FIELD OF INVENTION
The present invention relates to design of a dispersion compensating optical fiber link suitable for broadband dispersion compensation of Dense Wavelength Division Multiplexed (DWDM) optical signals transmitted through the International Telecommunication Union (ITU) specified standard G.655 signal transmission fibers.
PRIOR ART:
The phenomenal growth of Internet traffic has led to an explosive growth in demand for bandwidth (B W), and this in turn has led to need for higher and higher bit transmission rates, longer distances, and scalability with an eye for future addition of new customers, locations and services. At present, the key for increasing the transmission carrying capacity of an optical fiber lies in employing DWDM transmission by transmitting signals using multiple wavelengths spanning ?a wavelength range of more than 100 nm offered by various optical fiber amplifiers like erbium-doped fiber amplifiers (EDFA) in the band 1530-1610 nm and thulium-doped fiber amplifiers (TDFA) in the band 1480-1530 nm. Terabit transmission has already been demonstrated through this route in a few laboratory scale hero experiments.
Although the advent of fiber amplifiers took care of fiber losses, DWDM transmission brought to the fore two more limitations to fiber optic transmission, namely: chromatic dispersion and nonlinear effects.
Chromatic dispersion amounts to broadening of a temporal (i.e. time-dependent) pulse with propagation through an optical fiber due to its dispersive nature. Pulse broadening can lead to overlapping of adjacent pulses at high bit

rates when the pulses are transmitted too close to each other, due to which the receiver fails to distinguish these digital signals as individual pulses. However, chromatic dispersion is a linear effect, which can be cancelled by inserting another component whose dispersion is equal in magnitude and opposite in sign to that of the signal carrying fiber. The measure of dispersion is a parameter represented by D, which is a function of wavelength, X.
Another important parameter in case of DWDM transmission is the dispersion slope (S), which is the slope of the D versus X curve for a given fiber. For broadband dispersion compensation required in case of DWDM transmission, the dispersion slope of the signal transmission fiber and that of the dispersion-compensating device has to be matched. Amongst the various techniques proposed in the literature for dispersion compensation [Jopson and Gnauck, 1995; Danziger and Askegard, 2001], the most promising is the use of a dispersion compensating fiber (DCF), which is a fiber having a large dispersion coefficient D opposite in sign to that of the signal fiber [Antos and Smith, 1994; Akasaka et al, 1996; Thyagarajan et al, 1996; Auguste et al, 1998; Tsuda et al, 1998; Grunner Nielson et al, 2000; and Srikant, 2001].
On the other hand, nonlinear effects result in the generation of additional spectral components within the transmitted pulse which can lead to increased distortion of pulses, or cross-talk between different wavelength channels in case of multi-channel transmission. Nonlinear effects arise when the optical intensity, which is ratio of optical power per unit area of the propagating beam, becomes large. Such effects are likely to arise in a fiber because of its small cross-section, especially in case of multi-channel propagation, when the presence of several signals increases the net optical power transmitted through the fiber. Light
propagates in a fiber in the form of a guided mode having a fixed transverse intensity distribution determined by its refractive index profile; thus the power in the guided mode is effectively distributed across a region characterized by the mode effective area (Aeff) [Agrawal, 1995].
Thus, designing a fiber with a relatively large Aeff can reduce sensitivity to nonlinear optical effects. Moreover, nonlinear effects like four wave mixing (FWM) and cross phase modulation (XPM) [Agrawal, 1995], which are detrimental to multi-wavelength channel transmission, can be greatly reduced by leaving a small residual local dispersion all along an optical fiber link [Uchida, 2002]. Thus, although chromatic dispersion can lead to inter-symbol interference, nevertheless its absence would lead to severe signal distortion caused by nonlinearities like FWM and XPM. Realization of these facts led to the development of the so-called non-zero dispersion shifted fibers for signal transmission, classified by ITU standards as G.655 fibers, which are designed to exhibit a small dispersion D in the range 2 < D (ps/nm.km) < 6 in the C-band to counter nonlinear effects. There are two variants of G.655 fibers available commercially at present, each satisfying the primary ITU specifications pertaining to the limits set for the dispersion coefficient, but having additional features like a low dispersion slope in case of Truewave-RS™ fiber made by OFS and a large Aeff in case of the LEAF™ (large effective area fiber) made by Corning, meant for facilitating DWDM transmission. In addition, another type of signal fiber has recently been brought out by the company Alcatel under the brand name Teralight™, which is similar to the G.655 variety, having a dispersion ranging from 3-12 ps/nm.km across the wavelength range 1480-1610 nm, whose operating wavelength range has been extended to include the S-band as well.
