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The parametric optimization of Digital Backward Propagation (DBP) algorithm for mitigating fiber transmission impairments is proposed and numerically demonstrated for phase modulated signals in mixed-optical fiber transmission link. The optimization of parameters i.e. dispersion (D) and non-linear coefficient (γ) offer improved eye-opening (EO). We investigate the optimization of iterative and non-iterative symmetric split-step Fourier method (S-SSFM) for solving the inverse non-linear Schrödinger equation (NLSE). Optimized DBP algorithm, with step-size equal to fiber module length i.e. one calculation step per fiber span for obtaining higher computational efficiency, is implemented at the receiver as a digital signal processing (DSP) module. The system performance is evaluated by EO-improvement for diverse in-line compensation schemes. Using computationally efficient non-iterative symmetric split-step Fourier method (NIS-SSFM) upto 3.6 dB referenced EO-improvement can be obtained at 6 dBm signal launch power by optimizing and modifying DBP algorithm parameters, based on the characterization of the individual fiber types, in mixed-optical fiber transmission link.

We propose and numerically investigate logarithmic step-size distribution for implementing efficient digital backward propagation (DBP) algorithm using split-step Fourier method (SSFM). DBP is implemented in N-channel 112Gbit/s/ch dual-polarization quadrature-phase-shift-keying (DP-QPSK) transmission over 2000km standard single mode fiber (SMF) with no in-line optical dispersion compensation. This algorithm is compared with constant step-size modified DBP (M-SSFM) algorithm in terms of efficiency, complexity and computational time. Furthermore, we investigate the same-capacity and same-bandwidth-efficiency transmission systems with 28GBaud and 56GBaud per-channel rates. The logarithmic step-size based DBP algorithm depicts efficient mitigation of chromatic dispersion (CD) and non-linear (NL) impairments. Benefit of the logarithmic step-size is the reduced complexity and computational time for higher baud rates.Recent numerical and experimental studies have shown that coherent optical QPSK (CO-QPSK) is the promising candidate for next-generation 100Gbit/s Ethernet (100 GbE) [1]. Coherent detection is considered efficient along with digital signal processing (DSP) to compensate many linear effects in fiber propagation i.e. CD and polarization-mode dispersion (PMD) and also offers low required optical signal-to-noise ratio (OSNR). Despite of fiber dispersion and non-linearities, which are the major limiting factors, next-generation optical systems are employing higher order modulation formats in order to fulfil the ever increasing demand of capacity requirements [2]. DSP techniques are gaining increasing importance as they allow for robust long-haul transmission with mitigation of fiber transmission impairments at the receiver [3,4]. One of these methods is digital backward propagation (DBP) which can jointly mitigate CD and NL. Pioneering concepts on DBP can be found in [5,6]. A number of investigations have been done with coherent detection and split-step Fourier method (SSFM) [7][8][9][10][11][12][13][14][15][16]. It is demonstrated that DBP in a single-channel transmission can be improved by using modified split-step Fourier method (M-SSFM) [17,18]. Modification is done by shifting the non-linear operator calculation point Nlpt (r) along with the optimization of dispersion D and non-linear coefficient γ to get the optimized system performance. Recent investigations [14,15] shows the promising impact of DBP on higher order modulation formats, upto 256-QAM. However actual implementation of the DBP algorithm is now-a-days extremely challenging due to its complexity. The performance is mainly dependent on the computational step-size (h) for multi-channel (WDM) and higher baud-rate transmissions [19,20].In order to reduce the computations of the algorithm, by increasing the step-size (i.e. reducing the number of DBP calculation steps per fiber span), ultra-low-loss-fiber (ULF) is used [19] and a promising method called correlated DBP (CBP) has been introduced [21]. In the aforementioned investigations there is a trad...

Abstract:We investigate the performance of carrier phase estimation (CPE) and digital backward propagation (DBP) in compensating fiber nonlinearity for 224Gbps polarization-multiplexed quadrature-amplitudemodulation coherent systems with level of 4 and 16 (PM-4-QAM and PM-16-QAM) over standard single-mode fiber (SSMF) uncompensated link. The results from numerical simulation show the individual performance of CPE and DBP as well as their mutual influence. With DBP compensation, required CPE tap number for optimal performance can be reduced by 50% for 4-QAM signal and 67% for 16-QAM signal compared to linear compensation. On the other hand, employing CPE compensation after DBP also allows to reduce DBP steps. In the mentioned PM-16-QAM system, 60% reduction in the required number of DBP steps to achieve BER=10 −3 is possible, with a step-size of 200km, which reveals great potential to reduce the complexity for future real time implementation. 1814-1818 (2012). 17. L. Du and A. Lowery, "Improved single channel back-propagation for intra-channel fiber non-linearity compensation in long-haul optical communication systems," Opt.

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