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 Cn2 [m-2/3] ranging from 10-15 to 10-13.
We report on a set of measurements made in December 2005 by researchers from the University of Central Florida, SPAWAR's Innovative Science and Technology Experiment Facility (ISTEF), Harris Corporation, NASA Kennedy Space Center, and Northrop Grumman. The experiments were conducted on the Shuttle Landing Facility (SLF) at Kennedy Space Center (KSC) over terrestrial paths of 1, 2, and 5 km. The purpose of the experiments was to determine the atmospheric-induced beam spreading and beam wander at various ranges. Two lasers were used in the experiments. Both were a pulsed 1.06 μm laser; however, one was single-mode and the other was multi-mode. Beam profiles were recorded near the target position. Simultaneous measurements of Cn2, wind speed and direction, humidity, visibility, temperature, and surface temperature profiles were all recorded.
We report on measurements made at the Shuttle Landing Facility (SLF) runway at Kennedy Space Center of receiver aperture averaging effects on a propagating optical Gaussian beam wave over a propagation path of 1,000 m. A commercially available instrument with both transmit and receive apertures was used to transmit a modulated laser beam operating at 1550 nm through a transmit aperture of 2.54 cm. An identical model of the same instrument was used as a receiver with a single aperture that was varied in size up to 20 cm to measure the effect of receiver aperture averaging on Bit Error Rate. Simultaneous measurements were also made with a scintillometer instrument and local weather station instruments to characterize atmospheric conditions along the propagation path during the experiments.
The Shuttle Landing Facility runway at the Kennedy Space Center in Cape Canaveral, Florida is almost 5 km long and 100 m wide. Its homogeneous environment makes it a unique and ideal place for testing and evaluating EO systems. An experiment, with the goal of characterizing atmospheric parameters on the runway, was conducted in June 2005. Weather data was collected and the refractive index structure parameter was measured with a commercial scintillometer. The inner scale of turbulence was inferred from wind speed measurements and surface roughness. Values of the crosswind speed obtained from the scintillometer were compared with wind measurements taken by a weather station.
Atmospheric turbulence over long horizontal paths perturbs phase in the pupil of an optical communications receiver, and also can cause severe intensity scintillations. We describe a real time wavefront compensation system using PC technology to perform all wavefront control tasks. This system uses a modal correction scheme, and we report the first measurements of residual wavefront taken approximately 1 meter above ground level at 1 km range. The effects of turbulence, scintillations and control bandwidth on the correction are all examined.
We report an experimental study of the use of DuPont photopolymer Holographic Recording Film to record high resolution reflection holograms of an integrated circuit chip with an initial film-to-object separation of 1.5 millimeters. A two-step H1-H2 recording sequence is used to transfer H1 images to the H2 film plane. Because of emulsion shrinkage, the optimum H2 recording wavelength is approximately 10 nm shorter than the H1 recording wavelength. The H2 reflection holograms, which are incoherently illuminated and viewed through a conventional microscope, reconstruct high resolution images with clearly resolved micron sized features.
We describe four measurement techniques to characterize the performance of an optical wireless channel. The characteristics of each technique are illustrated through experimental measurements. These four techniques include: (1) bit-error rate measurements, (2) measurements of the mean and variance of the received optical power from which the bit- error rate can be estimated, (3) measurements of the fade rate and fade duration histograms of the optical carrier, (4) measurements of the intensity and phase distributions of atmospheric propagated laser beams using a Shack-Hartmann wavefront sensor.
Adaptive optics systems could be used to maintain the quality of a communication laser beam propagated near
ground over a few kilometers of turbulent atmosphere. Such an adaptive optics system may incorporate a Shack-
Hartmann sensor to measure the wavefront of an arriving beacon laser beam before precompensating that of the
communication laser beam through a deformable mirror. We present experimental measurements of the wavefront
of a laser beam propagated over distances of 0.94 and 2.4 km acquired using a 1200-subaperture Shack-Hartmann
sensor. These data were acquired at 1 kllz frame rate during 2 second time intervals over the span of several days. We
acquired and analyzed 41 sets of data. Our analysis shows that the atmosphere is not stationary over these 2second
intervals, that the statistics of the wavefront may look very different for the same measured value of the atmosphere
structure constant (C) and does not always follow the theoretical predictions based on Kolmogorov turbulence, and
that the subtraction of the first 11 Zernike polynomials (first-order correction) improves the wavefront significantly
unlike the subtraction of next 1 1 polynomials (third-order correction).
