Radiation effects data on commercially available optical fiber: database summary

Presented here are the results of a survey performed at NASA Goddard Space Flight Center on radiation data for commercially available optical fiber. The survey includes data from the past decade that can still be used for currently available parts. The objective of this work was to provide a current central location for relevant radiation data on optical fiber that could be useful for space flight projects. Only valid part numbers that are currently relevant to the manufacturers producing the optical fiber and parts that have been verified as changing number but not process, are included in this study. Presented are the currently available optical fiber types with valid part numbers and manufacturer, the relevant radiation data, and the corresponding data reference. Also included in this summary is the recent unreleased radiation data from experiments conducted at Goddard Space Flight Center on OFS (formerly Spectran) 62.5/125/250 micron optical fiber. This database will serve as a tool for engineers when selecting radiation tested optical fiber for future and current optical fiber systems for space flight applications.

INTRODUCTION Over the past two decades, a great deal of optical fiber radiation data has been published. Although much of the early work established the standard for how the testing would be conducted in later years, the optical fibers tested ten years ago or more are no longer produced. The push for greater bandwidth in telecommunications has focused the market into a situation where products are rapidly changing as improvements are made and new applications are designed. In addition, many manufacturers and vendors are becoming extinct with the subsequent saturation of the telecommunications market. Those manufacturing plants that remain in production are rapidly changing ownership. Therefore, all this perpetual change is making parts tracking a challenge for those parts that do remain in demand. In an attempt to sort out what data from the past can still be applied to currently produced optical fiber, a survey of available part numbers and manufactures was conducted to verify that the data in publication is still relevant to procurable products. Having a central database for current products with relevant radiation data will save a great deal of research time and the cost of testing for space flight projects that require performance information on commercially available optical fiber. The results of this survey, to date, are presented here.

II. CRITERIA FOR PRODUCTS INCLUDED IN DATABASE
After conducting a thorough literature search for all available data on optical fiber, it became apparent that not all papers that presented optical fiber results included part numbers that could be traced to a source for procurement. In many cases preforms or materials were tested that would be used for optical fiber but not a product that could be directly traced to a manufacturers part number for an optical fiber. Because this database is for the purpose of parts selection, the majority of the data included in the summary pertains to a final product with a part number. In a few cases, the data presented is representative of a family of optical fiber that can be traced to a manufacturers part number for fiber that only differs by dimension. In the case of fiber family data, the data is included to provide a general sense of how a fiber end product from the given family would perform. However, a majority of the survey data listed is for specific optical fiber parts (that are not in a cable configuration(. This is due to the fact that a variety of parameters will affect the radiation susceptibility of an optical fiber including drawing process and outside fiber coatings. [3] In addition the lot-to-lot variability will result in enough uncertainty to dictate that other variables be limited for part selection purposes. Therefore, most of the parts and references included in this survey are for products that exist currently such that the information can be verified through the manufacturer. Parts are included if the process by which they have fabricated is unchanged even if the part number has. During this survey, each manufacturer provided information to verify the parts included here if the parts were not currently in their catalog..

III. BACKGROUND ON VENDOR CHANGES
Although it would be very difficult to capture the entire history of company procurements of other companies that produce optical fiber, some historical details are briefly discussed. One of the main vendors for graded index 100/140 micron optical fiber for space flight was Spectran. Spectran was bought by Lucent SFT in 1999, was procured by Furukawa Electric in 2002 and for a brief time was called OFS Fitel as part of the Specialty Photonics Division. They have recently settled on the name "OFS". However, the Spectran part numbers are unchanged since prior to the Lucent SFT name change. Certain Spectran part numbers contained in references 1 and 3, that were fabricated in the specialty optical fiber manufacturing plant in Sturbridge Massachusetts, are mostly obsolete now with the exception of a few.
Siecor was formed from a venture between Siemens and Corning and was primarily a cabling plant in the United States but had a fiber manufacturing plant in Germany. Corning later absorbed company Siecor and called it Corning Cable Systems. This entity currently remains another division of Corning. Corning licensed their patent for polarization maintaining (PM) fiber to Fujikura who now supplies Corning with their PM fiber. There have been slight modifications to the drawing process due to the large quantities that were required in the past five years from Fujikura. There have also been slight variations of the dopant concentration in the fiber but the basic dopant structure has remained unchanged. The Corning PM fiber data is included in this database for reference purposes but is not completely transferable to the current Fujikura products. The company Litespec was closed in May of 2002 and will no longer represent parent company Sumitomo Electric Lightwave Corp. Finally, the parent company of Plasma Optical Fibre is Draka Fibre Technology B.V. in the Netherlands.