Although these fibers are suitable for DWDM transmission up to 10 Gbit/s in the C-band without requiring dispersion compensation, but at higher bit rates e.g. 40 Gbit/s, or for operation in the L-band, dispersion compensation would become inevitable in their case as well. A variety of DCFs have been proposed to achieve this objective [Grunner Nielson et al, 2000; Srikant, 2001; Aikawa et al, 2003], but most prior art designs have focused mainly on slope compensation and suffer from drawbacks like a small Aeff, small mode spot-size and large bend-losses. A small Aeff would result in greater susceptibility to nonlinear effects, and mismatch in the spot-size of the DCF and signal fiber would result in increased splice losses. Moreover, bend-loss can become critical in terms of overall insertion loss, since a DCF is normally co-located with the amplifier in the form of a compact spool of small bend diameter. Since a DCF inherently exhibits higher transmission losses due to higher germania (GeO2) content (which is necessary to achieve a negative dispersion coefficient) additional losses therefore need to be minimized in order to achieve greater efficiency and low insertion losses.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 shows the variation of the refractive index profile of the DCFs designed for the G.655 fibers.
Figure 2 shows the transverse cross-section of the fiber. Layers 1, 2 and 3 constitute the core of the fiber, whereas layer 4 is the cladding. Layers 1 and 2 are composed of silica (SiO2) doped with GeO2, whereas layers 2 and 4 are of pure SiO2.
Figure 3 shows the dispersion as a function of wavelength for DCFl, DCF3 and DCF6 designed for the G.655 fibers across the C-band.
Figure 4 shows the dispersion as a function of wavelength for DCF2, DCF4 and DCF7 designed for the G.655 fibers across the L-band.
Figure 5 shows the dispersion of DCF5 as well as the average dispersion of DCF5 + G.655 fiber as a function of wavelength across the S-band. In estimating average dispersion, the ratio of the length of the G.655 fiber to that of DCF5 has been taken as 33.96.
Figure 6 shows the average dispersion spectrum across the C-band of a link comprising of G.655 fiber and DCF. The ratio of the length of the G.655 fiber to that of the DCF is 61.31 in the case of DCF1, 37.45 for DCF3 and 21.74 for DCF6.
Figure 7 shows the average dispersion spectrum across the L-band of a link comprising of G.655 fiber and DCF. The ratio of the length of the G.655 fiber to that of the DCF is 23.4 in the case of DCF2, 27.78 for DCF4 and 14.18 for DCF7.
DESCRIPTION OF THE INVENTION:
The present invention relates to a dispersion compensated broadband optical communication link operating in the optical amplifiers bands (S-band (1480-1530 nm), C-band (1530-1565 nm), and L-band (1570-1610 nm), the said link comprising a signal fiber and a dispersion compensating fiber, wherein the said signal fiber has a dispersion coefficient D greater than 2 ps/nm.km and dispersion slope S greater than 0.05 ps/nm2.km in the operating amplifier band, and the said dispersion compensating fiber consists of a segmented core having three layers to attain a negative dispersion coefficient and negative dispersion
slope targeted to achieve broadband dispersion compensation of said signal fiber with low sensitivity to nonlinear optical effects through a large mode effective area Aeff, a substantially large FOM (Figure of Merit), and low sensitivity to bend loss.
The main objective is to design a dispersion-compensated broadband fiber optic link comprising a G.655 signal fiber and a segmented core DCF. The primary requirement for this is to design a DCF having a negative dispersion coefficient that can cancel the dispersion accumulated by signals propagating through G.655 fiber across the S-, C-, and L-bands of the optical fiber amplifiers. Due to the wide variety of commercially available G.655 fibers, we have optimized our DCF design separately for all the three varieties of standard G.655 fibers mentioned earlier.
