KEYWORDS: Receivers, Digital signal processing, Power consumption, Tunable filters, Reconstruction algorithms, Optical filters, Crosstalk, Signal detection, Optical engineering, Design
Simplified coherence technology based on direct detection has been a hot topic because of its simple structure and low cost. However, some recently proposed simplified coherent receivers (SCRs) require extremely complex mathematical operations, resulting in high power consumption of digital signal processing. We design an asymmetric SCR (ASCR) based on a twin-single-sideband signal, and the field reconstruction algorithm only requires one Hilbert operation, avoiding the nonlinear mathematical operation and the digital up-sampling. Compared to the asymmetric direct detection receiver, the power consumption of the ASCR in the part of optical field reconstruction and dispersion compensation can be reduced by 79.83% under the same sensitivity. Besides, an optimal design of the optical filter and the channel equalization for the ASCR is proposed.
For satellite-to-ground downlink optical communication systems, it is desirable to reduce the size, weight, and power profiles of the space terminals by employing high sensitivity and large collection area ground terminals. Multiaperture coherent receivers utilizing spatial diversity techniques can satisfy this requirement and have many advantages over the single large aperture coherent receivers. The influence of both atmospheric turbulence and weather conditions on the bit error rate performance of multiaperture coherent receivers utilizing maximum ratio combining, equal gain combining, and selection combining techniques is systematically investigated and compared for the satellite-to-ground downlink communication system. The results are useful for the design and adaptive optimization of the multiaperture coherent receiver-based downlink communication systems.
A digital carrier synchronization module with high working frequency is indispensable for high-speed digital coherent optical receivers to recover the transmitted symbols. We proposed a method to increase the working frequency of the digital carrier synchronization (DCS) module based on the commonly used M’th power algorithms. Parallel architecture can increase the throughput of digital signal processing (DSP) modules for a given working frequency. pipelined architecture (PA) leads to a reduction in the critical path, and thus it can be exploited to increase the throughput of DSP modules by increasing the working frequency. It is demonstrated that in PA the working frequency is not limited by the computation time of the M’th power subfunction with the highest complexity because it is feedforward and thus pipelining registers can be introduced to reduce the critical path inside it. Instead, the phase unwrapping subfunction (PUS) becomes the bottleneck of the working frequency because it requires the immediately preceding result and cannot be implemented in PA, which results in the longest critical path among the DCS module. To solve this problem, we propose a feedforward look-up-table-based PUS design that can greatly reduce the critical path and increase the working frequency. Experimental DCS implementation in a Xilinx Virtex7 field programmable gate array shows that with this method the working frequency of the DCS module for quadrature phase-shift keying (QPSK) signals can be increased by 63.8%. Furthermore, using experimental and simulation data, it is demonstrated that the performance of the DCS module with increased working frequency is close to that of the off-line DCS algorithms for QPSK signals.
Based on the well-known simulation software VPI TransmissionMaker, a comprehensive training program for senior undergraduates majoring in optical communication and optical network technology was developed by the author after detailed study of the teaching difficult and key points in the discipline. Aiming at solving practical scientific and engineering problems, the program helped our students to develop the ability of acquiring and applying knowledge by designing optical devices, optical signal processing algorithms and optical fiber communication systems. Furthermore, innovation is inspired by introducing competition mechanism among project teams. The program was validated through four years of use and achieved good results.
This paper represents a power transfer function (PTF)-based chromatic dispersion (CD) monitoring method applicable to signals with different duty cycles and optical signal-to-noise ratio (OSNR). Our method relies on phase-mismatched four wave mixing to provide a steep PTF that maps the CD experienced by the signal onto the average power of the output signal. The steep PTF greatly enhances the monitoring sensitivity, thus solves the problem that former PTF-based methods cannot be applied to signals with high duty cycles and low OSNR because of low output contrast. Furthermore, because the optical power required is low, our method is optically efficient and easy-to-implement. Numerical simulations are then used to demonstrate the effectiveness and efficiency of this monitoring method.
