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Chalcogenide hollow fibers
Journal of Non.Crystalline Solids 77 & 78 (1985) 1277-1280 North-Holland, Amsterdam
1277
CHALCOGENIDE HOLLOW FIBERS A. BORNSTEIN* and N. CROITORU+ *Dept. of Solid State Physics, Israel Atomic Energy Commission, Soreq Nuclear Research Centre, ISRAEL +N. C r o i t o r u , Dept. of E l e c t r o n i c Devices & M a t e r i a l s & Electromaqnetic Radiation, Tel-Aviv U n i v e r s i t y , Ramat-Aviv 69978, ISRAEL Chalcogenide glass hollow f i b e r s were f a b r i c a t e d by drawing from a glass c y l i n d e r tube made from As-Se or As-Ge-Sb chalcoqenide qlass. Very precise control was maintained during drawing to obtain uniform hollow f i b e r s . The tube was produced from oxyqen-free raw m a t e r i a l s inside an argon atmosphere glove box. The outside diameters of the f i b e r s were 300 to 500 ~m and the inside diameters I00 to 300 ~m. The f i b e r s were small, l i g h t , and f l e x i b l e and could transmit CO9 l a s e r l i g h t as well as v i s i b l e l i q h t . The a t t e n u a t i o n of the d i e l e c t r i c hol?ow f i b e r s remained constant even when bent to small r a d i i of 2 cm. The f i b e r s have an a t t e n u a t i o n of 0.5 dB/cm. 1.
INTRODUCTION The recent i n t e r e s t in developing i n f r a r e d o p t i c a l waveguides has been
stimulated by advances in the use of CO2 laser f o r surgery as well as f o r cutt i n g , weldinq and heat treatment.
The i n f r a r e d wavelenqth (~ : 10.6 ~m) of CO2
lasers is s t r o n q l y absorbed by most tissues, p e r m i t t i n g clean l o c a l i z e d cuts. One d i f f i c u l t y ,
however, has slowed down the development of the CO2 l a s e r in
many procedures I - the absence of good q u a l i t y o p t i c a l f i b e r which transmits t h a t wavelength. The hollow leaky f i b e r is one of the best solutions f o r t r a n s m i t t i n g IR power at wavelengths where m a t e r i a l s with low losses are not a v a i l a b l e . We examine here the c i r c u l a r d i e l e c t r i c
hollow waveguide (Fig. I ) , with
inner radius r and outer radius ( r + d) (d-wall thickness}. The free space r e f r a c t i v e index is no and the wall r e f r a c t i v e index is n = a.n ° (a > I ) .
n(r)
n (ri
nol I ooI, no t r+d rI
o, rI
FIGURE I C i r c u l a r d i e l e c t r i c hollow wavequide 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
rid
A. Bornstein, N. Croitoru / Chalcogenide hollow fibers
1278
The wall of the hollow f i b e r is made of low-loss glass m a t e r i a l .
We are look-
ing f o r a mode whose energy concentrates in the region of the center of the tube, i . e . ,
the leaky mode.
2. THEORETICAL AND EXPERIMENTAL RESULTS Using the transverse transmission l i n e model, the attenuation constant m of the leaky mode was obtained by M. Miyagi and S. Nishida 2.
Since the r e f r a c t i v e
index of chalcogenide qlass is hiqh (n = 2.6), we can take only the TE mode. The values mmin' mmax' and mav f o r a TE mode w i l l be qiven by the r e l a t i o n s : U~ amax = 4~r2
(I)
where r is the inner radius of the tubes (Fio. I ) and U =(2m + I / 2 ) ~ / 2 f o r the TE mode.
amin = 2 (2~) 3
, (_~_~)2U av
2~
(
a2x3 _ l)r 4
(2)
~2
(3)
(a 2 _ I )'~ r 3
M a r c a t i l i I got the expression (3) when he calculated the loss in a metal tube where the l i g h t travels through only the inner side of the tube. the dependence of m on d.
