Spatial-diversity schemes are applied to improve signal quality of coherent free-space optical transmission systems with uncorrelated phase noise. We compare the performance of conventional schemes (MRC, SDC) to a newly proposed one (X-MRC).

The increasing demand for fast and license-free satellite feeder links has put optical free-space communications on the research agenda. To compensate for atmospheric turbulence utilizing spatial diversity and post-DSP combining, a DSP is needed that works reliably even at very low SNRs. For this purpose, a custom multi-format data-aided equalizer is developed and its performance is analyzed under various conditions in a dual-polarization coherent transmission system. Numerical simulations and a field test over a 3.2-km free-space optical link are used to validate the DSP design, and show stable DSP performance for a 32 GBd coherent transmission at SNR-values down to 0 dB, even for DP-16QAM modulation.

We report on a demonstration of a bidirectional, multi-aperture free-space optics (FSO) system and discuss general system design aspects. A multi-aperture terminal is used for spatial diversity. On both sides, a real-time signal combining takes place on field-programmable gate arrays. The combined signals are weighted by their corresponding channel powers. Small form-factor C-band pluggable modules provide the opto-electrical interfaces of the 10-Gbit/s on/off-keying signals. A custom-built multi-channel bit error tester is used on both terminals for performance determination. Each aperture has an optical tracking system, is coupled to a bidirectional single-mode fiber and emits an optical power of 4 dBm.

Optical wireless communications gains new interest in the recent years, due to high potential for use cases in the industrial internet of things. In order to address lower power consumption requirements at the mobile transmitter, an on-off-keying based physical layer has been developed by the IEEE task group 802.15.13, supporting a wide range of symbol rates from 12.5 to 200 MBaud. While providing robustness against high-pass filtering effects in analogue optical frontends, the currently used 8b10b line-coding causes a large overhead, notably limiting the achievable throughput. In this work, we investigate scrambler-based alternatives for the line-coding that reduce overhead at the cost of less-balanced signaling. We analyze statistical properties and thoroughly test the performance using a frontend-model derived from measurements on an optical frontend prototype. Accordingly, the 64b67b scheme is proposed as an optional line coding in the IEEE P802.15.13 project.

Linearization of a high -bandwidth coherent driver module is demonstrated to enable the use of larger swing for driving signals. It is verified for symbol rates and modulation formats up to 80-GBd and 256-QAM, leading up to 3.7-dB higher transmitter output power compared to linear predistortion.

We experimentally compared terabit transmission employing 16QAM Nyquist WDM and QPSK Time-Frequency Packing. The two modulation schemes have been transmitted over the same link configuration. The latter showed slightly better performance in terms of spectral efficiency and reach, at the expense of a more expensive and complex hardware.

We investigate CAZAC sequences with positive and negative chirp for the use as training sequences in data-aided channel estimation. While both sequences show the same linear performance, the right choice of chirp is critical in case of nonlinear transmission. A performance difference of up to 2 dBQ was experimentally verified.

This paper describes the general architecture for a sliceable bandwidth variable transponder as identified within the IDEALIST European project. The capability of generate super-channels (optical connections with several adjacent optical sub-carriers) and the slice-ability (super-channels generated together but independently routed in the network towards different destinations) are the key elements of the considered architecture.

We present an experimental investigation of different Kerr nonlinearity mitigation schemes in a 2-span dispersion-compensated link of 160-km length for 4 × 28-GBd single-polarization 16-QAM signal transmission. We consider optical phase conjugation (OPC) based on mid-link spectral inversion, digital backpropagation (DBP), and transmission of phase-conjugated twin-waves with either digital coherent superposition (DCS) or all-optical coherent superposition employing phase-sensitive amplification (PSA). The experiments are performed in a single-channel and a wavelength-division multiplexing (WDM) scenario. For single-channel operation, phase-conjugated twin waves with DCS provided superior performance compared to all the other mitigation schemes with a maximum Q 2 -factor improvement of 1.7 dB compared to signal transmission without any nonlinearity mitigation. The DBP, however, showed a higher nonlinear threshold than all the other schemes in our experiments. In the WDM scenario, DBP showed worse performance than the other mitigation schemes whereas the DCS again provided a higher Q 2 -factor improvement than the other techniques with a maximum Q 2 -factor improvement of 1.9 dB. The PSA and the mid-link OPC showed similar nonlinearity mitigation performance both in single-channel and WDM scenarios.

