- Email: [email protected]
Performance of a dielectric-coated monolithic hollow metallic waveguide
MaterialsLetters North-Holland
16 (1993) 1X1-156
Performance of a dielectric-coated metallic waveguide
monolithic hollow
P. Bhardwaj ‘, O.J. Gregory a, C. Morrow b, G. Gu b and Ken Burbank a ’ Department of Chemical and Materials Engineering, b Surgilase Inc., Warwick, RI 02886, USA
University of Rhode Island, Kingston, RI 02881, USA
Received 9 October 1992; in final form 3 December 1992
A dielectric-coated monolithic hollow metallic waveguide was developed for the transmission of CO* laser light which employs a hollow silver tube as the support onto which a dense, defect-free native silver halide coating was formed. This waveguide exhibited high-power transmission and low loss of CO2 laser light when tested straight and bent. The performance of this hollow metallic waveguide is discussed in terms of surface finish, dielectric film thickness and dielectric film quality.
1. Introduction
, metal support
Hollow waveguides have considerable promise for transmitting high-power COZ laser radiation in industrial and medical applications, including the cutting, welding and heat treatment of metals and ceramics as well as many surgical procedures. Hollow core waveguides are advantageous in such applications because of their potentially low-cost high-power capability, high stability and high coupling efficiency at the input end of the waveguide without considerable reflection [ 11. Various types of hollow waveguides have been proposed for this purpose, including metallic circular waveguides, metallic plate parallel waveguides, whispering gallery waveguides, glass hollow waveguides, dielectric-coated metallic waveguides and dielectric tube waveguides [ 11. Dielectric-coated hollow metallic waveguides are reported to have lower losses and higher power capability than any other type of hollow waveguide [ l111. A schematic of a bent dielectric-coated hollow metallic waveguide with light bouncing from the inner wall is shown in fig. 1. A theoretical analysis of the propagation of electromagnetic waves through hollow waveguides was initially described in detail by Marcatili and Schmeltzer [2] and was later extended to include losses in dielectric-coated (circular) metallic waveguides [ 3,4]. In a more recent analysis of the trans150
Fig. 1.Schematic of cross section of a bent hollow metallic waveguide showing light bouncing from the inner wall in the direction of the guide axis.
mission properties and attenuation constants for dielectric-coated metallic waveguides, the losses in the dielectric-coated metallic waveguides have been attributed to IR absorption by the dielectric material and variations in thickness of the dielectric layer [ $61. Of particular concern was the temperature increase in these hollow waveguides due to these losses and other excitation-dependent losses [ 10,121. A variety of hollow waveguides were studied with re-
0167-577x/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 16, number
2,3
MATERXALS LETTERS
spect to these losses and dielectric-coated hollow metallic waveguides were determined to be superior than other hollow waveguides in this respect [ 10,121. Dielectric-coated hollow metallic waveguides are the most promising waveguides for the high-power transmission of CO2 laser light in medical and industrial applications, although other types of hollow waveguides, such as hollow glass fibers, sapphire fibers and optical fibers made of polycrstalline halide and chalcogenide glasses have also been developed [ 13- 18 1. Rigid hollow waveguides made of hollow alumina ceramic tubes encased within a stainless steel jacket have been used extensively in medicine for the transmission of CO2 laser light in rigid endoscopic applications [ 131. However, these waveguides are limited in both length and power handling and can readily overheat or melt when the laser light is not properly launched into the waveguide. Also, the ceramic tubes preclude use in flexible applications due to their inherent lack of ductility. In addition, some hollow metallic waveguides tend to overheat easily when light is not launched into the waveguide properly, thus limiting the maximum input power to the waveguide. A monolithic hollow metallic waveguide was recently developed employing a unique fabrication technique where an enhancing dielectric film is formed directly over the interior of a solid substrate monolithic tube. This provides a low-cost and highly efficient manufacturing method for hollow flexible and rigid waveguides for infrared radiation. Silver was selected as the substrate material for the monolithic waveguide because it has extremely high thermal conductivity, excellent ductility and excellent optical characteristics (reflectivity through the spectrum). Silver also reacts readily with halogens (chlorine, bromine and iodine) to form dense native halide films of controllable thickness and results in a very uniform coating with excellent infrared and adhesion properties. In addition, local heating due to tight bends, thermal lensing at high powers and coating damage will be dissipated by the high thermal conductivity of silver, thus avoiding potentially catastrophic local failures and hot spots. One of the practical fabrication problems found in hollow metallic waveguides has been the poor finish on the inner surface of the silver tube, which results from the metal forming processes used to fabricate
March
1993
the support tube. To improve the surface finish, a chemical polishing method was developed for the asreceived silver tubes, which significantly improved the transmission characteristics of the uncoated metal tube [ 1,9]. A native silver halide (n z 2.2) coating on the inner surface was formed by direct reaction with halogens. This waveguide exhibited low transmission losses when used straight and when bent to small radii of curvature. This performance was attributed to the quality and uniformity of the native silver halide films (silver bromide, silver iodide and silver chloride) formed on the silver surface and the surface finish of the silver support. The performance of these dielectric-coated monolithic hollow metallic waveguides was evaluated and the results are presented within.
