An analysis is presented of the charge dynamics involved in the poling of electro-optic polymer multi-layer materials. Specifically addressed are the electrical properties of the cladding layers in comparison to those of the active core layer which lead to the largest protection against premature dielectric breakdown during poling, the largest fraction of the applied DC poling field falling across the core during poling, and the largest fraction of the applied AC filed during modulation. Also presented is an in-depth analysis of the voltage-divider effect and the relative merits of conductive versus regular claddings for achieving efficient poling and minimal optical loss.
Empirical data regarding the radiation induced responses of Mach Zehnder interferometric electro-optic polymer based modulators (PBMs) operating at 1310 and 1550 nm and broadband InP quantum dot (QD) polymer photodetectors (PPDs) operating into the near infrared (NIR) are reported. Modulators composed of spun-on materials and hybrid electostatically self assembled (ESA) and spun-on NLO materials are examined for changes to their half-wave voltage and insertion losses following a gamma-ray total dose of 163 krad(Si) and irradiation by 25.6 MeV protons at a fluence of ~1011 cm-2. Pre- and post- irradiation responses of ESA grown polymer detectors using InP QDs are examined for photovoltage degradation and aging effects. The data indicates and excellent potential for developing polymer based photonic (PBP) devices with increased radiation resistance suitable for transition to photonic space applications.
This paper presents an update concerning the properties of a new class of nanostructured materials that exhibit the combined properties of low mechanical modulus and high electrical conductivity. Such "Metal RubberTM" materials are formed by molecular-level self-assembly processes. Material synthesis and properties are described. Potential applications for space-based photonics and electronics are in flexible polymer-based electrodes and opto-electronic devices.
When one thinks of suitability in space environments, irradiation tolerance typically is the first property that comes to mind. In addition to irradiation tolerance, however, suitable photonic materials must also possess the desired optical and electromagnetic properties for optimal device performance. Extracted and purified deoxyribonucleic acid (DNA), derived from salmon, has been investigated for use in photonic applications and has shown promise as an excellent optical waveguide material. In this paper we present the properties of DNA that are applicable for both ground and space based photonic applications. Such properties include optical loss, temperature stability, refractive index, resistivity, dielectric constant, microwave insertion loss and gamma ray irradiation tolerance.
Liquid crystal spatial light modulators are emerging as a potential replacement to traditional optical beam steering methods. The performance of these devices for optical communication systems in the radiation environment for geostationary orbits (GEO) are of interest for applications in the next generation of satellites. As an initial investigation to the study presented, several liquid crystals were irradiated to total dose levels consistent with expected GEO environments. While prior irradiation work has been done on spatial light modulators none is known to include a first look at a liquid crystal and CMOS backplane. Parameters of retardation, contrast ratio and primary power current were monitored at incremental stages during the test and are presented.
A parallel analysis of radiation-induced and thermal-induced degradation of polyethyleneterephtalate (PET) films is presented. The complexity of the degradation process is analyzed as a first step in a better understanding of the effect of combined temperature and radiation on PET. electron spin resonance spectrometry, DC electrical measurements, differential scanning calorimetry, and mechanical tests were used to analysze the effect of different ioninzing radiation (such as gamma, electrons, and accelerated ions) on thin films of PET. Data on the thermal analysis of PET are presented and analyzed. This study aims to a better understanding and modeling of complex degradation processes, required for a more reliable assessment of the behavior of polymers subjected to the space environment.
