SPLICE LOSS IN NON-ZERO DISPERSION-SHIFTED FIBERS

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1 SPLICE LOSS IN NON-ZERO DISPERSION-SHIFTED FIBERS Mary E. White Sheila A. Cooper Corning Incorporated Corning, New York ABSTRACT As non-zero dispersion-shifted fibers (NZ-DSF), and the more recently introduced optimized type of NZ-DSF with large effective area, Corning LEAF fiber become more prevalent in the marketplace, system designers need access to accurate information regarding splice loss. This paper examines laboratory splice loss data, modeled field performance data, and splice loss measurement considerations. Fusion splicing practices and equipment set-up are also examined. All NZ-DSF designs, including LEAF fiber require a renewed understanding of splice loss measurement. This paper provides an understanding of the issues outlined above with the objective of providing insights on how carriers can achieve consistent low loss. INTRODUCTION Large effective area fiber permits more power to be transmitted, resulting in longer distances, higher TDM rates and better system performance either through higher Signal-tonoise ratio (SNR) or lower Bit-error-rate (BER), than standard NZ-DSF. One important factor in network design that needs to be considered is splice loss budget. To better understand the effect on splice loss budget modeled data for both passive alignment systems, which mechanically align the fibers, and active alignment systems, which line up the fiber cores to minimize loss, will be provided. LEAF fiber installation allows longer distances ( to 15%) between repeaters without degrading system transmission performance versus NZ-DSF with small effective area. Most systems would typically be made up of cables ranging from 6 km to 12 km in length. This would translate into between 8 cable splice points and 15 cable splice points. By using LEAF fiber, overall system splice loss would be lower than that of standard NZ-DSF. With fifteen years of experience deploying standard single-mode optical fiber (SMF) in their networks, carriers and other end users have developed a practical understanding of the challenges associated with splicing optical fibers. However, new opportunities in longdistance high data rate applications have raised the question of the use of non-zero dispersionshifted fibers (NZ-DSF) which now are the fibers of choice for these applications. In addition, a recently introduced optimized type of NZ-DSF with large effective area, Corning LEAF fiber, also must be considered. Laboratory Splice Loss Data Laboratory data shows acceptable splice loss results for active alignment splicing. A study using six LEAF fibers was completed at Corning s Center for Fiber-Optic Testing. This study included six between each of the fifteen combinations of LEAF fibers available. An active alignment splicer was used. Limited data from subsequent studies show substantially lower splice loss results. The fibers selected for the study included a wide range of mode-field diameters and core/clad offsets as shown in Table 1.

2 Table 1: Characteristics for fiber used in study Mode Field Diameter (m) Core/Clad Offset (m) Fiber # Fiber # Fiber # Fiber # Fiber # Fiber # A summary of the results is shown in Table 2 and the distribution is shown in Figure 1. Table 2: Laboratory LEAF fiber splice loss study results Active Alignment Results Number of Splices 90 Average Splice Loss() Standard Deviation () 0.02 Maximum Loss() Splice Loss Distribution Splice Loss () More Figure 1: Laboratory Splice Loss Study Distribution Cumulative (%) Modeled Splice Loss Data When predicting splice loss for two fibers, there are two important factors: the size of the light carrying portion of the fiber and the location of the light carrying portion of one fiber with respect to the other. Typically, the size of the light carrying portion of the fiber is described by the mode-field diameter. The location of one fiber with respect to another is accounted for in the splice loss equations. These equations account for lateral offset, angular misalignment, and end separation. For standard single-mode fibers as well as the dispersion shifted family of fibers, equations derived from theoretical models have been quite accurate in predicting splice loss. Through the use of Monte Carlo techniques, splice loss distributions can be predicted for wide ranges of mode-field diameters and core-clad offsets. Use of the standard equations is still an excellent predictor of splice loss performance for LEAF fibers. With Monte Carlo techniques and standard splice loss equations, a distribution of the expected splice losses for LEAF fiber to LEAF fiber splicing can be created. These Monte Carlo results represent the expected distribution of losses for passive alignment splicers. Since active alignment splicers are commonly used for high data rate systems, an active alignment splice loss model was also developed. The active alignment splice loss model is based on worst case actual laboratory data. This data has few splice losses less than 0.02 and shows that the degree of alignment achieved between any given pair of fibers varies from splice to splice. Using this information, a transform using a distribution of degrees of alignment is applied to the theoretical splice loss distribution. The result is a predicted distribution of splice losses for active alignment splicing. The Monte Carlo simulations were performed using mode field diameters ranging from 9.0 m to 10.0 m and core-clad offsets as large as 0.8 m. Every effort has been made not to underpredict splice loss distributions with this model. As specifications are tightened, particularly for core-clad offset, splice performance will improve. The LEAF fiber Monte Carlo results for

