This PDF file contains the front matter associated with SPIE Proceedings Volume 9616, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Radiation effects are well know to cause significant degradation in polymer materials. Low earth orbit (LEO) radiation exposures cause ionization potentials that can undermine mechanical properties of polymers. In particular, small scale degradations can undermine carbon / polymer composite mirrors used in imaging applications. High specularity surface finish is required for optical mirrors and that surface is vulnerable to radiation ionization degradation thereby undermining the optical performance of the mirror in that environment. Experiments involving radiation ionization and its effects on replicated carbon/polymer composite mirrors will be will be discussed; 6 replicated carbon/polymer composite mirrors on the Materials on the International Space Station Experiment, MISSE 7 and MISSE 8, the replicated RICH mirror the Alpha Magnetic Spectrometer (AMS-02) and testing on the RICH 1 replicated mirrors in the LHCb experiment. Results are favorable for optically coated composite mirrors in terms of mirror figure, reflectivity and surface finish, but no so on uncoated polymer mirrors.

Molecular composition of distant stars is explored by observing absorption spectra. The star produces blackbody radiation that passes through the molecular cloud of vaporized material surrounding the star. Characteristic absorption lines are discernible with a spectrometer, and molecular composition is investigated by comparing spectral observations with known material profiles. Most objects in the solar system—asteroids, comets, planets, moons—are too cold to be interrogated in this manner. Molecular clouds around cold objects consist primarily of volatiles, so bulk composition cannot be probed. Additionally, low volatile density does not produce discernible absorption lines in the faint signal generated by low blackbody temperatures. This paper describes a system for probing the molecular composition of cold solar system targets from a distant vantage. The concept utilizes a directed energy beam to melt and vaporize a spot on a distant target, such as from a spacecraft orbiting the object. With sufficient flux (~10 MW/m²), the spot temperature rises rapidly (to ~2 500 K), and evaporation of all materials on the target surface occurs. The melted spot creates a high-temperature blackbody source, and ejected material creates a molecular plume in front of the spot. Bulk composition is investigated by using a spectrometer to view the heated spot through the ejected material. Spatial composition maps could be created by scanning the surface. Applying the beam to a single spot continuously produces a borehole, and shallow sub-surface composition profiling is also possible. Initial simulations of absorption profiles with laser heating show great promise for molecular composition analysis.

Laser ablation of a Near Earth Object (NEO) on a collision course with Earth produces a cloud of ejecta which exerts a thrust on the asteroid, deflecting it from its original trajectory. The DE-STAR system provides such a thrust by illuminating an Earth-targeting asteroid or comet from afar with a stand-off system consisting of a large phased-array laser in Earth orbit. A much smaller version of the same system called DE-STARLITE travels alongside the target, operating in a stand-on mode, slowly deflecting it over a long period. Such a stand-on system would also permit directing the thrust in any desired direction through careful positioning of the laser relative to the asteroid. We present orbital simulations comparing the effectiveness of both systems across a range of laser and asteroid parameters. Simulated parameters include magnitude, duration and, for the stand-on system, direction of the thrust, as well as the size and orbital characteristics of the target asteroid. These simulations indicate that deflection distance is, in general, proportional to the magnitude of thrust, proportional to the square of the laser on time, and inversely proportional to the mass. Furthermore, deflection distance shows strong dependence on thrust direction with optimal direction varying with the asteroid's orbital eccentricity. As one example, we consider a 325 m asteroid in an orbit of eccentricity e=0.2; given 15 years of warning, a force of just 2 N from a stand-on DE-STARLITE system is sufficient to deflect the asteroid by 2 Earth radii. We discuss numerous scenarios and discuss a practical implementation of such a system consistent with current launch vehicle capabilities.

