**FIBER OPTIC CABLE ASSEMBLIES FOR SPACE FLIGHT II:**

** THERMAL AND RADIATION EFFECTS**

MELANIE N. OTT

Technology Validation Assurance Group,

Goddard Space Flight Center / Swales Aerospace

E208A Building 11

Greenbelt, MD 20771

301-286-0127

melanie.ott@gsfc.nasa.gov

http://misspiggy.gsfc.nasa.gov/tva

KENNETH A. LABEL

Radiation Effects Group

Goddard Space Flight Center

C116D Building 11

Greenbelt, MD 20771

301-286-9936

kenneth.label@gsfc.nasa.gov

M. ANN GARRISON-DARRIN

Assistant for Technology, Electrical Systems Center

Goddard Space Flight Center

S-18A Building 6

Greenbelt, MD 20771

301-286-8884

ann.darrin@gsfc.nasa.gov

***Presented at the Aerospace Electrical Interconnect System
Conference***

***Williamsburg Virginia, Oct. 1997.***

ABSTRACT

Goddard Space Flight Center is conducting a search for space flight
worthy fiber optic cable assemblies that will benefit all projects at
all of the NASA centers. This paper is number two in a series of papers
being issued as a result of this task to define and qualify space grade
fiber optic cable assemblies. Though to qualify and use a fiber optic
cable in space requires treatment of the cable assembly as a system, it
is very important to understand the design and behavior of its parts.
This paper addresses that need, providing information on cable
components shrinkage testing and radiation testing results from recent
experiments at Goddard Space Flight Center. This work is an extension of
the "lessons learned" reported in the first paper of this series
entitled "Fiber Optic Cable Assemblies for Space Flight: Issues and
Remedies," published and presented at the AIAA World Congress in Anaheim
CA, on October 15, 1997.\[1\]

INTRODUCTION

As part of the NASA Advanced Interconnect Program, GSFC has been
investigating alternatives for fiber optic communication applications.
In particular the MIL-STD-1773 is being implemented more readily in
space flight than ever, possibly due to several successful missions
using the communications bus. Previously this communications system used
100/140 micron multimode fiber from Corning with a cable made by Brand
Rex. Due to the discontinuation of this fiber by Corning, it became
necessary to acquire a new space flight grade fiber optic cable and
conduct qualification testing and analysis to determine its stability
during missions of different duration. Therefore, the goal of this
program has been to determine which commercially available fiber optic
cable assemblies could be best suited for general space flight missions
of approximately 10 years in duration. This paper discusses the recent
findings on cable related issues and experiments that have been used to
better quantify the performance of commercially available fiber and
cable. Two issues in particular, the shrinking of the materials used in
cable components and the radiation effects on available fiber, left many
unanswered questions in the first paper issued on this topic \[1\]. As a
follow on to that investigation these two subjects have been examined
more closely for clarification. Several different cables are examined
here for shrinkage characteristics and one type of available multimode
hermetic acrylate fiber from Spectran is examined for radiation
hardness.

PART ONE: THERMAL EFFECTS AND TESTING

BACKGROUND

Shrinking of Cable Components: In the first paper giving status on this
investigation \[1\] it was established that fiber optic cable components
experience an overall permanent shrinkage as a result of thermal
cycling. This effect is different from the tendency of these materials
to contract and expand in temperature extremes. This dimensional
transient effect in a changing thermal environment is characterized by a
materials thermal coefficient of expansion. The type of shrinkage being
discussed here is permanent. Once the cable components have shrunk in
size, post thermal cycling they do not return to the previous state.

There are several theories for why the permanent shrinkage occurs with
one being that the extrusion process for depositing these materials on
top of an optical fiber induces stresses that tend to relax and result
in shrinkage at high temperature extremes. In wire cable this does not
greatly effect performance although it can create a reliability hazard
where as in optical fiber constructions it has large effects on signal
transmission integrity as well as reliability.

