- Email: [email protected]
Loss reduction in optical fibers
Journal of Non-Crystalline Solids 42 (1980) 247-260 © North-Holland Publishing Company
LOSS R E D U C T I O N
IN O P T I C A L
FIBERS
F. T. Stone, Bell L a b o r a t o r i e s B. K. Tariyal, W e s t e r n Electric Norcross, Georgia U.S.A.
To reduce the loss of optical fibers to the m i n i m u m p o s s i b l e value, the m a n u f a c t u r e r must have an accurate estimate of that value together w i t h an u n d e r s t a n d i n g of the m e c h a n i s m s that raise fiber loss above that value. In this paper we separate fiber loss into its a b s o r p t i o n and scattering components, p r e s e n t estimates of the m i n i m u m a t t a i n a b l e values of these components for m u l t i m o d e g r a d e d - i n d e x fibers, and examine m a n y m e c h a n i s m s that cause excess loss in such fibers.
1.0
INTRODUCTION
A l t h o u g h e x t r e m e l y low loss fibers have been r e p o r t e d [1-4], m a k i n g such fibers r o u t i n e l y remains a challenge. To p r o d u c e low-loss fibers, the m a n u f a c t u r e r needs tight control over the preformm a k i n g and f i b e r - d r a w i n g processes; the proper i m p l e m e n t a t i o n of those controls requires an u n d e r s t a n d i n g of the effect on 10ss of a large number of m a t e r i a l s and p r o c e s s i n g variables. In this paper we d i s c u s s loss m e c h a n i s m s in o p t i c a l fibers, e s t i m a t i o n s based on e x p e r i m e n t of the lowest a t t a i n a b l e (intrinsic) fiber 10ss, techniques used to d i a g n o s e the p r e s e n c e of excess (extrinsic) loss, and e x a m p l e s of excess loss. Our study is r e s t r i c t e d to gradedindex m u l t i m o d e fibers w i t h g e r m a n i u m - b o r d s i l i c a t e or germaniump h o s p h o s i l i c a t e cores and silica claddings. We c o n c e n t r a t e on the I < 1 ~m region; the techniques we describe, however, are a p p l i c a b l e to studying l o n g - w a v e l e n g t h behavior also. 2.0
CLASSIFICATIONS
OF F I B E R LOSS
Loss in fibers is c a u s e d by a b s o r p t i o n and scattering. Further s e p a r a t i o n of fiber loss into its intrinsic and extrinsic c o m p o n e n t s provides insight into the fundamental limitations of d i f f e r e n t fiber designs. We define the intrinsic loss for a p a r t i c u l a r m a t e r i a l system as the lowest a t t a i n a b l e loss for that system; hence extrinsic loss is the loss added on to this m i n i m u m figure, w h i c h can be e l i m i n a t e d by careful processing. 2.1
INTRINSIC
AND EXTRINSIC
LOSS
We arrive at an a p p r o x i m a t i o n to the intrinsic loss by m e a s u r i n g v e r y - l o w - l o s s samples made u n d e r a v a r i e t y of p r o c e s s i n g conditions. C o m p a r i n g such r e s u l t s w i t h those o b t a i n e d from models of fiber loss (Rayleigh scattering, b a n d - e d g e absorption) and w i t h those o b t a i n e d from b u l k - g l a s s samples can p r o v i d e further t h e o r e t i c a l u n d e r s t a n d i n g
247
248
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
Exact figures for intrinsic loss are difficult to obtain; hence, our point-of-view is to regard present work as providing state-ofthe-art estimates. The term intrinsic loss can be misleading since the lowest loss for a particular fiber depends not only on the materials and geometry of the fiber but also on the processing history. Because the intrinsic loss of fibers made with different compositions varies significantly, determining such variations can help the fiber designer pick the optimum fiber for the specified job. The presence of extrinsic loss can be detected by comparing a fiber's loss with the intrinsic-loss estimate. The type of extrinsic loss mechanisms must then be found (impurity absorption, core-cladding boundary scattering, etc.). To do that, diagnostic apparatus needs to be developed. 2.2
ABSORPTION AND SCATTERING
Since fiber loss can be separated into its scattering and absorption components and those components c~n further be divided into their intrinsic and extrinsic parts, we can s.~eak, of intrinsic and extrinsic scattering, and intrinsic and extrlnslc absorption. 2.2.1
INTRINSIC
SCATTERING
Compositional and density fluctuations with dimensions much smaller than an optical wavelength are unavoidably frozen into the fiber during the rapid quenching that occurs after draw. Hence, Rayleigh scattering from these fluctuations is termed intrinsic. Since the magnitude of the fluctuations depends on processing parameters such as draw termperature and draw speed, considering the Rayleigh scattering from these fluctuations as inherent in a particular material system is not strictly correct. Two useful expressions have been derived for Rayleigh scattering in a single component glass, one by Pinnow et al [5] and'Schroeder et al [6], ~S,D = and one by Stacy
(8~3/314)nSp282kTF
[7] and Maurer ~S,D =
(i)
[8],
(8~3/314) (n2-1)
8kTF
•
(2)
In these formulae, n is the refractive index, p the photoelastic constant, and 8 the isothermal compressibility evaluated at the fictive temperature. Because the fictive temperature is the temperature of the glass before rapid cooling, it is clear that ~S,D depends somewhat upon processing conditions. In a single-component gl~ss the sub-microscopic fluctuations in density cause Rayleigh scattering; in multi-component glasses such fluctuations in composition can also cause Rayleigh scattering. Olshansky [9] has derived an expression for Rayleigh scattering from compositional fluctuations in the GeO2-SiO 2 binary system: eS,C = 3"3C(I-C)/14 where C is the fractional
GeO 2 concentration.
(3) For low GeO 2
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
249
concentrations, Eq. (3) predicts 0.033 d B / k m loss increase per mole % GeO 2 at ~ = 1 ~m provided the GeO 2 does not affect the scattering due to density fluctuations. 2.2.2
INTRINSIC A B S O R P T I O N
The exponential e x t e n s i o n of the UV and IR absorption bands into the optically transparent w a v e l e n g t h region [i0] leads to absorption losses in this region of the form:
~A,UV = aoe
~o/~
(4)
for the UV component and ~A,IR = ale
-~i/~
(5)
for the IR component. In high-silica glasses the main c o n t r i b u t o r s to the band-edge absorptions are the dopants. Schultz [ii] and Keck et al [12] m e a s u r e d absorptions of the form of Eq. (4), w h i c h they attributed to Ge. In Sect. 5.1 of this paper we present results on a b s o r p t i o n in low-loss fibers that can be described over wide spectral regions by terms of the form of Eq. (4). 2.2.3
EXTRINSIC S C A T T E R I N G
Bubbles, striae (air-lines), u n d i s s o l v e d particles, c o r e - c l a d d i n g boundary roughness, and longitudinal variations of the core geometry or c o m p o s i t i o n all cause excess scattering. If the p e r t u r b a t i o n s caused by these scattering m e c h a n i s m s are small compared with a wavelength, they p r o d u c e R a y l e i g h scattering; otherwise they produce scattering w i t h a different spectral variation. All these scattering m e c h a n i s m s must be d i a g n o s e d and corrected if low-loss fibers are to be made. 2.2.4
EXTRINSIC A B S O R P T I O N
Excess a b s o r p t i o n can be divided into three categories: OH absorption, o t h e r - t h a n - O H impurity absorption (mainly transition metal ions), and a b s o r b i n g defects (color centers) introduced either during the d r a w i n g process or by the UV from the drawing furnace or the lamps used to cure certain coatings. Each of the above mechanisms has d i s t i n c t i v e spectral properties, w h i c h aid in locating the p a r t i c u l a r absorber present. 2.3
COATING-INDUCED MICROBENDING
In this paper we discuss only m i c r o b e n d i n g [13-14] introduced b y the fiber coating. Such c o a t i n g - c a u s e d m i c r o b e n d i n g depends on the type of coating, the physical state of the fiber during measurement, and the time the fiber has been in that p a r t i c u l a r state. We discuss all three of these in Sect. 6. 3.0
MEASUREMENT
TECHNIQUES
A c c u r a t e routine m e a s u r e m e n t s and more elaborate d i a g n o s t i c techniques are required to u n d e r s t a n d and control fiber loss. Fiber m e a s u r e m e n t s can be c o n v e n i e n t l y d i v i d e d into d i f f e r e n t i a l and i n t e g r a t e d types: D i f f e r e n t i a l m e a s u r e m e n t s examine only a portion of the fiber's response, for instance, the loss of a p a r t i c u l a r
250
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
group of modes in a m u l t i m o d e fiber or the loss of a small segment of a long length of fiber; thus they can d e t e r m i n e w h e t h e r a p a r t i c u l a r transverse or l o n g i t u d i n a l region of the fiber is exhibiting excess loss. Integrated m e a s u r e m e n t s provide data on the average behavior of the fiber, for instance, the total loss of a long length or the average loss of a p a r t i c u l a r mode d i s t r i b u t i o n (usually chosen to be close to the e q u i l i b r i u m mode distribution). Such m e a s u r e m e n t s are m o s t useful when studying low-loss fibers w i t h little length or mode dependence. In addition, m e a s u r e m e n t s can be separated into those that use beam optics to excite the fiber and those that use fiber optics. 3.1
MEASUREMENT
OF LONG F I B E R LENGTHS
Techniques to m e a s u r e the loss of long lengths of fiber include the two point m e t h o d [15], and o p t i c a l - t i m e - d o m a i n r e f l e c t o m e t r y (OTDR) [16]. These m e t h o d s u s u a l l y provide the average loss of a neare q u i l i b r i u m mode distribution, but when little m o d e - m i x i n g is present, they can yield m o d e - d e p e n d e n t results. Spectral m e a s u r e m e n t s can be made using w h i t e - l i g h t sources together w i t h a m o n o c h r o m a t o r or a filter set, or tunable lasers. The O T D R also provides valuable i n f o r m a t i o n on any length d e p e n d e n c e of the loss. 3.2
MEASUREMENT
OF SHORT F I B E R LENGTHS
Fiber c a l o r i m e t e r s [17-20] and a scattering cube [21] have been d e v e l o p e d to m e a s u r e the loss of short lengths of fiber. In addition, an integrating sphere has been used to m e a s u r e the scattering from various lengths of fiber [22]. M o d e - d e p e n d e n t loss [23-24] can be best studied in short samples w h e r e m o d e coupling is minimal. When m e a s u r i n g loss in short samples, however, care m u s t be taken to remove the energy p r e s e n t in the u n u s u a l l y lossy modes that p r o p a g a t e only short d i s t a n c e s [25]. If that is done, the m o d e - d e p e n d e n t a b s o r p t i o n and scattering results for short lengths of fiber p r o v i d e valuable data to study basic properties of the fiber glass and to d i a g n o s e causes of extrinsic fiber loss. 4.0 Inada
SCATTERING [26]
LOSSES
suggested
a model
for fiber
loss:
= A/I 4 + B + Cfl)
.
