Refractive index profiling technique

A monochromatic light beam scans a silica reference and a translucent body, both submerged in index matching fluid and the deflection angle of the beam is measured as it exits the reference, the translucent body and the fluid. This deflection angle of the light beam exiting the reference is compared with the undeflected beam passing through only the fluid. The difference, if any, is used to offset the measurements of the deflection angles of the light beam exiting the translucent body to correct them so that the profile is referenced to the index of silica.

TECHNICAL FIELD 
This invention relates to a method and apparatus for profiling the 
refractive index of a fiber optic preform, or other translucent body 
having an unknown graded or step profile of interest, relative to the 
refractive index of a reference having a known index of refraction, and 
more particularly, to a nondestructive method and apparatus for profiling 
the refractive index of an unclad fiber optic preform or component of a 
preform relative to the refractive index of a silica reference. 
BACKGROUND OF THE INVENTION 
Optical fiber is produced by drawing the fiber from a preform. The 
refractive index profile of the drawn fiber is substantially the same as 
the profile of the preform from which it is drawn. Deviations from the 
desired refractive index existing in the preform will thus be repeated in 
the fiber and may result in unacceptable transmission characteristics. It 
is therefore prudent to profile the refractive index of the preform before 
drawing fiber from it to avoid wasting time and money producing worthless 
scrap. 
Historically, refractive index profile measurements have been relative 
index measurements; the refractive index of the fiber core relative to the 
doped or undoped silica cladding. It is this relative difference which 
gives an optical fiber its light guiding properties. 
As currently practiced, the manufacture of single mode preforms comprising 
a core and a clad by the "modified chemical vapor deposition" (MCVD) 
process has associated cost penalties. This process requires many 
depressed cladding layers to be built up before the core layers are 
deposited. These depressed cladding layers are needed to obtain the proper 
core-to-clad ratio and acceptable loss. 
A more economical alternative preform manufacturing technique uses high 
purity fluorinated silica tubes as starting substrates for MCVD together 
with rod-in-tube technology as described in U.S. patent application, Ser. 
No. 099,441, Continuation under Rule 60 of Ser. No. 856,739, filed on 
Sept. 23, 1987 in the names of J. W. Baumgart et al and assigned to AT & T 
Technologies, Inc., which is now U.S. Pat. No. 4,820,322 issued on Apr. 
11, 1989. By starting with fluorinated tubes the deposition of many fewer 
depressed cladding layers are required, rendering MCVD more competitive. 
The above alternative, as well as others that are being used such as the 
"outside vapor deposition" (OVD) process, could present profiling problems 
that current generation preform profilers would not be able to handle. 
In the OVD process, the glass precursor vapor is introduced into a 
hydrolyzing flame and particulate material is formed. This material 
emanating from the flame is directed toward a mandril on which it is 
deposited. Following such deposition, the deposited material is 
consolidated into a transparent glass, the mandril removed and the 
resultant hollow tube collapsed to form a solid, cylindrical optical fiber 
preform which may not have a silica cladding. 
The current method and apparatus for nondestructively determining the 
refractive index profile of an optical fiber preform have been disclosed 
in U.S. Pat. No. 4,227,806 granted to Lawrence S. Watkins (incorporated by 
reference herein). This approach works well with a preform having a core 
and a silica clad. However, prior art apparatus is not capable of 
accurately profiling the refractive index of an unclad preform, or 
component of a preform relative to silica or another reference. 
Prior art apparatus is shown schematically in FIG. 1. Tank 10 has heads 11 
and 12 supporting side windows 13 and 14. A preform 15 having a core 16 
and cladding 17 is suspended in a fluid such as index matching oil 18 with 
its longitudinal axis parallel to the plane of windows 13 and 14 by a 
support (not shown). 
Oil 18 with its index of refraction close to that of cladding 17 is usually 
used instead of air when accuracy is required to avoid unpredictable 
variations in the refraction of beam 19 caused by non-circularity of the 
surface of cladding 17. 
In operation, a narrow monochromatic beam of light 19 from light source 20, 
focused at the center of core 16, is scanned across preform 15 such that 
beam 19, as it scans, is always substantially perpendicular to a plane 
containing the longitudinal axis of preform 15. The refraction angle of 
beam 19 exiting preform 15 is detected by detector 21. A computer 22 
programmed to process the detected data constructs the refractive index 
profile and displays it by employing an output device such as plotter 23. 
In the case of a fluorinated silica tube to be used as a substrate for 
MCVD, there is a need to profile the down-doped tubes as part of an 
initial quality check. Again, however, there is no silica reference 
without which the amount of fluorine down-doping could not be reliably 
measured. A silica jacket could be collapsed over an unclad object, such 
as a fluorinated tube or an OVD preform, in order to accurately measure 
its index depression relative to silica. This, of course, would be a 
time-consuming, inefficient and wasteful test procedure. Destructive 
tests, such as slicing and direct analysis could also be employed with 
obvious disadvantages. 
