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ABSTRACT: Phase control is crucial to the operation of coherent beam combining systems, whether for laser radar or high-power beam combining. We have recently demonstrated a design for a multi-aperture, coherently combined, synchronized- and phased-array slow light laser radar (SLIDAR) that is capable of scanning in two dimensions with dynamic group delay compensation. Here we describe in detail the optical phase locking system used in the design. The phase locking system achieves an estimated Strehl ratio of 0.8, and signals from multiple emitting apertures are phase locked simultaneously to within π/5 radians (1/10 wave) after propagation through 2.2 km of single-mode fiber per channel. Phase locking performance is maintained even as two independent slow light mechanisms are utilized simultaneously.
Optics Express 06/2013; 21(11):13094-104. · 3.59 Impact Factor
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ABSTRACT: We describe a high-speed physical random number generator based on a hybrid Boolean network with autonomous and clocked logic gates, realized on a reconfigurable chip. The autonomous logic gates are arranged in a bidirectional ring topology and generate broadband chaos. The clocked logic gates receive input from the autonomous logic gates so that random numbers are generated physically that pass standard randomness tests without further postprocessing. The large number of logic gates on reconfigurable chips allows for parallel generation of random numbers, as demonstrated by our implementation of 128 physical random number generators that achieve a real-time bit rate of 12.8Gbits/s.
Physical Review E 04/2013; 87(4-1):040902. · 2.26 Impact Factor
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ABSTRACT: We describe the design of a temporal imaging system that simultaneously
reshapes the temporal profile and converts the frequency of a photonic
wavepacket, while preserving its quantum state. A field lens, which imparts a
temporal quadratic phase modulation, is used to correct for the residual phase
caused by field curvature in the image, thus enabling temporal imaging for
phase-sensitive quantum applications. We show how this system can be used for
temporal imaging of time-bin entangled photonic wavepackets and compare the
field lens correction technique to systems based on a temporal telescope and
far-field imaging. The field-lens approach removes the residual phase using
four dispersive elements. The group delay dispersion (GDD) $D$ is constrained
by the available bandwidth $\Delta\nu$ by $D>t/\Delta\nu$, where $t$ is the
temporal width of the waveform associated with the dispersion $D$. This is
compared to the much larger dispersion $D\gg \pi t^2/8$ required to satisfy the
Fraunhofer condition in the far field approach.
03/2013;
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ABSTRACT: We study experimentally the synchronization patterns in time-delayed directed Boolean networks of excitable systems. We observe a transition in the network dynamics when the refractory time of the individual systems is adjusted. When the refractory time is on the same order of magnitude as the mean link time delays or the heterogeneities of the link time delays, cluster synchronization patterns change, or are suppressed entirely, respectively. We also show that these transitions occur when we change the properties of only a small number of driver nodes identified by their larger in degree; hence, the synchronization patterns can be controlled locally by these nodes. Our findings have implications for synchronization in biological neural networks.
Physical Review Letters 03/2013; 110(10):104102. · 7.37 Impact Factor
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ABSTRACT: We realize autonomous Boolean networks by using logic gates in their
autonomous mode-of-operation on a field-programmable gate array. This allows us
to implement time-continuous systems with complex dynamical behaviors that can
be conveniently interconnected into large-scale networks with flexible
topologies that consist of time-delay links and a large number of nodes. We
demonstrate how we realize networks with periodic, chaotic, and excitable
dynamics and study their properties. Field-programmable gate arrays define a
new experimental paradigm that holds great potential to test a large body of
theoretical results on the dynamics of complex networks, which has been beyond
reach of traditional experimental approaches.