(6)
The first term represents the R a y l e i g h scattering contribution, the second represents w a v e l e n g t h - i n d e p e n d e n t scattering (microbending loss, for example), and the last represents s p e c t r a l l y - m o r e - c o m p l i cated absorption. Usually C(1) is n e g l e c t e d in the spectral regions 0.8 to 0.9 ~m and 1.05 to 1.15 Bm and A and B are d e t e r m i n e d by fitting total-loss data in these regions to Eq. (7): = A/I 4 + B .
(7)
In following this procedure, however, effects of the tails of the OH r e s o n a n c e s are n e g l e c t e d as well as the systematic error introduced by b a n d - e d g e absorption. 4.1
COMPARISON SCATTERING
OF A AND B VALUES LOSS DATA
OBTAINED
FROM T O T A L LOSS AND
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
251
T h e e r r o r i n t r o d u c e d by a b s o r p t i o n terms, C(l), w a s e x a m i n e d in a r e c e n t s t u d y [27] t h a t c o m p a r e d A a n d B as d e t e r m i n e d f r o m t o t a l loss d a t a w i t h A a n d B g o t t e n f r o m d i r e c t l y m e a s u r e d s c a t t e r i n g - l o s s data. T h e a v e r a g e of the r e s u l t s for s u c h a s t u d y a r e p r e s e n t e d in T a b l e I. As c a n be seen: (i) T h e A c o e f f i c i e n t s for p h o s p h o r u s TABLE
I
A V E R A G E V A L U E S OF T H E A A N D B C O E F F I C I E N T S O B T A I N E D F R O M T O T A L L O S S DATA, S C A T T E R I N G LOSS DATA, A N D S C A T T E R I N G LOSS D A T A P L U S M E A S U R E D UV-BAND-EDGE ABSORPTION T o t a l Loss Data
Scattering Loss D a t a
Scattering Loss P l u s Eq. (4) %
1.54+0.05 0.81~0.35
1.33+0.02 0.7950.22
1.45 0.76
.9984+.0040
.9995+.0004
1.31+0.13 1.0050.20
1.04+0.10 0.69~0.40
.984+.021
.9995+.0004
GeO2-B203-SI02* A B Correlation Coefficient
.9996
GeO2-P205-SIO2** A B Correlation Coefficient *Average results **Average results %Average results ~o = 5.3 ~m; for
1.35 0.63 .9995
for 3 fibers: A = 1 . 2 6 ~ 0 . 0 2 % for 6 fibers: A = 1 . 2 4 + 0 . 0 6 % for the b o r o n - d o p e d f i ~ r s : e_ = 3 . 2 x 1 0 -4 a n d phosphorus-doped fibers: s o = ~ . 3 x 1 0 -3 a n d ~o=5.0
~m.
d o p e d f i b e r s a r e s m a l l e r t h a n t h e A c o e f f i c i e n t s of b o r o n - d o p e d fibers. B o t h c l a s s e s of f i b e r s h a d n e a r l y t h e s a m e n o r m a l i z e d m a x i m u m i n d e x d i f f e r e n c e A. (2) The A coefficients from total-loss d a t a a r e l a r g e r t h a n the A c o e f f i c i e n t s from scattering-loss data. For the p h o s p h o r u s - d o p e d fibers, the d i s c r e p a n c y c a n be l a r g e l y a t t r i b u t e d to the m e a s u r e d U V - b a n d - e d g e a b s o r p t i o n ; for the b o r o n d o p e d fibers, OH a n d o t h e r i m p u r i t y a b s o r p t i o n a l s o a f f e c t s the A c o e f f i c i e n t s o b t a i n e d f r o m t o t a l - l o s s data. As s u g g e s t e d by the t h i r d c o l u m n in T a b l e I, h o w e v e r , the s y s t e m a t i c e f f e c t of U V - b a n d e d g e a b s o r p t i o n a c c o u n t s for m o s t of the d i s c r e p a n c y . (3) The v a r i a t i o n in the r e s u l t s for the f i b e r s w i t h p h o s p h o r u s is g r e a t e r t h a n t h a t for b o r o n - d o p e d fibers. The phosphorus-doped fibers used to o b t a i n the d a t a of T a b l e I h a d a p p r o x i m a t e l y the same A v a l u e s b u t d i f f e r e n t r a t i o s of g e r m a n i u m to p h o s p h o r u s . We h a v e m e a s u r e d l a r g e v a r i a t i o n s in s c a t t e r i n g a n d a b s o r p t i o n as p h o s p h o r u s c o n t e n t is c h a n g e d ; hence, we a t t r i b u t e t h a t w i d e r v a r i a t i o n to s u c h c h a n g e s in p h o s p h o r u s c o n t e n t . (4) T h e c o r r e l a t i o n c o e f f i c i e n t s are m u c h higher when scattering-loss d a t a is u s e d to find A a n d B. Because of t h e s y s t e m a t i c e f f e c t U V - b a n d - e d g e a b s o r p t i o n has on A a n d B v a l u e s w h e n s u c h a b s o r p t i o n is n o t s u b t r a c t e d o u t of the s p e c t r a l loss d a t a u s e d to find t h e s e v a l u e s , an i m p r o v e d m o d e l for f i b e r loss in the s h o r t e r w a v e l e n g t h r e g i o n m i g h t be = A / l 4 + B + e e IO/l o
+ C(I)
.
(8)
252
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
For fibers doped w i t h phosphorus, Eq. (8) is a reasonable model out to at least 1.06 ~m, w i t h C(~) due almost e n t i r e l y to OH absorption. For fibers doped w i t h boron, the effect of the IR- and U V - b a n d - e d g e absorptions are about equal at 1.06 ~m; hence, Eq. (8) is not strictly correct in that region. 4.2
RAYLEIGH
SCATTERING
RESULTS
We have shown that the A and B c o e f f i c i e n t s m u s t be o b t a i n e d from s c a t t e r i n g - l o s s data to be accurate. Nevertheless, if the core c o m p o s i t i o n d o e s n ' t vary from fiber to fiber, the U V - b a n d - e d g e absorption will affect A by a p p r o x i m a t e l y the same amount; hence A values gotten from total-loss data can be used to m o n i t o r the m a n u f a c t u r i n g process p r o v i d e d one recognizes that they differ from the actual R a y l e i g h - s c a t t e r i n g values. This is useful b e c a u s e total-loss data can be obtained more c o n v e n i e n t l y than s c a t t e r i n g - l o s s data. The A and B c o e f f i c i e n t s of Eq. (7) are quite useful in characterizing a f i b e r - m a k i n g process. In Table II [29] we p r e s e n t data on A and B for fibers with G e O 2 - B 2 0 3 - S i O 2 cores and silica claddings. As can be seen, a l a t h e - t o - l a t h e v a r i a t i o n in A occurs, w h i c h is of the same order as the v a r i a t i o n in A for a single l a t h e . Because A was obtained from total-loss data, variations in OH i n c o r p o r a t i o n from lathe to lathe as well as variations in R a y l e i g h scattering could explain the difference. TABLE
II
A AND B C O E F F I C I E N T S OBTAINED LOSS DATA F O R B O R O N - D O P E D Lathe A B
F R O M TOTAL FIBERS
i*
Lathe
1.54+0.12 1.5650.41
2*
1.62+0.10 1.5450.36
*Ten preforms from each lathe. Each p r e f o r m y i e l d e d fiber. For all 90 km of fiber A = 1.26+0.02%.
4.5 km of
The v a r i a t i o n in B is m u c h larger than the v a r i a t i o n in A (see also Table I), p r i m a r i l y because of coating effects including coating n o n u n i f o r m i t y and hardness, and v a r i a t i o n s in the physical state of the c o a t i n g (as will be d i s c u s s e d in Sect. 6.1). 4.2.1
ESTIMATE
OF INTRINSIC
MODE-DEPENDENT
SCATTERING
The m o d e - d e p e n d e n t loss plot of fiber 2 shown in Fig. 1 is an archetype for scattering from a good g e r m a n i u m - b o r o s i l i c a t e fiber. The i n t e r p r e t a t i o n of plots such as in Fig. 1 is as follows: A laser beam a p p r o p r i a t e l y focused to p r o d u c e a small spot-size (~6 ~m) and small a n g u l a r - d i v e r g e n c e (N.A. ~0.07) beam excites the fiber (core diameter = 55 ~m). W h e n the b e a m is focused at the center of the core (r/a = 0) of the g r a d e d - i n d e x fiber, p r i m a r i l y low-order m o d e s are excited, w h i c h are confined to the inner region of the core. As the b e a m is scanned to the c o r e - c l a d d i n g b o u n d a r y (r/a = i), higher order m o d e s are excited, w h i c h have energy d i s t r i b u t i o n s that more c o m p l e t e l y fill the core. Hence, assuming only R a y l e i g h scattering, we c a l c u l a t e A = 1.385 from the 7.9 d B / k m scattering loss of the loworder modes (which p r o p a g a t e in the h i g h - g e r m a n i u m - c o n t e n t core center) and A = 1.315 from the 7.5 d B / k m scattering loss of the highorder m o d e s (which fill the core). Fiber 2 has A = 1 . 2 9 % implying a
253
F.T. Stone, B.K. Tariyal / Loss Reduction in optical Fibers
14 G e O 2 - B 2 0 3 - $ i O 2 CORE UNCOATED FLEERS = 647nm 12
J 10 O O
FIBER
j
2
I
I
I
I
l
I
S
10
15
20
25
27.5
m a x i m u m GeO 2 content of 13 mole %. A s s u m i n g the low-order modes see the m a x i m u m GeO 2 and the h i g h - o r d e r modes see, on average, one-third the maximum, Eq. (3) predicts a 1/3 x 13 x 0.033 dB/ km = 0.143 d B / k m loss increase at 1 ~m in going from high-ordermode to low-order-mode excitation. That figure is twice th m e a s u r e d value of ~A = 0.07 ~m ~dB/km; hence, other GeO~ effects and the fining effect ot B203 are significant. The m e a s u r e d loss increase for the fibers w i t h phosphorus is about half that of fibers w i t h boron indicating P205 has a strong fining effect.
BEAM POSITION (/4m)
EXTRINSIC SCATTERING: DISTRIBUTED SCATTERING MECHANISMS
4.3 Fig. I. M o d e - d e p e n d e n t scattering of a low-loss fiber (2) and a fiber w i t h c o r e - c l a d d i n g boundary roughness (i).
