Digital Optical Communications

Chinese Optics Letters, 2012

We present a novel coherent transceiver for optical differential phase-shift keying/differential quadrature phase-shift keying (DPSK/DQPSK) signals based on heterodyne detection and electrical delay interferometer. A simulation framework is provided to predict a theoretical sensitivity level for the reported scheme. High sensitivity of-45.18 dBm is achieved for 2.5-Gb/s return-to-zero (RZ)-DPSK signal, and high sensitivities of-36.83 dBm (I tributary) and-35.90 dBm (Q tributary) are observed for 2.5-GBaud/s RZ-DQPSK signal in back-to-back configuration. Transmission for both signals over 100 km is also investigated. Experimental results are discussed and analyzed.

The proposed paper utilized the concept of Coherent detection. A field received by advances in Digital Signal Processing (DSP), has renewed interest in optical communication systems with spectrally efficient modulation formats. Starting with the point of view in comparison between the DP QPSK and DP 16-QAM analyses of Phase Noise in terms of Average Bit Error Rate (BER) and Optical Signal to Noise Ratio (OSNR) has been done. OSNR component is used in order to introduce noise in the dual polarization Optical coherent receiver system. The Noise is analyzed under the influence of different filters. Finally the best filter with best result have chosen in order to have minimum phase noise. To improve the performance of coherent receiver, a DSP algorithms like Constant Modulous Algorithm (CMA) and Blind phase search algorithm are used to compensate Propagation Mode Dispersion (PMD),Chromatic Dispersion (CD) and to achieve high data rate.

Journal of Lightwave Technology, 1993

In this paper the error probability of a singlechannel coherent optical DPSK transmission system based on repeterless links in the presence of fiber chromatic dispersion and Kerr effect is evaluated. An accurate model for both the optical signal propagation and the probability distribution of the receiver decision variable is adopted by using a numerical solution of the non-linear Shrodinger equation and the characteristic function method. The obtained results show that the selection of an optimized IF filter bandwidth is crucial in order to obtain the best system performance. When chromatic dispersion dominates its combination with Kerr effect causes best performances to be achieved in normal dispersion region while, when Kerr effect is the most relevant limitation factor for the system performances, the lowest error probability is attained in anomalous regime. As a matter of fact in this regime Kerr effect and chromatic dispersion tends to partially compensate each other. In any case the error probability curves shows a relative minimum that is always attained in the anomalous region. Finally the maximum link length is limited by the presence of Kerr effect independently from the amount of transmitted optical power to be shorter than a threshold length if an error probability of is to be achieved.

Digital Coherent Receiver for Optical Transmission - Lazaro Hermoso, 2009

Keywords: Coherent Detection, Local Oscillator, Sensitivity, Shot noise, Polarization Multiplexing. Abstract: The purpose of this project is to study the basis of optical coherent detection associated with digital signal processing and to design and implement such a receiver. The investigated coherent can be used to detect arbitrary complex modulation formats as it is capable of splitting polarization multiplexed signals and provides the real and imaginary parts of the optical field. After coherent detection, the signal is digitize to perform DSP in order to digitally obtain the clock from the signal received and compensate for the linear impairments that previously affected the signal. For that purpose the digital coherent receiver implemented is composed by three modules: the first module is a clock recovery that can obtain the clock signal from the received signal. A second module that, using Digital Signal Processing (DSP), is able to compensate Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD) using adaptive filters. These filters are programmed to use two kinds of adaptive algorithms: LMS and CMA, which can be previously selected. The third module is in charge of the phase noise estimation to compensate for the rotation that causes this impairment in the constellation sent. Two different architectures for phase noise estimation are compared. After designing the receiver in Matlab® we validate its functionality. For that we use the VPItransmissionMaker TM tool in which we can simulate a realistic optical communication system, and, introducing our designed receiver model into the communication system the receiver‟s capability to compensate CD, PMD and Phase Noise is studied. Furthermore, changing the optical communication system properties, we put the receiver ‟s robustness to test. We also change different configurations of the receiver and study how it influences the reception of the signal.

IEEE Photonics Technology Letters, 2004

We propose and demonstrate a novel approach to detect optical differential phase-shift keying signals. The technique is based on differential phase-to-polarization conversion in a polarization-maintaining fiber, so that the polarization-modulated signal can be detected by using a polarizer and a common intensity modulation receiver.

The effect of polarization mode dispersion (PMD) imposes serious restriction on the allowable distance and ultimate hurdle for the massive deployment of multi-gigabit transmission rate in an optical communication system. An analytical approach is presented to determine the impact of signal phase distortion due to PMD in a single mode fiber (SMF) on the bit error rate (BER) performance of an optical continuous phase FSK (CPFSK) transmission system with MZI based direct detection receiver. The probability density function (pdf) of the random phase fluctuation due to PMD and group velocity dispersion (GVD) at the output of the receiver is determined analytically. Based on the pdf of the random phase fluctuation the BER performance results are evaluated at a bit rate of 10 Gb/s fiber with dispersion co-efficient Dc=15 ps/km-nm for different values of the mean differential group delay (DGD). The computed results show that the direct detection CPFSK system suffers a significant amount of ...

Journal of Optical Communications, 1999

The sensitivity penalty of a direct detection 10 Gbit/s optical CPFSK receiver due to laser phase noise, interferometric intensity noise and chromatic dispersion is determined analytically at a BER of 10~9. The analysis is based on linear approximation of the output phase of a linearly filtered CPFSK signal. The significant contributory role of the laser phase noise with increasing value of chromatic dispersion index is highlighted. The theoretical results have been found to be in excellent agreement with the simulation results.

Optics Communications, 2011

We demonstrate three novel all-optical signal processing methods for RZ-DPSK signals using a saturated SOA. The two logically opposite pulse streams from a demodulated noisy RZ-DPSK signal counter-propagate in the SOA. This configuration decreases the fluctuations in the demodulated marks while preventing the spaces from getting amplified. Photonic balancing provides functionality analogous to electrical balanced detection by reducing the required OSNR, and saturated asymmetric filtering removes the in-band ASE noise before detection. These methods are demonstrated experimentally by observing eye diagrams and BER measurements at 10.7 Gb/s and 22.3 Gb/s.

The drive for higher performance in optical fiber systems has renewed interest in coherent detection. We review detection methods, including noncoherent, differentially coherent, and coherent detection, as well as a hybrid method. We compare modulation methods encoding information in various degrees of freedom (DOF). Polarization-multiplexed quadrature-amplitude modulation maximizes spectral efficiency and power efficiency, by utilizing all four available DOF, the two field quadratures in the two polarizations. Dual-polarization homodyne or heterodyne downconversion are linear processes that can fully recover the received signal field in these four DOF. When downconverted signals are sampled at the Nyquist rate, compensation of transmission impairments can be performed using digital signal processing (DSP). Linear impairments, including chromatic dispersion and polarization-mode dispersion, can be compensated quasi-exactly using finite impulse response filters. Some nonlinear impairments, such as intra-channel four-wave mixing and nonlinear phase noise, can be compensated partially. Carrier phase recovery can be performed using feedforward methods, even when phase-locked loops may fail due to delay constraints. DSP-based compensation enables a receiver to adapt to time-varying impairments, and facilitates use of advanced forward-error-correction codes. We discuss both single-and multi-carrier system implementations. For a given modulation format, using coherent detection, they offer fundamentally the same spectral efficiency and power efficiency, but may differ in practice, because of different impairments and implementation details. With anticipated advances in analog-to-digital converters and integrated circuit technology, DSP-based coherent receivers at bit rates up to 100 Gbit/s should become practical within the next few years.