Fig. 2 shows
This dependence is periodic and the distance ~ be-
tween two maxima i s : ~ : x / 2 / a 2 _ l , which is half of the wavelength in the matt e r i a l of the w a l l .
Fig. 2 also shows that mmin is much less s e n s i t i v e to d
than mmax' In the same f i g u r e we see the t h e o r e t i c a l losses versus d, and the three special values of mmin' mav' and amax"
I t is c l e a r from Fig. 2 that i f
one wishes to get mmin' one must keep the d f l u c t a t i o n w i t h i n less than ~. For the 10.6 um wavelength and a chalcogenide glass f i b e r with a r e f r a c t i v e index of 2.6, the f l u c t u a t i o n can be ±I ~m, which is not easy to achieve. The t h e o r e t i c a l mmin loss c o e f f i c i e n t calculated from expression (2) f o r As2Se3 at I0 um and a r e f r a c t i v e index I00 um is 0.3 dB and f o r a tube radius m that this loss is rather low, compared At f i r s t i t was thought that hollow filling
of 2.6 f o r a d i e l e c t r i c tube radius of of 500 ~m is 5 x I0-4 ~dB --, nne can see dB to the I0 ~- in As2Se3 qlass f i b e r s 3. core f i b e r s were very promising in f u l -
some of the requirements f o r endoscopic u t i l i z a t i o n .
Several m e t a l l i c
types of f i b e r s were proposed, such as helical c i r c u l a r wavequides4 or d i e l e c t r i c coated m e t a l l i c wavequides2.
However, when examininq these f i b e r s ,
AI Bornsteot, N. Croitoru / Chalcogenide hollow fibers negative aspects were found.
1279
They are n e i t h e r r e a l l y small nor l i q h t nor
f l e x i b l e enough to be used in an endoscope.
. -
X
-
_
_
amo -
,u®? I x3 - OImin=2~'~"~"-# (aT_l) " ~ WALL
-"d
THICKNESS
FIGURE 2 The dependence of IOQ lOSS on d (wall thickness) We, t h e r e f o r e , s t a r t e d research on hollow chalcogenide glass f i b e r s t h a t could be expected to have most of the d e s i r a b l e f e a t u r e s . The f i b e r s which we prepared were drawn from a glass c y l i n d e r tube made from As-Se or As-Ge-Sb chalcogenide glass. obtain uniform hollow f i b e r s .
Very precise control was needed to
The tube was produced from oxygen-free raw
m a t e r i a l s inside an argon atmosphere qlove box3 ' 5 .
The outside diameters of
the f i b e r s were 300 to 500 um and the inside diameters I00 to 300 um.
The
length of the pulled f i b e r s could be made up to tens of meters. A f t e r the f i b e r was produced, i t was checked under the microscope.
The
o u t e r surface was examined at several places alonq the f i b e r to determine i t s radius and surface q u a l i t y , with special a t t e n t i o n to surface defects~ such as a i r bubbles and scratches.
The end surfaces were examined to observe whether
they were clean and smooth. Two types of experiments were performed in order to measure the a t t e n u a t i o n due to the bending of the f i b e r .
The experimental setup allowed the change of
one parameter only - the bend radius. around a c y l i n d e r to form a 90o anqle.
In the f i r s t
method, the f i b e r was bent
Cylinders of various diameters were
used, down t o 2 Cmo In the second method, the f i b e r had one f u l l
winding and by
increasing the distance between the i n l e t and o u t l e t , the winding diameter was
A. Bornstein, N. Croitoru/Chalcogenide hollowfibe~
1280
changed down to 2 cm.
No s i q n i f i c a n t influence (less than 10%) of the bending
on the output power was determined in the measurement r e s o l u t i o n .
In order to
measure the output scattering pattern, the end of the f i b e r was mounted at the center of a r o t a t i n g t a b l e , on which the IR detector was i n s t a l l e d so that the detector could move in an arc centered at the f i b e r end. dB From the above mentioned measurements a power-loss of 0.5 ~-~ was found.