Extrinsic scattering may be caused by m e c h a n i s m s distributed m o r e - o r - l e s s u n i f o r m l y along the fiber or scattering centers located at only a few places along the fiber. In Fig. 2a we show the crosssection of a fiber that exhibited an extreme amount of scattering and in Fig. 2b we show a m o d e - d e p e n d e n t loss plot of this fiber, w h i c h indicates excessive loss for the higher order modes caused by the elliptical shape of the c r o s s - s e c t i o n of the core. The m o d e - d e p e n d e n t s c a t t e r i n g - l o s s plot of a fiber with core-cladd i n g - b o u n d a r y roughness is presented in Fig. 1 together w i t h the same plot for a good fiber. The high loss for the v e r y - h i g h - o r d e r
SAMPLE E
TOTAL LOSS (dR/Kml
I
I
0.5
0
O.S
NORMALIZED POSITION OF EXCITING REAM (r/a)
Fig.
2 (a) M i c r o g r a p h of c r o s s - s e c t i o n of e l l i p t i c a l - c o r e (b) M o d e - d e p e n d e n t loss plot of the fiber.
fiber.
modes localizes the p r o b l e m to the c o r e - c l a d d i n g region. Because the high loss is caused by scattering, impurity absorption at the c o r e - c l a d d i n g b o u n d a r y can be e l i m i n a t e d as a possible cause (see
254
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
Sect. 5.2.1). M i c r o s c o p i c e x a m i n a t i o n of fiber nal variations in the c r o s s - s e c t i o n of the core than those of fibers such as fiber 2. 4.4
EXTRINSIC
SCATTERING:
LOCALIZED
1 revealed longitudithat were much larger
SCATTERING
MECHANISMS
The optical time domain r e f l e c t o m e t e r [16] provides data on the optical loss vs. position along the fiber. Hence such a plot, an e x a m p l e of which is shown in Fig. 3a, pinpoints regions of excess loss. The fiber may then be examined in the high-loss region to seek the loss m e c h a n i s m responsible. In the case of the fiber of Fig. 3a, a large bubble had been drawn into a long air-line, the start of w h i c h is shown in Fig. 3b. The air-line was 19 m long and at its center filled an appreciable region of the core, a rather extreme example of this class of fiber defect.
LOSS pIOFIL[ O+ F I I I I l , I H LOffO AII.LINI OITAINID WITH OpTI+AL IIMI DOMAIN I I F t I C T O M I T I I 3o i
~
~o
i
io
z
[
o 00OO
0,IOO
O 200
0 ]OO
O400
OSOO
0600
• 700
0 ilO0
LIMOTH.KILOmlTIIS
Figure
3.
a) b)
Optical time domain r e f l e c t o m e t e r loss plot of a fiber with a long air line. Air line begins at far right of micrograph. As is shown in Fig. 4, exciting a fiber that is w o u n d on a drum w i t h a visible laser also helps locate discrete scattering centers. In that figure, the laser was focused at the c o r e - c l a d d i n g boundary. When the e x c i t a t i o n was m o v e d to the central region of the core, the bright scattering center disappeared, indicating a defect at or near the c o r e - c l a d d i n g region. 5.0
ABSORPTION
LOSS
To achieve low losses, a b s o r p t i o n in fibers must be reduced to its intrinsic limit. Thus OH and other impurity c o n t a m i n a t i o n and p r o c e s s i n g - r e l a t e d damage, such as that caused by UV radiation [30, 31], m u s t be minimized. Fig. 4 P h o t o g r a p h of a fiber w o u n d on a drum, showing bright scattering center (arrow).
255
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
5.1
EXPONENTIAL UV-BAND-EDGE ABSORPTION
Numerous a b s o r p t i o n m e a s u r e m e n t s of b o r o n - d o p e d and p h o s p h o r u s - d o p e d fibers reveal the short w a v e l e n g t h (l < 1.06 pm) behavior shown in Fig. 5 (for b o r o n - d o p e d fibers) and Fig. 6 (for p h o s p h o r u s - d o p e d fibers).
+f so
O , O ~ - 1 2 0 ] - $ ~ ~ COtlS
=........,
20
a,:e
i0
-i
s : ]1.1o
~
~- so
10
so
-" 2o o
a = 1 s,vo-3, 4 1 / k
<
IO
io
.... ;:°r:
I I
104,
0.713 017G 0.147
I
01 Oil
I 10
12
II 1.4
OS61 o111
I )4
I / k ~'--11
Fig. loss core
5. Spectral a b s o r p t i o n of germanium-borosilicatefiber.
0711 0676 0647
os*$
0S31
047~
o47~.
!
r +I
~
~o
2~
I.o
I2
I¢ i/X
1.
I|
2o
~
~--5
Fig. 6. Spectral absorption loss of g e r m a n i u m - p h o s p h o s i l i c a t e - c o r e fiber.
For b o r o n - d o p e d fibers the exponential behavior extends to approxim a t e l y 700 nm; for longer w a v e l e n g t h s small impurity absorptions (<0.i dB/km) and the IR-band-edge a b s o r p t i o n (Eq. 5) due to boron cause the curve to reach a m i n i m u m at a p p r o x i m a t e l y 1.2 ~m and then begin to rise e x p o n e n t i a l l y [i]. The UV e x p o n e n t i a l - b a n d - e d g e absorption shown in Fig. 5 depends on g e r m a n i u m c o n t e n t : The boron in the fiber was constant (at 1%) throughout the core, but as seen in Fig. 5, the exponential a b s o r p t i o n decreases for higher order modes (At = 20 ~m) that propagate in a region of the core w i t h a lower average g e r m a n i u m content. Also the exponential fit to the data for the smallest-A fiber examined (A = 1.16%) had lo = 5.1 ~m and s o = 4.6xi0 -4 for Ar = o, a somewhat lower a b s o r p t i o n than that of the fiber in Fig. 5. For p h o s p h o r u s - d o p e d fibers, the a b s o r p t i o n losses m e a s u r e d were s i g n i f i c a n t l y higher than for b o r o n - d o p e d fibers. A c o m p a r i s o n of results presented in Figs. 5 and 6 for a typical fiber from each class indicates the m a g n i t u d e of the effect. The fibers with phosphorus had a b s o r p t i o n curves that were exponential to 1.06 ~m; however, the m a g n i t u d e of the a b s o r p t i o n varied widely, most probably caused by the formation of stable a b s o r p t i o n centers in w h i c h phosphorus plays a necessary part. Hence, the intrinsic a b s o r p t i o n of such fibers depends s i g n i f i c a n t l y on phosphorus content. For the b o r o n - d o p e d fibers and u n d a m a g e d fibers with low (< 2%) phosphorus c o n t e n t (such as.in Fig. 6), the exponential a b s o r p t i o n is near that a t t r i b u t e d to Ge 4+ in g e r m a n i u m - s i l l c a t e glasses [ii].
256
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
5.2
IMPURITY A B S O R P T I O N
A recent study [28] in g e r m a n i u m - b o r o s i l i c a t e fibers concludes that impurities are c o n c e n t r a t e d in the c o r e - c l a d d i n g boundary region or along the axis of the fiber, i.e., the impurities appear to diffuse from the starting tube into the outer deposited layers or to enter the innermost deposited layers of the p r e f o r m before complete collapse. The evidence for this conclusion is the observation of excess a b s o r p t i o n in very low- or very high-order modes. For germaniump h o s p h o s i l i c a t e fibers similar results have been observed; however, the presence of color centers in the h i g h - g e r m a n i u m - c o n t e n t central region of the core can obscure impurity a b s o r p t i o n present there. 5.2.1
B A R R I E R L A Y E R STUDY
The use of thick barrier layers to reduce impurity diffusion from the starting tube to the fiber core has already been noted [32-34]. As an example of impurity absorption, we present additional results showing the effect of b a r r i e r layer thickness on impurity diffusion into the core. From a large number of fibers with thin (5 d B / k m at 0.82 ~m. The average m o d e - d e p e n d e n t a b s o r p t i o n of these eight is plotted in Fig. 7 for the h i g h e r - o r d e r modes. Four fibers were then taken from a b a t c h having thick (>5 ~m) g e r m a n i u m - b o r o s i l i c a t e barrier layers, with no other selection criteria applied. The m o d e - d e p e n d e n t a b s o r p t i o n for these four is also shown in Fig. 7. The increasing a b s o r p t i o n for the t h i n - b a r r i e r - l a y e r samples as h i g h e r - o r d e r modes are excited indicates impurity levels gradually rising as the corecladding boundary (27.5 ~m) is approached.
GeO2-B203-SIO2 4
a AVERAGE OF i THIN--BARRIERLAYER SAMPLES o AVERAGE OF 4 THICK-BARRIERLAYER SAMPLiS
~. 3
[ INDICATES RANGE OF RESULTS
2
)~ = 647nla
_~
. i
//
CORES /
TI
GeO2-P2Os-S~O 2 ~- = 1.24"~ = 647.m
15
"('1/~ //
COATED (EXPOSED TO ONE
,
!i '°
///
EIGHT SAMPLES
.,,..,.,,.u...
/
// /
iS)
I
/
.,9, ,
s UNCOATED (NO UV)
I 0
i $
I 10
I 15
I 20
A
BEAM POSITION (nm)
Fig. 7. Mode -dependent-loss plots showing effect of impurities diffusing in from the starting tube in thin-barrierlayer fibers. 5.3
¸ BEAM POSIT'ON~i
Fig. 8. M o d e - d e p e n d e n t - l o s s plots showing large UV-induced loss for low-order modes propagating in highg e r m a n i u m - c o n t e n t central region of core.
U V - I N D U C E D DAMAGE IN G E R M A N I U M - P H O S P H O S I L I C A T E
FIBERS
Significant UV-caused damage has recently been noted in a wide
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
257
v a r i e t y of fibers [30]. In m o d e r a t e l y doped fibers (A ~ 1.3%) coated w i t h a U V - c u r e d polymer, such damage was o b s e r v e d in g e r m a n i u m - p h o s p h o s i l i c a t e but not in g e r m a n i u m - b o r o s i l i c a t e fibers [31]. An i n t e r e s t i n g result for 1.3%-A g e r m a n i u m - p h o s p h o s i l i c a t e fibers is p r e s e n t e d in Fig. 8. The UV causes a b s o r p t i o n centers that are far m o r e n u m e r o u s in the h i g h - g e r m a n i u m - c o n t e n t central region of the core, as is shown by the large increase in l o w - o r d e r - m o d e loss. B e c a u s e the p h o s p h o r u s c o n t e n t of this fiber is c o n s t a n t throughout the core, the increase in loss is due to the increase in germanium; however, the p r e s e n c e of phosphorus is n e c e s s a r y b e c a u s e the loss increase is a b s e n t in similar fibers w i t h b o r o n r e p l a c i n g the phosphorus. The c o m p a r i s o n shown in Fig. 8 was one of five made: in three cases excess loss similar to that of Fig. 8 appeared; in two cases such loss could not be discerned. Hence other v a r i a b l e s are involved, w h i c h must be determined. The spectral d e p e n d e n c e of the added loss is e x p o n e n t i a l w i t h a slope greater than that m e a s u r e d for the U V - b a n d - e d g e a b s o r p t i o n s of Figs. 5 and 6. From such data the added 820-nm loss caused by one coatingcure dose of UV has been e s t i m a t e d at <0.i d B / k m [32] p r o v i d e d a sufficient t h i c k n e s s of c o a t i n g covers the fiber: The screening effect of the c o a t i n g is critical because the loss increase of bare irradiated fibers is enormous [30]. 6.0
COATING-INDUCED
LOSS
A c o a t i n g for the fibers m u s t be chosen that adds a m i n i m u m amount of m i c r o b e n d i n g loss. The d e t e r m i n a t i o n of the added loss from c o a t i n g as well as the a d d e d loss caused by other stages in the c a b l i n g p r o c e s s is c o m p l i c a t e d by the d e p e n d e n c e of this added loss on the physical state of the fiber and its previous history. 6.1
EFFECT
OF THE STATE OF THE F I B E R ON A D D E D
LOSS
C o a t i n g - i n d u c e d loss d e p e n d s on the physical c o n f i g u r a t i o n of the fiber (wound on a small d r u m or a larger drum, or loosely laid in a container) and the time the fiber has been in that state.
i FliER1011 11~1 0
- -
-O.5
i -I.O
.
