Dispersion Compensating Fiber Module Kazuhiko Aikawa,1 Junji Yoshida,1 Susumu Saitoh,1 Manabu Kudoh,1 and Kazunari Suzuki2 Two types of slope compensating and dispersion compensating fiber (SC-DCF) are developed and fabricated into SC-DCF modules to impel the progress of dense wavelength division multiplexing (DWDM) transmission system, where there are two requirements for improving the performance of SC-DCF modules. The first requirement is to lower the insertion loss of the SC-DCF module in optical fiber transmission line. Low-loss SC-DCF module contributes to the relaxation in optical gain requirement of optical amplifiers, which improves the noise figure of the optical amplifiers as well as the overall performance of the optical fiber transmission systems. Downsizing of the SC-DCF module is the other requirement. DWDM transmission system consists of a large number of devices; thus, downsizing each device is one of the major developmental challenges. These two demands have different properties; thus, it is difficult to achieve both demands simultaneously. As a result, each of the new SC-DCF has been individually developed to meet these requirements. It is found that the newly developed SCDCF modules have high performance for practical applications and will steadily replace conventional SC-DCF modules. Recently dense wavelength division multiplexing (DWDM) transmission technology has made tremendous progress on transmission capacity, which is defined by optical signal modulation method and bandwidth used for transmission. As the transmission bandwidth is fixed, allowable dispersion values are in inverse proportion to the square of transmission speed, and therefore the faster the transmission bit rate speed, the more important the compensation of accumulated chromatic dispersion over transmission bandwidth. In the case of non-return-to-zero method, the allowable residual dispersion is about 100 ps/nm for a signal of 40 Gbit/s overall transmission bandwidth 1). In order to widen bandwidth of optical fiber transmission systems, dispersion compensation over transmission bandwidth for the accumulated chromatic dispersion of standard single-mode fiber (SSMF) transmitted over long distances is one of the key assignments. The transmission fiber is an SSMF having zero dispersion at a wavelength of 1310 nm and a dispersion of +17 ps/nm/km at a wavelength of 1550 nm. So far, several dispersion compensation techniques have been proposed: a pre-chirping of light source 2), introducing a spectral inversion at the middle of transmission span 3), a chirped fiber Bragg grating, 4) and dispersion compensating fiber 5) 6). In view of optical properties for transmission system, production cost, 1 Applied Optics Products Division 2 Aomori Fujikura Kanaya Ltd. 16 and module assembly, for a long time slope compensating and dispersion compensating fiber (SC-DCF) has been the most advantageous and widely used method as dispersion compensation devices. Figure 1 shows schematic diagram of chromatic dispersion compensation in optical transmission system. SC-DCF modules are inserted into the transmission line at uniform intervals and compensate the accumulated chromatic dispersion so that optical signals are adjusted within small residual dispersion required by the transmission channel so as not to cause any distortion of optical signal. One of the reference indexes of dispersion and dispersion slope compensation ability of SC-DCF is relative dispersion slope (RDS). The RDS is given by accumulated chromatic dispersion 1. Introduction Transmission fiber SC-DCF module Optical amplifier transmission distance Fig. 1. Schematic diagram of residual chromatic dispersion compensation. Panel 1. Abbreviations, Acronyms, and Terms. SC-DCF–Slope Compensating and Dispersion Compensating Fiber DWDM–Dense Wavelength Division Multiplexing SSMF–Standard Single-Mode Fiber RDS–Relative Dispersion Slope S … ……………… (1) D where S is the dispersion slope and D is the chromatic dispersion of SC-DCF and SSMF at operation wavelength. When the RDS of SC-DCF equals the RDS of SSMF, the