2. Experimental 2.1. Waveguide preparation Dielectric-coated hollow metallic waveguides were fabricated for the efficient transmission of both CO2 and HeNe laser light. Silver was drawn into 1 mm inner diameter tubing and served as the support and reaction tube for the formation of the native silver halide films. The as-received silver tubes did not exhibit an adequate surface finish for such optical applications due to microstructural defects from the metal forming processes used to fabricate the tubes. Thus, additional finishing steps were required, including the use of non-abrasive polishing techniques. Chemical polishing of the as-received silver tubes yielded optimal surfaces and was therefore incorporated into the fabrication sequence. The detailed description of the polishing procedure and schematic of apparatus is given in ref. [ 19 1. Native silver halide coatings (n = 2.2) were formed on the silver surfaces by the direct reaction of the metal with the halogens in liquid and vapor form. The process parameters included fluid flow rate, reaction time, substrate temperature and concentration of halogen vapor in the nitrogen diluted stream for the vapor-solid reaction process. A detailed description of the reaction processes and a schematic of apparatus used for these reactions is also given in reference [ 19 1. 151
Volume 16, number
MATERIALS
2,3
To determine the thickness of the dielectric films formed on the silver tubes, the weight change due to chemical reaction was measured and correlated with thickness. The change in weight due to the formation of a continuous silver halide film of theoretical density is related to film thickness by the following equations: At= Aml (P*G -PUBIC) , where At is the film thickness and Am/xDL weight change per unit area, and MW,,IP,, pAgeff =‘*’
MWAgX/pAsX’
(1)
is the
(2)
wherep the is density and MW the molecular weight. The density of the silver halide film (AgX) was assumed to be the same as the bulk theoretical density. 2.2. Transmission
tests
The dielectric-coated hollow metallic waveguides were tested for the transmission of CO2 light both straight and bent. These waveguides were 60 cm long with a 1 mm inner diameter. A maximum power of 20 W was used for the transmission tests. The experimental setup for evaluation of transmission losses consisted of a high-power CO2 ( 100 cw ) laser, ZnSe lenses with 200 mm focus length to couple the laser light into the waveguide, a power meter (Laser Instrumentation Ltd.) to measure the laser power and an optical bench with associated supports to mount and align the various components. The laser light was focused at the input end of the waveguide and transmitted through the waveguide. Power was measured at the output end of the waveguide and subsequently remeasured without the waveguide in the light path. The relative transmission was determined from the ratio of the powers measured. The bend tests were performed by keeping the first 20 cm of the waveguide straight and bending the rest of the waveguide at a predetermined radii of curvature using stainless steel sleeves. 3. Results and discussion The transmission of 10.6 l.trn was dependent on 152
LETTERS
March
1993
the surface roughness of the silver support, dielectric film quality, including grain size and uniformity, and dielectric film thickness. In addition, it was essential that the waveguide be free of any moisture and particulate contamination as a result of processing, since both of these can severely affect the optical properties of the silver halide film and thus the performance. Ideally, the metal tubes should have sufficient temper and stiffness to avoid permanent bends or tiny local bends of the tube during processing. It was determined that the tiny local bends in the monolithic tubes are particularly harmful to the transmission of 10.6 urn light in the waveguide. The transmission of CO2 laser light was particularly sensitive to surface defects, especially when bent. Some of these surface defects can be polished away and others cannot. In general, topographical features including those from plastic flow of the metal such as drawing marks, were removed by polishing. However, localized defects such as second phase inclusions were difficult to eliminate.
