Ind. Eng. Chern. Prod. Res. Dev., Vol. 18,No. 4, 1979
243
SYMPOSIA SECTION I.
Symposium on Polymers for Optical Fiber Systems M. J. Bowden and L. Blyler ACS/CSJ Chemical Congress, Honolulu, Hawaii, April 1979
Effect of Secondary Coating Technique of Low-Loss Optical Fibers on Transmission Loss Hldeo Suzuki' and Hiroshi Osanal The Fujikura Cable Works, Ltd., 1440, Mutsuzaki, Sakura-shi, Chiba-ken, 285, Japan
Secondary coating of optical fibers has been investigated for the purpose of establishing the coating techniques which are able to preserve the excellent transmission properties of bare fibers. Several kinds of plastic resins as the material of secondary coating have been coated on fibers by two different drawdown ratios. It has been determined that the drawdown ratio has a great effect on the increase of excess loss of fiber, and also the excess losses of optical fiber depend on the types of plastic resins which are coated on optical fibers as secondary coatings. Several plastics such as highdensity polyethylene, lowdensity polyethylene, and polypropylene have a large effect of drawdown ratio on the excess loss of fiber, but several plastics such as Nylon 12, polyester elastomer, and polyurethane elastomer scarcely have any effect of drawdown ratio on the excess loss of fiber. Nylon 12 has been finally chosen as a coating material, and the reliability of optical fiber which has been coated with Nylon 12 has been tested.
Introduction Optical fibers are generally coated with two plastic layers. One is the primary coating which protects optical fibers from various mechanical damages because glass fibers are very fragile. The other is the secondary coating which makes the handling of fibers easy. If there is no secondary coating, optical fibers are too small in diameter to handle in the usual plant operations. In order to keep excellent characteristics of optical fibers, the secondary coating should be extruded on the fibers without an increase of transmission losses and the changing of bandwidth. Furthermore, reliability of optical fibers under various severe environments over a long time should be guaranteed. From the viewpoint of maintaining excellent characteristics in optical fibers, we have studied properties of several kinds of plastic resins as the secondary coating materials and the extrusion techniques of the plastic resins. As the result of the study, it has been ascertained that the extrusion method of secondary coating of resin has a great effect on the increase of losses of fibers, and also the excess losses of optical fibers depend on the types of plastic resins which are coated on the optical fibers as secondary coatings. This paper presents the details of the above study and the results of reliability tests of optical fibers coated with the plastic resin which was selected as the secondary coating material on the basis of the study. Tested Specimen Graded index fibers were used in the study. The structure and fiber parameters of the graded index fibers used in the experiment are illustrated in Figure 1.
All of the tested fibers were high silica optical fibers whose preforms were made by the MCVD method and drawn by a high-purity carbon resistance furnace (Kobayashi et al., 1977). Each fiber was coated with urethane resin as a primary coating and with soft silicone rubber as a buffer layer which reduces microbendings of fiber (Naruse et al., 1977). The thicknesses of each layer were 2 and 150 pm, respectively. Typical losses of tested fibers at 0.85 and 1.10 pm were 2.6 and 1.0 dB/km, respectively, and typical bandwidth a t 6 dB down in an electrical signal was about 800 MHz km. Various kinds of plastic resins with 0.9 mm outside diameter were tightly extruded on the buffer layer of fibers. Secondary coating resin was extruded with the same type of conventional extruder that was used in an insulation of copper conductor for telecommunication cable. It is, however, considered that the tension of the fiber should always be kept under 100 g in order to prevent breakage. The extruder was equipped with a screw of 25 mm diameter and an ordinary type of die and guider tip. There exist two methods in a common extrusion technique. One is a pressure die extrusion and the other is a tubing die extrusion. In this study, the tubing die extrusion was applied on secondary coating for the reason of easy operation and easy protection of fiber from damages. Figure 2 shows the two methods in a common extrusion technique. Several kinds of plastic resins as secondary coating material were chosen among many plastic resins. Table I shows the plastic resins used in the experiment and their properties. Effect of Draw-Down Ratio and Plastic Resin on Transmission Loss The draw-down ratio was considered to have a large
0019-7890/79/1218-0243$01.00/00 1979 American Chemical Society
244
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
Table I. Plastic Resin Used in t h e Experiment
plastic resin
abbreviation
density, g/cm3
high density polyethylene-A high density polyethylene-B high density polyethylene-C low density polyethylene-A low density polyethylene-B polypropylene ethylene-vinyl acetate copolymer polyurethane elastomer polyester elastomer Nylon 1 2
HDPE-A HDPE-B HDPE-C LDPE-A LDPE-B PP EVA PUE PEE Ny 1 2
0.94 0.95 0.96 0.92 0.92 0.90 0.93 1.21 1.17 1.01
-Plastic
resin Silicone rubber Urethane resin jacketing
L
,~ a;::-
R e f r a c t i v e index difference
Figure 1. Structure and fiber parameters of graded index fiber.
