A multi-input multi-output (MIMO) algorithm based on higher-order Poincaré spheres is demonstrated for space-division multiplexing (SDM) systems. The MIMO algorithm is modulation format agnostic, robust to frequency offset and does not require training sequences. In this approach, the space-multiplexed signal is decomposed in sets of two tributary signals, with each set represented in a higher-order Poincaré sphere. For any arbitrary complex modulation format, the samples of two tributaries can be represented in a given higher-order Poincaré sphere with a symmetry plane. The crosstalk along propagation changes the spatial orientation of this plane and, therefore, it can be compensated by computing and realigning the best fit plane. We show how the transmitted signal can be successfully recovered using this procedure for all possible combinations of tributaries. Moreover, we analyze the convergence speed for the MIMO technique considering several optical-to-noise ratios.
We present a calculation of the required number of bits to be received in a system of communications in order to achieve a given level of confidence. The calculation assumes a binomial distribution function for the errors. The function is numerically evaluated and the results are compared with the ones obtained from Poissonian and Gaussian approximations. The performance in terms of the signal-to-noise ratio is also studied. We conclude that for higher number of errors in detection the use of approximations allows faster and more efficient calculations, without loss of accuracy.
The tunable mode conversion between symmetric and antisymmetric optical modes induced by acoustic flexural waves in optical microwires is proposed. The acoustic frequency dictates the resonant coupling between the optical modes, and consequently, the modes in which the mode conversion occurs. On the other hand, the efficiency of the mode conversion can be controlled by the acoustic wave amplitude. Moreover, symmetric modes can also be excited by a double resonant coupling between a symmetric and an antisymetric mode, and further between the antisymmetric mode excited and another symmetric mode, leading to a mode conversion between symmetric modes.
We show how to generate, encode, transmit and detect single photons. By using single photons we can address two of the more challenging problems that communication engineers face nowadays: capacity and security. Indeed, by decreasing the number of photons used to encode each bit, we can efficiently explore the full capacity to carry information of optical fibers, and we can guarantee privacy at the physical layer. We present results for single and entangled photon generation. We encode information in the photons polarization and after transmission we retrieve that information. We discuss the impact of fiber birefringence on the photons polarization.
In this paper, the implementations of clock and carrier recovery in digital domain are analyzed. Hardware implementation details, resources estimation and real-time results are presented. Analog-to-Digital Converters (ADC), operating at 1.25Gsa/s, and a Virtex-6 Field-Programmable Gate Array (FPGA), have been used, allowing the implementation of a real-time Quadrature Phase Shift Keying (QPSK) system operating at 1.25Gb/s. The real-time mode operation is successfully demonstrated over 80 km of Standard Single Mode Fiber (SSMF).
In classical cryptography, the bit commitment scheme is one of the most important primitives. We review the state of the art of bit commitment protocols, emphasizing its main achievements and applications. Next, we present a practical quantum bit commitment scheme, whose security relies on current technological limitations, such as the lack of long-term stable quantum memories. We demonstrate the feasibility of our practical quantum bit commitment protocol and that it can be securely implemented with nowadays technology.
In this paper we present an experimental characterization of a highly nonlinear silica fiber. It includes the determination of the fiber effective nonlinear parameter using the nonlinear four-wave mixing process. From the experimental results, a value γ = 10.6 W−1 km−1 has been obtained for the nonlinear parameter in the co-polarized case, which was reduced to γ = 9.4 W−1 km−1 in the case of a decorrelated state of polarization between pump and signal waves. A threshold power of 17.8mW was found for the stimulated Brillouin scattering process. A Raman gain curve extending over a frequency range of 25 THz, with a peak value shifted by 13.1 THz from the pump frequency, was also measured.
Optical regeneration is a key technology for next generation high-speed optical networks. All-optical regeneration can increase the reach of transmission systems without expensive optical-to-electrical signal conversion. Among various regeneration schemes, the Mamyshev regenerator attracted particular attention due to its simplicity and robustness. In this paper, we report an all-optical regeneration of a 40 Gbit/s return-to-zero signals. The regenerator proposed is based on the standard Mamyshev regenerator, which the temporal intensity profile and the average power are recovered. This device allows regenerating the signal without wavelength shift, decreasing the complexity and cost when compared with others 2-R regenerators reported. The input signal is first spectrally broadened, by self-phase modulation, after passing through a highly nonlinear fiber. Afterwards, the signal is amplified by a bidirectional erbium doped fiber amplifier, and offset spectral backscattering sliced by a fiber Bragg grating. In the second stage, the signal is spectral broadening and filtered recovering the input wavelength. The transfer function for the regenerator proposed is measured, and the all-optical regeneration is assessed by means of bit-error-rate measurements as well as real-time observation of the signal.
