Free-Space Optical Communications Technologies
This Free-space Long Range Optical Communications subtopic seeks innovative technologies for advancing free-space optical communications by pushing future data volume returns to and from space missions in multiple domains. Modulation and signaling techniques for long range optical communications should serve multiple functions that include the laser beams serving as (i) high-rate data carriers, (ii) high precision range and range-rate monitoring, and (iii) time-transfer.
Specific metrics are return data rates >100 Gb/s (cislunar, i.e., geosynchronous equatorial orbit (GEO) or lunar orbit to ground), >10 Gb/s (Earth-Sun L1 and L2), >1 Gb/s per astronomical units squared (AU2) (deep space), and >1 Gb/s (planetary lander to orbiter and/or interspacecraft). Ground-to-space forward data rates >25 Mb/s to farthest Mars ranges. High-precision centimeter to subcentimeter ranging, and picosecond-level time transfer.
Systems to satisfy the goals above require agile; cost-effective; low size, weight, and power (SWaP); and space-qualified optical transceivers with auxiliary assemblies for laser beam pointing control and thermal control. The ability to easily integrate the transceivers to diverse space platforms is highly desired. Interoperability by conforming to emerging signaling standards is also sought for the communications signaling.
NASA-validated optical communications architectures include ground transceivers that transmit and receive laser signals to and from space. Cost-effective technologies for ground assets that can point to and collect signal efficiently, as well as instrumentation for these ground transceivers including robust lasers transmitters and receivers, with considerations for mitigating atmospheric turbulence are required. This subtopic is broadly divided between Flight and Ground technologies for Free Space Optical Communications (FSOC). Innovation priorities are listed in order below:
- Lowering SWaP.
- Solutions for pointing narrow laser beams from space platforms.
- Technology choices that ease space qualification for radiation, random vibrations, and thermal-vacuum.
- Innovations leading to large aperture diameters for collecting faint optical signals through atmospheric turbulence while operating under daytime conditions.
- Kilowatt-class ground laser transmitter with narrow pulses and high repetition rate.
- Partial- or full-correcting aberrations of laser signals traversing Earth's turbulent atmosphere.
- Coherent receivers for multi-Gb/s data rates for space-to-ground optical links.
FLIGHT LASER TRANSCEIVERS:
Low-mass, high-Effective Isotropic Radiated Power (EIRP) laser transceivers for links over planetary distances with:
- 30- to 50-cm clear aperture diameter telescopes for laser communications.
- Targeted mass of opto-mechanical assembly per aperture area, less than 200 kg/m2.
- Cumulative wave-front error and transmission loss not to exceed 2 dB.
- Advanced thermo-mechanical designs to withstand planetary launch loads and spaceflight thermal environments, at least -20 to 70 °C operational range.
- Design to mitigate stray light while pointing transceivers 3° from the edge of Sun.
- Survive direct Sun pointing for extended duration (few hours to days).
Transceivers fitting the above characteristics should support robust link acquisition tracking and pointing characteristics, including point-ahead implementation from space for beacon-assisted and/or "beaconless" architectures.
- Acquisition, tracking, and pointing architectures that can operate with dim laser beacons (irradiance of a few picowatts per square meter at entrance of flight aperture) from Mars farthest ranges.
- Pointing loss allocations not to exceed 1 dB (pointing errors associated loss of irradiance at target less than 20%).
- Vibration isolation/suppression systems that can be integrated to the optical transceiver in order to reject high-frequency base disturbance by at least 50 dB.
- Receiver field-of-view (FOV) of at least 1-mrad angular radius for beacon-assisted acquisition, tracking, and pointing.
- As a goal, additional focal plane with wider FOV (>10 mrad) to support onboard astrometry is desired.
- Beaconless pointing subsystems for space-to-ground operations.
- Assume integrated spacecraft microvibration angular disturbance of 150 µrad (<0.1 to ~500 Hz).
- Integrated launch lock and latching mechanism.
Low-complexity small-footprint agile laser transceivers for bidirectional optical links:
- >1 to 10 Gb/s at a nominal link range of 1,000 to 20,000 km for planetary lander/rover-to-orbiter.
- >10 to 100 Gb/s at a nominal link range of 1,000 to 40,000 km for space-to-Earth optical links.
- 10 to 100 Gb/s space-to-space crosslinks.
- Disruptive low-SWaP technologies for space or planetary/lunar surface over extended mission duration.
HIGH WALL-PLUG EFFICIENCY FLIGHT LASER TRANSMITTERS:
High-Gb/s laser transmitters:
- 1,550-nm wavelength.
- Lasers, electronics, and optical components ruggedized for extended space operations.
- Build-in redundancy and other fail-safe measures.
- High rate 10 to 100 Gb/s for cislunar.
- 1 Gb/s for deep space.
- Integrated modem functions conforming to emerging optical communications Consultative Committee for Space Data Systems (CCSDS) standards.