melt index, g/10 min 0.2 0.4 6.5 0.2 1.3 2.0 1.7
E
tensile strength, kg/mm' elongation, % 2.1 1.6 2.0 2.4 1.8 3.6 1.5 5.0 3.5 4.8
0 HDPE-A
d
4 HDPE-B
m
0 HDPE-C a LDPE-A 0 LDPE-B
:
800 600 500 600 500 600 500 600 800 200
0 PP A EVA A Ny12 P PUE V PEE
E 5 5 m :4
Pressure die e x t r u s i o n
-2
3
z2 8 '
Tip
W
0 Draw-down
Tubing d i e e x t r u s i o n
0 HDPE-A 0
Tip
ratio
Figure 3. Relation between the excess loss and the draw-down ratio.
0
HDPE-B HDPE-C LDPE-A LDPE-B
PP A EVA A Ny12 v PUE PEE
'
Figure 2. Two methods in a common extrusion technique.
effect on the loss of coated fibers, and the relation between the draw-down ratio and the excess loss was studied. The draw-down ratio is defined as the ratio of the cross-sectional area of the tube, as extruded, to the cross-sectional area of the coating. Several kinds of plastic resins have been coated on fibers as the secondary coating with the two different draw-down ratios. The relations between the excess loss and the draw-down ratio are shown in Figure 3. It is clear from Figure 3 that the draw-down ratio has a great effect on the increase of losses of fibers, and also the excess losses of optical fibers depend on the types of plastic resins which are coated on optical fibers as the secondary coating. Several plastics, such as high density polyethylene, low density polyethylene, and polypropylene bring the large increase of losses with a high draw-down ratio. However, several plastics, such as Nylon 12, polyester elastomer, polyurethane elastomer, and ethylene-vinyl acetate copolymer, scarcely produce excess loss with any draw-down ratios. Excess losses of the coated fibers at low temperature are shown in Figure 4. These fibers were coated with various plastic resins with a small draw-down ratio. Although the excess losses due to coating were very small for all the coated fibers as shown in Table 11, the excess losses of coated fibers at low temperature depended on the types of plastic resins which were coated on optical fibers as secondary coating. The transmission losses of fibers coated with some kinds of plastic resins, which have a large effect of draw-down ratio on the excess loss of fiber, greatly increased at low temperature.
Figure 4. Excess loss of the coated fibers at low temperature. Table 11. Excess Loss Due to Coating (dB/km a t 0.84 .u m ,) ~. plastic HDPE-A HDPE-B HDPE-C resin 0.1 excess 0.3 0.1 loss EVA Ny 1 2 plastic PP resin 0.2 0 excess 0.3 loss
LDPE-A LDPE-B 0.1
0
PUE
PEE
0.2
0.2
As is well known, the main causes of the excess loss of optical fibers are attributed to the microbendings of fibers (Gardner, 1975). In the process of secondary coating, microbendings of optical fibers were considered to be brought by the rough surface and shrinkage of coated resins which are caused by molecular orientation of plastic resins at the stage of the extrusion process. This molecular orientation strongly relates to the draw-down ratio of resin, when optical fibers are coated with the resin. In order to clarify the relation between the loss increase and microbendings, the heat shrinkage and surface rough-
$l;.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979 245
-.
c
c d N I
0 0
z..
'
0 HDPE-A PP HDPE-8 A EVA HDPErC Ny12 I LDPE A PUE LDPE-B PEE
0'
'
-- '0.61 m C
! i
a
$ 5
U
o
a 4
a ; 3
I
.
/O
1,
I
0-
c
a
o-o-o-
..,
o-o-o* 0
/. , .
, o ~ .,
,
100 60 D r a w - d o w n ratio
,
,
,
I
160
Figure 7. Relation between the excess loss and the draw-down ratio of Nylon 12.
