Demonstration of a 90 Tb/s, 234.8 km, C+L band unrepeatered SSMF link with bidirectional Raman amplification.

This work demonstrates the use of bidirectional Raman amplification to achieve an unrepeatered 234 km link using standard single mode fibers and with a capacity×distance product of 21.132 Pb/s·km. A throughput above 90 Tb/s is achieved with an 87 nm wavelength-division multiplexed signal carrying 424 PDM-64QAM signals at 24.5 GBaud across C and L bands. Transmission is supported using up to 12 Raman pumps per propagation direction, covering a wavelength range between 1410.8 nm and 1502.7 nm and with total power under 2.6 W.

S-, C- and L-band transmission over a 157†nm bandwidth using doped fiber and distributed Raman amplification.

We investigate optical transmission of an ultra-wideband signal in a standard single mode fiber. Using a near continuous optical bandwidth exceeding 157 nm across the S-, C- and L-bands, we combine doped-fiber amplifiers covering S, C and L-bands with distributed Raman amplification to enable high-quality transmission of polarization division multiplexed (PDM)-256-quadrature-amplitude modulation (QAM) signals over a 54 km standard single-mode fiber. We receive 793 × 24.5 GBd signals from 1466.34 nm to 1623.57 nm and measure a data rate estimated from the generalized mutual information (GMI) of 256.4 Tb/s and an LDPC decoded throughput of 244.3 Tb/s. The measured data rates exceed the highest previously measured in a single mode fiber, showing the potential for S-band transmission to enhance achievable data rates in optical fibers.

Real-time transmission of geometrically-shaped signals using a software-defined GPU-based optical receiver.

A software-defined optical receiver is implemented on an off-the-shelf commercial graphics processing unit (GPU). The receiver provides real-time signal processing functionality to process 1 GBaud minimum phase (MP) 4-, 8-, 16-, 32-, 64-, 128-ary quadrature amplitude modulation (QAM) as well as geometrically shaped (GS) 8- and 128-QAM signals using Kramers-Kronig (KK) coherent detection. Experimental validation of this receiver over a 91 km field-deployed optical fiber link between two Tokyo locations is shown with detailed optical signal-to-noise ratio (OSNR) investigations. A net data rate of 5 Gbps using 64-QAM is demonstrated.

Compensation of inter-core skew in multi-core fibers with group velocity dispersion.

This work proposes an approach for the compensation of inter-core skew in homogeneous single-mode multi-core fiber links. We adjust the wavelengths of the transmitted spatial channels in such a way that the skew induced by group velocity counters inter-core skew. This approach is demonstrated experimentally using a 111 Gb/s spatial super channel (4 spatial channels at 27.8 Gb/s) on a 10.1 km 19-core multi-core fiber. It is shown that inter-core skew may be compensated without the need for devices such as variable optical delay lines or electronic buffers.

Experimental evaluation of the impact of the light source on the measurement of short-term average crosstalk in homogeneous single-mode multi-core fibers.

This work compares the random time-varying crosstalk in homogeneous multi-core fibers measured using different types of light sources with linewidths ranging from 100 Hz to 2.5 MHz. We show that the frequency stability of the light source plays a significant role on the quality of short-term average crosstalk measurements with no observable impact from laser linewidth. We also compare the use of filtered amplified spontaneous emission noise and coherent light sources for crosstalk measurements. The former are shown to enable average crosstalk measurements in short time periods. In contrast, measurements using coherent light sources require long measurement periods to reach similar results.

Simple method for optimizing the DC bias of Kramers-Kronig receivers based on AC-coupled photodetectors.

We propose and evaluate a method to estimate the DC bias required for AC-coupled Kramers-Kronig receivers. The proposed method is based on a spectral analysis of the reconstructed signal without requiring an evaluation of the signal quality. The proposed method is described analytically and demonstrated experimentally using 12.5 GBaud 16-ary quadrature-amplitude modulated signals in back-to-back and after 100 km transmission.

