Low V_π modulators are desirable in RF photonic and phased array radar applications. In general, there is still a need for optical modulators that have lower drive voltage, lower loss, and large bandwidth to decrease complexity, expense, and size in other parts of the packaged transmission system. This is particularly important for space based applications where reducing launch weight is crucial. Polymer modulators potentially enable space-based RF photonics because low V_π can be achieved by modifying the organic constituents of the material. Additionally, polymers tend to have relatively low loss tangent and good RF-optical velocity match, which enables broadband devices. One fundamental issue for polymer modulator usability in space is the resistance of the materials to radiation. Previous reports have shown a small but measurable change in modulator properties on irradiation with gamma-rays and protons. Herein we report on the fabrication of polymer modulators, the results of irradiation, and potential lifetimes in earth orbits.

We have employed new molecular media for volumetric data storage by means of two photon absorption. The optical and spectroscopic characteristics of the materials that are relevant to volumetric optical storage are described. In addition, fatigue characteristics of the photochromic media and substrates used in the fabrication of the storage devices are presented. These include temperature, read/write/erase cycles and space environments such as heavy ion and proton radiation, and heat under vacuum.

Phthalocyanines (Pcs) show exceptional stability against high temperatures (up to 900°C, for certain metallophthalocyanines), harsh chemical environments (strong acids and bases), γ-radiation (up to 100 MRad) and neutron radiation (up to 10¹⁹ thermal neutrons/cm²). On the other hand, Pcs exhibit a number of unique physical properties, including semi-conductivity, photoconductivity, large linear and nonlinear optical coefficients, and the ability of photo-switch between two different forms, in case of non-symmetrical metal-free Pcs. This has led to an advancement of phthalocyanine-based prototype field-effect transistors, gas- and photo-sensors, solar cells, optical power limiters, and optical memory devices (CDs). For increasing the capacity of carriers of information, it has been suggested to use the effect of simultaneous two-photon absorption (2PA), which can allow for writing and reading information in many layers, thus resulting in Terabyte (TB) disks. Our estimation of the signal-to-noise ratio shows, however, that for fast (MB/s) processing, molecular 2PA cross section must be extremely large, σ₂ > 10³ - 10⁴ GM (1GM = 10^-50 cm⁴ s), which has not been achieved yet in any photochromic material. In this paper we demonstrate, for the first time, that some specially designed non-symmetric metal-free phthahlocyanines are almost ideally suited for TB rewritable memory due to their extremely high, resonantly enhanced, 2PA cross section (~ 104 GM) in near-IR region and their intrinsic ability of reversible photo-tautomerization at lowered (~ 100 K) temperatures. We discuss how the special technical specifications, such as short pulse laser excitation and lowered working temperature, can be satisfied for space and terrestrial application.

Polyimides (PIs) such as Kapton are used extensively in spacecraft thermal blankets, solar arrays, and space inflatable structures. Atomic oxygen (AO) in low Earth orbit (LEO) causes severe degradation of Kapton. SiO₂ coatings impart remarkable oxidation resistance and have been widely used to protect Kapton, yet imperfections in the SiO₂ application process and micrometeoroid/debris impact in orbit damage the SiO₂ coating leading to Kapton erosion. A polyimide that is self-passivating by the formation of a silica layer upon exposure to AO has been achieved by the copolymerization of a polyhedral oligomeric silsesquioxane (POSS) diamine with the Kapton monomers, pyromellitic dianhydride and 4,4'-oxydianiline, resulting in POSS-Kapton-polyimide. The self-passivating properties have been shown by monitoring a 1 micron deep scratch in POSS-PIs after exposure to AO. Kapton H, SiO₂-coated Kapton HN, and 8.75 weight % Si₈O₁₁ cage "main-chain" POSS-polyimide (8.75 wt % Si₈O₁₁ MC-POSS-PI) were exposed to equivalent AO fluences before and after being scratched. During the first AO exposure and outside of the scratch, these samples eroded 5.0 microns, 0 microns, and less than 200 nm respectively. During the second AO exposure, the samples eroded an additional 5.0 microns within the scratch and outside of the scratch, 7.0 microns within the scratch and 0 microns outside of the scratch, and 200 nm within the scratch and 0 microns outside of the scratch respectively. Surface analysis of MC-POSS-PI films exposed to a hyperthermal O-atom beam shows evidence for the formation of a SiO₂ passivation layer upon AO exposure. This is exemplified by erosion yields of 3.5 and 7 wt % Si₈O₁₁ MC-POSS-PI samples which were 3.7 and 0.98 percent, respectively, of the erosion yield for Kapton H at a fluence of 8.5 x 10²⁰ O atoms cm^-2. Comparison of MC-POSS-PIs and "side-chain" POSS-PI (SC-POSS-PI) shows that these polymers have similar resistance to atomic oxygen and physical properties similar to Kapton H. Erosion yields and imaging of POSS-PIs flown on MISSE1, in a sample tray exposed to all elements (AO, UV light) of the space environment, demonstrated the greatly extended lifetime of POSS-PIs over polyimide.

