This PDF file contains the front matter associated with SPIE Proceedings Volume 8876, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Asteroids and comets that cross Earth’s orbit pose a credible risk of impact, with potentially severe disturbances to Earth and society. Numerous risk mitigation strategies have been described, most involving dedicated missions to a threatening object. We propose an orbital planetary defense system capable of heating the surface of potentially hazardous objects to the vaporization point as a feasible approach to impact risk mitigation. We call the system DE-STAR for Directed Energy System for Targeting of Asteroids and exploRation. DE-STAR is a modular phased array of kilowatt class lasers powered by photovoltaic's. Modular design allows for incremental development, test, and initial deployment, lowering cost, minimizing risk, and allowing for technological co-development, leading eventually to an orbiting structure that would be developed in stages with both technological and target milestones. The main objective of DE-STAR is to use the focused directed energy to raise the surface spot temperature to ~3,000K, allowing direct vaporization of all known substances. In the process of heating the surface ejecting evaporated material a large reaction force would alter the asteroid’s orbit. The baseline system is a DE-STAR 3 or 4 (1-10km array) depending on the degree of protection desired. A DE-STAR 4 allows for asteroid engagement starting beyond 1AU with a spot temperature sufficient to completely evaporate up to 500-m diameter asteroids in one year. Small asteroids and comets can be diverted/evaporated with a DESTAR 2 (100m) while space debris is vaporized with a DE-STAR 1 (10m).

Since the discovery, that a tightly focused femtosecond laser beam can induce a highly localized and permanent refractive index change in a wide range of dielectrics, ultrafast laser inscription has found applications in many elds due to its unique 3D and rapid prototyping capabilities. These ultrafast laser inscribed waveguide devices are compact and lightweight as well as inherently robust since the waveguides are embedded within the bulk material. In this presentation we will review our current understanding of ultrafast laser - glass lattice interactions and its application to the fabrication of inherently stable, compact waveguide lasers and astronomical 3D integrated photonic circuits.

We propose a directed energy orbital planetary defense system capable of heating the surface of potentially hazardous objects to the evaporation point as a futuristic but feasible approach to impact risk mitigation. The system is based on recent advances in high efficiency photonic systems. The system could also be used for propulsion of kinetic or nuclear tipped asteroid interceptors or other interplanetary spacecraft. A photon drive is possible using direct photon pressure on a spacecraft similar to a solar sail. Given a laser power of 70GW, a 100 kg craft can be propelled to 1AU in approximately 3 days achieving a speed of 0.4% the speed of light, and a 10,000 kg craft in approximately 30 days. We call the system DE-STAR for Directed Energy System for Targeting of Asteroids and exploRation. DE-STAR is a modular phased array of solid-state lasers, powered by photovoltaic conversion of sunlight. The system is scalable and completely modular so that sub elements can be built and tested as the technology matures. The sub elements can be immediately utilized for testing as well as other applications including space debris mitigation. The ultimate objective of DE-STAR would be to begin direct asteroid vaporization and orbital modification starting at distances beyond 1 AU. Using phased array technology to focus the beam, the surface spot temperature on the asteroid can be raised to more than 3000K, allowing evaporation of all known substances. Additional scientific uses of DE-STAR are also possible.

The development of a broadband IR focal plane array poses several challenges in the area of detector design, material, device physics, fabrication process, hybridization, integration and testing. The purpose of our research is to address these challenges and demonstrate a high-performance IR system that incorporates a HgCdTe-based detector array with high uniformity and operability. Our detector architecture, grown using molecular beam epitaxy (MBE), is vertically integrated, leading to a stacked detector structure with the capability to simultaneously detect in two spectral bands. MBE is the method of choice for multiplelayer HgCdTe growth because it produces material of excellent quality and allows composition and doping control at the atomic level. Such quality and control is necessary for the fabrication of multicolor detectors since they require advanced bandgap engineering techniques. The proposed technology, based on the bandgap-tunable HgCdTe alloy, has the potential to extend the broadband detector operation towards room temperature. We present here our modeling, MBE growth and device characterization results, demonstrating Auger suppression in the LWIR band and diffusion limited behavior in the MWIR band.

