Liquid-core light guide designed to withstand interior bubble formation from temperature-induced volumetric variations

A termination for a light guide having optical cladding tubing and a liquid core includes a volumetric change device configured to allow fluid flow out of the tubing whenever the liquid core expands and to allow fluid flow into the tubing whenever the liquid core contracts, an end plug fixed to the tubing to retain the liquid in the liquid core within the tubing and the end piece and a crimping piece for holding the end plug in place.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a design for core liquid retention for 
liquid core light guides and more particularly to a design for core liquid 
retention during temperature cycling between 0.degree. F. and 140.degree. 
F. for liquid core light guides. 
2. Related Prior Art 
In the use of liquid core light guides problems exist in temperature 
induced volumetric variations. Typically, the liquids that are used 
significantly change volume as a function of temperature, specifically the 
range of 0.degree. F. (-18.degree. C.) to 140.degree. F. (60.degree. C.). 
During the heating of a LLG (Liquid-core Light Guide) containing such a 
fluid, the pressure created by volumetric expansion of the liquid is 
sufficient to slightly deform (i.e., elongate) the Teflon.TM. tubing. This 
deformation is permanent, resulting in the formation of a small bubble 
when the liquid core light guide is returned to normal room temperature 
and the liquid undergoes volumetric contraction. This effect is rooted in 
the significant difference between the larger volumetric expansion 
coefficient of the liquid as compared to that of the FEP (Teflon.TM.) 
tubing. The resultant bubble is large enough to drastically reduce the 
delivered light intensity from the liquid core light guide. 
In a present commercial liquid core light guides, most commonly Series 300 
from Lumatec of Munich Germany, the core liquid has a volumetric expansion 
coefficient that closely matches that of the Teflon.TM. tubing. 
Several liquids are available that can be used to make a liquid core light 
guide transmit efficiently over a range from 350 to 800 nm., namely Dupont 
Syltherm XLT, General Electric SF-96-50, and Fluka UV-grade mineral oil. 
The first two are silicone oils (poly-dimethyl-siloxanes) and have been 
found to be radiation-hard under a 1,000 hour irradiation from 
ultra-violet (UV) light (centered at 365 nm) that varied in intensity from 
1-4 W/cm.sup.2. The third is not radiation-hard, but is a very efficient 
transmitter of visible and near-IR light. All three are relatively 
inexpensive. However, none of these possible liquids have a volumetric 
expansion coefficient that closely matches that of the Teflon.TM. tubing 
and were undesirable for operational use throughout the aforementioned 
temperature range because of the deleterious effects due to this 
difference. 
In addition, three new high index of refraction fluids (1.49-1.58) have 
been identified that can be used for efficient visible light transmission, 
and moreover, these fluids can used as low toxicity, non-flammable liquid 
scintillator bases. These are first, phenyl-xylyl-ethane (PXE), second, 
linear alkylbenzene (LAB) and third, isopropyl-biphenyl (IPB). With the 
addition of typical scintillator fluors such as PTP, BPBD, POPOP, bis-MSB, 
TPB, BBQ, Y7 and 3HF, one can create an active scintillating medium for 
use in Nuclear Science applications requiring the flexibility afforded by 
the liquid core light guide. In the case of PXE and LAB, such liquid 
scintillators can be obtained from Fisher Scientific under the 
Scintisafe.TM. trademark. 
When compared to a commercial reference standard liquid core light guide 
such as series 300 from Lumatec of Germany, a UV-grade liquid core light 
guide can deliver up to 85% of the UV intensity (365 nm) of the Lumatec 
liquid core light guide at a much lower cost. However, the liquid core 
light guide cannot withstand a temperature cycling test such as the 
following. First, from room temperature, cool the liquid core light guide 
to 0.degree. F. for 72 hours, and then return to room temperature. Second, 
proceed to heat the liquid core light guide to 1400.degree. F. for 72 
hours, and then return to room temperature. 