A
maximum transmitted power of 0.5 kW/cm2 was obtained f o r a maximum f i b e r length of 20 cm, i . e . the incident Dower density was 5 kW/cm2 (at which the hollow f i b e r was not damaged).
This r e s u l t is f a r from the t h e o r e t i c a l l i m i t of the
hollow d i e l e c t r i c f i b e r 1'2
In order to reach this l i m i t , f i b e r f a b r i c a t i o n
must be much b e t t e r c o n t r o l l e d to get a smooth and perfect surface and g e t constant radius.
We must also confine the energy of the laser to a correct
mode and launch i t more e f f i c i e n t l y . 3.
CONCLUSION Chalcogenide glass hollow f i b e r s were produced that s a t i s f y some of the
demands f o r medical use.
The f i b e r s are small, l i g h t , and f l e x i b l e enough to
be f i t t e d to endoscopes.
They can transmit CO2 laser l i g h t as well as v i s i b l e
l i g h t and t h e i r attenuation remains constant, even when bent to a small radius of 2 cm.
The f i b e r s can transmit high-power density in a small spot size.
However, the e f f i c i e n c y of the power transmission should be improved, in order to reach the t h e o r e t i c a l l i m i t and be suitable f o r medical a p p l i c a t i o n s . REFERENCES I
E.A.J. M a r c a t i l i and R.A. Schmeltzer, Bell System J. 43 (1964) 1783.
2
M. Miyagi and S. Nishida, IEEE Trans. Microwave Theory & Techniques MI128 (1980) 536.
3
A. Bornstein, N. Croitoru and E. Marom, Proc. Soc. Photo-Opt. I n s t r . Engs., Adv. in Infrared Fiber I I , 320 (1982) 402.
4) Eo Carmine, Proc. Soc. Photo-Opt. I n s t r . Eng., Adv. in Infrared Fiber I I , 320 (1982) 70. 5) A. Bornstein, N. Croitoru and E. Marom, Proc. Soc. Photo-Opt. I n s t r . Enqs., Infrared Optical Materials and Fibers I I I , 484 (1984) 99. 6) A. Bornstein, N. Croitoru and Eo Marom, J. Non-Cryst. Sol. (1985) in press.
1277
CHALCOGENIDE HOLLOW FIBERS A. BORNSTEIN* and N. CROITORU+ *Dept. of Solid State Physics, Israel Atomic Energy Commission, Soreq Nuclear Research Centre, ISRAEL +N. C r o i t o r u , Dept. of E l e c t r o n i c Devices & M a t e r i a l s & Electromaqnetic Radiation, Tel-Aviv U n i v e r s i t y , Ramat-Aviv 69978, ISRAEL Chalcogenide glass hollow f i b e r s were f a b r i c a t e d by drawing from a glass c y l i n d e r tube made from As-Se or As-Ge-Sb chalcoqenide qlass. Very precise control was maintained during drawing to obtain uniform hollow f i b e r s . The tube was produced from oxyqen-free raw m a t e r i a l s inside an argon atmosphere glove box. The outside diameters of the f i b e r s were 300 to 500 ~m and the inside diameters I00 to 300 ~m. The f i b e r s were small, l i g h t , and f l e x i b l e and could transmit CO9 l a s e r l i g h t as well as v i s i b l e l i q h t . The a t t e n u a t i o n of the d i e l e c t r i c hol?ow f i b e r s remained constant even when bent to small r a d i i of 2 cm. The f i b e r s have an a t t e n u a t i o n of 0.5 dB/cm. 1.
INTRODUCTION The recent i n t e r e s t in developing i n f r a r e d o p t i c a l waveguides has been
stimulated by advances in the use of CO2 laser f o r surgery as well as f o r cutt i n g , weldinq and heat treatment.
