Lightwave primer

This paper presents an introduction to the principles of lightwave system engineering. The treatment is historical rather than categorical-lightwave systems are described in terms of their evolution through four generations of technology, from a first generation operating at 0.85 μm wavelength over multimode fiber to a fourth generation employing coherent techniques at 1.55 μm. Basic engineering considerations such as fiber dispersion and receiver sensitivity are introduced early, then refined as the discussion progresses toward higher-performance, more sophisticated systems. The fundamental mechanisms that limit the performance of a given technology are quantified, and a figure of merit, the product of bit rate times maximum repeater spacing, is estimated. Values of this product range from about 2 Gbits/s . km for first-generation technology to roughly 900 Gbits/s . km for coherent systems.     

Multi Gbit/s bit parallel WDM transmission using dispersion managedfibers

This paper demonstrates the feasibility of a bit parallel WDM (BP-WDM) system using dispersion managed fibers. An expression for the total bit skew as a function of the fiber dispersion and system bandwidth is derived and compared with experimental results. A 4 bit×10 Gbit/s-per bit BP-WDM transmission experiment over 30 km DMF is used and an aggregate bite rate×distance product of 1.2 Tbit/s-km is obtained. The total bit skew of the system is reduced to one half of the bit period. We believe that systems using BPWDM will be useful for computer interconnects in high-speed parallel systems and ring networks     

120 km lightwave transmission experiment at 1 Gbit/s using a new long-wavelength avalanche photodetector

A transmission experiment was performed to study the high-speed potential of a long-distance gigabit lightwave system employing a single-frequency 1.5 ?m laser and a new long wavelength III?V avalanche photodetector. A 120 km length of standard production single mode fibre was used in the test, in which a bit error rate of 2×10?10 was achieved at 1 Gbit/s. This distance is the longest reported to date for rates higher than 500 Mbit/s, and the product of bit rate and distance (120 km Gbit/s) is the highest value achieved for any bit rate. The distance was limited by the 32.3 dB loss of the transmission path together with a total of 5.6 dB in power penalties associated with chirp broadening of the laser line.     

The maximum bit rates of soliton communication systems with lumpedamplifiers and filters in different distances

The maximum bit rate of a soliton communication system with lumped amplifiers and optical filters is considered. When the dispersion of the fiber varies from one amplifier spacing to another amplifier spacing, the maximum bit rate is significantly reduced. To overcome the effect, the amplitude of the soliton is amplified so that it is still the average soliton corresponding to the fiber dispersion for an amplifier spacing. Thus, the maximum bit rate is only slightly less than that without the dispersion variations. For a given distance, the maximum bit rate limited by the stability and soliton interaction is obtained. The result is compared with that limited by the Gordon-Haus effect. For shorter transmission distance, the maximum bit rate is limited by the stability and soliton interaction. For longer transmission distance, the maximum bit rate is limited by the Gordon-Haus effect     

Optimal transmission condition of nonlinear optical pulses insingle-mode fibers

Optimal conditions for transmission of nonlinear optical pulses in single-mode fibers are presented. When an optical pulse propagates in a fiber, it suffers fiber loss, group velocity dispersion, and self-phase modulation. An optimal output pulse can be obtained by choosing a suitable optical carrier wavelength and an initial input pulse. The system under optimal conditions not only has a more stable performance than the dispersion-free system, but also achieves maximum transmission bit rate for a fixed transmission distance. A bit-length product up to 8550 Gb/s-km or more can be achieved by using dispersion-shifted fibers without amplification     

A product-coded WDM coding system

Forward error correction is a feasible approach for reducing the bit error floor of lightwave systems, arising from the fact that a simple single-error-correcting code can reduce the error floor from O(P _e) to O(P_e²). We propose a novel wavelength demultiplexer (WDM) coding system using a product code to improve the performance of WDM systems with an error floor caused by fiber dispersion or system noise     