During the cooling cycle, the liquid is seen to contract leaving a vacuum 
air bubble. Upon reheating (slowly) to room temperature, the bubble 
disappears and the liquid core light guide operates normally. However, 
after the light guide is heated and then allowed to cool as in the second 
step, one sees a bubble developing even though no leakage of fluid is 
detected. Studies of the volumetric expansion coefficient and measurements 
of the pressure developed during the heating of the light guide suggest 
strongly that the FEP Teflon.TM. is permanently deformed during the 
heating. This apparent increase in the tubing volume results in a slight 
underfilling of the tubing after the cooling to room temperature. 
If the light guide can be used in normal room temperatures, for example 
from 600.degree. to 80.degree. F., then the normal design of the light 
guide can be used. For cases where the light guide operates in normal 
temperature conditions, but may be exposed to extreme temperatures for a 
short time, such as during shipping, or in cases where the light guide may 
have to operate in abnormal temperature conditions, an alternate design is 
required. 
The following patents are indicative of the state of the art addressing 
this problem of bubble formation during the cooling process after heating. 
U.S. Pat. No. 5,33,227, titled "Optical Waveguide Hose", issued to Minoru 
Ishiharada et al., relates to an optical waveguide hose including a hollow 
tubular cladding, a fluid core in the cladding, the fluid having a higher 
index of refraction than the cladding, and sealing plugs mated with 
opposite end opening of the cladding. In one form, the core fluid is 
filled in the cladding under positive pressure. In another form, a sheath 
of gas barrier material encloses the outer periphery of the cladding. The 
pressurized fluid core of the gas barrier sheath prevents air from 
penetrating into the core, allowing the hose to maintain its light 
transmission function. 
International Patent Application, PCT publication number WO 95/121138, 
published on May 4, 1995, titled "Liquid Core Optical Waveguide", applied 
for by Frederick Harold Eastgate, relates to an optical waveguide for 
transmitting radiation. In one embodiment the waveguide includes a 
flexible tube of a material that is substantially transparent to the 
radiation, and a liquid core filling the tube having a refractive index 
greater than that of the tube material. In another embodiment the 
waveguide includes a tube having an inner lining, and a liquid core 
filling the tube and having a refractive index greater than that of the 
lining material. In a still further embodiment there is an optical 
waveguide for transmitting radiation and functioning as a non-imaging 
concentrator. The waveguide includes a tube having an input end and/or an 
output end that is greater than the diameter of the tube body. 
U.S. Pat. No. 3,995,934, titled "Flexible Light Guide", issued to Gunther 
Nath, relates to a flexible light guide of the liquid filled type with a 
liquid supply container outside the light guide. In accordance with one 
form of the device described, a flexible light guide has a column, 
operating as an optic fiber or a liquid, which absorbs as little as 
possible of the wavelength range to be transmitted, with a predetermined 
index of refraction. This column is surrounded by a flexible tube of 
plastics material, which, in the wavelength range to be transmitted, has a 
somewhat lesser index of refraction than the liquid. In accordance with 
the device described, such a light guide has a supply container filled at 
least partly with the liquid and connected with the interior of the 
flexible tube. 
The presence of the supply container ensures that the flexible tube is 
always completely filled with the light conducting liquid, even if in the 
course of time liquid should be lost from the tube. The supply container 
furthermore ensures satisfactory filling of the flexible tube and high 
transmission of the light guide even on bending of the flexible tube and 
in the case of high thermal loading of the liquid column owing to high 
intensities of radiation. 
The foregoing methods and apparatus, although each effective for its 
limited purpose, do not solve the problem presented of permanent 
deformation of the Teflon.TM. tubing caused by the pressure created by the 
volumetric expansion of the liquid during the heating of a liquid-core 
light guide, resulting in the formation of a small bubble when the light 
guide is returned to normal room temperature. 