Recent interest in liquid crystal spatial light modulators as a potential replacement to traditional optical beam steering methods have engendered experiments to determine the technology's resistant to gamma radiation such as may be encountered in a space environment. We previously investigated the effects of exposure of liquid crystal devices to ionizing radiation to total dose levels consistent with a 14-year mission at geostationary orbits (GEO). We reported on the parameters of retardation, contrast ratio and primary power current, which were monitored at various dosing intervals for liquid crystal cells and a spatial light modulator. Here we present for the first time measurements of spatial light modulators' beam steering characteristics taken while they are undergoing gamma irradiation. We examine data on angular deflection, intensity, and beam spread for the liquid crystal spatial light modulators obtained during irradiation. The modulators were in continuous operation during irradiation at approximately 23 Rad (Si)/s, and, again the total ionizing dose reached levels consistent with 14 years at GEO. We observed minimal to no degradation in performance, either from dose rate effects or from total ionizing dose, in these environments.

Space electronic equipment, and NASA future exploration missions in particular, require improvements in solar cell efficiency and radiation hardness. Novel nano-engineered materials and quantum-dot array based photovoltaic devices promise to deliver more efficient, lightweight solar cells and arrays which will be of high value to long term space missions. In this paper, we describe issues related to the development of the quantum-dot based solar cells and comprehensive software tools for simulation of the nanostructure-based photovoltaic cells. Some experimental results used for the model validation are also reviewed. The novel modeling and simulation tools for the quantum-dot-based nanostructures help to better understand and predict behavior of the nano-devices and novel materials in space environment, assess technologies, devices, and materials for new electronic systems as well as to better evaluate the performance and radiation response of the devices at an early design stage. The overall objective is to investigate and design new photovoltaic structures based on quantum dots (QDs) with improved efficiency and radiation hardness. The inherently radiation tolerant quantum dots of variable sizes maximize absorption of different light wavelengths, i.e., create a "multicolor" cell, which improves photovoltaic efficiency and diminishes the radiation-induced degradation. The QD models described here are being integrated into the advanced photonic-electronic device simulator NanoTCAD, which can be useful for the optimization of QD superlattices as well as for the development and exploring of new solar cell designs.

Highly efficient IR detectors and photo-voltaic solar cells that incorporate nanotechnology composed of nanostructures and nanoparticles (including quantum dots) will play an important role in advanced photonic space applications. While the development of Si-based solar cells has successfully evolved into an efficient and economical technology these devices are predicted to soon reach their theoretical 29% limit efficiency. Alternative organic/polymer solar cells and IR detectors incorporating quantum dots and various nanoparticle or nanostructure materials are emerging which are expected to eventually outperform current state-of-the-art detectors and solar cell devices. By tailoring the QD design wavelength-optimized detectors and detector arrays operating over the UV-IR range can be realized. Specific examples for achieving near-IR photovoltaic and photoconductive detectors with high quantum efficiencies are presented along with brief examples of empirical data reported for assessing the radiation resistance of QD nanocrystalline devices for application in space environments.

Being the new frontier of science and technology, as the near earth space begins to attract attention, low cost and rapidly deployable earth observation satellites are becoming more important. Among other things these satellites are expected to carry out missions in the general areas of science and technology, remote sensing, national defense and telecommunications. Except for critical missions, constraints of time and money practically mandate the use of commercial-off-the-shelf (COTS) components as the only viable option. The near earth space environment (~50-50000 miles) is relatively hostile and among other things components/devices/systems are exposed to ionizing radiation. Photonic devices/systems are and will continue to be an integral part of satellites and their payloads. The ability of such devices/systems to withstand ionizing radiation is of extreme importance. Qualification of such devices/systems is time consuming and very expensive. As a result, manufacturers of satellites and their payloads have started to ask for radiation performance data on components from the individual vendors. As an independent manufacturer of both passive and active specialty silica optical fibers, Nufern is beginning to address this issue. Over the years, Nufern has developed fiber designs, compositions and processes to make radiation hard fibers. Radiation performance data (both gamma and proton) of a variety of singlemode (SM), multimode (MM), polarization maintaining (PM) and rare-earth doped (RED) fibers that find applications in space environment are presented.