$ 2 c
a 1
= 0 0
10
Ave= 0.1 dB/km
?O
D r a w - d o w n ratio
Min.
0 dWkm
Figure 5. Relation between the heat shrinkage and the draw-down ratio. t
a
0 HDPE-A
d
x
'
"t c
0
PP HDPE-8 A EVA HDPE-C A N y 1 2 LDPE-A v PUE LDPE-B PEE
l 0 o w 0 0.1 0.2 0.3 0.4 0.5 Excess loss CdBlkm at 0.84 p m )
/*
0
Figure
Histogram of excess losses due to sec0n-x-y coating.
n
" 4
/"
/
Z 2 C
0
I
1 2 Surface roughness Cy)
Figure 6. Relation between the surface roughness and the excess loss a t -10 OC.
ness of coated resin were investigated. The results of heat shrinkage are shown in Figure 5. It is clear from the results of Figure 5 that the heat shrinkages greatly depend upon the draw-down ratio. In other words, the molecular orientation strongly depends on the draw-down ratios. On the other hand, it has been found that the rough surface of a secondary coating scarcely has an effect upon the increase of the excess loss of fibers. From the above mentioned results, we are convinced that the surface roughness is not as effective on the increase of the excess loss as the heat shrinkage based on the molecular orientation a t extruding process. Further investigation, however, has clarified that surface roughneM causes the increase of the excess loss a t low temperatures under 0 OC. The relation between the surface roughness and the increase of excess loss at -10 "C is shown in Figure 6. The increase of the losses was nearly proportional to the surface roughness, as is shown in Figure 6. From these results, it is considered that the heat shrinkage which is brought by the molecular orientation of extrusion and surface roughness of coated resin has a great effect on the increase of fiber losses. Among the coating resins examined, Nylon 12, polyester elastomer, and polyurethane elastomer are the most appropriate resins for the secondary coating process. Reliability of Fibers Coated with Nylon 12 Considering the extrudability and stiffness of resin, Nylon 12 was finally chosen as the secondary coating material. Figure 7 shows the relation between the excess loss and the draw-down ratio of Nylon 12. It can be recognized from Figure 7 that Nylon 12 scarcely produces excess loss
v)
::o
,
W
0
1
2
3
4 Time ( m o n t h s )
5
6
Figure 9. Results of the reliability tests of optical fiber coated with Nylon 12. Table 111. Test Conditions test items
test condition
temperature: 80 C, 100 'C, 1 2 0 " C period: 6 months temperature: 60 " C relative humidity: 95% period: 6 months heat cycling test temperature: - 20 " C and 60 C cycle: 5 h/cycle period: 6 months
heat aging test (air oven) humidity test
with even very high draw-down ratios. This means that Nylon 12 hardly causes microbending of fiber by molecular orientation, and it is predicted from this fact that Nylon 12 can be easily applied to various conditions of secondary coating of optical fibers, such as coating diameter and coating thickness, etc. A number of graded index optical fibers have been coated with Nylon 1 2 and tested. Figure 8 shows the histogram of excess losses of these fibers due to secondary coating. These graded index fibers are similar in structure to those used in the experiment. The average excess loss due to coating was only 0.1 dB/km and the maximum excess loss was 0.3 dB/km at 0.84 l m . There were many fibers whose losses were not changed by the coating. Using these fibers, the reliability of the optical fiber which has been coated with Nylon 12 has been tested. The test conditions are shown in Table I11 and the test results are shown in Figure 9.