Both thermal effects mentioned (transient and permanent) have an
attenuation effect on the performance of the cable. In addition to
attenuating an optical transmission, the permanent shrinkage will also
impact the reliability of a termination by pulling away from the
connector that it is bonded to or by pulling the connector away from the
fiber. In either case the permanent shrinkage is a concern and limiting
that shrinkage is a goal for manufacturers of fiber optic cable for
harsh environments. For single mode applications the transient effects
of thermal shrinkage can have destabilizing effects on the transmitted
phase of the signal and the power, whereas the permanent effect will
cause a static phase shift and a greater permanent attenuation. Due to
this effect it is necessary to mitigate the expansion and contraction
transient thermal effects in a single mode application, by choosing a
suitable cable configuration. This "suitable" cable configuration should
have a lose or "looser" tube buffer, as opposed to the tight tube
configuration more common to slower optical communication systems that
can utilize multimode fiber. However, for multimode applications the
overall shrinkage is the issue of concern here for tight tube and loose
tube buffer configurations.

Although projects and manufacturers have used preconditioning procedures
to limit the shrinkage of fluoropolymers it has been found that in some
cases these procedures can be inadequate to limiting the shrinkage when
preconditioned for the usual eight cycles. In other words, the shrinkage
of a cable's components is specific to each cable type. Thermal
preconditioning may limit very little of the shrinkage, and the cable
may continue to shrink by the large amounts after a preconditioning
procedure has been used.

DISCUSSION

Five different types of cables candidates where measured for overall
cable component shrinkage: W.L. Gore Prototype 1, Spectran Flight Guide,
Brand Rex OC-1008, and Northern Lights Hytrel jacketed single mode cable
and multimode cable. The W.L. Gore FON 1008 multimode cable was
monitored for static optical changes in attenuation after thermal
cycling. The W.L. Gore 8388 Prototype 1, is called such for the reason
that this cable in its exact configuration was improved during this
testing and another version is soon to be released. Therefore, the
results of this cable can be used for information purposes or as a
reference but do not reflect the performance of the final space flight
version of the new prototype being developed by W. L. Gore currently.

The Brand Rex OC-1008 has been used in past space flight projects at
Goddard Space Flight Center but contains the multimode optical fiber
that has since been discontinued by Corning. Brand Rex now makes this
same cable configuration with polyimide coated optical fiber from
Spectran in place of the acrylate multimode from Corning used in the
OC-1008.

The W.L. Gore 8388 Prototype 1, the Brand Rex cable and the Spectran
Flight Guide were tested up to 60 thermal cycles from -30C to 140C at
1C/min with a 5 min dwell at both temperature extremes. This rate was
decided by EIA standards that state to limit thermal shock, cables
should be heated at a rate less than 40C/hour. This test was run at the
lowest rate possible for the thermal chamber being used in this
experiment. It is also worth noting that this thermal chamber is used
frequently for thermal preconditioning of space flight cable assemblies.
The Hytrel jacketed cable by Northern Lights and the W.L. Gore FON 1008
was tested to 36 thermal cycles.

Each cable that was not optically monitored, was cut to approximately a
3 meter piece and measured for overall length. Each cable that was
unterminated had all cable components cut to the same length. Those that
were terminated were measured for output power. The FON 1008 cable was
terminated with ST connectors and 100 meters for each of the two samples
was used. One sample of the Hytrel jacketed cable from Northern Lights
was terminated for optical measurements and was a length of
approximately 4 m. The summary table below describes the parameters of
importance for the cables and some results from the thermal cycle
testing.

