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     

Semiconductor laser optical amplifiers for multichannel coherentoptical transmission

The application of semiconductor laser optical amplifiers in multichannel coherent optical transmission systems is investigated. The amplifiers considered (λ=1.3 μm) exhibit a gain of 24 dB at a grain ripple <2 dB and a 3-dB bandwidth of about 4000 GHz. The characteristics of these amplifiers and transmission experiments with these amplifiers are described. The investigations concern noise accumulation in an amplifier chain, generation of echoes due to backward gain in cascaded amplifiers, crosstalk in multichannel transmission, and the effect of gain saturation due to spontaneous emission. A good fit is shown between the advantages of multichannel coherent optical transmission systems and the properties of semiconductor laser optical amplifiers, which are very promising for future long-haul optical transmission systems     

Over 1000 km FSK heterodyne transmission system experiment usingerbium-doped optical fiber amplifiers and conventional single-modefibers

The influence of spontaneous emission noise on coherent transmission systems using multistage erbium-doped optical fiber amplifiers is experimentally examined. A frequency-shift keying (FSK) heterodyne transmission experiment was successfully performed at 560 Mb/s through 1028 km of fiber using ten cascaded fiber amplifiers and conventional single-mode fibers with a zero dispersion wavelength of around 1.3 μm. In the experiment, no transmission penalty due to accumulated spontaneous emission noise or to fiber chromatic dispersion was observed     

Single-mode fiber components

A variety of optical components have been made directly from single-mode fibers. These components can be combined to construct in-line all-fiber systems such as fiber gyroscopes, fiber Raman lasers, local area networks, or coherent lightwave systems. This paper reviews the present state of passive and active single-mode fiber components. Passive components are polarizers, directional couplers, filters, and Faraday rotators. Active components require external control or optical power and include modulators, polarization controllers, frequency shifters, and amplifiers. Polarization is usually important in fiber components and discussions of polarization-maintaining and ordinary single-mode fibers are included.     

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     

High-capacity coherent lightwave systems

Recent theoretical work on coherent optical detection systems is reviewed and experimental results in high-speed coherent transmission are summarized. The theoretical advantages and limitations of the various modulation and detection formats are discussed and experimental progress towards the implementation of these systems is reviewed. The most significant obstacles to the attainment of quantum-noise limited detection at higher speeds are seen to be the requirement of uniform frequency response from electronic components and the local oscillator laser power requirement, which increases as the square of the bit rate. To make full use of the single-mode fiber bandwidth, frequency-division multiplexing of many moderate-rate channels is a very promising technique for local systems. For long-distance applications, frequency multiplexing is still possible but is limited by the need for optical amplifiers or wavelength-selective multiplexers     

Lightwave systems with optical amplifiers

Fiber-optic communication systems using semiconductor laser amplifiers are investigated theoretically and experimentally. The noise and bit-error-rate characteristics of lightwave systems with optical amplifiers are calculated and the dependence of system performance on amplifier characteristics such as optical bandwidth, noise figure, gain, etc., is shown. Experimental results for both a 4-Gb/s optical preamplifier as well as coherent and direct detection systems with four inline amplifiers are presented     

IM-DD long-distance multichannel WDM transmission experiments usingEr-doped fiber amplifiers

We have demonstrated multichannel wavelength-division multiplexed optical signal transmission experiments employing Er-doped fiber amplifiers. Four-channel transmission of 459 km and 1500 km experiments and two-channel transmission of 4550 km experiment are described, and the transmission penalty is analyzed. We also discuss the effect of four-wave mixing. The difference of the transmission fiber clearly shows the difference of the optical spectrum due to four-wave mixing after long-distance transmission. The results show the possibility of long-distance multidestination optical communication systems with wavelength-division multiplexed signals combined with Er-doped fiber amplifiers     

Optimum configuration for cascaded fiber amplifiers in attenuationlimited transmission systems

Fiber amplifiers play an important part in optical transmission systems to overcome the impact of attenuation. Together with the transmitter and receiver the positions of amplifiers on the optical path as well as the design of the amplifiers itself determine the bit error probability of the digital communication link. We present a simple method how to derive the optimum configuration for an optical transmission link with cascaded fiber amplifiers for an attenuation limited system. The bit error probability is calculated in dependence of the positions of the line amplifiers and the lengths of the doped fibers for systems with and without booster and optical preamplifier. In a first step we search the optimum configuration by numerical minimization of the bit error probability for a given transmission length, transmitter power and optical receiver. We have found a very simple rule how to determine the lengths of the transmission fibers and the doped fibers of the amplifiers for the minimum bit error probability when the length of the first transmission fiber is given. Therefore, the search for the optimum configuration reduces to the search of the appropriate length of the first transmission fiber which results in an enormous reduction of computing effort. We have investigated the effect on the bit error probability when one deviates from the optimum configuration. For the case, when the transmission length is reduced me have found that the bit error probability decreases always, when the lengths of the single transmission fibers are cut     

Transmission limitations due to self-phase modulation in opticalPSK heterodyne detection systems employing chromatic dispersionequalization

Even if the amplitude of a phase-modulated optical signal is constant before transmission, amplitude modulation is caused by fiber chromatic dispersion. As a result, self-phase modulation (SPM) is induced. In optical heterodyne detection, SPM cannot be compensated for by the delay equalizer (electrical domain) used to compensate fiber chromatic dispersion. However, the transmission distance limitation of multi-repeatered coherent transmission systems has not been investigated in the presence of SPM. This paper theoretically and experimentally investigates the transmission distance achievable with a phase-shift-keying (PSK) heterodyne detection system employing in-line optical amplifiers and delay equalization. The calculated results show that equalization is effective when γP₀/2B²|β₂|<10 in the normal dispersion regime, and γP₀/2B²|β ₂|<15 in the anomalous dispersion regime. Furthermore, the increase in transmission distance achieved by using equalization is experimentally shown in an 8 Gb/s PSK heterodyne transmission experiment using a conventional single-mode (SM) fiber and in-line fiber amplifiers     