It was also attempted to use index matching oil 18 as a surrogate for the 
silica cladding, but for reasons that will be discussed hereinafter, this 
procedure does not provide accurate results. 
Thus, there is a need for an efficient, accurate and nondestructive method 
and apparatus for constructing, relative to silica, the refractive index 
profiles of unclad preforms, unclad components of preforms and other 
unclad transparent bodies. 
SUMMARY OF THE INVENTION 
The instant invention solves the foregoing problem with a method and 
apparatus which allows the construction of the refractive index profile of 
a translucent body, such as an unclad preform or component of a preform, 
by scanning the translucent body and a reference having a known index of 
refraction, such as a silica rod, with a narrow beam of monochromatic 
light, detecting the refraction angles of the beam exiting both the 
translucent body and the reference and processing the detected refraction 
angle data to enable the profile with the desired reference to be 
constructed. These and other objects, features and advantages will be 
better understood from a consideration of the following detailed 
description taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION 
The present invention is explained in the context of nondestructively 
constructing the index of refraction profile of an unclad optical fiber 
preform, or component of a preform, relative to the refractive index of a 
reference such as fused silica. However, such description is for purposes 
of exposition and not for limitation. Unclad preforms and other objects, 
such as a graded index rod (GRIN) lenses having index profiles which are 
graded, step or otherwise, could be profiled and other parameters 
susceptible of being determined with the detected data could be computed. 
FIG. 2 shows the index profile of a multi-mode clad preform. The prior art 
method used simply consists of reconstructing the profile of the whole 
preform including its cladding. By this method the profile is referenced 
to the index 24 of index matching fluid 18 in preform tank 10. However, 
since the cladding is explicitly included in the profile, it is clear by 
how much the core index 25 varies from the clad index 26. 
Scanning collimated light beam 19 across preform 15 does not take 
significant time. However, computer processing time can be appreciable. 
Hence, it is usually preferable to preprocess the refraction angle data 
and then reconstruct the profile. The preprocessing step corrects the data 
for any index mismatch 27 between fluid 18 and cladding 17. If there is no 
mismatch 27 between index oil 18 and cladding 17, beam 19 will pass 
straight through without being refracted. On the other hand, the 
refraction due to any mismatch 27 can be detected and preprocessed by 
computer 22 to eliminate its effect in plotting the profile. 
Referring to FIGS. 3 and 4, this correction accomplishes two functions. 
First, as shown in FIG. 3 for the multi-mode fiber of FIG. 2, it enables 
the profile reconstruction to be truncated near the core-clad interface 
thereby saving processing time. Profiling the entire clad conveys no 
additional information. Second, preprocessing automatically references the 
reconstructed core profile to the index of the cladding. 
FIG. 4 shows a preprocessed refractive index profile of a single mode 
depressed clad preform. The depressed clad index 28 and the core index 29 
are both referenced to the refractive index 30 of the pure silica clad. 
A slight index mismatch 27 between index oil 18 and cladding 17 is often 
useful in precisely locating the surface of cladding 17 as beam 19 would 
be deflected as it enters and exits cladding 17. 
In any event, it would not be very practical in a manufacturing environment 
to attempt to keep the refractive index of oil 18 precisely matched to the 
index of cladding 17 as the refractive index of index matching oil 18 is a 
relatively sensitive function of temperature compared with the index of 
silica which is the usual material of cladding 17. 
Referring to FIG. 5, it can be seen that even over a range of 5.degree. C. 
the index mismatch 27' between the index matching oil and the silica 
varies by as much as 0.002 which is quite significant. 
It is readily seen that the prior art apparatus would not be capable of 
profiling the refractive index of a body relative to a silica reference if 
the object did not have a silica clad. However, this is precisely the 
problem that was confronted when it was attempted to profile the 
refractive index of preform components to be used in the rod-in-tube 
process previously mentioned. 
Absolute refractive index measurements could be made if a surrogate silica 
reference could be substituted for the silica cladding. In this regard, 
the linearity of the change of the index of refraction of oil with change 
in temperature, as shown in FIG. 5, suggests an oil index determination 
scheme based on an on-line oil temperature measurement technique. However, 
as previously mentioned, it turns out there are several problems with this 
approach. The most important problem is that the oil index cannot be 
computed precisely enough. Furthermore, the oil index may be affected by 
contaminants that build up in the preform tank over time, and there is 
even some question about the long term stability of the oil itself. None 
of these factors are addressed by simply measuring the temperature of the 
oil. 
The preferred embodiment of the herein disclosed invention circumvents the 
foregoing problems. Referring to FIG. 6, a fused silica rod 31 is 
suspended in index matching oil 18 by a support (not shown). The 
longitudinal axis of rod 31 is contained in the same plane as the 
longitudinal axis of unclad tube 32, which plane is perpendicular to light 
beam 19 and parallel to scan axis 33 of light beam 19. 