SUMMARY OF THE INVENTION 
The present invention provides a method is described to deal with liquids 
that significantly change volume as a function of temperature, 
specifically the range of 0.degree. F. (-18.degree. C.) to 140.degree. F. 
(60.degree. C.). It is found that during the heating of a liquid core 
light guide containing such a fluid, the internal pressure created by the 
liquid expansion is sufficient to slightly deform (i.e., elongate) the 
Teflon.TM. tubing. This deformation in the Teflon.TM. tubing is permanent, 
resulting in the formation of a small bubble when the liquid core light 
guide is returned to normal room temperature. This effect is rooted in the 
significant difference between the larger volumetric expansion coefficient 
of the liquid as compared to that of the tubing. The resultant bubble is 
large enough to drastically reduce the delivered light intensity from the 
light guide. 
The present invention provides a practical method to prevent this effect by 
preventing any bubble formation during the temperature cycling between 
0.degree. F. (-18.degree. C.) and 140.degree. F. (60.degree. C.). By 
preventing bubble formation during the temperature cycling process, the 
liquid core light guide can be operated throughout this temperature range 
without deleterious effects. This improvement allows a wider range of 
fluids to be used as the liquid core. 
If the light guide can be used in normal room temperatures, for example 
from 60.degree. to 80.degree. F., then the normal design of the light 
guide can be used. For cases where the light guide is normally used at 
room temperature, but may be exposed to extreme temperatures for a short 
time, such as during shipping or in cases where the light guide may have 
to operate in abnormal temperature conditions, an alternate design has 
been developed. 
Important features of the present invention are, first, to provide a 
flexible, elastic outer sheathing to the FEP Teflon.TM. tubing and, 
second, to allow a slight leakage of the core liquid into the space 
between the teflon and the sheath while crimping around the concentric 
layers of sheathing, teflon and the end plug. The FEP Teflon.TM. tubing, 
acts as the light guide optical cladding. The sheath will act as a bladder 
or concentric buffer allowing the core liquid to change volume without 
deforming the teflon tubing. Furthermore, when the liquid core light guide 
is put through a freeze cycle, the buffer volume will allow the central 
FEP tubing to remain filled with the fluid, thereby allowing the light 
guide to operate throughout the desired temperature range. The central FEP 
Teflon.TM. tube has two small incisions made at diametrically opposite 
points to allow for fluid flow between the FEP tubing and the outer 
concentric fluoroelastomer tubing. The incisions are small enough to 
minimize any disturbance in the optical path. As temperature variations 
force volumetric changes in the fluid, the liquid can seep between this 
concentric buffer and inner FEP tubing without exerting excess force upon 
the inner FEP tubing. This structure is only necessary at one end of the 
light guide. Crimping rings are applied at two points so as to seal in 
place the end plug, FEP tubing and the fluoroelastomer bladder. 
An alternate embodiment of the present invention may include a short piece 
of teflon tubing for example, twice the length of the end plug, connected 
to the end plug. The inner teflon tubing and the sheath are both filled 
with the core liquid. The short piece of teflon is inserted loosely into 
the inner teflon tube to permit fluid communication between the inner 
tubing and the outer sheath. 
A further alternate embodiment may include a short piece of teflon tubing 
is omitted, and the endplug inserted slightly into the inner Teflon.TM. 
tube. The crimp is around the sheath and endplug, leaving the inner 
Teflon.TM. tube free to permit fluid communication between the inner tube 
liquid and the sheath liquid. 
An additional alternate embodiment of the present invention may include a 
design where the inner Teflon.TM. tubing and sheath are the same lengths, 
the crimp is around all three materials, but a small incision is made in 
the tubing to allow core liquid seepage and while permitting minimal 
interruption in the optical cladding path. 
Another alternate embodiment of the present invention may include a design 
where all the tubing is FEP Teflon.TM.. In this case, the pressure relief 
is provided by a short section of corrugated FEP Teflon.TM.. To maintain 
optical continuity, an FEP Teflon.TM. tubing insert slightly longer than 
the corrugated section is used to guide the injected light past the 
corrugated section. 
