High Performance Dispersion and Dispersion Slope Compensating Fiber Modules for Non-zero Dispersion Shifted Fibers Kazuhiko Aikawa, Ryuji Suzuki, Shogo Shimizu, Kazunari Suzuki, Masato Kenmotsu, Masakazu Nakayama, Keiji Kaneda and Kuniharu Himeno Slope compensating and dispersion compensating fibers (SC-DCF) for a low dispersion slope non-zero dispersion shifted fiber () and a large effective area were designed and fabricated. The SC-DCFs realized dispersion slope compensation ratios of 100 % in the C-band or L-band while the fibers maintained dispersion and bend loss similar to those of SC-DCFs for standard single-mode fibers (S-SMF). In addition to the dispersion characteristics, thermal coefficients of dispersion for the fibers were measured. It has been confirmed that the coefficient is proportional to dispersion slope even for the fibers with large negative dispersion slopes. Nonlinearities of the SC-DCFs were evaluated. It was confirmed that nonlinear phase shifts and stimulated Brillouin scattering thresholds of the fibers were similar to those of SC-DCFs for S-SMF. The fibers were packaged into modules, and loss spectra, temperature characteristics and reliabilities on the modules were evaluated. As a result, it has been confirmed that the modules have good optical performance and high reliability. 1. Introduction In order to accommodate the rapid increase of demand for data communications, optical fiber transmission systems have required their capacity expansion. The evolution of dense wavelength-division multiplexing (DWDM) technology is essential to use an optical fiber bandwidth efficiently. In a DWDM transmission system, expanding an operating wavelength range and dispersion compensation technique over the wavelength range are important. Various technologies for dispersion compensation have been investigated so far, such as dispersion compensating fibers (DCF) utilizing a fundamental mode or a higher order mode 1), a fiber Bragg grating 2), a virtual image phased array 3) and planar waveguide-based devices 4). A slope compensating and dispersion compensating fiber (SC-DCF) utilizing a fundamental mode has good performances of wide operating wavelength range, low polarization mode dispersion (PMD), low dispersion ripple and low multiple path interference, compared with other types of dispersion compensation technologies. As a result, fundamental-mode SC-DCF is currently a suitable device for practical broadband dispersion compensation 5) 6) 7). There have been many reports on SC-DCFs and their modules for the standard single-mode fiber (S- SMF) 8) 9) 10) 11) 12) 13). In addition, SC-DCFs for two types of non-zero dispersion shifted fibers (), which have low dispersion slope or large effective area (Aeff), have been reported 14) 15) 16) 17). However, no detailed information such as nonlinearity or packaged performance on the fibers have been presented so far. In this report, we present the characteristics of SC- DCFs and their modules for the two types of NZ- DSFs in detail. Fabricated SC-DCFs showed dispersion slope compensation ratios of 100% in the C-band or L-band while the fibers maintained similar dispersion and bending loss to those of SC-DCF for S-SMF. Also, the thermal coefficients and nonlinearities of the fibers were measured. The fibers were packaged into modules, and loss spectra, temperature characteristics and reliabilities on the modules were evaluated. As a result, it has been confirmed that the modules have good optical performance and high reliability. 2. Basics of Dispersion Slope Compensation and Fiber Design One measure representing the dispersion and dispersion slope compensation ability of SC-DCF is a relative dispersion slope (RDS), which is defined as the Fujikura Technical Review, 2003 5
