Integrated Optical System

Mission

The Laser Communications Relay Demonstration (LCRD) project is NASA's multi-year demonstration of laser communication between multiple ground stations and a geosynchronous satellite. LCRD will provide two years of high data rate optical communications in an operational environment, demonstrating that optical communications can meet NASAs growing need for higher data rates while also enabling lower power, lower mass communications systems on spacecraft. In addition, LCRDs architecture will serve as a developmental test bed for additional methods, including symbol coding, ranging, link layer protocols, and network layer protocols. LCRD will serve as a test bed for future NASA missions.

The space terminal is capable of simultaneously communicating with two ground stations. There are currently two optical ground stations. The primary ground station is located at JPL's Table Mountain Observatory. It will use the 1-m Optical Communication Telescope Laboratory (OCTL) telescope. Optical Ground station 1 (OGS1) consists of multiple systems including the existing OCTL telescope, the communication and beacon lasers, the monitor and control system, laser safety and the the Integrated Optical System (IOS). The IOS system has two major functions. The first is to relay the received light from the telescope to the ground modem's single mode fiber. Its second function is relay light from the beacon and communication lasers to the telescope.

Hardware

The AO system is a two deformable mirror (DM) design with a Low Order DM (LODM) correcting low spatial frequencies large amplitude aberrations and a High Order DM (HODM) correcting high spatial frequencies small amplitude aberrations. Both DMs have diameters on the order of 1 cm, reducing the size, cost and complexity of the remaining optical train components. The AO system allocates 29 actuators across the diameter of the 1-m OCTL primary to meet the specified atmospheric turbulence correction. A Shack-Hartmann WFS measures atmospheric distortions using 20% of the received power. The speed of the WFS measurement is critical for the AO system to keep up with the rapidly changing atmospheric turbulence conditions at the 20º elevation and nominal 5.2 cm r₀ (referenced at zenith at 500 nm). Our models have shown that we need to have frame rates on the order of 10 kHz to achieve our desired level of performance in the specified atmospheric conditions. The WFS camera is an off the shelf InGaAs camera. Each lenslet in the WFS will illuminate an array of 2 × 2 detectors on the WFS focal plane array.

The system also has an atmospheric turbulence simulator (ATS). A mirror can be slid into the beam path to inject light from the ATS into the AO system. The beam has the same optical properties as the downlink from the telescope and forms pupil images at the same locations. The ATS consists of two spinning phase plates with computer controlled rotation rates to simulate a variety of turbulence conditions. By swapping out lenses and by changing their position, we can vary the Fried’s Parameter, and Rytov number. We can change the Greenwood frequency by changing the rotational speed of the phase plates. The ATS allows the IOS AO system to be tested in the lab over a wide variety of atmospheric conditions.

Software

The true innovation in the IOS lies in the software, which is the next evolutionary step of the software developed for the Palm 3000 AO system on the Palomar 5-m telescope. The system design is divided into four main components: a command/automation server, a device driver server, the real-time control component, and the GUI. A publish/subscribe communication method is used to transfer messages between components, a particularly effective method for systems with components running in physically separate locations. The real-time component uses a Digital Signal Processor (DSP) board with eight on-board DSP chips. Data is moved directly from the frame grabber to the DSP board via direct memory access (DMA), enabling the required frame rates. For the IOS, we have implemented a solid-state device in the IOS control computer capable of capturing all data types at the high log rates required for telemetry analysis.

Performance

To assess the system's performance we measured the amount of power coupled into the fiber at a range of input powers. The data were taken with two configurations of the ATS with an r₀ value of 5.2cm and effective elevation angles of 20° and 48°. Both sets of measurements had the ATS configured for a Greenwood frequency of 32 Hz. The first angle is the required mission elevation angle and the second is the predicted elevation angle of the mission. Input power was computed to be at the front of the IOS. The data are plotted below. The solid line is for the actual mission conditions, while the dashed line is for the more stringent mission requirement. There are two lines for 48°, because we took the data on two separate days. In all cases, the IOS couples the required amount of power and with considerable margin in the brightest cases.

Status

The IOS has been delivered to the OCTL telescope and is operating well. It is awaiting the launch of the spacecraft.

Publications

First results from the adaptive optics system from LCRD's Optical Ground Station One L.C. Roberts Jr., G.L. Block, S.F. Fregoso, H. Herzog, S.R. Meeker, J.E.Roberts, G.D. Spiers, J.A. Tesch, T.N. Truong, J.D. Rodriguez, & A. Bechter
Proc. AMOS Conference, (2018)

A laser communication adaptive optics system as a testbed for extreme adaptive optics Lewis C. Roberts, Gary Block, Santos Fregoso, Harrison Herzog, Seth R. Meeker, Jennifer E. Roberts, Joshua Rodriguez, Jonathan Tesch, Tuan Truong
Proc. SPIE, vol. 10703, 107031S (2018)
Copyright 2018 Society of Photo Optical Instrumentation Engineers. https://doi.org/10.1117/12.2314304

The adaptive optics and transmit system for NASA's Laser Communications Relay Demonstration project Lewis C. Roberts Jr., Rick Burruss, Santos Fregoso, Harrison Herzog, Sabino Piazzola, Jennifer E. Roberts, Gary D. Spiers, Tuan N. Truong
Proc. SPIE, vol. 9979, 99790I (2016)
Copyright 2016 Society of Photo Optical Instrumentation Engineers. http://dx.doi.org/10.1117/12.2238589

Overview of Optical Ground Station 1 of the NASA Space Communications and Navigation Program W. T. Roberts, D. Antsos , A. Croonquist, S. Piazzolla, L.C. Roberts Jr., V. Garkanian, T. Trinh, M. W. Wright, R. Rogalin, J. Wu, and L. Clare
Proc. SPIE, vol. 9739, 97390B (2016)
Copyright 2016 Society of Photo Optical Instrumentation Engineers. http://dx.doi.org/10.1117/12.2217465

Performance Predictions for the Adaptive Optics System at LCRD's Ground Station 1
Lewis C. Roberts Jr., Rick Burruss, Jennifer E. Roberts, Sabino Piazzolla, Sharon Dew, Tuan Truong, Santos Fregoso, Norm Page
Imaging and Applied Optics 2015, OSA Technical Digest (2015), paper JW4F.4.

Recent Developments in Adaptive Optics for the LCRD Optical Ground Station at Table Mountain
Keith E. Wilson & Lewis C. Roberts Jr.
Proc. International Conference on Space Optical Systems and Applications (ICSOS) 2014, S4-1 (2014)

Conceptual design of the adaptive optics system for the laser communication relay demonstration ground station at Table Mountain
Lewis C.Roberts Jr., Norman Page, Rick Burruss, Tuan N. Truong, Sharon Dew, Mitchell Troy
Proceedings of the SPIE, vol. 8610, 86100N (2013)
Copyright 2013 Society of Photo Optical Instrumentation Engineers. http://dx.doi.org/10.1117/12.2008981