Digital coherent transceivers have revolutionized optical fiber communications due to their superior performance offered compared to intensity modulation and direct detection based alternatives. As systems employing digital coherent transceivers seek to approach their information theoretic capacity, the use of multilevel modulation formats combined with appropriate forward error correction becomes essential. Given this context, in this tutorial paper, we, therefore, explore the digital signal processing (DSP) utilized in a coherent transceiver with a focus on multilevel modulation formats. By way of an introduction, we open by discussing the photonic technology required to realize a coherent transceiver. After discussing this interface between the analog optical channel and the digital domain, the rest of the paper is focused on DSP. We begin by discussing algorithms that correct for imperfections in the optical to digital conversion, including IQ imbalance and timing skew. Next, we discuss channel equalization including means for their realization for both quasi-static and dynamic channel impairments. Synchronization algorithms that correct for the difference between the transmitter and receiver oscillators both optical and electrical are then discussed and issues associated with symbol decoding highlighted. For most of the cases, we start with polarization division multiplexed quadrature phase-shift keying format as a basis and then discuss the extension to allow for high-order multilevel formats. Finally, we conclude by discussing some of the open research challenges in the field.

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Digital Signal Processing for Coherent Transceivers Employing Multilevel Formats

We demonstrate a novel integrated silicon and ultra-low-loss Si3N4 waveguide platform. Coupling between layers is achieved with (0.4 ± 0.2) dB of loss per transition and a 20 nm 3-dB bandwidth for one tapered coupler design and with (0.8 ± 0.2) dB of loss per transition and a 100 nm 3-dB bandwidth for another. The minimum propagation loss measured in the ultra-low-loss waveguides is 1.2 dB/m in the 1590 nm wavelength regime.

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Silicon on ultra-low-loss waveguide photonic integration platform.

The realization of gigahertz bandwidth modulators out of silicon-based technology in the early 2000s marked a cornerstone of silicon photonics development. While modulation speeds have since progressed well above 50 GHz and satisfy the bandwidth requirements of current and emerging modulation formats, concurrently obtaining low drive voltages and low insertion losses remains a very active area of research. While modulators generally come in two categories, direct absorption and those relying on embedded phase shifters, the focus of this paper lies on the latter capable of supporting both complex-valued modulation and optically broadband operation. The paper provides an overview of the current state of the art, as well as of currently explored improvement paths. First, common phase shifter configurations, aspects related to electrical driving, and associated power consumption are reviewed. Slow-wave, resonant, and plasmonic enhancements are further discussed. The reader is familiarized with the optimization of these devices and provided with a sense of the limitations of current technology and the potential of novel hybrid material integration.

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High-Speed Silicon Photonics Modulators

We present a comparative analysis of three popular digital filters for chromatic dispersion compensation: a time-domain least mean square adaptive filter, a time-domain fiber dispersion finite impulse response filter, and a frequency-domain blind look-up filter. The filters are applied to equalize the chromatic dispersion in a 112-Gbit/s non-return-to-zero polarization division multiplexed quadrature phase shift keying transmission system. The characteristics of these filters are compared by evaluating their applicability for different fiber lengths, their usability for dispersion perturbations, and their computational complexity. In addition, the phase noise tolerance of these filters is also analyzed.

Chromatic dispersion compensation in coherent transmission system using digital filters

A distributed III-V-on-Si electroabsorption modulator based on an asymmetric segmented electrode has been developed on the hybrid silicon platform for the 1.3 μm transmission window. The measured modulation response shows a 2 dB drop at 67 GHz and an extrapolated 3 dB bandwidth of 74 GHz. Large signal measurements show clearly open eye diagrams at 50 Gb/s. An extinction ratio of 9.6 dB for back to back transmission and an extinction ratio of 9.4 dB after 16 km transmission were obtained with a drive voltage of 2.2 V.

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Over 67 GHz bandwidth hybrid silicon electroabsorption modulator with asymmetric segmented electrode for 1.3 μm transmission.

A complete ETDM system based on monolithically integrated transmitter and receiver modules was demonstrated with BER performance below FEC threshold up to 107 Gb/s. ETDM signal at 112 Gb/s with clear eye opening was also observed.

100 Gb/s complete ETDM system based on monolithically integrated transmitter and receiver modules

We report the first field transmission experiment using the APRZ modulation format for 40-Gb/s transmission through 540 km SSMF, which confirms the improved nonlinear tolerance of APRZ. The optimum phase-modulation amplitude in this experiment is π/2.

40-Gb/s Field Transmission through 540 km SSMF Using the APRZ Modulation Format

Picosecond 4×40 GHz multi-colored optical pulses have been generated using a single two-arm modulated Mach-Zehnder modulator. In the same time the pulses can be aligned in the time domain to form a uniform 160 GHz multi-colored pulse train.

4×40 GHz multi-colored optical pulse generation using single two-arm modulated Mach-Zehnder modulator

The performance of long-haul high speed coherent optical fiber communication systems is significantly degraded by the laser phase noise and the equalization enhanced phase noise (EEPN). In this paper, the analysis of the one-tap normalized least-mean-square (LMS) carrier phase recovery (CPR) is carried out and the close-form expression is investigated for quadrature phase shift keying (QPSK) coherent optical fiber communication systems, in compensating both laser phase noise and equalization enhanced phase noise. Numerical simulations have also been implemented to verify the theoretical analysis. It is found that the one-tap normalized least-mean-square algorithm gives the same analytical expression for predicting CPR bit-error-rate (BER) floors as the traditional differential carrier phase recovery, when both the laser phase noise and the equalization enhanced phase noise are taken into account.

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Close-form expression of one-tap normalized LMS carrier phase recovery in optical communication systems

http://ieeexplore.ieee.org/iel5/5361200/5376978/05377278.pdf

Jie Li

Papers

100 Gb/s complete ETDM system based on monolithically integrated transmitter and receiver modules

40-Gb/s Field Transmission through 540 km SSMF Using the APRZ Modulation Format

4×40 GHz multi-colored optical pulse generation using single two-arm modulated Mach-Zehnder modulator

Close-form expression of one-tap normalized LMS carrier phase recovery in optical communication systems

4×40 GHz multi-colored optical pulse generation using single two-arm modulated Mach-Zehnder modulator