Spatial Mode-Division Multiplexing for High-Speed Optical Coherent Detection Systems

Spatial modedivision multiplexing is emerging as a potential solution to further increasing optical fiber capacity and spectral efficiency. We report a dualmode, dualpolarization transmission method based on modeselective excitation and detection over a twomode fiber. In particular, we present 107 Gbit/s coherent optical OFDM （COOFDM） transmission over a 4.5 km twomode fiber using LP and LP. modes in which mode separation is performed optically.

Simple real-time high-sensitivity heterodyne coherent optical transceiver at intraplane satellite communication

In this paper,we demonstrate a high-sensitivity and real-time heterodyne coherent optical transceiver for intraplane satellite communication,without digital-to-analog converter(DAC)devices and an optical phase lock loop(OPLL).Based on the scheme,a real-time sensitivity of-49 dBm is achieved at 5 Gbps QPSK.Because DAC is not needed at the transmitter,as well as OPLL at the receiver,this reduces the system cost.Furthermore,the least required Rx ADC bit-width is also discussed.Through theoretical analysis and experimental results,our cost-effective transceiver satisfies the scenario and could be a promising component for future application.

Recent progress on high-speed optical transmission

The recently reported high spectral efficiency （SE） and high-baud-rate szgnal transmission are all Daseo on digital coherent optical communications and digital signal processing （DSP）. DSP simplifies the re- ception of advanced modulation formats and also enables the major electrical and optical impairments to be processed and compensated in the digital domain, at the transmitter or receiver side. In this paper, we summarize the research progress on high-speed signal generation and detection and also show the progress on DSP for high-speed signal detection. We also report the latest progress on multi-core and multi-mode multiplexing.

Compensating for Nonlinear Effects in Coherent-Detection Optical Transmission Systems

Fiber nonlinearity is one of the most important limiters of capacity in coherent optical communications. In this paper, we review two nonlinear compensation methods： digital backward propagation （BP） and nonlinear electrical equalizer （NLEE） based on the timedomain Volterra series. These compensation algorithms are implemented in a singlechannel 50 Gb/s coherent optical singlecarrier frequency division multiplexed （CO-SCFDM） system transmitting over 10 × 80 km of standard singlemode fiber （SSMF）.

Parallel Spectral-Domain Optical Coherence Tomography for Non-Scattering Object Imaging

The parallel spectral-domain optical coherence tomography（PSDOCT） is described for highspeed optical coherence tomography（OCT） without lateral scanning. In this setup, the self-elimination of auto-correlation（AC） interference algorithm was used for eradicating the AC interference and ghost images. However, when performed in free space OCT, this algorithm still generated a weak DC component. The algorithm was improved by adding the background intensity part to compensate for the mutual interference between object and reference arms. The results demonstrate that the DC component can be eradicated. Compared with conventional QCT and complex Fourier-domain optical coherence to- mography, the advantages of PSDOCT with the improved algorithm in free space are that it has no moving parts to generate consecutive phase shift, the structure of the object can be reconstructed immediately and automatically, and the speed is approximately 16 times faster than those of the other two in the same case.

Joint modulation format identification and OSNR monitoring based on Stokes vector distribution features for digital coherent optical receivers

For joint modulation format identification(MFI)and optical signal-to-noise ratio(OSNR)monitoring,a simple and intelligent optical communication performance monitoring method is proposed,and the feasibility is demonstrated by digital coherent optical communication experiments.The experiment results show that for all modulation formats,including 28 GBaud polarization division multiplexing(PDM)QPSK/8-QAM/16-QAM/64-QAM,100%MFI accuracies are achieved even at OSNR values lower than the corresponding theoretical 20%forward error correction limit,as well as the high accuracies for OSNR monitoring.Furthermore,the proposed scheme has a reasonable monitoring level when chromatic dispersion and fiber nonlinear effects are varied.

Integrated coherent beam combining system for orbital-angular-momentum shift-keying-based free-space optical links

Orbital-angular-momentum(OAM)multiplexing technology offers a significant dimension to enlarge communication capacity in free-space optical links.The coherent beam combining(CBC)system can simultaneously realize OAM multiplexing and achieve high-power laser output,providing substantial advantages for long-distance communication.Herein,we present an integrated CBC system for freespace optical links based on OAM multiplexing and demultiplexing technologies for the first time,to the best of our knowledge.A method to achieve flexible OAM multiplexing and efficient demultiplexing based on the CBC system is proposed and demonstrated both theoretically and experimentally.The experimental results exhibit a low bit error rate of 0.47%and a high recognition precision of 98.58%throughout the entire data transmission process.By employing such an ingenious strategy,this work holds promising prospects for enriching ultra-long-distance structured light communication in the future.