3. Film thickness effects The theoretical thickness of the silver halide film that was required for this application was determined with TF Calc. software using appropriate optical constants [ 20,2 11. To obtain an optimum film thickness for the transmission of 10.6 urn laser light, the average polarized light (both s and p polarization) transmission was calculated as a function of total film thickness. In these calculations, it was assumed that the silver halide films were theoretically dense and had an index of refraction of 2.2 and that the silver substrate (support) had a real index of refraction of 14.4 and an imaginary index of 56.9. The theoretical (calculated) single bounce reflectivity as a function of film thickness and angle is shown in fig. 2. The experimental relative transmission of CO1 laser light as a function of silver halide film thickness for our waveguide is shown in fig. 3. The relative transmission of this dielectric-coated waveguide was compared to transmission predicted from theoretical consideration (for single bounce), the results of which are shown in fig. 2. The experimental plot is impacted by potentially more than one bounce, cou-
MATERIALS LETTERS
Yoiume 16, number 2,3
f
70.\____1 0.5
Fib
1.0
Thickruss
1.5
2.5
2.0
of Dielectric
3.0
Creating (pn)
Fig. 2. Singlebounce COz laser light (IO.6 pm) reflecting from dielectric-coatedsilver surfaceas a function of film thickness. The film has index of refraction n, = 2.20 and k, = 0.0 and the substrate has index ofrefraction n2= 14.4 and kz= 56.9.
March I993
mission of the waveguide decreases rapidly when the silver halide film thickness exceeds 1.2 pm. Further increases in film thickness did not result in a transmission maximum that was predicted from theoretical considerations I The thickness of the silver halide fiIm was critical in fabricating a low-loss waveguide. Film thicknesses less than 0.4 pm yielded excellent transmission (up to 95%) when tested straight, but the transmission decreased rapidly as the waveguide was bent, even by a small amount. This waveguide exhibited exceilent transmission, with almost unmeasurable loss for the silver halide film thickness range 0.4-1.0 pm when tested straight and variations in silver halide thickness within this range did not lead to appreciable changes in iransmission. Thus, considerable process fatitude was achieved using this fabrication method, at least when the waveguide was tested in straight mode. Waveguides with silver halide film thicknesses in the range 0.4-1.0 pm exhibited low transmission losses when bent at large radii as well. When thicker silver halide films (greater than I pm) were employed, the transmission losses were appreciable even when tested straight. 3.2. Bond tests
0.4
0.6
Dielectric
0.8
film
1.0
thickness
1.2
1.4
(pm)
Fig. 3. Relative straight t~nsmiss~o~ as a function of dielectric fitm thickness for silver hatide film ( n z 2.2) in a 60 cm waveguide with 1 mm inner diameter.
When the dielectric-coated holiow metallic waveguides are tested bent, light bounces repeatediy off the tube wall as it propagates down the tube as shown by the ray diagram in fig. 1. Fig. 4 shows the results of calculations of the incident bounce angle (#) and the number of internal bounces as a function of radius of curvature over a 50 cm long monolithic hollow bent silver waveguide with a 1 mm inner diameter. This calculation is based on the assumption of a laser beam propagating in the direction of the guide axis and simple geometric considerations 12 1]_ The number of bounces is related to the radius of curvature as follow:
N=I+L/2(R-6/2)cos-‘[(R-d/2)/R], pling efficiency as well as film properties. The calculations were based on the optical properties of highquality silver halide films that are rek-&ively free of defects and thus, deviations from this can be explained in part by difference in film density and defect content in these thin films. The relative trans-
(3)
where N is the number of bounces, L is the length of the waveguide, R is the radius of curvature and d is the inner diameter of the hoHow waveguide. Due to the number of bounces, the transmission is much more dependent on silver halide film thickness and halide film thickness variations. In addition, the sur-
Volume 16, number 2,3
80.0’
1
“““’
10
MATERIALS LETTERS
“““’
100
“““‘*
1000
“““‘m
March 1993
. ..-’ D
10000100000
Radius of Curvature of Bent Waveguide Fig. 4. Graphic representation of the incident bounce angle (@) and number of internal bounces (#) as a function of radius of curvature over a 50 cm long hollow bent waveguide with 1 mm inner diameter for a laser beam propagating in the direction of guide axis.
face roughness of the silver support is even more critical to the performance of a bent hollow waveguide. A series of bend tests were made on waveguides having different surface finishes. The inner surface of each of the polished silver support tubes is shown in fig. 5. These silver supports were then processed by reacting with halogen. The relative transmissions of these waveguides as a function of 1/R (the radius of curvature) are compared in fig. 6. The results show the effect of inner surface roughness of the silver support on the relative transmission, which is much more pronounced when the waveguides are tested bent. For example, comparing the relative transmission of these waveguides for a 20 cm radius of curvature bend, a decrease in relative transmission from 90 to 30% was observed with a surface finish shown in fig. 5a. The improved surface finish shown in fig. 5b, resulted in a decrease in relative transmission from 95 to 60%. However, a decrease in transmission from 98% to only 85% was observed for waveguides prepared with an optimized surface finish shown in fig. 5c. The bending performance of the dielectric-coated 154
Fig. 5. (a)-(c) SEM micrographs of inner surface of polished silver support tubes before silver halide film formation.