We report on laboratory studies of the effectiveness of directed energy planetary defense as a part of the DESTAR (Directed Energy System for Targeting of Asteroids and exploRation) program. DE-STAR [1][5][6] and DE-STARLITE [2][5][6] are directed energy "stand-off" and "stand-on" programs, respectively. These systems consist of a modular array of kilowatt-class lasers powered by photovoltaics, and are capable of heating a spot on the surface of an asteroid to the point of vaporization. Mass ejection, as a plume of evaporated material, creates a reactionary thrust capable of diverting the asteroid’s orbit. In a series of papers, we have developed a theoretical basis and described numerical simulations for determining the thrust produced by material evaporating from the surface of an asteroid [1][2][3][4][5][6]. In the DE-STAR concept, the asteroid itself is used as the deflection "propellant". This study presents results of experiments designed to measure the thrust created by evaporation from a laser directed energy spot. We constructed a vacuum chamber to simulate space conditions, and installed a torsion balance that holds an "asteroid" sample. The sample is illuminated with a fiber array laser with flux levels up to 60 MW/m2 which allows us to simulate a mission level flux but on a small scale. We use a separate laser as well as a position sensitive centroid detector to readout the angular motion of the torsion balance and can thus determine the thrust. We compare the measured thrust to the models. Our theoretical models indicate a coupling coefficient well in excess of 100 μN/Woptical, though we assume a more conservative value of 80 μN/Woptical and then degrade this with an optical "encircled energy" efficiency of 0.75 to 60 μN/Woptical in our deflection modeling. Our measurements discussed here yield about 45 μN/Wabsorbed as a reasonable lower limit to the thrust per optical watt absorbed.

Asteroids that threaten Earth could be deflected from their orbits using directed energy to vaporize the surface, because the ejected plume creates a reaction thrust that alters the asteroid’s trajectory. One concern regarding directed energy deflection is the rotation of the asteroid, as this will reduce the average thrust magnitude and modify the thrust direction. Flux levels required to evaporate surface material depend on surface material composition and albedo, thermal, and bulk mechanical properties of the asteroid, and rotation rate. The observed distribution of asteroid rotation rates is used, along with an estimated range of material and mechanical properties, as input to a 3D thermal-physical model to calculate the resultant thrust vector. The model uses a directed energy beam, striking the surface of a rotating sphere with specified material properties, beam profile, and rotation rate. The model calculates thermal changes in the sphere, including vaporization and mass ejection of the target material. The amount of vaporization is used to determine a thrust magnitude that is normal to the surface at each point on the sphere. As the object rotates beneath the beam, vaporization decreases, as the temperature drops and causes both a phase shift and magnitude decrease in the average thrust vector. A surface integral is calculated to determine the thrust vector, at each point in time, producing a 4D analytical model of the expected thrust profile for rotating objects.

Arrays of phase-locked lasers have been developed for numerous directed-energy applications. Phased-array designs are capable of producing higher beam intensity than similar sized multi-beam emitters, and also allow beam steering and beam profile manipulation. In phased-array designs, individual emitter phases must be controllable, based on suitable feedback. Most current control schemes sample individual emitter phases, such as with an array-wide beam splitter, and compare to a master phase reference. Reliance on a global beam splitter limits scalability to larger array sizes due to lack of design modularity. This paper describes a conceptual design and control scheme that relies only on feedback from the array structure itself. A modular and scalable geometry is based on individual hexagonal frames for each emitter; each frame cell consists of a conventional lens mounted in front of the fiber tip. A rigid phase tap structure physically connects two adjacent emitter frame cells. A target sensor is mounted on top of the phase tap, representing the local alignment datum. Optical sensors measure the relative position of the phase tap and target sensor. The tap senses the exit phase of both emitters relative to the target normal plane, providing information to the phase controller for each emitter. As elements are added to the array, relative local position data between adjacent phase taps allows accurate prediction of the relative global position of emitters across the array, providing additional constraints to the phase controllers. The approach is scalable for target distance and number of emitters without loss of control.