Infrared detector arrays operating in space must be able to withstand defect-inducing proton radiation without performance degradation. Therefore, it is imperative that the proton-radiation hardness of infrared detector materials be investigated. Photoluminescence (PL) is sensitive to defects in materials, and thus can be used to quantify the effects of proton-radiation-induced defects. The excitation intensity-dependent PL was used to examine of a set of InAs/InAsSb superlattices before and after 63-MeV-proton irradiation. A proton dose of 100 kRad(Si) was applied to a different piece of each superlattice sample. The low-temperature excitation intensity dependent PL results reveal minimal increases in the carrier concentration, non-radiative recombination, and the PL full-width half-maximum. These results suggest that InAs/InAsSb superlattices are quite tolerant of proton irradiation and may be suitable for space infrared detector arrays.

For space-based imaging systems radiation tolerance to both displacement damage and total ionizing dose (TID) radiation effects continues to be a major performance concern. Here, the TID and proton irradiance tolerance of mid wave infrared interband cascade infrared photodetectors (ICIPs) based on InAs/GaSb type II strained-layer superlattice (T2SLS) absorbers is presented. Protons of energy of 63 MeV were used to irradiate the unbiased ICIP detectors at room temperature to a proton fluence of 7.5 x 10¹¹ protons/cm², corresponding to a TID of 100 kRads(Si). A comparison of the detector performance of a variety of ICIPs with different numbers of electron barrier sizes cascade stages is presented. Performance of detectors of varying size was characterized by dark current and quantum efficiency measurements at different temperatures. Results show changes, increase in dark current and a reduction in the quantum efficiency, consistent with an increase in the trap density.

Interband cascade infrared photodetectors (ICIPs) potentially offer mid-wave infrared detection at very high operating temperatures due to their nearly ideal photovoltaic operation. An ICIP typically makes use of several cascade stages grown in series, each of which consists of an active absorption region with a mid-wave cutoff wavelength, an intra-band relaxation region for electron transport and an inter-band tunneling region to enable electron transport to the next stage. The latter two also effectively act as a hole-barrier (hB) and an electron-barrier (eB), respectively, forming a preferential path for each carrier. Here, an ICIP with a relatively large eB was investigated. One of the key parameters to measure for detector performance is the noise spectrum, particularly to observe the behavior at low frequencies where the noise is often much larger than estimates based on the ideal shot noise expression would predict. This paper presents the results of noise spectrum measurements of differently sized ICIP devices, taken using an external trans-impedance amplifier with a cooled, internal impedance converter and a cooled feedback resistor. Measurements were taken at different operating temperatures and voltage biases in order to determine the noise-dependence on each.

The objective of the Materials International Space Station Experiment (MISSE) is to study the performance of novel materials when subjected to the synergistic effects of the harsh space environment for several months. MISSE missions provide an opportunity for developing space qualifiable materials. Several laser and lidar components were sent by NASA Langley Research Center (LaRC) as a part of the MISSE 7 mission. The MISSE 7 module was transported to the international space station (ISS) via STS 129 mission that was launched on Nov 16, 2009. Later, the MISSE 7 module was brought back to the earth via the STS 134 that landed on June 1, 2011. The MISSE 7 module that was subjected to exposure in space environment for more than one and a half year included fiber laser, solid-state laser gain materials, detectors, and semiconductor laser diode. Performance testing of these components is now progressing. In this paper, the results of performance testing of a laser diode module sent by NASA Langley Research Center on MISSE 7 mission will be discussed. This paper will present the comparison of pre-flight and post-flight performance curves and discuss the effect of space exposure on the laser diode module. Preliminary findings on output power measurements show that the COTS laser diode characteristics did not undergo any significant performance degradation.

Human missions to Mars present some unique challenges for photonics devices. These devices will have exposure to many different space environments. During assembly they will be exposed to the earth orbiting environment. Upon departure they will need to function through the Earth’s Van Allen Radiation Belt. While the general interplanetary environment is less challenging than the radiation belt, they will operate in this environment for 18 months, subject to sudden saturation from solar flares. These components must continue to function properly through these saturation events presenting quite a challenge to photonic components, both optical and electronic. At Mars, the orbital environment is more benign than the Earth’s. Components used as part of the landing vehicles must also deal with the pervasive dust environment for 3 – 6 months. These assembly and mission execution environments provide every form of space environmental challenges to photonic components. This paper will briefly discuss each environment and the expectations on the components for successful operation over the life of the mission.

