Tianwai Bo

Dual modulation of an electro-absorption modulation laser (D-EML) is a compact and monolithically integrable optical single sideband (SSB) transmitter scheme. However, the quality of the output signal generated by the D-EML scheme could be degraded by the nonlinear distortions resulting from the cascade modulation of the directly modulated laser (DML) and electro-absorption modulator (EAM). In this Letter, we propose an effective approach to mitigate the modulation nonlinearity with the digital pre-distortion (DPD) technique to improve the signal quality and enhance the tolerance against fiber chromatic dispersion for the optical SSB signal generated by the D-EML transmitter. Significant improvement in signal-to-noise ratio and bit error rate performance at various transmission distances has been demonstrated with a 56-Gb/s optical SSB transmission system. The experimental results also show that the proposed scheme achieves power budget enhancements of ∼1.1 dB and ∼2.3 dB compared with the conventional SSB signals with and without the offset filtering scheme.

We propose and experimentally demonstrate an inter-core skew measurement method for multi-core fibers (MCFs) based on broadband incoherent interferometry of spectrum-sliced amplified spontaneous emission (ASE) noise. The technique avoids the need for narrow-linewidth lasers and tolerates the random power fading due to coherent coupling among neighboring cores. By utilizing a calibrated interference spectrum to extract the delay value through cascaded filtering and fitting, the proposed technique achieves sub-picosecond measurement accuracy. Both the back-to-back and 41-km MCF experiments demonstrate stable performance with root mean square errors of <60 fs, comparable to the typical measurement scheme based on a vector network analyzer. The simplicity, robustness, and sub-picosecond precision make the proposed scheme an ideal solution for non-intrusive, online inter-core skew monitoring in multi-core fibers.

Inter-channel skew (ICS) measurement is essential for synchronization, optical performance monitoring (OPM), and scalability in space-division multiplexing (SDM) systems based on multi-core fibers (MCFs). We propose a high-accuracy and low-complexity ICS measurement method based on digital nonlinear curve fitting of cross-correlation (Xcorr) peaks. Without increasing the sampling rate and with only negligible additional computational overhead, the method achieves skew estimation accuracy equivalent to upsampling by a factor exceeding 100, while operating at a native sampling rate of 20 GSa/s. The proposed method achieves root-mean-square errors (RMSEs) of 207.97 fs and 177.68 fs on 12-km and 41-km 7-core fibers, respectively, as validated experimentally. Overall, our proposed method achieves high measurement accuracy with low computational complexity, paving a feasible path toward real-time online ICS monitoring in optical transmission links.

With the advent of higher baud rates and modulation orders in optical fiber communication systems, device nonlinearities have emerged as a critical limiting factor. In this Letter, we propose a simplified probability-maintained (PM) notch signal generation technique, along with its associated post-processing method, to enable accurate nonlinearity measurement. The feasibility of the proposed method is verified through comprehensive simulations and experimental comparisons with the existing methods. This method offers the advantages of low-complexity probing signal generation and can be readily implemented using a real-time spectrum analyzer (RTSA). It experimentally achieves an estimation error within 0.5 dB when the noise power ratio (NPR) is around −20 dB, thereby facilitating precise nonlinearity characterization at frequencies of interest.

The performance of the current high-speed optical transmission system could be limited by the bandwidth of optical transceivers. Linear equalization is a feasible way to mitigate the inter-symbol interference (ISI) caused by the bandwidth limitation, but it enhances high-frequency noise, thereby degrading receiver performance. One effective way to cope with noise enhancement is based on the noise-whitening approaches at either the transmitter side or receiver side. However, conventional noise whitening requires a maximum likelihood sequence estimation (MLSE) equalizer to correct the ISI introduced intentionally, while its computation complexity is beyond the power of cost-sensitive applications. In this paper, we propose a novel selective noise decorrelation (SND) equalizer positioned after the linear equalizer to correct most of the errors in a feedforward manner. By leveraging the noise patterns to predict the likelihood of symbol errors and applying a weighted average of neighboring noise for decorrelation only when it is necessary, the SND equalizer provides an alternative approach to mitigate the noise enhancement issue without complicated MLSE equalization or noise whitening. We demonstrate its capability through proof-of-concept experiments at various bandwidth limitation ratios. The proposed SND equalizer outperforms the feedforward equalizer as well as the decision feedback equalizer in all cases. Moreover, it achieves comparable performance as the MLSE-based approach with a much lower computational complexity. Based on analysis, the proposed SND equalizer needs a mere 6.25% of the PF-MLSE equalizer’s computational load.

