Binhuang Song

Experimental Layered/Enhanced ACO-OFDM Short-Haul Optical Fiber Link

Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is theoretically more optically power efficient than dc-biased OFDM (DCO-OFDM); unfortunately, its spectral efficiency is halved to accommodate the clipping distortion. Recently, layered/enhanced ACO-OFDM has been developed to mostly regain the lost spectral efficiency, and theoretically compared with other modulation formats. In this letter, we experimentally demonstrate L/E-ACO-OFDM for the first time over a fiber link. We transmit over a 19.8-km single mode fiber at 4.375 Gb/s, and show a 2-dB improvement in signal quality for the same laser bias current and received optical power over DCO-OFDM.

Hardware-efficient signal generation of layered/enhanced ACO-OFDM for short-haul fiber-optic links

Layered/enhanced ACO-OFDM is a promising candidate for intensity modulation and direct-detection based short-haul fiber-optic links due to its both power and spectral efficiency. In this paper, we firstly demonstrate a hardware-efficient real-time 9.375 Gb/s QPSK-encoded layered/enhanced asymmetrical clipped optical OFDM (L/E-ACO-OFDM) transmitter using a Virtex-6 FPGA. This L/E-ACO-OFDM signal is successfully transmitted over 20-km uncompensated standard single-mode fiber (S-SMF) using a directly modulated laser. Several methods are explored to reduce the FPGAs logic resource utilization by taking advantage of the L/E-ACO-OFDMs signal characteristics. We show that the logic resource occupation of L/E-ACO-OFDM transmitter is almost the same as that of DC-biased OFDM transmitter when they achieve the same spectral efficiency, proving its great potential to be used in a real-time short-haul optical transmission link.

Subband Pairwise Coding for Robust Nyquist-WDM Superchannel Transmission

Nyquist wavelength-division-multiplexed coherent optical superchannel systems closely pack a set of rectangular-shaped channels to realize high-spectral-efficiency transmission. In long-haul fiber links, dividing each of these channels into multiple subbands, each a few-gigahertz wide, can increase tolerance to fiber nonlinearity. However, these low-bandwidth subbands will be sensitive to laser drift, which could cause them to overlap, creating interchannel interference (ICI). In this paper, we propose the application of pairwise precoding across the middle and edge subbands to improve their ICI tolerance. The proposed scheme is verified in superchannel experiments, and shows about 1.5-dB Q2 improvement after 3840-km transmission for QPSK modulation and 1-dB Q2 gain with 16-QAM modulation after 800-km transmission.

Sub-band pairwise coding for inter-channel-interference mitigation in superchannel transmission systems

By pairing the mid and edge sub-bands of each channel, the penalty due to inter-channel-interference can be mitigated for a superchannel. Experimental results show that the proposed method offers 1.5-dB Q2 improvement after 3840-km transmission with 1GHz channel overlap.

Improved polarization dependent loss tolerance for polarization multiplexed coherent optical systems by polarization pairwise coding

Polarization dependent loss (PDL) causes imbalanced optical signal to noise ratio (OSNR) of the two polarizations, thus remains one of the major bottlenecks for next-generation polarization-division-multiplexed (PDM) coherent optical transmission systems. In this paper, we investigate Pairwise Coding for adaptive PDL mitigation in PDM coherent optical systems. By pre-coding across two polarizations, the PDL-induced performance degradation can be largely mitigated without any coding overhead. We present details of the coding and de-coding design, and also derive the analytical symbol/bit error rate of the Polarization Pairwise Coding scheme, which can be used to predict the performance gain as well as for optimal rotation angle calculation. Simulation results verify that Pairwise Coding achieves substantial system performance gains over a wide range of PDL values. Compared with other digital coding techniques, Polarization Pairwise Coding shows improved performance than Walsh-Hadamard transform since it maximizes the coordinate diversity; and also Pairwise Coding is computationally much simpler to decode compared with the Golden and Silver Codes, therefore is practical for current 100-Gb/s and future 400-Gb/s and 1-Tb/s digital coherent transceivers.

Banded all-optical OFDM super-channels with low-bandwidth receivers.

We propose a banded all-optical orthogonal frequency division multiplexing (AO-OFDM) transmission system based on synthesising a number of truncated sinc-shaped subcarriers for each sub-band. This approach enables sub-band by sub-band reception and therefore each receivers electrical bandwidth can be significantly reduced compared with a conventional AO-OFDM system. As a proof-of-concept experiment, we synthesise 6 × 10-Gbaud subcarriers in both conventional and banded AO-OFDM systems. With a limited receiver electrical bandwidth, the experimental banded AO-OFDM system shows 2-dB optical signal to noise ratio (OSNR) benefit over conventional AO-OFDM at the 7%-overhead forward error correction (FEC) threshold. After transmission over 800-km of single-mode fiber, ≈3-dB improvement in Q-factor can be achieved at the optimal launch power at a cost of increasing the spectral width by 14%.

Pairwise coding to mitigate polarization dependent loss

By pre-coding data across polarizations, the performance degradation due to PDL can be largely mitigated without any coding overhead. Experimentally, this technique shows more than 2-dB Q2 improvement for PDM-QPSK with 6-dB worst-case PDL.

Efficient IFFT implementation in an ACO-OFDM transmitter

A computationally efficient IFFT implementation for asymmetrically clipped optical OFDM transmitter is proposed and implemented in a Field-Programmable Gate Array. The overall computational complexity is reduced by 68% in a real-time transmitter.

Compact 4×5 Gb/s silicon-on-insulator OFDM transmitter

We characterize an integrated silicon 4×5 Gb/s OFDM transmitter PIC (2.1×4.8 mm²) with four modulators and an optical Fourier transform. This PIC features a channel spacing of 5 GHz and an 80-GHz free spectral range.

Real-Time Demonstration of Augmented-Spectral-Efficiency DMT Transmitter Using a Single IFFT

Increasing the power and spectral efficiency in intensity-modulated direct-detection short-haul fiber-optic links enables higher data rates in power- and bandwidth-limited optical communication systems. Augmented spectral efficiency discrete multi-tone (ASE-DMT) can improve the spectral efficiency of pulse-amplitude-modulated DMT while maintaining its power advantage over dc-biased DMT, whose transmitter requires only one inverse fast Fourier transform (IFFT) with Hermitian symmetric inputs. Although the ASE-DMT transmitter requires multiple IFFTs, we show how these can be mapped onto a single IFFT, by using both the real and imaginary outputs of the IFFT and by extracting some signals from within the IFFTs structure. Using only one IFFT, we first demonstrate a real-time PAM4-encoded optical ASE-DMT transmitter with a net data rate of 18.4 Gb/s. When implemented in a FPGA, using a single IFFT saves 30% of logic resources, compared with a four-IFFT ASE-DMT transmitter. Finally, a 1550-nm directly modulated laser is used to evaluate its optical transmission performance with offline signal processing in the receiver. Without using any optical amplifiers, the ASE-DMT signal can be successfully transmitted over 10-km standard single-mode fiber (SSMF), but fails over 20-km SSMF due to the influence of fiber dispersion and laser chirp.