In this paper we describe recent tests of a unidirectional laser communication link operating in strong atmospheric turbulence conditions, near ground level, over horizontal path distances up to 2.4 km. The source is a 1.3 micrometers Nd:YAG, diode-pumped laser which is fiber-coupled to an electro-optic modulator. The transmitter includes an active tip-tilt system which maintains rough link alignment and compensates for slow pointing drift. The transmitted light is collected by a 7 cm diameter receiver aperture and focused onto a InGaAs p-i-n photodetector. The receiver includes a CCD camera for coarse alignment of the link. The optical carrier is modulated by a 160 Mbps digital signal using quadrature amplitude modulation (QAM) at a subcarrier frequency of 140 MHz or by a 678 Mbps digital on-off keyed (OOK) signal. The link receiver includes an electronic automatic gain control circuit to compensate for atmospheric induced signal fading. The electronic test equipment at the transmitter and the receiver is connected through an RF ethernet link to enable automated measurements of symbol error rate as a function of optical power, modulation format, and energy-per-bit to noise-density ratio. We report on the measurement of bit error rates and demonstrate error- free operation of the communication link using 160 Mbps QAM over 0.9 km and 678 Mbps OOK modulation over 2.4 km under certain atmospheric conditions.
We report an experimental study of the use of DuPont photopolymer Holographic Recording Film (HRF) to record high resolution reflection holograms. The goals of this work are to investigate techniques for recording images of the 3D surfaces of objects, such as integrated circuit chips, without the use of imaging optics and step and repeat techniques, and to investigate techniques for reconstructing these images through a conventional or slightly modified optical microscope. We use the Denisyuk reflection geometry to record single wavelength and multiple wavelength contact-copy and near-contact holograms of an integrated circuit chip. We present photographs of the holographic reconstructions as generated and viewed through a conventional microscope. The images demonstrate color contrast and clearly resolved features of less than 1 micron. In addition, we present an experimental investigation and theoretical modeling of the grating formation process in the DuPont photopolymer HRF. Experiments are described which measure the real-time growth and accompanying real-time shrinkage of reflection gratings. A theory is outlined that includes coupling of the writing beams with the real time growth and shrinkage of nonuniform gratings throughout the thickness of the recording emulsion. Results are presented for three types of DuPont photopolymer HRF.
In this paper, we describe the architecture and performance of a unidirectional, terrestrial laser communication link designed to operate over a range of 100 m to 1 km. The optical carrier is provided by a 1.3 micrometer laser which is modulated using an electro-optic modulator. Quadrature amplitude modulation (QAM) is used to modulate 150 Mbps input binary data at a subcarrier frequency of 140 MHz. At the receive side of the link, the optical signal is detected with a InGaAs p-i-n photodetector. Because the pointing of our low-cost link is passive, atmospheric fluctuations will induce amplitude fading of the optical carrier. The bit error rate performance of the QAM format is particularly sensitive to amplitude fading. Therefore, the performance of our link is sensitive to the effects of atmospheric turbulence. We discuss how these effects can be minimized by an appropriate choice of the beam diameter with respect to the receiver diameter and by incorporating an automatic gain control circuit. We present experimental results which demonstrate the operation of the link with 16-QAM and 256-QAM over a 100 m path with an atmospheric structure constant (Cn2) of 3 multiplied by 10-13 m-2/3.
OASLMs incorporating an a-Si:H photosensor and ferroelectric liquid crystal have proved their high performance in optoelectronic processing systems. The final step in their development is to establish a long mean time between failures, consistent responsibility, gain, resolution, and gray level response. Some subtle properties of the alignment layers, the liquid crystals, and the operating conditions create bottlenecks to producing consistently performing devices. We examine these bottleneck issues and explore ways to extend the reliability of device performance.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.