Showing papers on "Polarization mode dispersion published in 2017"

K-means-clustering-based fiber nonlinearity equalization techniques for 64-QAM coherent optical communication system.

Abstract: In this work, we proposed two k-means-clustering-based algorithms to mitigate the fiber nonlinearity for 64-quadrature amplitude modulation (64-QAM) signal, the training-sequence assisted k-means algorithm and the blind k-means algorithm. We experimentally demonstrated the proposed k-means-clustering-based fiber nonlinearity mitigation techniques in 75-Gb/s 64-QAM coherent optical communication system. The proposed algorithms have reduced clustering complexity and low data redundancy and they are able to quickly find appropriate initial centroids and select correctly the centroids of the clusters to obtain the global optimal solutions for large k value. We measured the bit-error-ratio (BER) performance of 64-QAM signal with different launched powers into the 50-km single mode fiber and the proposed techniques can greatly mitigate the signal impairments caused by the amplified spontaneous emission noise and the fiber Kerr nonlinearity and improve the BER performance.

Polarization Multiplexing With the Kramers-Kronig Receiver

Abstract: We present two schemes extending the Kramers-Kronig-receiver to polarization multiplexed transmission. One of the schemes relies on the Stokes receiver and allows for complex-valued polarization multiplexed transmission without using a local oscillator, and active polarization control at the receiver. It uses three photodiodes and requires three analog-to-digital converters (ADCs). The other scheme uses a local oscillator, and its implementation is suitable in all cases where the transmitter and the receiver operate at the same wavelength, so that the local oscillator signal can be extracted from the transmit laser. It uses only two photodiodes and two ADCs, and does not require optical hybrids. The performance of the first (Stokes-based) scheme is studied numerically, whereas that of the second scheme is studied both numerically and experimentally.

Chromatic dispersion compensation techniques and characterization of fiber Bragg grating for dispersion compensation

Abstract: Pulse spreading due to the dispersion causes the overlapping of the transmitted pulses at the receiver end known as inter symbol interference (ISI). The ISI thus limits transmission of high speed data. We are living in the age of bandwidth hungry and high speed applications, for which optical networks form the most important part because of its high bandwidth. In optical networks chromatic dispersion (CD) is one of the main obstacle in high speed transmission. Hence this CD is compensated by various approaches throughout the transmission system. A review of all the main approaches is presented in this paper. Characterization of fiber Bragg grating for dispersion compensation is done using the reflection spectrum and group delay response analysis.

Ultrahigh-Resolution Optical Vector Analysis Based on Optical Single-Sideband Modulation

Abstract: Knowing magnitude, phase, and polarization responses is of great importance for fabrication and application of optical devices. A large variety of parameters such as insertion loss, dispersion, group delay, polarization-dependent loss, and polarization mode dispersion can be obtained based on these responses. Conventional approaches achieve the optical spectral responses by sweeping the wavelength of a laser source. Restricted by the low-wavelength accuracy and poor wavelength stability of the wavelength-swept laser source, the resolution of the optical vector analyzers (OVAs) are usually poor (>1.6 pm). To achieve ultrahigh resolution measurement, an OVA based on optical single-sideband (OSSB) modulation has been proposed and developed, which potentially has a sub-Hz resolution. However, electrical-to-optical and optical-to-electrical conversions are required to implement the electrical frequency sweeping and to detect the phase and magnitude information in the electrical domain, which limits the spectral measurement range, accuracy, and dynamic range. In the past decade, great efforts have been devoted to deal with these problems. In this paper, techniques for constructing high-performance OSSB-based OVAs are discussed with an emphasis on the system architectures and operation principles for improving the spectral measurement range, accuracy, and dynamic range of the measurement system. Possible future research directions are also discussed.

Asynchronous and synchronous dual-wavelength pulse generation in a passively mode-locked fiber laser with a mode-locker.

Abstract: The evolution from asynchronous to synchronous dual-wavelength pulse generation in a passively mode-locked fiber laser is experimentally investigated by tailoring the intracavity dispersion. Through tuning the intracavity-loss-dependent gain profile and the birefringence-induced filter effect, asynchronous dual-wavelength soliton pulses can be generated until the intracavity anomalous dispersion is reduced to ∼8 fs/nm. The transition from asynchronous to synchronous pulse generation is then observed at an elevated pump power in the presence of residual anomalous dispersion, and it is shown that pulses are temporally synchronized at the mode-locker in the cavity. Spectral sidelobes are observed and could be attributed to the four-wave-mixing effect between dual-wavelength pulses at the carbon nanotube mode-locker. These results could provide further insight into the design and realization of such dual-wavelength ultrafast lasers for different applications such as dual-comb metrology as well as better understanding of the inter-pulse interactions in such dual-comb lasers.

