Showing papers on "Optical polarization published in 2010"

Digital Coherent Optical Receivers: Algorithms and Subsystems

Abstract: Digital coherent receivers have caused a revolution in the design of optical transmission systems, due to the subsystems and algorithms embedded within such a receiver. After giving a high-level overview of the subsystems, the optical front end, the analog-to-digital converter (ADC) and the digital signal processing (DSP) algorithms, which relax the tolerances on these subsystems are discussed. Attention is then turned to the compensation of transmission impairments, both static and dynamic. The discussion of dynamic-channel equalization, which forms a significant part of the paper, includes a theoretical analysis of the dual-polarization constant modulus algorithm, where the control surfaces several different equalizer algorithms are derived, including the constant modulus, decision-directed, trained, and the radially directed equalizer for both polarization division multiplexed quadriphase shift keyed (PDM-QPSK) and 16 level quadrature amplitude modulation (PDM-16-QAM). Synchronization algorithms employed to recover the timing and carrier phase information are then examined, after which the data may be recovered. The paper concludes with a discussion of the challenges for future coherent optical transmission systems.

Probing the Inner Jet of the Quasar PKS 1510?089 with Multi-Waveband Monitoring During Strong Gamma-Ray Activity

Abstract: We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of the quasar PKS 1510 – 089, concentrating on eight major γ-ray flares that occurred during the interval 2009.0-2009.5. The γ-ray peaks were essentially simultaneous with maxima at optical wavelengths, although the flux ratio of the two wave bands varied by an order of magnitude. The optical polarization vector rotated by 720° during a five-day period encompassing six of these flares. This culminated in a very bright, ~1 day, optical and γ-ray flare as a bright knot of emission passed through the highest-intensity, stationary feature (the "core") seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate down the jet at an apparent speed of 22c and emit strongly at γ-ray energies as a months-long X-ray/radio outburst intensified. We interpret these events as the result of the knot following a spiral path through a mainly toroidal magnetic field pattern in the acceleration and collimation zone of the jet, after which it passes through a standing shock in the 43 GHz core and then continues downstream. In this picture, the rapid γ-ray flares result from scattering of infrared seed photons from a relatively slow sheath of the jet as well as from optical synchrotron radiation in the faster spine. The 2006-2009.7 radio and X-ray flux variations are correlated at very high significance; we conclude that the X-rays are mainly from inverse Compton scattering of infrared seed photons by 20-40 MeV electrons.

Omnidirectional, polarization-insensitive and broadband thin absorber in the terahertz regime

Abstract: A nearly omnidirectional THz absorber for both transverse electric (TE) and transverse magnetic (TM) polarizations is proposed. Through the excitation of the magnetic polariton in a metal-dielectric layer, the incident light is perfectly absorbed in a thin thickness that is about 25 times smaller than the resonance wavelength. By simply stacking several such structural layers with different geometrical dimensions, the bandwidth of this strong absorption can be effectively enhanced due to the hybridization of magnetic polaritons in different layers.

Spin-imbalance in a one-dimensional Fermi gas

Abstract: Superconductivity and magnetism generally do not coexist. Changing the relative number of up and down spin electrons disrupts the basic mechanism of superconductivity, where atoms of opposite momentum and spin form Cooper pairs. Nearly forty years ago Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) proposed an exotic pairing mechanism in which magnetism is accommodated by the formation of pairs with finite momentum. Despite intense theoretical and experimental efforts, however, polarized superconductivity remains largely elusive. Unlike the three-dimensional (3D) case, theories predict that in one dimension (1D) a state with FFLO correlations occupies a major part of the phase diagram. Here we report experimental measurements of density profiles of a two-spin mixture of ultracold (6)Li atoms trapped in an array of 1D tubes (a system analogous to electrons in 1D wires). At finite spin imbalance, the system phase separates with an inverted phase profile, as compared to the 3D case. In 1D, we find a partially polarized core surrounded by wings which, depending on the degree of polarization, are composed of either a completely paired or a fully polarized Fermi gas. Our work paves the way to direct observation and characterization of FFLO pairing.