-I.S
~
-E.O
°
-2.5
12
I
t*
:
J
L
14 i
0
20
40
60
Fig. 9. The loss of coated fibers w o u n d on sixinch-diameter reels vs. the time on the reels.
|JOIN FUTURE MEASUREMENTS HERE~20 HRS.) 80 1100 IR0 140 1"0 180 200 220 240 260 MOUESAFTERUIAWINO
The effect the r e l a x a t i o n time has on the loss m e a s u r e d is shown in Fig. 9. A f t e r d r a w i n g and c o a t i n g (with c o a t i n g W of Table IV) the fibers were w o u n d on s i x - i n c h - d i a m e t e r reels w i t h m a n y o v e r l a p p i n g turns and m e a s u r e d [35] at the times shown in Fig. 9. After
258
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
approximately 120 h o u r s (5 days) all s u b s e q u e n t m e a s u r e m e n t s w e r e g i v e n 120 h o u r s to relax.
the m e a s u r e d loss s t a b i l i z e d ; hence, m a d e a f t e r the f i b e r c o a t i n g w a s
To d e t e r m i n e the e f f e c t of w i n d i n g c o n d i t i o n s , the loss of s e v e n f i b e r s w a s m e a s u r e d w i t h the f i b e r s w o u n d on s i x - i n c h - d i a m e t e r reels a n d l o o s e l y laid in a l a r g e b a r r e l . The measurement results shown in T a b l e III [36] w e r e all t a k e n a f t e r the 1 2 0 - h o u r w a i t i n g period. The a v e r a g e loss d e c r e a s e on g o i n g f r o m reel to b a r r e l m e a s u r e m e n t s w a s 0.69 dB/km, a l m o s t all of w h i c h a p p e a r e d as a d e c r e a s e in B (0.62 dB/km) a n d v e r y l i t t l e of w h i c h a p p e a r e d as a d e c r e a s e in A (0.058 ~ m 4 - d B / k m ) . T h e r e s u l t s for the f i b e r u n w o u n d in a b a r r e l s h o u l d be u s e d as the u n c a b l e d loss w h e n t r y i n g to d e t e r m i n e the a d d e d loss a g i v e n c o a t i n g c a u s e s in a c o m p l e t e d cable.
TABLE LOSS AT Fiber*
825
~m:
III R E E L VS.
Loss** R B 4.70 4.62 5.05 4.53 5.59 4.74 5.16 4.50 8.15 6.76 5.60 4.30 4.96 4.93
20 21 22 23 24 25 26
A*** R 1.50 1.67 1.77 1.48 1.56 1.73 1.66
*All f i b e r s had 5 5 - ~ m c o r e d i a m e t e r s * * R = Reel, B = B a r r e l ***Taken from total-loss data
6.2
EFFECT
OF D I F F E R E N T
BARREL B*** R B 1.39 1.30 1.37 0.93 1.78 i.i0 1.87 1.27 4.71 3.44 1.76 0.61 1.25 1.23
B 1.49 1.63 1.65 1.44 1.49 1.67 1.59
and
ll0-~m
cladding
diameters
COATINGS
In an e x p e r i m e n t to d e t e r m i n e t h e r e l a t i v e m e r i t s of d i f f e r e n t fiber c o a t i n g s , four m a t e r i a l s w e r e a p p l i e d to f i b e r d r a w n f r o m f o r t y - s i x d i f f e r e n t p r e f o r m s (4.3 k m of f i b e r p e r preform) in such a m a n n e r t h a t the e f f e c t of p r e f o r m - t o - p r e f 0 r m variations was minimized. C o a t i n g s W, X, and Z of T a b l e IV w e r e U V - c u r e d w i t h c o a t i n g W h a v i n g the l o w e s t e l a s t i c m o d u l u s , c o a t i n g Z h a v i n g t h e h i g h e s t (four times t h a t of W), and c o a t i n g X b e i n g i n t e r m e d i a t e . Coating Y was a hot m e l t c o a t i n g w i t h an e l a s t i c m o d u l u s an o r d e r of m a g n i t u d e l o w e r t h a n the others. As c a n be seen in T a b l e IV, the v a l u e s of A are n e a r l y the same for f i b e r s c o a t e d w i t h d i f f e r e n t m a t e r i a l s , b u t the
TABLE FIBER Coatinq
FOR FOUR
Loss
W X Y Z *From
LOSS
4.56+0.80 4.8750.40 5.5751.19 6.5852.08 total
loss
data
IV
DIFFERENT
COATINGS
A* 1.65+0.13 1.6350.08 1.6850.13 1.7650.12
B* 0.81+0.63 1.2650.42 1.8751.06 2.66~2.10
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
259
values of B vary widely, illustrating the effect that the coatings can have on extrinsic scattering loss. This does not imply that any particular coating is necessarily superior or inferior to some other coating, but it does mean that, depending upon their application parameters, coatings can introduce varying amounts of extrinsic loss, r e s u l t i n g in an overall increase in the coated fiber loss. For coatings W, X, and Z, the coating-induced loss decreased with decreasing elastic modulus; coating Y, which had the lowest modulus, could not be applied uniformly, leading to the high added loss shown. 7.0
CONCLUSIONS
To pursue a program aimed at reducing fiber loss, one needs a theoretical understanding of loss mechanisms and an array of routine and specialized measurement techniques. Measurement results then lead to estimates of the lowest attainable (or intrinsic) loss and the dependence of loss on fiber composition and processing. Using these results mechanisms causing excess loss can be identified and eliminated. REFERENCES [i] [2] [3] [4] [5] [6] [7] [8] [9] [i0] [ii] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Osanai, H., Shioda, T., Mariyama, T., Araki, S., Horiguchi, M., Izawa, T., and Takata, H., Electron. Lett. 12, 549 (1976). Horiguchi, M., and Osanai, H., Electron. Lett. 12, 310 (1976). Kawachi, M., Kawana, A., and Miyashita, T., Electron. Lett. 13, 442 (1977). Miya, T., Terunuma, Y., Hosaka, T., and Miyashita, T., Electron. Lett. 15, 106 (1979). Pinnow, D. A., Rich, T. C., Ostermayer, Jr., F. W., and DiDomenico, Jr., M., Appl. Phys. Lett. 22, 527 (1973). Schroeder, J., Mohr, R., Macedo, P. B., and Montrose, C. J., J. Am. Ceram. Soc. 56, 510 (1973). Stacey, K. A., Light--Scattering in Physical Chemistry (Academic Press, New York, 1956), p. 8. Maurer, R. D., Proc. IEEE 61, 452 (1973). Olshansky, R., Rev. Mod. Phys. 51, 341 (19J9). Urbach, F., Phys. Rev. 92, 1324 (1953). Schultz, P. C., Presented at the XIth International Conference on Glass, Prague, 1977. Keck, D. B., Maurer, R. D., and Schultz, P. C., Appl. Phys. Lett. 22, 307 (1973). Gardner, W. B., Bell Syst. Tech. J. 54, 457 (1975). Gloge, D., Bell Syst. Tech. J. 54, 245 (1975). Cherin, A. H., Cohen, L. G., Horden, W. S., Burrus, C. A., and Kaiser, P., Appl. Opt. 10, 2359 (1974). Barnoski, M. K., and Jensen, S. M., Appl. Opt. 15, 2112 (1976). Cohen, R. L., West. K. W., Lazay, P. D., and Simpson, J., Appl. Opt. 13, 2522 (1974). Zaganiaris, A., Appl. Phys. Lett. 25, 345 (1974). White, K. I., Opt. Quant. Electron. 8, 73 (1976). Stone, F. T., Gardner, W. B., and Lovelace, C. R., Opt. Lett. 2, 48 (1978). Tynes, A. R., Appl. Opt. 9, 2706 (1970). Keck, D. B., Schultz, P. ~., and Zimar, F., Appl. Phys. Lett. 21, 215 (1972). Olshansky, R., and Oaks, S. M., Appl. Opt. 17, 1830 (1978). Stone, F. T., Appl. Opt. 17, 2825 (1978). Stone, F. T., and Krawarik, P. H., Appl. Opt. 18, 756 (1979). Inada, K., Opt. Comm. 19, 436 (1975).
260
[27] [28] [29] [30]
[31] [32] [33] [34] [35] [36]
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
Philen, D. L., and Stone, F. T., presented at Conference on Physics O f Fiber Optics, Chicago, 1980, to be published in Advances in Ceramic Engineering, Am. Ceram. Soc., 1980. Stone, F. T., unpublished results. Tariyal, B. K., Partus, F. P., Sprow, R. B., Kalish, D., and Jefferies, J. A., unpublished results. Blyler, L. L., Jr., DiMarcello, F. V., Simpson, J. R., Sigety, E. A., Hart, A. C., Jr., and Foertmeyer, V. A., Proc. XIIth Int'l Congress on Glass, Albuquerque, 1980. To be published in a Special Issue of J. Non-Crytal. Solids, North Holland, Amsterdam, 1980. Stone, F. T., and Eichenbaum, B. R., Same Proceedings as [30] Kawachi, M., Horiguchi, M., Kawana, A., and Miyashita, T., Electron. Lett. 13, 247 (1977). Tariyal, B. K., ~-Effect of Barrier Layer Thickness on the Optical Loss of Step and Graded Index Fibers," Presented at the 80th Annual Mtg. of the Am. Ceram. Soc., Detroit, 1978. Saifi, M. A., to be published. Boggs, L. M., unpublished results. Wilson, L., unpublished results.
