Yiran Ma

107 Gb/s coherent optical OFDM transmission over 1000-km SSMF fiber using orthogonal band multiplexing

Coherent optical OFDM (CO-OFDM) has emerged as an attractive modulation format for the forthcoming 100 Gb/s Ethernet. However, even the spectral-efficient implementation of CO-OFDM requires digital-to-analog converters (DAC) and analog-to-digital converters (ADC) to operate at the bandwidth which may not be available today or may not be cost-effective. In order to resolve the electronic bandwidth bottleneck associated with DAC/ADC devices, we propose and elucidate the principle of orthogonal-band-multiplexed OFDM (OBM-OFDM) to subdivide the entire OFDM spectrum into multiple orthogonal bands. With this scheme, the DAC/ADCs do not need to operate at extremely high sampling rate. The corresponding mapping to the mixed-signal integrated circuit (IC) design is also revealed. Additionally, we show the proof-of-concept transmission experiment through optical realization of OBM-OFDM. To the best of our knowledge, we present the first experimental demonstration of 107 Gb/s QPSK-encoded CO-OFDM signal transmission over 1000 km standard-single- mode-fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated system employs 2x2 MIMO-OFDM signal processing and achieves high electrical spectral efficiency with direct-conversion at both transmitter and receiver.

1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access

A 1-Tb/s single-channel coherent optical OFDM (CO-OFDM) signal consisting of continuous 4,104 spectrally-overlapped subcarriers is generated using a novel device of recirculating frequency shifter (RFS). The RFS produces 320.6-GHz wide spectrum using a single laser with superior flatness and tone-to-noise ratio (TNR). The 1-Tb/s CO-OFDM signal is comprised of 36 uncorrelated orthogonal bands achieved by adjusting the delay of the RFS to an integer number of OFDM symbol periods. The 1- Tb/s CO-OFDM signal with a spectral efficiency of 3.3 bit/s/Hz is successfully received after transmission over 600-km SSMF fiber without either Raman amplification or dispersion compensation.

Coherent optical OFDM: has its time come? (Invited)

There has been growing interest in coherent optical orthogonal frequency-division multiplexing (CO-OFDM). We aim to present the case that the time for CO-OFDM has come, in terms of the demand from todays ever-advancing optical networks and the availability of its underlying technologies. We first lay out the signal processing aspect for CO-OFDM and then show a 2×2 multiple-input-multiple-output (MIMO) OFDM representation for the single-mode fiber optical channel. Through numerical simulation and experimental demonstration, we further present and discuss various MIMO-OFDM systems focusing on their polarization-mode dispersion resilience. Among those systems, the 2×1 MIMO-OFDM configuration incorporating polarization-time coding shows promise for optical access networks and broadcast networks. Finally, another class of the frequency-domain equalization techniques, namely, incoherent or coherent optical single-carrier frequency-domain equalization (CO-SCFDE) is discussed and experimentally demonstrated.

Phase Noise Effects on High Spectral Efficiency Coherent Optical OFDM Transmission

There are three major advantages for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission using digital signal processing. First, coherent detection is realized by digital phase estimation without the need for optical phase-locked loop. Second, OFDM modulation and demodulation are realized by the well-established computation-efficient fast Fourier transform (FFT) and inverse FFT. Third, adaptive data rates can be supported as different quadrature amplitude modulation (QAM) constellations are software-defined, without any hardware change in transmitter and receiver. However, it is well-known that coherent detection, OFDM, and QAM are all susceptible to phase noise. In this paper, theoretical, numerical, and experimental investigations are carried out for phase noise effects on high spectral efficiency CO-OFDM transmission. A transmission model in the presence of phase noise is presented. By using simulation, the bit error rate floors from finite laser linewidth are presented for CO-OFDM systems with high-order QAM constellations. In the experiments, the phase noise effects from both laser linewidth and nonlinear fiber transmission are investigated. The fiber nonlinearity mitigation based on receiver digital signal processing is also discussed.

Theoretical and experimental study on PMD-supported transmission using polarization diversity in coherent optical OFDM systems.