3 both passive and active alignment systems are shown in Figure 2 and Figure 3. Table 3: Yields vs. Number of Splices for Passive Alignment Splicing Passive Alignment Yields Leaf to Leaf Monte Carlo Results for Passive Alignment Splicing % % % Splice Loss () Cumulative Figure 2: LEAF Fiber Monte Carlo Results for Passive Alignment Splicing 3 25% 15% 5% Leaf to Leaf Monte Carlo Results for Active Alignment Splice Loss () 0.19 Cumulative Cumulative Figure 3: LEAF Fiber Monte Carlo Results for Active Alignment Splicing The impact of the splice loss distribution on system budgets and installation costs can be very large. The difference between active and passive alignment yields are shown in Table 3and Table 4. Cumulative Criterion Budget () Single Splice Yield with 4 with 7 with 10 with % 99.6% % 97.8% 99.5% % 79.9% 84.5% 88.2% 91.6% Table 4: Yields vs. Number of Splices for Active Alignment Splicing Criterion Budget () Active Alignment Yields Single Splice Yield with 4 with 7 with 10 with % % % For active alignment splicing, even with a 0.10 splice loss budget, single fiber splicing should result in 98.4% yield. For series of four or more, the overall yield is predicted to be 10. The need to break and remake a splice typically will be a result of fusion splicing practices as opposed to fiber parameters. Use of splicer estimated loss or uni-directional OTDR measured loss need to be understood in light of these results. Splice Loss Measurement As early experience with SMF and recent experience with standard NZ-DSF indicated, unidirectional OTDR measurement of splice loss may be deceiving. To obtain accurate, reliable splice loss values, bi-directional measurements

4 must be made. The uni-directional measurement error is primarily a function of the differences in the mode-field of the two fibers. This effect will be slightly more pronounced in all standard NZ-DSF designs, due to the smaller mode field diameter (MFD) than in LEAF fiber. The measurement error is shown in Figure Calculated versus OTDR Uni-directional Splice Loss After stripping, the fiber should be wiped clean using an alcohol wipe. Make sure all coating and debris are removed before inserting the fiber into the cleaver. Once cleaved, the fiber end should not be wiped clean. Wiping may place debris on the fiber end face that could cause problems with splicing. For best results, insert the cleaved fiber into the splicer alignment holder before repeating the process for the other fiber. Loss (/km) SMF LEAF NZDSF Equipment maintenance is also important. The stripper should be in good condition and the proper size. The cleaver should have a sharp blade that is perpendicular to the fiber axis. If more than a few bad cleaves are noted, the cleaver blade may need to be cleaned, rotated or replaced Mode-Field Diamter Mismatch (m) Splice Loss LEAF Side A LEAF Side B LEAF Splice Loss NZDSF Side A NZDSF Side B NZDSF Splice Loss SMF Side A SMF Side B SMF Figure 4: Error Associated with Unidirectional Splice Loss Measurement Splice loss is more sensitive to cleave end angle when splicing NZ-DSF compared to standard single-mode fiber. Laboratory studies show that end angles greater than 2 degrees can more than double the splice loss as compared to standard single-mode fiber, as indicated in Figure 5. Fusion Splicing Practices 0.3 Splicing conditions have a much greater impact on splice loss when splicing NZ-DSF. These more complex fiber designs are more sensitive to critical parameters such as clean splicing conditions, the end angle of the cleave, and splicing procedures. Additional attention needs to be given to both the equipment and the work area to achieve consistently low splice loss. Average Blind Splice Loss () Dispersion-Shifted Single-Mode Fiber Standard Single-Mode Fiber There are a number of things to keep in mind when preparing the fiber for splicing. Cleanliness plays an important part in achieving quality. The work area and all splicing tools should be kept clean and free of debris such as cable filling compound stripped coating or fiber ends. The stripper and cleaver should be cleaned frequently so that the fiber is not damaged during splicing preparation. Multiple passes with the stripping tool can also damage the fiber, possibly causing it to break during cleaving, splicing or installation. The splice equipment should be kept clean and dust free. Debris in the fiber alignment area (V-grooves) may cause fiber misalignment that would result in a poor quality splice End Angle (degrees) Figure 5: The Effect of End Angle on Splice Loss As high data rate applications have evolved, splice equipment manufacturers have continuously improved the equipment they provide so that it is capable of splicing the new, more complex fiber designs. Adjusting machine settings to optimize results as well as developing algorithms to provide more accurate splice loss

