The interest for short-reach links of the kind needed for inter-data-center communications has fueled in recent years the search for transmission schemes that are simultaneously highly performing and cost effective. In this work we propose a direct-detection coherent receiver that combines the advantages of coherent transmission and the cost-effectiveness of direct detection. The working principle of the proposed receiver is based on the famous Kramers–Kronig (KK) relations, and its implementation requires transmitting a continuous-wave signal at one edge of the information-carrying signal spectrum. The KK receiver scheme allows digital postcompensation of linear propagation impairments and, as compared to other existing solutions, is more efficient in terms of spectral occupancy and energy consumption.

Kramers–Kronig coherent receiver

Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

Advanced modulation formats combined with digital signal processing and direct detection is a promising way to realize high capacity, low cost and power efficient short reach optical transmission system. In this paper, we present a detailed investigation on the performance of three advanced modulation formats for 100 Gb/s short reach transmission system. They are PAM-4, CAP-16 and DMT. The detailed digital signal processing required for each modulation format is presented. Comprehensive simulations are carried out to evaluate the performance of each modulation format in terms of received optical power, transmitter bandwidth, relative intensity noise and thermal noise. The performance of each modulation format is also experimentally studied. To the best of our knowledge, we report the first demonstration of a 112 Gb/s transmission over 10km of SSMF employing single band CAP-16 with EML. Finally, a comparison of computational complexity of DSP for the three formats is presented.

Experimental study of PAM-4, CAP-16, and DMT for 100 Gb/s Short Reach Optical Transmission Systems

Short range optical data links are experiencing bandwidth limitations making it very challenging to cope with the growing data transmission capacity demands. Parallel optics appears as a valid short-term solution. It is, however, not a viable solution in the long-term because of its complex optical packaging. Therefore, increasing effort is now put into the possibility of exploiting higher order modulation formats with increased spectral efficiency and reduced optical transceiver complexity. As these type of links are based on intensity modulation and direct detection, modulation formats relying on optical coherent detection can not be straight forwardly employed. As an alternative and more viable solution, this paper proposes the use of carrierless amplitude phase (CAP) in a novel multiband approach (MultiCAP) that achieves record spectral efficiency, increases tolerance towards dispersion and bandwidth limitations, and reduces the complexity of the transceiver. We report on numerical simulations and experimental demonstrations with capacity beyond 100 Gb/s transmission using a single externally modulated laser. In addition, an extensive comparison with conventional CAP is also provided. The reported experiment uses MultiCAP to achieve 102.4 Gb/s transmission, corresponding to a data payload of 95.2 Gb/s error free transmission by using a 7% forward error correction code. The signal is successfully recovered after 15 km of standard single mode fiber in a system limited by a 3 dB bandwidth of 14 GHz.

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Multiband Carrierless Amplitude Phase Modulation for High Capacity Optical Data Links

Backpropagation has been shown to be the most effective method for compensating intra-channel fiber nonlinearity in long-haul optical communications systems. However, effective compensation is computationally expensive, as it requires numerous steps and possibly increased sampling rates compared with the baud rate. This makes backpropagation difficult to implement in real-time. We propose: (i) low-pass filtering the compensation signal (the intensity waveform used to calculate the nonlinearity compensation) in each backpropagation step and (ii) optimizing the position of the nonlinear section in each step. With numerical simulations, we show that these modifications to backpropagation improve system performance, reducing the number of backpropagation steps and reducing the oversampling for a given system performance. Using our ‘filtered backpropagation’, with four backpropagation steps operating at the same sampling rate as that required for linear equalizers, the Q at the optimal launch power was improved by 2 dB and 1.6 dB for single wavelength CO-OFDM and CO-QPSK systems, respectively, in a 3200 km (40 × 80km) single-mode fiber link, with no optical dispersion compensation. With previously proposed backpropagation methods, 40 steps were required to achieve an equivalent performance. A doubling in the sampling rate of the OFDM system was also required. We estimate this is a reduction in computational complexity by a factor of around ten.

Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems

Intrachannel nonlinearity is considered a major distortion in high-capacity transmission systems particular for the case without inline optical chromatic dispersion compensation. In this paper, a low-complexity intrachannel nonlinear compensator operating at the symbol rate is proposed based on the nonlinear perturbation predistortion for the dual-polarization quadrature phase-shift keying (DP-QPSK) systems. Compared with the widely studied backpropagation algorithm, the proposed algorithm achieves comparable performance with significantly reduced complexity and halved sampling speed in digital signal processing and digital-to-analog converters. The proposed algorithm is demonstrated in a 43 Gb/s DP-QPSK transmission experiment over 1500 km. In addition to the experimental demonstration, numerical simulation verifies that the proposed algorithm is quite robust by tolerating significant uncertainties of link parameters and span-by-span inhomogeneity in the links.

Multiplier-Free Intrachannel Nonlinearity Compensating Algorithm Operating at Symbol Rate

We have experimentally demonstrated 469-Gbps transmission over 30-km SMF using DMT on a LAN-WDM system for 400-Gbps Ethernet. Only four directly modulated lasers and direct detectors as optical devices were necessary to achieve the results.

Experimental Demonstration of 448-Gbps+ DMT Transmission over 30-km SMF

100 Gb/s optical IM-DD transmission over 10 km SMF is experimentally demonstrated by using discrete multi-tone and CMOS DAC core. The limiting factors towards further reach extension are investigated.

100 Gb/s optical IM-DD transmission with 10G-class devices enabled by 65 GSamples/s CMOS DAC core

One stage per span is commonly considered as the lower boundary for implementation complexity of nonlinear equalizer based on digital backpropagation. The proposed method overcomes this boundary and improves the efficiency by about four times.

Implementation efficient nonlinear equalizer based on correlated digital backpropagation

We present a digital frequency offset compensator for optical coherent QPSK receivers. The compensation accuracy reaches ± 30 MHz while the range covers ± 5 GHz. This would realize coherent receivers with common off-the-shelf tunable lasers.

Lei Li

Papers

Multiplier-Free Intrachannel Nonlinearity Compensating Algorithm Operating at Symbol Rate

Experimental Demonstration of 448-Gbps+ DMT Transmission over 30-km SMF

100 Gb/s optical IM-DD transmission with 10G-class devices enabled by 65 GSamples/s CMOS DAC core

Implementation efficient nonlinear equalizer based on correlated digital backpropagation

Wide-range, Accurate and Simple Digital Frequency Offset Compensator for Optical Coherent Receivers