Space based laser remote-sensing for Earth observation and planetary atmospheres has traditionally relied on the mature diode-pumped solid-state laser and nonlinear frequency conversion technology. We highlight representative examples, including ongoing space mission programs at Fibertek. Key design issues are highlighted, and the lessons learned from a multi-disciplinary design process addressing the space-qualification requirements. Fiber laser/amplifier system provides an agile optical platform for space based laser applications ‒ space lasercom, space-based Earth (or planetary) remote sensing, and space-based imaging. In particular we discuss ongoing efforts at Fibertek on a space-qualifiable, high-performance 1.5-μm Er-doped fiber laser transmitter for inter-planetary lasercom. Design and performance for space qualification is emphasized. As an example of an agile laser platform, use of above fiber laser/amplifier hardware platform for space based sensing of atmospheric CO₂ is also highlighted.

A prototype miniaturized digital sun sensor (miniDSS) was developed by TNO. It is expected to be launched on QuadSat for in-orbit demonstration. The single-chip sun sensor comprises an application specific integrated circuit (ASIC) on which an active pixel sensor (APS), read-out and processing circuitry as well as communication circuitry are combined. The sun sensor consumes only 65 mW, has a volume of 69x52x14 mm³ and a mass of just 72 grams. Although the miniDSS is a miniaturized and low-power device, the accuracy is not compromised by this. The uncalibrated accuracy is in the order of a few hundreds of a degree, across the field of view of 102°x102°. The sensor is albedo insensitive.

We study the effects of temperature changes on the operating wavelength of all-polymer microresonator lasers, particularly on multilayered defect distributed feedback and distributed Bragg reflector lasers. The parameters that change the operating wavelength are discussed with comparisons between experiments and simulations.

Current strategies for diverting threatening asteroids require dedicated operations for every individual object. We propose a stand-off, Earth-orbiting system capable of vaporizing the surface of asteroids as a futuristic but feasible approach to impact risk mitigation. We call the system DE-STAR (Directed Energy System for Targeting of Asteroids and exploRation). DE-STAR is a modular phased array of laser amplifiers, powered by solar photovoltaic panels. Lowcost development of test systems is possible with existing technology. Larger arrays could be tested in sub-orbital demonstrations, leading eventually to an orbiting system. Design requirements are established by seeking to vaporize the surface of an asteroid, with ejected material creating a reaction force to alter the asteroid’s orbit. A proposed system goal would be to raise the surface spot temperature to <3,000K, evaporating all known substances. Engagement distance required for successful diversion depends on the asteroid’s mass, composition and approach velocity. Distance to focus and desired surface spot temperature then determine laser array size. Volatile-laden objects (such as comets) ~100m wide and approaching at 5km/s could be diverted by initiating engagement at ~0.05AU, requiring a laser array of ~100m side length. Phased array configuration allows multiple beams, so a single DE-STAR of sufficient size would be capable of targeting several threats simultaneously. An orbiting DE-STAR could serve diverse scientific objectives, such as propulsion of kinetic asteroid interceptors or other interplanetary spacecraft. Vaporization of debris in Earth orbit could be accomplished with a ~10m array. Beyond the primary task of Earth defense, numerous functions are envisioned.