Degradation processes in confined polymeric films, with a thickness smaller or equal to 100 nm are of particular importance for future space missions and microelectronics applications. A simplified theoretical model for the evolution of free radicals in such films is proposed. The model takes into account the dependence of the glass transition temperature (TG) on the film thickness as well as the dependence of TG on the average molecular mass of the polymer (Fox-Flory equation), by exploiting the blob concept. It is assumed that the film thickness controls blobs' size. The time and temperature evolution of free radicals is desxcribed by dividing the main physical and chemical processes into two statistically independent steps. In the first step, the reactants diffuse towards a nanometer sized reaction volume. In the second step the proper chemical reaction between reactants occurs. Two possible chemical reactions are considered: the deactiviation of free radicals through chemical reactions with small molecules or free electrons and the recombination of free radicals. It is supposed that the diffusion of free radicals is a self-diffusion process that obeys a Williams-Landel-Ferry like temperature dependence. The temperature dependence of the diffusion coefficient of small molecules was assumed to obey a simple Arrhenius like dependence. This provides a simple theoretical approach for the modeling of the physical properties thin polymeric films subjected to degradation processes within the glass transition range and may be refined to assess the lifetime of such films in extreme environments.
The National Aeronautics and Space Administration's (NASA) Marshall Space Flight Center (MSFC) continues research into the utilization of photonic materials for spacecraft propulsion. Spacecraft propulsion, using photonic materials, will be achieved using a solar sail. A solar sail operates on the principle that photons, originating from the sun, impart pressure to the sail and therefore provide a source for spacecraft propulsion. The pressure imparted to a solar sail can be increased, up to a factor of two, if the sun-facing surface is perfectly reflective. Therefore, these solar sails are generally composed of a highly reflective metallic sun-facing layer, a thin polymeric substrate and occasionally a highly emissive back surface. Near term solar sail propelled science missions are targeting the Lagrange point 1 (L1) as well as locations sunward of L1 as destinations. These near term missions include the Solar Polar Imager and the L1 Diamond. The Environmental Effects Group at NASA's Marshall Space Flight Center (MSFC) continues to actively characterize solar sail material in preparation for these near term solar sail missions. Previous investigations indicated that space environmental effects on sail material thermo-optical properties were minimal and would not significantly affect the propulsion efficiency of the sail. These investigations also indicated that the sail material mechanical stability degrades with increasing radiation exposure. This paper will further quantify the effect of space environmental exposure on the mechanical properties of candidate sail materials. Candidate sail materials for these missions include Aluminum coated Mylar TM, TeonexTM, and CP1 (Colorless Polyimide).
These materials were subjected to uniform radiation doses of electrons and protons in individual exposures sequences. Dose values ranged from 100 Mrads to over 5 Grads. The engineering performance property responses of thermo-optical and mechanical properties were characterized. The contribution of Near Ultraviolet (NUV) radiation combined with electron and proton radiation was also investigated.
The NASA Marshall Space Flight Center is currently evaluating polymer based components for application in launch vehicle and propulsion system avionics systems. Organic polymers offer great advantages over inorganic corollaries. Unlike inorganics with crystalline structures defining their sensing characteristics, organic polymers can be engineered to provide varying degrees of sensitivity for various parameters including electro-optic response, second harmonic generation, and piezoelectric response. While great advantages in performance can be achieved with organic polymers, survivability in the operational environment is a key aspect for their practical application. The space environment in particular offers challenges that must be considered in the application of polymer based devices. These challenges include: long term thermal stability for long duration missions, extreme thermal cycling, space radiation tolerance, vacuum operation, low power operation, high operational reliability. Requirements for application of polymer based devices in space avionics systems will be presented and discussed in light of current polymer materials.
This paper reports the radiation tolerance of small form factor multimode optical transceivers designed for extended temperature and vibration environments. The transceivers are based on specifically designed VCSELs and PIN photodiodes and packaged such that the transceiver functions over an extended temperature range (beyond -40 to +85°C) without affecting performance or dynamic range. These transceivers are capable of multi-Gigabit per second transmission with bit error ratio of less than 1e-12 in non-irradiated conditions. To characterize these transceivers, we have measured their response to dose rate, total dose, and neutron fluence environments. For the dose rate test, we report the upset, latch-up, and burnout thresholds of the transceiver. In the total dose test, we measure the total dose level that does not significantly affect the transceiver and characterize the degradation for increasing total dose levels. For the neutron fluence test, we measure the fluence level that does not significantly affect the transceiver and characterize the degradation for increasing fluence levels. With this radiation data on the performance limits and penalties of the transceiver, high speed fiber optic links may now be considered for platforms previously off limits due to their irradiated environments.
