使用调制相移法测量光纤色散的研究

Nowadays, the coherent optical communication system plays an important role in communication field because of large capability and bandwidth. A coherent optical communication, based on high-order modulation and digital signal processing technologies, consists of optical transmitters, optical fiber lines, optical amplifiers and optical receivers. In the high-speed coherent optical communication system, the phase noise from the transmitter laser and the local oscillator laser can significantly degrade the performance of the signal transmission and detection, especially for the systems using high-order modulation format, such as m-ary phase shift keying (mPSK) and m-ary quadrature amplitude modulation (m-QAM). Therefore, investigations on laser phase noise compensation algorithm based on digital signal processing technologies has become more and more significant. In this work, a multi-ring carrier phase recovery algorithm is developed for compensating the laser phase noise in optical fiber communication systems using high-order modulation formats. Degradations on the performance of communication systems due to the laser phase noise have been investigated. The system performance using the proposed algorithm and the conventional Viterbi-Viterbi algorithm were also evaluated in 9-channel and 15- channel, 32-Gbaud, Nyquist-spaced QPSK, 16-QAM, 64-QAM and 256-QAM coherent transmission systems with considering the impact of the laser phase noise. It is found that the phase noise leads to stricter constraints on the linewidths of transmitter-side and receiver-side lasers, and it can greatly degrade the achievable information rates in communication systems. Besides, compared to the conventional Viterbi-Viterbi algorithm, which is usually applied in the QPSK system, our proposed algorithm can also well mitigate the laser phase noise in 16-QAM, 64-QAM and 256-QAM optical communication systems.
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As an important application of laser technology, optical fiber communications are key enablers for the current information age. With the emergence of new technologies including Internet of Things, big data, cloud computing, virtual reality, and artificial intelligence, there are growing needs in the society for high capacity information transmission, which puts forward higher requirements for the optical fiber communication technology. Based on a systematic review of the development status of the technology in China and abroad, this paper analyzes the problems and challenges it faces in the application of specific scenarios, and prospects the development of some related technologies in the future. Through analysis, the challenges in optical fiber communication systems with an ultra-large capacity can be solved by increasing the transmitted power, raising the bandwidth of optical amplifiers, and conducting research on low-loss optical fibers and technologies related to space division multiplexing. The cost problems of the systems in other scenarios are also considered. In general, the optical fiber communication technology is evolving toward the direction of ultra-large capacity, intelligence, and integration. It is hoped to realize intelligent monitoring of network parameters, as well as ultra-long distance and ultra-large capacity transmission. With the development of devices and integration technology, the optical fiber communication industry continues to move forward toward the goal of high performance and low cost.

Guest Editorial

The efficient and accurate evaluation of the transmission performance of high-capacity optical communication systems has always attracted significant research attentions. The enhanced Gaussian noise (EGN) model is considered as an excellent solution to predict the system performance taking into account linear and nonlinear transmission impairments. Since the conventional form of the EGN model is complicated and intractable for a fast computation, the closed-form simplification has been regarded as a direction to significantly reduce the computational complexity. However, the accuracy of such a closed-form EGN model becomes a main concern in the application of ultra-wideband optical communication systems. In this work, we have investigated the accuracy of the closed-form EGN model for ultra-wideband optical fiber communication systems, where the performance of the system using electronic dispersion compensation, multi-channel nonlinearity compensation and full-field nonlinearity compensation has been evaluated in terms of symbol rate, number of channels and signal power. Our work will provide an insight on the application of the EGN model in next-generation ultra-wideband long-haul optical fiber communication networks.

Characterization and Modeling of SAGCM InP/InGaAs Avalanche Photodiodes for Multigigabit Optical Fiber Communications

Optical signal to noise ratio (OSNR) is a critical performance parameter of an optical fiber communication system. A method for OSNR monitoring based on coherent optical sampling technology is presented. The estimated error of a 10-GBd QPSK signal is <1 dB with the actual OSNR in range of 12 to 30 dB. The influence of uneven spectrum of a local oscillator on the measured accuracy is investigated, and a compensated algorithm for such unevenness is provided. Our method has the advantage of lower complexity and can be extended to higher symbol rate communication system requiring no high bandwidth ADC hardware.

Stimulated Brillouin scattering (SBS) is one of important nonlinear effects in optical fiber communication systems. Due to its low threshold, it limits the optical power that couples into the fiber. In this paper, Stimulated Brillouin scattering and the effect of phonon loss to SBS in optical fiber communication systems are investigated. In experiments we measure first the power of backward Stokes wave and output light under various input powers of the systems with different lengths of fibers. Then for observing the effect of the phonon blocker to the SBS threshold, we repeat the same experiments but with a phonon blocker inserted at different locations of the fiber link. The experiments indicate that adding phonon blocker to the fiber is able to increase the SBS threshold, and the blocker at different locations makes different effects.

At present, optical fiber communication systems are developing in the direction of large capacity, high speed, long distance, etc., making nonlinear effects and polarization mode dispersion (PMD) and other optical fiber performance defects that were negligible for low-rate systems become the main factors limiting system upgrades and transmission distances. Polarization mode dispersion is an important factor affecting the performance of optical fiber communication systems, and it has become one of the main obstacles to the development of high-speed and long-distance optical fiber communication systems. It can cause pulse widening in the digital transmission system, thereby limiting the transmission rate of the system. Therefore, the influence of polarization mode dispersion on the transmission system and the compensation method are analyzed in this paper.