This article reports the work on next generation transponders for optical networks carried out within the last few years. A general architecture supporting super-channels (i.e., optical connections composed of several adjacent subcarriers) and sliceability (i.e., subcarriers grouped in a number of independent super-channels with different destinations) is presented. Several transponder implementations supporting different transmission techniques are considered, highlighting advantages, economics, and complexity. Discussions include electronics, optical components, integration, and programmability. Application use cases are reported.

Four-dimensional (4-D) set-partitioning quadrature amplitude modulation (4-D SP-QAM) has emerged as an interesting option for cost- and resource-efficient realization of bandwidth variable transceivers in elastic optical networks. In this invited paper, we review the principles for generation of 4-D SP-QAM signals, and describe options for forward error correction coding of 4-D SP-QAM signals and for realization of the digital signal processing in the coherent receiver. Furthermore, we report on the experimental realization of 4-D 512-ary and 2048-ary SP-QAM signals at a symbol rate of 28 GBd and investigate their performance in a Nyquist-WDM scenario. In transmission experiments over standard single-mode fiber, we compare the reach and spectral efficiency of five-carrier Nyquist-WDM signals modulated by various 4-D SP-QAM formats and polarization-division multiplexed (PDM) QAM formats. Of these modulation formats, the one with the lowest spectral efficiency is 128-ary SP-QAM encoding 7 bits/4-D symbol and the one with the highest spectral efficiency is PDM-64QAM encoding 12 bits/4-D symbol. By switching the modulation format, the spectral efficiency can be optimized for a specific reach with a granularity of 0.56 bit/s/Hz.

Kerr nonlinearity compensation by optical phase conjugation is demonstrated in a WDM PDM 16-QAM system. Improved received signal quality is reported for both dispersion-compensated and dispersion-uncompensated transmission and a comparison with digital backpropagation is provided.

We experimentally investigate the performance of Kerr nonlinearity mitigation by employing two different nonlinearity mitigation approaches in a 4x28-GBd single-polarization 16-QAM signal transmission system over a 400-km dispersion-compensated link. One scheme employs digital coherent superposition of phase-conjugated twin waves at the receiver whereas the other scheme is based on a digital backpropagation employing the split-step Fourier method. The results for both nonlinearity mitigation schemes are compared in the single-channel and the WDM scenario. Both schemes show significant signal quality-factor improvements compared to the signal transmission without any nonlinearity mitigation technique.

In this contribution we experimentally investigate the performance of the four-dimensional 2048-ary set-partitioning quadrature amplitude modulation (4-D 2048SP-QAM) format in a Nyquist-WDM (NWDM) transmission system and compare it with PDM-64QAM at a symbol rate of 28 GBd in back-to-back and after transmission over standard singlemode fiber (SSMF). Three NWDM super-channels are generated, each formed by multiplexing five wavelengths with 35 GHz spacing. A gross bit rate of 1.54 Tb/s and 1.68 Tb/s per NWDM super-channel is obtained for 4-D 2048SPQAM and PDM-64QAM, respectively. Furthermore, the performance of the 2048SP-QAM format is evaluated in presence of soft-decision forward error correction (SD-FEC) based on turbo product code (TPC) with 21.3% overhead.