A change in the optical polarization associated with a γ-ray flare in the blazar 3C 279

Abstract: It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight1. The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (γ)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and γ-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 105 gravitational radii.

Spectrally Efficient Long-Haul Optical Networking Using 112-Gb/s Polarization-Multiplexed 16-QAM

Abstract: We discuss the generation, wavelength-division-multiplexed (WDM) long-haul transmission, and coherent detection of 112-Gb/s polarization-division-multiplexed (PDM) 16-ary quadrature amplitude modulation (16-QAM) at a line rate of 14 Gbaud and spectral efficiencies beyond 4 b/s/Hz. We describe the (off-line) digital signal processing and blind filter adaptation algorithms used in our intradyne receiver and characterize its performance using both simulated and measured 16-QAM waveforms. We measure a required optical signal-to-noise ratio of 20.2 dB (0.1-nm reference bandwidth; 10-3 bit-error ratio), 3.2-dB off the theoretical limit. We study the effects of finite analog-to-digital converter resolution, laser frequency offset, laser phase noise, and narrowband optical filtering. Our experiments on a 25-GHz WDM grid (4.1-b/s/Hz spectral efficiency) reveal a 1-dB penalty after 7 passes though reconfigurable optical add/drop multiplexers (ROADMs) and an achievable transmission reach of 1022 km of uncompensated standard single-mode fiber. At a spectral efficiency of 6.2 b/s/Hz (16.67-GHz WDM channel spacing) a transmission reach of 630 km is attained.

Broadband light absorption enhancement in thin-film silicon solar cells.

Abstract: Currently, the performances of thin film solar cells are limited by poor light absorption and carrier collection In this research, large, broadband, and polarization-insensitive light absorption enhancement was realized via integrating with unique metallic nanogratings Through simulation, three possible mechanisms were identified to be responsible for such an enormous enhancement A test for totaling the absorption over the solar spectrum shows an up to approximately 30% broadband absorption enhancement when comparing to bare thin film cells

Alignment, Rotation, and Spinning of Single Plasmonic Nanoparticles and Nanowires Using Polarization Dependent Optical Forces

Abstract: We demonstrate optical alignment and rotation of individual plasmonic nanostructures with lengths from tens of nanometers to several micrometers using a single beam of linearly polarized near-infrared laser light. Silver nanorods and dimers of gold nanoparticles align parallel to the laser polarization because of the high long-axis dipole polarizability. Silver nanowires, in contrast, spontaneously turn perpendicular to the incident polarization and predominantly attach at the wire ends, in agreement with electrodynamics simulations. Wires, rods, and dimers all rotate if the incident polarization is turned. In the case of nanowires, we demonstrate spinning at an angular frequency of ∼1 Hz due to transfer of spin angular momentum from circularly polarized light.

Nonpolar and Semipolar III-Nitride Light-Emitting Diodes: Achievements and Challenges

Abstract: It has been several years since InGaN/GaN light-emitting diodes (LEDs) on nonpolar and semipolar orientations were first demonstrated. Prominent performance and inherent potential of these crystallographic orientations have been revealed as bulk-GaN substrates of arbitrary orientations became available for epitaxial device growth. At this point in time, we intend to survey the progress made to date and prospect the future requirements for further device improvements. The discussion begins with a historical background: how nonpolar/semipolar orientations were introduced to III-nitride LEDs and why they are beneficial. The discussion then provides information on elementary crystallography and piezoelectricity in addition to the electronic band structure of wurtzite crystals. Later in this paper, LED reports are collected to develop comprehensive knowledge of the past research efforts and trends. Nonpolar and semipolar orientations provide not only high LED performances, e.g., optical output power and wavelength ranges, but also unique functions, e.g., polarized light emission, which will explore new fields of applications.