5 estimations are just two improvements available to the end-user. There are many manufacturers of splicing equipment. Two equipment manufacturers recommendations are included below. Other splicing equipment manufacturers should be contacted directly for specific recommendations. Profile alignment splicers optically align the cores of the fibers to be spliced. The set-up of the splicer can be adjusted to improve performance. For older models such as the Fujikura FSM-20CS and FSM-20CSII, there are two switches that may be changed that will improve splicer performance with NZ-DSF. The first switch changes the cleave angle alarm threshold from 5 to 3, which is the smallest end-angle alarm available for this model. Tightening this parameter will decrease distortions of the fiber cores at the splice point, thereby minimizing the associated splice loss. The second switch changes the splice loss estimation algorithm to account for the smaller core and mode-field diameter of NZ-DSF (in comparison to standard single-mode fibers). The splice loss estimation algorithm does not affect actual splice loss; it does, however, provide the user with a more accurate estimation of actual splice loss. Corning recommends bidirectional OTDR splice loss measurement. For newer models such as the Fujikura FSM- 30S, one needs only to select the DS AUTO fiber splicing mode and change the cleave angle alarm threshold to optimize splicer performance. In this mode, the splicer automatically monitors and adjusts the arc power, which may be helpful in field conditions. The splice loss estimation algorithm is also automatically updated to account for the more complex fiber design. The user also can select the cleave angle alarm threshold; 2 is the recommended threshold for NZ-DSF. For Siecor s M90 fusion splicer with local injection and detection (LID) system, there are no changes to the splicer set-up required for single-fiber fusion splicing. These splicers optimize the transmitted power through the splice and the fuse time during splicing. Each works equally well with standard single-mode and dispersion-shifted fiber. Good cleaving is of course important, and the LID system has a built-in alarm to indicate high end-angles (angles > 1.6). Of particular value to the installer are the choices available for splice loss estimation. With these options, a more accurate estimate of splice loss is available for the NZ-DSF designs. For the M90 splicer, Table 5 shows how to choose the best option for splice loss estimation. Table 5: Settings for Siecor s M90 Splicer Settings Available SMF For LEAF choose: SMF SMF DS DS SMF DS LS LS The splice loss estimation algorithm does not affect actual splice loss. The same low loss can be achieved using splicers that have only the SMF option. Bi-directional OTDR splice loss measurement is recommended. CONCLUSIONS LEAF fiber can consistently provide lower loss with active alignment systems than other NZ-DSF designs. With attention to splicing practices and equipment set-up, LEAF fiber offers the same ease of installation as standard single-mode fiber designs. System designers can feel confident that they can take advantage of all the benefits of the LEAF design in their high data rate systems and still achieve excellent splice performance. ACKNOWLEDGEMENTS The authors wish to acknowledge Doug Duke from Alcoa Fujikura Ltd. and Todd Rhyne from Siecor for their helpful information and advice. REFERENCES 1. Sheila A. Cooper, J. Jeff Johnson, Randy L. McClure and Mary E. White, Splicing Non- Zero Dispersion-Shifted Fiber, Outside Plant, December, 1997, pp

6 2. Alcoa Fujikura Ltd., Splicing Guidelines for Dispersion-Shifted Fibers or Non-Zero Dispersion-Shifted Fibers, Alcoa Fujikura Splicing Applications Notes, OFSP-AN-002. BIOGRAPHIES and in her current position is the principal statistician for the Product Engineering Department. Her experience includes optical, environmental, mechanical and field applications testing. Mary E. White is a 1991 graduate of Auburn University where she received a B.S. in Mechanical Engineering. She joined Corning Incorporated in 1995 as an Applications Engineer in the Telecommunications Products Division responsible for providing technical support to optical fiber cable and end-user customers. She has published several other papers on applications in telecommunications and her experience includes cable manufacturing and field applications. Sheila A. Cooper is a Senior Product Engineer in Corning Incorportated s Telecommunications Products Division (TPD). She received her B.S. in Physics from Michigan Technological University. She has worked in TPD since 1988,