In order to cancel the accumulated dispersion of all the different signal wavelengths within the operating wavelength range by means of the same length of dispersion compensating fiber, the slope of the DCF has to be negative (i.e. opposite to that of the G.655 fibers) and the following condition has to be satisfied
Kdcf=Ktx,
where Kdcf = Ddcf/Sdcf, Ktx = Dtx/Stx, and Ddcf, Sdcf, Dtx, Stx being the dispersion coefficients and slopes of the DCF and the signal transmission fiber, respectively. In practice, it is extremely difficult to satisfy the above condition across the entire operating wavelength region spanning an amplifier band. However, since most receivers can tolerate a small amount of residual dispersion, and also since the D versus wavelength curve is almost linear for the G.655 fibers across the amplifier band, one typically targets a match in the K values roughly around the median wavelength of the operating band. In order to achieve this objective, we have chosen the segmented profile for the DCF as shown in Figs. 1 and 2. The presence
of the annular ring of germania doped silica (third layer) is responsible for achievement of a negative dispersion slope and the various profile parameters are adjusted to achieve the best possible match for the k values. In addition to this, we have also tried to maximize the negative dispersion coefficient D and mode effective area Aeff of our designed DCF. A larger magnitude of D ensures that a smaller length of the dispersion compensating fiber would be required for canceling the dispersion of a given length of the signal fiber. The choice of an intermediate layer of pure silica (as opposed to fluorine doped silica used in prior art DCF designs) ensures a reasonably large Aeff and low bend-loss sensitivity. And finally, we have also targeted achievement of a mode spot size (p) that is as close as possible to that of the relevant signal fiber.
Table 1 lists some important performance parameters related to our designed DCFs. Since the various G.655 signal fibers have slightly different dispersion characteristics, as mentioned in the previous section, the dispersion characteristics of the DCFs have to be tailored in accordance with the dispersion characteristics of the individual signal fiber to achieve slope compensation. Moreover, since a DCF would typically be co-located with the relevant amplifier, we have optimized the DCFs separately for the S-, C- and L- amplifier bands. Thus, DCF1 and DCF2 have been optimized for canceling the dispersion of LEAF™, and operating in the C- and L-bands respectively. DCF3 and DCF4 have been optimized for Truewave-RS™ in the C- and L-bands respectively. And finally, DCF5, DCF6 and DCF7 have been optimized for Teralight™ in the S-, C- and L-bands respectively. In Figs. 3-5 we have plotted the dispersion spectra of these DCFs, and in Figs. 5-7 we have plotted the average dispersion spectra of the link comprising of the DCF and the relevant signal transmission fiber.
Table 1: Important performance parameters of the designed DCFs

(Table Removed)
*Bend-loss for a single turn of diameter 32 mm #Values correspond to those at 1510 nm (S-band)

†Values correspond to those at 1550 nm (C-band) ‡Values correspond to those at 1590 nm (L-band)
As can be seen from Figs. 3-5, for an optimized set of the refractive index profile parameters, the DCFs are characterized by a dispersion coefficient D that is more negative than -110 ps/nm.km over the given optical fiber amplifier band. Also, due to appropriate slope matching of the DCFs with that of the relevant signal transmission fibers, the average dispersion of the link is substantially reduced compared to that of the signal transmission fiber alone (see Figs. 5-7). Also, the designed DCFs are characterized by an Aeff is at least 40 µm2 at the center wavelength of a given amplifier band (and greater than 30 µm2 across the overall amplifier band(s)), which is larger than that of prior art DCFs (for which typical
values range between 15-25 µm2 at 1550 nm) and indicates reduced sensitivity to detrimental nonlinear effects. Moreover, these DCFs have a bend-loss < 0.1 dB for a single turn of bend-diameter of 32 mm for all operating wavelength ranges, and is within the tolerance limits (typically quoted as being less than 0.5 dB for a single turn of bend radius of 32 mm) specified for signal transmission fibers by fiber manufacturers. In addition, the mode spot-sizes that have been attained for these fibers are greater than that of prior art DCFs, thereby ensuring relatively smaller splice-losses of these DCFs with signal transmission fibers. Thus a link comprising of the presently designed DCF along with the relevant signal fiber will have lesser sensitivity to non-linear effects and smaller insertion losses resulting in improved performance of the systems carrying such broadband dispersion compensated links.