chromatic dispersion and the dispersion slope of SSMF are compensated simultaneously. As shown in Fig. 2, SC-DCF has negative dispersion and dispersion slope so as to compensate for the chroRDS = Operating wavelength range (C-band : 1525~1565nm) SSMF dispersion (ps/nm/km) 25 0 -25 Residual dispersion -50 -75 SC-DCF -100 -125 -150 1300 1400 1500 1600 wavelength (nm) Fig. 2. Chromatic dispersion of fibers and residual dispersion. Table 1. Optical properties of SSMF, conventional standard SC-DCF, and conventional SC-DCF for compact modules. SSMF Standard SC-DCF SC-DCF for compact modules Item Unit Wavelength µm Fiber diameter µm 125 125 90 Coating diameter µm 250 240 175 Chromatic dispersion ps/nm/km +17 -115 -115 RDS nm-1 0.0034 0.0034 0.0034 Attenuation dB/km - 0.40 0.53 Figure of merit (FOM) ps/nm/dB - 288 217 Aeff µm2 80 21 17 1.55 matic dispersion of SSMF over a wide wavelength range. Previously, two types of SC-DCF have been produced, as shown in Table 1: SC-DCF with cladding diameter of 125 μm, which has same dimension of SSMF, and SC-DCF with reduced cladding diameter of 90 μm for compact module. To impel the progress of DWDM transmission system, there are two requirements for improving the performance of SC-DCF modules. The first requirement is a lowering of insertion loss of the SC-DCF module in optical fiber transmission systems. Low-loss SC-DCF module contributes to the relaxation in optical gain requirement for an optical amplifier, which improves the noise figure of the optical amplifier as well as the overall performance of the optical fiber transmission systems. Downsizing of the SC-DCF module is the other requirement. DWDM transmission system consists of a large number of devices, and therefore downsizing each device is one of the major developmental challenges. Particular features of each SC-DCF are demanded; thus, it is difficult to achieve both demands simultaneously. In this paper, two types of new SC-DCFs, which meet two different requirements mentioned above, are presented. In the second section, we mention developing an approach of the SC-DCF module with low insertion loss, and in the third section, we present the compact SC-DCF module by using SC-DCF with reduced fiber diameter. The final section is devoted to summary. 2. Low-loss SC-DCF module 2.1 Optical fiber design FOM ; Figure of merit : dispersion compensating ability per unit loss Fujikura Technical Review, 2011 FOM–Figure of Merit Aeff–Effective Area SBS–Stimulated Brillouin Scattering IL–Insertion Loss MFD–Mode Field Diameter The key optical properties of SC-DCF are chromatic dispersion, RDS, figure of merit (FOM), bending loss, cutoff wavelength, and effective area (Aeff). There are design tradeoffs among these properties; therefore, it is difficult to realize that all of the optical properties are maximized all together. For example, SC-DCF with large chromatic dispersion tends to show large bending loss and small Aeff. We adopt a segment core type to refractive index profile of SC-DCF as shown in Fig. 3. The profile design has a central core with small diameter and highly GeO2-doped silica glass, the de17 (A) Center core (C) Ring core (B) Trench core Fig. 3. Refractive index profile of SC-DCF. 18 sated, we focus on the design of SC-DCF so as to have chromatic dispersion and Aeff as large as possible in the range of allowable bending loss and cutoff wavelength. We numerically calculate the optical properties of SC-DCF for various dimensions and deltas of the refractive index profile and finally achieve target properties of SC-DCF. 2.2 Optical properties of SC-DCF Optical properties of prototype SC-DCF are shown in Table 2. The cladding diamiter of the SC-DCF is the same as SSMF. At the wavelength of 1550 nm, the SCDCF is in chromatic dispersion of -152 ps/nm/km, the attenuation of 0.40 dB/km, FOM of 380 ps/nm/ dB, RDS of 0.0034 nm-1 and Aeff of 21 mm2. These results achieved the optical properties that meet our expectations. The wavelength dependence of chromatic dispersion is shown in Fig. 4. Besides, the prototype SC-DCF has the same attenuation as the