The i n f r a r e d wavelenqth (~ : 10.6 ~m) of CO2
lasers is s t r o n q l y absorbed by most tissues, p e r m i t t i n g clean l o c a l i z e d cuts. One d i f f i c u l t y ,
however, has slowed down the development of the CO2 l a s e r in
many procedures I - the absence of good q u a l i t y o p t i c a l f i b e r which transmits t h a t wavelength. The hollow leaky f i b e r is one of the best solutions f o r t r a n s m i t t i n g IR power at wavelengths where m a t e r i a l s with low losses are not a v a i l a b l e . We examine here the c i r c u l a r d i e l e c t r i c
hollow waveguide (Fig. I ) , with
inner radius r and outer radius ( r + d) (d-wall thickness}. The free space r e f r a c t i v e index is no and the wall r e f r a c t i v e index is n = a.n ° (a > I ) .
n(r)
n (ri
nol I ooI, no t r+d rI
o, rI
FIGURE I C i r c u l a r d i e l e c t r i c hollow wavequide 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
rid
A. Bornstein, N. Croitoru / Chalcogenide hollow fibers
1278
The wall of the hollow f i b e r is made of low-loss glass m a t e r i a l .
We are look-
ing f o r a mode whose energy concentrates in the region of the center of the tube, i . e . ,
the leaky mode.
2. THEORETICAL AND EXPERIMENTAL RESULTS Using the transverse transmission l i n e model, the attenuation constant m of the leaky mode was obtained by M. Miyagi and S. Nishida 2.
Since the r e f r a c t i v e
index of chalcogenide qlass is hiqh (n = 2.6), we can take only the TE mode. The values mmin' mmax' and mav f o r a TE mode w i l l be qiven by the r e l a t i o n s : U~ amax = 4~r2
(I)
where r is the inner radius of the tubes (Fio. I ) and U =(2m + I / 2 ) ~ / 2 f o r the TE mode.
amin = 2 (2~) 3
, (_~_~)2U av
2~
(
a2x3 _ l)r 4
(2)
~2
(3)
(a 2 _ I )'~ r 3
M a r c a t i l i I got the expression (3) when he calculated the loss in a metal tube where the l i g h t travels through only the inner side of the tube. the dependence of m on d.
Fig. 2 shows
This dependence is periodic and the distance ~ be-
tween two maxima i s : ~ : x / 2 / a 2 _ l , which is half of the wavelength in the matt e r i a l of the w a l l .
Fig. 2 also shows that mmin is much less s e n s i t i v e to d
than mmax' In the same f i g u r e we see the t h e o r e t i c a l losses versus d, and the three special values of mmin' mav' and amax"
I t is c l e a r from Fig. 2 that i f
one wishes to get mmin' one must keep the d f l u c t a t i o n w i t h i n less than ~. For the 10.6 um wavelength and a chalcogenide glass f i b e r with a r e f r a c t i v e index of 2.6, the f l u c t u a t i o n can be ±I ~m, which is not easy to achieve. The t h e o r e t i c a l mmin loss c o e f f i c i e n t calculated from expression (2) f o r As2Se3 at I0 um and a r e f r a c t i v e index I00 um is 0.3 dB and f o r a tube radius m that this loss is rather low, compared At f i r s t i t was thought that hollow filling
of 2.6 f o r a d i e l e c t r i c tube radius of of 500 ~m is 5 x I0-4 ~dB --, nne can see dB to the I0 ~- in As2Se3 qlass f i b e r s 3. core f i b e r s were very promising in f u l -
some of the requirements f o r endoscopic u t i l i z a t i o n .
Several m e t a l l i c
types of f i b e r s were proposed, such as helical c i r c u l a r wavequides4 or d i e l e c t r i c coated m e t a l l i c wavequides2.
However, when examininq these f i b e r s ,
AI Bornsteot, N. Croitoru / Chalcogenide hollow fibers negative aspects were found.
1279
They are n e i t h e r r e a l l y small nor l i q h t nor
f l e x i b l e enough to be used in an endoscope.
. -
X
-
_
_
amo -
,u®? I x3 - OImin=2~'~"~"-# (aT_l) " ~ WALL
-"d
THICKNESS
FIGURE 2 The dependence of IOQ lOSS on d (wall thickness) We, t h e r e f o r e , s t a r t e d research on hollow chalcogenide glass f i b e r s t h a t could be expected to have most of the d e s i r a b l e f e a t u r e s . The f i b e r s which we prepared were drawn from a glass c y l i n d e r tube made from As-Se or As-Ge-Sb chalcogenide glass. obtain uniform hollow f i b e r s .
