Object-Oriented Matlab Adaptive Optics (OOMAO) is a Matlab toolbox dedicated to Adaptive Optics (AO) systems. It is based on a small set of classes representing the source, atmosphere, telescope, wavefront sensor, Deformable Mirror (DM) and an imager of an AO system. The original OOMAO toolbox was developed for modeling the performance of adaptive optics systems utilizing plane-waves. We have extended the capabilities of the OOMAO toolbox by adding a new method, a Gaussian-beam source, to an existing class, source. This new source method is needed to use OOMAO for modeling laser communications uplinks that are currently under development by many institutions. In this paper, we describe the method that we newly added, namely, Gaussian-beam source uplink, and present several numerical examples. These include results from the simulation for uplink pre-compensation of lasers. We will also show how well uplink works in and outside of the isoplanatic patch.
An experimental demonstration of a quantum-optimal receiver for optical binary signals, developed as a joint effort by the Jet Propulsion Laboratory and the California Institute if Technology, is described in this article. A brief summary of the classical, quantum-optimal, and quantum near optimal solutions to detecting binary signals is first presented. The components and experimental setup used to implement the receivers is then discussed. Experimental performance and results for both optimal and near-optimal receivers are presented and compared to theoretical limits. Finally, experimental shortcomings are discussed along with possible solutions and future direction.
The results of field experiments designed to demonstrate key enabling concepts of optical array receivers are presented,
in a field environment similar to actual operating conditions. The theoretical framework and analytical evaluation of
optical array receivers have been developed and documented in previous articles. This article describes the
implementation and field evaluation of a two-element optical array receiver incorporating photon-counting, signal
conditioning, high-speed digital sample distribution, adaptive delay compensation and sample combining operations
required for array reception of intensity-modulated optical pulse-position modulated (PPM) signals. Data collected in the
field was processed offline to determine communications performance of a two-element optical array, and demonstrate
the inherent advantages of optical array reception.
The performance of a coherent free-space optical communications system operating in the presence of turbulence is
investigated. Maximum Likelihood Detection techniques are employed to optimally detect Pulse Position Modulated
signals with a focal-plane detector array, and reconstruct the turbulence-degraded signals. The experimental
demonstration of this project and results may be divided in three parts; two of which have already been explained in
previous publications [1]. This latest paper shows the final experimental results, including investigation of
performance of the Coherent Optical Receiver Experiment (CORE) performed at the laboratory facilities at JPL. Bit
Error Rate (BER) is presented for single and multichanel optical receivers, where quasi-shot noise limited performance
is achieved, under simulated turbulence conditions using non-coherent post-detection processing techniques.
Theoretical BER expressions are compared with experimental obtained BER results and array combining gains are
presented. Receiver sensitivity in terms of photons per bit (PPB) is examined; BER results are shown as a function of
signal to noise ratios, (SNR), as well as a function of photons per symbol, and photons per bit.
An optical receiver concept for binary signals with performance approaching the quantum limit at low average signal energies is developed and analyzed. A conditionally nulling receiver that reaches the quantum limit in the absence of background photons has been devised by Dolinar, however this receiver requires ideal optical combining and complicated real-time shaping of the local field, hence tends to be difficult to implement at high data rates. A simpler nulling receiver that approaches the quantum limit without complex optical processing, suitable for high-rate operation has been suggested earlier by Kennedy. Here we formulate a vector receiver concept that incorporates the Kennedy receiver together with a physical beamsplitter, but also utilizes the reflected signal component to improve signal detection. It is found that augmenting the Kennedy receiver with classical coherent detection at the auxiliary beamsplitter output, and optimally processing the vector observations, always improves on the performance of the Kennedy receiver alone, significantly so at low average photon rates. This is precisely the region of operation where modern codes approach channel capacity. It is also shown that the addition of background radiation has little effect on the performance of the coherent receiver component, suggesting a viable approach for near quantum-limited performance in high background environments.
