Description:
OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Directed Energy (DE);Space Technology
The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.
OBJECTIVE: The objective of this project is to develop and demonstrate key components that would increase the spatial coherence of laser beacons (i.e., generate smaller laser beacons) to help improve the performance of adaptive optics systems for ground-to-space imaging applications. The final design should not require a light source external to the system itself (i.e., light from a star or satellite) to preserve the dim object imaging capability of a laser beacon adaptive optics system. For this effort, we are primarily focused on continuous wave sodium beacons in a side-launched or bi-static configuration. That said, pulsed sodium or Rayleigh beacons, are also of interest. The primary focus of this topic is to develop, build, and test the necessary adaptive optics components to achieve reliable pre-compensation of the beacon. It is also highly desired that an on-sky demonstration of the system be completed in conditions that are representative of typical sites for ground-based observations of earth-orbiting satellites. The system demonstration can be performed on government, university, or civilian telescopes; however, our primary goal is to demonstrate the pre-compensation system on the island of La Palma in the Canary Islands, Spain.
DESCRIPTION: AFRL supports the US Space Force in researching and developing effective, affordable techniques to identify, track, and characterize satellites in Earth orbit. Radar, although it is expensive to build and operate, works for satellites in low-Earth orbit. However, because of the distances involved, only a few specialized ground-based radars are capable of tracking satellites in geosynchronous orbit. Compared to ground-to-space radars, ground-based optical telescopes are less expensive to build and operate; in addition, they work well for satellites in all orbits. However, atmospheric turbulence limits the resolution and effectiveness of ground-based optical telescopes. Laser-beacon adaptive optics is an established technique to overcome the effects of atmospheric turbulence. However, there remain significant challenges to improving the utility and effectiveness of laser beacon adaptive optics for defense applications. There are two main types of laser beacons used in adaptive optics, Rayleigh beacons and sodium beacons. Rayleigh beacons are formed by scattering light from molecules of nitrogen and oxygen lower in the atmosphere; typical altitudes range from 10 km to 20 km. Pulsed lasers are typically used for Rayleigh beacons so that the light may be sampled from a particular altitude using a technique called range gating. Because Rayleigh scattering is much stronger for shorter wavelengths of light, common wavelengths for Rayleigh beacons are 355 nm and 532 nm. Because Rayleigh beacons rely on scattering from air molecules, they are limited to relatively low altitudes where the density of air molecules is higher. Light from the beacon traverses a cone of air above the telescope, with the beacon at the apex of the cone and the telescope pupil at the base of the cone. If a Rayleigh beacon is used for a larger telescope, the cylindrical column of air above the telescope will not be well sampled. Because of this cone effect, Rayleigh beacons are suitable only for smaller telescopes of up to 2 m in diameter. Sodium beacons are formed from scattering light from a layer of ionic sodium that is centered at an altitude of 90 km above the ground. Because of their high altitude, sodium beacons sample a much larger cone of air when compared to Rayleigh beacons. So, they are better suited for use with large telescopes. Typical current laser beacon systems for astronomical applications do not compensate the outgoing laser beam to correct for atmospheric turbulence. As a result, the laser beacon can be large and extended, especially when compared to an unresolved point source, like a star. For most astronomical observatories, this is not a problem, because they are located in places with weak atmospheric turbulence. However, observatories for space situational awareness (SSA) are typically located in places with stronger atmospheric turbulence, so their laser beacons are typically larger than those at astronomical observatories. A larger laser beacon results in lower sensitivity of the laser-beacon wavefront sensor. To make matters worse for SSA observatories, when a ground-based telescope tracks a satellite in low-Earth orbit, it must slew quickly across the sky. This, in effect, creates a situation that is equivalent to a strong wind blowing across the aperture of the telescope. This means the adaptive optics system must operate at a higher frame rate and higher gain to compensate for atmospheric turbulence. The combination of these two factors means a laser beacon for SSA purposes must be much brighter and smaller than a laser beacon for astronomy. Thus, AFRL is seeking development of systems to generate smaller more spatially coherent laser beacons to help improve the performance of adaptive optics systems for ground-to-space imaging applications.
PHASE I: As this is a Direct-to-Phase-II (D2P2) topic, no Phase I awards will be made as a result of this topic. To qualify for this D2P2 topic, the Government expects the applicant to demonstrate feasibility by means of a prior “Phase I-type” effort that does not constitute work undertaken as part of a prior SBIR/STTR funding agreement. "Phase I-type" deliverables include a report that thoroughly describes concepts, analyses, and simulations for laser beacon components that are suitable for SSA ground-to-space imaging applications. These analyses and simulations must show that the proposed components are effective and affordable. The report should describe the components at a level suitable for a conceptual design review. (See https://en.wikipedia.org/wiki/Engineering_design_process#Concept_Generation) The report shall include a plan for demonstrating the laser components on-sky, in conditions that are representative of typical sites for ground-based observations of earth-orbiting satellites. (Since this is a D2P2 topic, this section describes the content expected to substantiate that the proposer's technology is currently at an acceptable stage to award a D2P2.)
PHASE II: Phase II deliverables include a detailed design of laser beacon pre-compensation components that are suitable for SSA ground-to-space imaging applications. This design must illustrate that the proposed components are effective and affordable. The design documents should describe the components at a level suitable for preliminary and critical design reviews. (See https://en.wikipedia.org/wiki/Design_review_(U.S._government)#Preliminary_Design_Review_(PDR), and https://en.wikipedia.org/wiki/Design_review_(U.S._government)#Critical_Design_Review_(CDR)) After successful completion of the PDR and CDR, a prototype system will be built, tested in the lab environment. A detailed test plan shall also be developed for demonstrating the laser pre-compensation components on-sky, in conditions that are representative of typical sites for ground-based observations of earth-orbiting satellites. As cost and schedule constraints allow, the prototype pre-compensation system shall be demonstrated on-sky at a government, university, or civilian observatory. The proposer will not include the sodium beacon laser, launch telescope, gimbals, and safety systems in their proposal, as these components could be made available, depending on the location for the on-sky demonstration. Currently, the goal is to support on-sky testing on the island of La Palma in the Canary Islands, Spain
PHASE III DUAL USE APPLICATIONS: A Phase III effort would require identifying a suitable transition partner, which could be a government program office, a government contractor or other commercial entity, or a civilian astronomical observatory. Potential phase III applications include other defense SSA observatories in the US, Europe, and Australia; civilian astronomical observatories that wish to observe at visible wavelengths, which requires improved adaptive optics performance; and ground-to-space laser communications research facilities.
REFERENCES:
- Laser beacons or laser guide stars, https://en.wikipedia.org/wiki/Laser_guide_star;
- Drummond, Jack & Telle, John & Denman, Craig & Hillman, Paul & Spinhirne, Jim & Christou, Julian. (2004). Photometry of a Sodium Laser Guide Star from the Starfire Optical Range. II. Compensating the Pump Beam. Publications of the Astronomical Society of the Pacific. 116. 952-964. 10.1086/425595.
KEYWORDS: sodium beacons; laser beacons; laser beacon coherence; adaptive optics