In this paper we present results from the study of optical signal regeneration using Mamyshev type regenerator. We have performed the simulations and experimental characterization of regenerator by obtaining it`s transfer function and output optical signal to noise ratio measurements for two different filters - fixed and a tunable optical filter. Investigated regenerator setup consists of a high power erbium doped fiber amplifier, highly nonlinear fiber and a single stage optical filtering. Signal used for regeneration was an on-off keying return to zero code 40 Gbps pulse sequence. To find out optimum filter pass-band shift from signal`s central wavelength the regenerator`s transfer function was measured. Results show that highest output signal to noise ratio improvement for the fixed filter is at 0.6nm shift and amplifier output power set to 63 mW. While the tunable filter shift is 0.7nm at the 100 mW power level.
The all-optical control of light parameters such as intensity profile, power, phase, modes, wavelength, and state of polarization is viewed as a cornerstone for the feasibility of future high-speed fiber-optic communication networks. In particular, the ability to repolarize light with an almost unitary efficiency represents a huge advantage over the standard polarizers that waste 50% of the unpolarized light. In this paper, we show how to obtain a nonlinear lossless repolarization of light in low-birefringence optical fibers. A characterization of the stimulated Raman scattering based polarization pulling process is also presented, with particular focus on the optimization of several parameters such as fiber length, polarization mode dispersion coefficient, correlation length, and signal and auxiliar pump power regimes. Finally, we compare the co-propagating and counter-propagating signal and pump schemes, presenting the main advantages and disadvantages of each scenario and its suitability for particular applications.
The rapid increase on the information sharing around the world, leads to an utmost requirement for capacity and bandwidth. However, the need for security in the transmission and storage of information is also of major importance. The use of quantum technologies provides a practical solution for secure communications systems. Quantum key distribution (QKD) was the first practical application of quantum mechanics, and nowadays it is the most developed one. In order to share secret keys between two parties can be used several methods of encoding. Due to its simplicity, the encoding into polarization is one of the most used. However, when we use optical fibers as transmission channels, the polarization suffers random rotations that may change the state of polarization (SOP) of the light initially sent to the fiber to a new one at the output. Thus, in order to enable real-time communication using this encoding method it is required the use of a dynamic control system. We describe a scheme of transmission of quantum information, which is based in the polarization encoding, and that allows to share secret keys through optical fibers without interruption. The dynamic polarization control system used in such scheme is described, both theoretically and experimentally. Their advantages and limitations for the use in quantum communications are presented and discussed.
Quantum communications can provide almost perfect security through the use of quantum laws to detect any
possible leak of information. We discuss critical issues in the implementation of quantum communication systems
over installed optical fibers. We use stimulated four-wave mixing to generate single photons inside optical fibers,
and by tuning the separation between the pump and the signal we adjust the average number of photons per pulse.
We report measurements of the source statistics and show that it goes from a thermal to Poisson distribution with
the increase of the pump power. We generate entangled photons pairs through spontaneous four-wave mixing.
We report results for different type of fibers to approach the maximum value of the Bell inequality. We model
the impact of polarization rotation, attenuation and Raman scattering and present optimum configurations to
increase the degree of entanglement. We encode information in the photons polarization and assess the use
of wavelength and time division multiplexing based control systems to compensate for the random rotation of
the polarization during transmission. We show that time division multiplexing systems provide a more robust
solution considering the values of PMD of nowadays installed fibers. We evaluate the impact on the quantum
channel of co-propagating classical channels, and present guidelines for adding quantum channels to installed
WDM optical communication systems without strongly penalizing the performance of the quantum channel. We
discuss the process of retrieving information from the photons polarization. We identify the major impairments
that limit the speed and distance of the quantum channel. Finally, we model theoretically the QBER and present
results of an experimental performance assessment of the system quality through QBER measurements.
Quantum laws can be used to implement secure communication channels; this has been named quantum cryptography.
In quantum cryptography the security does not depend of limited computational power, but is inherent
to the laws that govern the propagation and detection of single and entangled photons. We show how single
and entangled photon-pairs can be efficiently generated using four-wave mixing in optical fibers. We analyze the
source statistics, degree of entanglement and impact of spontaneous Raman scattering. By coding information
in the photons polarization we are able to transmit quantum information over 20 km of standard single mode
In this work we develop an analysis of polarization control schemes suitable for quantum key distribution systems.
Both time division multiplexing and wavelength division multiplexing based schemes are considered. A model
for the optimization of the temporal separation between reference pulses and polarization encoded photons
is presented. The model accounts for the reference pulse shape, the single photon detector gate width, and
the respective temporal separation between them. The theoretical results are validated through experimental
measurements. These results can be used to optimize the performance of polarization control schemes and
therefore to optimize the polarization encoded quantum key distribution systems.
A single-photon source based on the stimulated four-wave mixing (SFWM) process in optical fibers is presented.
At the output of the source, the state of polarization (SOP) of the photons can be adjusted in order to obtain
any linear polarization. A theoretical model to describe the average photon counts recorded in the avalanche
photodiodes (APDs) is presented. The experimental results show an accurate detection of two non-orthogonal
linear SOPs after propagation through a 60 km quantum channel, and good agreement with theory. This source,
operating in a low power regime, can be used for quantum key distribution (QKD) using polarization-encoding
in quantum communications.