SA Photonics has developed a family of compact Fiber Optic Gyroscopes (FOGs) for platform stabilization applications. The use of short fiber coils enables the high update rates required for stabilization applications but presents challenges to maintain high performance. We are able to match the performance of much larger FOGs by utilizing several innovative technologies. These technologies include source noise reduction to minimize Angular Random Walk (ARW), advanced digital signal processing that minimizes bias drift at high update rates, and advanced passive thermal packaging that minimizes temperature induced bias drift while not significantly affecting size, weight, or power. In addition, SA Photonics has developed unique distributed FOG packaging technologies allowing the FOG electronics and photonics to be packaged remotely from the sensor head or independent axis heads to minimize size, weight, and power at the sensing location(s). The use of these technologies has resulted in high performance, including ARW less than 0.001 deg/rt-hr and bias drift less than 0.004 deg/hr at an update rate of 10 kHz, and total packaged volume less than 30 cu. in. for a 6 degree of freedom FOG-based IMU. Specific applications include optical beam stabilization for LIDAR and LADAR, beam stabilization for long-range free-space optical communication, Optical Inertial Reference Units for HEL stabilization, and Ka band antenna pedestal pointing and stabilization. The high performance of our FOGs also enables their use in traditional navigation and positioning applications. This paper will review the technologies enabling our high-performance compact FOGs, and will provide performance test results.

In this work, blackening of metals is performed using a commercialized mode-locked femtosecond fiber laser. Different types of surface structures are produced with varying laser scanning conditions (scanning speed and pitch). The modified surface morphologies are characterized using Scan Electron Microscope (SEM) and the blackening effect was investigated both visually and through spectral measurements. Spectral measurements show that the reflectance of the processed materials decreased sharply in a wide wavelength range and varied at different rate for different scanning pitch and speed. Above 95% absorption over the entire visible wavelength range has been demonstrated for the surface structures and the absorption for specific wavelengths can go up to 98.6%. It is found that the enhanced absorption of the black metal is due to light trapping and a variety of micro- and nano-scale surface structures. This study shows the great potential applications such as constructing sensitive detectors and sensors, solar energy absorber and biomedicine. Keywords: Femtosecond laser, fiber laser, blackening, direct writing, nanostructure, light trapping.

Pulse fiber lasers constitute a promising optical transmitter technology for remote sensing applications characterized by tight size, power consumption, and ruggedness constraints. In this paper, we review laser architecture and component solutions that support power scaling of efficient fiber-based sources towards long-range operation consistent with imaging and/or chemical sensing from space-based platforms.

Tailored protective coatings have the potential for tremendous technical and affordability benefits to ground, air and space systems because of their ability to reduce surface contamination, minimize icing, reduce friction, and to reduce corrosion for a wide variety of applications and missions. The thermal and radiation environment of space systems also pose unique challenges to protective coatings because of the space environment’s large temperature variations, the plasma environment and solar UV and Xrays. Contamination may accumulate on sensors inhibiting accurate and timely data acquisition and their efficiency can be seriously affected by contamination buildup. For polymeric materials, not all properties are affected to the same degree by radiation but are often localized at a specific molecular bond. Both hydrophilic and hydrophobic coating approaches may be important to address specific design requirements. Hydrophilic materials are composed of polar molecules and have been used to defog glass and enable oil spots to be swept away with water. Hydrophobic molecules tend to be nonpolar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules often cluster and are difficult to wet with liquids. This paper presents an overview of various types of contamination that adheres to critical air and space surfaces and potential coatings phenomenology that may be used to eliminate contamination.

We present initial work to develop an extensible model for spacecraft environmental interactions. The starting point for model development is a rarefied gas dynamics model for hyperthermal atomic oxygen. The space envi- ronment produces a number of challenging stimuli, including atomic oxygen, but also charged particles, magnetic fields, spacecraft charging, ultraviolet radiation, micrometeoroids, and cryogenic temperatures. Moreover, the responses of spacecraft to combinations or sequences of these stimuli are different from their responses to single stimuli.

New multi-stimulus test facilities such as the Space Threat Assessment Testbed at the USAF Arnold Engi- neering Development Complex make understanding the similarities and differences between terrestrial test and on-orbit conditions increasingly relevant. The extensible model framework under development is intended to host the variety of models needed to describe the multiphysics environment, allowing them to interact to produce a consistent unified picture. The model framework will host modules that can be validated individually or in combination.