LOSS R E D U C T I O N
IN O P T I C A L
FIBERS
F. T. Stone, Bell L a b o r a t o r i e s B. K. Tariyal, W e s t e r n Electric Norcross, Georgia U.S.A.
To reduce the loss of optical fibers to the m i n i m u m p o s s i b l e value, the m a n u f a c t u r e r must have an accurate estimate of that value together w i t h an u n d e r s t a n d i n g of the m e c h a n i s m s that raise fiber loss above that value. In this paper we separate fiber loss into its a b s o r p t i o n and scattering components, p r e s e n t estimates of the m i n i m u m a t t a i n a b l e values of these components for m u l t i m o d e g r a d e d - i n d e x fibers, and examine m a n y m e c h a n i s m s that cause excess loss in such fibers.
1.0
INTRODUCTION
A l t h o u g h e x t r e m e l y low loss fibers have been r e p o r t e d [1-4], m a k i n g such fibers r o u t i n e l y remains a challenge. To p r o d u c e low-loss fibers, the m a n u f a c t u r e r needs tight control over the preformm a k i n g and f i b e r - d r a w i n g processes; the proper i m p l e m e n t a t i o n of those controls requires an u n d e r s t a n d i n g of the effect on 10ss of a large number of m a t e r i a l s and p r o c e s s i n g variables. In this paper we d i s c u s s loss m e c h a n i s m s in o p t i c a l fibers, e s t i m a t i o n s based on e x p e r i m e n t of the lowest a t t a i n a b l e (intrinsic) fiber 10ss, techniques used to d i a g n o s e the p r e s e n c e of excess (extrinsic) loss, and e x a m p l e s of excess loss. Our study is r e s t r i c t e d to gradedindex m u l t i m o d e fibers w i t h g e r m a n i u m - b o r d s i l i c a t e or germaniump h o s p h o s i l i c a t e cores and silica claddings. We c o n c e n t r a t e on the I < 1 ~m region; the techniques we describe, however, are a p p l i c a b l e to studying l o n g - w a v e l e n g t h behavior also. 2.0
CLASSIFICATIONS
OF F I B E R LOSS
Loss in fibers is c a u s e d by a b s o r p t i o n and scattering. Further s e p a r a t i o n of fiber loss into its intrinsic and extrinsic c o m p o n e n t s provides insight into the fundamental limitations of d i f f e r e n t fiber designs. We define the intrinsic loss for a p a r t i c u l a r m a t e r i a l system as the lowest a t t a i n a b l e loss for that system; hence extrinsic loss is the loss added on to this m i n i m u m figure, w h i c h can be e l i m i n a t e d by careful processing. 2.1
INTRINSIC
AND EXTRINSIC
LOSS
We arrive at an a p p r o x i m a t i o n to the intrinsic loss by m e a s u r i n g v e r y - l o w - l o s s samples made u n d e r a v a r i e t y of p r o c e s s i n g conditions. C o m p a r i n g such r e s u l t s w i t h those o b t a i n e d from models of fiber loss (Rayleigh scattering, b a n d - e d g e absorption) and w i t h those o b t a i n e d from b u l k - g l a s s samples can p r o v i d e further t h e o r e t i c a l u n d e r s t a n d i n g
247
248
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
Exact figures for intrinsic loss are difficult to obtain; hence, our point-of-view is to regard present work as providing state-ofthe-art estimates. The term intrinsic loss can be misleading since the lowest loss for a particular fiber depends not only on the materials and geometry of the fiber but also on the processing history. Because the intrinsic loss of fibers made with different compositions varies significantly, determining such variations can help the fiber designer pick the optimum fiber for the specified job. The presence of extrinsic loss can be detected by comparing a fiber's loss with the intrinsic-loss estimate. The type of extrinsic loss mechanisms must then be found (impurity absorption, core-cladding boundary scattering, etc.). To do that, diagnostic apparatus needs to be developed. 2.2
ABSORPTION AND SCATTERING
Since fiber loss can be separated into its scattering and absorption components and those components c~n further be divided into their intrinsic and extrinsic parts, we can s.~eak, of intrinsic and extrinsic scattering, and intrinsic and extrlnslc absorption. 2.2.1
INTRINSIC
SCATTERING
Compositional and density fluctuations with dimensions much smaller than an optical wavelength are unavoidably frozen into the fiber during the rapid quenching that occurs after draw. Hence, Rayleigh scattering from these fluctuations is termed intrinsic. Since the magnitude of the fluctuations depends on processing parameters such as draw termperature and draw speed, considering the Rayleigh scattering from these fluctuations as inherent in a particular material system is not strictly correct. Two useful expressions have been derived for Rayleigh scattering in a single component glass, one by Pinnow et al [5] and'Schroeder et al [6], ~S,D = and one by Stacy
(8~3/314)nSp282kTF
[7] and Maurer ~S,D =
(i)
[8],
(8~3/314) (n2-1)
8kTF
•
(2)
In these formulae, n is the refractive index, p the photoelastic constant, and 8 the isothermal compressibility evaluated at the fictive temperature. Because the fictive temperature is the temperature of the glass before rapid cooling, it is clear that ~S,D depends somewhat upon processing conditions. In a single-component gl~ss the sub-microscopic fluctuations in density cause Rayleigh scattering; in multi-component glasses such fluctuations in composition can also cause Rayleigh scattering. Olshansky [9] has derived an expression for Rayleigh scattering from compositional fluctuations in the GeO2-SiO 2 binary system: eS,C = 3"3C(I-C)/14 where C is the fractional
GeO 2 concentration.
(3) For low GeO 2
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
249
concentrations, Eq. (3) predicts 0.033 d B / k m loss increase per mole % GeO 2 at ~ = 1 ~m provided the GeO 2 does not affect the scattering due to density fluctuations. 2.2.2
INTRINSIC A B S O R P T I O N
The exponential e x t e n s i o n of the UV and IR absorption bands into the optically transparent w a v e l e n g t h region [i0] leads to absorption losses in this region of the form:
~A,UV = aoe
~o/~
(4)
for the UV component and ~A,IR = ale
-~i/~
(5)
for the IR component. In high-silica glasses the main c o n t r i b u t o r s to the band-edge absorptions are the dopants. Schultz [ii] and Keck et al [12] m e a s u r e d absorptions of the form of Eq. (4), w h i c h they attributed to Ge. In Sect. 5.1 of this paper we present results on a b s o r p t i o n in low-loss fibers that can be described over wide spectral regions by terms of the form of Eq. (4). 2.2.3
EXTRINSIC S C A T T E R I N G
Bubbles, striae (air-lines), u n d i s s o l v e d particles, c o r e - c l a d d i n g boundary roughness, and longitudinal variations of the core geometry or c o m p o s i t i o n all cause excess scattering. If the p e r t u r b a t i o n s caused by these scattering m e c h a n i s m s are small compared with a wavelength, they p r o d u c e R a y l e i g h scattering; otherwise they produce scattering w i t h a different spectral variation. All these scattering m e c h a n i s m s must be d i a g n o s e d and corrected if low-loss fibers are to be made. 2.2.4
EXTRINSIC A B S O R P T I O N
Excess a b s o r p t i o n can be divided into three categories: OH absorption, o t h e r - t h a n - O H impurity absorption (mainly transition metal ions), and a b s o r b i n g defects (color centers) introduced either during the d r a w i n g process or by the UV from the drawing furnace or the lamps used to cure certain coatings. Each of the above mechanisms has d i s t i n c t i v e spectral properties, w h i c h aid in locating the p a r t i c u l a r absorber present. 2.3
COATING-INDUCED MICROBENDING
In this paper we discuss only m i c r o b e n d i n g [13-14] introduced b y the fiber coating. Such c o a t i n g - c a u s e d m i c r o b e n d i n g depends on the type of coating, the physical state of the fiber during measurement, and the time the fiber has been in that p a r t i c u l a r state. We discuss all three of these in Sect. 6. 3.0
MEASUREMENT
TECHNIQUES
A c c u r a t e routine m e a s u r e m e n t s and more elaborate d i a g n o s t i c techniques are required to u n d e r s t a n d and control fiber loss. Fiber m e a s u r e m e n t s can be c o n v e n i e n t l y d i v i d e d into d i f f e r e n t i a l and i n t e g r a t e d types: D i f f e r e n t i a l m e a s u r e m e n t s examine only a portion of the fiber's response, for instance, the loss of a p a r t i c u l a r
250
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
group of modes in a m u l t i m o d e fiber or the loss of a small segment of a long length of fiber; thus they can d e t e r m i n e w h e t h e r a p a r t i c u l a r transverse or l o n g i t u d i n a l region of the fiber is exhibiting excess loss. Integrated m e a s u r e m e n t s provide data on the average behavior of the fiber, for instance, the total loss of a long length or the average loss of a p a r t i c u l a r mode d i s t r i b u t i o n (usually chosen to be close to the e q u i l i b r i u m mode distribution). Such m e a s u r e m e n t s are m o s t useful when studying low-loss fibers w i t h little length or mode dependence. In addition, m e a s u r e m e n t s can be separated into those that use beam optics to excite the fiber and those that use fiber optics. 3.1
MEASUREMENT
OF LONG F I B E R LENGTHS
Techniques to m e a s u r e the loss of long lengths of fiber include the two point m e t h o d [15], and o p t i c a l - t i m e - d o m a i n r e f l e c t o m e t r y (OTDR) [16]. These m e t h o d s u s u a l l y provide the average loss of a neare q u i l i b r i u m mode distribution, but when little m o d e - m i x i n g is present, they can yield m o d e - d e p e n d e n t results. Spectral m e a s u r e m e n t s can be made using w h i t e - l i g h t sources together w i t h a m o n o c h r o m a t o r or a filter set, or tunable lasers. The O T D R also provides valuable i n f o r m a t i o n on any length d e p e n d e n c e of the loss. 3.2
MEASUREMENT
OF SHORT F I B E R LENGTHS
Fiber c a l o r i m e t e r s [17-20] and a scattering cube [21] have been d e v e l o p e d to m e a s u r e the loss of short lengths of fiber. In addition, an integrating sphere has been used to m e a s u r e the scattering from various lengths of fiber [22]. M o d e - d e p e n d e n t loss [23-24] can be best studied in short samples w h e r e m o d e coupling is minimal. When m e a s u r i n g loss in short samples, however, care m u s t be taken to remove the energy p r e s e n t in the u n u s u a l l y lossy modes that p r o p a g a t e only short d i s t a n c e s [25]. If that is done, the m o d e - d e p e n d e n t a b s o r p t i o n and scattering results for short lengths of fiber p r o v i d e valuable data to study basic properties of the fiber glass and to d i a g n o s e causes of extrinsic fiber loss. 4.0 Inada
SCATTERING [26]
LOSSES
suggested
a model
for fiber
loss:
= A/I 4 + B + Cfl)
.
