In this paper, we present the measurement results of a spectrally efficient 2.56Tb/s free-space data link using orbital
angular momentum (OAM) beams. This link includes 32 independent 20 Gbaud/s 16-quadrature-amplitude-modulation
data streams, each encoded on a different OAM beam, which are mode-, polarization- and space-division multiplexed as
one collocated beam. We measured the bit-error rate (BER) curves of all 32 channels, all of which can achieve a BER of
<2×10-3. The performance degradation due to the spatial multiplexing using concentric ring scheme is analyzed.
Additionally, the effect of the pre-filtering is investigated, and negligible penalty is observed.
An ideal intensity-modulated photon-counting channel can achieve unbounded photon information efficiencies
(PIEs). However, a number of limitations of a physical system limit the practically achievable PIE. In this paper,
we discuss several of these limitations and illustrate their impact on the channel. We show that, for the Poisson
channel, noise does not strictly bound PIE, although there is an effective limit, as the dimensional information
efficiency goes as e-ePIE
beyond a threshold PIE. Since the Holevo limit is bounded in the presence of noise, this
illustrates that the Poisson approximation is invalid at large PIE for any number of noise modes. We show that a
finite transmitter extinction ratio bounds the achievable PIE to a maximum that is logarithmic in the extinction
ratio. We show how detector jitter limits the ability to mitigate noise in the PPM signaling framework. We
illustrate a method to model detector blocking when the number of detectors is large, and illustrate mitigation of
blocking with spatial spreading and filtering. Finally, we illustrate the design of a high photon efficiency system
using state-of-the-art photo-detectors and taking all these effects into account.
Coherent states achieve the Holevo capacity of a pure-loss channel when paired with an optimal measurement,
but a physical realization of this measurement scheme is as of yet unknown, and it is also likely to be of high
complexity. In this paper, we focus on the photon-counting measurement and study the photon and dimensional
efficiencies attainable with modulations over classical- and nonclassical-state alphabets. We analyze two binary-modulation
architectures that improve upon the dimensional versus photon efficiency tradeoff achievable with
the state-of-the-art coherent-state on-off keying modulation. We show that at high photon efficiency these
architectures achieve an efficiency tradeoff that differs from the best possible tradeoff--determined by the Holevo
capacity--by only a constant factor. The first architecture we analyze is a coherent-state transmitter that relies
on feedback from the receiver to control the transmitted energy. The second architecture uses a single-photon
number-state source.
Optical communication at the quantum limit requires that measurements on the optical field be maximally
informative, but devising physical measurements that accomplish this objective has proven challenging. The
Dolinar receiver exemplifies a rare instance of success in distinguishing between two coherent states: an adaptive
local oscillator is mixed with the signal prior to photodetection, which yields an error probability that meets
the Helstrom lower bound with equality. Here we apply the same local-oscillator-based architecture with an
information-theoretic optimization criterion. We begin with analysis of this receiver in a general framework for
an arbitrary coherent-state modulation alphabet, and then we concentrate on two relevant examples. First, we
study a binary antipodal alphabet and show that the Dolinar receiver's feedback function not only minimizes
the probability of error, but also maximizes the mutual information. Next, we study ternary modulation consisting
of antipodal coherent states and the vacuum state. We derive an analytic expression for a near-optimal
local-oscillator feedback function, and, via simulation, we determine its photon information efficiency (PIE). We
provide the PIE versus dimensional information efficiency (DIE) trade-off curve and show that this modulation
and the our receiver combination performs universally better than (generalized) on-off keying plus photon
counting, although, the advantage asymptotically vanishes as the bits-per-photon diverges towards infinity.
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.
In this paper we derive the capacity of Pulse Position Modulation (PPM) on a general soft output, memoryless channel, and evaluate the capacity formula for a variety of optical channel models, including AWGN, Webb, and Webb plus Gaussian distributions. Unlike a typical RF link, the optical channel has correlated signal and noise, complicating the statistical model to the point that capacity and code performance cannot be summarized by a single SNR parameter. Nevertheless, we are able to define a small set of fundamental parameters (two for AWGN and three for Webb) which are sufficient to determine the capacity. Numerical results indicate that over a wide range of operating points, a single fundamental parameter dominates the capacity calculation. A second contribution of the paper is the description of the relationship between the fundamental parameters and a multitude of physical parameters that describe the laser, channel, and detector. Using this relationship and the gradient of capacity, the sensitivity of capacity with respect to each fundamental and physical parameter is derived. This enables engineers to focus laser and detector development efforts in areas that will result in the largest capacity increases.
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