ratio of dispersion slope to chromatic dispersion as follows: S RDS =... (1) D where D and S are a dispersion and a dispersion slope per unit length of an optical fiber. If the RDS of SC-DCF is the same as that of a transmission fiber, it is possible to compensate the dispersion slope of the transmission fiber completely by adjusting the length of the SC-DCF so as to compensate the total dispersion of the transmission fiber. A slope-compensating rate can be expressed by the following equation using RDS: RDS SC DCF Slope compensating rate =... (2) RDS TMF where RDSSC-DCF and RDSTMF are the RDSs of SC-DCF and a transmission fiber. Table 1 shows the typical dispersion characteristics and RDSs of various types of transmission fibers including the two types of NZ- DSFs. DCF module should have low insertion loss, low polarization dependent loss (PDL), low PMD, and low optical nonlinearity with a slope compensation rate of about 100 % maintained. In addition to these characteristics, DCF should have a large chromatic dispersion coefficient and a low bending loss to minimize Table 1. Dispersion Characteristics of Transmission Fibers Transmission Wavelength Dispersion Dispersion RDSTMF No. slope fiber (nm) (ps/nm/km) (ps/nm2 /km) (nm 1 ) 1,550 17 0.058 0.0034 S-SMF 1,590 19 0.055 0.0029 1 Low dispersion 1,550 4.5 0.045 0.010 2 slope 1,590 6.3 0.045 0.007 3 Large effective 1,550 4.2 0.085 0.020 area 4 1,590 7.6 0.085 0.011 (a) (b) (c) Fig. 1. Refractive Index Profile of SC-DCF. the size of its module since DCF modules are generally mounted in a rack in a terminal and repeater office. However, there are design trade-offs among chromatic dispersion coefficient, cutoff wavelength, effective area and bending loss. In particular, it is difficult to achieve low bending loss for a large RDS SC- DCF. Fig. 1 shows a typical refractive index profile of SC-DCF. While we changed dimensions and index deltas of the index profile, we searched optimum index parameters to achieve the target RDS of two types of the s, maximum negative dispersion coefficient and minimum bending loss. The target chromatic dispersion was 70 ps/nm/km in terms of optimum RDS, cutoff wavelength, bending loss and module size. The respective target RDSs were the same as those of corresponding transmission fibers. Table 2 shows the optical characteristics achieved for the sets of optimized index profile parameters. 3. Optical Characteristics of SC-DCF We fabricated SC-DCFs for the two types of NZ- DSFs based on the sets of the index profile parameters. Table 3 shows the typical optical characteristics of fabricated SC-DCFs. Each fiber has the same RDS as the target RDS, so that all the dispersion slope compensation rates at the center wavelength of the operating wavelength are approximately 100 %. The Table 2. Simulation Result of Optical Characteristics SC-DCF for low SC-DCF for large Item Unit dispersion slope effective area A B C D Wavelengh nm 1,550 1,590 1,550 1,590 Dispersion Table 3. Optical Characteristics of Fabricated SC-DCFs SC-DCF for low SC-DCF for large Item Unit dispersion slope effective area A B C D Wavelengh nm 1,550 1,590 1,550 1,590 RDS nm 1 0.010 0.007 0.020 0.011 Dispersion ps/nm/ km ps/nm/ km 70 70 70 70 RDS nm 1 0.010 0.007 0.020 0.011 Aeff µm 2 =15 > = > 15 = > 12 = > 15 Bending db/m, <5 <5 <5 <5 loss 2R=20mm 70 72 68 75 Attenuation db/km 0.35 0.35 0.60 0.41 PMD ps/ km 0.06 0.08 0.12 0.09 Aeff µm 2 16.8 16.7 13.2 16.6 Bending db/m, 1.0 0.8 4.0 2.0 loss 2R=20mm 6