Volume 16, number
MATERIALS
2,3
LETTERS
March 1993
guides having the dielectric film thickness range 0.5 0.8 urn exhibited a relative transmission (80-85%) for a 20 cm radius of curvature. The performance characteristics of this monolithic waveguide are compared to bending losses of 1 m long Ge-Ni and Ge-Ag hollow metallic waveguides [9] in fig. 7.
4. Conclusions
I 0
1
2
3
4
5
6
l/R (m) Radius of curvature of bent waveguide Fig. 6. Bending losses in monolithic waveguides having different inner surface finishes. The waveguides have optimum dielectric film thickness.
hollow metallic waveguide also depended on the dielectric film thickness. The waveguide exhibited a range of film thicknesses which yielded excellent performance, similar to the case of straight transmission. However, this variation (process latitude) was reduced in the waveguides tested bent. Wave-
Preliminary transmission measurements of this novel dielectric-coated hollow monolithic metallic waveguide were dependent on the inner surface finish of the silver support, the silver halide film thickness and resulting thin film quality. A significant improvement in transmission was achieved after chemical polishing (non-abrasive polishing) of the as-received hollow silver support. Waveguides having dielectric (n z 2.2) film thicknesses in the range 0.4- 1 urn exhibited excellent transmission characteristics when tested straight. However, the transmission of these waveguides was more sensitive to the film thickness and surface roughness when tested bent. An optimum silver halide film thickness of 0.5 0.8 urn resulted in low transmission losses when tested either straight or bent. This recently developed monolithic hollow metallic waveguide, which incorporates an enhancing dielectric film that is formed directly over the interior of a solid substrate monolithic tube, has excellent potential for low-cost and highly efficient waveguides for infrared radiation.
References [ 1] M. Miyagi, A. Hongo and Y. Matsuura, SPIE, Vol. AT 1, p.
“”
0
6
l/R (m) Radius of curvature of bent waveguide Fig. 7. Relative transmission as a function of I/R.The bending losses of other hollow waveguides are also presented [ 91.
258. [2] E.A.J. Marcatali and R.A. Schmeltzer, Bell Syst. Tech. J. 43 (1964) 1783. [3] J.W. Carlin and P. D’agostino, Bell Syst. Tech. J. 50 ( 1971 ) 1631. [4] J.W. Carlin and P. D’agostino, Bell Syst. Tech. J 52 ( 1973) 453. [ 51M. Miyagi and S. Kawakami, IEEE J. Lihgtwave Tech. LT2,No.2 (1984) 116. [6] M. Miyagi, IEEE J. Lightwave Tech. LT-3, No. 2 (1985) 3. [ 71 M. Miyagi, A. Hongo and S. Kawakami, Appl. Phys. Letters 43 (1983) 430.
155
Volume 16, number 23
MATERIALS LETTERS
[8]Y. Matsuura, M. Miyagi and A. Hongo, Optics Laser Technol. 22 (1990) 141. [9] A. Hongo, K Morosawa, T. Shiota, Y. Matsuura and M. Miyagi, IEEE J. Quantum Electron. 26 (1990) 1510, f 101Y. Hiratani, M. Miyagi and S. Nishida, Optics Laser Teehnd. June (‘1985) 135. I1 I JE. Gamire, T. McMohan and M. Bass, IEEE J. Quantum Electron. 16 (1980) 23. [ 121 S. Karasawa, M. Miyagi and S. Nishida, Appl. Optics 26 (1987) 4581. [ 131 US patent number 4,9 17,083, April 17,199O. If41 S.J. Saggese, J.A. Harrington and C.H. Sigel Jr., Optics Letters I6 (1991) 27.
156
March 1993
[ 151 J.A. Harrington and CC. Gregory, Optics Letters IS (1990) 541. [ 161 E. Gamire, T. McMohan and M. Bass, Appl. Optics 15 (1976) 145. ] 171 A. Bornstein and N. Craitoru, Appl. Optics 25 ( 1986) 335. i I8 1M. Saito, M. Takizawa and M. Miyagi, J. Lightwave Technol. 7 (1989) I%. [ I9f US Patent application pending for “Monolithic hohow waveguide and method and apparatus for making the same”. [ 20 ] TFCalcTM,Software Spectra Inc., Thintilm Design software for the Macintosh Computer Version 2.3 (December 1988). I21 JSurgilase Engineering Research and Development project.