- High-photon-efficiency lasers for planetary distances with high peak-to-average power for regular or augmented M-ary pulse position modulation (PPM) with M = 4, 8, 16, 32, 64, 128, and 256 operating at near-infrared (NIR) wavelengths, preferably 1,550 nm, with average powers from 5 to 50 W.
- Subnanosecond pulse.
- Low-pulse jitter (<25 ps).
- Long lifetime and reliability operating in space environment (>5 years and as long as 20 years).
- High modulation extinction ratio (>30 dB).
- High polarization extinction ratio (>20 dB).
- 1- to 10-GHz linewidth.
- Integrated modem functions.
- >10% wall-plug efficiency, direct current (DC)-to-optical, including support electronics with description of approach for stated efficiency of space-qualifiable lasers. Multiwatt to 20 W Erbium Doped Fiber Amplifier (EDFA), or alternatives, with high-gain bandwidth (>30 nm, 0.5-dB flatness) concepts will be considered.
- Operational thermal range 0 - 50 °C.
- Storage -15 to 65 °C.
- Radiation tolerance greater than 100 krad is required (including resilience to photodarkening).
Space-qualified high-speed receivers and low-light-level sensitive acquisition, tracking, and pointing detectors and detector arrays.
- NIR wavelengths: 1,064 and/or 1,550 nm.
- Sensitive to low irradiance incident at flight transceiver aperture (~ femtowatts per square meter to picowatts per square meter) detection.
- Low subnanosecond timing jitter and fast rise time.
- Novel hybridization of optics and electronic readout schemes with built-in preprocessing capability.
- Characteristics compatible with supporting time-of-flight or other means of processing laser communication signals for high-precision range and range-rate measurements.
- Tolerant to space radiation effect (total dose >100 krad), displacement damage and single-event effects.
NOVEL TECHNOLOGIES AND ACCESSORIES:
- Center wavelength (CWL) 1,064 or 1,550 nm.
- Space-qualified, 0.1 to 1.3 nm, noise equivalent bandwidth with ~90% throughput, large spectral range out-of-band blocking (~40 dB).
- Reliable tuning over limited range.
- Thermally stable with well characterized temperature dependence of passband.
Novel Photonics Integrated Circuit (PIC) devices targeting space applications with the objective of reducing SWaP of modulators without sacrificing performance. Proposed PIC solutions should allow improved integration and efficient coupling to discrete optics, when needed.
Concepts for offering redundancy to laser transmitters in space.
- Low-loss, high-power multiplexing devices that can handle up to 20 W of optical power per channel and tens to 100 W of optical power output.
- Optical fiber routing of high average powers (tens of watts) and high peak powers (1 to 10 kW).
- Redundancy in actuators and optical components.
- Reliable optical switching.
- Innovative applications of machine learning to ease flight operations of Deep Space Optical Communications (DSOC) transceivers, for example, to achieve improved pointing performance from space.
GROUND ASSETS FOR OPTICAL COMMUNICATIONS:
Low-cost large-aperture receivers for faint optical communication signals from deep space subsystem technologies:
- Demonstrate innovative subsystem technologies for >10-m-diameter deep space ground collector.
- Capable of operating to within 3° of solar limb.
- Better than 10-µrad spot size (excluding atmospheric seeing contribution).
- Desire demonstration of low-cost primary mirror segment fabrication to meet a cost goal of less than $35K per square meter.
- Low-cost techniques for segment alignment and control, including daytime operations.
- Partial adaptive correction techniques for reducing the FOV required to collect signal photons under daytime atmospheric "seeing" conditions.
- Adaptive optics for uplink laser transmission in order to be able to transmit low-beam divergence lasers with near diffraction limited performance.
- Innovative adaptive techniques not requiring a wave-front sensor and deformable mirror of particular interest.
- Mirror cleanliness monitor and control systems.
- Active metrology systems for maintaining segment primary figure and its alignment with secondary optics.
- Large-core-diameter multimode fibers with low temporal dispersion for coupling large optics to detectors remote (30 to 100 m) from the large optics.
1,550-nm sensitive photon counting detector arrays compatible with large-aperture ground collectors with a means of coupling light from large aperture diameters to reasonably sized detectors/detector arrays, including optical fibers with acceptable temporal dispersion.
- Integrated time tagging readout electronics for >5 gigaphotons/s incident rate.
- Time resolution <50 ps, 1-sigma.
- Highest possible single-photon detection efficiency, at least 50% at highest incident photon-flux rates.
- Total detector active area >0.3 to 1 mm2.
- Integrated dark rate <3 megacounts/s.
- Subnanometer noise equivalent bandwidths.
- Tunable in a limited range in the 1,550-nm spectral region.
- Transmission losses <0.5 dB.
- Clear aperture >25 mm and acceptance angle >40 mrad or similar etendue.
- Out-of-band rejection of >50 dB at 0.7 to 1.8 µm.