At the start of the scan of monochromatic light beam 19, which is typically 
a two milliwatt helium neon laser, along scan axis 33, beam 19 goes 
through at least part of rod 31. Light beam 19 is focused at the axis of 
tube 32 as described in U.S. Pat. No. 4,726,677 to Werner J. Glantschnig 
et al (incorporated by reference herein). If the refractive index of oil 
18 happens to match that of rod 31, beam 19 will not be refracted at 
surface 34 as shown by beam 35 exiting rod 31. On the other hand, if there 
is an index mismatch, between oil 18 and rod 31, beam 19 will be slightly 
refracted either toward or away from the normal to surface 34 of rod 31 as 
shown by beams 35' and 35". At any rate, if beam 19 is refracted at 
surface 34, as can be established by comparing the measurements obtained 
with beam 19 passing through both rod 31 and oil 18 with measurement 19' 
taken with beam 19 passing through only oil 18, the index difference 
between oil 18 and rod 31, and hence between the index matching oil and 
silica, can be computed. Once this difference is known, the problem is 
solved. The refractive index profile of unclad transparent body 32 can be 
plotted with the proper offset so that the index profile of body 32 is 
referenced to the index of silica rather than that of index matching oil 
18. 
The diameter of silica rod 31 should be large enough to permit several 
hundred data points to be measured as light beam 19 is scanned across it. 
In the preferred embodiment, a one-half inch diameter rod 31 was used 
effectively. As the data points were spaced only twenty microns apart, 
approximately eight hundred measurements can be taken to assure that an 
accurate profile is constructed. In addition, Snell's Law, upon which the 
refractive angle computations of computer 22 are based, presupposes 
surface 34 to be flat compared with the width of light beam 19. Beam 19 is 
approximately fifty microns wide so that surface 34 of one-half inch rod 
31 is, for all practical pruposes, flat. 
It was also determined that, although not critical, a one-quarter inch 
spacing between rod 31 and transparent body 32 was adequate if there is 
not too large a refractive index mismatch between silica rod 31 and oil 
18. This spacing should be large enough to avoid interference between beam 
19 exiting rod 31 and body 32, and vice-versa. 
It was also found preferable, that while computer 22 is calculating the 
offset to be used in plotting the refractive index of body 32, that the 
scan of beam 19 be stalled in the gap between rod 31 and body 32. Of 
course, this was a matter of choice rather than necessity. 
Referring to FIGS. 7, 11 and 12, in order to check the effectiveness of 
this invention, index matching oil 18 was heated, and while it was cooling 
down and its index of refraction increasing, a standard MCVD single mode 
fiber preform with a silica cladding was profiled three times using the 
method and apparatus of this invention. During the measurements, nothing 
was assumed about the composition of the preform. The invention was being 
checked to see if it would be able to determine the proper index offset 
such that the silica cladding would fall on the zero index reference line 
in the profile plot. The results of this check are shown in FIGS. 7, 11 
and 12. Even though the mismatch 36 between the oil and the silica was 
different for each temperature, the profiler of this invention 
automatically determined the correct index offset in each case. 
FIGS. 8 and 9 show profile measurements obtained with the present invention 
for objects which could not have been properly and efficiently profiled 
without this invention. FIG. 8 is the profile of an unclad preform made 
with the OVD process. It has a fluorinated cladding extending all the way 
to its surface. Prior to this invention, it would have been necessary to 
have collapsed a silica jacket over this preform in order to obtain an 
accurate profile of the index depression of the fluorinated clad 
referenced to the index of silica, the zero line in the profile. 
FIG. 9 is the profile of a fluorinated tube made with the OVD process. 
Again, the measure of its index depression relative to the index of silica 
could not have been determined by nondestructive prior art methods or 
apparatus. 
Alternatively, referring to FIG. 10, instead of a rod, a fused silica prism 
37 is cemented to the inside of window 14 using UV curable epoxy. The 
epoxy was selected to be compatible with the oil so as not to deteriorate 
over time. The selection was made with the aid of R. P. Cargille 
Laboratories, Inc. of Cedar Grove, N.J., the supplier of index matching 
oil 18. 
Surface 38 of prism 37 in this embodiment performs the same function as 
does surface 34 of rod 31 in the preferred embodiment. However, it was 
found that as angle 39 increased, the deflection of light beam 19 
increased providing a greater deflection for a given index mismatch 
between oil 18 and silica. The greater prism angle 39, the greater the 
accuracy of the profile obtained. However, as rod 31 in effect provides an 
infinite range of angles, the accuracy of the profiles obtained using rod 
31 instead of prism 37 were as much as an order of magnitude more 
accurate. Other modifications and embodiments may be made by those skilled 
in the art without departing from the spirit or scope of the invention.