Space-division multiplexed transmission in the S-band over 55 km few-mode fibers.

Transmission of highly spectral efficient 24.5 GBaud quadrature phase shift keying and 16- and 64-quadrature amplitude modulated signals in the S-band between 1492 nm and 1518 nm wavelength is demonstrated over 55 km few-mode fibers. The carrier lines for S-band transmission were generated by a single wideband optical comb source with more than 120 nm optical bandwidth. While the three-mode fiber was originally optimized for C- and L-band transmission, we show that differential mode delay and mode-dependent loss show only a minor wavelength dependence within the measured S-band channels. However, the transceiver sub-system, including S-band optical amplifiers as well as a reduced optical signal-to-noise ratio of the comb source, leads to a significant Q-factor penalty for channels towards the edges of the S-band optical amplifiers below 1495 nm and above 1515 nm wavelength.

Master-slave carrier recovery for M-QAM multicore fiber transmission.

Master-slave carrier recovery is a digital signal processing technique that uses correlated phase noise in multi-channel receivers to eliminate redundant carrier recovery blocks. In this paper we experimentally investigate the performance of master-slave carrier recovery for multicore fiber transmission in the presence of inter-channel nonlinear interference. Using a triple parallel loop setup we jointly receive three spatial channels in a 7-core fiber for transmission distances of up to 1600 km. We find that an increased launch power causes a moderate penalty on the slave channels. Furthermore, we study the penalty from a non-zero inter-core skew.

Long distance crosstalk-supported transmission using homogeneous multicore fibers and SDM-MIMO demultiplexing.

We propose and evaluate the use of spatial-division multiplexing (SDM) multiple input multiple output (MIMO) systems to support long distance transmission using single-mode homogeneous multicore fibers. We show that on a uniform link with per-span inter-core skew compensation, the required SDM-MIMO memory length corresponds to the largest inter-core skew per span on the link. Furthermore, we show that with inter-core skew compensation, the required memory length of the SDM-MIMO is nearly constant with the transmission distance for accumulated crosstalk below -11 dB. We experimentally demonstrate the use of SDM-MIMO with a memory length of 20 ns on a long distance transmission link using 20 GBaud PDM-QPSK signals. We achieve a reach of 9780 km, which corresponds to a 9% improvement over the case without SDM-MIMO. We also show that the use of SDM-MIMO is applicable to the transmission of signals with higher modulation order, achieving transmission reach improvements of 14% for 20 GBaud PDM-16QAM and PDM-64QAM signals.

Optical subcarrier processing for Nyquist SCM signals via coherent spectrum overlapping in four-wave mixing with coherent multi-tone pump.

As one of the promising multiplexing and multicarrier modulation technologies, Nyquist subcarrier multiplexing (Nyquist SCM) has recently attracted research attention to realize ultra-fast and ultra-spectral-efficient optical networks. In this paper, we propose and experimentally demonstrate optical subcarrier processing technologies for Nyquist SCM signals such as frequency conversion, multicast and data aggregation of subcarriers, through the coherent spectrum overlapping between subcarriers in four-wave mixing (FWM) with coherent multi-tone pump. The data aggregation is realized by coherently superposing or combining low-level subcarriers to yield high-level subcarriers in the optical field. Moreover, multiple replicas of the data-aggregated subcarriers and the subcarriers carrying the original data are obtained. In the experiment, two 5 Gbps quadrature phase-shift keying (QPSK) subcarriers are coherently combined to generate a 10 Gbps 16 quadrature amplitude modulation (QAM) subcarrier with frequency conversions through the FWM with coherent multi-tone pump. Less than 1 dB optical signal-to-noise ratio (OSNR) penalty variation is observed for the synthesized 16QAM subcarriers after the data aggregation. In addition, some subcarriers are kept in the original formats, QPSK, with a power penalty of less than 0.4 dB with respect to the original input subcarriers. The proposed subcarrier processing technology enables flexibility for spectral management in future dynamic optical networks.

