ThA3FOM W-shaped DCF. Fig. 1. Attenuation and dispersion spectra of high s 8 "E 0 N E Dispersion (pdnmlkm) ThA3 Fig. 2. Dispersion value versus n2 value.were demanded for DCF. Large FOM was requested to decrease optical signal-to-noise ratio (SNR). High negative dispersion was required to shorten the necessary length for compact packaging. Table 1 shows characters of DCFs optimized for above requirements. High negative dispersion (-163 ps/nm/km) and high FOM (323 ps/nm/dB) were realized with W-shaped index profiles.Recently, WDM technique has been studied actively for higher bit-rate transmission such as 10 Gbit/s system etc. For this technique, slope compensation and suppressed nonlinearity were desired. In order to discuss the flat compensation at a wide wavelength range, a dispersion slope compensation rate, shown with the Eq. (l), is proposed,where slope was defined as the difference of dispersion values from 1.53 pm to 1.56 pm divided by a wavelength interval. The nearer to 100% this rate is, the higher the compensating efficiency is. Table 1 also shows the compensating rate of DFCF. Because matched clad DCF could not give negative slope, Wshaped DCF was suitable for WDM transmission. On the WDM transmission, optical power density becomes very large because plural optical signals are inputted into a fiber at the same time. Then a pulse is distorted by nonlinear phenomenon (e.g., cross-phase modulation, XPM; four-wave mixing, FWM; and stimulated Brillouin scattering, SBS), therefore suppression of nonlinearity is important. XPM is the thirdorder nonlinear phenomenon and can be estimated from nonlinear refraction rate (n,) of fiber? We measured nz values of W-shaped DCF and matched clad DCF using XPM method. Figure 2 shows the relation of dispersion values versus n, values of each profile. The n2 values of W-shaped DCF were smaller than those of matched clad DCFs at the same dispersion value, which was caused by the higher negative dispersion of W-shaped profile. In this point of view, W-shaped DCF is also better than matched clad DCF. FWM was not observed due to the large dispersion values of both SMF and DCF. Because threshold value of SBS was bigger than XPM, suppression of XPM was equated with that of SBS.It is concluded that W-shaped index profile is suitable for DCF and adequate for WDM transmission from the following points of view: (1) low nonlinearity, (2) compact module, (3) flat compensation. 1. Y. Akasaka et al., in OFC'95 Technical Digest, paper ThH3. 2. Y. Akasaka et al., in Proc. ECOC'95, paper We.B.2.4. 3. R. H. Stolen et al., in OFC'95 Technical Digest, paper FD1.In this paper we report on measurements of the nonlinear coefficient n2/Aeff (nonlinear refractive index/effective area) of different fibers of interest in transmission systems at 1550 nm: large-effective area fiber dispersion-shifted fiber (LEAF),' dispersion-compensation fiber (DCF), and standard dispersionSynthesizer
Multiwavelength SystemsOver the last four years most of the current North American terrestrial long-haul fiber network has been upgraded to operate at 1.7 to 2.4 Gbfs. Long term trends indicate that fiber data rates double every 2 years. The introduction of 10 Gb/s systems is expected in 1996 and we can predict that there will be a need for 40 Gb/s by the end of the decade.In the past, increasing the data rate (TDM) has been the method of choice to reach higher fiber capacities, but now systems that use multiple wavelength channels (WDM) at both 2.4 and 10 Gb/s are being considered [ 11. This change has been driven by a number of factors the most critical of which has been the introduction of the Er-doped fiber amplifier (EDFA). A single EDFA, in contrast to an optoelectronic repeater, can ampllfy multiple wavelength channels. Secondly, the maximum fiber dispersion that can support a given data rate distance product goes down as the square of the data rate. Most of the installed fiber network is unshifted fiber which has a dispersion of between 15-20 ps/nm km in the 1550 nm window. With this dispersion, 2.4 Gb/s can be carried over 500 km, but 10 Gb/s transmission will require dispersion compensation. At 40 Gb/s the dispersion tolerance will be so small that dispersion compensation may even be required for dispersion-shifted fiber. Also, in a TDM system at an add-drop point, all of the traffic must be processed and the add-drop must operate at the trunk-line rate. For a WDM add-drop, only one wavelength need be dropped while the bulk of the traffic is untouched and the add-drop electronics need only operate at the individual channel rate. Finally, in a WDM system the upgrade to higher fiber capacity can be gradual as individual wavelengths are added only as needed on a particular route. Multiwavelength AmplifiersThe shape of the gain spectrum is critical for a multiwavelength amplifier. The S N R of the low gain channels w i l l limit the number of amplifiers that can be cascaded. In Er-Al-silica, a reasonably flat gain spectrum can be obtained between 1540 and 1560 nm. To go below 1540 nm requires a gain shaping filter. Other requirements on the amplifier are: high gain, to support long spans between amplifiers, efficient use of pump, so that many wavelength channels can be camed, and low noise, so that the amplifers can be cascaded. The combination of all of these requirements favors 980 nm pumping and requires the use of multi-stage amplifier designs. We have built a multi-stage amplifier for multi-channel amplification between 1549 and 1561 nm. Over that range there is less than a dB of gain variation with 34 dB gain. Operating with 34 dB gain, 72% of the pump power delivered to the gain fibers is converted into signal, and the noise figure is 4.2 dB.Numerical modeling can be used to study the behavior of multiwavelength amplifier cascades. Figure 1 shows the predicted optical path Q, in dB, for each of 8 channels at the end of a 500 km system with 4 amplifiers. The Q values are all above 25 dB and they vary ...
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