Free space optics (FSO) can provide high data rates with efficient use of power. However, small platforms may not be able to support the payload requirements of a conventional FSO terminal. An alternative FSO terminal uses a modulating retro-reflector (MRR). MRRs shift most of the power, weight, and pointing requirements to one end of the link. With a MRR configuration, it is possible to establish a two-way FSO link using a single laser transmitter. The MRR terminal of these systems can be small, lightweight, and low power. The MRR maintains the small beam divergence of a conventional optical communications link, but gains the loose pointing advantage of an RF link, reducing the pointing requirements. Communication needs in space present many asymmetric scenarios in which a MRR architecture could be beneficial. This paper describes some of the current capabilities and limitations of MRR systems, as well as applications to space links. An evaluation of the radiation tolerance of modulators is presented.

The European Space Agency has nominated the laser altimeter as one of the principal devices for planetary research for the next decade. The first mission in view is Mercury with scheduled launch in 2010. The device should be capable to range over the distances 400 to 1000 km and to acquire the information about the probe altitude above the planet surface and about the surface terrain profile with the precision of the order of one meter. The requirements on the device are rather strict: total mass below 5 kilograms, power consumption below 10 Watts. Recently, the Technology Demonstrator of the altimeter is under development at German Air and Space Agency, Institute of Planetary Research, Germany. The altimeter Technology Demonstrator is based on the diode pumped frequency doubled Nd:YAG laser delivering 50 mJ at 532 nm in 3 nanosecond long pulses with the repetition rate of 20 kHz. The solid state echo signal detector in photon counting mode will be used. The optical part of the altimeter is scaled down to simulate the real background count rate scenario and to reduce the energy budget link by a factor of 104 at the same time. The demonstrator should be capable to range objects at distances 0 - 5 kilometers in both night and day time. We are presenting the concept, design and construction of the timing system part of the laser altimeter technology demonstrator, which has been developed at the Czech Technical University in Prague optimized for photon counting altimeter concept. The timing system has interval resolution 0.25 ns, stability and linearity ~0.1 ns, epoch resolution 100 ns and accuracy 1 μs, and programmable range gate.

Re-entry bodies are subject to extreme conditions, among them the rigorous shock, vibration, and loading characteristics that can often induce noise or loss of measurement. Restrictions by the Department of Energy on spark sources within a sealed body require the exclusive use of fiber optics for sensing. A joint effort between Los Alamos National Laboratory and Lambda Instruments has developed and evaluated a white light interferometric fiber sensor to address these concerns while measuring displacements between high explosive components in potential flight applications. The sensor offers advantages with electro-magnetic immunity, non-contact sensing elements, and high sensitivity to movement. Gap values are calculated from the extrema of the sinusoidal wavelength pattern created by the Fabry-Perot cavity between the lens and explosive surface, collected by an optical spectrum analyzer and interpreted by an external computer. This paper focuses on the interferometric concept and experimental data received from the unit in real-time during centrifuge tests. Results from single and multimode versions are presented and reported in their effectiveness for 0-2 millimeter measurements.

With the advent of interplanetary missions, requirements posed on satellite instrumentation are becoming increasingly difficult to meet. The main problems are associated with the very limited mass and volume budgets in combination with extreme temperatures and radiation environment. All of the new requirements will have to be met without loosing the prime instrument or sensor performance. This requires a fundamentally different approach. Through the use of Microsystems engineering in combination with clever design both volume and mass can be decreased by more than an order of magnitude while drastically increasing ruggedness. In the end this approach may lead to a paradigm shift in the sense that very capable and rugged sensors will be procure also for more benign missions due to the cost advantages of using MST. The paper will focus on application of MST technologies for sunsensors generally used in the Attitude and orbit control subsystem of satellites and the possible gains (both technological and application wise).