Spacecraft accelerate by directing propellant in the opposite direction. In the traditional approach, the propellant is carried on board in the form of material fuel. This approach has the drawback of being limited in Delta v by the amount of fuel launched with the craft, a limit that does not scale well to high Delta v due to the massive nature of the fuel. Directed energy photon propulsion solves this problem by eliminating the need for on-board fuel storage. We discuss our system which uses a phased array of lasers to propel the spacecraft which contributes no mass to the spacecraft beyond that of the reflector, enabling a prolonged acceleration and much higher final speeds. This paper compares the effectiveness of such a system for propelling spacecraft into interplanetary and interstellar space across various laser and sail configurations. Simulated parameters include laser power, optics size and orbit as well as payload mass, reflector size and the trajectory of the spacecraft. As one example, a 70 GW laser with 10 km optics could propel a 1 kg craft past Neptune (~30 au) in 5 days at 4% the speed of light, or a 1 g “wafer-sat” past Mars (~0.5 au) in 20 minutes at 21% the speed of light. However, even lasers down to 2 kW power and 1 m optics show noticeable effect on gram-class payloads, boosting their altitude in low Earth orbits by several kilometers per day which is already sufficient to be of practical use.

In the nearly 60 years of spaceflight we have accomplished wonderful feats of exploration and shown the incredible spirit of the human drive to explore and understand our universe. Yet in those 60 years we have barely left our solar system with the Voyager 1 spacecraft launched in 1977 finally leaving the solar system after 37 years of flight at a speed of 17 km/s or less than 0.006% the speed of light. As remarkable as this is, we will never reach even the nearest stars with our current propulsion technology in even 10 millennium. We have to radically rethink our strategy or give up our dreams of reaching the stars, or wait for technology that does not exist. While we all dream of human spaceflight to the stars in a way romanticized in books and movies, it is not within our power to do so, nor it is clear that this is the path we should choose. We posit a technological path forward, that while not simple; it is within our technological reach. We propose a roadmap to a program that will lead to sending relativistic probes to the nearest stars and will open up a vast array of possibilities of flight both within our solar system and far beyond. Spacecraft from gram level complete spacecraft on a wafer (“wafer sats”) that reach more than ¼ c and reach the nearest star in 15 years to spacecraft with masses more than 10⁵ kg (100 tons) that can reach speeds of near 1000 km/s such systems can be propelled to speeds currently unimaginable with our existing propulsion technologies. To do so requires a fundamental change in our thinking of both propulsion and in many cases what a spacecraft is. In addition to larger spacecraft, some capable of transporting humans, we consider functional spacecraft on a wafer, including integrated optical communications, optical systems and sensors combined with directed energy propulsion. Since “at home” the costs can be amortized over a very large number of missions. The human factor of exploring the nearest stars and exo-planets would be a profound voyage for humanity, one whose non-scientific implications would be enormous. It is time to begin this inevitable journey beyond our home.

The long standing approach to space travel has been to incorporate massive on-board electronics, probes and propellants to achieve space exploration. This approach has led to many great achievements in science, but will never help to explore the interstellar medium. Fortunately, a paradigm shift is upon us in how a spacecraft is constructed and propelled. This paper describes a mission concept to get to our Sun’s Gravity Lens at 550AU in less than 10 years. It will be done by using DE-STAR, a scalable solar-powered phased-array laser in Earth Orbit, as a directed energy photon drive of low-mass wafersats. [1] [2] [3] [4] [5] With recent technologies a complete mission can be placed on a wafer including, power from an embedded radio nuclear thermal generator (RTG), PV, laser communications, imaging, photon thrusters for attitude control and other sensors. As one example, a futuristic 200 MW laser array consisting of 1 - 10 kw meter scale sub elements with a 100m baseline can propel a 10 gram wafer scale spacecraft with a 3m laser sail to 60AU/Year. Directed energy propulsion of low-mass spacecraft gives us an opportunity to capture images of Alpha Centauri and its planets, detailed imaging of the cosmic microwave background, set up interstellar communications by using gravity lenses around nearby stars to boost signals from interstellar probes, and much more. This system offers a very large range of missions allowing hundreds of wafer scale payload launches per day to reach this cosmological data reservoir. Directed Energy Propulsion is the only current technology that can provide a near-term path to utilize our Sun’s Gravity Lens.