previous SC-DCF of 0.40 dB/km; its chromatic dispersion is also 1.3 times that of the previous one. As per the results mentioned above, we succeed to develop the new SC-DCF with high FOM and equal Aeff compared with conventional SC-DCF. The attenuation spectrum of the SC-DCF is shown in Fig. 5. This increased attenuation due to Table 2. Optical properties of prototype SC-DCF. Item Unit Measured value Wavelength mm 1.55 Fiber diameter mm 125 Attenuation dB/km 0.40 Chromatic dispersion ps/nm/km -152 nm RDS -1 0.0034 FOM ps/nm/dB 380 Cutoff wavelength mm 1.55 Aeff mm2 21 Bending loss (bending condition F20 mm¥10 turns) dB/m 3 -135 dispersion (ps/nm/km) pressed area surrounding the central core, which is fluorine-doped silica glass and is known as a trench core, and, finally, the outer ring with a lower refractive index than that of the central core. By optimizing distribution of the refractive index profile, negative dispersion and dispersion slope are realized. Consequently, it becomes possible to compensate accumulated dispersion of the transmission fiber over a wide wavelength range 6) 7). The reference index of dispersion compensating ability per unit loss is FOM given by D FOM SC-DCF = SC-DCF … ………… (2) LSC-DCF where DSC-DCF is chromatic dispersion and LSC-DCF is loss of fiber. When SC-DCF has high FOM by realizing large chromatic dispersion, the fiber length of SCDCF used in SC-DCF module is shorter than previous fiber length. Shortening fiber length used in the module is one of the reasons to decrease the insertion loss of the SC-DCF module. Additionally, we have to take into account nonlinear effects in optical fibers. In the case of long-distance transmission, optical signal with high input power is necessary for the enhancement of signal-to-noise ratio. However, high input power induces nonlinear effects in optical fiber and will cause distortion of optical signal. Specifically, stimulated Brillouin scattering (SBS) and self-phase modulation are the two major nonlinear effects to cause problems in SC-DCF. The threshold of SBS depends on homogeneity of the longitudinal fiber structure and fiber length. When the fiber has inhomogeneous stricture or short fiber length, SBS is suppressed 8). Nonlinear phase shift is used to evaluate an effect of self-phase modulation 9). The nonlinear phase sift is expressed as 2p n2 F= L P ……………… (3) l A eff eff 0 where λ is wavelength, n2 is nonlinear refractive index, Leff is effective fiber length, which is calculated by the actual fiber length and the attenuation of the fiber, and P0 is input power. Lowing the input power, shortening the fiber length, and the large Aeff are all effective for suppressing the nonlinear effect. In the condition that residual dispersion is compen- -145 -155 -165 1525 1535 1545 1555 1565 wavelength (nm) Fig. 4. Chromatic dispersion characteristics of SC-DCF. bending loss over the wavelength of 1570 nm is not observed. 2.3 Optical properties of SC-DCF module Appearance view of SC-DCF module is shown in Fig. 6. The SC-DCF module consists of a reel-wound SC-DCF and two connectors at either end of SC-DCF. Thus, module insertion loss (IL) is the sum of attenuation of SC-DCF, splice loss between SC-DCF and SSMF, and reflection loss at the end of connectors, which is expressed as follows, IL module = LSC-DCF + a + b…………… (4) where LSC-DCF is attenuation of the SC-DCF, a is total splice loss, and b is reflection loss at the end of connectors which is estimated around 0.3 dB. There are differences of electromagnetic field distribution between SSMF and SC-DCF. SSMF has Gaussian electromagnetic field distribution and mode field diameter (MFD) of around 10 μm. On the other hand, SC-DCF has particular refractive index profile in Fig. 3 and a small core with a high refractive index, which brings on non-Gaussian electromagnetic field distribution and small MFD of around 5 μm. By employing an intermediate fiber, which has similar refractive index profile and MFD as the SC-DCF, the intermediate fi- ber can be spliced with low loss to both SC-DCF and SSMF, thus reducing the splice loss. In