This paper reports on a novel optical linearized directional coupler modulator in stoichiometric lithium niobate (SLN). The linearized design has important applications in analog and RF communications systems where fiber optic link performance depends critically on the spurious-free dynamic range of the modulator. Newly available SLN has several distinct advantages over the congruently grown crystals commonly used for high speed integrated optic devices, including higher electrooptic coefficient and better ferroelectric properties. The higher electrooptic coefficient yields lower drive voltage, while the enhanced ferroelectric properties enable better velocity-matched electrode structures using domain inverted waveguides. This paper addresses the operation of the linearized directional coupler design, and the critical advantages of the SLN substrate for implementing high-speed operation using velocity-matching.
Optical fiber sensors offer a number of advantages for spacecraft applications, including freedom from electromagnetic interference, light weight, and low power consumption. One application is strain sensing, where high sensitivity and bandwidth and the ability to individually interrogate tens of multiplexed sensors via a single fiber lead has been demonstrated. This paper will describe 2 recent NRL uses of distributed strain sensing using arrays of fiber Bragg gratings (FBGs) on spacecraft parts, structures, and ground test hardware: distributed dynamic strain monitoring of a lightweight reflector during acoustic qualification tests and high-frequency, high-sensitivity strain measurements of a latch fixture. A second fiber sensor being seriously considered for spacecraft is the interferometric fiber optic gyroscope (IFOG). Although its performance in a benign environment is quite attractive, deployment of this and other optical fiber sensors requires addressing issues such as the deleterious effects of the space radiation environment. These challenges, unique to this application, will be discussed.
Future thermonuclear fusion reactors need remote-handled equipment for maintenance tasks, since the stringent environmental conditions prohibit direct human interventions. Fiber-optic technology is considered since many years as a potential reliable alternative to conventional electronic transmission lines. Recently we demonstrated the feasibility of transmitting analog data with a hybrid opto-electronic link at 850 nm, up to total doses of several MGy. However, for bidirectional communications under these severe conditions, we still need to characterize the corresponding photo detector response and design an adapted radiation tolerant amplifier. We therefore assessed in-situ the radiation response of commercially available p-i-n type Si-detectors, at a constant temperature of about 60°C, first under gammas rays up to a total dose of about 10 MGy and under neutrons up to a fluence of about 7•1015/cm2. We also performed similar tests with InGaAs photodiodes at different wavelengths, in order to assess their use in radiation tolerant coarse wavelength division multiplexing (CWDM) transmission architectures. Our results indicate that the wavelength dependence of the detectors' response under gamma radiation remains almost unchanged. We observed no catastrophic failure for these InGaAs devices, nor for the Si devices up to 10 MGy. The increase of the dark current is the most obvious radiation effect, particularly under neutrons. In this paper we also present temperature dependent measurements and hence assess to what extend both ionizing and particle radiation affect the photodiodes reliability.
Chemical sensors play a very important role when it comes to information gathering about the environment we live in. Conducting polymers have been used as transducer elements in many sensor devices as they offer great design flexibility, ease of processing and excellent environmental stability. Conducting polymer polypyrrole has found applications in the area of chemical sensing, primarily because of the conductivity modulation that comes about in it due to interaction with gases. In this paper, conducting polymer thin films are applied to optical fibers as a chemo-chromic transducer to sense toxic gases like ammonia, dimethylmethylphosphonate (a chemical precursor to nerve gas sarin) and organic vapors like acetone. The developed sensor device is based on the modified cladding or coating approach. In the sensor design, a small section of the optical fiber cladding is replaced by the conducting polymer polypyrrole. The optical property changes that come about in polypyrrole due to the presence of the gas leads to a change in the transmission properties of the fiber and hence gas sensing via intensity modulation.
