (CRVCLs). In conventional VCSELs, the extent to which the gain volume may be increased is limited by the onset of multi-mode operation. Results indicate that this limitation is circumvented in a coupled-resonator device allowing high power fundamental-mode operation.

The continuous wave operation of 1.3 μm vertical cavity surface emitting laser (VCSEL) grown on GaAs substrates is achieved up to 55 °C, as motivated by demands of emerging VCSEL network applications. These VCSELs employ the mature AlGaAs/GaAs distributed Bragg reflector mirror technology, including selective oxidation for efficient cavity designs. By incorporating a tunnel junction near the optical cavity, both mirrors are doped n-type, which provides the benefits of low optical loss.

The authors report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits bistable behavior in the light output versus injection current. Large bistability regions over current ranges as wide as 18 mA have been observed with on/off contrast ratios of greater than 20 dB. The position and width of the bistability region can be varied by changing the bias to the top cavity. Switching between on and off states can be accomplished with changes as small as 250 {micro}W to the electrical power applied to the top cavity. Theoretical analysis suggests that the bistable behavior is the response of the nonlinear susceptibility in the top cavity to the changes in the bottom intracavity laser intensity as the bottom cavity reaches the thermal rollover point.