We report on time-of-flight (TOF) hole mobility measurements in an aged discotic columnar liquid crystal, Hexakis(pentyloxy)triphenylene (HAT5). The experimental data was fit to an interfacial trapping model based on Van de Walle’s approximations. The theory accurately reproduces the TOF transients of delayed charge release near the optically excited material/electrode interface. Interfacial trapping appears only in the aged materials, but the bulk mobility is the same as that of the pristine material. We also discuss preliminary results of TOF photocurrent transients of HAT5 exposed to ozone.

Understanding the fundamental mechanisms behind the radiation resistance of polymers and molecules would allow us to tailor new materials with enhanced performance in space and adverse environments. Previous studies of the radiation effects on polymer-based photonic materials indicate that they are very dependent on the choice of polymer-host and guest-chromophores. The best results have been reported from the combination of CLD1 as a guest-chromophore doped in APC as host polymer, where improvement of the performance was observed upon gamma-irradiation at moderate doses. In this paper, we report on the different complementary tools that have been tried to characterize the origin of such enhancement: characterization of the linear and nonlinear response, characterization of chemical properties, and application of an all-optical protocol. We derive some general conclusions by contrasting the results of each characterization, and propose complementary experiments based on microscopy techniques.

High performance infrared sensors are vulnerable to slight changes in defect densities and locations. For example in a space application where such sensors are exposed to proton irradiation capable of generating point defects the sensors are known to suffer performance degradation. The degradation can generally be observed in terms of dark current density and responsivity degradations. Here we report results of MWIR HgCdTe/CdZnTe single element diodes dark current densities before and after exposure to 63MeV protons at room temperature to a total ionizing dose of 100 kRad(Si). We find the irradiated diodes as a group show some signs of proton-induced damage in dark current.

In this work, we report on the design, fabrication, and characterization of MWIR unipolar barrier photodetectors based on InAs/GaSb Type-II superlattice. We have designed, fabricated, and characterized band-structure engineered MWIR photodetectors based on the pBiBn architecture. The devices have been characterized using the most relevant radiometric figures of merits. At 200 K, the peak value of detectivity is 1.2 x 1011 Jones at an applied bias voltage of -0.5 V.

The continuous effort to improve space-based infrared (IR) detectors has led to a search for greater fundamental understanding of radiation damage phenomena effects on key material properties. The material parameter of interest in this paper is the minority carrier recombination lifetime (MCRL), which is directly related to detector performance and can be empirically determined. As radiation damage is incurred upon a detector structure, the MCRL can be significantly affected, and tracking this in a step-wise, in-situ fashion at a radiation source can reveal rates of defect introduction. This has been accomplished by the development of a portable MCRL measurement system employing time resolved photoluminescence (TRPL) while maintaining operational temperatures. Using this methodology is more insightful than the so-called ‘bag tests’ (i.e. characterization before and after a single 100krad dosage) due to complex parameter changes witnessed with annealing as temperatures change. In addition to the system description, MCRL data on IR detectors from its inaugural deployments at a proton radiation source are analyzed and reveal a linear relationship between inverse MCRL and proton fluence.

