Radiation Testing of Commercial off the Shelf 62

The 62.5/125/250 micron optical fiber manufactured by Lucent SFT was tested for gamma radiation resistance at the NASA Goddard Space Flight Center Cobalt chamber. The fiber was tested with 1300 nm light, at 20.7 rads/min and 5 rads/min at –25°C and at 32.3 rads/min and 5 rads/min at +25°C. Data was recorded during exposure until the attenuation reached levels below that of the optical detection data acquisition system. All gathered data is presented here along with extrapolation equations for other dose rates. Background In the past for space flight applications,100/140 micron optical fiber was used for optical communication purposes. This fiber has been characterized for space flight environments under various conditions.[1-2] For several years, there has more interest in usage of commercial optical fiber products because of the limited availability of 100/140 micron optical fiber. Over seven years ago, Corning ceased manufacturing the 100/140 fiber and since then Lucent SFT (formerly Spectran Specialty Fiber) has been the only manufacturer of this fiber. NASA has been asked to look into other options for purposes of providing alternatives to the 100/140 micron graded index fiber for space flight environments. Specifically, the question has been asked: how does 62.5/125 perform in a space radiation environment? The dopants used in the 62.5/125 micron optical fiber are different from those used in the radiation hardened 100/120/172 micron fiber being used currently. Therefore, it was expected that there would be a difference in the radiation performance but until now, answering the question about how much of a difference in performance there would be, was based on speculation with no data. Experimental Set-up The radiation exposure was conducted at Goddard Space Flight Center’s Cobalt gamma radiation chamber. Four tests total were conducted to assess the performance of the 62.5/125 micron commercial fiber using two dose rate conditions for each thermal condition. In each case the fiber was tested until light could no longer be detected with the optical detection system used for in-situ monitoring throughout exposure. Table 1 summarizes the tests conducted. Table 1: Summary of experiments conducted Designation Test Length Temperature Dose Rate Total Dose* Input Level Spool A 100 m -25°C 20.7 rads/min 33.12 Krads .84 μwatt Spool B 100 m -25°C 5 rads/min 26.0 Krads .84 μwatt Spool C 100 m +25°C 32.3 rads/min 45.22 Krads .65 μwatt Spool D 100 m +25°C 5 rads/min 35.0 Krads .65 μwatt * Total dose is based on the last detectable transmission through each of the fiber spools. In Table 1 the details of the experiments are shown with the designation for each spool under test. The last column is the input light level registered prior to the 100 m spool or the output of the lead in cables coupling the source to the spools themselves. The level used for monitoring the transmission of light through the fiber during exposure is required to be below 1 microwatt to limit photobleaching effects.[2] Two of the spools of 62.5/125 micron fiber were tested at –25°C and tested at two different dose rates; 20.7 rads/min for spool A and 5 rads/min for spool B. With typical germanium doped multimode fiber, this thermal environment represents a harsher scenario than room temperature since the lower temperatures inhibit thermal annealing of the radiation induced attenuation. At room temperature or +25°C, the other two spools were tested at 32.3 rads/min for spool C and 5 rads/min for spool D. For all spools tested, data was recorded prior to testing and every minute during exposure. All spools were placed inside of lead boxes (to eliminate secondary low energy level reflections) and remained there through the duration of the test. Lead in and lead out cables were used to couple the source and detection equipment, through the chamber wall, to where the spools were located in the chamber. A thermal chamber (with enough room for one spool) was used in addition, to maintain a temperature of –25° C during and after the radiation exposure. The RIFOCS 752L dual wavelength LED source was used at 1300 nm. The signal was attenuated with the JDS HA9 optical attenuator such that all incoming light to the spools was less than 1 microwatt of optical power. The HP8166 multichannel optical power meter was used for detection of the output signal from the spools. Experimental Results Two exposure sessions were conducted to collect data for the two thermal and dose rate condition combinations, since the thermal chamber could only be used to test one spool at a time. The results of all testing conducted are graphed in Figure 1. Figure 1: Complete data set of radiation induced attenuation recorded for each spool in units of dB/m vs. time . It is evident by Figure 1 that all the spools experienced large radiation induced attenuation. The data has been scaled such that it is represented in dB/m units. Typical space flight applications will use 3 to 10 m of fiber when connecting instruments on a spacecraft for communication purposes. This would result in 1.5 to 5 dB for the short cables at the largest total dose registered here of ~ 25-40 Krads. However, the dose rates used for this testing are not exactly the dose rates expected for space flight. They may represent some periods of time but it is not usually the case that high dose rates such as these are maintained for any long duration over a few hours. The exposure at high dose rate in this testing was maintained for 3.5 to 4.5 days depending on the dose rate. The purpose of this was to collect enough data to formulate an extrapolation equation for other dose rates. In Figure 1, it is evident that when held at the same dose rate but at different temperatures, the fiber performs as expected for germanium doped multimode where the colder thermal environment represents a harsher case for a fiber during radiation exposure. Spool B and spool D are both exposed at 5 rads/min but spool B experiences more induced attenuation since the temperature is much colder at –25°C. For analysis the data was categorized into two segments by thermal condition and an extrapolation equation for each condition was formulated to make predictions for losses using other dose rates and total doses.[3] Results of radiation exposure at –25oC: Data set one is the radiation induced attenuation gathered at –25°C for spools A and B. The data for spool A and spool B is shown in Figure 2.