Design theory of long-distance WDM dispersion-managed transmissionsystem

This paper describes a novel design theory of long distance wavelength division multiplexed (WDM) dispersion-managed optical transmission systems. Assuming that the transmission distance, bit rate, and number of WDM channels are initially known, we investigate the optimum dispersion allocation and input power per channel to achieve the minimum channel spacing. Based on the design guidelines for single-channel and multichannel systems, we establish the optimal design strategy. Details of the design procedure are demonstrated for 2.5-, 5-, and 10-Gb/s 10000 km WDM systems by using computer simulations. Next, we study the impact of the fiber dispersion slope on the usable wavelength span, and show that the attainable capacity of the representative 5-Gb/s 10000 km WDM system employing the postcompensation scheme can not exceed 100 Gb/s. Finally, we propose several techniques to approach the ultimate capacity of the WDM system and show that up to 1 Tb/s (200×5 Gb/s) 10000 km system can be implemented without utilizing the in-line dispersion slope compensation scheme. We also discuss the 10 Gb/s-10000 km WDM system employing in-line dispersion slope compensation     

An all-optical long-distance multi-Gbytes/s bit-parallel WDMsingle-fiber link

An all-optical long-distance (>30 km) bit-parallel wavelength division multiplexed (WDM) single-fiber link with 12 bit-parallel channels having 1 Gbyte/s capacity has been designed. That system functionally resembles an optical fiber ribbon cable, except that all the bits pass on one fiber-optic waveguide. This single-fiber bit parallel wavelength link can be used to extend the (speed x distance) product of emerging cluster computer networks, such as, the MyriNet, SCI, Hippi-6400, ShuffleNet, etc. Here, the detailed design of this link using the commercially available Corning DS (dispersion-shifted) fiber is given. To demonstrate the viability of this link, two WDM channels at wavelengths 1530 and 1545 nm carrying 1 ns pulses on each channel were sent through a single 25.2-km long Corning DS fiber. The walkoff was 200 ps, well within the allowable setup and hold time for the standard ECL logic which is 350 ps for a bit period of 1 ns. This result implies that 30 bit-parallel beams spaced 1 nm apart between 1530-1560 nm, each carrying 1 Gbits/s signal, can be sent through a 25.2-km Corning DS fiber carrying information at a 30 Gb/s rate     

WDM coding for high-capacity lightwave systems

An interchannel parallel coding scheme in the wavelength domain-the WDM coding system-is proposed. The system differs from the usual serial coding systems and provides many advantages. First, data channels are completely unaltered in the coding process, rendering it very suitable for practical lightwave systems with standard bit rate. Second, parallel encoding/decoding systems are simpler than those of serial coding systems, being easier to be implemented in high-speed optical systems. Third, compared with serial coding, WDM coding is able to reduce heavily the number of encoding/decoding pairs. For example, a (15, 11) Hamming coded WDM system reduces the number from 12×11=132 to 1 at the line rate of STS-12. Fourth, the WDM coding system could offer infinite coding gain in dispersion-limited lightwave systems. Finally, WDM coding systems could correct single-channel burst error. The system performance was evaluated and the system limitation imposed by bit-skew among wavelength channels was analyzed. The results indicated that a 15-channel Hamming coded WDM system can reduce the uncoded BER from 10^-9 to 3×10^-17 and the distance limitation imposed by bit-skew is 250 km if a dispersion-shifted fiber is used and a channel span of 30 nm is assumed     

Chromatic dispersion limitations in coherent lightwave transmissionsystems

Computer simulation is used to evaluate the chromatic dispersion limitations for both various coherent lightwave transmission systems and direct-detection on-off keying (OOK) systems. The results show that for a 2-dB dispersion penalty the maximum modulation rate ranges from 5 to 9 Gb/s for systems operating at 1.55 μm with 15 ps/km-nm of chromatic dispersion and 100 km of fiber. The effect is less severe for OOK systems and most severe in coherent detection systems. Simulation results are in agreement with a available experimental data     

