We demonstrate a balanced-homodyne LADAR receiver employing a phase-sensitive amplifier (PSA) to raise the
effective photon detection efficiency (PDE) to nearly 100%. Since typical LADAR receivers suffer from losses in the
receive optical train that routinely limit overall PDE to less than 50% thus degrading SNR, PSA can provide significant
improvement through amplification with noise figure near 0 dB. Receiver inefficiencies arise from sub-unity quantum
efficiency, array fill factors, signal-local oscillator mixing efficiency (in coherent receivers), etc. The quantum-enhanced
LADAR receiver described herein is employed in target discrimination scenarios as well as in imaging applications. We
present results showing the improvement in detection performance achieved with a PSA, and discuss the performance
advantage when compared to the use of a phase-insensitive amplifier, which cannot amplify noiselessly.
A 10 Gbps aperture agnostic data buffer has been developed to mitigate packet loss over highly scintillated FSO links
operating in a hybrid FSO/RF network. The buffer incorporates a custom IP packet inspection and scheduling processor.
Packet buffering and transmission scheduling is determined from link availability and a QoS parameter in the IP header
based upon RFC 2474 (Differentiated Services). Buffer metric parameters are monitored and could be provided to the
network management system. Integration of the novel buffer into the FSO link along with improved network routers
allows operation under strong scintillation conditions at fade margins as low as 8 dB. We present the salient performance
characteristics of a buffered FSO modem with VOA-emulated atmospheric fading statistics. Application test cases,
including a TCP/IP MPEG-4 video stream, have been emulated both to determine the effects of packet loss, latency and
intra-packet jitter introduced by buffering and to optimize traffic flow settings.
Combined RF and optical communication within a heavily scintillated atmosphere requires special modems that can
accommodate significant signal fading. A hybrid network (10 Gbps 1550 nm FSO and RF transmission) has been
developed and a link quality parameter is used to assist the network routers with the path cost calculation algorithm.
Fading statistics, determined by field experiments, are emulated in the laboratory network by a statistically-driven VOA.
COTS hardware (FEC and a special amplifier) enable a 35 dB dynamic range. The special modem and its performance
within a multi-node network are presented.
B-PPM formatting for trans-atmospheric optical communication is compared experimentally to OOK (NRZ) at a single
channel data rate of 1.25Gbps in deep fading conditions. Unlike low data rate transmission using M-ary PPM
formatting, high-speed B-PPM formatting does not benefit from the theoretical improvement that has been realized at
low data rate. Although B-PPM can indeed benefit from a threshold set to near-zero, the high speed transmission
precludes the implementation of a traditional Maximum Likelihood Detection circuit that compares the integrated power
of each slot. At high speed, one has to rely on signal strength alone within the bit period which degrades the contrast
between a "one" and a "zero." Moreover, the need for twice the bandwidth for B-PPM significantly limits available
components such as APDs. More important, however, is the fact that during deep fades clock resynchronization
dominates at high data rate. The primary question to be addressed is: Does B-PPM formatting really provide sufficient
margin compared to NRZ to merit its use in deep fading atmospheric conditions? By building a special dual transceiver
system, we have been able to propagate both B-PPM and NRZ formatted signals co-linearly on two C-band wavelengths
centered close to 1550nm. Under field testing we measured the BER, including signal resynchronization, using special
InGaAs, high-speed, multimode pigtailed, APD-based detectors in the receiver. The data were collected on fully
instrumented horizontal paths of 1km and 500m with C<sub>n</sub>
<sup>2</sup> [m<sup>-2/3</sup>] ranging from 10<sup>-15</sup> to 10<sup>-13</sup>.