02/2013;
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ABSTRACT: We report on an ultra-high-frequency (>1 GHz), piecewise-linear chaotic system designed from low-cost, commercially available electronic components. The system is composed of two electronic time-delayed feedback loops: A primary analog loop with a variable gain that produces multi-mode oscillations centered around 2 GHz and a secondary loop that switches the variable gain between two different values by means of a digital-like signal. We demonstrate experimentally and numerically that such an approach allows for the simultaneous generation of analog and digital chaos, where the digital chaos can be used to partition the system's attractor, forming the foundation for a symbolic dynamics with potential applications in noise-resilient communications and radar.
Chaos (Woodbury, N.Y.) 12/2012; 22(4):043112. · 1.80 Impact Factor
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ABSTRACT: We describe a high-speed physical random number generator based on a hybrid
Boolean network with autonomous and clocked logic gates, realized on a
reconfigurable chip. The autonomous logic gates are arranged in a bidirectional
ring topology and generate broadband chaos. The clocked logic gates receive
input from the autonomous logic gates so that random numbers are generated
physically that pass standard randomness tests without further post-processing.
The large number of logic gates on reconfigurable chips allows for parallel
generation of random numbers, as demonstrated by our implementation of 128
physical random number generators that achieve a real-time bit rate of 12.8
Gbit/s.
09/2012;
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ABSTRACT: We demonstrate theoretically and experimentally that excitable systems can be
built with autonomous Boolean networks. Their experimental implementation is
realized with asynchronous logic gates on a reconfigurabe chip. When these
excitable systems are assembled into time-delay networks, their dynamics
display nanosecond time-scale spike synchronization patterns that are
controllable in period and phase.
08/2012;
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ABSTRACT: We demonstrate steady-state, mirrorless superradiance in a cold vapor pumped
by weak optical fields. Beyond a critical pump intensity of 1 mW/cm$^2$, the
vapor spontaneously transforms into a spatially self-organized state: a density
grating forms. Scattering of the pump beams off this grating generates a pair
of new, intense optical fields that act back on the vapor to enhance the atomic
organization. We map out experimentally the superradiant phase transition
boundary and show that it is well-described by our theoretical model. The
resulting superradiant emission is nearly coherent, persists for several
seconds, displays strong temporal correlations between the various modes, and
has a coherence time of several hundred $\mu$s. This system therefore has
applications in fundamental studies of many-body physics with long-range
interactions as well as all-optical and quantum information processing.
02/2012;
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ABSTRACT: We use an information-theoretic method developed by Neifeld and Lee [J. Opt. Soc. Am. A 25, C31 (2008)] to analyze the performance of a slow-light system. Slow-light is realized in this system via stimulated Brillouin scattering in a 2 km-long, room-temperature, highly nonlinear fiber pumped by a laser whose spectrum is tailored and broadened to 5 GHz. We compute the information throughput (IT), which quantifies the fraction of information transferred from the source to the receiver and the information delay (ID), which quantifies the delay of a data stream at which the information transfer is largest, for a range of experimental parameters. We also measure the eye-opening (EO) and signal-to-noise ratio (SNR) of the transmitted data stream and find that they scale in a similar fashion to the information-theoretic method. Our experimental findings are compared to a model of the slow-light system that accounts for all pertinent noise sources in the system as well as data-pulse distortion due to the filtering effect of the SBS process. The agreement between our observations and the predictions of our model is very good. Furthermore, we compare measurements of the IT for an optimal flattop gain profile and for a Gaussian-shaped gain profile. For a given pump-beam power, we find that the optimal profile gives a 36% larger ID and somewhat higher IT compared to the Gaussian profile. Specifically, the optimal (Gaussian) profile produces a fractional slow-light ID of 0.94 (0.69) and an IT of 0.86 (0.86) at a pump-beam power of 450 mW and a data rate of 2.5 Gbps. Thus, the optimal profile better utilizes the available pump-beam power, which is often a valuable resource in a system design.