The modified Gerchberg-Saxton (GS) algorithm provides an effective solution for electrical dispersion pre-compensation (pre-EDC) in short reach intensity modulation and direct detection (IM/DD) optical fiber transmission systems. To further enhance its performance under bandwidth-limited IM/DD systems, a joint bandwidth limitation and electrical dispersion pre-compensation (pre-BL-EDC) scheme is proposed. In this scheme, an equalization filter for bandwidth limitation (BL) is integrated into the modified GS pre-EDC, and a 2-tap finite impulse response (FIR) filter is introduced to improve the pre-compensation performance. In this way, simultaneous pre-compensation of BL and chromatic dispersion (CD) can be achieved at the transmitter, eliminating the need for any post-equalization at the receiver. Simulation results in a C-band 56 Gbit/s on-off keying (OOK) system demonstrate that the pre-BL-EDC can improve the bit error ratio (BER) performance and reduce device bandwidth requirements. Under back-to-back (BTB) transmission, bandwidth limitation pre-compensation (pre-BLC) improves receiver sensitivity by 2.2 dB, 1.8 dB, and 1.4 dB at the hard-decision forward error correction (HD-FEC) threshold for 24 GHz, 32 GHz, and 40 GHz bandwidths, respectively, compared to without pre-compensation. Under 80 km transmission, the receiver sensitivity of pre-BL-EDC is 3.2 dB, 2.8 dB and 1.0 dB higher than that of modified GS pre-EDC for the above corresponding bandwidths. Moreover, pre-BL-EDC allows for longer transmission distances in bandwidth-limited systems. Taking 32 GHz bandwidth as an example, for transmission distances ranging from 10 km to 100 km, the BER can be below the HD-FEC threshold using pre-BL-EDC, whereas it cannot be achieved using modified GS pre-EDC. The proposed scheme is further validated by an experimental system with 3-dB bandwidth of 9 GHz at different baud rates over a 50 km transmission. For OOK signals at 20 Gbit/s, 28 Gbit/s and 32 Gbit/s, the receiver sensitivity gains of pre-BL-EDC compared to modified GS pre-EDC are 1.0 dB, 1.0 dB and 2.0 dB at the HD-FEC threshold, respectively.

As a fundamental limiting factor in the space division multiplexing (SDM) system based on weakly coupled multi-core fiber (MCF), inter-core crosstalk (IC-XT) requires measuring conveniently and efficiently during MCF deployment. In this paper, we propose an effective IC-XT measurement method based on optical time domain reflectometer (OTDR). By connecting the designed bridge single-mode fibers (SMFs) structure at the beginning end of the MCF and launching optical pulse signal into one core under test at the far end, multiple loopback IC-XT peaks can be read on the OTDR, from which the IC-XT levels from the core under test to all other cores can be simultaneously calculated. Two bridge schemes for IC-XT measurement are proposed and experimentally verified on a 41 km 7-core fiber. The first scheme supports bidirectional IC-XT measurement, while the second performs better in the OTDR dynamic range and computational complexity. Both schemes demonstrate good measurement accuracy, with < 1 dB of errors. We believe that the proposed method will certainly provide convenient and efficient IC-XT evaluation during future MCF deployment.

The optical single sideband (SSB) transmitter based on dual modulation of an electro-absorption modulation laser (D-EML) has attracted considerable attention for its capability of monolithic integration and high output power. A model-based modulation method has been developed recently for generating high-quality optical SSB signals with this D-EML scheme. However, this method requires accurate characterization of the EML’s chirps and pre-compensation for frequency responses of all-optical/electrical components, as well as the path difference between two driving signals. This imposes notable requirements on the transmitter characterization in practical applications. In this paper, we propose an adaptive method to approach the required responses of the pre-compensation filters for this optical SSB transmitter. This method avoids cumbersome device characterization and shows great resilience to the variation of system parameters. By using the proposed adaptive method, we generate a 56 Gb/s optical SSB orthogonal frequency-division multiplexed signal with the sideband suppression power ratio exceeding 21 dB. It is convenient with this method to switch the devices, i.e., directly modulated laser (DML) or electro-absorption modulator (EAM), to be pre-compensated for the optical SSB signal generation. Moreover, this method exhibits good tolerance to the path delay (±15 ps) between DML and EAM, as well as modulation depth. We also successfully transmit this signal over 80 km long standard single-mode fiber.

The optical in-phase and quadrature modulator (IQM) is typically utilized as an optical single-sideband (SSB) transmitter for its simple structure and low cost. It requires the modulator biases to be slightly offset for generating the optical carrier. However, this poses significant challenges in the bias control circuits, as most off-the-shelf automatic bias control (ABC) modules are designed to stabilize the IQM at its carrier suppression point for coherent optical communication. In this paper, a novel arbitrary ABC scheme is proposed to stabilize the carrier-to-signal power ratio (CSPR) of an optical SSB signal over long-term operation. This scheme uses SSB rather than double-sideband (DSB) sinusoidal signals as the dither signals. It does not violate the minimum phase condition, thus avoiding performance penalties compared to the conventional ABC scheme in which the DSB dither signal is used. We also develop a closed-form expression to reveal the relationship between the control parameter in the ABC algorithm and the CSPR of the optical SSB signal. By this means, the optical SSB transmitter can automatically adjust its bias voltages and stabilize them at a given CSPR value. The experimental results with ∼80 Gb/s optical SSB signal show that our proposed ABC scheme is effective in stabilizing the bias of the IQM at any point and maintains the CSPR value of the optical SSB signal over hours. It is also shown that the proposed method leads to 1 dB improvement in the receiver sensitivity compared with the traditional ABC technique.