The present invention provides a termination for a light guide having 
optical cladding tubing and a liquid core that includes a volumetric 
change device configured to allow fluid flow out of the tubing whenever 
the liquid core expands and to allow fluid flow into the tubing whenever 
the liquid core contracts, an end plug fixed to the tubing to retain the 
liquid in the liquid core within the tubing and the end piece and a 
crimping piece for holding the end plug in place. In a first embodiment, 
the volumetric change device includes a bladder section over the tubing 
adjacent to the end plug, the bladder section held over the tubing at one 
end by the crimping piece, a second crimping piece holding the bladder 
section over the tubing at another end of the bladder section and a 
channel portion in the tubing to permit fluid communication between liquid 
in the tubing and liquid in the bladder section. In a second embodiment, 
the volumetric change device includes a flexible outer sheath extending 
over the length of the tubing and held in place over the end plug by the 
crimping piece and an insert portion arranged to be fixed to the end plug 
and loosely fitted to the tubing to permit fluid communication between 
liquid in the tubing and liquid in the flexible outer sheath. In a third 
embodiment, the tubing fits loosely over the end plug and the volumetric 
change device includes a flexible outer sheath extending over the length 
of the tubing and held in place over the end plug by the crimping piece. 
In a fourth embodiment, the tubing fits under the end plug and the tubing 
is held in place by the crimping piece and the volumetric change device 
includes a flexible outer sheath extending over the length of the tubing 
and held in place over the end plug by the crimping piece and a channel 
portion in the tubing to permit fluid communication between liquid in the 
tubing and liquid in the bladder section. In a fifth embodiment, the 
tubing includes a corrugated section immediately prior to the end plug and 
the volumetric change device includes an insert portion arranged to be 
fixed to the end plug and extending into the tubing covering the 
corrugated section to provide an optical cladding path.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention provides a method and apparatus that deals with 
liquids that significantly change volume as a function of temperature, 
specifically the range of 0.degree. F. (-18.degree. C.) to 140.degree. F. 
(60.degree. C.). It is found that during the heating of a liquid core 
light guide containing such a fluid, the internal pressure created by the 
liquid expansion is sufficient to slightly deform (i.e., elongate) the 
Teflon.TM. tubing. This deformation in the Teflon.TM. tubing is permanent, 
resulting in the formation of a small bubble when the liquid core light 
guide is returned to normal room temperature. This effect is rooted in the 
significant difference between the larger volumetric expansion coefficient 
of the liquid as compared to that of the tubing. The resultant bubble is 
large enough to drastically reduce the delivered light intensity from the 
light guide. 
The present invention provides a practical method to prevent this effect by 
preventing any bubble formation during the temperature cycling between 
0.degree. F. (-18.degree. C.) and 140.degree. F. (60.degree. C.). By 
preventing bubble formation during the temperature cycling process, the 
liquid core light guide can be operated throughout this temperature range 
without deleterious effects. This improvement allows a wider range of 
fluids to be used as the liquid core. In a present commercial liquid core 
light guides, most commonly Series 300 from Lumatec of Munich Germany, the 
core liquid has a volumetric expansion coefficient that closely matches 
that of the Teflon.TM. tubing. This fortuitous equality is no longer 
necessary with the designs of the present invention. 
Several liquids are available that can be used to make a liquid core light 
guide transmit efficiently over a range from 350 to 800 nm., namely Dupont 
Syltherm XLT, General Electric SF-96-50, and Fluka UV-grade mineral oil. 