Applied Optics 11/2011; 50(32):6063-72. · 1.41 Impact Factor
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ABSTRACT: We report a new nonlinear optical process that occurs in a cloud of cold atoms at low-light-levels when the incident optical fields simultaneously polarize, cool, and spatially-organize the atoms. We observe an extremely large effective fifth-order nonlinear susceptibility of χ(⁵) = 7.6 × 10⁻¹⁵ (m/V)⁴, which results in efficient Bragg scattering via six-wave mixing, slow group velocities (∼ c/10⁵), and enhanced atomic coherence times (> 100 μs). In addition, this process is particularly sensitive to the atomic temperatures, and provides a new tool for in-situ monitoring of the atomic momentum distribution in an optical lattice. For sufficiently large light-matter couplings, we observe an optical instability for intensities as low as ∼ 1 mW/cm² in which new, intense beams of light are generated and result in the formation of controllable transverse optical patterns.
Optics Express 11/2011; 19(23):22535-49. · 3.59 Impact Factor
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ABSTRACT: We observe a nonlinear optical process in a gas of cold atoms that
simultaneously displays the largest reported fifth-order nonlinear
susceptibility \chi^(5) = 1.9x10^{-12} (m/V)^4 and high transparency. The
nonlinearity results from the simultaneous cooling and crystallization of the
gas, and gives rise to efficient Bragg scattering in the form of
six-wave-mixing at low-light-levels. For large atom-photon coupling strengths,
the back-action of the scattered fields influences the light-matter dynamics.
This system may have important applications in many-body physics, quantum
information processing, and multidimensional soliton formation.
08/2011;
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ABSTRACT: We study ultra-broadband slow light in a warm Rubidium vapor cell. By working
between the D1 and D2 transitions, we find a several-nm window centered at
788.4 nm in which the group index is highly uniform and the absorption is small
(<1%). We demonstrate that we can control the group delay by varying the
temperature of the cell, and observe a tunable fractional delay of 18 for
pulses as short as 250 fs (6.9 nm bandwidth) with a fractional broadening of
only 0.65 and a power leakage of 55%. We find that a simple theoretical model
is in excellent agreement with the experimental results. Using this model, we
discuss the impact of the pulse's spectral characteristics on the distortion it
incurs during propagation through the vapor.
07/2011;
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ABSTRACT: We observe interference between the optical precursors and the main signal for small optical depth α0 L 1, in which the main signal cannot be entirely absorbed. Since the main signal oscillates at the carrier frequency of the input pulse and precursors oscillate at medium resonance frequency, in our case carrier frequency dependence of the total transmitted field is observed as a form of modulation patterns oscillating at the detuning frequency. To distinguish between the Sommerfeld and Brillouin precursors for the case of weakly dispersive off-resonance medium, we utilize asymptotic precursor theory under the assumption of small detuning.
Journal of Modern Optics 06/2011; 58(10):865-872. · 1.17 Impact Factor
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ABSTRACT: Forward stimulated Brillouin scattering (FSBS) is observed in a standard 2-km-long highly nonlinear fiber. The frequency of FSBS arising from multiple radially guided acoustic resonances is observed up to gigahertz frequencies. The tight confinement of the light and acoustic field enhances the interaction and results in a large gain coefficient of 34.7 W(-1) at a frequency of 933.8 MHz. We also find that the profile on the anti-Stokes side of the pump beam have lineshapes that are asymmetric, which we show is due to the interference between FSBS and the optical Kerr effect. The measured FSBS resonance linewidths are found to increase linearly with the acoustic frequency. Based on this scaling, we conclude that dominant contribution to the linewidth is from surface damping due to the fiber jacket and structural nonuniformities along the fiber.
