Digital filters underpin the performance of coherent optical receivers which exploit digital signal processing (DSP) to mitigate transmission impairments. We outline the principles of such receivers and review our experimental investigations into compensation of polarization mode dispersion. We then consider the details of the digital filtering employed and present an analytical solution to the design of a chromatic dispersion compensating filter. Using the analytical solution an upper bound on the number of taps required to compensate chromatic dispersion is obtained, with simulation revealing an improved bound of 2.2 taps per 1000ps/nm for 10.7GBaud data. Finally the principles of digital polarization tracking are outlined and through simulation, it is demonstrated that 100krad/s polarization rotations could be tracked using DSP with a clock frequency of less than 500MHz.

Digital filters for coherent optical receivers.

Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

Digital coherent receivers have caused a revolution in the design of optical transmission systems, due to the subsystems and algorithms embedded within such a receiver. After giving a high-level overview of the subsystems, the optical front end, the analog-to-digital converter (ADC) and the digital signal processing (DSP) algorithms, which relax the tolerances on these subsystems are discussed. Attention is then turned to the compensation of transmission impairments, both static and dynamic. The discussion of dynamic-channel equalization, which forms a significant part of the paper, includes a theoretical analysis of the dual-polarization constant modulus algorithm, where the control surfaces several different equalizer algorithms are derived, including the constant modulus, decision-directed, trained, and the radially directed equalizer for both polarization division multiplexed quadriphase shift keyed (PDM-QPSK) and 16 level quadrature amplitude modulation (PDM-16-QAM). Synchronization algorithms employed to recover the timing and carrier phase information are then examined, after which the data may be recovered. The paper concludes with a discussion of the challenges for future coherent optical transmission systems.

Digital Coherent Optical Receivers: Algorithms and Subsystems

Continuous real-time measurements are shown from a coherent 40 Gb/s transmission system that uses Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) modulation. Digital compensation is used for dispersion and polarization effects, with little performance degradation created by 150 ps of rapidly varying 1st-order PMD.

Real-time measurements of a 40 Gb/s coherent system

We discuss the use of a coherent digital receiver for the compensation of linear transmission impairments and polarization demultiplexing in a transmission system compatible with a future 100-Gb/s Ethernet standard. We present experimental results on the transmission performance of 111 Gbit/s POLMUX-RZ-DQPSK. For a dense WDM setup with channels carrying 111 Gbit/s with a 50 GHz channel spacing (2.0 bits/s/Hz), we show the feasibility of 2375 km transmission. This is enabled through coherent detection which results in excellent noise performance, and subsequent electronic equalization which provides the high tolerance to polarization mode dispersion and chromatic dispersion (CD). Furthermore, we discuss the impact of sampling and digital signal processing with either 1 or 2 samples/bit. We show that when combined with low-pass electrical filtering, 1 sample/bit signal processing is sufficient to obtain a large tolerance towards CD. The proposed modulation and detection techniques enable 111 Gbit/s transmission that is directly compatible with the existing 10 Gbit/s infrastructure.

Coherent Equalization and POLMUX-RZ-DQPSK for Robust 100-GE Transmission

We have demonstrated the DWDM transmission of 70 100Gb/s channels using CP-DQPSK and coherent detection with real-time DSP and FEC. The channel selection was performed with the coherent receiver only, without using any optical demultiplexing.

DWDM transmission of 70 100Gb/s CP-DQPSK channels over 2000km of uncompensated SMF with real-time DSP and coherent channel selection

Coherent technology can be operated with independent digital subcarriers to realize point-to-point and point-to-multipoint optical networks. Enabled by configurable software management, it creates a simple, scalable, low-cost solution, compatible across network, vendors, and generations. © 2022 The Author(s)

Digital Subcarriers: A Universal Technology for Next Generation Optical Networks

Characterisation of an RZ-DQPSK transmitter using coherent detection

We optimized the 15–tap equalizer filter of a 43Gb/s coherent polarization multiplexed QPSK receiver and compared simulation results with measurements using an FPGA-based realtime receiver Optimization extends the chromatic dispersion tolerance from 4,000ps/nm to 7,000ps/nm

Optimization of the chromatic dispersion equalizer of a 43Gb/s realtime coherent receiver

MLSE offers adaptive non-linear receiver-sided equalization for channels with memory. New areas of application considered are directly modulated DFB laser, D(B/Q)PSK, and duo-binary transmission systems. Reduced computational complexity is a new research area.

C.R.S. Fludger

Papers

DWDM transmission of 70 100Gb/s CP-DQPSK channels over 2000km of uncompensated SMF with real-time DSP and coherent channel selection

Digital Subcarriers: A Universal Technology for Next Generation Optical Networks

Characterisation of an RZ-DQPSK transmitter using coherent detection

Optimization of the chromatic dispersion equalizer of a 43Gb/s realtime coherent receiver

Review of Recent Progress in MLSE Receiver Technologies