108 Gb/s OFDMA-PON With Polarization Multiplexing and Direct Detection

Abstract: In this paper, we propose and experimentally demonstrate the first single-? 40 Gb/s and 108 Gb/s multiple-input multiple-output orthogonal frequency-division multiple access (OFDMA) passive optical networks (PON) architecture for next-generation PON systems based on OFDM, polarization multiplexing (POLMUX), and direct detection. Superior performance was exhibited after 20 km SSMF transmission and a 1:32 optical split. The novel POLMUX approach greatly simplified receiver-end hardware compared to coherent detectors, while increasing spectral efficiency to enable 40+ Gb/s data rates. Moreover, the proposed solution achieved the highest single-wavelength downstream transmission reported to date in any PON system. As such, the introduced architecture may be viewed as a highly attractive candidate for next-generation optical access.

Wavelength-Selective Switches for ROADM Applications

Abstract: The trends, architecture, and performance of wavelength-selective switches (WSS) are analyzed in the context of their application to reconfigurable optical add/drop multiplexer (ROADM)-based optical networks. The resulting analyses define the requirements for the latest generation of ROADM systems and provide insight into the critical specifications of this technology. In addition, the current trends for WSS technology are reviewed in the context of synergies with the strengths of different switching technologies.

Broadband CPW-Fed Circularly Polarized Square Slot Antenna With Lightening-Shaped Feedline and Inverted-L Grounded Strips

Abstract: A novel design is described for a circularly polarized square slot antenna (CPSSA). Circular polarization (CP) operations can be attained using a lightening-shaped feedline protruded from the signal line of the feeding coplanar waveguide (CPW). The CP bandwidth can be significantly enhanced by implanting a pair of inverted-L grounded strips into the slot and adjusting the dimensions of the lightening-shaped feedline, whereas the impedance bandwidth can be greatly enlarged through tuning embedded vertical and horizontal stubs. The designed antenna was measured to exhibit a CP bandwidth of as high as 48.8%.

Polarization-entangled photons produced with high-symmetry site-controlled quantum dots

Abstract: Entangled photons are efficiently generated from highly symmetric, site-controlled InGaAs/GaAs quantum dots grown in inverted pyramids. Fine-structure splitting of the intermediate exciton level is suppressed without the application of electric, magnetic or strain fields. Polarization entanglement is demonstrated by measurements of the two-photon density matrix and the confirmation of several entanglement criteria.

High-resolution microwave frequency dissemination on an 86-km urban optical link

Abstract: We report the first demonstration of a long-distance ultra-stable frequency dissemination in the microwave range. A 9.15-GHz signal is transferred through an 86-km urban optical link with a fractional frequency instability of 1.3×10−15 at 1-s integration time and below 10−18 at one day. The optical link phase noise compensation is performed with a round-trip method. To achieve such a result we implement light polarisation scrambling and dispersion compensation. This link outperforms all the previous radio-frequency links and compares well with recently demonstrated full optical links.

Optical Communications for High-Altitude Platforms

Abstract: This paper contains a review of technologies, theoretical studies, and experimental field trials for optical communications from and to high-altitude platforms (HAPs). We discuss the pointing, acquisition, and tracking of laser terminals and describe how laser beams with low divergence can be used to transmit data at multi-Gigabits per second. Investigating the influence of the atmosphere, background light, and flight qualification requirements on system design, we explain why the data rates in free-space optical communications are still significantly below those possible in today's terrestrial fiber-based systems. Techniques like forward-error correction, adaptive optics, and diversity reception are discussed. Such measures help to increase the data rate or link distance while keeping the bit error ratio and outage probability of the optical HAP communication system low.

