Signal linewidth broadening due to fibre nonlinearities in long-haul coherent optical fibre communication systems

Signal spectral linewidth broadening due to the Kerr effect in long-haul coherent optical fibre communication systems is examined theoretically and experimentally. By recirculating loop experiments, it is shown that linewidth broadening becomes serious in a system of more than a few thousand kilometres.<>     

Gain characteristics of coherent optical amplifiers using aMach-Zehnder interferometer with Kerr media

This paper investigates the gain characteristics of coherent optical amplifiers that amplify only one of two quadrature phase components in an input signal light according to the phase of pump light. It is constructed around a Mach-Zehnder interferometer with Kerr media. To heuristically obtain the design parameters of the coherent optical amplifier, small-signal analysis is adapted to the Mach-Zehnder interferometer with Kerr media. The theoretical results are then compared to experimental results and shown to agree well, which confirms the validity of this design approach. By utilizing low-loss and high-nonlinearity silica fiber as the Kerr medium and optimizing its length, a coherent optical amplifier is constructed that yields high-gain (up to 26 dB) operation     

相干光纤通信系统对激光器线宽的要求

通过仿真相干光接收机载波相位估计性能,研究了采用不同调制格式时对所使用激光器线宽的要求,分析了16-QAM采用Star和Square两种不同星座图时线宽要求差异的原因.仿真结果表明:对于既定的比特速率,采用高阶调制格式的系统降低了线路符号速率,但要求激光器线宽更窄;同一调制格式对激光器线宽的要求与所采用的星座图有关.     

Long-haul coherent optical fiber communication systems usingoptical amplifiers

Studies on long-haul coherent optical fiber communication systems with in-line optical amplifier repeaters are made theoretically and experimentally. By theoretical calculation it was found that coherent systems can achieve wider dynamic range for an amplifier input power as compared with the intensity-modulation direct-detection (IM-DD) systems. The feasibility of such systems using traveling-wave semiconductor laser amplifiers (TWSLAs) and erbium-doped fiber amplifiers (EDFAs) was investigated, and 546 km, 140 Mb/s CPFSK transmission using TWSLAs and 1028-km, 560-Mb/s CPFSK transmission using EDFAs were successfully demonstrated     

Change of field spectrum of signal light due to fibre nonlinearities and chromatic dispersion in long-haul coherent systems using in-line optical amplifiers

The change of a field spectrum of signal light in long-haul coherent optical fibre communication systems using in-line amplifiers is measured in the normal and anomalous dispersion regions of a fibre. Results show that the interplay of nonlinear Kerr effect and fibre chromatic dispersion cause a fatal change in the field spectrum in the anomalous dispersion region.<>     

Power penalties due to multiple Rayleigh backscattering in coherenttransmission systems using in-line optical amplifiers

The power penalties due to double Rayleigh backscattering are calculated for coherent lightwave systems with optical in-line amplifiers. It is shown that coherent systems are less sensitive to Rayleigh noise than high-speed direct-detection systems. For PSK heterodyne systems, the maximum gain of in-line amplifiers that can be achieved without optical isolation is limited to about 25 dB     

Crosstalk penalities due to coherent Rayleigh noise inbidirectional optical communication systems

The statistical properties, i.e. power spectral densities and probability density functions, of the noise due to Rayleigh backscattered light and due to interference of backscattered and reflected light are derived from the autocorrelation function of the optical source field. The crosstalk penalties due to Rayleigh backscattering in single- and double-source bidirectional systems are calculated. It is shown that it is possible to realize a bidirectional system with commercially available connectors in which Rayleigh backscattering is the dominant crosstalk contribution. The measured crosstalk penalties and bit-error probabilities in a double-source bidirectional system confirm the theoretical results     

The effect of laser linewidth on coherent optical receivers with nonsynchronous demodulation

Recent theoretical analysis of the effect of using lasers with significant linewidths in coherent optical fiber transmission systems has shown how the design of the optical receiver, particularly the IF stage, affects the sensitivity. This paper reviews this theory for two classes of coherent systems-those making a differential phase measurement, and those not using phase information-where the requirements on laser linewidth are not stringent. We discuss the factors that affect the performance of systems with significant laser phase noise. Wherever possible, we compare the theoretical results with published system experiments. We show that our theory fits the experimental data well and indicates why experimental results have fallen short of the theoretical limits.     

The performance limit of coherent OTDR enhanced with optical fiberamplifiers due to optical nonlinear phenomena

This paper theoretically and experimentally clarifies the limit of incident optical pulse power in coherent optical time-domain reflectometry (C-OTDR) enhanced with optical fiber amplifiers. The critical pulse power, at which the performance of C-OTDR is degraded by the effect of optical nonlinear phenomena in a single-mode optical fiber, depends on the amplified optical pulse waveform and the pulse width. For a pulse width of 1 μs or longer, the incident pulse power is limited by the effect of self-phase modulation (SPM). When an optical pulse having a power gradient within the pulse width is incident to a single-mode optical fiber, the optical frequency of the backscattered signal is shifted by SPM, and the center frequency of the signal moves outside the receiver band, so the sensitivity of C-OTDR is degraded. For a pulse width of 100 ns, the incident optical pulse power is limited by four-wave mixing (FWM) which transfers the energy from the incident optical pulse to Stokes and anti-Stokes light as a result of the interaction between the incident optical pulse and amplified spontaneous emission. This paper also demonstrates the high performance of C-OTDR enhanced with EDF A's with 48, 44, 39, and 29 dB single-way dynamic ranges for pulse widths of 10 μs, 4 μs, 1 μs, and 100 ns, respectively, limited by the effect of SPM or FWM. These results are believed to be the best performance of C-OTDR with EDFA's     