The Study on Heterodyne and Phase Diversity Coherent Optical Fiber Communication Systems and Their Components

The research in Coherent Optical Fiber Communication Systems (COFCS) using long waveband semiconductor laser diodes as transmitter and local oscillator has evolved gradually from an esteric subject studied in just a few communication laboratories around the world to the field demonstration stage. This is mainly because of the possibility of receiver sensitivity improvement reaching 10～20 dB, that of frequency division multiplexing (FDM) with very fine frequency separation and the possibility of using electronic equalization network to compensate for the effect of optical pulse dispersion in the Single Mode Fiber (SMF). The author of the dissertation has engaged in the study on COFCSs since 1987. Undergoing 3 years of hard work, the finished research work includes two parts: Part I is the study on heterodyne COFCSs and their key components; Part II is the study on two branch phase diversity COFCS and its components. In the first part, using the Lamb′s semi classical theory and the model of vector field, the effects of various kinds of parameters, such as cavity detuning, cavity′s Q factor and the transverse magnetic field strength, on the intramode beat frequency tuning characteristics are analyzed. On the basis of the theoretical analysis, the equipment of 1 523 nm He Ne Stabilized Transverse Zeeman Lasee (STZL) with high frequency stability and certain continuous single frequency tuning range, has been established domestically for the first time. With the development of long waveband semi conductor laser diodes fabricated domestically, the author mainly deals with the long waveband COFCSs using semiconductor laser diodes as transmitter and local oscillator. To counter the poor spectrum characteristics of the conventional double heterojunction semi conductor laser diodes, the characteristics of 1.5 um InGaAsP GRINROD External Cavity Semi conductor Lasers (ECSL) in the case of strong and weak feedback conditions, is experimentally studied. Then the GRINROD Dissertation completed Apr.1991ECSLs with complete closed and compact structure are developed. Using this kind of lasers and other components made domestically, the long waveband large deviation FSK heterodyne single filter/ envelope detection systems and small deviation FSK heterodyne single filter/delay line frequency discriminator detection system are achieved for the first time. In those experiments, the frequency modulation characteristics, line width, single frequency continuous tuning range and frequency stability of the GRINROD ECSL are measured by means of IF spectrum. Meanwhile, the IF receiving circuits are optimized for obtaining stable transmission properties. The main research achievements in part II are the theoretical analysis of the diversity COFCSs. Firstly, the Bit Error Rate (BER) performance of two branch homodyne phase diversity ASK, DPSK and two branch homodyne phase polarization DPSK receivers are analyzed by means of characteristic function model. In the analysis, the effects of pre and post detection filters on the system performance are especially studied. Secondly, the impacts of the phase noise, shot noise, polarization mismatch and imbalance of branch circuits on the two branch homodyne phase diversity ASK receiver, and that of modulation frequency deviation on FSK receiver are analyzed in terms of Gaussian approximation. Lastly, an improved all fiber 90 degree optical hybrid is fabricated. Utilizing the hybrid and 1.5 um InGaAsP GRINROD and grating ECSLs, a low IF, two branch phase diversity FSK coherent optical fiber transmission system is experimented for the first time in China. Guan Kejian Born in Jan. 1963. He received his Ph. D degree in Apr. 1991, in Radio Engineering, Beijing Uni versity of Posts and Telecommunications.