Four Wave Mixing-Based Time Lens for Orthogonal Polarized Input Signals

Abstract: We present highly efficient time-lenses for orthogonal polarized signal waves with a specific state of polarization. Our four-wave mixing based time-lenses exploit polarization mode dispersion to compensate for chromatic dispersion. The results reveal that the efficiency of the time-lens is increased fivefold for both states of polarization. We also compensate for any polarization-dependent losses in the system by tailoring the pump state of polarization. Our time-lenses can be implemented in current telecommunication systems together with polarization division multiplexing in order to reach higher bandwidth.

Harmonic dissipative soliton resonance pulses in a fiber ring laser at different values of anomalous dispersion

Abstract: The pulse dynamics of harmonic mode-locking in a dissipative soliton resonance (DSR) region in an erbium-doped fiber ring laser is investigated at different values of anomalous dispersion. The fiber laser is mode-locked by a nonlinear polarization rotation technique. By inserting 0–200 m anomalous dispersion single-mode fiber in the laser cavity, the cavity length is changed from 17.3 to 217.3 m, and the corresponding dispersion of the cavity ranges from −0.27 to −4.67 ps2. The observed results show that the tuning range of repetition rate under a harmonic DSR condition is highly influenced by the cavity dispersion. Furthermore, it is found that, by automatically adjusting their harmonic orders, the lasers can work at certain values of repetition rate, which are independent of the cavity length and dispersion. The pulses at the same repetition rate in different laser configurations have similar properties, demonstrating that each achievable repetition rate represents an operation regime of harmonic DSR lasers.

Semi-Analytical Modelling of Linear Mode Coupling in Few-Mode Fibers

Abstract: This paper reviews and extends a method for the semi-analytical solution of the coupled linear differential equations that describe the linear mode coupling arising in few-mode fibers due to waveguide imperfections The semi-analytical solutions obtained proved to be accurate when compared to numerical solution methods These solutions were integrated into a multisection model with split-steps for mode dispersion and mode coupling Simulations using this model matched the analytical predictions for the statistics of group-delays in few-mode fiber links, considering different coupling regimes with and without mode delay management

Digital backpropagation accounting for polarization-mode dispersion.

Abstract: Digital backpropagation (DBP) is a promising digital-domain technique to mitigate Kerr-induced nonlinear interference. While it successfully removes deterministic signal-signal interactions, the performance of ideal DBP is limited by stochastic effects, such as polarizationmode dispersion (PMD). In this paper, we consider an ideal full-field DBP implementation and modify it to additionally account for PMD; reversing the PMD effects in the backward propagation by passing the reverse propagated signal also through PMD sections, which concatenated equal the inverse of the PMD in the forward propagation. These PMD sections are calculated analytically at the receiver based on the total accumulated PMD of the link estimated from channel equalizers. Numerical simulations show that, accounting for nonlinear polarization-related interactions in the modified DBP algorithm, additional signal-to-noise ratio gains of 1.1 dB are obtained for transmission over 1000 km.

Highly birefringent, highly negative dispersion compensating photonic crystal fiber

Abstract: A triangular lattice dispersion compensating photonic crystal fiber is presented in this paper. The fiber produces high birefringence and operates at fundamental mode only. The full vector finite element method with a perfectly matched absorbing layer boundary condition is applied to investigate the guiding properties of the proposed fiber. The designed fiber demonstrates that it is possible to obtain a very large negative dispersion of −9486.1 ps/(nm·km) at 1550 nm wavelength with a negative dispersion more than −7000 ps/(nm·km) over the entire C-band (1530–1565 nm), which is suitable for broadband dispersion compensation. The birefringence is about 4.13×10−2 at 1550 nm wavelength, which is also very high. All these properties make this fiber very suitable in the area of broadband dispersion compensation and polarization-maintaining applications.