Optical hydrogen sensors are intrinsically safe since they produce no arc or spark in an explosive environment caused by the leakage of hydrogen. Safety remains a top priority since leakage of hydrogen in air during production, storage, transfer and distribution creates an explosive atmosphere for concentrations between 4% (v/v) - the lower explosive limit (LEL) and 74.5% (v/v) - the upper explosive limit (UEL) at room temperature and pressure. Being a very small molecule, hydrogen is prone to leakage through seals and micro-cracks. Hydrogen detection in space application is very challenging; public acceptance of hydrogen fuel would require the integration of a reliable hydrogen safety sensor. For detecting leakage of cryogenic fluids in spaceport facilities, Launch vehicle industry and NASA are currently relying heavily on the bulky mass spectrometers, which fill one or more equipment racks, and weigh several hundred kilograms. An optical sensor system can decrease pay load while monitoring multiple leak locations in situ and in real time. In this paper design of ormsoil approach for developing a completely reversible optical hydrogen sensors for aerospace application is being discussed.

This paper describes the method used to evaluate single-mode optical connectors under consideration for military avionics platforms. This testing is described in terms of the appropriate fiber optics test procedures (FOTPs) from the TIA/EIA-455 series.

NASA is conducting a series of component-level tests, to better understand the reliability and the effects of a spacebased environment on the operation of diode-pumped, solid-state lasers by simulating the unique and harsh environment of launch, vacuum and radiation exposure of a typical mission. We report on our continuing work on high-power, laser-diode arrays (LDA) which are used as an energy source for several proposed and currently flying diode-pumped solid-state lasers missions (ICESAT, MESSENGER and LRO.) The laser-diode arrays are a critical component which can determine the reliability of the whole laser system. NASA needs reliability and performance data for these components to minimize the risks for space-based laser programs. We are concentrating on laser diode arrays emitting at 808 nm and operating quasi-cw with peak powers of ~100 watts per bar at 100 amps. The laser diode arrays are operated with a duty cycle from 0.6% to 2% and current pulses from 50 to 100 amps peak. We studied the effects of power cycling and temperature cycling on the performance of the diode arrays. We also conducted vacuum test as well as vibration and radiation tests. The laser diode arrays have accumulated more then 5.0 billion pulses during some of these tests and continue to operate within specifications.

Two-dimensional periodic structures with nanometer feature sizes have been widely used in many photonic devices. The profile and size of the nanostructure elements can greatly affect optical performances. Various practical subwavelength structures working in the visible and near infrared region have been fabricated using electron-beam writing or laser interference techniques. In this paper, we present a new technique which use nanosphere lithography (NSL) and reactive-ion etching to fabricate two-dimensional nanostructures with tunable nanoelement size and profile.

Carbon nanotubes have come under intense theoretical and experimental investigation focused on both their transport and photonic properties. Of recent interest is the observation that when a gate voltage is applied perpendicular to the axis of the nanotube, it can lead to spin-orbit interaction (SOI). This is of the same nature as the Rashba-Bychkov SOI at an asymmetric semiconductor heterojunction. We have calculated the "band states" for a cylindrical nanotube in the presence of SOI and then used the results to determine the collective plasma oscillations corresponding to electron transitions between the spin-split subbands for single-wall and multiwall nanotubes. These collective plasma modes determine the peak positions for the intersubband absorption coefficient as well as the energy loss spectrum. We show that the plasmon excitations for coaxial tubules have a region of instability which could lead to amplification in the energy transfer spectrum. The frequency regime where this instability occurs could be in the terahertz range. This source of radiation may be suitable for use in space-based detectors because of the durability and robustnes of carbon nanotubes.

Linear and nonlinear optical phenomena in quantum dot (QD) systems caused by interlevel transitions are investigated theoretically. The electron-electron (e-e) interaction is taken into account by employing the self-consistent field approach in the quasistatic limit. It is shown that presence of metal surface, and especially another resonant system, can dramatically enhance the effect of the e-e interaction on the optical phenomena. The conditions for the intrinsic optical bistability in QD systems caused by the e-e interaction are discussed. The obtained results can find applications for designing, manufacturing, and exploiting nanooptoelectronics devices, in part, all-optical components like QD-based optical switches and optical transistors.

The development of technologies in the terahertz spectrum or the very-far-infrared region has been slow mainly because of lack of convenient detectors and lasers. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1 - 10 THz). Quantum well, barrier, and doping parameters are optimized in terms of operating temperature, absorption, and detectivity. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance (BLIP). These devices are suited for a variety of applications, especially in conjunction with the newly developed THz quantum cascade lasers. One of such example is THz free space communication.