Several spectral efficient solutions for next-generation flexible optical networks are under investigation in order to cope with scalability issues while traffic demand is continuously increasing. Concerning transmission, Nyquist wavelength-division multiplexing (NWDM)-that confines the bandwidth within the Nyquist frequency of the signal-recently gained a momentum as one of the most suitable solutions for transmission over backbone networks and commercial solutions are available by now. Besides NWDM, other transmission techniques have been proposed to approach or overcome the Nyquist limit, thus further increasing the spectral efficiency (SE). Among them, time-frequency packing (TFP) is one candidate. This method builds a superchannel, whose subcarriers significantly overlap in frequency or time or both. This leads to an increased SE at the expenses of additional complexity within the transceiver to compensate for the introduced intersymbol interference. In this paper, we experimentally compare, for the first time, NWDM and TFP when employing the same identical test-bed. The experiment considered the two different terabit superchannels: a polarization multiplexed (PM)-16 quadrature amplitude modulation for the case of NWDM case, and a PM-quadrature phase-shift keying for TFP. The comparison and assessment of the results is carried out first in back-to-back configuration and, afterward, by propagating them over a recirculating loop consisting of a standard single-mode fiber, including spectrum selective switch to emulate node filtering.

The ability to adaptively change the employed modulation format in order to realize transmission with flexible bit rate and spectral efficiency depending on the actual requirements of the network is a key feature for future optical networks. Data-aided channel estimation and equalization methods were shown to enable modulation format transparent digital signal processing and are therefore essential building blocks in future optical transponders. In this contribution, we investigate a popular set of training sequences, namely constant amplitude and zero autocorrelation (CAZAC) sequences, for channel estimation in nonlinear optical transmission systems. These sequences are widely used in wireless communication systems and have recently been adopted for optical systems as well. CAZAC sequences are similar to linearly chirped signals and thus can be generated with either positive or negative chirp. We show both numerically as well as experimentally that CAZAC sequences with positive and negative chirp exhibit similar performance in the case of a linear channel while their performance strongly differs in the case of nonlinear transmission. We find that the interplay of chromatic dispersion and fiber nonlinearities during transmission can cause large performance differences between two CAZAC sequences having opposite signs of the chirp. A difference in Q-factor of up to 2 dB between the two CAZAC sequences is experimentally observed for an EDFA amplified standard single-mode fiber link with 80 km spans. Hence, the sign of the frequency chirp of the CAZAC sequence has to be carefully chosen depending on the actual dispersion map of the link and the signal parameters.

We report on the experimental realization of 28-GBd four-dimensional 512-ary setpartitioning QAM signals and investigate them in a Nyquist-WDM scenario. In transmission experiments over standard single-mode fiber, we compare them with 28-GBd PDM-16QAM and PDM-32QAM Nyquist-WDM signals.

We extensively compare the performance of single-carrier digital back-propagation applied to different modulation formats and data-rates when different fiber types are considered in an ultra dense WDM system.

We present a 107-GBd coherent optical system for the generation and reception of m-ary phase-shift keyed (PSK) and quadrature-amplitude modulated (QAM) signals, including Nyquist pulse shaping. It is based on phase-stable optical time-division multiplex (OTDM) of two tributaries, which are modulated at half the target symbol rate, optical Nyquist filtering, and a broadband digital coherent receiver. The system is analyzed in back-to-back experiments with binary PSK (BPSK) modulation, quadrature-PSK (QPSK) modulation, as well as with 16-ary QAM. Compared to all-ETDM implementations, the transmitter is based on independent electro-optical modulation of two time-division multiplex tributaries and subsequent optical interleaving after modulation. Although this scheme implies increased hardware complexity, it enables lower penalties with respect to theory compared to all-ETDM experiments presented so far. This is due to the reduced hardware speed requirements. The measured penalties at bit-error ratios of 1 × 10 -3 are 0.5, 1.5, and 6 dB for single-polarization BPSK, QPSK, and 16QAM, respectively. Furthermore, the employed phase-stable optical multiplexer allows use of standard receiver DSP at the broadband digital coherent receiver without need for special treatment of the individual OTDM tributaries.