Very precise control was needed to
The tube was produced from oxygen-free raw
m a t e r i a l s inside an argon atmosphere qlove box3 ' 5 .
The outside diameters of
the f i b e r s were 300 to 500 um and the inside diameters I00 to 300 um.
The
length of the pulled f i b e r s could be made up to tens of meters. A f t e r the f i b e r was produced, i t was checked under the microscope.
The
o u t e r surface was examined at several places alonq the f i b e r to determine i t s radius and surface q u a l i t y , with special a t t e n t i o n to surface defects~ such as a i r bubbles and scratches.
The end surfaces were examined to observe whether
they were clean and smooth. Two types of experiments were performed in order to measure the a t t e n u a t i o n due to the bending of the f i b e r .
The experimental setup allowed the change of
one parameter only - the bend radius. around a c y l i n d e r to form a 90o anqle.
In the f i r s t
method, the f i b e r was bent
Cylinders of various diameters were
used, down t o 2 Cmo In the second method, the f i b e r had one f u l l
winding and by
increasing the distance between the i n l e t and o u t l e t , the winding diameter was
A. Bornstein, N. Croitoru/Chalcogenide hollowfibe~
1280
changed down to 2 cm.
No s i q n i f i c a n t influence (less than 10%) of the bending
on the output power was determined in the measurement r e s o l u t i o n .
In order to
measure the output scattering pattern, the end of the f i b e r was mounted at the center of a r o t a t i n g t a b l e , on which the IR detector was i n s t a l l e d so that the detector could move in an arc centered at the f i b e r end. dB From the above mentioned measurements a power-loss of 0.5 ~-~ was found.
A
maximum transmitted power of 0.5 kW/cm2 was obtained f o r a maximum f i b e r length of 20 cm, i . e . the incident Dower density was 5 kW/cm2 (at which the hollow f i b e r was not damaged).
This r e s u l t is f a r from the t h e o r e t i c a l l i m i t of the
hollow d i e l e c t r i c f i b e r 1'2
In order to reach this l i m i t , f i b e r f a b r i c a t i o n
must be much b e t t e r c o n t r o l l e d to get a smooth and perfect surface and g e t constant radius.
We must also confine the energy of the laser to a correct
mode and launch i t more e f f i c i e n t l y . 3.
CONCLUSION Chalcogenide glass hollow f i b e r s were produced that s a t i s f y some of the
demands f o r medical use.
The f i b e r s are small, l i g h t , and f l e x i b l e enough to
be f i t t e d to endoscopes.
They can transmit CO2 laser l i g h t as well as v i s i b l e
l i g h t and t h e i r attenuation remains constant, even when bent to a small radius of 2 cm.
The f i b e r s can transmit high-power density in a small spot size.
However, the e f f i c i e n c y of the power transmission should be improved, in order to reach the t h e o r e t i c a l l i m i t and be suitable f o r medical a p p l i c a t i o n s . REFERENCES I
E.A.J. M a r c a t i l i and R.A. Schmeltzer, Bell System J. 43 (1964) 1783.
2
M. Miyagi and S. Nishida, IEEE Trans. Microwave Theory & Techniques MI128 (1980) 536.
3
A. Bornstein, N. Croitoru and E. Marom, Proc. Soc. Photo-Opt. I n s t r . Engs., Adv. in Infrared Fiber I I , 320 (1982) 402.
4) Eo Carmine, Proc. Soc. Photo-Opt. I n s t r . Eng., Adv. in Infrared Fiber I I , 320 (1982) 70. 5) A. Bornstein, N. Croitoru and E. Marom, Proc. Soc. Photo-Opt. I n s t r . Enqs., Infrared Optical Materials and Fibers I I I , 484 (1984) 99. 6) A. Bornstein, N. Croitoru and Eo Marom, J. Non-Cryst. Sol. (1985) in press.