Given the rapid demand for higher and higher bandwidth and the contention for radio frequency (RF) spectrum allocations, the need for space-based Free Space Optical Communications (FSOC) systems is ever increasing. We have previously presented design concepts for a lightweight, small, and high performance two-axis gimbal focusing on the use of commercial off-the-shelf (COTS) subsystems. Our efforts to design a small gimbal with 100 micro-radian pointing accuracy for FSOC have resulted in an unconventional optical fiber wrap design in order to achieve the low optical noise needed to meet the design goals. However, the design raised concerns about stress induced in the optical fibers. To verify our design assumptions and the actual gimbal performance, a mockup of the fiber wrap configuration was subjected to a Cyclical Stress Test conceived to mimic the desired on-orbit lifetime. At regular intervals throughout the test duration, optical power measurements were collected to characterize the degradation of the fiber wrap fiber. The results of the Cyclic Stress Test validated assumptions regarding the design performance, and provided insights used to modify the design prior to hardware implementation into compact free space optical communications gimbals.
Numerous Free Space Optical Communications (FSOC) applications use fine tracking to achieve precise jitter stabilization necessary for high data rate communications. In addition, precision pointing mechanism are often required for point-ahead of the transmit laser. Prior systems have used either fast steering mirrors (FSMs), piezoelectric fiber optic positioners, or inertially stabilized platforms each of which has its own advantages and disadvantages for different applications.
We developed a small form-factor, high performance FSM capable of meeting both high bandwidth stabilization requirements as well as high precision pointing necessary for the point-ahead function. The current design achieves a 2.5 kHz closed-loop optical track bandwidth, <5 μrad/mrad accuracy, and better than 15 nm rms surface figure error. Because there is no single approach to FSOC architecture, we designed the FSM to be easily scaled and customized for various applications ranging from FSOC, image stabilization, and scanning. Simple choices and customization of the FSM components including the mirror substrate, flexure, feedback sensors, and actuator design can provide custom designs for various applications. Analysis tools were developed to quickly trade the multitude of design parameters that influence performance. This paper reviews the FSM design, performance, and qualification test results, and trade space available to customize the FSM. We present analysis and test data from a couple of design variations to show how our design and analysis approach allows the FSM to be quickly adapted to various performance and environmental requirements.
Along with advantages in higher data rates, spectrum contention, and security, free space optical communications can provide size, weight, and power (SWaP) advantages over radio frequency (RF) systems. SWaP is always an issue in space systems and can be critical in applying free space optical communications to small satellite platforms. The system design of small space-based free space optical terminals with Gbps data rates is addressed. System architectures and requirements are defined to ensure the terminals are capable of acquisition, establishment and maintenance of a free space optical communications link. Design trades, identification of blocking technologies, and performance analyses are used to evaluate the practical limitations to terminal SWaP. Small terminal design concepts are developed to establish their practicality and feasibility. Techniques, such as modulation formats and capacity approaching encoding, are considered to mitigate the disadvantages brought by SWaP limitations, and performance as a function of SWaP is evaluated.
Numerous Deep Space Optical Communications (DSOC) demonstrations are planned by NASA to provide the basis for future implementation of optical communications links in planetary science missions and eventually manned missions to Mars. There is a need for a simple, robust precision optical stabilization concept for long-range free space optical communications applications suitable for optical apertures and masses larger than the current state of the art. We developed a stabilization concept by exploiting the ultra-low noise and wide bandwidth of ATA-proprietary Magnetohydrodynamic (MHD) angular rate sensors and building on prior practices of flexure-based isolation. We detail a stabilization approach tailored for deep space optical communications, and present an innovative prototype design and test results. Our prototype system provides sub-micro radian stabilization for a deep space optical link such as NASA’s integrated Radio frequency and Optical Communications (iROC) and NASA’s DSOC programs. Initial test results and simulations suggest that >40 dB broadband jitter rejection is possible without placing unrealistic expectations on the control loop bandwidth and flexure isolation frequency. This approach offers a simple, robust method for platform stabilization without requiring a gravity offload apparatus for ground testing or launch locks to survive a typical launch environment. This paper reviews alternative stabilization concepts, their advantages and disadvantages, as well as, their applicability to various optical communications applications. We present results from testing that subjected the prototype system to realistic spacecraft base motion and confirmed predicted sub-micro radian stabilization performance with a realistic 20-cm aperture.
Data transmits via optical communications through fibers at 10’s of Terabits per second. Given the recent rapid explosion for bandwidth and competing demand for radio frequency (RF) spectrum allocations among differing interests, the need for space-based free space optical communications (FSOC) systems is ever increasing. FSOC systems offer advantages of higher data rates, smaller size and weight, narrower beam divergence, and lower power than RF systems. Lightweight, small form factor, and high performance two-axis gimbals are of strong interest for satellite FSOC applications. Small gimbal and optical terminal designs are important for widespread implementation of optical communications systems; in particular, for satellite-to-satellite crosslinks where the advantages of more secure communications links (Lower Probability of Intercept (LPI)/Lower Probability of Detect (LPD)) are very important. We developed design concepts for a small gimbal focusing on the use of commercial off-the-shelf (COTS) subsystems to establish their feasible implementation against the pointing stabilization, size, weight and power (SWaP), and performance challenges. The design drivers for the gimbal were weight, the elevation and azimuth field of regards, the form factor envelope (1U CubeSats), 100 μrad pointing accuracy, and 10 degrees per second slew capability. Innovations required in this development included a continuous fiber passed through an Azimuth Fiber Wrap and Elevation Fiber Wrap, overcoming typical mechanical and stress related limitations encountered with fiber optic cable wraps. In this presentation, we describe the configuration trades and design of such a gimbal.
Thermal optical software has been written and used to reduce surface temperature and optical transmission thermal distortion interferometry data. A high reflectance mirror on a fused silica substrate was irradiated by a high intensity laser beam at 1.3 micrometers . Surface temperature and optical transmission data that anchor the software are presented in this paper. In addition, a novel method of computing the optical transfer function from the interferometer data is discussed.
A series of multilayer mirrors was exposed to a high power laser to measure absorption of the coatings and to test for thermal distortion. A high power chemical oxygen iodine laser with a wavelength of 1.315 micrometers was used to irradiate a variety of high reflectivity mirrors. The mirror coatings were multilayers of Ta2O5/SiO2 and Si3N4/SiO2 as well as aluminum enhanced with Nb2O5/SiO2. The dielectric layers were deposited by modulated reactive-dc-magnetron sputtering on fused silica substrates. The coated samples were placed in a vacuum chamber and monitored with a thermal imaging camera and an interferometer during irradiation. Absorption levels as low as 10 ppm were observed and the maximum distortion of the wave front was less than (lambda) /10 at 0.633 micrometers for the best parts.