this case, SCDCF module needs four points of arc fusion splice. Table 3 shows the optical properties of fabricated SC-DCF modules. The insertion loss of module No. 5 is 3.7 dB at the wavelength of 1550 nm, which is reduced by over 40 % from the previous module with insertion loss of about 6 dB. Hence, it can be seen that SC-DCF has high FOM and low splice loss. Figure 7 shows compensated chromatic dispersion of total transmission line under the condition that SSMF is 80 km in length. It is conformed that residual dispersion is within ± 5 ps/nm over the wavelength range of 1525 nm to 1565 nm. As a result, the SC-DCF module, which has low insertion loss and adequate ability of dispersion compensation, is realized. In comparison with nonlinear phase shift of the previous SC-DCF, the factors 2 p/l and n2/ Aeff in Eq. (3) are almost conserved in the case of prototype SC-DCF. Furthermore, total fiber length used for the prototype SC-DCF module is reduced by two-thirds of the previous SC-DCF module. For example, when prototype 9 km SC-DCF compensates 80 km SSMF, the previous SC-DCF needs the length of 16 km for compensating the same length of SSMF. Input power is also reduced to about 60 %, Table 3. Optical properties of SC-DCF modules. attenuation (dB/km) 0.8 Item Unit No.1 No.2 No.3 No.4 No.5 No.6 0.6 Dispersion ps/nm 1550 nm -150 -350 -651 -1003 -1370 -1700 km 0.4 Fiber length of compensated SSMF 1475 1500 1525 1550 wavelength (nm) Fig. 5. Attenuation spectrum of SC-DCF. 1575 20 40 60 80 100 nm-1 0.0037 0.0035 0.0040 0.0036 0.0034 0.0033 1550 nm RDS 0.2 1450 10 Module insertion loss dB 1550 nm ps Module PMD * 1525 nm - 1565 nm 0.9 1.4 1.8 2.7 3.7 4.6 0.1 0.1 0.1 0.1 0.2 0.3 residual dispersion (ps/nm) *PMD : Polarization mode dispersion 20 10 0 -10 -20 1525 1535 1545 1555 1565 wavelength (nm) Fig. 6. SC-DCF module. Fujikura Technical Review, 2011 Fig. 7. Residual dispersion after chromatic dispersion compensation for 80 km SSMF. 19 2.4 Reliability of SC-DCF module Figure 8 shows temperature dependence of insertion loss of fabricated module No. 4 at measurement wavelength of 1545 nm. In this evaluation test, environmental temperature variation range is 115 °C from -40 °C to +75 °C. This range covers operating temperature from -5 °C to +70 °C, and storage temperature from -40 °C to +75 °C. The maximum value of the module insertion loss appears at the lowest limit temperature of -40 °C ; however, this variation of the module insertion loss disappears when the environmental temperature returns back to room temperature of 25 °C. It is confirmed that the temperature variation of the module insertion loss is lass than 0.05 dB over the operating temperature range. In addition, the fabricated SC-DCF modules are evaluated by reliability tests based on Telcordia standard as shown in Table 4 10) 11). Table 5 shows the result of reliability tests. No variation of optical properties is observed through all tests. 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Insertion loss Temperature 0 4 8 12 20 time (h) Fig. 8. Temperature dependence on insertion loss of No.4 module. Table 4. Test item and condition of reliability test. No. Item Test condition 1 Vibration Frequencies range 10 to 500 Hz, 1.5 G along each axis 2 Shock 4 inch drop unpackaged, 30 inch drop packaged 3 Thermal cycle -40 °C/+85 °C, 1000 cycle 4 Damp and heat 85 °C and 85 %RH for 1000 h Table 5. Measurement variation of optical characteristics after each test. No. Item 3. Compact SC-DCF module Variation of Variation of Variation Variation insertion dispersion of PMD of PDL* loss (ps/nm) (ps) (dB) (dB) Wavelength 1550 nm 3.1 Optical fiber design 1 Vibration <0.2 <1.0 <0.2 <0.02 For the downsizing of the SC-DCF module, reduction of fiber diameter and large dispersion coefficient are necessary 12). With the goal that new module will be half the current size of SC-DCF module, we focus on the design of SC-DCF so as to achieve fiber dimension as small as possible and chromatic dispersion as large as possible in the range of allowable bending loss, cutoff wavelength, and micro-bending loss due to lateral pressure at winding process. We numerically calculate the optical properties of SC-DCF and finally achieve target properties of SCDCF. 