High-speed DPSK coherent systems in the presence of chromaticdispersion and Kerr effect

The error probability for a single-channel coherent optical differential phase-shift keying (DPSK) transmission system based on repeaterless links in the presence of fiber chromatic dispersion and Kerr effect is evaluated. An accurate model for both the optical signal propagation and the probability distribution of the receiver decision variable is obtained by using a numerical solution of the nonlinear Shrodinger equation and the characteristic function method. The results show that the selection of an optimized IF filter bandwidth is crucial to obtaining the best system performance. When chromatic dispersion dominates, the best performance is achieved in the normal dispersion region whereas when the Kerr effect has the most limiting effect on system performances, the lowest error probability is attained in the anomalous regime. The maximum link length is limited by the presence of Kerr effect, independently of the amount of transmitted optical power, to be shorter than a threshold length if an error probability of 10^-9 is to be achieved     

Performance degradation due to cross-phase modulation in optical fdm ask coherent detection systems

The performance degradation due to cross-phase modulation in optical FDM ASK coherent detection systems has been investigated in this paper. The effect of modulation statistics and phase tracking of the receivers is taken into account. Based on the theoretical treatment, power penalty and allowable launched power per channel have been estimated.     

Optical Kerr shutter using organic nonlinear optical materials in capillary waveguides

Fundamental experiments conducted on optically gated optical switches using organic materials with large optical nonlinearities are reported. An optical Kerr shutter was constructed, whose main part is a long-length and miniature-core glass capillary, filled with a solution of charge transfer organic material of 4-(N,N-diethylamino)- beta -nitrostyrene (DEANST). The capillary waveguide made it possible to enlarge the medium length and to reduce the beam diameter, realizing about 10/sup 4/ times greater efficiency than the conventional CS/sub 2/ optical Kerr shutter using a 1-mm thick cell. A response time of 4 ps was observed and gating power was reduced to 10 W.<>     

Transfer characteristics of Brillouin fibre amplifiers for use in self-homodyne coherent optical transmission systems

Presents for the first time, the basic input-output characteristics of a Brillouin fibre amplifier, and shows that these are in good agreement with the first experimental results reported recently by other workers on a novel and potentially very important application of Brillouin amplification in a coherent optical fibre transmission system. For this type of application, theoretical results are also presented on the optical pump power dependence of the amplifier gain per unit optical pump power.<>     

Forward-scatter frequency-derived distributed optical fibre sensing using the optical Kerr effect

A novel technique is proposed for forward-scatter frequency-derived distributed sensing along a birefringent singlemode optical fibre. This uses a pump probe scheme to generate a signal frequency in forward-scattered light by means of the optical Kerr effect. The sensing action has been demonstrated by applying transverse pressure to a section of the fibre.<>     

Liquid-core optical fibre for voltage measurement using Kerr effect

Liquid-core optical fibre for voltage measurement, based on the Kerr effect, is shown. The measurements are performed for 50 Hz alternating voltage, i.e. for 23?130 kV/m electric fields.     

OTDR in optical transmission systems using Er-doped fiber amplifiers containing optical circulators

Fault location in optical amplifier transmission systems is described. Optical time-domain reflectometry (OTDR) cannot be used for an optical transmission line containing traditional Er-doped fiber amplifiers (EDFAs) because they contain optical isolators. The authors propose an OTDR scheme that uses new EDFAs containing optical circulators and return transmission lines. The new EDFAs support both OTDR and digital signal transmission. A 280.9 km transmission line containing three of the proposed EDFAs was constructed and tested. Experimental results demonstrated the feasibility of OTDR fault location and 1.8-Gb/s digital signal transmission.<>     

Weakly coherent optical systems using lasers with significant phasenoise

Current theories for coherent receiver performance as affected by phase noise are compared, the main prediction of these theories are highlighted, and the reasons why many coherent system experiments have failed to approach the level of performance which is theoretically possible are considered. Different theoretical predictions are compared under idealized receiver conditions concerning the appropriate IF bandwidth, the penalty in receiver sensitivity resulting from laser phase noise, and other system properties. In general, good agreement is found. It is shown how these results differ for realistic receiver parameters, and optimized front-end design using tuning of parasitics is suggested     

Limitations in long haul IM/DD optical fibre systems caused by chromatic dispersion and nonlinear Kerr effect

The power penalty induced by dispersion, Kerr nonlinearity and attenuation is numerically investigated for long haul IM/DD systems with cascaded optical amplifiers. The authors demonstrate that the eye opening is critically dependant on the input mean power.<>     

Bit-dependent degradation caused by stimulated Brillouin scatteringin CPFSK coherent optical repeaterless systems with booster amplifiers

An interesting phenomenon due to stimulated Brillouin scattering (SBS) is experimentally found in a CPFSK heterodyne system with a booster amplifier. We find that the demodulated waveform amplitudes of just a few distinguished bits are strongly suppressed leading to a significant increase in BER. It is confirmed that the BER degradation can be reduced by adjusting the threshold voltage bias of the receiver decision circuit