16 (1993) 1X1-156
Performance of a dielectric-coated metallic waveguide
monolithic hollow
P. Bhardwaj ‘, O.J. Gregory a, C. Morrow b, G. Gu b and Ken Burbank a ’ Department of Chemical and Materials Engineering, b Surgilase Inc., Warwick, RI 02886, USA
University of Rhode Island, Kingston, RI 02881, USA
Received 9 October 1992; in final form 3 December 1992
A dielectric-coated monolithic hollow metallic waveguide was developed for the transmission of CO* laser light which employs a hollow silver tube as the support onto which a dense, defect-free native silver halide coating was formed. This waveguide exhibited high-power transmission and low loss of CO2 laser light when tested straight and bent. The performance of this hollow metallic waveguide is discussed in terms of surface finish, dielectric film thickness and dielectric film quality.
1. Introduction
, metal support
Hollow waveguides have considerable promise for transmitting high-power COZ laser radiation in industrial and medical applications, including the cutting, welding and heat treatment of metals and ceramics as well as many surgical procedures. Hollow core waveguides are advantageous in such applications because of their potentially low-cost high-power capability, high stability and high coupling efficiency at the input end of the waveguide without considerable reflection [ 11. Various types of hollow waveguides have been proposed for this purpose, including metallic circular waveguides, metallic plate parallel waveguides, whispering gallery waveguides, glass hollow waveguides, dielectric-coated metallic waveguides and dielectric tube waveguides [ 11. Dielectric-coated hollow metallic waveguides are reported to have lower losses and higher power capability than any other type of hollow waveguide [ l111. A schematic of a bent dielectric-coated hollow metallic waveguide with light bouncing from the inner wall is shown in fig. 1. A theoretical analysis of the propagation of electromagnetic waves through hollow waveguides was initially described in detail by Marcatili and Schmeltzer [2] and was later extended to include losses in dielectric-coated (circular) metallic waveguides [ 3,4]. In a more recent analysis of the trans150
Fig. 1.Schematic of cross section of a bent hollow metallic waveguide showing light bouncing from the inner wall in the direction of the guide axis.
mission properties and attenuation constants for dielectric-coated metallic waveguides, the losses in the dielectric-coated metallic waveguides have been attributed to IR absorption by the dielectric material and variations in thickness of the dielectric layer [ $61. Of particular concern was the temperature increase in these hollow waveguides due to these losses and other excitation-dependent losses [ 10,121. A variety of hollow waveguides were studied with re-
0167-577x/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 16, number
2,3
MATERXALS LETTERS
spect to these losses and dielectric-coated hollow metallic waveguides were determined to be superior than other hollow waveguides in this respect [ 10,121. Dielectric-coated hollow metallic waveguides are the most promising waveguides for the high-power transmission of CO2 laser light in medical and industrial applications, although other types of hollow waveguides, such as hollow glass fibers, sapphire fibers and optical fibers made of polycrstalline halide and chalcogenide glasses have also been developed [ 13- 18 1. Rigid hollow waveguides made of hollow alumina ceramic tubes encased within a stainless steel jacket have been used extensively in medicine for the transmission of CO2 laser light in rigid endoscopic applications [ 131. However, these waveguides are limited in both length and power handling and can readily overheat or melt when the laser light is not properly launched into the waveguide. Also, the ceramic tubes preclude use in flexible applications due to their inherent lack of ductility. In addition, some hollow metallic waveguides tend to overheat easily when light is not launched into the waveguide properly, thus limiting the maximum input power to the waveguide. A monolithic hollow metallic waveguide was recently developed employing a unique fabrication technique where an enhancing dielectric film is formed directly over the interior of a solid substrate monolithic tube. This provides a low-cost and highly efficient manufacturing method for hollow flexible and rigid waveguides for infrared radiation. Silver was selected as the substrate material for the monolithic waveguide because it has extremely high thermal conductivity, excellent ductility and excellent optical characteristics (reflectivity through the spectrum). Silver also reacts readily with halogens (chlorine, bromine and iodine) to form dense native halide films of controllable thickness and results in a very uniform coating with excellent infrared and adhesion properties. In addition, local heating due to tight bends, thermal lensing at high powers and coating damage will be dissipated by the high thermal conductivity of silver, thus avoiding potentially catastrophic local failures and hot spots. One of the practical fabrication problems found in hollow metallic waveguides has been the poor finish on the inner surface of the silver tube, which results from the metal forming processes used to fabricate
March
1993
the support tube. To improve the surface finish, a chemical polishing method was developed for the asreceived silver tubes, which significantly improved the transmission characteristics of the uncoated metal tube [ 1,9]. A native silver halide (n z 2.2) coating on the inner surface was formed by direct reaction with halogens. This waveguide exhibited low transmission losses when used straight and when bent to small radii of curvature. This performance was attributed to the quality and uniformity of the native silver halide films (silver bromide, silver iodide and silver chloride) formed on the silver surface and the surface finish of the silver support. The performance of these dielectric-coated monolithic hollow metallic waveguides was evaluated and the results are presented within.
