Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

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200 Gbps/Lane IM/DD Technologies for Short Reach Optical Interconnects

Intensity modulation and direct detection (IM/DD) together with pulse amplitude modulation (PAM) is a cost-efficient solution for intra-datacenter links. However, sophisticated digital signal processing (DSP) is inevitable to reach the demanded high data rates for these short-reach applications. In this article, we compare different DSP schemes that enable rates of 200 Gb/s/λ and beyond. Those schemes include Tomlinson–Harashima precoding, partial response signaling, Viterbi equalization and post-filtering together with Volterra nonlinear equalization. Extensive experimental measurements are shown in which the DSP approaches are compared at gross rates between 180 Gb/s and 300 Gb/s. The experiments are done in C-band for fiber links of up to 2 km length. As modulation formats, PAM4, PAM6, and PAM8 are considered. Net bit rates well over 200 Gb/s are shown for all formats at a range of 1 km single mode fiber using a transmission system consisting of currently available commercial components. The optimum DSP configuration as well as the best modulation format varies along the investigated data rates. The best solution is shown to change from partial response PAM4 for lower rates to PAM6 and PAM8 at high bit rates.

DSP for High-Speed Short-Reach IM/DD Systems Using PAM

In this paper, we propose a nonlinear Tomlinson-Harashima pre-coding (THP) scheme for nonlinear distortion suppression in direct-detected double sideband (DSB) PAM-4 transmission systems. Based on the traditional THP, the feedback term is modified by introducing nonlinear components. In this way, more accurate feedback can be obtained to mitigate the signal distortions, especially the nonlinear distortions including the signal-to-signal beating interference and nonlinear power series caused by chromatic dispersion and square-law detection. Meanwhile, we also propose to only reserve the nonlinear kernels with adjacent tap products in nonlinear THP, for the purpose of computation complexity reduction. To verify the effectiveness, transmissions of double sideband (DSB) PAM-4 signal in 1550nm window are experimentally demonstrated. Volterra FFE is adopted on the receiver side to suppress linear and nonlinear pre-cursors. We optimize various parameters of hardware and apply appropriate simplification to the nonlinear THP kernels. The results indicate that, the proposed nonlinear THP can lead to up to three folds BER reduction, compared to the conventional linear THP. Finally, with the combination of proposed nonlinear THP and conventional Volterra FFE, we successfully transmit 84-Gbps PAM-4 and 107-Gbps PAM-4 respectively over 80 km and 40 km under the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3.

Nonlinear Tomlinson-Harashima precoding for direct-detected double sideband PAM-4 transmission without dispersion compensation

85-Gbaud PAM-8 transmission with BER below HD-FEC limit under 20-GHz bandwidth limitation is achieved applying NL-MLSE based on Volterra filter. NL-MLSE also achieves 3.84-Tbps O-band transmission using 4-λ LAN-WDM and SDM over 2-km 4-core fiber.

255-Gbps PAM-8 Transmission under 20-GHz Bandwidth Limitation using NL-MLSE Based on Volterra Filter

In this paper, a joint digital signal processing (DSP) scheme including partial response shaping and trellis coded modulation (TCM) technology is proposed to mitigate the bandwidth limitation of the long-reach underwater wireless optical communication (UWOC) system. The similarity of spectral responses between the partial response filter and the communication system is utilized to suppress the enhancement of noise and distortion induced by the full-response feed-forward equalizer. A random interleaver and a precoder are applied to whiten the noise and distortion. In a laboratory experiment, the minimal required received optical power of -29.8 dBm (at a BER of $3.80 \times {10^{ - 3}}$ ) is realized at a data rate of 500 Mbps. In a standard swimming pool, the proposed system successfully achieves a data rate of 500 Mbps with a transmission distance up to 150 m, at which the total attenuation is measured to be 53.47 dB. Such a data rate, enabled by the proposed joint DSP scheme, is 8.0%, 9.2%, and 14.2% higher than those achieved by solely using partial response shaping scheme, TCM scheme, and least square equalization scheme, respectively. To the best of our knowledge, this is the first time to employ the combination of partial response shaping and TCM technology in UWOC for high-speed long-reach transmission.

150 m/500 Mbps Underwater Wireless Optical Communication Enabled by Sensitive Detection and the Combination of Receiver-Side Partial Response Shaping and TCM Technology

While ultrahigh-baud-rate optical signals are effective for extending the transmission distance of large capacity signals, they also reduce the number of wavelengths that can be arranged in a band because of their wider bandwidth. This reduces the flexibility of optical path configuration in reconfigurable optical add/drop multiplexing (ROADM) networks. Particularly in colorless, directionless and contentionless (CDC)-ROADM, the effect reduces the add/drop ratio at a node. Multiband ROADM systems are an effective countermeasure for overcoming this issue, but they make the node configuration more complicated and its operation more difficult. In this paper, we analyze the challenges of C + L band CDC-ROADM and show that optical switch devices that operate over multiple bands are effective in meeting them. For this purpose, we built a C + L band CDC-ROADM node based on C + L band wavelength-selective switches and multicast switches and confirmed its effectiveness experimentally. In particular, to simplify the node configuration, we propose a reduction in the number of optical amplifiers used for node loss compensation and experimentally verify its feasibility.

First demonstration of a C + L band CDC-ROADM with a simple node configuration using multiband switching devices

We propose a NL-MLSE scheme using a third-order Volterra filter that estimates channel non-linearities to improve bandwidth-limitation and chromatic-dispersion tolerances. NL-MLSE with three memories achieves 192-Gb/s PAM-4 transmission with 96-Gsample/s DAC under a 22-GHz bandwidth limitation and 19-ps/nm chromatic dispersion.

96-Gbaud PAM-4 Transmission with 1 sample/symbol under 22-GHz Bandwidth Limitation Using NL-MLSE Based on Third-Order Volterra Filter

We propose a simple nonlinear trellis-coded-modulation to realize spectral-shaping for short-reach IM-DD transmission. Simulation results show the improvement of bandwidth-limitation and chromatic-dispersion tolerances. Experimental results show the achievement of 7% HD-FEC threshold in 184-Gb/s transmission.

92-Gbaud PAM4 Transmission using Spectral-Shaping Trellis-Coded-Modulation with 20-GHz Bandwidth Limitation

We achieve 180-Gb/s PAM-4 transmission under 32-GHz bandwidth limitation applying MLSE based on 3-memory T-spaced 2nd order Volterra filter. We experimentally demonstrate 2.88-Tb/s O-band transmission using 4-λ LAN-WDM and SDM over 2-km 4-core fiber.

Hiroki Taniguchi

Papers

255-Gbps PAM-8 Transmission under 20-GHz Bandwidth Limitation using NL-MLSE Based on Volterra Filter

First demonstration of a C + L band CDC-ROADM with a simple node configuration using multiband switching devices

96-Gbaud PAM-4 Transmission with 1 sample/symbol under 22-GHz Bandwidth Limitation Using NL-MLSE Based on Third-Order Volterra Filter

92-Gbaud PAM4 Transmission using Spectral-Shaping Trellis-Coded-Modulation with 20-GHz Bandwidth Limitation

Achievement of 90-Gbaud PAM-4 with MLSE Based on 2 nd Order Volterra Filter and 2.88-Tb/s O-band Transmission Using 4-λ LAN-WDM and 4-Core Fiber SDM