2 Shock <0.2 <1.0 <0.2 <0.02 3 Thermal cycle <0.2 <1.0 <0.2 <0.02 4 Damp and heat <0.2 <1.0 <0.2 <0.02 3.2 Optical properties of SC-DCF Optical properties of previous SC-DCF with reduced fiber diameter, target values of prototype SC-DCF, and measurement values of prototype SC-DCF are shown in Table 6. The dimension of the prototype SC-DCF is reduced, in cladding diameter of 80 mm and coating diameter of 145 mm. At the wavelength of 1550 nm, the SC-DCF is in chromatic dispersion of -160 ps/nm/ km, the attenuation of 0.53 dB/km, FOM of 300 ps/ nm/dB, RDS of 0.0034 nm-1 and Aeff of 17 mm2. The SC-DCF has reduced cladding diameter and thin coating; therefore, micro-bending loss and me20 16 100 80 60 40 20 0 -20 -40 -60 24 temperature (°C) variation of insertion loss (dB, @1545nm) because module insertion loss is reduced from 6 dB 6) to less than 4 dB. From the results gathered so far, the nonlinear phase shifted down to 30 % of the previous one. In addition to the shortened fiber length of fabricated SC-DCF module, the threshold of SBS has improved from 7.5 dBm 6) to 8.5 dBm. *PDL : Polarization depending loss chanical strength are key characteristics. The attenuation of the fiber is shown in Fig. 9. Increased loss at a wavelength longer than 1575 nm, which is due to bending loss of the fiber, is not observed. Figure 10 shows Weibull distribution plots of prototype fiber. The curve of prototype SC-DCF is in line with SSMF. This result indicates that the SC-DCF has reliable mechanical strength. Coating dimension of prototype SC-DCF is 0.83 times that of the previous one, and chromatic dispersion of the SC-DCF is 1.39 times that of the previous one. As a result, occupied winding volume of the SCDCF in SC-DCF module is 0.6 times that of the previous one. 3.3 Optical properties of SC-DCF module SC-DCF module for compensation of 100 km SSMF is fabricated by using prototype SC-DCF. The SC-DCF is wound on a reel and optical connectors are spliced Table 6. Optical properties of conventional 90μm SC-DCF, target 80μm SC-DCF for compact modules and prototype 80μm SC-DCF. 80μm SC-DCF (Target value) 90μm SC-DCF (Conventional) 80μm SC-DCF (Prototype results) Item Unit Wavelength nm Fiber diameter mm 90 80 80 Coating diameter mm 175 145 145 Dispersion ps/nm/km -115 £ -150 -160 0.0034 0.0034 0.0034 RDS nm -1 1550 Attenuation dB/km 0.53 - 0.53 FOM ps/nm/dB 220 ≥ 300 302 Aeff mm2 17 17 17 PMD ps/km1/2 0.1 0.1 0.1 persion is within ±10 ps/nm. 3.4 Reliability of SC-DCF module Figure 12 shows temperature dependence of module insertion loss at measurement wavelength of 1550 nm. The maximum value of the module insertion loss appears at lowest limit temperature of -40 °C. It is confirmed that the temperature variation of module insertion loss is less than 0.1 dB. The fabricated SC-DCF modules are evaluated with reliability tests as shown in Table 4 mentioned in Section 2. No variance of optical properties is observed through all tests. 4. Conclusion We have designed and developed two types of SCDCF and fabricated SC-DCF modules by using each prototype SC-DCF. One is a SC-DCF module with low insertion loss by using the prototype SC-DCF, which has higher FOM and equal Aeff compared with the previous one. As a result, the length of SC-DCF becomes shorter than that of the previous one, and thus nonlinear effect in the fiber is suppressed. The other is a compact SC-DCF module by using the prototype SCDCF, which has small fiber diameter and large dispersion coefficient. As a result, the dimension of SC-DCF becomes smaller than that of the previous one. FabriIn(ln(1/(1-F))) F : Failure probability at both ends of SC-DCF. The module dimension, which is dominated by the size of flanges put on both sides of the reel, is diameter of 138 mm and width of 23 mm. A technique for reducing the splice loss between SC-DCF and SSMF is applied. The direct connection loss between SC-DCF and SSMF is less than 0.2 dB, which is lower than that