A few comments... C. T. Mueller

Executive summary:

=================

Point 1:

\-\-\-\-\-\--

The \"Root Cause Team,\" also called \"Team Two\", was formed to find
why an increasing number of links made by Boeing using fiber optic
cable, vintage 1998/1999, had excessive insertion loss. We have
agreement that the reason is the presence of etch

pits extending into the core of the fiber, caused by a corrosive agent
attacking the glass through a (micron-scale) breach in the carbon
coating.

We do not have agreement as to what is causing the breach in the carbon
coating, or what is the source of the corrosive agent. We do not have
agreement as to why the frequency of these etch pits was low (one per
several kilometers) in 1996-vintage cable, and high (one per several
meters) in 1997/1998-vintage fiber. We do not have agreement as to what
is to be done to keep etch pits away forever more in newly made cable.

We have agreement re the existence of a new, previously unknown problem:
excessive degradation of fiber breaking strength. The strength-testing
facility at Boeing-HB has detailed the damage to the strength
distribution curve associated with each step in

the cabling activity. There are dramatic variations between two lots of
carbon-coated fiber, and between two lots of a comparison fiber (OC1260:
not carbon-coated, but with a

polyimide plastic coating). We do not have agreement as to what the
cause of the damage is, but there are obvious physical differences in
the polyimide/carbon coatings on these two fibers. We do not have
agreement as to what is to be done to keep this kind of damage away
forever more.

We are issuing this report to document what we have established, and
what is so far unresolved.

Discussion:

==========

Jeannette Plante has a synthesis of all the emails and technical
interchange meetings and telcons relating to the work of Team Two (The
\"Root Cause Team\"), and has added a summary of Team Two\'s findings
and conclusions on the root cause of the fiber flaws. She circulated
early drafts by email before the technical interchange meeting (TIM) in
Huntington Beach in early May, and brought a mature draft with her to
this meeting where it was discussed in detail. Indeed, some sections
were discussed line by line.

The May TIM discussion demonstrated that there were substantial areas of
agreement, as well as substantial areas of non-agreement. BICCGeneral
sent Ms Plante a three page memo detailing their objections.
Spectran/Lucent sent a similar memo. Since her return, Ms Plante has
been working steadily and for long hours re-writing the May report in
the light of the TIM and these memos. She is nearly finished with what
she is able to

do without new experiments or substantiative new scholarship. This
present document is a highly abreviated version of the one Ms Plante
will release shortly.

Ms Plante\'s re-written report does not presently identify \"the root
cause of the root cause\" in a way that everyone will accept.

That is, we know that the ROOT CAUSE of the insertion loss first noticed
about a year ago is an etch pit made by a corrosive agent attacking the
fiber through a localized breach

in the carbon coating. These were rare in 1996-era cable (we can say
that since they were not noticed in insertion loss records of links made
from this cable, and also since Boeing-HB inspected some 10 km of
1996-era cable and found precisely one etch pit), but commonplace in
1998/1999-era cable (many links were found to have excessive insertion
losses, and many glows & OTDR echoes). Many etch pits have been imaged
(using optical microscopes and SEM) and, in every case for which the
image permits examination, there is a feature within the polyimide
directly above the etch pit. In some cases, the polyimide feature is a
thinned zone. In others, the feature is a cavity

of some kind with a hole in the carbon directly beneath it. In both
cases, external fluids (if any) have easy access to the glass, relative
to an unfeatured polyimide coating. (This was directly confirmed in one
experiment last Fall. \[not sure what experiment you are referring
to...) It was determined that the etching process was preferential,
etching faster in the germanium rich core vs. the undoped glass
cladding. I think all parties agree with this.

David DiGiovanni (and others too) have demonstrated that exposure of
fiber, with localized breaches deliberately created in the polyimide and
carbon coatings, to hydrogen fluoride acid creates etch pits that are
identical in all respects with the etch pits seen in Boeing\'s damaged
links. Further, it is well-known that hydrogen fluoride acid is created
from FEP and water during a high temperature extrusion of FEP. But not
all parties agree that the corrosive agent is hydrogen fluoride created
during the extrusion of the FEP buffer around the fiber. The byproducts
of the reaction have been identified to be Si-rich balls and Ge-rich
rods on the surface of the polyimide coating directly above an etch or
bubble in the glass. Not all parties agree on the cause of the localized
breaches in the carbon coating, or the associated features in the
polyimide coating. (Various investigators have seen \"bubbles\" within
the polyimide coating, but these would not, themselves, provide a breach
in the carbon coating: something else is needed beside polyimide
bubbles.) And bubbles have now been seen in fiber prior to the cabling
process, each located directly above a pinhole in the carbon coating. It
seems likely that substantially more effort will be required to reach
agreement on these points. So we do not have the \"root cause of the
root cause\" documented satisfactorily.