Optical polarization characteristics of ultraviolet (In)(Al)GaN multiple quantum well light emitting diodes

Abstract: The polarization of the in-plane electroluminescence of (0001) orientated (In)(Al)GaN multiple quantum well light emitting diodes in the ultraviolet-A and ultraviolet-B spectral range has been investigated. The intensity for transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with decreasing emission wavelength. This effect is attributed to rearrangement of the valence bands at the Γ-point of the Brillouin zone with changing aluminum and indium mole fractions in the (In)(Al)GaN quantum wells. For shorter wavelength the crystal-field split-off hole band moves closer to the conduction band relative to the heavy and light hole bands and as a consequence the transverse-magnetic polarized emission becomes more dominant for deep ultraviolet light emitting diodes.

Monolithic Polarization and Phase Diversity Coherent Receiver in Silicon

Abstract: In this paper, we realized a monolithic silicon photonic integrated circuit (PIC) for polarization and phase diversity coherent detection. The PIC includes two polarization beam splitters, two 90° optical hybrids, and four pairs of balanced photodiodes implemented as integrated germanium detectors. We tested the PIC using polarization-division multiplexed quadrature phase-shift keyed signals at 43 and at 112 Gb/s.

Mitigation of Fiber Nonlinearity Using a Digital Coherent Receiver

Abstract: Coherent detection with receiver-based DSP has recently enabled the mitigation of fiber nonlinear effects. We investigate the performance benefits available from the backpropagation algorithm for polarization division multiplexed quadrature amplitude phase-shift keying (PDM-QPSK) and 16-state quadrature amplitude modulation (PDM-QAM16). The performance of the receiver using a digital backpropagation algorithm with varying nonlinear step size is characterized to determine an upper bound on the suppression of intrachannel nonlinearities in a single-channel system. The results show that for the system under investigation PDM-QPSK and PDM-QAM16 have maximum step sizes for optimal performance of 160 and 80 km, respectively. Whilst the optimal launch power is increased by 2 and 2.5 dB for PDM-QPSK and PDM-QAM16, respectively, the Q-factor is correspondingly increased by 1.6 and 1 dB, highlighting the importance of studying nonlinear compensation for higher level modulation formats.

100G and beyond with digital coherent signal processing

Abstract: The demand for increased bandwidth is ever present. Coherent technology coupled with advanced modulation formats and digital signal processing is a key enabler for optical communication systems at 100 Gb/s and beyond. This article reviews the attributes of coherent systems in light of the challenges faced by system designers to realize increased bit rates for next-generation optical systems.

Real-Time Software-Defined Multiformat Transmitter Generating 64QAM at 28 GBd

Abstract: We demonstrate a software-defined real-time optical multiformat transmitter. Here, eight different modulation formats are shown. Data rate and modulation formats are defined through software accessible look-up tables enabling format switching in the nanosecond regime without changing the transmitter hardware. No data are lost during the switching process. SP-64 quadrature amplitude modulation at 28 Gbd has been generated and tested. This allows us to generate a 336-Gb/s real-time pseudorandom bit sequence in a dual polarization setup.

UWB-Over-Fiber Communications: Modulation and Transmission

Abstract: The distribution of ultra-wideband (UWB) signals over optical fiber, or UWB over fiber (UWBoF), is proposed to extend the area of coverage and to offer the availability of undisrupted service across different networks. Various techniques have been reported recently for optical UWB pulse generation, but the study on the implementation of different modulation schemes based on these UWB pulses has just started. In addition, the influence of fiber dispersion on the power spectral density (PSD) of an UWB signal, and the bit-error-rate performance of an UWBoF system with different modulation schemes have not been systematically investigated. In this paper, we perform a comprehensive investigation of techniques to implement on-off keying (OOK), bi-phase modulation (BPM), pulse-amplitude modulation (PAM), pulse shape modulation (PSM), and pulse-position modulation (PPM) based on a phase modulator and an asymmetric Mach-Zehnder interferometer (AMZI). The AMZI is electrically reconfigurable by employing a polarization modulator (PolM). UWB signals with OOK, BPM, PAM, PSM, and PPM are realized by adjusting the polarization controllers in the AMZI and the amplitude of the electrical drive signal to the PolM. The UWB signals with OOK, BPM, PAM, and PSM are transmitted over a wired (single-mode fiber) and wireless link. Error-free operation is obtained for all the modulation schemes. The power penalties of transmission are less than 1.8 dB. The fiber dispersion on the PSD of the UWB signals is also theoretically studied and experimentally evaluated. An excellent agreement between the theoretical and the experimental results is achieved. The system is potentially integratable, which may provide a simple and cost-effective solution for UWBoF applications.

Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes

Abstract: We review, compare and extend recent studies searching for evidence for a preferred cosmological axis. We start from the Union2 SnIa dataset and use the hemisphere comparison method to search for a preferred axis in the data. We find that the hemisphere of maximum accelerating expansion rate is in the direction (l,b) = (309°+23°−3°,18°+11°−10°) (Ω0m = 0.19) while the hemisphere of minimum acceleration is in the opposite direction (l,b) = (129°+23°−3°,−18°+10°−11°) (Ω0m = 0.30). The level of anisotropy is described by the normalized difference of the best fit values of Ω0m between the two hemispheres in the context of ΛCDM fits. We find a maximum anisotropy level in the Union2 data of ΔΩ0m max / Ω0m = 0.43±0.06. Such a level does not necessarily correspond to statistically significant anisotropy because it is reproduced by about 30% of simulated isotropic data mimicking the best fit Union2 dataset. However, when combined with the axes directions of other cosmological observations (bulk velocity flow axis, three axes of CMB low multipole moments and quasar optical polarization alignment axis), the statistical evidence for a cosmological anisotropy increases dramatically. We estimate the probability that the above independent six axes directions would be so close in the sky to be less than 1%. Thus either the relative coincidence of these six axes is a very large statistical fluctuation or there is an underlying physical or systematic reason that leads to their correlation.

Dual encryption scheme of images using polarized light

Abstract: We propose and analyze a dual encryption/decryption scheme, motivated by recent interest in polarization encoding. Compared to standard optical encryption methods, which are based on phase and amplitude manipulation, this encryption procedure relying on Mueller-Stokes formalism provides large flexibility in the key encryption design. The effectiveness of our algorithm is discussed, thanks to a numerical simulation of the polarization encryption/decryption procedure of a 256 gray-level image. Of additional special interest is the immunity of this encryption algorithm to brute force attacks.

Orthogonal frequency division multiple access PON (OFDMA-PON) for colorless upstream transmission beyond 10 Gb/s

Abstract: In this paper, we overview the fundamental principles of next-generation optical Orthogonal Frequency Division Multiple Access (OFDMA)-PON systems, with a particular focus on upstream architectures capable of achieving 10+ Gb/s colorless upstream transmission. We also propose a novel OFDMA-PON architecture that satisfies these requirements and is capable of exceeding 10 Gb/s upstream transmission over a single wavelength. It is first analytically shown that optical carrier suppression at the optical network units (ONUs) combined with coherent detection at the optical line terminal (OLT) successfully eliminates both in- and cross-polarization beating noise that would otherwise be generated at the OLT and would fundamentally limit upstream transmission performance. A centralized light source OFDM-based PON architecture with source-free ONUs and OLTside coherent detection is then presented and experimentally verified to achieve 20 Gb/s/λ transmission over a class B+ optical distribution network (20 km SSMF with an additional 1:32 optical split.) By thus providing very high-speed, flexible, colorless upstream transmission, the proposed architecture is an attractive candidate for next-generation PON systems capable of cost-efficiently delivering heterogeneous services.

Cavity-involved plasmonic metamaterial for optical polarization conversion

Abstract: We experimentally demonstrate a plasmonic assisted Fabry–Perot cavity in a metal/insulator/metal trilayer structure with L-shaped hole arrays inside, which significantly contribute to the mechanism to realize a nearly complete polarization conversion (=0.93) in optical transmissions at near-infrared wavelength. This interesting property is found arising from an overlap of the cavity and plasmonic modes in two orthogonal polarization states. This discovered physics remarkably endows this plasmonic metamaterial with good optical performance and looser fabrication requirement, not only indicating practical applications but also providing fruitful inspirations in future nanophotonic designs.