Ultra-high negative dispersion compensating square lattice based single mode photonic crystal fiber with high nonlinearity

Abstract: This paper presents dispersion tailoring of photonic crystal fibers creating artificial defect along one of the regular square axes. A finite element method (FEM) has been enforced for numerical investigation of several guiding properties of the PCF covering a broad wavelength range about 1340–1640 nm over the telecommunication windows. According to simulation, the proposed PCF has obtained a strictly single-mode fiber, which has an ultra-high negative dispersion of about −584.60 to −2337.60 ps/(nm-km) and also possible to cover the highest nonlinearity order of 131.91 W−1 km−1 under the operating wavelength. Moreover, the proposed PCF structure experimentally focuses on higher nonlinear coefficient, which successfully compensates the chromatic dispersion of standard single mode in entire band of interest and greatly applicable to the optical transmission system. Additionally, the single mode behavior of S-PCF is explicated by employing V parameter. In our dispersion sensitive analysis, this fiber is significantly more robust due to successfully achieve ultra-high negative dispersion, which gains more promiscuous compared to the prior best results.

Design of Highly Nonlinear Few-Mode Fiber for C-Band Optical Parametric Amplification

Abstract: We present a thorough description of the dominating intramodal and intermodal four-wave mixing interactions occurring in a highly nonlinear few-mode fiber and describe their phase matching conditions. Those interactions that result in few-mode parametric amplification using a single-frequency pump are of particular interest. Thus, based on the phase matching conditions of such interactions, we outline the dispersion properties that a fiber should possess in order to achieve few-mode parametric amplification, while having minimal modal crosstalk. Accordingly, we design and optimize two fibers such that they meet the dispersion requirements for parametric amplification of two and four spatial modes, respectively. The two-mode fiber provides a maximum differential modal gain (DMG) of 0.21 dB across the C-band with a minimum gain of 9.5 dB per mode, while the four-mode fiber provides a maximum DMG of 1.51 dB with a minimum gain of 6.5 dB per mode over a 19 nm bandwidth in the C-band. The designed fibers are highly nonlinear dispersion-shifted few-mode fibers that provide both high nonlinearity and low dispersion for several modes in the C-band, which have not been demonstrated simultaneously to date. We also take practical fabrication issues into account and analyze and compare the tolerances of the structural parameters of both fibers to small deviations from their optimal values.

Performance analysis of spectrally encoded hybrid WDM-OCDMA network employing optical orthogonal modulation format against eavesdropper

Abstract: In this paper, the spectrally encoded hybrid WDM-OCDMA network employing 60 Gbps non-return to zero/differential quadrature phase shift keying (NRZ/DQPSK) orthogonally modulated data signal operating over 100 km SMF+DCF has been proposed. In proposed hybrid WDM-OCDMA network, the orthogonal modulation formats are used to enhance per channel capacity and the spectral amplitude optical coding for enhancing the confidentiality of data from unauthorized user or eavesdropper. The impact of input power, transmission distance and energy per bit to noise power spectral density ratio ( E b /N 0 ) on the performance of WDM-OCDMA network in terms of output optical power, timing diagram, BER, Q-factor and probability of error free code detection has been investigated. It can also be determined that orthogonal modulation formats are promising option to increase per channel capacity as compared to conventional modulation format and these are less vulnerable to chromatic dispersion (CD) and polarization mode dispersion (PMD). Meanwhile, the performance of proposed hybrid network is compared with existing OCDMA network which show the feasibility of proposed WDM-OCDMA network in future generation optical networking.

Impact of Optical Phase Conjugation on the Nonlinear Shannon Limit

Abstract: Compensation of the detrimental impacts of nonlinearity on long-haul wavelength division multiplexed system performance is discussed, and the difference between transmitter, receiver, and in-line compensation analyzed. We demonstrate that ideal compensation of nonlinear noise could result in an increase in the signal-to-noise ratio (measured in dB) of 50%, and that reaches may be more than doubled for higher order modulation formats. The influence of parametric noise amplification is discussed in detail, showing how increased numbers of optical phase conjugators may further increase the received signal-to-noise ratio. Finally, the impact of practical real world system imperfections, such as polarization mode dispersion, are outlined.