Type-II Strained Layer Superlattice (T2SLS) infrared photodetectors have been in ongoing development over the last decade with the goal of achieving lower dark currents and higher operating temperatures when com- pared to mercury cadmium telluride (MCT) detectors. The theoretically longer Auger recombination lifetime of T2SLS has potential to lower dark current but the presence of Shockley-Read-Hall (SRH) defects limits the recombination lifetime far below the Auger-limit. In order to reduce SRH-recombination, unipolar barriers have been incorporated into the energy bands of T2SLS materials in different forms, such as nBn, to improve performance. Here, noise spectra are presented for varyingly sized, near 90% quantum efficiency, nBn mid-wave infrared (MWIR) detectors with superlattice absorbing layers grown by MBE. Noise spectrum measurements are used to evaluate device performance and reveal mechanisms contributing to low frequency noise that often exceeds predictions based on ideal shot noise. Voltage and temperature dependent noise spectra were taken using an external trans-impedance amplifier with an internal, cooled impedance converter and feedback resistor.

InAs/GaSb type-II strained-layer superlattice (T2SLS) materials are being considered for space-based infrared detector applications. However, an inadequate understanding of the role of carrier transport, specifically the vertical mobility, in the radiation tolerance of T2SLS detectors remains. Here, progress towards a vertical transport study of proton-irradiated, p-type InAs/GaSb T2SLS materials using magnetoresistance measurements is reported. Measurements in the growth direction of square mesas formed from InAs/GaSb superlattice material were performed using two distinct contact geometries in a Kelvin mode setup at variable magnetic fields, ranging from -9 T to 9 T, and temperatures, ranging from 5 K and 300 K. The results here suggested multi-carrier conduction and a field-dependent series resistance from the contact layer were present. The implications of these results and the plans for future magnetoresistance measurements on proton-irradiated T2SLS materials are discussed.

We are developing VCSEL technology producing >100mW in single frequency at wavelengths 780nm, 795nm and 850nm. Small aperture VCSELs with few mW output have found major applications in atomic clock experiments. Using an external cavity three-mirror configuration we have been able to operate larger aperture VCSELs and obtain >70mW power in single frequency operation.

The VCSEL has been mounted in a fiber pigtailed package with the external mirror mounted on a shear piezo. The package incorporates a miniature Rb cell locker to lock the VCSEL wavelength. This VCSEL operates in single frequency and is tuned by a combination of piezo actuator, temperature and current. Mode-hop free tuning over >30GHz frequency span is obtained. The VCSEL has been locked to the Rb D2 line and feedback control used to obtain line-widths of <100kHz.

We have designed and completed initial testing on a laser source suitable for atomic interferometry from compact, robust, integrated components. Our design is enabled by capitalizing on robust, well-commercialized, low-noise telecom components with high reliability and declining costs which will help to drive the widespread deployment of this system. The key innovation is the combination of current telecom-based fiber laser and modulator technology with periodicallypoled waveguide technology to produce tunable laser light at rubidium D1 and D2 wavelengths (and expandable to other alkalis) using second harmonic generation (SHG). Unlike direct-diode sources, this source is immune to feedback at the Rb line eliminating the need for bulky high-power isolators in the system. In addition, the source has GHz-level frequency agility and in our experiments was found to only be limited by the agility of our RF generator. As a proof-of principle, the source was scanned through the Doppler-broadened Rb D2 absorption line. With this technology, multiple channels can be independently tuned to produce the fields needed for addressing atomic states in atom interferometers and clocks. Thus, this technology could be useful in the development cold-atom inertial sensors and gyroscopes.

This paper presents a novel architecture for a high performance atomic clock based on the use of miniature optical whispering gallery mode (WGM) resonators. Following the approach of stabilizing a laser local oscillator to an optical transition in an atom or ion, as used in advanced atomic clock, a semiconductor laser is used for stabilization to the D₁ line of Rb atoms, held in a small vapor cell. The laser is self-injection locked to a WGM resonator to reduce its linewidth. To produce the RF output of the clock, a second WGM resonator excited with a second cw semiconductor laser produces an optical frequency comb that is demodulated on a fast photodiode. Locking the resonator that generates the frequency comb to the laser stabilized to the Rb transition transfers the stability of the atomic transition to the RF output of the clock. In this way, a miniature all-optical atomic clock is realized. Details of the operation of the clock and application of the architecture to other atomic systems, such as a ytterbium ion, will be described.