(6)
The first term represents the R a y l e i g h scattering contribution, the second represents w a v e l e n g t h - i n d e p e n d e n t scattering (microbending loss, for example), and the last represents s p e c t r a l l y - m o r e - c o m p l i cated absorption. Usually C(1) is n e g l e c t e d in the spectral regions 0.8 to 0.9 ~m and 1.05 to 1.15 Bm and A and B are d e t e r m i n e d by fitting total-loss data in these regions to Eq. (7): = A/I 4 + B .
(7)
In following this procedure, however, effects of the tails of the OH r e s o n a n c e s are n e g l e c t e d as well as the systematic error introduced by b a n d - e d g e absorption. 4.1
COMPARISON SCATTERING
OF A AND B VALUES LOSS DATA
OBTAINED
FROM T O T A L LOSS AND
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
251
T h e e r r o r i n t r o d u c e d by a b s o r p t i o n terms, C(l), w a s e x a m i n e d in a r e c e n t s t u d y [27] t h a t c o m p a r e d A a n d B as d e t e r m i n e d f r o m t o t a l loss d a t a w i t h A a n d B g o t t e n f r o m d i r e c t l y m e a s u r e d s c a t t e r i n g - l o s s data. T h e a v e r a g e of the r e s u l t s for s u c h a s t u d y a r e p r e s e n t e d in T a b l e I. As c a n be seen: (i) T h e A c o e f f i c i e n t s for p h o s p h o r u s TABLE
I
A V E R A G E V A L U E S OF T H E A A N D B C O E F F I C I E N T S O B T A I N E D F R O M T O T A L L O S S DATA, S C A T T E R I N G LOSS DATA, A N D S C A T T E R I N G LOSS D A T A P L U S M E A S U R E D UV-BAND-EDGE ABSORPTION T o t a l Loss Data
Scattering Loss D a t a
Scattering Loss P l u s Eq. (4) %
1.54+0.05 0.81~0.35
1.33+0.02 0.7950.22
1.45 0.76
.9984+.0040
.9995+.0004
1.31+0.13 1.0050.20
1.04+0.10 0.69~0.40
.984+.021
.9995+.0004
GeO2-B203-SI02* A B Correlation Coefficient
.9996
GeO2-P205-SIO2** A B Correlation Coefficient *Average results **Average results %Average results ~o = 5.3 ~m; for
1.35 0.63 .9995
for 3 fibers: A = 1 . 2 6 ~ 0 . 0 2 % for 6 fibers: A = 1 . 2 4 + 0 . 0 6 % for the b o r o n - d o p e d f i ~ r s : e_ = 3 . 2 x 1 0 -4 a n d phosphorus-doped fibers: s o = ~ . 3 x 1 0 -3 a n d ~o=5.0
~m.
d o p e d f i b e r s a r e s m a l l e r t h a n t h e A c o e f f i c i e n t s of b o r o n - d o p e d fibers. B o t h c l a s s e s of f i b e r s h a d n e a r l y t h e s a m e n o r m a l i z e d m a x i m u m i n d e x d i f f e r e n c e A. (2) The A coefficients from total-loss d a t a a r e l a r g e r t h a n the A c o e f f i c i e n t s from scattering-loss data. For the p h o s p h o r u s - d o p e d fibers, the d i s c r e p a n c y c a n be l a r g e l y a t t r i b u t e d to the m e a s u r e d U V - b a n d - e d g e a b s o r p t i o n ; for the b o r o n d o p e d fibers, OH a n d o t h e r i m p u r i t y a b s o r p t i o n a l s o a f f e c t s the A c o e f f i c i e n t s o b t a i n e d f r o m t o t a l - l o s s data. As s u g g e s t e d by the t h i r d c o l u m n in T a b l e I, h o w e v e r , the s y s t e m a t i c e f f e c t of U V - b a n d e d g e a b s o r p t i o n a c c o u n t s for m o s t of the d i s c r e p a n c y . (3) The v a r i a t i o n in the r e s u l t s for the f i b e r s w i t h p h o s p h o r u s is g r e a t e r t h a n t h a t for b o r o n - d o p e d fibers. The phosphorus-doped fibers used to o b t a i n the d a t a of T a b l e I h a d a p p r o x i m a t e l y the same A v a l u e s b u t d i f f e r e n t r a t i o s of g e r m a n i u m to p h o s p h o r u s . We h a v e m e a s u r e d l a r g e v a r i a t i o n s in s c a t t e r i n g a n d a b s o r p t i o n as p h o s p h o r u s c o n t e n t is c h a n g e d ; hence, we a t t r i b u t e t h a t w i d e r v a r i a t i o n to s u c h c h a n g e s in p h o s p h o r u s c o n t e n t . (4) T h e c o r r e l a t i o n c o e f f i c i e n t s are m u c h higher when scattering-loss d a t a is u s e d to find A a n d B. Because of t h e s y s t e m a t i c e f f e c t U V - b a n d - e d g e a b s o r p t i o n has on A a n d B v a l u e s w h e n s u c h a b s o r p t i o n is n o t s u b t r a c t e d o u t of the s p e c t r a l loss d a t a u s e d to find t h e s e v a l u e s , an i m p r o v e d m o d e l for f i b e r loss in the s h o r t e r w a v e l e n g t h r e g i o n m i g h t be = A / l 4 + B + e e IO/l o
+ C(I)
.
(8)
252
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
For fibers doped w i t h phosphorus, Eq. (8) is a reasonable model out to at least 1.06 ~m, w i t h C(~) due almost e n t i r e l y to OH absorption. For fibers doped w i t h boron, the effect of the IR- and U V - b a n d - e d g e absorptions are about equal at 1.06 ~m; hence, Eq. (8) is not strictly correct in that region. 4.2
RAYLEIGH
SCATTERING
RESULTS
We have shown that the A and B c o e f f i c i e n t s m u s t be o b t a i n e d from s c a t t e r i n g - l o s s data to be accurate. Nevertheless, if the core c o m p o s i t i o n d o e s n ' t vary from fiber to fiber, the U V - b a n d - e d g e absorption will affect A by a p p r o x i m a t e l y the same amount; hence A values gotten from total-loss data can be used to m o n i t o r the m a n u f a c t u r i n g process p r o v i d e d one recognizes that they differ from the actual R a y l e i g h - s c a t t e r i n g values. This is useful b e c a u s e total-loss data can be obtained more c o n v e n i e n t l y than s c a t t e r i n g - l o s s data. The A and B c o e f f i c i e n t s of Eq. (7) are quite useful in characterizing a f i b e r - m a k i n g process. In Table II [29] we p r e s e n t data on A and B for fibers with G e O 2 - B 2 0 3 - S i O 2 cores and silica claddings. As can be seen, a l a t h e - t o - l a t h e v a r i a t i o n in A occurs, w h i c h is of the same order as the v a r i a t i o n in A for a single l a t h e . Because A was obtained from total-loss data, variations in OH i n c o r p o r a t i o n from lathe to lathe as well as variations in R a y l e i g h scattering could explain the difference. TABLE
II
A AND B C O E F F I C I E N T S OBTAINED LOSS DATA F O R B O R O N - D O P E D Lathe A B
F R O M TOTAL FIBERS
i*
Lathe
1.54+0.12 1.5650.41
2*
1.62+0.10 1.5450.36
*Ten preforms from each lathe. Each p r e f o r m y i e l d e d fiber. For all 90 km of fiber A = 1.26+0.02%.
4.5 km of
The v a r i a t i o n in B is m u c h larger than the v a r i a t i o n in A (see also Table I), p r i m a r i l y because of coating effects including coating n o n u n i f o r m i t y and hardness, and v a r i a t i o n s in the physical state of the c o a t i n g (as will be d i s c u s s e d in Sect. 6.1). 4.2.1
ESTIMATE
OF INTRINSIC
MODE-DEPENDENT
SCATTERING
The m o d e - d e p e n d e n t loss plot of fiber 2 shown in Fig. 1 is an archetype for scattering from a good g e r m a n i u m - b o r o s i l i c a t e fiber. The i n t e r p r e t a t i o n of plots such as in Fig. 1 is as follows: A laser beam a p p r o p r i a t e l y focused to p r o d u c e a small spot-size (~6 ~m) and small a n g u l a r - d i v e r g e n c e (N.A. ~0.07) beam excites the fiber (core diameter = 55 ~m). W h e n the b e a m is focused at the center of the core (r/a = 0) of the g r a d e d - i n d e x fiber, p r i m a r i l y low-order m o d e s are excited, w h i c h are confined to the inner region of the core. As the b e a m is scanned to the c o r e - c l a d d i n g b o u n d a r y (r/a = i), higher order m o d e s are excited, w h i c h have energy d i s t r i b u t i o n s that more c o m p l e t e l y fill the core. Hence, assuming only R a y l e i g h scattering, we c a l c u l a t e A = 1.385 from the 7.9 d B / k m scattering loss of the loworder modes (which p r o p a g a t e in the h i g h - g e r m a n i u m - c o n t e n t core center) and A = 1.315 from the 7.5 d B / k m scattering loss of the highorder m o d e s (which fill the core). Fiber 2 has A = 1 . 2 9 % implying a
253
F.T. Stone, B.K. Tariyal / Loss Reduction in optical Fibers
14 G e O 2 - B 2 0 3 - $ i O 2 CORE UNCOATED FLEERS = 647nm 12
J 10 O O
FIBER
j
2
I
I
I
I
l
I
S
10
15
20
25
27.5
m a x i m u m GeO 2 content of 13 mole %. A s s u m i n g the low-order modes see the m a x i m u m GeO 2 and the h i g h - o r d e r modes see, on average, one-third the maximum, Eq. (3) predicts a 1/3 x 13 x 0.033 dB/ km = 0.143 d B / k m loss increase at 1 ~m in going from high-ordermode to low-order-mode excitation. That figure is twice th m e a s u r e d value of ~A = 0.07 ~m ~dB/km; hence, other GeO~ effects and the fining effect ot B203 are significant. The m e a s u r e d loss increase for the fibers w i t h phosphorus is about half that of fibers w i t h boron indicating P205 has a strong fining effect.
BEAM POSITION (/4m)
EXTRINSIC SCATTERING: DISTRIBUTED SCATTERING MECHANISMS
4.3 Fig. I. M o d e - d e p e n d e n t scattering of a low-loss fiber (2) and a fiber w i t h c o r e - c l a d d i n g boundary roughness (i).