Performance of High Speed Coherent Optical Communication Systems

Performance of high speed coherent optical communication systems has been investigated. A novel frequency tracking/locking automatical fre quency control (clock frequency tracking AFC, CFT AFC) scheme has been presented. The first Gb/s speed coherent optical communication system of China (1.1 Gb/s FSK optical communication system) has been achieved. The achievements of this paper are as follows: (1) Performance of coherent optical communication system has been investigated. A set of expressions are deduced. In the expressions, the effects of the LD line width, waveform of modulation signal, the signal phase jitter, intersymbol interference (ISI) and various kinds of noises are comprehensively included for the first time. (2) The jitter performance of coherent optical communication system is analyzed for the first time. The timing jitter theory of coherent optical communication system is achieved. (3) A novel frequency tracking/locking automatic frequency control (clock frequency tracking AFC, CFT AFC) scheme is presented. The scheme has three advantages compared with other AFC schemes: (a) frequency tracking/locking may be achieved efficiently; (b) timing clock may be extracted efficiently; and (c) the circuitry is simpler and steadier. (4) Performance of phase diversity (PD) optical communication systems is analyzed. The BER expressions of PD systems modulated under various kinds of signal waveforms are presented. The power penalties of PD systems resulted from signal phase jitter and ISI are deduced. (5) Performance of the light sources of coherent optical communication (external cavity LD and DFB LD) is analyzed. The non flat FM (frequency modulation) response of LD chip is compensated by external circuitry. The DFB LD module with 4.7 GHz FM response is achieved. (6) Compensation of fiber dispersion in optical communication is investigated. The practical fiber dispersion compensators are achieved. (7) The first Gb/s speed coherent optical Dissertation completed Apr. 1992communication system of China is achieved. The system includes: (a) a 1.1 Gb/s ～ 2.2 Gb/s PRBS high speed signal source; (b) DFB LD modules with 4.7 GHz FM response and 1.1 Gb/s FSK optical transmitter; (c) a low noise preamplifier with 4.2 GHz width; (d) CFT AFC circuitry, which system, CFT AFC scheme not only achieves frequency tracking/locking, but also depressed timing jitter efficiently. In fact, the receiver may extract the timing clock efficiently even received signal with infinite continuous “0”; (e) The LD line width is narrowed by electrical negative feedback of CFT AFC. The characteristics of the system include: (a) the DFB LD modules are taken as the light sources; (b) balanced PIN is taken as photoelectron detector of the receiver; (c) CFT AFC scheme is taken as AFC, timing extracting, and LD line width narrowing; (e) the system speed: 1.1 Gb/s, the wavelength of LD: 1.536 4 μm, the transmission distance: 3.87 km, and the receiver sensitivity: 34.2 dBm (BER=10 9 ). Huang Shouhua Born in Hubei province, 1956, graduated from Physics Department of Nanjing University in 1980. He received the ME degree in Wuhan Research Institute of Posts and Telecom (WRIPT), in 1986. He joined WRIPT in 1986. Since 1988, he has been working towards Ph.D. degree at Beijing University of Posts and Telecom. He is the authors of over 30 journal and conference papers.

Beyond 100 Gb/s Optical OFDM Transmitted in Optical Long-Haul System

100 G Ethernet is considered to become the next generation Ethernet standard for IP networks.Typical 100 Gb/s transmission systems and their performance are presented.Comparision and analysis for 100 Gb/s transmission systems have been discussed.It is demonstrated that optical OFDM can be used in future 100 Gb/s/ch and long-haul system.

Integrated nano-structured silicon waveguides and devices for high-speed optical communications

With progress in fabrication technology, integrated photonics plays an increasingly important role in high-speed optical communications, from monolithic transmitters and receivers for advanced optical modulation formats to on-chip subsystems for optical signal processing. We review our recent work on the highly tailorable physical properties of silicon waveguides for communication and signal processing applications, using slot structures. Controllable chromatic dispersion, nonlinearity, and polarization properties of the waveguides are presented, and the enabled wideband wavelength conversion, optical tunable delay, and signal processing of polarization-multiplexing data channels are discussed.

Research on Multi-System Ultra-High Speed Optical Signal Access System

In view of the small transmission capacity and single signal modulation format of the existing optical transmission system, this paper proposes an ultra-high-speed optical signal access scheme based on NEL0670, which can realize the transmission of 100 G DP-QPSK, 200 G DP-16QAM and 400 G DP-16QAM signals, and realize flexible and intelligent reception of multi-system optical signals.

The Possibility of Applying Spectral Redundancy in Coherent DWDM Systems for Increasing the Data Transmission Rate and Decreasing Nonlinear Effects and Double Rayleigh Scattering with Great Security of Data

A new method coherent DWDM system has been proposed. Standard method uses 90 wavelengths with channel capacity 100 G, so total capacity amounts to 9 Terabit, optical power is P. New “channel super” (CS) consists of 15 wavelengths, number of CS is 6, so total number of wavelengths is 90 too, total capacity is 6 Terabit, but optical power is 4/15P. Over ULH and EULH markets, standard approach will be forced to reduce the total capacity to 2.4 Terabit with optical power 4/15P as in proposed method. But the new can reach 6 Terabit with the same optical power 4/15P.