FFSS: The fast fiber simulator software

Abstract: We introduce FFSS, an optical fiber simulator entirely developed in MATLAB® which takes advantage of parallel calculation on graphic processing units (GPU). This software solves the nonlinear Schrodinger equation (NLSE) - the single-mode fiber wave equation - relying on the well-known split-step Fourier method (SSFM). FFSS operates on either the PMD-Manakov equation (PMD-ME) that averages out random birefringence and polarization mode dispersion (PMD), or on the dual-polarization (DP) NLSE including random birefringence by complementing the SSFM with the waveleplate model. We present results obtained by solving the SSFM with PMD-ME. We tested and benchmarked FFSS using GPU computation on two different hardware solutions, being able to show performance improvements up to 75× for single-precision calculation and 40× for double-precision calculation with respect to the use of computing architectures relying on standard CPU-only computations. We demonstrated the capabilities of FFSS by simulating propagation of 81 PM-QPSK WDM channels on the 50 GHz grid, representing a full C-band WDM comb. We considered a uniform, uncompensated and amplified fiber link, made of 30×100 km spans of standard single-mode fiber (SSMF). We show the accuracy of simulator outcomes obtained by double-precision calculations by comparison with results obtained using theoretical modeling of fiber propagation.

Chalcogenide waveguide structure for dispersion in mid-infrared wavelength

Abstract: We present a waveguide design with low dispersion in mid-infrared wavelengths. The design consists of slot-strip-slot structures horizontally, the strip structure is considered with high index and slot is considered with low index material. We show a dispersion of 0–350 ps/(kmnm) over a band width of 1375 nm, and the structure shows zero dispersions at 2512 and 3887 nm wavelength. The magnitude of dispersion can be fine-tuned by varying the waveguide parameters. Such a waveguide structure with low dispersion at mid-infrared wavelengths has a great potential for supercontinuum generation application. Apart this, we have also proposed dispersion compensation structure, the structure shows a high negative dispersion at 1510 nm wavelength. The structure should find application in the design of an integrated optic dispersion compensator for optical telecommunication and ultrafast waveguide lasers.

Reducing detection noise of a photon pair in a dispersive medium by controlling its spectral entanglement

Abstract: Chromatic dispersion is one of the main limitations to the security of quantum communication protocols that rely on the transmission of single photons in single-mode fibers. This phenomenon forces the trusted parties to define longer detection windows to avoid losing signal photons and increases the amount of detection noise that is being registered. In this work, we analyze the effects of chromatic dispersion on a photon pair generated via spontaneous parametric down-conversion and propagating in standard telecommunication fibers. We also present the possibility of reducing the detection noise by manipulating the spectral correlation of the pair. As an example, we show that our results can be used to increase the maximal security distance of a discrete-variable quantum key distribution scheme in which the photon source is located between the legitimate participants of the protocol.

Polarization conversion based on an all-dielectric metasurface for optical fiber applications

Abstract: Polarization conversion (PC) in optical fiber is a very important operation in practice. To date, however, PC in fiber is usually achieved by coupling an external bulk element, or using the birefringence results from mechanically squeezing or coiling the fiber. In this paper, we propose a distinct approach for PC in optical fiber by introducing an all-dielectric metasurface in it, which has been proven to be compact, efficient and robust. Based on this approach, nearly perfect PCs from the linear polarization fundamental mode, i.e. mode to various other polarization modes, are achieved, including the mode, left/right-handed circular polarization mode, and also vector modes with radial and azimuthal polarizations. In addition, the fabrication of this all-dielectric-based metasurface is compatible with semiconductor manufacturing technologies, which makes the PC presented here competitive against traditional ones, and may find potential applications in optical fiber elements and systems.

Polarization-entangled photon pair sources based on spontaneous four wave mixing assisted by polarization mode dispersion

Abstract: Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91 ± 0.01 observed in a 1,100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90 ± 0.01 from another (800-nm-wide) waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits.

Uncertainty analysis of BTDM-SFSW based fiber-optic time transfer

Abstract: In this paper, the uncertainty of time transfer based on a bidirectional time division multiplexing transmission over a single fiber with the same wavelength (BTDM-SFSW) is investigated via theoretical models and experimental measurements. According to the principle and system architecture, the uncertainty evaluation schemes for BTDM-SFSW based time transfer are presented and the uncertainty sources are identified accordingly. In order to avoid the effect of the temperature-dependent fiber delay, the measured time intervals at two sites are regarded as a whole to obtain an overall uncertainty of time interval measurements. For the uncertainty of time transfer modem calibration, aside from the type A uncertainty obtained under the applied calibration scheme, the system reproducibility against practical operation and the contribution of optical power-dependent receiving delays are also included. A mathematical model considering fiber dispersion, polarization mode dispersion (PMD) and the Sagnac effect is established to evaluate the uncertainty from the fiber link. The characteristics of the uncertainty sources in a long-distance fiber-optic time transfer testbed are then explored in detail. The combined expanded uncertainties with a coverage factor of 2 are calculated and experimentally validated over various non-calibrated fiber extensions.

