Free space optical communication requires laser transmitters with high output power, good beam quality, and high modulation extinction ratios. Current state-of-the-art transmitters utilize a single…

This is a grant award to Rose-Hulman Institute of Technology Inc., a private non-profit university in Terre Haute, Indiana, to develop a laser transmitter technology for free space optical communication. The $495,132.91 grant, awarded by the National Aeronautics and Space Administration (NASA), aims to create a highly efficient laser transmitter with high extinction ratio and simple drive electronics, which would be well-suited for satellite-based communication systems. The proposed approach integrates high-power, high-efficiency semiconductor laser designs with active modulators based on tapered semiconductor amplifiers. The goal is to prototype devices capable of over 1 Gbps data rates using pulse position modulation, while being compact, radiation-resistant, and compatible with wavelength division multiplexing. This award is not associated with any set-aside designation.

Free space optical communication requires laser transmitters with high output power, good beam quality, and high modulation extinction ratios. Current state-of-the-art transmitters utilize a single mode laser source which is modulated using several passive (absorption) modulators cascaded in series before amplification. Various levels of integration of these components have been demonstrated. While effective, this approach has two significant drawbacks: 1) complex drive electronics are required to generate and distribute synchronized signals to the modulator chain, and 2) the overall efficiency of the transmitter is extremely poor (since the laser source and amplifier are always on, and additional power is expended in the absorption of light in the modulator). As a result, this approach is not ideally suited to satellite-based free space communication, where system size, weight, and power (swap) must be minimized. This proposal seeks to develop a laser transmitter with high extinction ratio (>30 DB) that is capable of operating with both high efficiency and simple drive electronics. The approach leverages two technologies which, to date, have not been incorporated into photonic integrated circuit technology: 1) high power, high efficiency ingaasp-based epitaxial designs, and 2) active modulators based on tapered semiconductor amplifiers. State-of-the-art 1550 NM high power lasers can deliver>40% power conversion efficiency when operated at room temperature; by adopting a similar approach to the epitaxial design of the PIC, a significant improvement in the overall wallplug efficiency of the laser transmitter expected. The integration of tapered amplifiers to serve as active modulators will drive further improvement in efficiency while maintaining high speed operation with high extinction ratio. Prototype devices will be fabricated and tested to>1 gbps in a free space communication link utilizing pulse position modulation (PPM). The design (including drive electronics) is expected to be compact and highly resistant to radiation, making IT well-suited for spaceflight. Additional benefits offered by the approach include compatibility with: 1) on-chip integration of multiple channels for high speed wavelength division multiplexing, 2) other modulation schemes, and 3) high-gain tapered amplifiers for multi-watt chip output.