The Air Force has recently built a vacuum chamber for simulating LEO and GEO space environments for testing small satellites or satellite subsystems. Four natural environmental sources are provided: broadband solar flux, and narrowband electron, proton, and atomic oxygen fluxes; together with spacecraft charging; outgassing and attitude control Xe ion simulators. Although the chamber has diagnostic sensors to measure the fluxes from the sources at one plane, uncertainties remain about the details of the source emissions, their interactions, and impacts on the chamber. To help address these uncertainties models of the sources are being developed. We have modeled the Atomic Oxygen source and present results of our recent computer simulations. We motivate our choice of the dynamical DSMC code used, and present results such as the AO fluence time history and its spatial distribution over the test article; chamber pressure time history; flow velocity contours; the importance of particle-particle collisions; and attempts to model the motion of residual source-produced O⁺ ions in the geomagnetic field. We compare with existing measurements.

Protective thin film coatings are important for many near-Earth and interplanetary space systems applications using photonic components, optical elements, solar cells and detector-sensor front surfaces to name but a few environmentally at-risk technologies. The near-Earth and natural space environment consists of known degradation processes induced within these technologies brought about by atomic oxygen, micrometeorite impacts, space debris and dust, solar generated charged particles, Van Allen belt trapped particles, and galactic cosmic radiation. This paper will focus on presenting the results of an investigation based on simulated ion-induced defect-modeling and nuclear irradiation testing of several innovative hybrid-polymeric self-cleaning hydrophobic coatings investigated for application to space photonic components, lunar surface, avionic and terrestrial applications. Data is reported regarding the radiation resistance of several hybrid polymer coatings containing various loadings of nanometer-sized TiO₂ fillers for protecting sensors, structures, human and space vehicles from dust contamination found in space and on the Lunar and other planetary surfaces.

Polymer based optoelectronic materials and thin film devices exhibit great potential in future space applications due to their flexibility, light weight, large light absorption coefficient, and promising radiation tolerance in space environment as compared to their inorganic semiconductor counterparts. Since carbon-fluorine (C-F) chemical bonds are much stronger than the carbon-hydrogen (C-H) bonds, fluorinated polymer films offer great potential for space applications due their expected resistance to oxidation, thermal stability, excellent wear properties, and low coefficients of friction. Their use in a space environment is extremely attractive since they are expected to retain their lubricating characteristics in vacuum, unlike many solid lubricants. Current existing polymer photovoltaic materials and devices suffer low photoelectric power conversion efficiencies due to a number factors including poor morphologies at nano scale that hinder the charge separation and transport. This paper reports our recent work on a fluorinated DBfA type block copolymer system where the donor (D) block contains a donor substituted and hydrocarbon based polyphenylenevinylene (PPV), acceptor (fA) block contains a fluorinated and a sulfone acceptor substituted polyphenylenevinylene (f-PPV), and B is a non-conjugated and flexible bridge unit. Preliminary studies reveal DBfA exhibits better nano phase morphologies and over 100 times more efficient optoelectronic conversion efficiencies as compared to D/fA blend.

Quantum well infrared photodetectors are widely used in focal plane arrays operating at liquid nitrogen temperatures. Compared to quantum-well structures, quantum dot (QD) nanomaterials are more flexible to control photoelectron processes by engineering of the nanoscale potential profiles formed by charged quantum dots. Quantum dots with builtin charge (Q-BIC) suppress capture of photoelectrons by QDs and provide strong coupling to infrared radiation. We review design approaches, fabrication and characterization of photodetectors based on Q-BIC media with strong selective doping to increase the built-in dot charge. Characterization of Q-BIC media includes the structural, spectral (photoluminescence measurements) and electrical characterization (dark current, I-V measurements). After several design-growth-characterization cycles we reached relatively high density of quantum dots, small concentration of defects related to quantum dot growth, and suppressed carrier capture by QDs. Optimized Q-BIC media were used for fabrication of Q-BIC IR photodetectors. We studied spectral and temperature dependences of photoresponse and also its dependences on bias voltage and parameters of Q-BIC medium.