We recently demonstrated the first optical refrigerator based on anti-Stokes fluorescence. Optical refrigeration offers several advantages over more conventional cooling techniques including no vibration, no electromagnetic interference (EMI), low mass, and low volume. Analytical and experimental research is being performed to eliminate heating processes and maximize the cooling power in Ytterbium doped compounds. Current research is focused on developing new techniques and materials for optical cooling. We have developed a bulk cooling technique to measure the cooling capacity of materials without the need for applying dielectric mirror coatings. This method involves bulk cooling measurements with a thermal camera of potential samples for optical cooling. Test configurations include single pass, two pass, or multiple pass tests. Two pass and multiple pass tests use external mirrors to return the pump beam to the fluorescent element to enhance absorption. Comparison of bulk cooling in samples gives a clear indication of which samples will perform optimally for optical refrigeration. The results of measurements and analyses of various samples are presented and compared with photothermal deflection spectroscopy results.
Nanostructured OrigamiTM 3D Fabrication and Assembly Process is a
method of manufacturing 3D nanosystems using exclusively 2D litho tools. The 3D structure is obtained by folding a nanopatterned 2D substrate. We report on the materials, actuation, and modeling aspects of the manufacturing process, and present experimental results from fabricated structures.
Integrated holographics is a novel photonics technology made possible by recent advances in semiconductor manufacturing technology and planar waveguide fabrication. The technology's corner stone, the holographic Bragg reflector (HBR), is a slab-waveguide based, nanoscale, refractive-index structure that merges, for the first time, powerful features of holography, such as single-element spectral and spatial signal processing and overlay of multiple structures, with a highly integrated environment. As a building block for photonic circuits, the HBR's holographic signal mapping comprises a unique and novel way of on-chip signal routing and transport that is free-space-like but fully integrated. Signals propagate and overlap freely as they are imaged from active element to active element - an architecture that eliminates the need for constraining electronics-style channel-waveguides and associated space requirements and opens the door to unique integrated photonic circuits of very compact footprint. Photolithographic HBR fabrication was recently demonstrated to provide complete amplitude and phase control over individual HBR diffractive elements thus offering the powerful ability to implement almost arbitrary phase-coherent spectral filtering functions. This is enabling to a broad range of optics-on-a-chip devices including compact multiplexers, tailored passband optical filters, optical switch fabrics, spectral comparators, and correlator-based optical look-up tables.
Using detailed numerical simulations, and analytical theory, we study properties of micro-cavities which incorporate materials that exhibit Electro-magnetically Induced Transparency (EIT) or Ultra Slow Light (USL). We find that such systems, while being miniature in size (order wavelength), and integrable, can have some outstanding properties. In particular, they could have lifetimes orders of magnitude longer than other existing systems, and could exhibit non-linear all-optical switching at single photon power levels. Potential applications include miniature atomic clocks, and all-optical quantum information processing.
We describe our latest experimental and theoretical results on two promising nanophotonics geometries for sensor applications. These geometries are based on various combinations of nanohole and/or microdroplet arrays on the surfaces of metal films which support propagation of surface plasmon-polaritons. These novel geometries exhibit large enhancements of local electromagnetic field, which can be used in various nonlinear optical sensing arrangements. For example, liquid microdroplets on the gold film surface support surface plasmon whispering gallery modes. Local field enhancement due to excitation of such modes is determined by combination of both cavity electrodynamics and surface plasmon-polariton related effects. In addition, individual microdroplets have interesting imaging properties, which may be used in high-resolution visualization of individual viruses and cells.