of intermediate fiber technique. Intermediate fiber becomes unnecessary by employing this method, and thus the number of arc fusion splice points is decreased to two points. This method allows for compact storage of fiber connection. Table 7 shows the module properties of fabricated SC-DCF module. In comparison with previous SC-DCF module, the fabricated SC-DCF module has twice compensation ability at the same volume. This means that the dimension of fabricated SC-DCF module is 0.6 times that of the previous one. Downsizing and lowering insertion loss of SC-DCF module are realized, with a module dimension that is half of the previous SCDCF module, by using prototype SC-DCF and direct fusion splice technique. Residual chromatic dispersion after compensating for 100 km SSMF is shown in Fig. 11. Over a wide wavelength range of 1525 nm to 1565 nm, residual dis- 1.50 1.00 0.50 0.00 -0.50 SC-DCF with reduced cladding of 80µm SSMF -1.00 -1.50 -2.00 -2.50 -3.00 -3.50 1 5 failure stress (GPa) 10 Fig. 10. Weibull distribution of tensile strength in fabricated SC-DCF with small diameter. Table 7. Optical properties of compact SC-DCF modules. attenuation (dB/km) 0.8 0.6 0.4 0.2 1450 1475 1500 1525 wavelength (nm) 1550 1575 Fig. 9. Attenuation spectrum of SC-DCF with small diameter. Fujikura Technical Review, 2011 Item Unit Sample1 Sample2 Sample3 Sample4 Dispersion ps/nm 1550 nm -680 -1020 -1020 -1700 Fiber length of compensated SSMF km 40 60 60 100 RDS nm-1 1550 nm 0.0035 0.0034 0.0035 0.0034 Module insertion loss dB 1550 nm 2.7 4.0 3.9 6.2 Module PMD ps 1525-1565 nm 0.1 0.1 0.2 0.3 21 0 -10 -20 1525 1535 1545 1555 1565 wavelength (nm) Fig. 11. Residual dispersion after chromatic dispersion compensation for 100 km SSMF. cated compact SC-DCF module is reduced by half of the previous one. Each of the prototype SC-DCF has been developed, and we have confirmed the optical properties and reliability of SC-DCF modules. It is found that the newly developed SC-DCF modules have high performance for practical applications and will steadily replace the conventional SC-DCF modules. References 1) H. Toba, et. al.: “Next generation ultra-high-speed transmission technologies”, NTT R&D, Vol.48, No.1, pp.33 -41, 1999 2) N. Henmi, et. al.: “10Gb/s, 100km normal fiber transmission experiment employing a modified prechirp technique”, OFC’91 Technical Digest, Tu02, 1991 3) R. M. Jopson, et. al.: “Compensation of fiber chromatic dispersion by spectral inversion”, Electron. Lett., Vol.29, pp.576578, 1993 4) M. Sudo, et. al.: “Fabrication of Fiber Bragg Gratings for Dispersion Compensation”, Communications society conference of IEICE, C-3-47, 1998 5) K. Nishide, et. al.: “1.55 µm Single Mode Fibers with Large Chromatic Dispersion”, IEICE spring conference, C-575, 1989 22 2.0 Sample 1 Sample 2 Sample 3 Sample 4 Temperature 1.5 1.0 100 80 60 0.5 40 0.0 20 -0.5 0 -1.0 -20 -1.5 -40 -2.0 0 5 time (h) 10 temperature (°C) 10 variance of insertion loss (dB.@1550nm) residual dispertion (ps/nm) 20 -60 Fig. 12. Temperature dependence on insertion loss of Sample1–4 modules. 6) K. Aikawa, et. al.: “High-performance Dispersion-slope and Dispersion Compensation Modules”, Fujikura Technical Review, No.31, 2002 7) L. Grüner-Nielsen, et. al.: “Dispersion-compensating fibers”, J. Lightwave Technol., Vol.23, No.11, pp.3566-3578, 2005 8) A. Wada, et al.: “Suppression of stimulated Brillouin scattering by intentionally induced periodical residual-strain in single-mode optical fibers”, IEICE Trans. Commun. Vol.E76-B, No.4, pp.345-351, 1993 9) T. Kato, et. al.: “Design optimization of dispersion compensating fiber for NZ-DSF considering nonlinearity and packaging performance“, Proc. Optical Fiber Communication Conference, No.TuS6, 2001 10) Telcordia-GR-2854 Generic Requirements for Fiber Optic Dispersion Compensators (Issue 2, Dec. 1997) 11) Telcordia-GR-63 NEBS Requirements: Physical Protection (Issue 3, Mar. 2006) 12) P. Kristensen, et. al.: “Dispersion and slope compensating module for G.652 fiber with x4 reduced physical dimensions”, Proc. European Conf. on Opt. Commun., No. We4P.15, 2003