References:
1. Agrawal, G.P., 1995, the book Nonlinear Fiber Optics, Academic Press.
2. Akasaka, Y., R. Sugizaki, S. Arai, Y. Suzuki, and T. Kimiya, 1996, "Dispersion flat compensation fiber for dispersion shifted fiber," Europ. Conf. On Optical Commn.(ECOC'96), Oslo, pp. 221-224.
3. Antos, A.J. and D.K. Smith, 1994, "Design and characterization of dispersion compensating fiber based on LP01 mode," IEEE J. Lighwave Tech. Vol. LT-12, pp. 1739-1743.
4. Auguste, J.L., R. Jindal, J.M. Blondy, M. Clapeau, J. Marcou, B. Dussardier, G. Monnom, D.B. Ostrowsky, B.P. Pal, and K. Thyagarajan," 2000, "-1800 ps/nm.km chromatic dispersion at 1.55 micrometer in dual core fiber," Electron. Letts., vol. 36, pp. 1689-1691.
5. Danziger, Y. and D. Askegard, 2001, "High-order mode fiber - an innovative approach to chromatic dispersion management that enables optical networking in long-haul high-speed transmission systems," Opt. Networks Mag., vol. 2, pp. 40-50.
6. Gruner Nielsen, L., S.N. Knudsen, B. Edvold, T. Veng, D. Magnussen, C.C. Larsen, and H. Damsgaard," 2000, "Dispersion compensating fibers," Opt. Fib. Tech., vol. 6, pp. 164-180.
7. Srikant, V., 2001, "Broadband dispersion and dispersion slope compensation in high bit rate and ultra long haul systems, Proc. Of Optical Fiber Communications conference OFC'01, Anaheim, Calif.
8. Uchida, N., 2002, "Development and future prospects of optical fiber technology," IEICE Trans. Electronics, vol. E85-C, pp.868-880.

9. Thyagarajan, K., R.K. Varshney, P. Palai, A.K. Ghatak, and I.C. Goyal, 1996, "A novel design of a dispersion compensating fiber," IEEE Photon. Tech. Letts., vol. PTL-8, pp. 1510-1512.
10. Tsuda, T., Y. Akasaka, S. Sentsui, K. Aiso, Y. Suzuki, and T. Kamiya, 1998, "Broadband dispersion slope compensation of dispersion shifted fiber using negative slope fiber," Europ. Conf. On Optical Commn.(ECOC98), Madrid, pp. 233-234.
11. Aikawa K., R. Suzuki, S. Shimizu, K. Suzuki, M. Kenmotsu, M. Nakayama, K. Kaneda, and K. Himeno, 2003, "High performance dispersion and dispersion slope compensating fiber modules for non-zero dispersion shifted fibers," Fujikura Tech. Rev. vol. 32, pp 5-10.
12. For Fig. 2, Lindstrom, A. (Laser Communication Co.), 2000, "Defeating dispersion," Telephony, December issue.

We Claim
1. A dispersion compensated broadband optical communication link operating in the optical amplifier bands S-band (1480-1530 nm), C-band (1530-1565 nm), and L-band (1570-1610 nm), the said link comprising a signal fiber and a dispersion compensating fiber, wherein the said signal fiber has a dispersion coefficient D greater than 2 ps/nm.km and dispersion slope S greater than 0.05 ps/nm2.km in the operating amplifier band(s), and the said dispersion compensating fiber consists of a segmented core having three layers to attain a negative dispersion coefficient and negative dispersion slope meant to achieve broadband dispersion compensation of the said signal fiber with a low sensitivity to nonlinear optical effects through a large mode effective area Aeff, a substantially large FOM (Figure of Merit), and low sensitivity to bend loss.
2. A dispersion compensated broadband optical communication link as claimed in claim 1 wherein the relative refractive index difference between the core and the cladding of the said dispersion compensating fiber is determined by the formula:
(Formula Removed)
where  corresponds to the first layer and 2 corresponds to the third layer characterized in that the segmented core comprises a layer 1 of germania doped silica having a relative core cladding index difference , which is in the range of
0.0146 < 1 < 0.0165
and having a radius "a" which is in the range of
2.366 µm < a < 3.255 µm,
a second layer of pure silica surrounding the first layer and having a width "s" in the range of
2.95 µm < s < 7.28 µm,

surrounded by a third layer of germania doped silica with a relative core cladding index difference 2 in the range of
0.0027 < 2< 0.0037
and a width "d" in the range of
10.19 µm