Multikilowatt laser transmitters for use as ground beacon and uplink laser transmitters.
- NIR wavelengths in 1.0- or 1.55-µm spectral region.
- Narrow linewidths <0.3 nm.
- Capable of modulating with nanosecond and subnanosecond rise times.
- Low timing jitter and stable operation.
- High-speed real-time signal processing of serially concatenated PPM operating at a few bits per photon with user-interface outputs.
- 15- to 60-MHz repetition rates.
Expected TRL or TRL Range at completion of the Project: 2 to 5
Primary Technology Taxonomy:
- Level 1 05 Communications, Navigation, and Orbital Debris Tracking and Characterization Systems
- Level 2 05.1 Optical Communications
Desired Deliverables of Phase I and Phase II:
Desired Deliverables Description:
For all technologies, lowest cost for small volume production (5 to 20 units) is a driver. Research must convincingly prove technical feasibility (proof of concept) during Phase I, ideally with hardware deliverables that can be tested and/or compelling simulations, to validate performance claims, with a clear path to demonstrating and delivering functional hardware meeting all objectives and specifications in Phase II.
State of the Art and Critical Gaps:
The state of the art (SOA) for FSOC can be subdivided into near-Earth (extending to cislunar and translunar distances) and planetary ranges with the Lagrange points falling in between.
Near-Earth FSOC technology has matured through a number of completed and upcoming technology demonstrations from space. Transition from technology demonstration to an operational service demands low-SWaP, novel high-speed (10 to 100 Gbps) space-qualified laser transmitters and receivers. Transmitters and receivers servicing near-Earth applications can possibly be repurposed for deep space proximity links, such as landed assets on planetary surfaces to orbiting assets with distances of 5,000 to 100,000 km or intersatellite links. Innovative lightweight space-qualified modems for handling multiple optical-modulation schemes. Emerging photonics technologies that can benefit space FSOC applications are sought.
Deep space FSOC is motivated by NASA's initiative to send humans to Mars. Critical gaps following a successful technology demonstration will be lightweight 30- to 50-cm optical transceivers with a wide operational temperature range -20 to 70 °C over which wave-front error and focus is stable; high peak-to-average power space-qualified lasers with average powers of 20 to 50 W; and single photon-sensitive radiation-hardened flight detectors with high detection efficiency, fast rise times, and low timing jitter. The detector size should be able to cover 1-mrad FOV with an instantaneous FOV comparable to the transmitted laser beam width. Laser pointing control systems that operate with dim laser beacons transmitted from Earth or use celestial beacon sources. For DSOC, ground laser transmitters with high-average power (kilowatt class) but narrow linewidths (<0.25 nm) and high-variable repetition rates are required. Innovative optical coatings for large-aperture mirrors that are compatible with near-Sun pointing applications for efficiently collecting the signal and lowering background and stray light. Reliability through space-qualified materials and component selection and implementation of redundancy are highly sought after to enable sending humans to planetary destinations, as well as enable higher resolution science instruments. Deriving auxiliary optimetrics from the FSOC signals to support laser ranging and time transfer will also be critical for providing services to future human missions to Mars. High-rate uplink from the ground to Mars with high-modulation-rate high-power lasers are also currently lacking.
Relevance / Science Traceability:
A number of FSOC-related NASA projects are ongoing with launch expected in the 2021 to 2024 timeframe. The Laser Communication Relay Demonstration (LCRD) is an Earth-to-geostationary satellite relay demonstration that launched in 2021. The Illuma-T Project will follow to extend the relay demonstration to include a low Earth orbit (LEO) node on the International Space Station (ISS). In 2023, the Optical to Orion (O2O), Artemis II, demonstration will transmit data from the Orion crewed capsule as it performs a translunar trajectory and returns to Earth.
In 2022, the DSOC Project technology demonstration will be hosted by the Psyche Mission spacecraft extending FSOC links to AU distances.
These missions are being funded by NASA's Space Technology Mission Directorate (STMD) Technology Demonstrations Missions (TDM) program and Exploration Systems Development Mission Directorate (ESDMD) and Space Operations Mission Directorate (SOMD) Space Communications Navigation (SCaN) program.
Of the 6 technologies recently identified by NASA for sending humans to Mars, laser communications was identified (https://www.nasa.gov/directorates/spacetech/6_Technologies_NASA_is_Advancing_to_Send_Humans_to_Mars)
Laser Communications Relay Demonstration (LCRD) using two ground nodes and GEO space asset
Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal (ILLUMA-T) from ISS-to-GEO and ISS-to-Ground
Optical to Orion (O2O) optical communications DTO from on Artemis II with crewed Orion
Deep Space Optical Communications (DSOC), first demonstration of optical communications from planetary distances
Small Satellite Conference, 2019, NASA’s Terabyte Infrared Delivery (TBIRD) Program: Large-Volume Data Transfer from LEO