<table>
<tbody>
<tr class="odd">
<td><p>Manufacturer</p></td>
<td><p>W.L. Gore</p></td>
<td><p>Spectran</p></td>
<td><p>Brand Rex</p></td>
<td><p>Northern Lights Microcable</p></td>
<td><p>Northern Lights</p>
<p>Microcable</p></td>
<td><p>W. L. Gore</p></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td><p>Part Number</p></td>
<td><p>8388 Prototype 1</p></td>
<td><p>Flightguide</p></td>
<td><p>OC 1008</p></td>
<td><p>1-HY-MC-62CFD</p></td>
<td><p>1-HY-MC-10C</p></td>
<td><p>FON 1008</p></td>
<td><p>Total samples tested</p></td>
<td><p>4</p></td>
<td><p>6</p></td>
<td><p>4</p></td>
<td><p>3</p></td>
<td><p>3</p></td>
<td><p>2</p></td>
<td><p>Length of samples</p></td>
<td><p>~ 3 m</p></td>
<td><p>~ 3 m</p></td>
<td><p>~ 3 m</p></td>
<td><p>~ 3, 4 m</p></td>
<td><p>~ 3 m</p></td>
<td><p>100 m</p></td>
<td><p>Measurement</p></td>
<td><p>Length</p></td>
<td><p>Length</p></td>
<td><p>Length</p></td>
<td><p>Length, Optical</p></td>
<td><p>Length</p></td>
<td><p>Optical</p></td>
<td><p>Jacket Material</p></td>
<td><p>Fluoropolymer</p></td>
<td><p>Tefzel</p></td>
<td><p>Tefzel</p></td>
<td><p>Hytrel</p></td>
<td><p>Hytrel</p></td>
<td><p>Fluoropolymer</p></td>
<td><p>Outer Diameter</p></td>
<td><p>2.5 mm</p></td>
<td><p>.9 mm</p></td>
<td><p>2.75 mm</p></td>
<td><p>.9 mm</p></td>
<td><p>.9 mm</p></td>
<td><p>1.16 mm</p></td>
<td><p>Total Number of cycles</p></td>
<td><p>60</p></td>
<td><p>60</p></td>
<td><p>60</p></td>
<td><p>36</p></td>
<td><p>36</p></td>
<td><p>36</p></td>
<td><p>Ave Total % shrinkage</p></td>
<td><p>3</p></td>
<td><p>1.58</p></td>
<td><p>2.43</p></td>
<td><p>1.14</p></td>
<td><p>2.25</p></td>
<td></td>
<td><p>Total attenuation</p>
<p>(28 cycles)</p></td>
<td><p>--</p></td>
<td><p>--</p></td>
<td><p>--</p></td>
<td><p>--</p></td>
<td><p>--</p></td>
<td><p>-.25 dB/100m</p></td>
</tr>
<tr class="odd">
<td><p>Total Attenuation (36)</p></td>
<td><p>--</p></td>
<td><p>--</p></td>
<td><p>--</p></td>
<td><p>3.6 dB/4 m</p></td>
<td><p>--</p></td>
<td><p>.084 dB/100m</p></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
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<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
</tbody>
</table>

Table 1: Summary of Cables Tested with Shrinkage and Attenuation Results

**Figure 1: **

**Results of Cable Component Shrinkage of Spectran Flightguide, **

**Brand Rex OC-1008 and W.L. Gore 8388 Prototype 1**

**Figure 2**

**Shrinkage Results of Northern Lights Hytrel Coated Optical Fiber
Cable**

SUMMARY

From viewing Table 1, Figure 1 and Figure 2 it is apparent that the
cables that had the most amount of cable component shrinkage were of the
largest outer diameter. In third place for the greatest amount of
overall shrinkage was the Hytrel coated single mode cable from Northern
Lights which was only cycled 36 times and shrunk by large amount. Had
the cycling continued for this Northern Lights cable it may have ended
up shrinking more than the larger diameter cables had.

