Several optomechanical design trades have been conducted for the NASA Goddard Space Flight Center (GSFC) Direct Detection Laser Transceiver (DDLT) program. DDLT is an experiment designed to demonstrate high data rate direct detection optical communication technology for space applications. Weight, resonance frequency, and the accommodation of a complex thermal design are the primary transmitter design drivers. The receiver hermetic seal is the major optomechanical design issue. In addition to the transmitter /receiver trade studies, a flight qualified third mirror mode stabilized laser diode was developed. 1.0 INTRODUCTION DDLT is a two way optical communication link from ACTS (Advanced Communication Technology Satellite) in geosynchronous orbit to a ground station. The communication link is designed to be a very flexible communications experiment, with multiple data rates and formats. The data rates range from a high of 220 MBPS to a low of 1.72 MBPS, with a 10-6 bit error rate (BER). The modulation formats are all variants of pulse position modulation (PPM), such as manchester (2 -slot PPM), QPPM (4 -slot PPM), and 8 bit PPM. The upper data rate is designed to demonstrate the technology for satellite cross links such as NASA's Tracking and Data Relay Satellite (TDRS), where a high data rate is required. The 1.72 MBPS data rate and the 8 bit PPM data format are designed to demonstrate the very long range communications technology as would be necessary for an outer planet mission. The DDLT transmitter, operating with a broadened beam at a 54 kHz (square wave), acts as the acquisition beacon: once acquisition is complete, the transmitter switches to one of the selectable communication data rates. DDLT is a subsystem of the Laser Intersatellite Transmission Experiment (LITE), developed by MIT/Lincoln Labs for the Air Force Space Technology Center. The purpose of LITE is to demonstrate coherent detection optical communication technologies for space applications. LITE is a complete communication terminal, independent of DDLT; however, DDLT uses some of the LITE hardware components. The purpose of DDLT is to demonstrate a flight qualified direct detection transmitter and receiver. The LITE telescope, acquisition and tracking subsystem, and optical bench are used by DDLT. In addition, LITE provides DDLT with the required support structure. The LITE assembly is shown in Figure 1. The DDLT transmitter is shown on the left of the LITE bench. Although the DDLT receiver is not visible in this diagram, it is embedded within the LITE optical train. (It is very difficult to align due to its location.) ABSTRACT Several optomechanical design trades have been conducted for the NASA Goddard Space Flight Center (GSFC) Direct Detection Laser Transceiver (DDLT) program. DDLT is an experiment designed to demonstrate high data rate direct detection optical communication technology for space applications. Weight, resonance frequency, and the accommodation of a complex thermal design are the primary transmitter design drivers. The receiv...
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