IV. DATA RESOURCES INCLUDED IN STUDY
Data included in this summary was collected from a variety of sources with most of the information gathered from peer reviewed conference and journal publications. The dose rate and total dose parameters used for a majority of the experiments that are included in this database do not represent actual space flight radiation environments. This data is included however, for reference purposes. Where possible, this data that is more representative of military concerns instead of space flight conditions, was included with extrapolation results to lower dose rate conditions. Other high dose rate data that does not include extrapolation equations for a lower dose rate environment is present as well.
All available data is included provided the product tested can be traced to a part number and is still in production today. Many of the references used in this study include both valid and obsolete part numbers. For those interested in which parts are now obsolete, more information on this can be extracted by examining the references themselves and is left as an exercise for the reader. The focus for this database is to gather current part numbers and radiation data. The references themselves are included that have been already accepted in the public domain. The few publications that were not extracted from journals and proceedings are from authors and researchers that are well known for other relevant references that have been peer reviewed and published. Table I presents the summary of multimode optical fiber parts that are still available for procurement with a brief description and the reference that was used to provide the radiation data. Table II contains the collected information on single mode products and Table III contains the summary of polarization maintaining single mode products still in production. Tables IV-VII contain the actual radiation data results, details of testing, and extrapolation information with reference to usage in space flight applications. Tables IV -VI contain gamma radiation exposure data and Table VII contains two entries of neutron exposure results. Neutron exposure is less damaging to optical fiber performance than gamma radiation. Fiber type MM-D2 has data entries in Table VI for gamma  exposure and Table VII for neutron exposure for comparison purposes. For a more in-depth comparison on the differences between neutron and gamma radiation more information is available in reference 13.

V. SUMMARY OF OPTICAL FIBER IN DATABASE
In Tables IV-VI it is evident that in many cases extrapolation data is not available for space flight environments. The dose rates are too high or the references do not provide enough information such that an extrapolation can be made. The information included in these tables provides the parameters that are considered significant to the performance of an optical fiber during radiation exposure. In most cases unless otherwise noted, the data was collected using optical power of 1 microwatt or less. In some cases optical power used was not specified or the source used was from an optical time domain reflectometer (OTDR) instrument. It was noted in the details when a test was conducted using a source power greater than 1 microwatt if this information was available. Lengths for testing usually ranged from 50 m to 100 m and the data was scaled to km. In some cases the attenuation data was represented in terms of dB/m and for the sake of being consistent for this database was converted to dB/km. For guidance purposes, most space flight environment gamma exposure rates are typically less than 1 rad/min and more likely less than .01 rad/min. In a few other cases due to solar activity the rates can be quite high for short durations. For example, the actual dose rate requirements for the International Space Station (ISS) were 42 rads/min for two hours at -125ºC and then down to typical space flight exposure rates for the remainder of the mission [4]. Although it is less common to find higher dose rate requirements for space flight missions, almost all missions have unique requirements.
There are a variety of lengths required for space flight missions. ISS required lengths between 30 m and 100 m while missions such as the NASA Microwave Anisotropy Probe (MAP) and Earth Observing 1 (EO-1) required lengths of assemblies less than 3 m. For the Geoscience Laser Altimeter System (GLAS), most of the assemblies were less than 3 m with the exception of the delay line which was 2 km. In cases of short lengths and forgiving power budgets, even some of the "poor" performers can be used since the values here are in terms of km. Therefore, an optical fiber with an attenuation of 1 kdB/km at low temperature will be 3 dB total for a 3 m length and then could be even lower when the temperature is increased. When comparing performance, the lengths of assemblies, thermal requirements, operational wavelength, dose rate and total dose are the important parameters to consider for choosing a fiber based on the collected data presented here. Actual Experimental Data** ** Data was taken using 10 microwatts of power, where less than 1 microwatt is typical, therefore photobleaching is present. Actual Experimental Data ** ** Data was taken using 10 microwatts of power, where less than 1 microwatt is typical, therefore photobleaching is present.

VI. CONCLUSION
Presented here is a database of available optical fiber radiation testing performed in the last decade. The optical fiber products included in this study are currently available through a manufacturer catalog or by special order. Parts that are now considered obsolete by the manufacturer were not included in this survey. It is expected that the information supplied here will have to be updated every couple of years such that the parts list remains current with the available fiber technology and with the available radiation data. In future reports, more information on specialty fiber and fiber devices can be included such as rare earth doped optical fiber and fiber gratings. The actual database information is available at the website misspiggy.gsfc.nasa.gov/photonics and at www.nepp.nasa.gov.