Using current optical sources, a data pulse sent from Mars has a spot size of about 1,000 kilometers (km) by the time it reaches Earth. This size, assuming a beam size of 30-40 centimeters at the transmitter end, is a result of the beam spreading by diffraction. This huge increase in beam size forces one to transmit a large amount of energy in each pulse to ensure that at least one photon is captured by the receiving antenna on Earth. One approach to increasing the power of the signal received on Earth would be to modify the wave front of the beam from a Gaussian profile to a flat top profile. In addition, with more powerful lasers, it should be possible to reduce the pulse duration to the picosecond range, which would enable one to increase the data rates by a factor of a thousand or more. NREN plans to develop and simulate this technology for high-power fiber lasers capable of emitting such short pulses with high peak power levels. Furthermore, NREN plans to develop concepts for a beam-control module, designed to increase the on-axis power, as the data is transmitted across a distance of 400-million km.

With the eventual goal of landing humans on Mars, NASA is developing communication technologies that can transmit data between Earth and Mars at a bit rate high enough to provide video, still images and hyperspectral imaging. Current microwave transmitters can achieve, at best, a bit rate of only 10 to 20 megabits per second across interplanetary distances. For this reason, attention has focused on free-space optical communication systems that have the potential of achieving data rates as high as 10 gigabits per second between geo-stationary satellites and earthbound receivers.

NREN will investigate several approaches to increasing the power at the receivers. One approach would be to modify the wave front from a Gaussian profile to a flat top profile, which would increase the power of the signal at the receiver. Another approach to increasing deep-space data rates would be to use higher power lasers than those planned for the Mars Laser Communication Demonstration (MLCD) Project. In addition to increasing the received power, reliability of laser operation in deep space is a concern. In an effort to improve the resistance of the pump laser to radiation, the MLCD 976 nanometer diode pump that is based on planar quantum wells will be replaced by a new laser diode technology that is based on quantum dots.

Advanced Optical Sources for Deep-Space Data Transmission Applications

The Institute of Optics at University of Rochester

Microelectronics Research Center at The University of Texas in Austin

Optical Physics, Quantum Electronics, and Photonics at Cornell University