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
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.
We report the development of top illuminated InGaAs photodetectors pigtailed to 50 &mgr;m core multimode (MM) fibers. These PIN diodes, in conjunction with low dispersion graded index MM fibers, allow for low cost and rugged solutions for high speed digital and analog applications. Our PIN diodes have previously demonstrated high optical power handling capability at large signal bandwidths. Coupled with large collection efficiency of MM fibers, these devices are suitable for a diverse range of systems, including avionics, ultra-fast Ethernet, radio over fiber, optical backplanes and free space laser links. The effect of the MM fiber's transfer function and fiber misalignment on the photodetector response is addressed. The spatial and temporal filtering effects of the MM fiber and the photodiode are explored experimentally through a 40 Gb/s link. Enhancement in photodiode linearity due to MM fiber is also reported.