In free-space optical communications, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal. We propose to use focal-plane arrays to collect optical signals from different spatial modes simultaneously, and then recombine them optimally. Experimental setup for proof-of-concept coherent adaptive array detection experiment using 32-Pulse Position Modulated (PPM) signals under atmospheric turbulence has been completed. Adaptive combining of experimentally obtained heterodyned PPM signals with pulse-to-pulse coherence, in the presence of simulated atmospheric turbulence is demonstrated. The adaptively combined PPM signals are phased up via a Least-Mean-Square (LMS) algorithm suitably optimized to operate with PPM in the presence of additive shot-noise, and detected via a maximum likelihood software receiver. Convergence study of the algorithm is presented and results with simulated PPM signals and real PPM signals experimentally obtained at the laboratory are presented.
The conceptual design, theoretical performance, and experimental verification of a two-telescope optical array receiver currently under development at the Jet Propulsion Laboratory, is described in this paper. A brief summary of optical communications theory for array reception of pulsed laser signals is developed, and the impact of coding discussed. The development of the optical detection, array processing, and data-acquisition assemblies required for experimental demonstration is described, and preliminary results obtained in a field environment are presented and evaluated.
Adaptive combining of experimentally obtained heterodyned pulse position modulated (PPM) signals with pulse-to-pulse coherence, in the presence of simulated spatial distortions resembling atmospheric turbulence, is demonstrated. The adaptively combined PPM signals are phased up via an LMS algorithm suitably optimized to operate with PPM in the presence of additive shot-noise. A convergence analysis of the algorithm is presented, and results with both computer simulated and experimentally obtained PPM signals are analyzed.
KEYWORDS: Receivers, Telescopes, Space telescopes, Turbulence, Sensors, Signal detection, Optical arrays, Optical communications, Staring arrays, Chemical elements
An optical array receiver concept is developed and analyzed. It is shown that for ground-based reception, the number of array elements can be increased without any performance degradation, as long as the array telescope diameters exceed the coherence-length of the atmosphere. Maximum likelihood detection of turbulence-degraded signal fields is developed for the case of pulse-position modulated (PPM) signals observed in the presence of background radiation. Performance of optical array receivers is compared to single-aperture receivers with diameters ranging from 4 to 8 meters, both in the presence of turbulence and in a turbulence-free environment such as space. It is shown that in the absence of atmospheric turbulence, single-aperture receivers outperform receiver arrays when
significant background radiation is present. However, it is also
shown that for ground-based reception of deep-space signals, the number of array elements can be as great as several thousand without incurring any performance degradation relative to a large single-aperture receiver.
KEYWORDS: Signal detection, Signal to noise ratio, Heterodyning, Sensors, Homodyne detection, Modulation, Signal processing, Receivers, Oscillators, Error analysis
The performance of a coherent free-space optical communications system is investigated. Bit Error Rate (BER) performance is analyzed, and laboratory equipment and experimental setup used to carry out these experiments at the Jet Propulsion Laboratory are described. The key components include two lasers operating at 1064 nm wavelength for use with coherent detection, a 16 element (4X4) focal plane detector array, and data acquisition and signal processing assembly needed to sample and collect the data and analyze the results. Combining of the signals is accomplished using the least-mean-square (LMS) algorithm. Convergence of the algorithm for experimentally obtained signal tones is demonstrated in these initial experiments.
A new technique for evaluating the performance of quantum signals observed in the presence noise is described and evaluated. The quantum theory for detecting coherent-state signals has been developed previously, however the quantum "signal plus noise" problem has received little attention due to its complexity. Here we develop a discrete approximation to the coherent-state representation of signal-plus-noise density operators, and present solutions to optimum receiver performance in terms of quantum "measurement states" whose performance is optimized via generalized rotations in Hilbert space. An efficient algorithm for carrying out the required numerical optimization is described and applied to binary signals observed in the presence of noise, for which exact results are available for comparison. The algorithm is then applied to the detection of ternary signals observed in the presence of noise, a previously unsolved problem, and the performance of the optimum receiver characterized.