Polarization properties of the enhanced light transmission through a thin gold film perforated with an array of subwavelength elliptical holes have been studied. It is shown that broadband optical transmission can be achieved through such nanostructures due to the complex nature of the SPP Bloch modes related to a periodic lattice with a low symmetry primitive cell. The optical transmission is dependent on both the incident and transmitted light polarization states even at normal incidence. Using this feature it is therefore possible to tune the transmission spectrum by selecting the polarization of the incident and/or transmitted light. It is shown that such a nanostructure acts as a thin two-dimensional birefringent crystal with wavelength dependent principal optical axes: the property which is not encountered in natural crystals. A rotation of the polarization of the light transmitted through an array of subwavelength holes strongly depends on the thin layer of chiral material placed upon the nanostructured surface. The polarization rotation effect of the chiral molecules appeared to be coupled with the polarization properties of the metallic structure related to SPP excitations. Optical components based on nanostructured metallic systems can find numerous photonic applications space-based and terrestrial systems in extreme ambient conditions.
A Y-branch photonic crystal laser diode operating at 1.55 μm is presented. The photonic band gap of the three-layered device was calculated by the use of simulation software. The propagation constant was determined from simulation. In addition, a scattering matrix approach was used to model the resonant structure of the beam splitting laser, incorporating the effects of internal reflections on the cavity mode structure and mode spacing. Phase matching conditions were explored, taking into account all the different resonant sub-structures. The results from the phase matching analysis lead directly to an operating mode that reduces threshold current beyond that of the conventional ridge laser. Optical gain calculations in an InGaAsP-InP MQW structure show a threshold current of 0.5mA. This device could be configured to work as a combined optical amplifier and beam splitter. The paper ends with a discussion of how this would be possible.
We have developed chemical-based methods to produce binary assemblies of nanocrystals. The ordered arrays that result are superlattices that mimic the structures of known crystal phases. Applications of this new type of material extends into the realm of optical science and technology. The model is of single component nanocrystals in the 5-20 nm range, which build multicomponent structures of micrometer dimensions. The method presents the opportunity to choose from a variety of inorganic nanocrystals (e.g. semiconducting, magnetic) in order to prepare superlattices with uniquely tunable properties. Transition metal and transition metal oxide nanocrystals are nanometer dimension crystals composed of one or more metals from the d block of the periodic table, and oxygen. The nanocrystals have capping groups which render them discrete, stable, and enable them to be manipulated in a variety of media such as solvents or polymers. The nanocrystals are ideally monodisperse, uniform in composition, crystalline, and can be prepared over a range of sizes from 5-20 nm. The selection of composition for the nanocrystals is based on materials with known interesting properties (optical, electronic or electrical) in the bulk phase. Once fully characterized, the nanocrystals can be considered as components for the assembly of a nanostructured composite material designed to exhibit interesting collective properties with tunable control at the nanoscale.
Currently used optical filters exhibit strong limitations in the deep UV and shorter wavelength ranges. We propose an entirely different type of UV filter to solve many of the problems due to inadequate materials and fabrication techniques. These filters consist of three-dimensionally ordered Macroporous Silicon (MPSi), with the pores used as waveguide cores separated by the reflective silicon host. Ordered pores serve as a two-dimensional array of optical waveguides. Multilayer coating of the pore walls results in the band-pass, short-pass, or band-blocking transmittance spectra of MPSi filters. Such filters have a number of advantages. They do not exhibit spectral shifts of the passed or blocked spectral bands with the angle of incidence, permitting operation in tilted and divergent light beams to simplify optical system design and fabrication. Due to their structures (fewer and thinner layers on the pore walls required to gain the same level of rejection), the filters do not exhibit delamination problems and are well suited for operation at extreme temperatures (for space as well as for terrestrial environments). The fabrication process is different from that used for multilayer interference filters. This process permits the fabrication of filters up to 200mm in diameter that are suitable for wavelengths from longer than 400 nm to shorter than 100 nm. Far-UV filters can be manufactured as simply and economically as the near UV ones. The theory of light propagation through the MPSi layers is developed, the main predictions of the theory are experimentally validated, and the fabrication procedure for MPSi UV filters is reported.