We present the initial laboratory test results of the adaptive optics (AO) subassembly for the Low-Cost Optical Terminal (LCOT), a flexible communications ground terminal developed by Goddard Space Flight Center. LCOT will receive first light in 2023 testing. This terminal includes a 700mm commercial telescope, 1550nm receive instruments, and uplink transmit systems. Demodulating coherent formats requires AO to correct turbulence effects and allow coupling into single-mode fiber. General Atomics delivered the system to Goddard in September 2021, where engineers have evaluated performance. We describe laboratory testing, turbulence phase plate design, results, and AO field testing plans when installed on LCOT.
We present the status of ongoing work at NASA’s Goddard Space Flight Center (GSFC) to build a prototype, low-costof- production, flexibly-configured ground terminal for space optical communication. For laser telecommunication to be cost effective for future missions, a wide-spread global network of operationally responsive optical terminals should be established. There has been a decades-old need for a single modular open systems approach (MOSA) ground terminal architecture capable of supporting multiple space missions ranging from LEO to Lunar distances with 2-way laser communications. At the heart of LCOT’s design concept is the Free-Space Optical Subsytem (FSOS). The major subassemblies of LCOT/FSOS that address most optical comms configurations are : (1) Single 700mm F/12 Nasmyth folded Rx R-C Telescope, (2) Four independent 150mm diameter high-power all-reflective Tx beam directors (XOA), (3) Non-coherent direct detection Rx bench on starboard side of telescope (SOB), and (4) Coherent (possibly Quantum) optical communications bench on port side (POB). The Low-Cost Optical Terminal (LCOT) research and development (R&D) prototype is designed to be a generalized system that can be quickly field-reconfigured to support a wide variety of laser communications missions past, present, and future.
This paper provides the status of ongoing work at NASA-Goddard Space Flight Center (GSFC) to build a low-cost flexible ground terminal for optical communication. For laser communication to be cost-effective for future missions, a global network of flexible optical terminals must be put in place. There is a need for a single ground terminal design capable of supporting multiple missions ranging from LEO to lunar distances. NASA’s Low-Cost Optical Terminal (LCOT) has a single modular design that can be quickly reconfigured to support different laser communications missions. The LCOT prototype uses a 70cm commercially available telescope designed with optical and quantum communications in mind. This telescope is currently being integrated with a state-of-the-art adaptive optics system, and novel high-power laser amplifier demonstrate its utility as an optical communications receiver by receiving a downlink from the recently launched Laser Communication Relay Demonstration (LCRD). LCOT uses commercially available components wherever possible, and where commercial options are not available, the LCOT team works with vendors to create commercial options. This paper discusses the development progress for the blueprint of NASA’s future global ground terminal network.
We present the status of ongoing work at NASA-Goddard Space Flight Center (GSFC) to build a low-cost flexible ground terminal for optical communication. Previous laser communication missions at NASA have been supported by one-of-akind ground terminals built specifically for each mission. If NASA is to build a global network of optical terminals to enable widespread use of optical communications, then a blueprint for an economical ground terminal able to support a variety of missions is needed. With this goal in mind, NASA is constructing a ground terminal in Greenbelt, Maryland to enable testing of new ground terminal technologies from industry to academia.
The study of light carrying complex phase profiles, specifically orbital angular momentum (OAM), has been of interest for its use in free-space optical communications and remote sensing systems. Each of these applications requires a beam to propagate through the atmosphere, where optical turbulence is the main distorter of the beam. In this computational study, coherent Laguerre-Gaussian (LG) beams and partially coherent Im Bessel beams are propagated through atmospheric turbulence. The LG beams are propagated through turbulence using a split-step method for solving the Fresnel diffraction integral. Whereas for the the Im Bessel beams, the coherent mode representation is used, where each eigenmode is individually propagated through turbulence. The split-step algorithm is then modified to simulate optical turbulence by the use of phase screens. Beam metrics, in the form of intensity, scintillation, spot size, and OAM spectrum, are then calculated over a number of turbulence realizations. Three turbulence regimes are simulated that include the weak, moderate, and strong turbulence regimes along with two different initial beam sizes. The Im Bessel beam is simulated using three values of overall coherence ξ. The results for the metrics are plotted against propagation distance and OAM mode l. The resulting beam metrics show a strong dependence on turbulence strength, a weak dependence on OAM mode due to LG modes expanding with an extra prefactor of ι + 1, and no strong dependence on the overall coherence ξ.
Optical countermeasures are widely used nowadays and quite often a laser is used as the optical source. Unfortunately such a laser beam can become severely distorted by optical turbulence when propagating through the atmosphere, resulting in effects such as beam spreading, beam wander, irradiance fluctuations, and loss of spatial coherence. These effects can be (partially) overcome using knowledge of the atmospheric conditions, as well as techniques to correct for amplitude and phase distortions. Our research focuses on the characterization of the atmospheric conditions, using adaptive optics, an in-house developed multi-aperture transmissometer, as well as a plenoptic sensor using phase distortion algorithms to compensate for effects caused by (strong) turbulence conditions.