It was previously stated in paper one of this investigation \[1\] that a
typical preconditioning procedure consisted of 8 thermal cycles and that
this procedure would alleviate further materials shrinkage. In some
senses it is true especially with cables such as the W.L. Gore Prototype
and the Brand Rex cables. However, by looking at Figure 1 it is obvious
that while 8 cycles could limit the amount of shrinkage over temperature
cycling after this preconditioning, 18 cycles makes more of an impact.
The Spectran Flight Guide appears to shrink by almost the same amount
each cycle session. In reference 1, it is stated that after 50 cycles
the shrinkage would cease. Although it seems to continue shrinking after
the 52^nd^ thermal cycle all cables have shrunk by less than .1%
relatively by the 60^th^ thermal cycle. It is apparent that each cable
behaves slightly different from the others, which is not surprising
since each cable uses slightly different materials and fabrication
processes. While preconditioning makes a dramatic difference to the
shrinkage of the W.L. Gore prototype after the 18 cycle or even after
the 12^th^ cycle, the Spectran seems to be less affected in that it
continues to shrink each session by nearly the same amount but slowly
decreases relative to the previous cycling session. There is also a
large difference in the way both cables from Northern Lights perform
under thermal transients. Figure 3 plots the shrinkage of all cables
tested for comparison purposes. Although there is a line connecting the
% shrinkage of the Hytrel cables from cycle 24 to cycle 31, the cables
were put in the over just after cycle 24, so this part of the plot
deserves no attention.

**Figure 3**

**Shrinkage Results of all Cables Tested **

In Figure 4 the shrinkage of cable components is characterized by the
power output of the optical fiber. The assumption is that as the cable
components shrink microbend attenuation will cause a power loss of the
transmission signal. This effect is most obvious in the Northern Lights
Hytrel jacketed multimode cable. The power drops from 8.6 mwatts to 3.7
mwatts and then rises again to 5.3 mwatts after the 22^nd^ cycle. This
is also true for the W.L. Gore FON 1008. Although it loses very little
power over the thermal cycling process due to very little shrinkage of
this cable it also begins to rise again in power after cycle 28 for
sample one and cycle 14 for sample two. A logical reason for this could
be that the connector being bonded so well to the cable components
begins to pull back from the fiber, that does not shrink, and in doing
so pushes the fiber farther through the ferrule causing a protrusion,
allowing more light to be coupled through the gap that is usually
between the connector ferrules. It is apparent from this more sensitive
test, that the W.L. Gore FON 1008 shrinks very little in comparison with
some of the other candidates and loses little optical power of less than
.25 dB/100 meters. (Table 1). In summary it appears that the cables that
performed the best over thermal cycling was the W. L. Gore FON 1008, due
to the very low induced attenuation and the non detectable length
changes. Even though this 100 meter cable was not measured for detection
of length changes, it was evident that very little shrinkage was
occurring by viewing the area bonded to the connector. This type of
stress from the cable pulling back from the connector was however,
detectable in the Hytrel jacketed Northern Lights multimode cable which
actually by length measurements shrunk less than the unterminated
Northern lights single mode microcable.

**Figure 4**

**Optical Attenuation as a Result of Cable Component Shrinkage**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

PART TWO: RADIATION EFFECTS AND TESTING

BACKGROUND

It is a well known fact that an optical signal can attenuate when
transmitted through an optical fiber in a radiation environment. The
question with each optical fiber is: by how much does the signal
attenuate? Many parameters affect an optical fibers ability to withstand
a harsh radiation environment including: operating wavelength, materials
used for doping, temperature of operation, fabrication impurities,
coatings, draw speed, draw tension, dose rate and total dose.

Attenuation occurs in optical fiber as a result of ionizing radiation
generating color centers in the material. The color centers are areas in
the material that appear opaque to some wavelengths of light and
transparent to others. The color centers have high absorption of light
wavelengths in the UV portion of the electromagnetic spectrum and less
absorption at longer wavelengths in the infrared portion of the
spectrum. A fibers tendency to attenuate is sometimes suppressed by its
self healing abilities.

Optical fiber can thermally anneal and therefore "recover" from the
effect. This annealing is thermally activated and can offset the color
center generation better at higher temperatures than a lower
temperatures if the core dopant used in the fiber is not phosphorous or
boron. However, the dose rate must be slow enough for a fiber to reach
an equilibrium. This equilibrium is achieved when the competition
between the annealing properties and the generation of the color centers
evens out to a steady state attenuation. This behavior is called
"saturation". Fibers doped with germanium or simply optical fibers that
are pure silica are most prone to a saturation behavior with pure silica
core fiber being faster to saturate. Pure silica fiber is used in single
mode applications. Multimode applications that demand greater bandwidths
require graded index fiber which must be doped with some type of
material.