The fundamental performance limits and channel capacity of optical communications systems operating over the free space channel will be examined using quantum detection theory. The performance of the optimum quantum receiver for on-off keying (OOK) and optical binary phase shift keying (BPSK) is first examined as a pure state (no noise) problem. The classical capacity of the binary symmetric channel for these two modulation schemes is then evaluated for the optimum quantum receiver by making use of the concept of quantum measurement states. The performance of M-ary pulse position modulation, which requires a product state representation, is evaluated along with the performance of certain 'dense signal sets.' Performance comparisons with classical techniques shows over 5 dB improvement in some cases when quantum detection is employed. As a further application of the quantum detection theory, the capacity of the binary channel with on-off keyed modulation and quantum detection is evaluated, and shown to exceed the capacity obtained with classical photon counting.
Pulse position modulation (PPM) provides a means of using high peak power lasers for transmitting communications signals from planetary spacecraft to earth-based receiving stations. Large aperture (approximately 10 m diameter) telescopes will be used to collect and focus the laser communications signal originating from a deep space transmitter on to a PPM receiver. Large area (1 - 3 mm diameter) sensitive detectors, preceded by appropriate narrow (0.1 - 0.2 nm) optical band-pass filters and followed by low-noise, high-gain, amplifiers will serve as the PPM receiver front end. A digital assembly will form the backbone of the receiver. The PPM receiver will achieve and maintain slot synchronization based on sub slot sums generated by a field programmable-gated array (FPGA). Spacecraft dynamics and timing issues between the ground- based receiver and the transmitter on board the spacecraft must be taken into account. In the present report, requirements and design of a prototype PPM receiver being developed over the next year will be elaborated. The design is driven by the need to demonstrate and validate PPM reception using a variety of detectors under simulated conditions representative of those to be encountered in a deep space optical communications link.
A variety of avalanche photodiodes (APD's) were tested with pulse position modulated (PPM) Q-switched laser pulses incident on the detector, with varying amounts of attenuation. The detector output was recorded and post- processed in order to determine the signal and noise slot statistics, as well as, to estimate bit-error-rates (BER). The probability distribution functions predicted by a Webb+Gaussian model were compared to the measured slot statistics, as were theoretical BER curves. Allowing noise equivalent temperature to be a free fitting parameter yielded good fits between measurements and theory. All the measurements used 256-ary PPM and 10 - 25 ns slot widths, with a Q-switched Nd:YVO4 laser modulated at 50 K - 100 K pulses per second. A 3 mm diameter, silicon (Si) APD with 80% quantum efficiency (QE) at 532 nm displayed a sensitivity deteriorated to 18 photons/bit in the presence of 100 photons per 25 ns slot of background light. A 0.8 mm diameter near infrared (NIR) enhanced Si APD with QE of 0.38 displayed sensitivities of 23 - 32 photons/bit for a BER of 10-2 at 1064 nm in the absence of background light. Backgrounds of 400 photons per 25 ns slot degraded the sensitivity to approximately 58 photons/slot. Finally a 3 mm diameter NIR enhanced Si APD yielded a sensitivity of approximately 100 photons/bit 1064 nm for BER of 10-2 with no background present.
Ground-based reception of optical signals from space suffers from degradation of the optical phase-front caused by atmospheric turbulence, leading to a reduction in the effective diameter of the receiving aperture and to random fluctuations of the point spread function in the focal plane. A proportional increase in the receiver's field of view, required to collect all of the signal, also causes a corresponding increase in the amount of interfering background radiation, resulting in degraded communications performance. These problems may be mitigated through the use of an optical detector array assembly in the focal plane that can adaptively select areas of higher signal density while ignoring areas predominated by background noise. This concept is investigated for both Poisson photon counting detector arrays and avalanche photodiode arrays. Kolmogorov phase screen simulations are used to model the sample functions of the focal-plane signal distribution due to turbulence and to generate realistic spatial distributions of the received optical field. The optimum photon counting array detector is derived and approximated by a simpler suboptimum structure that replaces the continuous weighting function of the optimal receiver by a hard decision on the selection of the signal detector elements. It is shown that for photon counting receivers observing Poisson distributed signals, performance improvements of up to 5 dB can be obtained over conventional single detector photon counting receivers, when observing turbulent optical fields in high background environments. For the avalanche photodiode detector case, it is shown that gains of up to 4 dB may be achieved by using the array receiver rather than a single APD, but that a photon-counting array still performs about 5.5 dB better than an APD array.
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