Proceedings Article | 13 May 2008
Proc. SPIE. 6950, Laser Radar Technology and Applications XIII
KEYWORDS: Signal to noise ratio, Photodetectors, LIDAR, Sensors, Photons, Photodiodes, Receivers, Atmospheric turbulence, Atmospheric propagation, Signal detection
Active optical remote sensing has numerous applications including battlefield target recognition and tracking,
atmospheric monitoring, structural monitoring, collision avoidance systems, and terrestrial mapping. The maximum
propagation distance in LIDAR sensors is limited by the signal attenuation. Sensor range could be improved by
increasing the transmitted pulse energy, at the expense of reduced resolution and information bandwidth. Coherent
detection can operate at low optical power levels without sacrificing sensor bandwidth.
Utilizing a high power LO laser to increase the receiver gain, coherent systems provide shot noise-limited gain thereby
increasing the sensing range. To fully exploit high LO powers without incurring performance penalties due to the RIN
of the LO, high power handling balanced photodiodes are used. The coherent system has superior dynamic range,
bandwidth, and noise performance than small-signal APD-based systems.
Coherent detection is a linear process that is sensitive to the amplitude, phase and polarization of the received signal.
Therefore, Doppler shifts and vibration signatures can be easily recovered. RF adaptive filtering following
photodetection enables channel equalization, atmospheric turbulence compensation, and efficient background light
filtering.
We demonstrate a coherent optical transmission system using 15mA high power handling balanced photodetectors. This
system has an IF linewidth <1Hz, employing a proprietary phase locked loop design. Data is presented for 100ps pulsed
transmission. We have demonstrated amplitude and phase modulated 10Gb/s communication links with sensitivities of
132 and 72 photons per bit respectively. Investigations into system performance in the presence of laboratory induced
atmospheric turbulence are shown.