A predictive computational approach that limits use of DLTS experiments is presented, developed using the experimental data and proposed physics based models. Three-dimensional NanoTCAD simulations are used for physicsbased prediction of space radiation effects in III-V solar cells, and validated with experimentally measured characteristics of a p+n GaAs solar cell with AlGaAs window. The computed dark and illuminated I-V curves as well as corresponding performance parameters matched very well experimental data for 2 MeV proton irradiation at various fluences. We analyze the role of majority vs. minority and deep vs. shallow carrier traps in the solar cell performance degradation. The traps/defects parameters used in the simulations were derived from Deep Level Transient Spectroscopy (DLTS) data obtained at NRL. It was noticed that the degradation caused by deep traps observed in single-trap numerical tests exhibit a very similar trend to the degradation caused by a full spectrum of defect traps, but to a lesser degree. This led to the development of a method to accurately simulate the degradation of a solar cell by using only a single deep level defect whose density is calculated by the Stopping and Range of Ions in Matter (SRIM) code. Using SRIM, we calculated the number of vacancies produced by 2 MeV proton irradiation for fluences ranging from 6x1010 cm-2 to 5x1012 cm-2. Based on the SRIM results, we applied trap models in NanoTCAD and performed full I-V simulations from which the amount of degradation of performance parameters (Isc, Voc, Pmax) was calculated. The physics-based models using SRIM allowed obtaining good match with experimental data.

Electronic circuits alone cannot fully meet future requirements for speed, size, and weight of many sensor systems, such as digital radar technology and as a result, interest in integrated photonic circuits (IPCs) and the hybridization of electronics with photonics is growing. However, many IPC components such as photodetectors are not presently ideal, but germanium has many advantages to enable higher performance designs that can be better incorporated into an IPC. For example, Ge photodetectors offer an enormous responsivity to laser wavelengths near 1.55μm at high frequencies to 40GHz, and they can be easily fabricated as part of a planar silicon processing schedule. At the same time, germanium has enormous potential for enabling 1.55 micron lasers on silicon and for enhancing the performance of silicon modulators. Our new effort has begun by studying the deposition of germanium on silicon and beginning to develop methods for processing these films. In initial experiments comparing several common chemical solutions for selective etching under patterned positive photoresist, it was found that hydrogen peroxide (H₂O₂) at or below room temperature (20 C) produced the sharpest patterns in the Ge films; H₂O₂ at a higher temperature (50 C) resulted in the greatest lateral etching.

Recent progress in III-V multijunction space solar cell has led to Spectrolab’s GaInP/GaAs/Ge triple-junction, XTJ, cells with average 1-sun efficiency of 29% (AM0, 28°C) for cell size ranging from 59 to 72-cm². High-efficiency inverted metamorphic (IMM) multijunction cells are developed as the next space solar cell architecture. Spectrolab’s large-area IMM3J and IMM4J cells have achieved 33% and 34% 1-sun, AM0 efficiencies, respectively. The IMM3J and the IMM4J cells have both demonstrated normalized power retention of 0.86 at 5x10¹⁴ e-/cm² fluence and 0.83 and 0.82 at 1x10¹⁵ e-/cm² fluence post 1-MeV electron radiation, respectively. The IMM cells were further assembled into coverglass-interconnect-cell (CIC) strings and affixed to typical rigid aluminum honeycomb panels for thermal cycling characterization. Preliminary temperature cycling data of two coupons populated with IMM cell strings showed no performance degradation. Spectrolab has also developed semiconductor bonded technology (SBT) where highperformance component subcells were grown on GaAs and InP substrates separately then bonded directly to form the final multijunction cells. Large-area SBT 5-junction cells have achieved a 35.1% efficiency under 1-sun, AM0 condition.

Cyan Systems has recently developed an approach to focal plane assembly (FPA) architecture which represent a significant advancement in information extraction from the data as it is being collected. This approach utilizes sub-pixels which achieve a high degree of oversampling of the sensors Point Spread Function (PSF), well beyond the Nyquist limit for a critically sampled sensor. The data contained in an oversampled image has the obvious advantage of readily discriminating between focal plane and object generated artifacts as the first step in false alarm rejection. This effect is particularly useful at identification of radiation events. However there are further advantages that can be exploited through nearest neighbor subpixel correlation, and pooling that achieves significant noise reduction and therefore improved sensitivity. In Cyan’s architecture these processes are accomplished for the first time at the input to the preamp in the ROIC. This approach not only allows improved fidelity in imaging, but further reduces false alarm rates, improves detection ranges, and demonstrates an improved ability to track closely spaced objects. The small pixels that enable this approach also ensure improved radiation hardness reducing the capture cross section. The architecture has been modeled and simulations run which illustrate the dramatic improvements possible.

