Description:
TECH FOCUS AREAS: Directed Energy
TECHNOLOGY AREAS: Sensors; Electronics; Battlespace
OBJECTIVE: The objective of this project is to develop and demonstrate key components that would help make sodium-beacon or Rayleigh-beacon adaptive optics practical for military, ground-to-space imaging applications. Current commercial laser systems used to produce sodium and Rayleigh beacons were developed for astronomical applications. These commercial lasers are not suited for smaller military telescopes, which are typically installed in locations with much worse turbulence, when compared to astronomical telescopes. The objective is to develop these laser components and demonstrate them on-sky, in conditions that are representative of typical sites for ground-based observations of earth-orbiting satellites. These components could be demonstrated on government, university, or civilian telescopes.
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 military 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 by 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.
Lasers for bright Rayleigh and sodium beacons are large and heavy; they are difficult to mount on typical military telescopes, which tend to be much smaller than astronomical telescopes. In addition, military telescopes are typically deployed to locations where the atmospheric turbulence is much worse than locations for astronomical observatories. To make matters worse, 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. The combination of these two factors means a laser beacon for military purposes must be much brighter than a laser beacon for astronomy.
Another factor to consider is the risk that laser beacons pose to the safe operation of aircraft. Visible laser beacons are not eye-safe, thus considerable effort is necessary to avoid blinding aircraft pilots. Ultra-violet lasers are not transmitted by aircraft windscreens, but the silver mirror coatings typically used in telescopes do not reflect ultra-violet wavelengths well. Furthermore, the quantum efficiency of typical wave-front sensor cameras is low at ultra-violet wavelengths. Thus, AFRL is seeking development of key components that would help to make sodium-beacon or Rayleigh-beacon adaptive optics practical for military ground-to-space imaging applications. These components are listed below.
• On-telescope (side- or center-launched) Rayleigh beacon laser (ultra-violet and visible)
• Ultra-violet (eye-safe) laser beacon
• Uplink compensation of laser beacon to reduce beacon size
• Polychromatic laser beacon for sensing tilt and high-order aberrations
• Laser-beacon (Rayleigh and sodium) simulator for laboratory bench-top testing
• Hybrid Rayleigh-sodium beacon adaptive optics
• Tilt anisoplanatism compensation
• Electronic camera shutter or low-radio-frequency-interference Pockels cell for gating Rayleigh beacon return • Using adaptive optics telemetry in near-real-time for improving laser-beacon imaging and detection of closely spaced objects
• Advanced wave-front sensors and cameras for laser beacon adaptive optics
PHASE I: Phase I deliverables include a report that describes thoroughly concepts, analyses, and simulations for laser beacon components that are suitable for military 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 the references section for the contents of a conceptual design review.) 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.
PHASE II: Phase II deliverables include a detailed design of laser beacon components suitable for military ground-to-space imaging applications. This design must illustrate 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 the references section for the contents of preliminary and critical design reviews.)
The report shall include a detailed plan for demonstrating the laser components on-sky, in conditions representative of typical sites for ground-based observations of earth-orbiting satellites. As cost and schedule constraints allow, a prototype component shall be built, tested, and demonstrated on-sky at government, university, or civilian observatory.
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.
NOTES: 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 proposed tasks intended for accomplishment by the FN(s) in accordance with section 5.4.c.(8) of the Announcement and within the AF Component-specific instructions. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws. Please direct questions to the Air Force SBIR/STTR Help Desk: usaf.team@afsbirsttr.us
REFERENCES:
1. Laser beacons or laser guide stars https://en.wikipedia.org/wiki/Laser_guide_star;
2. Conceptual Design Review https://en.wikipedia.org/wiki/Engineering_design_processConcept_Generation
KEYWORDS: laser beacon; laser guide star; Rayleigh beacon; polychromatic beacon; adaptive optics; tilt anisoplanatism; wave-front sensor; electronic shutter