The first two are silicone oils (poly-dimethyl-siloxanes) and have been 
found to be radiation-hard under a 1,000 hour irradiation from 
ultra-violet (UV) light (centered at 365 nm) that varied in intensity from 
1-4 W/cm.sup.2. The third is not radiation-hard, but is a very efficient 
transmitter of visible and near-IR light. All three are relatively 
inexpensive. However, none of these possible liquids have a volumetric 
expansion coefficient that closely matches that of the Teflon.TM. tubing 
and were undesirable for operational use over throughout this temperature 
range because of the deleterious effects due to the difference in 
coefficients of expansion. 
In addition, three new high index of refraction fluids (1.49-1.58) have 
been identified that can be used for efficient visible light transmission, 
and moreover, these fluids can used as low toxicity, non-flammable liquid 
scintillator bases. These are first, phenyl-xylyl-ethane (PXE), second, 
linear alkylbenzene (LAB) and third, isopropyl-biphenyl (IPB). With the 
addition of typical scintillator fluors such as PTP, BPBD, POPOP, bis-MSB, 
TPB, BBQ, Y7 and 3HF, one can create an active scintillating medium for 
use in Nuclear Science applications requiring the flexibility afforded by 
the liquid core light guide. In the case of PXE and LAB, such liquid 
scintillators can be obtained from Fisher Scientific under the 
Scintisafe.TM. trademark. 
When compared to a commercial reference standard liquid core light guide, 
series 300 from Lumatec of Germany, a UV-grade liquid core light guide can 
deliver up to 85% of the UV intensity (365 nm) of the Lumatec LLG and at a 
much lower cost. However, previously, the liquid core light guide cannot 
withstand a temperature cycling test such as the following. First, from 
room temperature, cool the liquid core light guide to 0.degree. F. for 72 
hours, and then return to room temperature. Second, proceed to heat the 
liquid core light guide to 140.degree. F. for 72 hours, and then return to 
room temperature. 
During the cooling cycle, the liquid is seen to contract leaving a vacuum 
air bubble. Upon reheating (slowly) to room temperature, the bubble 
disappears and the liquid core light guide operates normally. However, 
after the light guide is heated and then allowed to cool as in the second 
step, one sees a bubble developing even though no leakage of fluid is 
detected. Studies of the volumetric expansion coefficient and measurements 
of the pressure developed during the heating of the light guide suggest 
strongly that the FEP Teflon.TM. is permanently deformed during the 
heating. This apparent increase in the tubing volume results in a slight 
underfilling of the tubing after the cooling to room temperature. 
If the light guide can be used in normal room temperatures, for example 
from 60.degree. to 80.degree. F., then the normal design of the light 
guide can be used. For cases where the light guide is normally used at 
room temperature, but may be exposed to extreme temperatures for a short 
time, such as during shipping or in cases where the light guide may have 
to operate in abnormal temperature conditions, an alternate design has 
been developed. 
FIG. 1 is a cross-sectional view of a first embodiment of the apparatus of 
the present invention. Liquid core light guide 12 is comprised of FEP 
tubing 14 with liquid core 16. FEP tubing 14 is closed at the illustrated 
end by plug 18 to retain liquid core 16. A flexible sheath or bladder 20 
is fitted over FEP tubing 14 and is held in place by two crimp rings 22 
and 24 at either end of bladder 20. Crimping ring 22 also holds plug 18 in 
place at the end of FEP tubing 14. FEP tubing 16 is provided with small 
incisions or channels 26 and 28 in the section covered by bladder 20 to 
permit fluid communication between the section 30.defined by bladder 20 
and liquid core 16 of light guide 12. 
An important feature of the present invention is to provide a flexible, 
elastic outer sheathing or bladder 20 to FEP tubing 14. The latter, FEP 
tubing 14, acts as the liquid core light guide 12 optical cladding. A 
second important feature is to allow fluid communication, even if only a 
slight leakage, of core liquid 16 into the space between FEP tubing 14 and 
the sheath or bladder 20, while crimping around the concentric layers of 
sheathing or bladder 20, FEP tubing 14 and end plug 18 with crimping rings 
22 and 24 to form section 30. The sheath or bladder 20 will act as a 
concentric buffer, allowing core liquid 16 to change volume without 
deforming FEP tubing 14. 