Theory of signal degradation in semiconductor laser amplifiers indirect detection systems

Theoretical studies are carried out on long haul direct detection optical fiber communication systems, with inline optical semiconductor amplifier repeaters. Calculations are made of the noise, eye diagrams, and bit-error-rate characteristics of lightwave systems with optical amplifiers. Indications are given of the effect of amplifier characteristics such as spontaneous noise and signal distortion due to gain saturation on the system performance. The nonlinear process within semiconductor laser amplifiers leads generally to pulse amplitude-temporal distortions due to gain saturation. This theoretical study demonstrates that the system penalty caused by these nonlinear effects appears progressively as the optical input power at each amplifier is increased. For example, nonregenerated fiber transmission using traveling wave semiconductor laser amplifiers was simulated, and results obtained at 0.5 and 2.5 Gb/s are presented. In order to improve the system performance, the influence of structure and bulk dimensions of the amplifier cavity is also considered     

Trunk and distribution network application of erbium-doped fiberamplifier

Recent research on erbium-doped fiber amplifiers and their applications to trunk and distribution networks is described. Simple methods to estimate amplification performances from only three basic erbium-doped fiber parameters are proposed. It is clarified that optical amplifiers greatly influence the development of future optical communication systems     

Optical time domain reflectometry on optical amplifier systems

The use of optical time-domain reflectometry (OTDR) on long-span fiber transmission systems containing in-line optical amplifiers is discussed. Having identified the specific requirements for OTDR equipment, measurements were carried out on systems of up to 300 km in length, containing three semiconductor laser amplifiers. The results demonstrate that OTDR can be used not only for fault location on fiber links several hundred kilometers in length but also as an alternative to standard system supervisory techniques, thus providing the potential for minimizing the hardware in the optical amplifier stages of future long-span fiber optic transmission systems     

Optical amplifiers transform long-distance lightwavetelecommunications

Optical amplifiers and wavelength-multiplexing technology are transforming lightwave communications by providing cost-effective upgrades that will increase immensely the transmission capacity of long-distance telecommunications networks. A new generation of undersea cable systems using fiber optical amplifiers as repeaters has been developed for transoceanic applications, yielding a capacity almost ten times larger than conventional systems using opto-electronic regenerators. Terrestrial long-haul networks will benefit significantly from amplified wavelength-multiplexed transmission systems designed to access the large inherent bandwidth in the installed fiber. Successful deployment of these advanced systems requires a thorough understanding of optical amplifiers and the optical fiber medium, as their requirements interrelate through optical bandwidth, noise, dispersion, optical nonlinearities, and their impact on signal transmission. While the first commercial WDM amplified lightwave systems are deployed for point-to-point applications, optical transparency and wavelength multiplexing will be exploited for networking leading to the higher functionality and improved cost-effectiveness expected of photonic networks     

Multi-stage erbium-doped fiber amplifier designs

Applications for erbium-doped fiber optical amplifiers have been multiplying in recent years, and increasingly complex structures have been introduced. We review characteristics and design parameters, the use of interstage elements, and various digital transmissions systems applications for multi-stage amplifiers. A notation is introduced to describe multi-stage fiber amplifiers     

Propagation of signal and noise in concatenated erbium-doped fiberoptical amplifiers

Signal propagation and noise accumulation in lightwave systems using saturated optical amplifiers as repeaters are analyzed. Numerical simulations of amplified spontaneous emission in concatenated erbium-doped fiber amplifiers indicate that a reach beyond 10000 km is possible with a 1.55-μm system in the absence of fiber nonlinearities. Distributed optical amplifiers are shown to have low noise, but require higher pump power than lumped amplifiers. Three operating modes of an amplifier lightwave system are identified and their relative signal power efficiency and noise performance are described     

Coherent reflectometry of optical fiber amplifiers

The reflectivity of commercially available erbium-doped fiber amplifiers (EDFAs) was measured with coherent optical frequency domain reflectometry (C-OFDR). Reflections at the output isolator as well as the distributed gain along the erbium-doped fiber could be observed thanks to the high amplified spontaneous emission (ASE) rejection due to the coherent detection. Gain figures obtained with the OFDR technique are in good agreement with single-pass direct gain measurements.     

Novel chromatic dispersion measurement method over continuousGigahertz tuning range

A method for the measurement of chromatic dispersion in a single-mode optical fiber is proposed and demonstrated. This method adopts the technique of external modulation of the distributed feedback (DFB) laser output and direct optical frequency sweep range monitoring by coherent heterodyne detection. The method is also applicable to the future optical communication system with narrowband optical elements such as optical amplifiers, whereas measurement is impossible given the present state of the art     

宽带增益平坦光纤Raman放大器的研究进展

宽带增益平坦光放大器是密集波分复用光通信系统提高信息容量的基本需求。在总结宽带掺铒光纤放大器的基础上 ,介绍了展宽光纤Raman放大器平坦增益带宽的基本方法和研究现状。     

光孤子通信系统（上）

本讲座介绍光孤子通信系统的概貌［１］，着重叙述光孤子的物理概念及其特性、光纤损耗对光孤子的影响以及对光孤子放大怎样补偿光纤损耗，从而使光孤子传输数千公里后仍保持波形和幅度不变。