Joint Scheme of Dynamic Polarization Demultiplexing and PMD Compensation Up To Second Order for Flexible Receivers

Abstract: Polarization division multiplexing (PDM) doubles the bitrate of flexible transponders, whereas the joint effects of fast state of polarization (SOP) rotations, first-order polarization mode dispersion (PMD), and second-order PMD (SOPMD) would severely degrade the system performance. In this paper, we propose and experimentally validate a novel dynamic polarization demultiplexing scheme using extended Kalman filter (EKF) in digital signal processing of flexible receivers. The idea comes from two-stage PMD compensator in optical domain to perform SOP tracking and equalization of first-order PMD and SOPMD. After parameters optimization of EKF, the scheme can track up SOP rotations more than 35 times and 50 times faster than that of constant modulus algorithm (CMA) and CMA/multiple modulus algorithm (MMA), respectively. More importantly, when SOP keeps a fast rotation of 1.00, 10.00, or 20.02 Mrad/s, the system tolerances of first-order PMD and SOPMD that the joint scheme achieves are 28.57 ps and 793 ps2 for 28 GBaud PDM-QPSK at optical signal-to-noise rate (OSNR) of 17 dB, and 50 ps and 4836 ps2 for 10 GBaud PDM-16 quadratic-amplitude modulation (QAM) at OSNR of 23 dB, respectively. In addition, the proposed scheme has a fast convergence speed of about 50 symbols (equal to 5 ns). The calculation complexities of each symbol that the proposed scheme consumes are just about 44% of CMA for PDM-QPSK, and 31% of CMA/MMA for PDM-16 QAM.

Long-Term Polarization Mode Dispersion Evolution and Accelerated Aging in Old Optical Cables

Abstract: Today’s optical networks are composed of thousands of kilometers of aging optical cables. Many of these cables are located in harsh environments, which contribute to induced birefringence of the fibers and a corresponding increase of polarization mode dispersion (PMD). This letter introduces derived statistics from the longest-known running evaluation of a PMD measuring campaign and an investigation into how higher optical power affects these aging systems. Results indicate strong seasonal dependence of PMD on temperature for an optical cable test bed exposed to atmospheric changes, leading to a 16% increase of a mean PMD value in summer. This fluctuation causes bit error rate limits to be exceeded for 10 and 40 Gbps non-return-to-zero signals, which is a critical issue for applications where high reliability is required. Moreover, due to the high optical power load within old optical infrastructures, a more than 0.15 dB increase of relative loss per year in tested routes, compared with reference routes, has been observed.

Optical Modulation: Advanced Techniques and Applications in Transmission Systems and Networks

Abstract: This books aims to present fundamental aspects of optical communication techniques and advanced modulation techniques and extensive applications of optical communications systems and networks employing single-mode optical fibers as the transmission system New digital techqniues such as chromatic dispersion, polarization mode dispersion, nonlinear phase distortion effects, etc will be discussed Practical models for practice and understanding the behavior and dynamics of the devices and systems will be included

Measurement of optical signal-to-noise-ratio in coherent systems using polarization multiplexed transmission

Abstract: A new method for measuring Optical Signal-to-Noise-Ratio (OSNR) in systems using polarization multiplexed transmission was investigated. The OSNR can be calculated from the correlation between spectral components in the optical spectrum of a transmission signal.

Low loss QKD optical scheme for fast polarization encoding

Abstract: We present a new optical scheme for BB84 protocol quantum key distribution (QKD). The proposed setup consists of a compact all-fiber polarization encoding optical scheme based on LiNbO3 phase modulators, single laser source and two single-photon detectors. The optical scheme consists of standard telecommunication components and is suitable for both fiber and free-space quantum communication channels. Low losses (~2 dB) in Bob’s device increase both the key generation rate and the distance limit. A new technique for solving the polarization mode dispersion (PMD) issue in LiNbO3 is implemented, allowing two crystals to neutralize the effect of each other. Several proof-of-concept experiments have been conducted at a 10 MHz repetition frequency over 50 km of standard optical fiber under laboratory conditions and over 30 km of urban fiber with high losses (13 dB), which is a link within a QKD network. To achieve this, calibration algorithms have been developed, allowing the system to work autonomously and making it promising for practical applications.

Polarization Mode Dispersion-Based Physical Layer Key Generation for Optical Fiber Link Security

Abstract: A novel physical layer secret key generation technique for point-to-point optical fiber link security is proposed. It can efficiently generate high entropy symmetric cryptographic keys based on Polarization Mode Dispersion in optical fiber links.