The ability of a fiber to self heal or anneal is aided by the light that
transmits down the fiber while in a radiation environment and this is
called "photobleaching". Continuous signal power over 5 to 10 microwatts
will stimulate this photobleaching effect thereby limiting the
attenuation caused by radiation. In the past, researchers were stating
that greater OH (hydroxyl) content in the fiber would desensitize the
fiber to radiation, but OH has a high absorption around 1390 nm. It is
also true the previously multimode optical communications were operating
at a wavelength around 850 nm and now systems are using 1300 nm making
the elevation of OH content not an option for systems that operate at
this longer wavelength. Manufacturers are well aware of the fact that
impurities introduced during the fiber drawing process will make a fiber
sensitive to radiation by allowing more opportunities for color centers
to form. Most maintain a clean environment to assure that no impurities
enter into the drawing fiber.

Coatings as well, have an affect on a fibers performance in a radiation
environment. In some cases hermetic coatings themselves help the
performance by disallowing the migration of impurities from the external
coatings and materials into the fiber itself. Polyimide coating has been
preferred over acrylate coating for the radiation hardness properties it
provides, however it has more to do with the process with which the
coating is fabricated. During the polyimide coating process the fiber is
heated to 400 - 500 C and coating the fiber with acrylate requires a UV
cure that could actually create a residual damage. For space flight
applications Goddard Space Flight Center prohibits any mechanical
stripping of the fiber coating and uses chemical stripping only. This
causes problems with the usage of chemicals because to strip the
polyimide, hot sulfuric acid is required and to strip acrylate a
methylene chloride solution similar to paint stripper is required and is
safer for everyday use. Therefore, part of the study here is dedicated
to determining the radiation hardness of hermetic acrylate coated
100/140 micron fiber that can be easily and safely chemically stripped.

Typical data results for radiation hardened graded index multimode fiber
from Spectran state that at a dose rate of 1300 rads/min the induced
attenuation reaches between 2 to 4 dB/Km for 10 Krads TID but after an
hour of "dark" recovery (meaning no photobleaching stimulation by the
optical source) drops to 1 to 3 dB/km. Space environments usually have a
much slower dose rate for ionizing radiation so better performance could
be expected under a slower dose rate and with photobleaching effects if
a greater power source is used.

Extrapolation Method:

For this set of experiments two dose rate tests were conducted such that
the extrapolation method developed at NRL by Friebele, Gingerich and
Griscom could be utilized.\[2\] The method states that radiation induced
attenuation can be extrapolated using the expression

.

Where A(D) is the expected induced attenuation, C is a constant
determined by dose rate, D is the total dose and f is a constant
independent of dose rate. The expression for C is described by

where F is the dose rate and n is the kinetic order of recovery. With
this information, the entire expression should take the form

\(1\)

where C~o~ and f are constant for all dose rates and total dose. If a
value is assumed for n, kinetic order (usually 1 or 2) and a minimum two
sets of data have been accumulated at two distinct dose rates, then the
equation can be solved and used to extrapolate for all total dose and
dose rates. Solving for f given two distinct data points of
(A~1~,F~1~,D~1~) and (A~2~,F~2~,D~2~) and knowing that C~o~ is the same
for all data points the solution is

\(2\)

Equation 2 can solved by using several data points from the actual data
which are values of attenuation, dose rate and a total dose. Averaging
these values a suitable f can be determined once a value is chosen for
n.

DISCUSSION

The data presented here is based on two total dose radiation tests one
conducted approximately at 50 rads/min and one conducted approximately
at 34 rads/min. Choosing two dose rates so close together was not by
choice but by necessity since the ^60^CO chamber's maximum dose rate was
no higher than 65 rads/min. The fiber tested in this experiment was an
acrylate hermetic coated 100/140 micron graded index fiber from Spectran
part number BF0544. For each of the two tests over 100 meters was
irradiated in a wound spool of diameter larger than 6 inches. Due to the
geometry the spool and the radiation distribution of the ^60^CO chamber
the dose rate on the spool was not uniform and therefore had to be taken
in to consideration. The geometry calculations approximated the two
radiation dose rates mentioned above.