Fast-light phenomena can enhance the sensitivity of an optical gyroscope of a given size by several orders of magnitude, and could be applied to other optical sensors as well. MagiQ Technologies has been developing a compact fiber-based fast light Inertial Measurement Unit (IMU) using Stimulated Brillouin Scattering in optical fibers with commercially mature technologies. We will report on our findings, including repeatable fast-light effects in the lab, numerical analysis of noise and stability given realistic optical specs, and methods for optimizing efficiency, size, and reliability with current technologies. The technology could benefit inertial navigation units, gyrocompasses, and stabilization techniques, and could allow high grade IMUs in spacecraft, unmanned aerial vehicles or sensors, where the current size and weight of precision gyros are prohibitive. By using photonic integrated circuits and telecom-grade components along with specialty fibers, we also believe that our design is appropriate for development without further advances in the state of the art of components.

Polymer based optoelectronic composites and thin film devices exhibit great potential in space applications due to their lightweight, flexible shape, high photon absorption coefficients, and robust radiation tolerance in space environment. Polymer/dye composites appear promising for optoelectronics applications due to potential enhancements in both light harvesting and charge separation. In this study, the optoelectronic properties of a series of molecular dyes paired with a conjugated polymer Poly(3-hexylthiophene-2,5-diyl) (P3HT) were investigated. Specifically, the solution PL quenching coefficients (K_sv) of dye/polymer follows a descending order from dyes of Chloro(protoporphyrinato)iron(III) (Hemin), Protoporphyrin, to meso-Tetra(4-carboxyphenyl)porphine (TCPP). In optoelectronic devices made of the P3HT/dye/PCBM composites, the short circuit current densities J_sc as well as the overall power conversion efficiencies (PCE) also follow a descending order from Hemin, Protoporphyrin, to TCPP, despite Hemin exhibits the intermediate polymer/dye LUMO (lowest unoccupied molecular orbital) offset and lowest absorption coefficient as compared to the other two dyes, i.e., the cell optoelectronic efficiency did not follow the LUMO offsets which are the key driving forces for the photo induced charge separations. This study reveals that too large LUMO offset or electron transfer driving force may result in smaller PL quenching and optoelectronic conversion efficiency, this could be another experimental evidence for the Marcus electron transfer model, particularly for the Marcus ‘inverted region’. It appears an optimum electron transfer driving force or strong PL quenching appears more critical than absorption coefficient for optoelectronic conversion devices.

Time Delay and Integration (TDI) is used to increase the Signal to Noise Ratio (SNR) in image sensors when imaging fast moving objects. One important TDI application is in Earth observation from space. In order to operate in the space radiation environment, the effect that radiation damage has on the performance of the image sensors must be understood. This work looks at prototype TDI sensor pixel designs, produced by e2v technologies. The sensor is a CCD-like charge transfer device, allowing in-pixel charge summation, produced on a CMOS process. The use of a CMOS process allows potential advantages such as lower power consumption, smaller pixels, higher line rate and extra on-chip functionality which can simplify system design. CMOS also allows a dedicated output amplifier per column allowing fewer charge transfers and helping to facilitate higher line rates than CCDs. In this work the effect on the pixels of radiation damage from high energy protons, at doses relevant to a low Earth orbit mission, is presented. This includes the resulting changes in Charge Transfer inefficiency (CTI) and dark signal.

A microscopic-level model is proposed for exploring degraded performance in electron transport and photodetection devices, based on pre-calculated results as initial conditions for meso-scale approaches, including ultra-fast displacement cascade, intermediate defect stabilization and cluster formation, and slow defect reaction and migration. The steady-state spatial distribution of point defects in a mesoscopic-scale layered system will be studied by taking into account the planar dislocation loops and spherical neutral voids as well. These theoretical efforts are expected to be crucial in fully understanding the physical mechanism for identifying defect species, performance degradations, and the development of mitigation strategies. Additionally, verification of the current model by device characterization is discussed.