Dispersion penalty for 1.3 μm lightwave systems with multimodesemiconductor lasers

The effect of fiber dispersion on the performance of lightwave systems is analyzed for the case where multimode semiconductor lasers operating near the zero-dispersion wavelength of the single-mode fiber are used as sources. Both the intersymbol interference and the mode-partition noise are considered in the discussion of dispersion-induced power penalties. The theory is in agreement with an experiment in which the bit error rate is measured for lasers at various bit rates. The tolerable limits on the deviation of the laser wavelength from the zero-dispersion wavelength are obtained for a 1.3-μm system operating at 1.7 Gb/s. Monte Carlo simulations are used to predict the effect of mode-partition noise on the performance of such high-speed lightwave communication systems     

基于10 Gb/s传输链路的40 Gb/s光传输实验研究

基于中国自然科学基金网(NSFCNet)的400 km×10 Gb/s光传输链路实现了40 Gb/s光传输,没有出现误码率(BER)平台,说明在常规的中短距离10 Gb/s系统可以直接升级至40 Gb/s系统,而不需要升级传输链路。但是,由于相对10 Gb/s系统而言40 Gb/s系统的色散容限非常小,在升级时必须精确补偿原有链路的色散,在接收机前一般需要加可调色散补偿单元。同时,还分析了光纤注入功率对系统性能的影响,结果表明在设计这种由10 Gb/s向40 Gb/s升级的系统时,不仅要考虑信号带宽增加带来信噪比要求的提高,而且必须充分考虑光纤非线性的影响。     

Waveform degradation due to dispersion effects in an optical CPFSKsystem

Waveform degradation due to polarization and chromatic dispersions in a single-mode fiber is calculated for a coherent CPFSK signal. For a single-mode fiber with polarization dispersion of ⩽1 ps, chromatic dispersion almost dominates the system. However, if a fiber has polarization dispersion of more than a few picoseconds and a chromatic dispersion of less than 0.1 ps/km/nm, which can be attained by using a dispersion-shifted fiber and/or by electric dispersion compensation, polarization dispersion will restrict transmission capacity. For instance, polarization dispersion of 5 ps will restrict a bit rate by ~60 Gb/s when chromatic dispersion is fully reduced using a dispersion-shifted fiber or applying electrical equalization     

Signal transmission by optical solitons in monomode fiber

A transmission rate of ≃1 Tbits/sec (≃0.1 Tbits/s) per 30 km can be achieved using envelope solitons with peak power of ≃10 W (≃10 mW) in a monomode optical fiber, respectively. Unlike the linear pulse in which the bit rate is limited by the group dispersion, the bit rate of soliton transmission is limited by the fiber loss and the input power. Conditions for achieving optimum transmission rate using solitons are theoretically obtained including the effects of fiber loss and second order group dispersion.     

Optical fiber-based dispersion compensation using higher ordermodes near cutoff

Higher order spatial modes in optical fibers exhibit large, negative chromatic dispersion when operated near their cutoff wavelength. By using a spatial mode-converter to selectively excite a higher order mode in specially designed multimode fiber, this dispersion can be used to compensate the positive dispersion in conventional single-mode fiber spans. In this paper, issues related to compensating fiber and mode-converter design are explored. Experimental measurements in specially designed two-mode fibers operated in LP₁₁ mode show negative dispersion as large as -70 ps/nm·km at 1555 nm. Pulse propagation and system experiments employing spatial mode-converters to excite LP₁₁ mode in a two-mode fiber demonstrate the feasibility of this technique for dispersion compensation in lightwave systems     

