Characterization of a twelve channel optical fiber, ribbon cable and MTP array connector assembly for space flight environments

Presented here is the second set of testing conducted by the Technology Validation Laboratory for Photonics at NASA Goddard Space Flight Center on the twelve optical fiber ribbon cable with MTP array connector for space flight environments. In the first set of testing the commercial 62.5/125 cable assembly was characterized using space flight parameters (published in SPIE Vol. 3440 ).[1] The testing showed that the cable assembly would survive a typical space flight mission with the exception of a vacuum environment. Two enhancements were conducted to the existing technology to better suit the vacuum environment as well as the existing optoelectronics and increase the reliability of the assembly during vibration. The MTP assembly characterized here has a 100/140 optical commercial fiber and non outgassing connector and cable components. The characterization for this enhanced fiber optic cable assembly involved vibration, thermal and radiation testing. The data and results of this characterization study are presented which include optical in-situ testing. 1. BACKGROUND This is a follow up paper to the publication entitled "Twelve channel optical fiber connector assembly: from commercial off the shelf to space flight use," SPIE vol. 3440. In the first paper the commercial version of the MTP and ribbon cable assembly was investigated and suggestions were made to enhance its performance. This paper investigates the results of those enhancements through performance testing. The twelve fiber ribbon cable with MTP array connector was selected to support the cable harnessing for the Spaceborne Fiber Optic Data Bus (SFODB) on space flight missions selected to utilize this communications system. The ribbon cable itself was selected with a Kynar jacket and manufactured by W.L. Gore. The MTP connector with the two enhancements mentioned above was manufactured by USCONEC. The terminations were fabricated by USCONEC using space flight procedures as adjusted to the array connectors and with compliance to NASA-STD-8739.5. Originally a materials analysis was conducted on the commercially available 62.5/125/250 micron ribbon cable and MTP array connector assembly to determine which materials would be suitable for a vacuum environment. The analysis led to the enhancements of the boot and ferrule boot components of the connector. A second enhancement to increase the reliability of the connection link during vibration was made to the commercially available cable/connector assembly. The ferrule size was changed from the 125 micron hole size to the 140 micron hole size to accommodate the 100/140 micron optical fiber that was preferred by the optoelectronics provider for the SFODB system.[1] The test plan chosen for this study was designed to bring out known failu re mechanisms of the cable assembly system and this sequence of environmental testing was developed based on previous research.[2-4] This characterization study was conducted in three sections 1) vibration characterization, 2) thermal characterization and 3) radiation characterization, respectively. The sequence of testing was conducted in this order with visual inspections before and after each test and optical performance monitoring conducted in-situ, for some channels, based on equipment availability. 2. EXPERIMENTAL SUMMARY Three mated pair connector/cable assemblies were tested of length 5.24 m and the sequence and summary of testing are in table 1. * [email protected], Sigma Research and Engineering at NASA Goddard Space Flight Center, Code 562, Greenbelt MD 20771, 301-286-0127, 301-552-6300 URL’s: misspiggy.gsfc.nasa.gov/tva/authored/fo_photonics.htm, and www.sigmaspace.com. Table 1: Cable Channels monitored during testing Test Cable Designation Channel Optically Monitored Channel Visual Inspection Vibration DUTA 1,3,5,6,8,10,12 All DUTB 1,3,5,6,8,10,12 All DUTC 1,3,5,6,8,10,12 All Thermal DUTA 1, 5, 8, 12 All DUTB 1, 5, 12 All DUTC 1,3,5,6,8,10,12 All Radiation DUTA 12 All DUTB 12 All A visual and optical performance verification was performed prior to any environmental testing on all Device Under Test (DUT) cable sets and reference fan out cables. Three mated pairs of cables were tested during vibration and thermal characterizations and two of the three cables were testing during radiation characterization. 3. VIBRATION CHARACTERIZATION The main purpose of the vibration testing was to verify that the MTP assemblies would in fact survive typical launch vehicle conditions. In addition however, an interrupt test was conducted to verify whether the connector assemblies could be in operation during a launch if necessary. The MTP ribbon connector assemblies were tested in three dimensions x, y, and z. The same fixturing and axis conventions were used as were used during the research conducted in 1998 [1]. The HP81552SM 1310 nm laser source was used for in-situ testing, coupled into a 1 X 8 100/140 micron diameter optical fiber coupler. The coupler mated to the input, reference fan out cable and then into lead cables that mated to the DUT mated pair, which was attached to the vibration shaker fixturing. During each axis test, channel six was monitored with an optical to electrical converter and oscilloscope in order to detect interrupts occurring larger than a 25 micro second sampling rate. The remaining channels were monitored via the HP8166 multichannel optical power multimeter for slow optical power changes during testing. The parameters used for this testing are summarized in Table 2 and are based on typical launch conditions for components under prototype testing. The actual vibration test level used to qualify components (small boxes) is 14.1 grms for the protoflight level. The actual survival test level used to test the spacecraft instrumentation is lower than the parameters specified here. Table 2: Vibration profile for MTP vibration testing Frequency (Hz) Protoflight Level 20 .052 g/Hz 20-50 +6 dB/octave 50-800 .32 g/Hz 800-2000 -6 dB/octave 2000 .052 g/Hz Overall 20.0 grms The duration for testing at each axis position was three minutes and the axis sequence of testing was x, y, z respectively. After each axis test was completed, all optical fiber end faces were visually inspected for damage. Optical data was collected during the testing and recorded via lap top computers to excel spreadsheet files using custom written Labview data acquisition programs. 