Direct phase control method for binary phase-shift keying space coherent laser communication

An optical phase locking method based on direct phase control is proposed.The core of this method is to synchronize the carrier by directly changing the phase of the local beam.The corresponding experimental device and the supporting algorithm were configured to verify the feasibility of this method.Phase locking can be completed without cycle skipping,and the acquisition time is 530 ns.Without an optical preamplifier,a sensitivity of-34.4 d Bm is obtained,and the bit error rate is 10^(-9) for 2.5 Gbit/s binary phase-shift keying modulation.The measured standard deviation of the phase error is 5.2805°.

Fiber Nonlinearity Mitigation by Improved Zero-Interpolation Method in Coherent Optical OFDM Systems

An improved zero-interpolation method with signal clipping to improve fiber nonlinearity tolerance in the long-haul coherent optical orthogonal frequency division multiplexing system （CO-OFDM） is presented. The new technique is implemented by interpolating zero subcarriers and selecting the odd subcarriers to carry data and clipping conventional CO-OFDM signal at zero. With such a scheme,the effect of fiber nonlinearity can be miti-gated,and the wanted signal carried on odd subcarriers is or-thogonal to clipping noise,which falls on even frequencies. Simu-lation shows that the system Q value is improved by more than 2 dB at the length of 960 km

Research of Homodyne Optical Fiber Communication System and Associated Technologies

Because it has the advantages of high sensitivity, and it is easy to demodulate and convenient to select in FDM system, the coherent optical fiber communication system is much suitable to be used in long distance optical communication systems and in optical fiber WANs. There are two major patterns in coherent optical fiber communication: heterodyne and homodyne. Compared with the heterodyne scheme, the homodyne optical fiber communication system has the following advantages: (1) The sensitivity of the homodyne receiver is higher than that of the heterodyne receiver. As we know, the PSK homodyne optical fiber communication system has the highest sensitivity in coherent optical fiber communication systems. So it is much suitable to be used in long distance optical communication systems or in FDM systems. (2) Because the homodyne receiver only uses the baseband to demodulate the transmitted signals, it occupies much narrower frequency domain than the heterodyne receiver does, which makes it more suitable to be used in multichannel systems. (3) The demodulation pattern used in homodyne receiver is much easier than that used in the heterodyne receiver, since it only needs the baseband demodulation. Usually we construct a homodyne receiver with an optical phase locked loop (OPLL). The research of the OPLL began at 1960′s and the study of the homodyne receiver has been made gradually. In 1984, the first homodyne optical fiber communication system was demonstrated in BTRL, in which the signal laser and the local laser were all 1.5 μm He Ne gas lasers, and the OPLL used was a balanced one. In 1989, L.G.Kazovsky demonstrated experimentally a homodyne receiver in Bellcore using two 1.3 μm Nd:YAG lasers as the signal laser and the local laser and also using a balanced OPLL. Because the linewidth of the normal semiconductor laser is too large and its frequency stability is much poorer, it is very difficult to construct a homodyne receiver with the semiconductor lasers. At the end of 1989, the first Dissertation completed Jul. 1992homodyne optical fiber communication system using two 1.5 μm external cavity semiconductor lasers as the signal laser and the local laser, respectively, was finished in AT&T Bell Lab by J. M. Kahn, in which the OPLL was also a balanced one. In China, the research of the homodyne optical fiber communication system was funded by the National Seventh Five Year Program and by the National Natural Science Foundation. The difficulties to construct a homodyne optical fiber communication system are listed as follows: (1) In homodyne communication systems, the signal laser′s frequency should be stable to avoid the penalty of the receiver′s BER and the crosstalk to other channels in a FDM system, and the local laser could be tuned widely and easily to cover all the signal lasers′ frequency domain. Both the signal laser and the local laser should be narrow in linewidth to decrease the influence of the laser′s phase noise on the BER of the receiver. (2) The modulation pattern used should be studied and chosen carefully because the requirements of different kinds of modulation on the laser, the receiver and the channel are different. (3) Since the construction of the linear OPLL (the balanced) and the nonlinear OPLL (the Costas OPLL, or the Decision Driven OPLL) are rather different, their requirements on the linewidths of the lasers are different too, we should study the theory and the construction of the OPLL carefully to select the suitable scheme to realize. (4) In a multichannel system (such as a FDM system), the influence of crosstalk between channels on the homodyne system should be researched carefully. The technology used to stabilize the channel interval should also be studied. In this thesis, the homodyne optical fiber communication system has been theoretically analyzed, the technical difficulties of constructing the system have been studied. Several kinds of external cavity semiconductor laser have been researched experimentally. Compared