A source from RIFOCS 252A dual source at the 1300 nm setting was used
which was rated for 10 microwatts but actually transmitted more power
than the rating. For converting the output power into a voltage an
optical to electrical converter P6703A for 1300nm was used from
Tektronix and data was recorded using a digital oscilloscope HP54201D,
(due to lack of equipment compatible with available lab data acquisition
software) and monitored as well with a nanovolt meter HP34420A. The test
lasted several days and each dose rate test was conducted separately due
to constraints of the radiation chamber. During the first hour of the
testing, data was recording from the nanovolt meter and throughout the
duration of the testing was recorded with the digital oscilloscope. The
test for each dose rate was run undisturbed for the first 100 Krads and
after this TID was reached the chamber was opened and shut to include
other experiments not associated with this one. The test conducted at 34
rads/min was disturbed more than a few times between the total dose of
100 Krad to 200 Krad. However, each time the chamber shutter was lowered
it wasn't for more than 5 to 10 minutes. These disturbances were
detectable by viewing the recorded data. Initial optical power output
measurements were recorded pre-irradiation such that all increases in
attenuation could be considered induced attenuation due to radiation.
During the first hour, output power was monitored to illustrate the
linear reaction of the attenuation at low total dose. Unfortunately this
data was not be logged by computer throughout the testing and had to be
taken by observation.

The results of the 50 rads/min test are in Figure 4 showing a linear
decline in power. Average change in power for first hour from the data
points graphed in Figure 4 were calculated at -2.35 nW/minute at 50
rads/min, -47 nW/Krad or .019dB/Krad induced attenuation.

**Figure 4**

**Power Output During First Hour of Radiation Testing at 50 rads/min**

SUMMARY

All the data captured was processed after the testing was complete and
the results of the actual data capture is graphed in Figure 6. The
extrapolation method was used via equation 2 and the results of the
calculated attenuation are graphed in Figure 6 as well. For these
calculations the best fit to the actual data was determined to have a
value of n=1 making f = .5345. These calculations were made by using
both the data from the 50 rads/min test and the 34 rads/min test shown
in Figure 7. The expression used for the extrapolation of this data is

\(3\)

where the units are dB/Km. Although the slower dose rate test did not
saturate at a lower total dose, with the test of 50 rads/min showing a
faster saturation, the slower rate test at 34 rads/min does show less
attenuation once it did reach saturation. The 50 rads/min test sample
saturated at a value of attenuation 12.21 dB/km at a total dose of 150
Krads. The 34 rads/min test sample saturated at 9.18 dB/Km at
approximately a total dose of 160 Krads. It is most likely the case that
frequent opening and lowering of the gamma ray chamber shutter during
testing may have delayed the saturation of the 34 rads/min test sample.
However, in most cases 100 Krads total dose is more than sufficient for
testing of parts for space flight and at that total dose the 50 rad/min
test sample had experienced an induced attenuation of 9.60 dB/Km and the
34 rads/min had experienced an induced attenuation of 6.45 dB/kKm at 100
Krads total dose.

Just to illustrate how this fiber performed when compared to the
Spectran specification, at 10 Krads the 50 rads/min sample had an
induced attenuation of 1.67 dB/km and the 34 rads/min sample had 1.05
dB/Km induced attenuation and both of these are below that of the rating
Spectran specifies values approximately between 2 to 4 dB/Km at a dose
rate of 1300 rads/min. This of course makes sense due to the slower dose
rates used in this experiment and the photobleaching effects that were
small, but present.