Optics Express 03/2011; 19(6):5339-49. · 3.59 Impact Factor
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ABSTRACT: We demonstrate a 5-GHz-broadband tunable slow-light device based on stimulated Brillouin scattering in a standard highly-nonlinear optical fiber pumped by a noise-current-modulated laser beam. The noisemodulation waveform uses an optimized pseudo-random distribution of the laser drive voltage to obtain an optimal flat-topped gain profile, which minimizes the pulse distortion and maximizes pulse delay for a given pump power. In comparison with a previous slow-modulation method, eye-diagram and signal-to-noise ratio (SNR) analysis show that this broadband slow-light technique significantly increases the fidelity of a delayed data sequence, while maintaining the delay performance. A fractional delay of 0.81 with a SNR of 5.2 is achieved at the pump power of 350 mW using a 2-km-long highly nonlinear fiber with the fast noise-modulation method, demonstrating a 50% increase in eye-opening and a 36% increase in SNR in the comparison.
Optics Express 01/2011; 19(2):687-97. · 3.59 Impact Factor
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ABSTRACT: ct: We introduce a new concept for stimulated-Brillouin-scattering-based slow light in optical fibers that is applicable for broadly-tunable frequency-swept sources. It allows slow light to be achieved, in principle, over the entire transparency window of the optical fiber. We demonstrate a slow light delay of 10 ns at 1.55 μm using a 10-m-long photonic crystal fiber with a source sweep rate of 400 MHz/μs and a pump power of 200 mW. We also show that there exists a maximal delay obtainable by this method, which is set by the SBS threshold, independent of sweep rate. For our fiber with optimum length, this maximum delay is ~38 ns, obtained for a pump power of 760 mW.
Optics Express 12/2010; 18(26):27263-9. · 3.59 Impact Factor
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ABSTRACT: We study an optoelectronic time-delay oscillator that displays high-speed chaotic behavior with a flat, broad power spectrum. The chaotic state coexists with a linearly stable fixed point, which, when subjected to a finite-amplitude perturbation, loses stability initially via a periodic train of ultrafast pulses. We derive approximate mappings that do an excellent job of capturing the observed instability. The oscillator provides a simple device for fundamental studies of time-delay dynamical systems and can be used as a building block for ultrawide-band sensor networks.
Physical Review Letters 03/2010; 104(11):113901. · 7.37 Impact Factor
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ABSTRACT: The theory of electromagnetism for wave propagation in vacuum, as embodied by Maxwell’s equations, contains physical constants
that can be combined to arrive at the speed of light in vacuum c. As shown by Einstein, consideration of the space–time transformation properties of Maxwell’s equations leads to the special
theory of relativity. One consequence of this theory is that no information can be transmitted between two parties in a time
shorter than it would take light, propagating through vacuum, to travel between the parties. That is, the speed of information
transfer is less than or equal to the speed of light in vacuum c and information related to an event stays within the so-called light cone associated with the event. Hypothetical faster-than-light
(superluminal) communication is very intriguing because relativistic causality would be violated. Relativistic causality is
a principle by which an event is linked to a previous cause as viewed from any inertial frame of reference; superluminal communication
would allow us to change the outcome of an event after it has happened.
01/2010: pages 175-204;
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ABSTRACT: We undertake a systematic study of the dynamics of Boolean networks to determine the origin of chaos observed in recent experiments. Networks with nodes consisting of ideal logic gates are known to display either steady states, periodic behaviour or an ultraviolet catastrophe where the number of logic-transition events circulating in the network per unit time grows as a power law. In an experiment, the non-ideal behaviour of the logic gates prevents the ultraviolet catastrophe and may lead to deterministic chaos. We identify certain non-ideal features of real logic gates that enable chaos in experimental networks. We find that short-pulse rejection and asymmetry between the logic states tend to engender periodic behaviour, at least for the simplest networks. On the other hand, we find that a memory effect termed 'degradation' can generate chaos. Our results strongly suggest that deterministic chaos can be expected in a large class of experimental Boolean-like networks. Such devices may find application in a variety of technologies requiring fast complex waveforms or flat power spectra, and can be used as a test-bed for fundamental studies of real-world Boolean-like networks.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 01/2010; 368(1911):495-513. · 2.77 Impact Factor