Digital-analog hybrid optical phase-lock loop for optical quadrature phase-shift keying

We analyze a feasible high-sensitivity homodyne coherent optical receiver for demodulating optical quadrature phase-shift keying(QPSK). A fourth-power phase-lock loop based on a digital look-up table is used. Considering the non-negligible loop delay, we optimize the loop natural frequency. Without error correction coding, a sensitivity of -37 dBm/-35 dBm is achieved, while the bit error rate is below 10-9 at 2.5 Gbaud/5 Gbaud rate.For the QPSK communication system, the bit rate is twice the baud rate. The loop natural frequency is 0.647 Mrad/s, and the minimized steady-state phase-error standard deviation is 3.83°.

Low-noise amplification of dissipative Kerr soliton microcomb lines via optical injection locking lasers

The dissipative Kerr soliton microcomb provides a promising laser source for wavelength-division multiplexing(WDM)communication systems thanks to its compatibility with chip integration.However,the soliton microcomb commonly suffers from a low-power level due to the intrinsically limited energy conversion efficiency from the continuous-wave pump laser to ultra-short solitary pulses.Here,we exploit laser injection locking to amplify and equalize dissipative Kerr soliton comb lines,superior gain factor larger than 30 dB,and optical-signal-to-noise-ratio(OSNR)as high as 60 dB obtained experimentally,providing a potential pathway to constitute a high-power chip-integrated WDM laser source for optical communications.

Use of polarization freedom beyond polarizationdivision multiplexing to support high-speed and spectral-efficient data transmission

Increasing the system capacity and spectral efficiency(SE)per unit bandwidth is one of the ultimate goals for data network designers,especially when using technologies compatible with current embedded fiber infrastructures.Among these,the polarizationdivision-multiplexing(PDM)scheme,which supports two independent data channels on a single wavelength with orthogonal polarization states,has become a standard one in most state-of-art telecommunication systems.Currently,however,only two polarization states(that is,PDM)can be used,setting a barrier for further SE improvement.Assisted by coherent detection and digital signal processing,we propose and experimentally demonstrate a scheme for pseudo-PDM of four states(PPDM-4)by manipulation of four linearly polarized data channels with the same wavelength.Without any modification of the fiber link,we successfully transmit a 100-Gb s−1 PPDM-4 differential-phase-shift-keying signal over a 150-km single-mode fiber link.Such a method is expected to open new possibilities to fully explore the use of polarization freedom for capacity and SE improvement over existing fiber systems.

Multi-Gigabit CO-OFDM System over SMF and MMF Links for 5G URLLC Backhaul Network

The 5G cellular network aims at providing three major services:Massive machine-type communication(mMTC),ultra-reliable low-latency communications(URLLC),and enhanced-mobile-broadband(eMBB).Among these services,the URLLC and eMBB require strict end-to-end latency of 1 ms while maintaining 99.999%reliability,and availability of extremely high data rates for the users,respectively.One of the critical challenges in meeting these requirements is to upgrade the existing optical fiber backhaul network interconnecting the base stations with a multigigabit capacity,low latency and very high reliability system.To address this issue,we have numerically analyzed 100 Gbit/s coherent optical orthogonal frequency division multiplexing(CO-OFDM)transmission performance over 400 km single-mode fiber(SMF)and 100 km of multi-mode fiber(MMF)links.The system is simulated over optically repeated and non-repeated SMF and MMF links.Coherent transmission is used,and the system is analyzed in a linear and non-linear regime.The system performance is quantified by bit error ratio(BER).Spectrally efficient and optimal transmission performance is achieved for 400 km SMF and 100 km MMF link.The results designate thatMMF links can be employed beyond short reach applications by using them in the existing SMF infrastructure for long haul transmission.In particular,the proposed CO-OFDM system can be efficiently employed in 5G backhaul network.The multi-gigabit capacity and lower BER of the proposed system makes it a suitable candidate especially for the eMBB and URLLC requirements for 5G backhaul network.