Flexible generation of 28 Gbps PAM4 60 GHz/80 GHz radio over fiber signal by injection locking of direct multilevel modulated laser to spacing-tunable two-tone light.

To meet the ever-increasing bandwidth demands in the future broadband wireless networks, the millimeter-wave (mm-wave) frequency region is being actively perused, owing to its broad bandwidth and high frequencies. In this paper, a photonic mm-wave system is proposed and experimentally demonstrated based on the injection locking of a direct multilevel modulated laser to a spacing-tunable two-tone light. Since the mm-wave frequency of the generated signal is locked to the frequency spacing of the injected two-tone light, it shows better frequency stabilization than the schemes based on two free-running lasers. Moreover, by simply tuning the tone spacing, the mm-wave frequency could be easily re-configured, offering flexibility in the mm-wave signal generation. Instead of using complex and expensive optical modulators, the multilevel modulation on the mm-wave data carrier is implemented through the direct multilevel modulation of a laser and the injection locking. A 28 Gbps four-level pulse amplitude modulation (PAM4) is realized by biasing a 10 G-class laser at a current far from the threshold, providing a cost-effective and simple mm-wave generation scheme. In the experiment, a photonic approach to generating 28 Gbps PAM4 60 GHz/80 GHz mm-wave signals is experimentally demonstrated. A power penalty of less than 0.2 dB is observed for the filtered-out PAM4 signals with respect to the original PAM4. Besides, an ultra-low phase noise of up to -98 dBc/Hz is obtained for the mm-wave carriers after the injection locking. The proposed scheme possesses the flexibility and frequency stability of the mm-wave frequency, and also has low cost and implementation complexity.

Crosstalk dynamics in multi-core fibers.

Inter-core crosstalk is a potential limitation on the achievable data-rates in optical fiber transmission systems using multi-core fibers. Crosstalk arises from unwanted coupling between cores of a homogenous multi-core fiber and it's average power has been observed to vary over time by 10s of decibels, potentially requiring an additional performance margin to achieve acceptable outage probability. Most investigations of crosstalk have so far only considered continuous wave laser light or amplified spontaneous emission as sources of crosstalk. In this paper, we theoretically and experimentally investigate the time-dependence of inter-core crosstalk in a homogeneous multi-core fiber when considering signals with various modulation formats and symbol rates. We find that crosstalk power fluctuations depend on the symbol rate, modulation and skew between cores. For carrier-free signals, such as quadrature amplitude modulation, the crosstalk power is nearly constant for expected conditions of multi-core transmission systems. However, carrier-supported signals, such as OOK, always induce time-varying crosstalk powers.

Performance of adaptive DD-OFDM multicore fiber links and its relation with intercore crosstalk.

Adaptive direct-detection (DD) orthogonal frequency-division multiplexing (OFDM) is proposed to guarantee signal quality over time in weakly-coupled homogenous multicore fiber (MCFs) links impaired by stochastic intercore crosstalk (ICXT). For the first time, the received electrical power of the ICXT and the performance of the adaptive DD-OFDM MCF link are experimentally monitored quasi-simultaneously over a 210 hour period. Experimental results show that the time evolution of the error vector magnitude due to the ICXT can be suitably estimated from the normalized power of the detected crosstalk. The detected crosstalk results from the beating between the carrier in the test core and ICXT originating from the carrier and modulated signal from interfering core. The results show that the operation of DD-OFDM systems employing fixed modulation can be severely impaired by the presence of ICXT that may unpredictable vary in both power and frequency. The system may suffer from deleterious impact of moderate ICXT levels over a time duration of several hours or from peak ICXT levels occurring over a number of minutes. Such power fluctuations can lead to large variations in bit error ratio (BER) for static modulation schemes. Here, we show that BER fluctuations may be minimized by the use of adaptive modulation techniques and that in particular, the adaptive OFDM is a viable solution to guarantee link quality in MCF-based systems. An experimental model of an adaptive DD-OFDM MCF link shows an average throughput of 12 Gb/s that represents a reduction of only 9% compared to the maximum throughput measured without ICXT and an improvement of 23% relative to throughput obtained with static modulation.