Silicon-Waveguide-Based Mode Evolution Polarization Rotator

Abstract: A mode-evolution-based polarization rotator in silicon waveguide was designed. The rotator's performance was studied under different launching conditions. The rotator with minimum length of 40 ?m was demonstrated to provide the polarization rotation with polarization extinction ratio of 15 dB. The insertion loss at the transition region was less than 1 dB.

Frequency and Polarization Characteristics of Correlated Photon-Pair Generation Using a Silicon Wire Waveguide

Abstract: We report the frequency and polarization characteristics of correlated photon pairs generated in a Si wire waveguide (SWW). We confirmed that the bandwidth for correlated photon-pair generation was at least >2.8 THz. Moreover, we carried out a classical four-wave mixing experiment using strong pump and idler lights to estimate the bandwidth for correlated photon-pair generation. The results indicated that it is possible to generate photon pairs over a bandwidth as large as ~12 THz. We also showed that the generation efficiencies of the signal and idler photons for the horizontal polarization mode were much higher than those for the vertical polarization mode. This is probably caused by the large efficiencies in the group indexes and the effective cross-sectional areas for the two polarization modes. Furthermore, the bandwidth for the correlated photon-pair generation in the vertical polarization mode was ~±1 THz, and this was much narrower than that for the horizontal polarization mode. The difference between the bandwidths of the two polarization modes indicates that the SWW dispersion for the vertical polarization mode is significantly larger than that for the horizontal polarization mode. We then confirmed that the noise photons generated by spontaneous Raman scattering in an SWW were suppressed to below the detection limit of our setup.

Analysis of RF-Pilot-Based Phase Noise Compensation for Coherent Optical OFDM Systems

Abstract: In coherent optical long-haul transmission systems, orthogonal frequency-division multiplexing represents a promising modulation format. However, due to long symbol length, laser phase noise can be a major impairment. In this manuscript, the RF-pilot-based phase noise compensation scheme is analyzed and compared to conventional common-phase error compensation. It has been shown that the RF-pilot-based phase noise compensation scheme allows for a considerable increase in tolerable laser linewidth as compared to conventional common-phase error compensation at the cost of an increase in system complexity. For a 112-Gb/s transmission scheme, the tolerable linewidth is increased by a factor of ten as compared to common-phase error compensation.

Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes

Abstract: We review, compare and extend recent studies searching for evidence for a preferred cosmological axis. We start from the Union2 SnIa dataset and use the hemisphere comparison method to search for a preferred axis in the data. We find that the hemisphere of maximum accelerating expansion rate is in the direction $(l,b)=({309^\circ}^{+23^\circ}_{-3^\circ}, {18^\circ}^{+11^\circ}_{-10^\circ})$ ($\omm=0.19$) while the hemisphere of minimum acceleration is in the opposite direction $(l,b)=({129^\circ}^{+23^\circ}_{-3^\circ},{-18^\circ}^{+10^\circ}_{-11^\circ})$ ($\omm=0.30$). The level of anisotropy is described by the normalized difference of the best fit values of $\omm$ between the two hemispheres in the context of \lcdm fits. We find a maximum anisotropy level in the Union2 data of $\frac{\Delta \ommax}{\bomm}=0.43\pm 0.06$. Such a level does not necessarily correspond to statistically significant anisotropy because it is reproduced by about $30%$ of simulated isotropic data mimicking the best fit Union2 dataset. However, when combined with the axes directions of other cosmological observations (bulk velocity flow axis, three axes of CMB low multipole moments and quasar optical polarization alignment axis), the statistical evidence for a cosmological anisotropy increases dramatically. We estimate the probability that the above independent six axes directions would be so close in the sky to be less than $1%$. Thus either the relative coincidence of these six axes is a very large statistical fluctuation or there is an underlying physical or systematic reason that leads to their correlation.