Extrapolating using Equation (3) the result of a total dose of 10 Krads
at a dose rate of 1300 rads/min yields an attenuation of 11.6 dB/Km.
This result is much higher than what Spectran has published in the
specification which could be for the reason that the extrapolation
method sometimes does not predict well at very low total doses and at
very high total doses. This is evident in both figures 5 and 6 by the
over estimate of the extrapolation curve in both tests at high dose
rates after the fiber has reached a saturation point and the over
estimate at the lower total doses in the 34 rad/min test. It is possible
that by conducting two tests both at low dose rate has caused faulty
results at higher dose rates but for space flight the lower dose rate
information is more realistic for planning purposes. It also appears
that the extrapolation curve fits the 50 rads/min test data better than
the 34 rads/min test data. The values for f and n were chosen based on
the best fit of the available data and it may have been quite different
with two dose rates much farther apart. It is also the case that due to
the experimental setup available the data collected can not be
considered exact but should be considered approximations. This is
another reason that the extrapolation method does not provide the
correct data to predict attenuation accurately for all dose rates and
total doses. It does however, show the trend and may be able to be used
for other low dose rates.

It was somewhat difficult to solve for f for this experiment, possibly
due to the slow dose rates used. The best choice had to be sorted out
from the available data instead of a simple averaging of all the data
point calculations. Some results were obviously incorrect and would not
work for extrapolation purposes.

After 24 hours the permanent induced attenuation for the 50 rad/min test
was 7.7 dB/Km. The attenuation for the 34 rad/min test was higher than
the saturation value of the induced attenuation during the radiation
testing therefore, it most likely had to do with the connector or
termination becoming damaged during removal from the radiation chamber.

Figure 6

Induced Attenuation Data and Extrapolated Attenuation for 50 rads/min on
BF0544 Spectran Multimode Acrylate Hermetic Optical Fiber

**Figure 7**

**Induced Attenuation Data and Extrapolated Attenuation for 34 rads/min
on BF0544 Spectran Multimode Acrylate Hermetic Optical Fiber**

CONCLUSION

The shrinkage testing of cable components illustrated how a
preconditioning procedure for fiber optic cable assemblies should be
tailored to fit the type of cable being used. In some cases there is no
need for a preconditioning procedure. In other cases 8 cycles provides
some protection against further shrinkage but 18 cycles provides enough
to consider further shrinkage much less of an issue. The Spectran Flight
Guide seemed to shrink continuously but by an amount much less than the
other types tested. The W.L. Gore FON 1008 that was optically tested as
well as inspected for stress from shrinkage around the connector did
quite well by causing very little loss in comparison to the Hytrel
jacketed multimode cable. It is probably important to once again point
out that the cable components may pull the connector back from the fiber
after a certain amount of shrinkage is attained and that a result of
this could be the fiber protruding out of the ferrule. Which is only one
of the reasons that preconditioning should be accomplished prior to
terminating the cable to a connector. Overall it is apparent that many
types of fiber optic cable components should be examined for shrinkage
before use and possibly preconditioned before termination such that
further shrinkage may be supressed but not eliminated. As shown it takes
many cycles to eliminate the shrinkage entirely and all possible steps
should be taken to be sure that during extrusion fabrication stresses
should be avoided as much as possible. Although preconditioning
procedures may not entirely remove the possibility of further shrinkage
in some cases in can be limited by a large amount.

The radiation results presented here on the Spectran BF0544 hermetic
acrylate coated 100/140 fiber show that in the low dose rate environment
of space this fiber would experience less than 10 dB/Km for a total dose
of 100 Krads (taken from the 50 rads/min data). The BF0544 experienced
less than 7 dB/Km for a total dose of 100 Krads at a dose rate of 34
rads/min. It is also important to keep in mind that in most space flight
applications the lengths of fiber optic cable used are approximately 10
meters and in very few applications are lengths as long as 100m used.
Another advantage of this optical fiber is that it did reach a
saturation point of induced attenuation. This implies that at a low
enough dose rate, like those used here, a maximum amount of attenuation
can be expected for any total dose.

One of the reasons that this optical fiber was chosen for investigation,
is that it does have the advantage of a coating that is strippable in a
methylene chloride solution. It was verified during these experiments
that the acrylate coating did strip off of the fiber easily in chemical
solution after 2 minutes of being immersed.