Furthermore, when liquid core light guide 12 is put through a freeze cycle, 
the buffer volume will allow central FEP tubing 14 to remain filled with 
liquid 16, thereby allowing light guide 12 to operate throughout the range 
of 0.degree. F. to 140.degree.. FEP tubing 14 has two small incisions 26 
and 28 made at opposite points to allow for fluid flow between FEP tubing 
14 and outer concentric fluoroelastomer bladder 20 (e.g., Viton.TM., 
Fluran.TM.). This elastometric tubing or bladder 20 is compatible with the 
liquid (that is, does not deteriorate the optical properties of the 
fluid), and acts as a pressure relief bladder. Incisions or channels 26 
and 28 are small enough to minimize any disturbance in the optical path. 
As temperature variations force volumetric changes in the fluid, liquid 16 
can seep between this concentric buffer, bladder 20 and FEP tubing 14 
without exerting excess force upon inner FEP tubing 14. This structure is 
only necessary at one end of light guide 12. Crimping rings 22 and 24 are 
applied at two points so as to seal in place end plug 18, FEP tubing 14 
and the fluoroelastomer bladder 20. 
It is important that sheath or bladder 20 section be both elastic and 
compatible with core liquid 16. Specifically, silicone oils are known to 
leach out plasticizers from materials such as polyethylene and PVC, 
resulting in a degradation in the optical transmission capability of the 
liquid and causing stress-cracking in the plastic. Rubbers such a 
chloroprene (Neoprene.TM.), isobutylene-isoprene (butyl), 
nitrile-butadiene (Nitril Buna N), and fluororubbers (VitonTM and 
Fluorel.TM.) are known to be compatible with the silicone oils. Other 
possible candidates include Tygon.TM., silicones, Silastic.TM., 
Fluran.TM., Nalgene.TM. (polyurethane), Norprene.TM., and PharMed.TM.. 
FIGS. 2 through 5 illustrate alternate embodiments of the present 
invention. In FIG. 2, a short piece 32 of FEP tubing is fixed to a portion 
of endplug 18. Piece 32 may, for example, be twice the length of endplug 
18. FEP tubing 14 and a flexible outer sheath 34 are both filled with core 
liquid 16. Short piece 32 of FEP is inserted into FEP tubing 14. The inner 
diameter and wall thickness of FEP tubing 14 is chosen so as to provide as 
close a fit as possible (while maintaining sufficient flexibility). In 
this way, the close fit provides an essentially uninterrupted optical 
cladding path while allowing the slow seepage of core liquid 16 between 
FEP tubing 14 and flexible outer sheath 20 during large temperature 
variations. 
In FIG. 3, short piece 32 of teflon tubing is omitted, and endplug 18 is 
inserted slightly into FEP tubing 14. Crimping ring 22 is around flexible 
outer sheath 20 and endplug 18. The seepage of core liquid 16 occurs at 
the insertion point of endplug 18 and FEP tubing 14. 
FIG. 4 illustrates a fourth embodiment of the present invention. FEP tubing 
14 and flexible outer sheath 20 are the same lengths, crimping ring 22 is 
around all three materials, but a small incision 34 is made in FEP tubing 
14 to allow core liquid 16 seepage and minimal interruption in the optical 
cladding path. 
FIG. 5 shows the embodiment of the present invention where all tubing is 
FEP teflon. In this case, the pressure relief is provided by a short 
section 36 of corrugated FEP Teflon.TM.. To maintain optical continuity, a 
short FEP tubing insert 38 is used to guide the injected light past 
corrugated section 36. 
While there has been illustrated and described a particular embodiment of 
the present invention, it will be appreciated that numerous changes and 
modifications will occur to those skilled in the art, and it is intended 
in the appended claims to cover all those changes and modifications which 
fall within the true spirit and scope of the present invention.