transmission loss of the C-band SC-DCF for the large effective area is about 0.60dB/km at 1,550 nm while those of other SC-DCFs are ranged from 0.35dB/km to 0.40dB/km at the wavelength. The high transmission loss of the C-band SC-DCF for the large effective area originates from the difference of refractive index profile. The effective area and bending loss of the fibers are approximately the same as the target values. The thermal coefficient of chromatic dispersion of an optical fiber is known to be dependent on its dispersion slope 18). Fig. 2 shows the relationship between the thermal coefficient of chromatic dispersion and dispersion slope for SC-DCFs and transmission fibers including the fabricated SC-DCFs. The thermal coefficient of chromatic dispersion for the SC-DCF for the large effective area is relatively high. For instance, if we consider their SC-DCF modules for compensation of 80km s and a temperature change of 30 degrees, the dispersion variation on the modules using fiber A and fiber B is about 3ps/nm, and those on modules using fiber C and fiber D are about 5ps/nm. When these modules are used for a 40Gb/s transmission system, it is necessary to take into account of the dispersion variation by temperature change especially for the SC-DCF for the large effective area. The length dependence of return loss for an optical fiber is expressed by the following equation: P out 1 exp ( 2αL) = CR... (3) Pin 2 α where CR is a Rayleigh scattering coefficient, α is a transmission loss of the fiber, L is a fiber length, Pin is an input power to the fiber, and Pout is a back scattering power. If the fiber length becomes long, the Thermal coefficient of chromatic dispersion (ps/nm/km/ C) Large effective area Low dispersion slope S-SMF C-band SC-DCF for S-SMF L-band SC-DCF for S-SMF 0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0.005 1.500 1.000 0.500 0.000 0.500 1.000 Dispersion slope (ps/nm 2 /km) Fiber A Fiber B Fiber C Fiber D Fig. 2. Relationship between Dispersion Slope and Thermal Coefficient of Chromatic Dispersion. return loss of the fiber is saturated to the following value: P out C R =... (4) Pin 2 α Fig. 3 shows the fiber length dependence of return loss for the fabricated SC-DCFs together with that for C-band SC-DCF for SMF. The return loss of the SC- DCFs are saturated about 27dBm or 28dBm as well as SC-DCF for SMF. It is confirmed that the return loss of the SC-DCFs for the s are almost the same as those of C-band SC-DCF for S- SMF. 4. Nonlinearities of SC-DCF In addition to dispersion compensation characteristics, evaluation of nonlinearities of the SC-DCF is important. Four-wave mixing among nonlinearities of SC-DCF is not a major factor to deteriorate transmission performance since the dispersion coefficient of SC-DCF is large. Self-phase modulation (SPM) and stimulated Brillouin scattering (SBS) are primary nonlinearities deteriorating (impairing) transmission performance. To obtain the basic information on SPM for the SC- DCF, we measured the nonlinear refractive index n2 in the fabricated SC-DCFs by dual frequency SPM technique 19) 20) 21). Nonlinear coefficient n2/aeff for fiber C was 2.5 10 9 /W, and n2/aeff for fiber A, B, and D were 1.7 10 9 /W. They correspond to nonlinear refractive index n2 for fiber C = 3.1 10 20 m 2 /W, and the n2 of fiber A, B, and D = 2.8 10 20 m 2 /W. Since n2/aeff for DCF with a positive dispersion slope and SC-DCF for S-SMF ranges from 1.2 to 1.5 10 9 /W, n2/aeff for the fabricated SC-DCFs for the s are higher than those of the positive slope DCF and the SC-DCF for S-SMF. Actual transmission impairment through SC-DCF should be evaluated by the nonlinear phase shift caused by SPM. Therefore, we calculated and compared the nonlinear phase shifts induced in SC-DCFs for 80 km of S-SMF and the s. The nonlinear phase shifts of the fabricated SC-DCF for the NZ- Return loss (db) 26 28 30 32 34 36 38 100 1,000 10,000 Fiber length (m) Fiber A Fiber B Fiber C Fiber D C-band SC-DCF for S-SMF 100,000 Fig. 3. Fiber Length Dependence of Return Loss. Fujikura Technical Review, 2003 7