Optimizing 10 Gbps optical communication system with duty cycle selection of return to zero pulse

We propose a method of modulation format identification based on compressed sensing using a high-order cyclic cumulant combined with a binary tree classifier. Through computing the fourth-order cyclic cumulant of the pretreated band signal, which is obtained by compressed sensing with the sampling rate much less than the Nyquist sampling value, the feature vector for classification is extracted. Simulations are carried out in the optical coherent fiber communication system with different modulation formats of multiple phase-shift keying and multiple quadrature amplitude modulation. The results indicate that this method can identify these modulation formats correctly and efficiently. Meanwhile, the proposed method is insensitive to laser phase noise and signal noise.

In the modern world, bandwidth and data rates are two most important parameters under consideration in any communication system. An optical fiber communication system is capable of transmitting the data at high bit rates and has a large bandwidth capacity. This high data rate and bandwidth capacity in an optical fiber communication system is further enhanced by using wavelength division multiplexing (WDM) in conjunction with the information transmission system, in which information from multiple sources are transferred using the same fiber, at the same time, but at different wavelengths. But the high capacity and data rates in optical WDM system is limited by nonlinear effects that occur in optical WDM system, which not only pose a limitation on the channel capacity and data rates but also degrades the performance of the data transmission system. In this paper, analysis has been done to investigate the efficiency of Four Wave Mixing (FWM) effect under the influence of different system parameters such as chromatic dispersion coefficient, channel spacing, transmission power level, effective area of fiber and fiber length in the form of input signals and output signals frequency spectrums.

The design, fabrication, optical and mechanical characteristics, and reliability of several kinds of the latching-type single-mode-fiber (SMF) switch are reported. Practical applications of these switches to optical fiber communication systems and optical testing and sensing systems are also demonstrated. In the developed switches, a movable fiber is actuated by the magnetic attraction force produced between permanent magnets and a micromagnetic-alloy pipe fixed around the fiber. The ultralight micropipes (0.13 (i.d.)-0.18 (o.d.)/spl times/5/sup L/ mm, 400 /spl mu/g) were successfully massproduced by a fiber-micromachining technique based on metallizing and etching of sacrificial fibers. We used a refractive-index-matching oil in the switches to reduce insertion loss and reflected light. The several hundred fabricated 1/spl times/2 standard SMF switches exhibited excellent optical characteristics, such as typical insertion loss of 0.36 dB, return loss of 49 dB and crosstalk of -70 dB. They exhibited stable latching-type operation with loss changes of below 0.1 dB during 10/sup 5/ switching operations with minimum driving power of 9 mW and switching time of 2 ms. The switches were subjected to endurance (10/sup 8/ switching operations), heat cycles, shock and vibration, and long-term (6.5 years) intermittent operation tests, and revealed no significant optical or mechanical degradation. A 1/spl times/8 latching-type SMF switch was constructed using seven 1/spl times/2 switches cascaded in three stages and uniform insertion losses of below 0.7 dB and return losses of over 47 dB at each of eight ports were obtained. The 1/spl times/2 and 1/spl times/8 switches have been applied to NTT's optical communication systems. We fabricated a compact and cost-effective wavelength switch by installing microfilter chips in the 1/spl times/2 switch and confirmed stable latching-type wavelength switching between 1.3- and 1.55-/spl mu/m optical signals. We also designed and fabricated a polarization-maintaining switch using polarization maintaining and absorption reducing (PANDA) fibers and confirmed stable polarization maintaining switching operations. These switches are applicable not only to optical fiber communication systems, but also to optical testing and sensing systems.

We study the effects of chaos attacking on optical fiber communication systems. We analysis the effects of chaos masking interference (CMI) and chaos phase interference (CPI). In CMI, the larger chaotic signal can mask fully the message signal and the smaller chaotic signal can also interfere the message signal by the state superposition-couple principle. In CPI, the characteristics of the chaotic signal can overprint the phase of the message signal by the cross-phase modulation so that chaos can interfere the message signal. At last, optical fiber communication systems with bit rates of 2.5 Gbit/s, 5 Gbit/s and 25 Gbit/s are simulated in CMI and CPI, respectively, to find that chaos can interfere effectively optical fiber communication systems.

Study on the regulation of amplitude for the optical soliton through nonlinear effects in the optical communication system

Nonlinear effects have been considered as the major limitations in coherent optical (CO) fiber transmission system. Digital signal processing (DSP)-based CO receiver with digital backpropagation (DBP) method has recently facilitated the compensation of fiber nonlinear impairments as well as compensating dispersion of optical fiber. In this paper, for the first time, a comprehensive design is presented for a newly introduced DSP-based CO receiver with the 8-quadrature amplitude modulation (8-QAM) circular-polarization division multiplexed (CPDM) fiber optic communication (FOC) system to investigate the impact of nonlinearities. We have examined the performance of nonlinearity compensating DSP-based CO receiver with DBP method and demonstrated that it can be effectively employed to mitigate the intrachannel nonlinearities in CPDM 8-QAM FOC system over 800-km SSMF link. Accordingly, by effectively compensating the fiber nonlinearities, we have analyzed the performance of bit error rate, optical signal-to-noise ratio (OSNR), optimum launch power, and investigate the link margin by evaluating OSNR margin for a specific launch power. Moreover, a CPDM technique helps as a very suitable means of maximizing the link capacity as well as enhancing the spectral efficiency of the FOC system.