IR photo detectors are in high demand for various military and civilian applications, such as airborne surveillance, remote sensing, environmental monitoring, and spectrometry. Recently InAs/GaSb type II superlattice (T2SL) has attracted numerous R and D interest since SLS is the only IR material that has a theoretical prediction of higher performance than HgCdTe. Here we report the improvement of SL photo diodes through a new design with highly-strained type-II superlattice (HS-T2SL). The HS-T2SL consists of a highly compressively strained thick InSb layer at InAs/GaSb interfaces. The presence of coherent strain shifts the band edges such that the SL energy gap is reduced. This reduced band gap is advantageous to photodetectors because longer cut-off wavelengths can be obtained with reduced layer thickness in the strained SL. The highly compressive strain in HS-T2SL also leads to an even higher optical absorption coefficient and lower dark current. Applying this new design resistance-area product (R₀A) is measured as high as 2.1 Ohm-cm² at 85K for 14.8-μm-cutoff photo diodes without any dark current suppression barriers. The fabricated 14.5μm-cutoff photo diode shows Johnson-noiselimited peak detectivity of 8.4×10¹⁰ cmHz^1/2/W at zero bias at 85K.

The performance of polymer-based electro-optic modulator is very dependent on the wavelength of operation of the device. Surprisingly, most of our intuitive understanding of the molecular performance in such devised are based on studies performed in the off-resonance regime, where the nonlinear optical response of the molecule is by assumption not dependent on the wavelength of operation; or/and two-level model extrapolation, where the response is assumed to be dominated by the contribution of only one excited state. In either case, the effects of quantum interference (cancellation and enhancement of the response due to interactions between multiple excited states) are ignored. In this paper we show how in complex molecules with more than one excited state, quantum interference effects plays an important role in determining the on-resonant response, and hence should not be ignored when studying the response of organic-based electro-optic materials. We use this principle to interpret previous experimental results on the performance of electro-optic modulations under enhanced radiation environments.

Knowledge of the spatial distribution and evolution of embedded charge in thin dielectric materials has important applications in semiconductor, high-power electronic device, high-voltage DC power cable insulation, high-energy and plasma physics apparatus, and spacecraft industries. Knowing how, where, and how much charge accumulates and how it redistributes and dissipates can predict destructive charging effects. Pulsed Electro-acoustic (PEA) measurements— and two closely related methods, Pressure Wave Propagation (PWP) and Laser Intensity Modulation (LIMM)— nondestructively probe such internal charge distributions. We review the instrumentation, methods, theory and signal processing of simple PEA experiments, as well as the related PPW and LIMM methods. We emphasize system improvements required to achieve high spatial resolution for in vacuo measurements of thin dielectrics charged using electron beam injection.

This work introduces the concept of a digital Wavelength Division Multiplexed (WDM) network for small avionic and space platforms. For packaging and heat transfer efficiency, all optical wavelength sources occupy a common location. Addressable wavelengths are allocated to each receiver, which may be reached by selection or tuning of a transmitter wavelength. Individual delays may be applied to assure synchronization at each receiver. The output of each individual source wavelength is pre-modulated with a clock signal. Signal modulation is applied by passing or rejecting the clock signals. Due to the simplicity of the modulation, the control plane functions can be merged with the data plane functions. Although the concept is based on a single data rate, the digital WDM LAN concept can possibly be extended to process packet and analog payloads.

Optical Frequency Domain Reflectometry (OFDR) is used to interrogate fiber sensors adhered to various structures. Temperatures in excess of 1000°C are observed on a thermal-barrier coated stainless steel test plate as it is exposed to a high-temperature torch. The surface temperature distribution is mapped with 5 mm spatial resolution at 100 Hz, revealing large spatial and temporal thermal gradients at coating defect locations. Results and response times are compared with conventional K type thermocouples. Also presented in this work, are real-time position, shape and twist measurements of a simple structure as it is subjected to various loads.