Training Symbol Assisted in-Band OSNR Monitoring Technique for PDM-CO-OFDM System

Abstract: We proposed and demonstrated an in-band optical signal-to-noise ratio (OSNR) monitoring method for a polarization-division-multiplexed coherent optical orthogonal frequency-division-multiplexing (PDM-CO-OFDM) system. OSNR of the PDM signals on both polarization tributaries can be simultaneously monitored by calculating the electrical SNR with low complexity by taking advantage of the uncorrelation property between the noise and training symbol, which is used for conventional frame synchronization. This OSNR monitoring technique is verified in 103.57-Gb/s 16 quadrature amplitude modulation-based PDM CO-OFDM transmission system with 240—1440-km standard single-mode fiber (SSMF) links. Experimental results show that after calibration, the OSNR estimation error is less than 0.6 dB in a dynamic range of 12—27 dB, the estimation remains accurate even after 1440-km SSMF transmission, regardless of the chromatic dispersion and polarization mode dispersion in the fiber. To further investigate the robustness of the proposed OSNR monitoring method, additional simulation is implemented on VPI and MATLAB platform. It turned out to be insensitive to nonlinear effect within reasonable range as our monitoring technique shows good performance with launching power below −2 dBm after 500-km SSMF transmission. The proposed technique could still accurately monitor the individual OSNR of each polarization tributary of the PDM signal even in the presence of 5-dB polarization-dependent loss.

Limitations on Hybrid WDM/OTDM Multicast Overlay System Imposed by Nonlinear Polarization Effect and its Mitigation

Abstract: This paper investigates the nonlinear polarization effect of 120-Gbps polarization and subcarrier multiplexed unicast signal and 40-Gbps multicast signal on transmission performance of hybrid WDM/OTDM multicast overlay system. The interaction between Kerr nonlinearity and polarization mode dispersion (PMD) induced nonlinear cross polarization modulation (XPolM) effect can significant limits the transmission performance of hybrid WDM/OTDM multicast overlay system. The effect of signal input power, extinction ratio, PMD, and differential group delay is studied in multicast enable and disable mode. Also, a mitigation technique of nonlinear polarization effect is introduced by use of group delay compensator, judicious addition of some PMD, and inline pre-DCF in the transmission channel. It is also found that the amplitude to phase interaction between multicast and unicast signal transmission can be suppressed by controlling the loss of orthogonality.

Simpplified extended Kalman filter phase noise estimation for CO-OFDM transmissions.

Abstract: We propose a flexible simplified extended Kalman filter (S-EKF) scheme that can be applied in both pilot-aided and blind modes for phase noise compensation in 16-QAM CO-OFDM transmission systems employing a small-to-moderate number of subcarriers. The performance of the proposed algorithm is evaluated and compared with conventional pilot-aided (PA) and blind phase search (BPS) methods via extensive an Monte Carlo simulation in a back-to-back configuration and with a dual polarization fiber transmission. For 64 subcarrier 32 Gbaud 16-QAM CO-OFDM systems with 200 kHz combined laser linewidths, an optical signal-to-noise ratio penalty as low as 1 dB can be achieved with the proposed S-EKF scheme using only 2 pilots in the pilot-aided mode and just 4 inputs in the blind mode, resulting in a spectrally efficient enhancement by a factor of 3 and a computational effort reduction by a factor of more than 50 in comparison with the conventional PA and the BPS methods, respectively.

Low loss QKD optical scheme for fast polarization encoding

Abstract: We present a new optical scheme for BB84 protocol quantum key distribution (QKD). The proposed setup consists of a compact all-fiber polarization encoding optical scheme based on LiNbO3 phase modulators, single laser source and two single-photon detectors. Optical scheme consists of standard telecommunication components and is suitable for both fiber and free-space quantum communication channels. Low losses (~2dB) in Bob's device increase both the key generation rate and distance limit. A new technique for solving polarization mode dispersion (PMD) issue in LiNbO3 is implemented, allowing two crystals to neutralize the effect of each other. Several proof-of-concept experiments have been conducted at 10 MHz repetition frequency over 50 km of standard optical fiber under laboratory conditions and over 30 km of urban fiber with high losses (13dB), which is a link within a QKD network. To achieve this, calibration algorithms have been developed, allowing the system to work autonomously and making it promising for practical applications.