In this paper, we conduct theoretical and experimental study on the PMD-supported transmission with coherent optical orthogonal frequency-division multiplexing (CO-OFDM). We first present the model for the optical fiber communication channel in the presence of the polarization effects. It shows that the optical fiber channel model can be treated as a special kind of multiple-input multiple-output (MIMO) model, namely, a two-input two-output (TITO) model which is intrinsically represented by a two-element Jones vector familiar to the optical communications community. The detailed discussions on various coherent optical MIMO-OFDM (CO-MIMO-OFDM) models are presented. Furthermore, we show the first experiment of polarization-diversity detection in CO-OFDM systems. In particular, a CO-OFDM signal at 10.7 Gb/s is successfully recovered after 900 ps differential-group-delay (DGD) and 1000-km transmission through SSMF fiber without optical dispersion compensation. The transmission experiment with higher-order PMD further confirms the immunity of the CO-OFDM signal to PMD in the transmission fiber. The nonlinearity performance of PMD-supported transmission is also reported. For the first time, nonlinear phase noise mitigation based on receiver digital signal processing is experimentally demonstrated for CO-OFDM transmission.

1-Tb/s Single-Channel Coherent Optical OFDM Transmission With Orthogonal-Band Multiplexing and Subwavelength Bandwidth Access

A 1-Tb/s single-channel coherent optical OFDM (CO-OFDM) signal consisting of continuous 4104 spectrally-overlapped subcarriers is generated using a recirculating frequency shifter (RFS). Theoretical and experimental analysis of the RFS is performed to study its effectiveness in extending OFDM bandwidth. In particular, the RFS produces a 320.6-GHz wide frequency comb from a single laser with superior flatness and tone-to-noise ratio (TNR). The 1-Tb/s CO-OFDM signal is consisted of 36 uncorrelated orthogonal bands achieved by adjusting the delay of the RFS to an integer number of OFDM symbol periods. The 1-Tb/s CO-OFDM signal with a spectral efficiency of 3.3 bit/s/Hz is successfully received after transmission over 600-km SSMF fiber without either Raman amplification or dispersion compensation.

1-Tb/s per channel coherent optical OFDM transmission with subwavelength bandwidth access

1-Tb/s per channel coherent optical OFDM transmission over 600-km SSMF fiber is demonstrated assisted with a recirculating frequency shifter. Record spectral efficiency and transmission reach is achieved at 1-Tb/s channel rate without Raman amplification.

Experimental Demonstration and Numerical Simulation of 107-Gb/s High Spectral Efficiency Coherent Optical OFDM

Orthogonal frequency-division multiplexing (OFDM) is a multicarrier modulation format in which the data are transmitted with a set of orthogonal subcarriers. Recently, this modulation format has been actively explored in the field of optical communications to take advantages of its high spectral efficiency and resilience to chromatic and polarization dispersion. However, to realize the optical OFDM at 100 Gb/s and beyond requires extremely high electronic bandwidth for the electronic signal processing elements. In this paper, we investigate orthogonal-band-multiplexed OFDM (OBM-OFDM) as a suitable modulation and multiplexing scheme for achieving bandwidth scalable and spectral efficient long-haul transmission systems. The OBM-OFDM signal can be implemented in either RF domain, or optical domain, or a combination of both domains. Using the scheme of OBM-OFDM, we show the successful transmission of 107 Gb/s data rate over 1000-km standard single-mode fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated OBM-OFDM system is realized in optical domain which employs 2times2 MIMO-OFDM signal processing and achieves high optical spectral efficiency of 3.3 bit/s/Hz using 4-QAM encoding. Additionally, we perform numerical simulation of 107-Gb/s CO-OFDM transmission for both single-channel and wavelength-division-multiplexed (WDM) systems. We find that the Q -factor of OBM-OFDM measured using uniform filling of OFDM subbands is in fact more conservative, in particular, is 1.2 dB and 0.4 dB lower than using random filling for single-channel and WDM systems, respectively.

Bit and Power Loading for Coherent Optical OFDM

We show the first experiment of bit and power loading for coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) systems. The data rate of CO-OFDM systems can be dynamically adjusted according to the channel condition. The system performance can be further improved through optimal power loading into each modulation band.

Real-time reception of multi-gigabit coherent optical OFDM signals.

Coherent Optical OFDM (CO-OFDM) has been demonstrated for delivering superior performance in spectral efficiency, receiver sensitivity, and polarization-dispersion resilience. Fueled by the rapid advancement in semiconductor technology, high-speed field-programmable gate arrays (FPGA) and analogue-to-digital-converters/digital-to-analogue converters (ADC/DACs) have been increasingly adopted for digital signal processing in optical communications. In this paper, we report the first FPGA-based real-time implementation of coherent optical OFDM (CO-OFDM) receiver with a transmission rate up to 3.1 Gb/s. Several basic aspects of CO-OFDM signal processing are described in detail, and the BER sensitivity performance are evaluated in real-time. To the best of our knowledge, we have achieved the record real-time reception date rate for a single-input single-output (SISO) coherent OFDM signal, in either RF domain or optical domain.