Extrinsic scattering may be caused by m e c h a n i s m s distributed m o r e - o r - l e s s u n i f o r m l y along the fiber or scattering centers located at only a few places along the fiber. In Fig. 2a we show the crosssection of a fiber that exhibited an extreme amount of scattering and in Fig. 2b we show a m o d e - d e p e n d e n t loss plot of this fiber, w h i c h indicates excessive loss for the higher order modes caused by the elliptical shape of the c r o s s - s e c t i o n of the core. The m o d e - d e p e n d e n t s c a t t e r i n g - l o s s plot of a fiber with core-cladd i n g - b o u n d a r y roughness is presented in Fig. 1 together w i t h the same plot for a good fiber. The high loss for the v e r y - h i g h - o r d e r
SAMPLE E
TOTAL LOSS (dR/Kml
I
I
0.5
0
O.S
NORMALIZED POSITION OF EXCITING REAM (r/a)
Fig.
2 (a) M i c r o g r a p h of c r o s s - s e c t i o n of e l l i p t i c a l - c o r e (b) M o d e - d e p e n d e n t loss plot of the fiber.
fiber.
modes localizes the p r o b l e m to the c o r e - c l a d d i n g region. Because the high loss is caused by scattering, impurity absorption at the c o r e - c l a d d i n g b o u n d a r y can be e l i m i n a t e d as a possible cause (see
254
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
Sect. 5.2.1). M i c r o s c o p i c e x a m i n a t i o n of fiber nal variations in the c r o s s - s e c t i o n of the core than those of fibers such as fiber 2. 4.4
EXTRINSIC
SCATTERING:
LOCALIZED
1 revealed longitudithat were much larger
SCATTERING
MECHANISMS
The optical time domain r e f l e c t o m e t e r [16] provides data on the optical loss vs. position along the fiber. Hence such a plot, an e x a m p l e of which is shown in Fig. 3a, pinpoints regions of excess loss. The fiber may then be examined in the high-loss region to seek the loss m e c h a n i s m responsible. In the case of the fiber of Fig. 3a, a large bubble had been drawn into a long air-line, the start of w h i c h is shown in Fig. 3b. The air-line was 19 m long and at its center filled an appreciable region of the core, a rather extreme example of this class of fiber defect.
LOSS pIOFIL[ O+ F I I I I l , I H LOffO AII.LINI OITAINID WITH OpTI+AL IIMI DOMAIN I I F t I C T O M I T I I 3o i
~
~o
i
io
z
[
o 00OO
0,IOO
O 200
0 ]OO
O400
OSOO
0600
• 700
0 ilO0
LIMOTH.KILOmlTIIS
Figure
3.
a) b)
Optical time domain r e f l e c t o m e t e r loss plot of a fiber with a long air line. Air line begins at far right of micrograph. As is shown in Fig. 4, exciting a fiber that is w o u n d on a drum w i t h a visible laser also helps locate discrete scattering centers. In that figure, the laser was focused at the c o r e - c l a d d i n g boundary. When the e x c i t a t i o n was m o v e d to the central region of the core, the bright scattering center disappeared, indicating a defect at or near the c o r e - c l a d d i n g region. 5.0
ABSORPTION
LOSS
To achieve low losses, a b s o r p t i o n in fibers must be reduced to its intrinsic limit. Thus OH and other impurity c o n t a m i n a t i o n and p r o c e s s i n g - r e l a t e d damage, such as that caused by UV radiation [30, 31], m u s t be minimized. Fig. 4 P h o t o g r a p h of a fiber w o u n d on a drum, showing bright scattering center (arrow).
255
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
5.1
EXPONENTIAL UV-BAND-EDGE ABSORPTION
Numerous a b s o r p t i o n m e a s u r e m e n t s of b o r o n - d o p e d and p h o s p h o r u s - d o p e d fibers reveal the short w a v e l e n g t h (l < 1.06 pm) behavior shown in Fig. 5 (for b o r o n - d o p e d fibers) and Fig. 6 (for p h o s p h o r u s - d o p e d fibers).
+f so
O , O ~ - 1 2 0 ] - $ ~ ~ COtlS
=........,
20
a,:e
i0
-i
s : ]1.1o
~
~- so
10
so
-" 2o o
a = 1 s,vo-3, 4 1 / k
<
IO
io
.... ;:°r:
I I
104,
0.713 017G 0.147
I
01 Oil
I 10
12
II 1.4
OS61 o111
I )4
I / k ~'--11
Fig. loss core
5. Spectral a b s o r p t i o n of germanium-borosilicatefiber.
0711 0676 0647
os*$
0S31
047~
o47~.
!
r +I
~
~o
2~
I.o
I2
I¢ i/X
1.
I|
2o
~
~--5
Fig. 6. Spectral absorption loss of g e r m a n i u m - p h o s p h o s i l i c a t e - c o r e fiber.
For b o r o n - d o p e d fibers the exponential behavior extends to approxim a t e l y 700 nm; for longer w a v e l e n g t h s small impurity absorptions (<0.i dB/km) and the IR-band-edge a b s o r p t i o n (Eq. 5) due to boron cause the curve to reach a m i n i m u m at a p p r o x i m a t e l y 1.2 ~m and then begin to rise e x p o n e n t i a l l y [i]. The UV e x p o n e n t i a l - b a n d - e d g e absorption shown in Fig. 5 depends on g e r m a n i u m c o n t e n t : The boron in the fiber was constant (at 1%) throughout the core, but as seen in Fig. 5, the exponential a b s o r p t i o n decreases for higher order modes (At = 20 ~m) that propagate in a region of the core w i t h a lower average g e r m a n i u m content. Also the exponential fit to the data for the smallest-A fiber examined (A = 1.16%) had lo = 5.1 ~m and s o = 4.6xi0 -4 for Ar = o, a somewhat lower a b s o r p t i o n than that of the fiber in Fig. 5. For p h o s p h o r u s - d o p e d fibers, the a b s o r p t i o n losses m e a s u r e d were s i g n i f i c a n t l y higher than for b o r o n - d o p e d fibers. A c o m p a r i s o n of results presented in Figs. 5 and 6 for a typical fiber from each class indicates the m a g n i t u d e of the effect. The fibers with phosphorus had a b s o r p t i o n curves that were exponential to 1.06 ~m; however, the m a g n i t u d e of the a b s o r p t i o n varied widely, most probably caused by the formation of stable a b s o r p t i o n centers in w h i c h phosphorus plays a necessary part. Hence, the intrinsic a b s o r p t i o n of such fibers depends s i g n i f i c a n t l y on phosphorus content. For the b o r o n - d o p e d fibers and u n d a m a g e d fibers with low (< 2%) phosphorus c o n t e n t (such as.in Fig. 6), the exponential a b s o r p t i o n is near that a t t r i b u t e d to Ge 4+ in g e r m a n i u m - s i l l c a t e glasses [ii].
256
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
5.2
IMPURITY A B S O R P T I O N
A recent study [28] in g e r m a n i u m - b o r o s i l i c a t e fibers concludes that impurities are c o n c e n t r a t e d in the c o r e - c l a d d i n g boundary region or along the axis of the fiber, i.e., the impurities appear to diffuse from the starting tube into the outer deposited layers or to enter the innermost deposited layers of the p r e f o r m before complete collapse. The evidence for this conclusion is the observation of excess a b s o r p t i o n in very low- or very high-order modes. For germaniump h o s p h o s i l i c a t e fibers similar results have been observed; however, the presence of color centers in the h i g h - g e r m a n i u m - c o n t e n t central region of the core can obscure impurity a b s o r p t i o n present there. 5.2.1
B A R R I E R L A Y E R STUDY
The use of thick barrier layers to reduce impurity diffusion from the starting tube to the fiber core has already been noted [32-34]. As an example of impurity absorption, we present additional results showing the effect of b a r r i e r layer thickness on impurity diffusion into the core. From a large number of fibers with thin (5 d B / k m at 0.82 ~m. The average m o d e - d e p e n d e n t a b s o r p t i o n of these eight is plotted in Fig. 7 for the h i g h e r - o r d e r modes. Four fibers were then taken from a b a t c h having thick (>5 ~m) g e r m a n i u m - b o r o s i l i c a t e barrier layers, with no other selection criteria applied. The m o d e - d e p e n d e n t a b s o r p t i o n for these four is also shown in Fig. 7. The increasing a b s o r p t i o n for the t h i n - b a r r i e r - l a y e r samples as h i g h e r - o r d e r modes are excited indicates impurity levels gradually rising as the corecladding boundary (27.5 ~m) is approached.
GeO2-B203-SIO2 4
a AVERAGE OF i THIN--BARRIERLAYER SAMPLES o AVERAGE OF 4 THICK-BARRIERLAYER SAMPLiS
~. 3
[ INDICATES RANGE OF RESULTS
2
)~ = 647nla
_~
. i
//
CORES /
TI
GeO2-P2Os-S~O 2 ~- = 1.24"~ = 647.m
15
"('1/~ //
COATED (EXPOSED TO ONE
,
!i '°
///
EIGHT SAMPLES
.,,..,.,,.u...
/
// /
iS)
I
/
.,9, ,
s UNCOATED (NO UV)
I 0
i $
I 10
I 15
I 20
A
BEAM POSITION (nm)
Fig. 7. Mode -dependent-loss plots showing effect of impurities diffusing in from the starting tube in thin-barrierlayer fibers. 5.3
¸ BEAM POSIT'ON~i
Fig. 8. M o d e - d e p e n d e n t - l o s s plots showing large UV-induced loss for low-order modes propagating in highg e r m a n i u m - c o n t e n t central region of core.
U V - I N D U C E D DAMAGE IN G E R M A N I U M - P H O S P H O S I L I C A T E
FIBERS
Significant UV-caused damage has recently been noted in a wide
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
257
v a r i e t y of fibers [30]. In m o d e r a t e l y doped fibers (A ~ 1.3%) coated w i t h a U V - c u r e d polymer, such damage was o b s e r v e d in g e r m a n i u m - p h o s p h o s i l i c a t e but not in g e r m a n i u m - b o r o s i l i c a t e fibers [31]. An i n t e r e s t i n g result for 1.3%-A g e r m a n i u m - p h o s p h o s i l i c a t e fibers is p r e s e n t e d in Fig. 8. The UV causes a b s o r p t i o n centers that are far m o r e n u m e r o u s in the h i g h - g e r m a n i u m - c o n t e n t central region of the core, as is shown by the large increase in l o w - o r d e r - m o d e loss. B e c a u s e the p h o s p h o r u s c o n t e n t of this fiber is c o n s t a n t throughout the core, the increase in loss is due to the increase in germanium; however, the p r e s e n c e of phosphorus is n e c e s s a r y b e c a u s e the loss increase is a b s e n t in similar fibers w i t h b o r o n r e p l a c i n g the phosphorus. The c o m p a r i s o n shown in Fig. 8 was one of five made: in three cases excess loss similar to that of Fig. 8 appeared; in two cases such loss could not be discerned. Hence other v a r i a b l e s are involved, w h i c h must be determined. The spectral d e p e n d e n c e of the added loss is e x p o n e n t i a l w i t h a slope greater than that m e a s u r e d for the U V - b a n d - e d g e a b s o r p t i o n s of Figs. 5 and 6. From such data the added 820-nm loss caused by one coatingcure dose of UV has been e s t i m a t e d at <0.i d B / k m [32] p r o v i d e d a sufficient t h i c k n e s s of c o a t i n g covers the fiber: The screening effect of the c o a t i n g is critical because the loss increase of bare irradiated fibers is enormous [30]. 6.0
COATING-INDUCED
LOSS
A c o a t i n g for the fibers m u s t be chosen that adds a m i n i m u m amount of m i c r o b e n d i n g loss. The d e t e r m i n a t i o n of the added loss from c o a t i n g as well as the a d d e d loss caused by other stages in the c a b l i n g p r o c e s s is c o m p l i c a t e d by the d e p e n d e n c e of this added loss on the physical state of the fiber and its previous history. 6.1
EFFECT
OF THE STATE OF THE F I B E R ON A D D E D
LOSS
C o a t i n g - i n d u c e d loss d e p e n d s on the physical c o n f i g u r a t i o n of the fiber (wound on a small d r u m or a larger drum, or loosely laid in a container) and the time the fiber has been in that state.
i FliER1011 11~1 0
- -
-O.5
i -I.O
.