We propose the design method of fiber parametric wavelength converters based on dispersion-flattened photonic crystal fibers (PCFs) with two zero-dispersion wavelengths (ZDWs). Analytical expressions of the optimum signal frequency and maximal pump tuning range are deduced. By our method, the tuning ranges can be considerably broadened with a relatively low pump power and short fiber. This is because the fourth-order dispersion coefficient of two ZDW PCFs can be 1-2 orders of magnitude larger than those of one ZDW fiber and are effectively utilized to compensate the linear phase mismatch due to the second-order dispersion, resulting in a low phase mismatch for a widely tunable pump. To exemplify the effectiveness of our method, a PCF based on lead-silicate glasses with two ZDWs spaced 127-nm apart is presented. Numerical simulations show that based on this PCF a transparent wavelength conversion with a 117-nm pump tuning range can be achieved with only a 3.7-m-long fiber and 0.532 W pump power.
In this paper, we show that the bandwidth of wavelength converters based on four-wave mixing can be considerably
broadened if photonic crystal fibers with two zero-dispersion wavelengths are used as the Kerr media. The design
method of fiber parametric wavelength converters based on photonic crystal fibers with two zero-dispersion wavelengths
is proposed. It is proved that when the input signal wavelength is close to the center of the two zero-dispersion
wavelengths, the maximal tuning range of the converted wave can be obtained. And the tuning range is approximately
two times the difference of the two zero-dispersion wavelengths, which in turn can selected by tailoring the geometrical
parameters of photonic crystal fibers. Numerical simulations show that after such converters can achieve ultra broadband
tuning range and high conversion efficiency with much lower pump power and shorter fiber.
In this paper an efficient method is proposed to maximize the tuning range of a fiber parametric wavelength converter
based on degenerated four-wave mixing. The physical mechanism behind efficient phase matching over a broad
wavelength range has been analyzed as a function of the fiber dispersion and nonlinear properties. Analytic expressions
of the optimum input signal wavelength and maximal bandwidth are deduced and it is revealed that by carefully selecting
fibers with appropriate zero dispersion wavelength and high-order dispersion coefficients the tuning range can be greatly
broadened and tailored to cover any WDM bands.
We prove that the tuning range of a one-pump fiber parametric wavelength converter based on degenerate four-wave mixing can be maximized by selecting fibers with appropriate zero dispersion wavelength and high-order dispersion coefficients. The maximal tuning range is proportional to the cube root of the product of the pump power and fiber nonlinear coefficient. If the effects of fourth-order dispersion are utilized, the tuning range can increase dramatically.
Due to the polarization sensitivity of Raman gain and the random nature of polarization mode dispersion (PMD) in the fiber, the gain of a fiber Raman amplifier (FRA) fluctuates over a wide range during a relatively long time. In this work, a numerical method of obtaining the probability distribution function of the random fluctuating Raman gain in a bidirectionally pumped FRA is presented for the first time, based on the vector theory of stimulated Raman scattering (SRS). After optimizing a bidirectionally pumped FRA to achieve the best tradeoff between an optical signal-to-noise ratio (OSNR) and nonlinear distortions, the proposed method is applied to study the impact of gain fluctuations. The results show that the fluctuations reach their peaks at two values of PMD parameters (Dp), respectively, when Dp is increased from zero. The heights of the two peaks are proportional to the power allocations for the forward and backward pumps, respectively. The results are useful for avoiding high gain fluctuations when bidirectional pumping is applied.
Due to the polarization sensitivity of Raman gain and the random nature of PMD in the fiber the gain of fiber Raman amplifier not only depends on the polarization states of the input signal and pump but also fluctuates over a wide range during a relative long time. The fluctuations are related to the PMD parameter of the fiber and the pumping scheme. In this paper the statistics of the gain fluctuations in an amplifier employing bi-directionally pumping schemes is studied. The ratio of forward to total pumping power is optimized to achieve best trade-off between the OSNR and nonlinear distortions beforehand. The results obtained are useful for the further optimization of bi-directionally pumped amplifiers.
In this paper we evaluate the influence of pumping schemes on the performance of fiber Raman amplifier bidirectionally pumped at multiple wavelengths. The numerical model we used includes bi-directional interactions between all signal and pumping wavelengths induced by stimulated Raman scattering, spontaneous Raman scattering and Rayleigh backscattering. We develop a effective algorithm to numerically solve the equations and show that by changing the ratio of forward to backward pumping power for each wavelength we can get very good tradeoff between gain flatness, optical signal to noise ratio and nonlinear effects.
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