DSFs were less than a half of those of the SC-DCF for S-SMF. Main reason for this is that the fiber lengths required for dispersion compensation of the s are shorter than those required for S-SMF. It is well known that an optical fiber with low loss and high longitudinal uniformity has a low threshold against SBS 22). We measured SBS threshold for several fiber lengths of fiber A, C and C-band SC-DCF for S-SMF. Fig. 4 shows the fiber length dependence of SBS threshold for the tested SC-DCFs. Fiber A shows almost the same fiber length dependence of the threshold as the SC-DCF for S-SMF. Although the SBS threshold of fiber C is higher than that of the SC- DCF for S-SMF, the fiber lengths required for the SC- DCF modules for the s are shorter than that of SC-DCF module for S-SMF. As a result, the SBS thresholds of SC-DCFs for the s are lower than that of SC-DCF for S-SMF when we compare these fibers in the lengths required for dispersion compensation of the same length of transmission fiber. 5. Optical Characteristics of SC-DCF Module SBS threshold (dbm) 14 13 12 11 10 9 8 7 6 C-band SC-DCF for S-SMF Fiber A Fiber C 0 5,000 10,000 15,000 20,000 Fiber length (m) Fig. 4. Fiber Length Dependence of SBS Threshold. Table 4. Optical Characteristics of Fabricated SC-DCF Modules SC-DCF module for SC-DCF module for Item Unit the low dispersion the large effective slope area Module- Module- Module- Module- A B C D Wavelengh nm 1,550 1,590 1,550 1,590 Dispersion ps/nm 356 499 316 595 RDS nm 1 0.010 0.007 0.020 0.011 Slopecompensation % 100 100 100 100 rate Insertion loss db 2.8 4.0 3.8 4.1 PMD ps 0.3 0.2 0.4 0.3 We fabricated four SC-DCF modules for dispersion compensation of 80 km s using the fabricated SC-DCF. The fibers were packaged into small modules. The dimensions of the packages were 195 195 45 mm. S-SMF was used as a pigtail fiber in these modules. It is well known that an actual splice loss between an optical fiber with a quasi-gaussian field distribution and an optical fiber with a non-gaussian field distribution is higher than a splice loss estimated from their mode field diameters (MFD) 23). The MFDs of the SC- DCFs are smaller than those of S-SMF and NZ-DCF. In addition, the field distribution of SC-DCF with a segmented core profile is a non-gaussian profile. Therefore, the splice loss between SC-DCF and S- SMF tends to be high. Mode-field conversion technique and an intermediate fiber with almost the same MFD as each SC-DCF are used for the splice between SC-DCF and the pigtail fiber. The average splice loss between the SC-DCF and the pigtail fiber was 0.5dB. Table 4 shows the optical characteristics of the fabricated modules using the splice method. Fig. 5 shows the attenuation spectra of module B for the low dispersion slope and module D for the large effective area. Degradation of insertion loss caused by a bending loss is not observed in the operating wavelength. Fig. 6 shows the dispersion spectra of the fabricated modules. The dispersion spectra of the all types show good performance in terms of dispersion compensation over a wide wavelength range. Insertion loss (db) 5.5 5.0 4.5 4.0 Module B Module D 3.5 1,525 1,545 1,565 1,585 1,605 1,625 Fig. 5. Insertion Loss Spectra of Fabricated SC-DCF Modules. Dispersion (ps/nm) 200 300 400 500 600 700 Module A Module C Module D Module B 800 1,525 1,545 1,565 1,585 1,605 1,625 Fig. 6. Chromatic Dispersion Characteristics of Fabricated SC-DCF Modules. 8