REFERENCES

\[1\] M. Ott, J. Plante, J. Shaw, M. A. Garrison Darrin "Fiber Optic
Cable Assemblies for Space Flight: Issues and Remedies," Paper number
975592 AIAA/SAE World Aviation Congress, Anaheim, CA 1997

\[2\] E. J. Friebele, M.E. Gingerich, D. L. Griscom, "Extrapolating
Radiation-Induced Loss Measurements in Optical Fibers from the
Laboratory to Real World Environments", 4^th^ Biennial Department of
Defense Fiber Optics and Photonics Conference, March 22-24, 1994.

\[3\] E. J. Friebele, "Survivability of Photonic Systems in Space" DoD
Fiber Optics Conference, McLean VA, March 24-27, 1992.

\[4\] C. E. Barnes "Technology Assessment: Radiation Hardened Fiber
Optic and Optoelectronic Devices and Systems," Report to the Defense
Nuclear Agency, April 1992.

ACKNOWLEGEMENTS:

We would like to thank the Advanced Interconnect Program for funding
this effort and providing this as a service to all NASA centers.

Special thanks are extended to the following for their help in
facilitating and supporting this effort: Hak Kim, Tony Sanders, George
Jackson, John Slonaker, Henning Leidecker, Jeannette Plante and Jack
Shaw of Goddard Space Flight Center, Karen Berg and Jonathan Loft of
Spectran, Doug Hardy and Joe Gallo of W.L. Gore, and Ken Hull from
Northern Lights.

**MELANIE N. OTT**

Melanie Ott is a senior systems engineer from Swales Aerospace with nine
years experience in fiber optic and bulk optical sensing systems and
four years experience in reliability of photonic parts for optical
communications and sensing systems for space flight. Her expertise with
photonic devices includes: volume holographic storage crystals,
integrated optical modulators, semiconductor sources, fiber optics,
optocouplers and passive fiber optic interconnects. Ott holds a Masters
in Electrical Engineering with Optics emphasis from Virginia
Polytechnic. Prior to working at GSFC she worked at NASA Langley
Research Center, the Fiber and Electro Optics Research Center (FEORC) in
Blacksburg and the Crystal Physics Laboratory at the Massachusetts
Institute of Technology. She currently works with the Technology
Validation Assurance Group at Goddard Space Flight Center
(http://misspiggy.gsfc.nasa.gov/tva).

When Ott is not an engineer she is a professional singer, songwriter,
and musician performing in the local MD area in a duo named
*AlterAcoustic*.

**KENNETH A. LABEL**

Ken LaBel is Group Leader for the Radiation Effects and Analysis Group
at NASA Goddard Space Flight Center (GSFC). He has been at GSFC since
1983 andhas worked in such diverse areas as ground system design,
spaceflight computers and data systems, fiber optic system development,
and radiation effects. Mr. LaBel has worked in the radiation effects
arena since 1990 and has authored or co-authored over 30 publications
ranging from the use and analysis of commercial microelectronics in the
space radiation environment to ground and spaceflight experimentation on
emerging technologies. Mr. LaBel is an IEEE member and is currently the
Senior Member-at-Large for the IEEE Radiation Effects Steering Group. In
his \"copious\" spare time, Mr. LaBel runs, bikes, skis, does
triathlons, plays golf and volleyball, enjoys reading and microbrews.

**MARGARET ANN GARRISON DARRIN**

For the past five years Ann Darrin has been the chief of the Assurance
Technologies Division at Goddard Space Flight Center overseeing efforts
in parts, electronic packaging and processes, and material engineering.
Ms. Darrin has extensive experience in microelectronics in both private
industry and the public sector. Ms. Darrin earned her Bachelor of Arts
from the Pennsylvania State University and her Masters in Technology
Management from the University of Maryland, University College. Ms.
Darrin is currently the newly appointed Assistant for Technology to the
Electrical Systems Center.