Xuebing Zhang

Experimental demonstration of 110-Gb/s unsynchronized band-multiplexed superchannel coherent optical OFDM/OQAM system

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 (< 20 MHz) 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.

Advanced modulation formats for 400-Gbps short-reach optical inter-connection

Besides the long-haul optical networks covering over thousands of kilometers for backbone transmission, short reach optical networks (SR-ONs) are widely deployed in metro-area for aggregation and accessing. The SR-ONs include the metro optical transport networks (Metro-OTN), optical access networks or other optical inter-connection systems with even shorter distance. As predicted, the growing bandwidth demanding from SR-ONs will be much more than that from the long-haul optical networks in the near future. Besides, there are tremendous amounts of optical terminals and end-users in SR-ONs compared with the long-haul transmission systems and thus will induce large cost and huge energy consumption. So, the power and cost efficiency should be the key consideration for SR-ONs besides the transmission performance. To improve the power and cost efficiency in SR-ONs, advanced modulations and detection techniques based on low power, low cost and integrated optical modulators should be utilized. In this paper, different advanced modulation formats have been discussed. 56Gbps PAM4, 112Gbps poly-binary and 100Gbps DMT that can be used to realize 400-Gbps SR-ONs for different applications have also been demonstrated respectively. In addition, low-cost and low-power opto-electronic components suitable for SR-ONs, the impairments induced by all kinds of defects and bandwidth limitation of opto-electronic components and the corresponding compensation techniques based on DSP algorithms have also been discussed in the experiments.

Transmission of 100-Gb/s DDO-OFDM/OQAM over 320-km SSMF with a single photodiode

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.

Experimental Demonstration of 429.96-Gb/s OFDM/OQAM–64QAM Over 400-km SSMF Transmission Within a 50-GHz Grid

We experimentally demonstrate a 429.96-Gb/s signal transmission over a 400-km standard single-mode fiber within the 50-GHz grid and successfully achieve spectral efficiency as high as 8.63 bit/s/Hz. Orthogonal frequency-division multiplexing/offset quadrature amplitude modulation with 64-quadrature amplitude modulation is selected as the modulation format to provide a “perfect” rectangular spectrum that efficiently reduces the channel crosstalk. No timing or frequency alignment is required for the subbands to form the superchannel.

Single photodiode direct detection system of 100-Gb/s OFDM/OQAM-64QAM over 80-km SSMF within a 50-GHz optical grid

We propose a novel guard-band-shared direct-detection (GBS-DD) scheme to improve the receiver spectrum efficiency (SE). The 100-Gb/s signal is modulated by 2 sub-bands, which are assigned onto two orthogonal polarizations. The central wavelengths of the two sub-bands are set as 10.84-GHz frequency space. The two sub-bands are then received simultaneously using a single conventional photodiode (PD) of 40-GHz bandwidth. Only one optical pilot carrier is inserted to beat with the 2 sub-bands on the two polarizations. When the 2 sub-band signal entering into the receiver, the signal-to-signal beat interference (SSBI) terms fall and overlap in the same guard band. As a consequence, the bandwidth usage of the PD is enhanced from 1/2 to 2/3. The 100-Gb/s signal is modulated using orthogonal frequency-division multiplexing based on offset quadrature-amplitude-modulation of 64-quadrature amplitude modulation (OFDM/OQAM-64QAM), and transmitted over 80-km standard single mode fiber (SSMF) within a 50-GHz optical grid. It is shown that the proposed GBS-DD scheme can be implemented by the current commercial optical/electrical devices.

Theoretical studies on the polarization-modulator-based single-side-band modulator used for generation of optical multicarrier

This paper focuses on the studies on the polarization- modulator-based single-side-band modulator (PSSBM) as well as its implementation in generation of frequency-locked multicarrier. The principle and properties of PSSBM have been analyzed in detail with theoretical and simulation results. Then, the PSSBM-based frequency-locked multicarrier generator (PSMCG) with recirculating frequency shifting loop has also been demonstrated via simulation. The results have a good agreement with the theoretical analysis. According to the results, multiple frequency-locked carriers with high quality can be achieved based on the proposed PSMCG. The generated carriers have the potential applications in different scenarios such as optical transmission, microwave photonics and so on.

Low-cost coherent receiver for long-reach optical access network using single-ended detection

A low-cost coherent receiver using two 2×3 optical hybrids and single-ended detection is proposed for long-reach optical access network. This structure can detect the two polarization components of polarization division multiplexing (PDM) signals. Polarization de-multiplexing and signal-to-signal beat interference (SSBI) cancellation are realized by using only three photodiodes. Simulation results for 40 Gb/s PDM-OFDM transmissions indicate that the low-cost coherent receiver has 3.2 dB optical signal-to-noise ratio difference compared with the theoretical value.

Pilot-based blind phase estimation for coherent optical OFDM system

In this paper, we propose an improved pilot-based blind (PBB) phase estimation method named mixed-polar-amplitude-modulation (MPAM-PBB) method for coherent optical orthogonal frequency-division multiplexing (CO-OFDM). In the previous PBB methods, all the selected so-called pilot subcarriers (PSs) are loaded with bipolar amplitude modulation (AM) format signal. In MPAM-PBB method, half of the selected so-called pilot subcarriers (PSs) are changed to be loaded with bipolar AM format signal, which will improve the SE. The performance of 3 phase noise estimation methods including the conventional pilot-aided-based (PAB) estimation method, PBB method and MPAM-PBB method are compared in a CO-OFDM transmission simulation. The results show that the proposed MPAM-PBB method can achieve comparable performance with PAB method and get higher SE of the system than PBB method without increasing the computational complexity.

Demonstration of Simultaneous 1-to-34 Multicasting of OFDM/OQAM 64-QAM Signal from Single Gaussian Mode to Multiple Orbital Angular Momentum (OAM) Modes

By exploiting single phase-only spatial light modulator, we demonstrate 1-to-34 multicasting of OFDM/OQAM 64-QAM signal from single Gaussian mode to multiple OAM modes. All 34-fold multicasted OAM channels show relatively low crosstalk (<-10 dB) from their neighboring OAM modes and achieve BER less than 2e-3.

Optical OFDM/OQAM for the Future Optical Communication

In this paper we review the prior works on optical OFDM/OQAM systems, including the coherent optical OFDM/OQAM, and direct detection optical OFDM/OQAM. Besides this, we prospects the future optical communication of OFDM/OQAM.