In this paper, we experimentally demonstrate the first 110-Gb/s multi-band superchannel coherent optical orthogonal frequency-division multiplexing based on offset quadrature amplitude modulation (OFDM/OQAM) system. Unlike the conventional orthogonal band-multiplexed OFDM system, no timing or frequency synchronization is required for the OFDM/OQAM system. We further investigate the influence of guard band, and find that very trivial guard band spacing (<20MHz) is required without any sensitivity performance or spectral efficiency degradation. Thus, the newly designed scheme would significantly reduce the implementation constrains for the band-multiplexed superchannel coherent optical OFDM system.

Coherent optical OFDM (CO-OFDM) combined with orthogonal band multiplexing provides a scalable and flexible solution for achieving ultra high-speed rate. Among many CO-OFDM implementations, digital Fourier transform spread (DFT-S) CO-OFDM is proposed to mitigate fiber nonlinearity in long-haul transmission. In this paper, we first illustrate the principle of DFT-S OFDM. We then experimentally evaluate the performance of coherent optical DFT-S OFDM in a band-multiplexed transmission system. Compared with conventional clipping methods, DFT-S OFDM can reduce the OFDM peak-to-average power ratio (PAPR) value without suffering from the interference of the neighboring bands. With the benefit of much reduced PAPR, we successfully demonstrate 1.45 Tb/s DFT-S OFDM over 480 km SSMF transmission.

In this paper, we proposed an improved mobile fronthaul architecture employing the SNR-improved delta-sigma digitization scheme and 4-level pulse amplitude modulation (PAM-4) format. Different from traditional 2-bit quantization, this scheme deploys two-fold delta-sigma quantization, which uses 1-bit delta-sigma modulator to quantize the signal, and another 1-bit delta-sigma modulator to quantize the in-band noise. A significant reduction of the in-band noise can be achieved only applying a differentiator, bringing about a much better noise shaping performance. Meanwhile, the two 1-bit streams can be combined and transmitted in a PAM-4 manner through an intensity modulation direct detection (IM-DD) channel. We have successfully experimentally demonstrated the digitization and transmission of 65536 quadrature amplitude modulation (QAM) baseband orthogonal frequency division multiplexing (OFDM) signal with sampling rate of 1.25 GSa/s over 20-km fiber with standard 10-Gbaud PAM-4 signal, with a signal to noise ratio (SNR) of 57.7 dB. Compared to the conventional 2-bit quantized OFDM scheme, experimental results show that a significant SNR improvement of 17.7 dB has been achieved by the proposed scheme. In addition, we also realized a transmission of 16384-QAM intermediate frequency (IF) signal at the center frequency of 3.5 GHz (with an SNR of 49.6 dB). An improvement of 15.9 dB can also be maintained.

Multipath interference (MPI) noise suppression is crucial for high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) transmission systems. In this paper, two algorithms are proposed to eliminate the MPI noise, and these algorithms can be realized in the receiver digital signal processing (DSP) module without changing the existing IMDD system architecture. The first algorithm (A1) suppresses the MPI noise by removing the fluctuation of MPI-impaired PAM4 signals. The second algorithm (A2) can estimate the MPI noise which is intensity-dependent, and then the estimated MPI noise is subtracted from the impaired signal. The complexity of the proposed algorithms is lower than that of the traditional high-pass filter (HPF) scheme. Simulation results indicate that the proposed schemes can effectively suppress the MPI noise and improve the transmission performance under different signal-interference ratios (SIR), different laser linewidth, and random optical delay of the reflected signal. Besides, the cascade of A1 and A2 can further improve the transmission performance, and it has a higher tolerance for laser linewidth compared to the traditional HPF scheme. With the help of the proposed scheme, 56 Gb/s PAM4 signals are experimentally transmitted over 15.5 km standard single-mode fiber under the SIR of 18 dB and the laser linewidth of 100 kHz, and the measured bit error rate (BER) can be below the 7% overhead hard-decision forward error correction limit. When the laser linewidth is increased to 4 MHz, the traditional HPF scheme becomes less effective and may degrade the transmission performance, but the proposed schemes are still effective and the SIR tolerance can be improved by about 3 dB. 100 Gb/s PAM4 under optical back-to-back transmission with 24 dB SIR can be achieved, and the BER performance can be increased from 4.63e-3 to 2.06e-3 with the help of the proposed schemes.

In this paper, we propose a novel approach to simultaneously receive multi-band 100-Gb/s direct-detection optical signal with only one polarization and one conventional 40-GHz photodiode. The modulation format of orthogonal frequency-division multiplexing based on offset quadrature amplitude modulation (OFDM/OQAM) is selected to provide signal spectrum with high side-lobe suppression ratio, which can effectively reduce the electrical sub-band frequency interference. The whole 100-Gb/s OFDM/OQAM signal is comprised of 6 sub-bands with 16- and 32-QAM formats loading. Only one guard band is required to accommodate the overlapped 6-band signal-to-signal beat interference (SSBI). The receiver bandwidth is mainly limited by the digital storage oscilloscope (DSO) of 33 GHz. The transmission distance over standard single mode fiber (SSMF) is up to 320 km.

High precise IQ skewcalibration of the coherent optical transceiver is crucial for high baud rate and high-order modulation formats optical transmission system. In this paper, a novel high precise optical coherent transceiver calibration method based on the specially designed interleaved in-phase (I) and quadrature-phase (Q) multi-tone signals is proposed. This method can simultaneously characterize the transmitter (Tx) and receiver (Rx) side IQ skew by just one measurement without using any additional device or training process. Because of the non-overlapping characteristic of these interleaved I and Q signals in the frequency domain, the IQ crosstalk in the coherent optical transceiver can be effectively avoided, contributing to high-precision calibration. Since the calibration process only focuses on the phase frequency response characteristic, the dynamic range of IQ skew calibration can easily reach dozens of picoseconds. Theoretical analysis and simulation results indicate that the measurement accuracy of the Tx skew and Rx skew is within ±0.2 ps in the proposed scheme, and the dynamic range of IQ skew calibration can reach ±40 ps. With the help of this IQ skew calibration method, the transmission performance improvement is experimentally verified in both single-polarization 30 GBaud 32QAM signal and dual-polarization 30 GBaud 16QAM signal transmission systems.