2. Experimental 2.1. Waveguide preparation Dielectric-coated hollow metallic waveguides were fabricated for the efficient transmission of both CO2 and HeNe laser light. Silver was drawn into 1 mm inner diameter tubing and served as the support and reaction tube for the formation of the native silver halide films. The as-received silver tubes did not exhibit an adequate surface finish for such optical applications due to microstructural defects from the metal forming processes used to fabricate the tubes. Thus, additional finishing steps were required, including the use of non-abrasive polishing techniques. Chemical polishing of the as-received silver tubes yielded optimal surfaces and was therefore incorporated into the fabrication sequence. The detailed description of the polishing procedure and schematic of apparatus is given in ref. [ 19 1. Native silver halide coatings (n = 2.2) were formed on the silver surfaces by the direct reaction of the metal with the halogens in liquid and vapor form. The process parameters included fluid flow rate, reaction time, substrate temperature and concentration of halogen vapor in the nitrogen diluted stream for the vapor-solid reaction process. A detailed description of the reaction processes and a schematic of apparatus used for these reactions is also given in reference [ 19 1. 151
Volume 16, number
MATERIALS
2,3
To determine the thickness of the dielectric films formed on the silver tubes, the weight change due to chemical reaction was measured and correlated with thickness. The change in weight due to the formation of a continuous silver halide film of theoretical density is related to film thickness by the following equations: At= Aml (P*G -PUBIC) , where At is the film thickness and Am/xDL weight change per unit area, and MW,,IP,, pAgeff =‘*’
MWAgX/pAsX’
(1)
is the
(2)
wherep the is density and MW the molecular weight. The density of the silver halide film (AgX) was assumed to be the same as the bulk theoretical density. 2.2. Transmission
tests
The dielectric-coated hollow metallic waveguides were tested for the transmission of CO2 light both straight and bent. These waveguides were 60 cm long with a 1 mm inner diameter. A maximum power of 20 W was used for the transmission tests. The experimental setup for evaluation of transmission losses consisted of a high-power CO2 ( 100 cw ) laser, ZnSe lenses with 200 mm focus length to couple the laser light into the waveguide, a power meter (Laser Instrumentation Ltd.) to measure the laser power and an optical bench with associated supports to mount and align the various components. The laser light was focused at the input end of the waveguide and transmitted through the waveguide. Power was measured at the output end of the waveguide and subsequently remeasured without the waveguide in the light path. The relative transmission was determined from the ratio of the powers measured. The bend tests were performed by keeping the first 20 cm of the waveguide straight and bending the rest of the waveguide at a predetermined radii of curvature using stainless steel sleeves. 3. Results and discussion The transmission of 10.6 l.trn was dependent on 152
LETTERS
March
1993
the surface roughness of the silver support, dielectric film quality, including grain size and uniformity, and dielectric film thickness. In addition, it was essential that the waveguide be free of any moisture and particulate contamination as a result of processing, since both of these can severely affect the optical properties of the silver halide film and thus the performance. Ideally, the metal tubes should have sufficient temper and stiffness to avoid permanent bends or tiny local bends of the tube during processing. It was determined that the tiny local bends in the monolithic tubes are particularly harmful to the transmission of 10.6 urn light in the waveguide. The transmission of CO2 laser light was particularly sensitive to surface defects, especially when bent. Some of these surface defects can be polished away and others cannot. In general, topographical features including those from plastic flow of the metal such as drawing marks, were removed by polishing. However, localized defects such as second phase inclusions were difficult to eliminate.