In addition, the strength-testing facility created at Boeing-HB has
established that the great majority of low-strength breaks do not
originate at etch pits, but rather at flaws at the surface of the fiber.
These flaws are at the interface between the glass cladding, carbon, and
the polyimide coating. Some of the photos made using optical microscopy
seem to show a surface feature that resembles a crack sometimes as large
as a few microns, and sometimes smaller, down to the limit of resolution
of the optical system used. There is the possibility that, if the images
were made with better resolution, there would ALWAYS be such a surface
feature, and I suspect this is the case. (SEM

images could be definitive. ) In every image that allows inspection over
the origin of the break, there is a feature within the polyimide coating
directly above the site of origin

of the fracture in the fiber. In some cases, this feature resembles a
nearly spherical cavity whose diameter approaches the thickness of the
polyimide coating, with small openings to the glass and to the outside.
In other cases, this feature more nearly resembles a tube extending all
the way through the polyimide coating. These flaws at the surface of the
glass cladding ARE DISTINCT FROM THE ETCH PITS. \[The term "a second
kind of flaw" conjures up the idea that we are looking at something that
is unrelated to the "rocket engine defect." It is plausible that all
defects in the polyimide fiber (bubbles with holes in the carbon layer,
are not etched in the cabling process. One can think of several reasons
for this. First, there might not be enough HF. Second, the physical path
for the HF through the polyimide and the relatively small opening in the
carbon coating could impede the process.\] We have discovered a second
kind of flaw. This second kind is not responsible for excessive
insertion loss in links, nor for glows induced by Visual Fault Finders,
nor for echoes seen by OTDRs (since they are present only at the surface
of the cladding, and do not intrude into the core); however, they have a
profound impact on the distribution of breaking strengths. I think all
parties would agree to this. The net effect of such flaws is to fracture
the fiber when it is placed under stress. Both Visual Fault Finders and
OTDR are good tools to detect fractures.

We do not have a consensus on what causes these fracture-initiation
flaws first seen in the Boeing strength-testing facility. So we have a
distinct \"second kind of root cause\" (causing low-strength breaks, not
the optical anomalies caused by the etch pits), and again no consensus
on the \"cause of the second kind of root cause.\"

Some, including Dave Gill believe, ~~mentioned that he feels~~ that the
surface flaws are produced by the same mechanism as the etch pits.
Henning Leidecker agrees

~~I agree.~~ And suggests ~~I suggest~~ that an ESD event punches a hole
through the polyimide, removes a patch of the carbon coating, and
fractures the surface of the glass cladding layer. (There is probably a
range in the size of the damage; it seems unlikely that each ESD event
would have the same size, and would create the same

damage.) If the strength distribution is measured at this moment, one
would find a serious reduction, caused by the surface cracks whose scale
is 0.1 to 10 microns. ESD experiments performed by Aerospace have shown
the damage from ESD on "defect-free" polyimide fiber to produce larger
size holes in the polymide with substantially more debris. The breakdown
voltages are typically higher than the electric fields measured near the
fiber during the respooling process. Aerospace recently discovered
bubbles in the polyimide coating similar to those observed by Boeing
above each low strength break in the mechanical testing. And below each
bubble is a distinct micron-size hole in the carbon. It is also noted
that these defects exist in the fiber without any substantial decrease
in mechanical strength, making the existence of surface cracks unlikely.

In some cases, there is subsequent exposure to enough hydrogen fluoride
and water that the fracture is turned into an etch pit. This replaces a
sharp-tipped surface fracture by a smooth chemically-polished
hemisphere, with a far smaller stress concentration factor: the strength
distribution associated just with these flaws (smooth-walled etch pits)
would be close to that of the original fiber. Thus, when one breaks many
specimens of this sort of fiber, one would rarely find an etch pit at
the break; rather, one would usually find a sharp-tipped (un-etched)
crack there. \[unclear as to how we know there are many such defects in
the fiber...\]

The sharp-tipped cracks are expected to grow with time, when there is
stress applied. Therefore, these features are especially dangerous to a
long life expectancy. Conventional \"n theory\" is used to explain the
life expectancy of fiber containing surface flaws, i.e. cracks, should
apply here. Unfortunately, the mechanical data performed on the ISS
fiber does not. But the long-term behavior of the etch pits, or the
fiber with holes in the polyimide/carbon layer, has not been
established: we can only make some guesses. For the case of the etched
pit, one guess is that the smooth, chemically-polished etch pit would
retain it low-stress concentration factor for a substantial time, but,
eventually, water (entering through the hole in the carbon
~~polyimide)~~ would corrode the walls, and begin to degrade the
strength. For the case of the fiber with holes in the polyimide/carbon
layer, we only know that the strength degrades substantially during the
cabling process. We are in unknown territory here, and we can expect
that we may stumble some number of times before we get this part of the
puzzle worked out. Even if everyone agreed that these ideas were
reasonable, we would have to agree that they are as yet untested and
therefore probably wrong. (Most ideas, even the plausible ones, are
wrong. There are many many ways to be wrong, and only one way to be
right.) We are in a position to go after these guesses, and construct
experiments (including damaging fiber with controlled ESD events, and
exposing fiber to HF, and breaking many fiber specimens to establish
distributions of strengths). I think this is a good idea, and that we
will locate convincing evidence for the \"cause of both root causes\"
\-\-- the cause of those flaws that cause etch pits, and the cause of
those flaws that cause low-strength breaks. I welcome discussion of what
we should do next.