Laser beams used in many open space applications, such as in defense, optical communication, and remote sensing, will subject to turbulence distortions that disrupt the intended beam profiles at the end of propagation. To guide the transmitted beam properly through an open space channel, adaptive optics (AO) are often used to implement beam corrections based on the reciprocity principles. In specific, if wave distortion from a remote spot can be determined and field conjugated at the site of the transmitter, the transmitted light will focus to the same spot at the receiver. Many experiments have demonstrated such a principle using a cooperative laser guide star on the target plane. However, finding or creating a well-defined guide star is impractical in real-world applications. The second best beacon choice is temporal glint signals that are relatively refined in geometry and brighter than ambient target illumination. To date, the best approach to extract information from arbitrary glint signals to instruct AO correction is still unknown. We propose the plenoptic sensor technique to extract phase distortion information from glint signals with minimum loss of information. In addition, as the addressed turbulence channel is typically a lateral path near the ground, we also validate the function of the plenoptic sensor in revealing the anisotropic state of turbulence.
In recent years, theoretical studies of optical propagation through turbulence exhibiting statistical anisotropy and a non- Kolmogorov energy spectrum, with a power law unequal to 11/3rds, have appeared regularly in the literature. In this study, we use split-step propagation wave optics simulations, together with FFT-based subharmonic phase screens to compare long-term beam spreading and scintillation index statistical projections using the Rytov method with theories leveraging coherence length-base perturbation terms. We demonstrate the latter method’s superior accuracy versus simulation for turbulence conditions above the weak fluctuation regime. The necessity of including low spatial frequency subharmonic components, with frequencies below the domain of simulation, to achieve agreement with theory for large magnitudes of the energy spectrum power law is demonstrated. Additionally, a new method of including low frequency components in the phase screens used in simulation with significant computation time improvements relative to the traditional subharmonic method is introduced.
Refraction effects can alter the appearance of an object near the horizon, resulting in sometimes severely distorted images of the object. The amount of distortion is determined by the curved trajectory that light rays follow through the atmosphere. Variations in the index of refraction, due to among others variations in the temperature of the atmosphere, are responsible for this curving of light rays. There are several models that can be used for to calculate the curvature of the light rays. Lehn has developed a simple model that traces light rays through a refracting atmosphere. The atmosphere is represented by spherical layers in which the temperature is taken constant. The intersections of the light ray and the boundaries are calculated in terms of the temperature and pressure of the atmosphere in this layer.
We present an experimental evaluation of a multi-aperture laser transmissometer system which profiles long-term laser beam statistics over long paths. While the system was originally designed to measure the aerosol extinction rate, the beam profiling capabilities of the transmissometer system also allows experimental observations of Gaussian beam statistics in weak and strong turbulence. Additionally, measurement of long-term beam spread at the receiver allows the system to estimate a path-averaged Cn2, including in strong turbulence regimes where scintillometers experience saturation effects. Additionally, a phase-frequency correlation technique for synchronizing with transmitter ON/OFF modulation in the presence of background ambient light is presented. In application, our ruggedized and weather resistant laser transmissometer system has significant advantages for the measurement and study of aerosol concentration, absorption, scattering, and turbulence properties over multi-kilometer paths, which are crucial for directed energy systems, ground-level free-space optical communication systems, environmental monitoring, and weather forecasting.
Recently, the number of optical systems that operate along near horizontal paths within a few meters of the ground has increased rapidly. Examples are LIDAR or optical sensors imbedded in a vehicle, long range surveillance or optical communication systems, a LIFI network, new weather monitoring stations, as well as directed energy systems for defense purposes. Near ground turbulence distortion for optical waves used in those systems cannot be well described by conventional turbulence and beam propagation theory. Phenomena such as anisotropy, micro mirage effects, a temporal negative relation between diurnal dips and altitude, and condensation induced measurement errors are frequently involved. As a result, there is a high risk of defective designs or even failures in those optical systems if the near ground turbulence effects are not well considered. To illustrate such risk, we make Cn2 measurements by different approaches and cross compare them with associated working principles. By demonstrating the reasons for mismatched Cn2 results, we point out a few guidelines regarding how to use the general anisotropy theorem and the risk of ignoring it. Our conclusions can be further supported by an advanced plenoptic sensor that provides continuous wavefront data.
Large temperature gradients are a known source of strong atmospheric turbulence conditions. Often times these areas of strong turbulence conditions are also accompanied by conditions that make it difficult to conduct long term optical atmospheric tests. The Shuttle Landing Facility (SLF) at the Kennedy Space Center (KSC) provides a prime testing environment that is capable of generating strong atmospheric turbulence yet is also easily accessible for well instrumented testing. The Shuttle Landing Facility features a 5000 m long and 91 m wide concrete runway that provides ample space for measurements of atmospheric turbulence as well as the opportunity for large temperature gradients to form as the sun heats the surface. We present the results of a large aperture LED scintillometer, a triple aperture laser scintillometer, and a thermal probe system that were used to calculate a path averaged and a point calculation of Cn2. In addition, we present the results of the Plenoptic Sensor that was used to calculate a path averaged Cn2 value. These measurements were conducted over a multi-day continuous test with supporting atmospheric and weather data provided by the University of Central Florida.
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