3. Film thickness effects The theoretical thickness of the silver halide film that was required for this application was determined with TF Calc. software using appropriate optical constants [ 20,2 11. To obtain an optimum film thickness for the transmission of 10.6 urn laser light, the average polarized light (both s and p polarization) transmission was calculated as a function of total film thickness. In these calculations, it was assumed that the silver halide films were theoretically dense and had an index of refraction of 2.2 and that the silver substrate (support) had a real index of refraction of 14.4 and an imaginary index of 56.9. The theoretical (calculated) single bounce reflectivity as a function of film thickness and angle is shown in fig. 2. The experimental relative transmission of CO1 laser light as a function of silver halide film thickness for our waveguide is shown in fig. 3. The relative transmission of this dielectric-coated waveguide was compared to transmission predicted from theoretical consideration (for single bounce), the results of which are shown in fig. 2. The experimental plot is impacted by potentially more than one bounce, cou-
MATERIALS LETTERS
Yoiume 16, number 2,3
f
70.\____1 0.5
Fib
1.0
Thickruss
1.5
2.5
2.0
of Dielectric
3.0
Creating (pn)
Fig. 2. Singlebounce COz laser light (IO.6 pm) reflecting from dielectric-coatedsilver surfaceas a function of film thickness. The film has index of refraction n, = 2.20 and k, = 0.0 and the substrate has index ofrefraction n2= 14.4 and kz= 56.9.
March I993
mission of the waveguide decreases rapidly when the silver halide film thickness exceeds 1.2 pm. Further increases in film thickness did not result in a transmission maximum that was predicted from theoretical considerations I The thickness of the silver halide fiIm was critical in fabricating a low-loss waveguide. Film thicknesses less than 0.4 pm yielded excellent transmission (up to 95%) when tested straight, but the transmission decreased rapidly as the waveguide was bent, even by a small amount. This waveguide exhibited exceilent transmission, with almost unmeasurable loss for the silver halide film thickness range 0.4-1.0 pm when tested straight and variations in silver halide thickness within this range did not lead to appreciable changes in iransmission. Thus, considerable process fatitude was achieved using this fabrication method, at least when the waveguide was tested in straight mode. Waveguides with silver halide film thicknesses in the range 0.4-1.0 pm exhibited low transmission losses when bent at large radii as well. When thicker silver halide films (greater than I pm) were employed, the transmission losses were appreciable even when tested straight. 3.2. Bond tests
0.4
0.6
Dielectric
0.8
film
1.0
thickness
1.2
1.4
(pm)
Fig. 3. Relative straight t~nsmiss~o~ as a function of dielectric fitm thickness for silver hatide film ( n z 2.2) in a 60 cm waveguide with 1 mm inner diameter.
When the dielectric-coated holiow metallic waveguides are tested bent, light bounces repeatediy off the tube wall as it propagates down the tube as shown by the ray diagram in fig. 1. Fig. 4 shows the results of calculations of the incident bounce angle (#) and the number of internal bounces as a function of radius of curvature over a 50 cm long monolithic hollow bent silver waveguide with a 1 mm inner diameter. This calculation is based on the assumption of a laser beam propagating in the direction of the guide axis and simple geometric considerations 12 1]_ The number of bounces is related to the radius of curvature as follow:
N=I+L/2(R-6/2)cos-‘[(R-d/2)/R], pling efficiency as well as film properties. The calculations were based on the optical properties of highquality silver halide films that are rek-&ively free of defects and thus, deviations from this can be explained in part by difference in film density and defect content in these thin films. The relative trans-
(3)
where N is the number of bounces, L is the length of the waveguide, R is the radius of curvature and d is the inner diameter of the hoHow waveguide. Due to the number of bounces, the transmission is much more dependent on silver halide film thickness and halide film thickness variations. In addition, the sur-
Volume 16, number 2,3
80.0’
1
“““’
10
MATERIALS LETTERS
“““’
100
“““‘*
1000
“““‘m
March 1993
. ..-’ D
10000100000
Radius of Curvature of Bent Waveguide Fig. 4. Graphic representation of the incident bounce angle (@) and number of internal bounces (#) as a function of radius of curvature over a 50 cm long hollow bent waveguide with 1 mm inner diameter for a laser beam propagating in the direction of guide axis.
face roughness of the silver support is even more critical to the performance of a bent hollow waveguide. A series of bend tests were made on waveguides having different surface finishes. The inner surface of each of the polished silver support tubes is shown in fig. 5. These silver supports were then processed by reacting with halogen. The relative transmissions of these waveguides as a function of 1/R (the radius of curvature) are compared in fig. 6. The results show the effect of inner surface roughness of the silver support on the relative transmission, which is much more pronounced when the waveguides are tested bent. For example, comparing the relative transmission of these waveguides for a 20 cm radius of curvature bend, a decrease in relative transmission from 90 to 30% was observed with a surface finish shown in fig. 5a. The improved surface finish shown in fig. 5b, resulted in a decrease in relative transmission from 95 to 60%. However, a decrease in transmission from 98% to only 85% was observed for waveguides prepared with an optimized surface finish shown in fig. 5c. The bending performance of the dielectric-coated 154
Fig. 5. (a)-(c) SEM micrographs of inner surface of polished silver support tubes before silver halide film formation.