3.1 Vibration Characterization Results For each mated pair cable assembly under test, two types of optical in-situ testing was conducted. For each DUT, six channels were monitored actively with the HP8166 for transmission interruptions or “static” losses that occur slowly while testing. One channel, channel six, of each mated pair was monitored with an optical to electrical (O/E) converter and digital oscilloscope to measure intermittent transmission losses. A 25 microsec sampling rate was the shortest sampling rate we attained for the in-situ “dynamic” monitoring of channel six. Again, the point was to monitor for events that signified a drop out of optical power transmission (or transient) through the mated pair. For each axis, this data was collected for each mated pair. The laser source was monitored during data acquisition from the HP8166 such that the variation of the laser power was subtracted from vibration induced variations in transmission. Concluding each axis test, a visual inspection of the MTP end-face was performed and as a result no damage was detected as compared to the initial state of each connector end-face recorded prior to testing. In Table 3 the data is summarized for each cable and axis test, in the order the tests were conducted. The data presented is for channel six of each cable set tested using a 25 microsec sampling rate. The test duration was 3 minutes or 180 seconds and data was recorded for several seconds just before and after the vibration test. In Figure 1, the axis designations are defined and Figure 2 shows pictures of the testing set up. With the exception of DUT A during the x axis orientation vibration test, most of the event losses were around or less than 0.1 dB and the events lasted no more than the sample rate of 25 microsec. Table 3: Vibration induced events (dynamic losses) for channel six of each cable assembly, mated pair for each axis DUT Test Set Vibration Test Axis* Loss events registering below noise level A X Less than 1.2 dB A Y Less than 0.1 dB A Z Less than 0.1 dB B X Less than 0.1 dB B Y Less than 0.1 dB B Z Less than 0.1 dB C X Less than 0.1 dB C Y Less than 0.1 dB C Z Less than 0.2 dB Figure 1: Axis designations Figure 2: Pictures of test set up for vibration testing. In Table 4, the vibration data is summarized for the remaining channels that were monitored optically during testing for “static” losses. The data summarized in Table 4 represents the slow losses measured at approximately a 5 sec sample rate. These are referred to as the static losses because they are slow changes in transmission performance that occur as a result of vibration induced effects. The source power drift noise, which was never greater than .03 dB maximum, at any given time, was subtracted out of the final data summary. x z y Table 4: Summary of MTP vibration data (static losses) on channels 1,3,5,8,10, & 12. Cable Assembly (DUT) Axis Test Transmission Loss Recorded A X Less than .08 dB A Y Less than .10 dB A Z Less than .11 dB B X Less than .13 dB B Y Less than .35 dB B Z Less than .03 dB C X Less than .15 dB C Y Less than .40 dB C Z Less than .28 dB Figures 3 and 4 display some of the data for DUT B summarized above in Table 4. Due to the large amount of data collected, only two graphs are included here to provide examples of the entire data set. These graphs represent the “static” losses as a result of vibration testing during the three minute test. In some cases it can be noted that the performance during the static or slower monitoring of the transmission is similar to the channel six dynamic high speed tests summarized in Table 3 and in other cases some of the monitored channels show a shift in alignment of the MTP interconnection during the testing. Some cable data shows some channels shift into a gain transmission situation while other cables show decrease in optical transmission. This can be seen in Figure 3 during the y vibration testing of DUT B and could be due to a twisting along the z direction. In Figure 4 the z axis testing of DUT B there seems to be a gain over the duration of the testing. This could indicate a movement along the x axis that could cause better alignment along this axis. Another cause for this would be if the connectors are not actually physically touching as they are supposed to do when inside the connecter adapter, they could be moving uniformly in the z direction and forcing a better physical contact between the two interfaces of each connector end. In all cases, the losses never exceeded 0.5 dB for any of the channels tested and none of the optical fiber interfaces were damaged as a result of the vibration launch conditions used in this experiment. This fact was verified through visual inspection. Optical transmission for MTP DUT B during X axis vibration testing -0.200 -0.100 0.000 0.100 0.200 0.300 0.400 0.500 1 4 7 10 13 16 19 22 25 28 31 34 37 data point (~5 sec/point) O p ti ca l t ra n sm is si o n ( d B ) Ch 1 Ch 3 Ch 5 Ch 8 Ch 10 Ch 12 Optical transmission for MTP DUT B during Y axis vibration testing -0.500 -0.400 -0.300 -0.200 -0.100 0.000 0.100 0.200 0.300 0.400 1 4 7 10 13 16 19 22 25 28 31 34 37 Data point (~ 5 sec/point) O p ti ca l t ra n sm is si o n ( d B )