We present a prototype photodetector in which the built-in "tunneling structure" serves as an internal gain mechanism for photon detection. Initial feasibility studies demonstrated that the new photon detector offers an optical responsivity as high as 3000 A/W peaked at λ=1.3 μm at less than 1 V bias applied. The measurements were carried out using a photospectrometer setup in a continuous mode at room-temperature. Very strong (> 1000) responsivity is also measured from visible to SWIR even with a simple optical coupling scheme that utilizes very thin absorber layers in the prototype devices. The dark current density is ~ 5×10^-10 A/μm² at the operating bias. Room-temperature NEP was calculated based on a shot noise measurement, yielding NEP of 4~ 5×10^-15 W/Hz^1/2 and D* of 2~3×10¹² cmHz^1/2/W, peaked at a bias of 0.3 V at a fixed wavelength of λ=1.3 μm.

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) 1024x1024 pixel quantum well infrared photodetector (QWIP) focal planes have been demonstrated with excellent imaging performance. The MWIR QWIP detector array has demonstrated a noise equivalent differential temperature (NEΔT) of 17 mK at a 95K operating temperature with f/2.5 optics at 300K background and the LWIR detector array has demonstrated a NEΔT of 13 mK at a 70K operating temperature with the same optical and background conditions as the MWIR detector array after the subtraction of system noise. Both MWIR and LWIR focal planes have shown background limited performance (BLIP) at 90K and 70K operating temperatures respectively, with similar optical and background conditions. In addition, we have demonstrated MWIR and LWIR pixel co-registered simultaneously readable dualband QWIP focal plane arrays. In this paper, we will discuss the performance in terms of quantum efficiency, NEΔT, uniformity, operability, and modulation transfer functions of the 1024x1024 pixel arrays and the progress of dualband QWIP focal plane array development work.

Carbon nanotubes are a unique material that can be either metallic or semiconducting, usually with a small bandgap inversely proportional to tube diameter and with interesting optical properties. However, their general randomness in length, diameter, and chirality, and the challenges in aggregating sufficiently large quantities of precisely uniform nanotubes, render its applications in optical detection so far unattainable beyond simple absorptive coating. The highly-ordered carbon nanotube array, as grown by the non-lithographic methods described here, surmounts many of these obstacles while presenting a geometry that is useful for focal plane array applications. A nanoporous alumina template assists the nanotube growth, which proceeds by carbon vapor deposition in a technique that is compatible with integration on silicon. We report on the experimental treatment of one possible platform for applying carbon nanotubes in infrared detection: a heterojunction photodiode with silicon. The nanotube-silicon heterojunction has rectifying characteristics that are consistent with silicon doping type, nanotube work function, and silicon-nanotube bandgaps. We investigate this hybrid nanostructure with spectral photocurrent measurements in the near and mid-infrared regime in both cooled and uncooled modes of detection. Transient photocurrent analysis suggests that both pyroelectric and direct optoelectronic effects are sources of photoresponse. First-principle theoretical treatments of nanotube-silicon heterojunction detection imply that performance parameters such as D* could be greatly optimized in future generations of samples. We explore the suitability of this detector prototype for spaceborne applications where many known properties of carbon, such as chemical and mechanical durability as well as strong covalent bonding and therefore radiation hardness, merit its consideration.

This paper reviews the recent progress of quantum-dot semiconductor optical amplifiers developed as ultrawide-band high-power amplifiers, high-speed signal regenerators, and wideband wavelength converters.

The photonics group in Code 562 at NASA Goddard Space Flight Center supports a variety of space flight programs at NASA including the: International Space Station (ISS), Shuttle Return to Flight Mission, Lunar Reconnaissance Orbiter (LRO), Express Logistics Carrier (ELC), and the NASA Electronic Parts and Packaging Program (NEPP). Through research, development, and testing of the photonic systems to support these missions much information has been gathered on practical implementations for space environments. Presented here are the highlights and lessons learned as a result of striving to satisfy the project requirements for high performance and reliable commercial optical fiber components for space flight systems. The approach of how to qualify optical fiber components for harsh environmental conditions, the physics of failure and development lessons learned will be discussed.

NASA's Goddard Space Flight Center (GSFC) cooperatively with Sandia National Laboratories completed a series of tests on three separate configurations of multi-fiber ribbon cable and MTP connector assemblies. These tests simulate the aging process of components during launch and long-term space environmental exposure. The multi-fiber ribbon cable assembly was constructed of non-outgassing materials, with radiation-hardened, graded index 100/140-micron optical fiber. The results of this characterization presented here include vibration testing, thermal vacuum monitoring, and extended radiation exposure testing data.