Single parity check-coded 16QAM over spatial superchannels in multicore fiber transmission.

We experimentally investigate single-parity check (SPC) coded spatial superchannels based on polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16QAM) for multicore fiber transmission systems, using a 7-core fiber. We investigate SPC over 1, 2, 4, 5 or 7 cores in a back-to-back configuration and compare the sensitivity to uncoded PM-16QAM, showing that at symbol rates of 20 Gbaud and at a bit-error-rate (BER) of 10-3, the SPC superchannels exhibit sensitivity improvements of 2.7 dB, 2.0 dB, 1.7 dB, 1.3 dB, and 1.1 dB, respectively. We perform both single channel and wavelength division multiplexed (WDM) transmission experiments with 22 GHz channel spacing and 20 Gbaud channel symbol rate for SPC over 1, 3 and 7 cores and compare the results to PM-16QAM with the same spacing and symbol rate. We show that in WDM signals, SPC over hl1 core can achieve more than double the transmission distance compared to PM-16QAM at the cost of 0.91 bit/s/Hz/core in spectral efficiency (SE). When sharing the parity-bit over 7 cores, the loss in SE becomes only 0.13 bit/s/Hz/core while the increase in transmission reach over PM-16QAM is 44 %.

Peta-bit-per-second optical communications system using a standard cladding diameter 15-mode fiber.

Data rates in optical fiber networks have increased exponentially over the past decades and core-networks are expected to operate in the peta-bit-per-second regime by 2030. As current single-mode fiber-based transmission systems are reaching their capacity limits, space-division multiplexing has been investigated as a means to increase the per-fiber capacity. Of all space-division multiplexing fibers proposed to date, multi-mode fibers have the highest spatial channel density, as signals traveling in orthogonal fiber modes share the same fiber-core. By combining a high mode-count multi-mode fiber with wideband wavelength-division multiplexing, we report a peta-bit-per-second class transmission demonstration in multi-mode fibers. This was enabled by combining three key technologies: a wideband optical comb-based transmitter to generate highly spectral efficient 64-quadrature-amplitude modulated signals between 1528 nm and 1610 nm wavelength, a broadband mode-multiplexer, based on multi-plane light conversion, and a 15-mode multi-mode fiber with optimized transmission characteristics for wideband operation.

Pump-probe analysis of the cross-phase modulation degradation induced by 10 Gbit/s amplitude-shift-keyed signals on 40 Gbit/s DPSK signals.

A study of the cross-phase modulation (XPM) degradation of differential-phase-shift-keyed (DPSK) signals due to amplitude-shift-keyed signals is performed using pump-probe simulation. Approximate expressions for the contributions of the XPM-induced intensity and phase modulation to the electrical current fluctuations at the differential-phase-exchange-keyed receiver are presented. It is shown that, unlike prior works and similar to intensity-modulated signals, the contribution of XPM-induced intensity modulation is dominant in systems using standard fiber or high residual dispersion.

Cross-phase modulation bandwidth in ultrafast fiber wavelength converters.

We propose a novel analytical model for the characterization of fiber cross-phase modulation (XPM) in ultrafast all-optical fiber wavelength converters, operating at modulation frequencies higher than 1 THz. The model is used to compare the XPM frequency limitations of a conventional and a highly nonlinear dispersion shifted fiber (HN-DSF) and a bismuth oxide-based fiber, introducing the XPM bandwidth as a design parameter. It is shown that the HN-DSF presents the highest XPM bandwidth, above 1 THz, making it the most appropriate for ultrafast wavelength conversion.