基于不同的非线性前色散补偿技术的比较

对非线性前色散补偿系统中的完全补偿、过补偿和欠补偿进行了数值模拟。数值结论表明：采用非线性前补偿不论是完全补偿、过补偿还是欠补偿，当入纤平均功率为1mW至15mW时20Gbit／s、最大半宽度为20ps的RZ脉冲经过100km的标准单模光纤后均能满足误码率小于10－9的要求。当入纤功率较大时，采用完全补偿经过一个放大距离后Q值最大，而且能够传输的放大周期也最多。     

Spectral width reduction using apodized cascaded fiber Bragg grating for post-dispersion compensation in WDM optical networks

This paper proposes a system that aims to reduce the spectral width, Δλ, of the optical signal at transmitter for WDM system over distance 100 km. Also, a chirped fiber Bragg grating (CFBG) at the receiver is used to compensate dispersion. The proposed system consists of four cascaded FBGs connected between light source and optical fiber. Many apodization functions are investigated to enhance the performance of the FBG in the proposed system, and Δλ is obtained at every stage and apodization function. The Q-factor and bit error rate (BER) are obtained at distances 30, 40 and 50 km. It is found that Cauchy apodization function is the best one that reduces the reflective spectral width, Δλ, and achieves a maximum Q-factor and minimum BER at distances 30, 40 and 50 km at the last stage.

     

Attenuation and bit-rate limitations in LED/single-mode fibertransmission systems

In this analysis, distortion of the spectral distribution of the light due to the wavelength dependence of the loss in the fiber is taken into account. The computations show that the system performance of a given fiber type may be characterized by three parameters, one for attenuation and two for dispersion. The attenuation in the fiber may be modeled by an average attenuation per unit length, whereas the bit rate that limits the sensitivity degradation in the receiver due to intersymbol interference to less than 1 dB may be modeled by a bit-rate distance product and a constant bit rate valid for long lengths of fiber. As a design aid in system planning, diagrams showing the three parameters as functions of the diode center wavelength and the spectral width for typical single-mode fibers have been prepared     

System performance of coherent transmission over cascaded in-linefiber amplifiers

The limitations of cascaded in-line amplifier systems using coherent modulation-demodulation schemes are examined by evaluating the product of the data rate and the transmission distance. The linear amplified spontaneous emission (ASE) accumulation is shown to make the maximum value of the data rate-distance product increase proportionally with the ratio of the amplifier output signal power to the noise figure. It is also shown that the Kerr-nonlinearity-induced phase noise limits the product of the data rate and the third power of the distance, the maximum value of which is inversely proportional to both the amplifier output signal power and the noise figure. The fiber dispersion is known to limit the product of the distance and the square of the data rate by causing waveform distortion. By taking these three relations into account, it is concluded that coherent signal transmission has a maximum in-line amplifier system length of 10³-10⁴ km in the gigabit-per-second range. Among these three factors, the nonlinearity-induced phase noise has the greatest impact     

Performance limits of unrepeatered systems using higher-order codirectional Raman pumping

Higher-order pumping schemes enhance the performance improvement provided by codirectional Raman amplifiers, which is achieved by pushing the maximum of the signal power deeper into the transmission fiber. This paper deals with the performance limits inherent to this technology.Simulation results reveal that the performance of an unrepeatered link operated with 10 Gbit/s NRZ signals increases with growing distance between the location of the maximum power and the fiber input up to distances of 70 km. For larger distances, the system performance starts to decrease. The amount of dispersion precompensation providing optimum system performance increases with increasing distance of the maximum to the fiber input. Furthermore, it is shown that the tolerance to variations of the precompensation is almost independent of the position of the maximum and, thus, the order of the pumping scheme. Nevertheless, the operation of commercial systems employing this kind of technologies becomes more complex since the system becomes more sensitive to variations of the residual dispersion. If the maximum of the signal power is reached after more than ≈50 km of propagation in the fiber, deviations of the residual dispersion from its optimum value exceeding ≈20 ps/nm/km lead to a performance degradation larger than 0.5 dB. However, it is also shown that the sensitivity to deviations of the residual dispersion can be reduced at cost of the resulting performance benefit.