This work presents a fundamental investigation of the surface conduction pathways occurring along etched sidewalls in devices fabricated from InAs and GaSb. Surface leakage currents are identified by their dependence on device size and thermal activation energy, and are characterized in terms of sheet conductance. InAs is found to have a temperature-independent sheet conductance of approximately 8×10^-8 mho×square. The sheet conductance of GaSb is comparable to that of InAs at room temperature, and when cooled it decreases with a thermal activation energy of 75 meV, which is approximately equal to the known separation between the valence band and surface Fermi level. The temperature dependence of the surface conductance of the two materials indicates that the surface of InAs is degenerate and the surface of GaSb is non-degenerate.

Optical cooling of solids is a promising and innovative method to provide cryogenic cooling to infrared sensors. Currently insulator crystals, specifically ytterbium-doped yttrium- lithium-fluoride (Yb:YLF), have shown the most promise for cooling to low temperatures. This method has demonstrated cooling below the National Institute of Standards and Technology (NIST) cryogenic temperature definition of less than 123 K. Optical refrigeration utilizes a phenomenon called anti-Stokes fluorescence to generate cooling power. Incident laser light is absorbed by the cooling crystal and photons are spontaneously emitted at a higher, and thus more energetic, frequency. The difference in frequency is proportional to the cooling power of the crystal. Anti-Stokes cooling is highly dependent on doping percentages and YLF crystal purity and structure. Space based infrared sensors and their coolers are operated in a radiation environment where protons, gamma, rays, heavy ions, and other radiation species are common and of varying severities depending on operational orbit. To ensure that radiative effects on cooling crystal performance are minimal, we irradiated two samples with 63 MeV protons to a total of ionized dose of 100 Krad (Si) and 1 Mrad (Si), and compared cooling crystal efficiency parameters before and after dosing.

Laser sources operating near a wavelength of four microns are important for a broad range of space and airborne applications. Efficient solid-state laser sources, demonstrating the highest output power, are based upon nonlinear optical (NLO) conversion using the NLO crystal ZnGeP₂. However, a related NLO crystal, CdSiP₂, is now under investigation by several groups around the world. A comparison of its figure of merit for high-power handling with other NLO candidates indicates its potential for higher performance. In addition, the crystal’s characteristics as well as efforts to understand the crystal’s defects that presently limit NLO performance are briefly discussed.

Owing to their ability to move a target in space without requiring propellant, laser-based deflection methods have gained attention among the research community in the recent years. With laser ablation, the vaporized material is used to push the target itself allowing for a significant reduction in the mass requirement for a space mission. Specifically, this paper addresses two important issues which are thought to limit seriously the potential efficiency of a laser-deflection method: the impact of the tumbling motion of the target as well as the impact of the finite thickness of the material ablated in the case of a space debris. In this paper, we developed a steady-state analytical model based on energetic considerations in order to predict the efficiency range theoretically allowed by a laser deflection system in absence of the two aforementioned issues. A numerical model was then implemented to solve the transient heat equation in presence of vaporization and melting and account for the tumbling rate of the target. This model was also translated to the case where the target is a space debris by considering material properties of an aluminium 6061-T6 alloy and adapting at every time-step the size of the computational domain along with the recession speed of the interface in order to account for the finite thickness of the debris component. The comparison between the numerical results and the analytical predictions allow us to draw interesting conclusions regarding the momentum coupling achievable by a given laser deflection system both for asteroids and space debris in function of the flux, the rotation rate of the target and its material properties. In the last section of this paper, we show how a reasonably small spacecraft could deflect a 56m asteroid with a laser system requiring less than 5kW of input power.