6. Residual Dispersion Compensated by SC- DCF Modules Fig. 7 shows the wavelength dependence of residual dispersion through 80km low dispersion slope compensated by the SC-DCF modules. The residual dispersion in the C-band or L-band is less than ± 5ps/nm. Therefore, the residual dispersion for a 400km transmission in the C-band or L-band is less than ± 25ps/nm. In terms of residual dispersion, the 400km transmission line compensated by the SC-DCF modules with the fabricated SC-DCF accommodate a 40Gb/s transmission system. Fig. 8 shows the wavelength dependence of residual dispersion through 80km large effective area NZ- DSF compensated by the SC-DCF modules. The residual dispersion in the C-band or L-band is less than ± 20 ps/nm, or ± 15ps/nm, respectively. Therefore, the transmission line with these modules for a 40Gb/s transmission system accommodates up to 240km in the C-band and 320km in the L-band, respectively. If the length of a transmission line is longer than these critical distances, the dispersion left at a wavelength should be compensated wavelength by wavelength after de-multiplexing. It is necessary to improve the wavelegth dependence of RDS to compensate only by SC-DCF modules less than the Residual dispersion (ps/nm) Residual dispersion (ps/nm) 50 25 0 25 Low dispersion slope + Module A Low dispersion slope + Module B 50 1,510 1,540 1,570 1,600 1,630 Fig. 7. Residual Dispersion through 80km Low Dispersion Slope Compensated by Module A and B. 50 25 0 25 Large effective area + Module C Large effective area + Module D 50 1,510 1,540 1,570 1,600 1,630 Fig. 8. Residual Dispersion through 80km Large Effective Area Compensated by Module C and D. required residual dispersion in a 40Gb/s transmission line over the critical distances. 7. Reliability Test We conducted reliability tests on the fabricated SC- DCF modules. Table 5 shows the test items and the test conditions of the reliability tests. Each test was performed sequentially. Table 6 shows the measured variations of the optical characteristics after each test. The variations after all tests were very small within measurement errors. As a result, we have confirmed that the fabricated modules have high reliabilities. 8. Conclusion SC-DCFs for a low dispersion slope and a large effective area both were designed and fabricated. The SC-DCFs showed dispersion slope compensation ratios of 100 % in the C-band or L-band while the fibers maintained dispersion and bend loss similar to those of SC-DCF for S-SMF. Nonlinearities of the fabricated SC-DCFs for the s were evaluated. It has been confirmed that nonlinear phase shifts and SBS thresholds for the fibers are similar to those of SC-DCF for S-SMF. The fibers were packaged into modules, and loss spectra, temperature characteristics and reliabilities on the modules were evaluated. As a result, it has Table 5. Test Item and Condition of Reliability Test No. Test item Test condition Frequency range from 10 to 500 Hz, 1 Vibration amplitude of vibration 1.5mm, 1.5 G along each axis. 2 Shock 4 inch drop unpackaged, 30 inch drop packaged 3 Thermal cycle 40 C/+85 C, 100 cycles 4 5 Low temperature storage Damp and heat storage 40 C for 72 h 85 C and 85% RH for 2,000 h Table 6. Measurement Variation of Optical Characteristics after Each Test Insertion Dispersion PMD PDL No. Test Item loss variation variation variation variation (db) (ps/nm) (ps) (db) Measurment 1,550 nm for C-band Module / wavelength 1,590 nm for L-band Module 1 Vibration <0.2 <1.0 <0.2 <0.02 2 Shock <0.2 <1.0 <0.2 <0.02 3 Thermal cycle <0.2 <1.0 <0.2 <0.02 Low 4 temperature <0.2 <1.0 <0.2 <0.02 5 storage Damp and heat storage <0.2 <1.0 <0.2 <0.02 Fujikura Technical Review, 2003 9