-I.S
~
-E.O
°
-2.5
12
I
t*
:
J
L
14 i
0
20
40
60
Fig. 9. The loss of coated fibers w o u n d on sixinch-diameter reels vs. the time on the reels.
|JOIN FUTURE MEASUREMENTS HERE~20 HRS.) 80 1100 IR0 140 1"0 180 200 220 240 260 MOUESAFTERUIAWINO
The effect the r e l a x a t i o n time has on the loss m e a s u r e d is shown in Fig. 9. A f t e r d r a w i n g and c o a t i n g (with c o a t i n g W of Table IV) the fibers were w o u n d on s i x - i n c h - d i a m e t e r reels w i t h m a n y o v e r l a p p i n g turns and m e a s u r e d [35] at the times shown in Fig. 9. After
258
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
approximately 120 h o u r s (5 days) all s u b s e q u e n t m e a s u r e m e n t s w e r e g i v e n 120 h o u r s to relax.
the m e a s u r e d loss s t a b i l i z e d ; hence, m a d e a f t e r the f i b e r c o a t i n g w a s
To d e t e r m i n e the e f f e c t of w i n d i n g c o n d i t i o n s , the loss of s e v e n f i b e r s w a s m e a s u r e d w i t h the f i b e r s w o u n d on s i x - i n c h - d i a m e t e r reels a n d l o o s e l y laid in a l a r g e b a r r e l . The measurement results shown in T a b l e III [36] w e r e all t a k e n a f t e r the 1 2 0 - h o u r w a i t i n g period. The a v e r a g e loss d e c r e a s e on g o i n g f r o m reel to b a r r e l m e a s u r e m e n t s w a s 0.69 dB/km, a l m o s t all of w h i c h a p p e a r e d as a d e c r e a s e in B (0.62 dB/km) a n d v e r y l i t t l e of w h i c h a p p e a r e d as a d e c r e a s e in A (0.058 ~ m 4 - d B / k m ) . T h e r e s u l t s for the f i b e r u n w o u n d in a b a r r e l s h o u l d be u s e d as the u n c a b l e d loss w h e n t r y i n g to d e t e r m i n e the a d d e d loss a g i v e n c o a t i n g c a u s e s in a c o m p l e t e d cable.
TABLE LOSS AT Fiber*
825
~m:
III R E E L VS.
Loss** R B 4.70 4.62 5.05 4.53 5.59 4.74 5.16 4.50 8.15 6.76 5.60 4.30 4.96 4.93
20 21 22 23 24 25 26
A*** R 1.50 1.67 1.77 1.48 1.56 1.73 1.66
*All f i b e r s had 5 5 - ~ m c o r e d i a m e t e r s * * R = Reel, B = B a r r e l ***Taken from total-loss data
6.2
EFFECT
OF D I F F E R E N T
BARREL B*** R B 1.39 1.30 1.37 0.93 1.78 i.i0 1.87 1.27 4.71 3.44 1.76 0.61 1.25 1.23
B 1.49 1.63 1.65 1.44 1.49 1.67 1.59
and
ll0-~m
cladding
diameters
COATINGS
In an e x p e r i m e n t to d e t e r m i n e t h e r e l a t i v e m e r i t s of d i f f e r e n t fiber c o a t i n g s , four m a t e r i a l s w e r e a p p l i e d to f i b e r d r a w n f r o m f o r t y - s i x d i f f e r e n t p r e f o r m s (4.3 k m of f i b e r p e r preform) in such a m a n n e r t h a t the e f f e c t of p r e f o r m - t o - p r e f 0 r m variations was minimized. C o a t i n g s W, X, and Z of T a b l e IV w e r e U V - c u r e d w i t h c o a t i n g W h a v i n g the l o w e s t e l a s t i c m o d u l u s , c o a t i n g Z h a v i n g t h e h i g h e s t (four times t h a t of W), and c o a t i n g X b e i n g i n t e r m e d i a t e . Coating Y was a hot m e l t c o a t i n g w i t h an e l a s t i c m o d u l u s an o r d e r of m a g n i t u d e l o w e r t h a n the others. As c a n be seen in T a b l e IV, the v a l u e s of A are n e a r l y the same for f i b e r s c o a t e d w i t h d i f f e r e n t m a t e r i a l s , b u t the
TABLE FIBER Coatinq
FOR FOUR
Loss
W X Y Z *From
LOSS
4.56+0.80 4.8750.40 5.5751.19 6.5852.08 total
loss
data
IV
DIFFERENT
COATINGS
A* 1.65+0.13 1.6350.08 1.6850.13 1.7650.12
B* 0.81+0.63 1.2650.42 1.8751.06 2.66~2.10
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
259
values of B vary widely, illustrating the effect that the coatings can have on extrinsic scattering loss. This does not imply that any particular coating is necessarily superior or inferior to some other coating, but it does mean that, depending upon their application parameters, coatings can introduce varying amounts of extrinsic loss, r e s u l t i n g in an overall increase in the coated fiber loss. For coatings W, X, and Z, the coating-induced loss decreased with decreasing elastic modulus; coating Y, which had the lowest modulus, could not be applied uniformly, leading to the high added loss shown. 7.0
CONCLUSIONS
To pursue a program aimed at reducing fiber loss, one needs a theoretical understanding of loss mechanisms and an array of routine and specialized measurement techniques. Measurement results then lead to estimates of the lowest attainable (or intrinsic) loss and the dependence of loss on fiber composition and processing. Using these results mechanisms causing excess loss can be identified and eliminated. REFERENCES [i] [2] [3] [4] [5] [6] [7] [8] [9] [i0] [ii] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Osanai, H., Shioda, T., Mariyama, T., Araki, S., Horiguchi, M., Izawa, T., and Takata, H., Electron. Lett. 12, 549 (1976). Horiguchi, M., and Osanai, H., Electron. Lett. 12, 310 (1976). Kawachi, M., Kawana, A., and Miyashita, T., Electron. Lett. 13, 442 (1977). Miya, T., Terunuma, Y., Hosaka, T., and Miyashita, T., Electron. Lett. 15, 106 (1979). Pinnow, D. A., Rich, T. C., Ostermayer, Jr., F. W., and DiDomenico, Jr., M., Appl. Phys. Lett. 22, 527 (1973). Schroeder, J., Mohr, R., Macedo, P. B., and Montrose, C. J., J. Am. Ceram. Soc. 56, 510 (1973). Stacey, K. A., Light--Scattering in Physical Chemistry (Academic Press, New York, 1956), p. 8. Maurer, R. D., Proc. IEEE 61, 452 (1973). Olshansky, R., Rev. Mod. Phys. 51, 341 (19J9). Urbach, F., Phys. Rev. 92, 1324 (1953). Schultz, P. C., Presented at the XIth International Conference on Glass, Prague, 1977. Keck, D. B., Maurer, R. D., and Schultz, P. C., Appl. Phys. Lett. 22, 307 (1973). Gardner, W. B., Bell Syst. Tech. J. 54, 457 (1975). Gloge, D., Bell Syst. Tech. J. 54, 245 (1975). Cherin, A. H., Cohen, L. G., Horden, W. S., Burrus, C. A., and Kaiser, P., Appl. Opt. 10, 2359 (1974). Barnoski, M. K., and Jensen, S. M., Appl. Opt. 15, 2112 (1976). Cohen, R. L., West. K. W., Lazay, P. D., and Simpson, J., Appl. Opt. 13, 2522 (1974). Zaganiaris, A., Appl. Phys. Lett. 25, 345 (1974). White, K. I., Opt. Quant. Electron. 8, 73 (1976). Stone, F. T., Gardner, W. B., and Lovelace, C. R., Opt. Lett. 2, 48 (1978). Tynes, A. R., Appl. Opt. 9, 2706 (1970). Keck, D. B., Schultz, P. ~., and Zimar, F., Appl. Phys. Lett. 21, 215 (1972). Olshansky, R., and Oaks, S. M., Appl. Opt. 17, 1830 (1978). Stone, F. T., Appl. Opt. 17, 2825 (1978). Stone, F. T., and Krawarik, P. H., Appl. Opt. 18, 756 (1979). Inada, K., Opt. Comm. 19, 436 (1975).
260
[27] [28] [29] [30]
[31] [32] [33] [34] [35] [36]
F.T. Stone, B.K. Tariyal / Loss Reduction in Optical Fibers
Philen, D. L., and Stone, F. T., presented at Conference on Physics O f Fiber Optics, Chicago, 1980, to be published in Advances in Ceramic Engineering, Am. Ceram. Soc., 1980. Stone, F. T., unpublished results. Tariyal, B. K., Partus, F. P., Sprow, R. B., Kalish, D., and Jefferies, J. A., unpublished results. Blyler, L. L., Jr., DiMarcello, F. V., Simpson, J. R., Sigety, E. A., Hart, A. C., Jr., and Foertmeyer, V. A., Proc. XIIth Int'l Congress on Glass, Albuquerque, 1980. To be published in a Special Issue of J. Non-Crytal. Solids, North Holland, Amsterdam, 1980. Stone, F. T., and Eichenbaum, B. R., Same Proceedings as [30] Kawachi, M., Horiguchi, M., Kawana, A., and Miyashita, T., Electron. Lett. 13, 247 (1977). Tariyal, B. K., ~-Effect of Barrier Layer Thickness on the Optical Loss of Step and Graded Index Fibers," Presented at the 80th Annual Mtg. of the Am. Ceram. Soc., Detroit, 1978. Saifi, M. A., to be published. Boggs, L. M., unpublished results. Wilson, L., unpublished results.