248
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4 , 1979
When secondary coating resin is exposed in the atmosphere at high humidity, the coated resin absorbs the humidity and its volume increases. As a result, the increase of volume is anticipated to cause microbendings of the fiber and to bring an increase of transmission loss. However, there was scarcely any change of transmission loss by the humidity test at 60 "C. Regarding the heat aging tests, the losses of coated fiber were increased at high temperatures such as 100 and 120 "C. It is believed that high temperature causes crystallization and oxidation of resin which results in the increase of density and it causes the loss increase. As the induction period for oxidation and crystallization greatly depends on temperature in which the test pieces are kept, high temperature causes severe changes of transmission characteristics of fibers. However, from the results of the reliability tests, it has been proved that the fibers coated with Nylon 12 are very stable for transmission losses in practical use. Conclusion The draw-down ratio has a great effect on the increase of loss of fiber and also the excess losses of optical fibers depend on the types of plastic resins which are coated on optical fibers as secondary coating. Furthermore, the transmission losses of fibers coated with some kinds of plastic resins, which have a large effect of draw-down ratio on the excess loss of fiber, greatly increase at low temperature. The draw-down ratio has an effect on the molecular orientation and the surface roughness, and the microbend-
ings of optical fibers are brought about by the rough surface and shrinkage of coated resin which are caused by molecular orientation of plastic resin. As Nylon 12, polyester elastomer, and polyurethane elastomer hardly cause the microbendings of fiber by molecular orientation and surface roughness, these plastic resins are suitable for secondary coating material. Nylon 12 has been used practically as a secondary coating material, and it has been proved that it is one of the best of these materials. Acknowledgment The authors would like to acknowledge Dr. N. Niizeki, Mr. K. Masuno, and Mr. N. Uchida, Electrical Communication Laboratory of N.T.T., for many useful suggestions, and Mr. S. Tanaka and Dr. K. Inada of The Fujikura Cable Works, Ltd., for their continuous encouragement through this work. Literature Cited Gardner, 9. W., Bell Syst. Tech. J., 13(6), 457-465 (1975). Kobayashi, T., Osanai, H., %to, M., Takata, H., Nakahara, M., "1977 International Conference on Integrated Optics and Optical Fiber Communication Technical Digest", pp 331-334 Tokyo, 1977. Naruse, T., Sugawara, Y., Masuno, K., Electr. Lett., 13(6), 153-154 (1977).
Received for review April 18, 1979 Accepted June 11, 1979 Presented a t the ACS/CSJ Chemical Congress, Division of Organic Coatings and Plastics Chemistry, 177th National Meeting of the American Chemical Society, Honolulu, HI, April 1979.
Plastic-Clad Fiber Using Optical Transmission Takeshl Kojima, Kenji Yagi," Kiyoshi Shlbuya, and Tetuo Sakanaka Showa Electric Wire & Cable Co., Ltd., Sagamiharashi, Kanaga wa, 229, Japan
Plasticclad fiber is made by coating a core of high-purity synthetic silica with a silicone resin. Properties of cladding silicones such as optical transmission, mechanical characteristics, and reliability for use with optical fibers were investigated. Optical transmission properties (loss,bandwidth, numerical aperture) of siliconeclad fibers are influenced by the components and purity of the cladding resin. Reliability of the transmission is increased by the buffering ability of silicone resin. Silicone coating is effective in preserving the inherently high pristine strength of the fiber. Fiber which is made by coating silica core (refractive index, 1.458; transmission loss, 2 dWkm) with silicone (refractive index, 1.405; loss, 1-2 dWm) has an attenuation of 3 dB/km, a bandwidth of more than 20 MHz/km, and an effective numerical aperture of 0.25. The mechanical strength of the fiber is 600 kg/mm2 or more.
Introduction Plastic-clad fiber is a fiber of the step-index type that consists of an inner core of high-purity synthetic quartz and an outer coating clad with plastic material which has a lower refractive index than the inner core. Plastic-clad fiber is used for optical transmission. Core quartz which has a total loss of transmission of less than 3 dB/km can readily be obtained, but when a fiber is made by coating a fused quartz core with resin, the properties of the cladding resin have a great effect on the characteristics of the fiber. The resin materials used as a fiber cladding should desirably have the following properties: (1)a lower index of refraction than the core quartz; (2) a higher transmissivity of rays; (3) a high capability of adhesion to quartz; (4)its properties should be unaffected by any change of condi0019-7890/79/1218-0246$01 .OO/O
tions in the environment (such as temperature, relative humidity, etc.); (5) easy coatability on the quartz core. For resins which have a lower index of refraction than quartz, fluoro resins or silicone resins are available. For resins which allow light to transmit with a higher transmission and are relatively readily available, dimethyl silicone resin can be used as a cladding. This report discusses the interrelationship between the properties of the dimethyl silicone resin and the characteristics of the plastic-clad fiber. Manufacture of Fiber The process for manufacturing plastic-clad fibers consists of drawing a rod of high-purity quartz under applied heating, applying a coating of silicone resin as a cladding to be formed on the quartz fiber, and allowing the coating to cure. The choice of the thickness of a coating or clad@ 1979 American Chemical Society