been confirmed that the modules have good optical performance and high reliability. References 1) C. D. Pool, et al.: Optical Fiber-Based Dispersion Compensation Using Higher Order Modes Near Cutoff, J. Lightwave Technol., Vol. 12, No. 10, pp.1746-1758, 1994 2) J. A. J. Fells, et al.: Widely Tunable Twin Fiber Grating Dispersion Compensator for 80 Gbit/s, Optical Fiber Conf., Paper PD11, 2001 3) M. Shirasaki, et al.: Compensation of Chromatic Dispersion and Dispersion Slope Using a Virtually Imaged Phased Array, Proc. Optical Fiber Conf., TuS1, 2001 4) C. K. Madsen, et al.: Compact Integrated Tunable Chromatic Dispersion Compensator with a 4000 ps/nm Tuning Range, Optical Fiber Conf., Paper PD9, 2001 5) T. Suzuki, et al.: Large-effective-area dispersion compensating fibers for dispersion accommodation both in the C and L band, OECC 00, Technical Digest, 14C4-4, pp.554-555, 2000 6) K. Aikawa, et al.: High-performance Dispersion-slope and Dispersion Compensation Module, Fujikura Technical Review, No.31, pp.59-64, 2002 7) M. J. Li, et al.: Recent Progress in Fiber Dispersion Compensators, Proc. European Conf. on Opt. Commun., Paper Th.M.1.1, Sept. 2001 8) A. J. Antos, et al.: Design and Characterization of Dispersion Compensating Fiber Based on the LP01 Mode, J. Lightwave Technol., Vol. 12, No. 10, pp.1739-1745, 1994 9) L. Grüner-Nielsen, et al.: Design and Manufacture of Dispersion Compensating Fiber for Simultaneous Compensation of Dispersion and Dispersion Slope, Proc. Optical Fiber Conf., WM13, 1999 10) G. E. Berkey, et al.: Negative Slope Dispersion Compensating Fibers, Proc. Optical Fiber Conf., WM14, 1999 11) Koyano Y, et al.: High Performance Fiber-based Dispersion Compensation Modules, Technical Report of IEICE, OCS96-68, pp.59-64, 1996 12) S. Shimizu, et al.: Dispersion Compensating Fiber Module for L-band with Low Nonlinearity, The 2001 IEICE General Conference, C-3-33, pp.198, 2001 13) M. Nakayama, et al.: Dispersion Slope and Dispersion Compensating Fiber Module for L-band, The 2001 IEICE Electronics Society Conference, C-3-105, pp.215, 2001 14) V. Srikant, et al.: Broadband Dispersion and Dispersion Slope Compensation in High Bit Rate and Ultra Long Haul Systems, Proc. Optical Fiber Conf., TuH1 2001 15) Quang Le N. T., et al.: New Dispersion Compensating Module for Compensation of Dispersion and Dispersion Slope of Non-zero Dispersion Fibers in the C-band, Proc. Optical Fiber Conf., TuH5, 2001 16) T. Kato, et al.: Design Optimization of Dispersion Compensating Fiber for Considering Nonlinearity and Packaging Performance, Proc. Optical Fiber Conf., TuS6, 2001 17) K. Aikawa, et al.: High Performance Dispersion and Dispersion Slope Compensating Fiber Modules for Non- Zero Dispersion Shifted Fibers, Technical Report of IEICE, OCS2002-7 pp.35-40, 2002 18) T. Kato, et al.: Temperature Dependence of Chromatic Dispersion in Various Types of Optical Fibers, Proc. Optical Fiber Conf., TuG7, 2000 19) A. Boskovic, et al.: Direct Continuous-wave Measurement of n2 in Various Types of Telecommunication Fiber at 1.55 µm, Opt. Lett., Vol. 21, pp.1966-1968, 1996 20) S. V. Chernikov, et al.: Measurement of Normalization Factor of n2 for Random Polarization in Optical Fiber, Opt. Lett., Vol. 21, pp.1559-1561, 1996 21) A. Nihonyanagi, et al.: Measurement of Nonlinear Refractive Index Coefficient of Several Types of Fibers by SPM Method, The 1999 IEICE General Conference, B-10-164, pp.525, 1999 22) D. Cotter, et al.: Stimulated Brillouin Scattering in Monomode Optical Fiber, J. Opt. Commun., Vol. 4, pp.10-19, 1983 23) K. Himeno, et al.: Splice Loss of Large Effective Area Fiber and It s Reduction by Mode Field Conversion, Proc. European Conf. on Opt. Commun., Vol. 1, pp.131-134, Sept.1997 10