Volume 16, number
MATERIALS
2,3
LETTERS
March 1993
guides having the dielectric film thickness range 0.5 0.8 urn exhibited a relative transmission (80-85%) for a 20 cm radius of curvature. The performance characteristics of this monolithic waveguide are compared to bending losses of 1 m long Ge-Ni and Ge-Ag hollow metallic waveguides [9] in fig. 7.
4. Conclusions
I 0
1
2
3
4
5
6
l/R (m) Radius of curvature of bent waveguide Fig. 6. Bending losses in monolithic waveguides having different inner surface finishes. The waveguides have optimum dielectric film thickness.
hollow metallic waveguide also depended on the dielectric film thickness. The waveguide exhibited a range of film thicknesses which yielded excellent performance, similar to the case of straight transmission. However, this variation (process latitude) was reduced in the waveguides tested bent. Wave-
Preliminary transmission measurements of this novel dielectric-coated hollow monolithic metallic waveguide were dependent on the inner surface finish of the silver support, the silver halide film thickness and resulting thin film quality. A significant improvement in transmission was achieved after chemical polishing (non-abrasive polishing) of the as-received hollow silver support. Waveguides having dielectric (n z 2.2) film thicknesses in the range 0.4- 1 urn exhibited excellent transmission characteristics when tested straight. However, the transmission of these waveguides was more sensitive to the film thickness and surface roughness when tested bent. An optimum silver halide film thickness of 0.5 0.8 urn resulted in low transmission losses when tested either straight or bent. This recently developed monolithic hollow metallic waveguide, which incorporates an enhancing dielectric film that is formed directly over the interior of a solid substrate monolithic tube, has excellent potential for low-cost and highly efficient waveguides for infrared radiation.
References [ 1] M. Miyagi, A. Hongo and Y. Matsuura, SPIE, Vol. AT 1, p.
“”
0
6
l/R (m) Radius of curvature of bent waveguide Fig. 7. Relative transmission as a function of I/R.The bending losses of other hollow waveguides are also presented [ 91.
258. [2] E.A.J. Marcatali and R.A. Schmeltzer, Bell Syst. Tech. J. 43 (1964) 1783. [3] J.W. Carlin and P. D’agostino, Bell Syst. Tech. J. 50 ( 1971 ) 1631. [4] J.W. Carlin and P. D’agostino, Bell Syst. Tech. J 52 ( 1973) 453. [ 51M. Miyagi and S. Kawakami, IEEE J. Lihgtwave Tech. LT2,No.2 (1984) 116. [6] M. Miyagi, IEEE J. Lightwave Tech. LT-3, No. 2 (1985) 3. [ 71 M. Miyagi, A. Hongo and S. Kawakami, Appl. Phys. Letters 43 (1983) 430.
155
Volume 16, number 23
MATERIALS LETTERS
[8]Y. Matsuura, M. Miyagi and A. Hongo, Optics Laser Technol. 22 (1990) 141. [9] A. Hongo, K Morosawa, T. Shiota, Y. Matsuura and M. Miyagi, IEEE J. Quantum Electron. 26 (1990) 1510, f 101Y. Hiratani, M. Miyagi and S. Nishida, Optics Laser Teehnd. June (‘1985) 135. I1 I JE. Gamire, T. McMohan and M. Bass, IEEE J. Quantum Electron. 16 (1980) 23. [ 121 S. Karasawa, M. Miyagi and S. Nishida, Appl. Optics 26 (1987) 4581. [ 131 US patent number 4,9 17,083, April 17,199O. If41 S.J. Saggese, J.A. Harrington and C.H. Sigel Jr., Optics Letters I6 (1991) 27.
156
March 1993
[ 151 J.A. Harrington and CC. Gregory, Optics Letters IS (1990) 541. [ 161 E. Gamire, T. McMohan and M. Bass, Appl. Optics 15 (1976) 145. ] 171 A. Bornstein and N. Craitoru, Appl. Optics 25 ( 1986) 335. i I8 1M. Saito, M. Takizawa and M. Miyagi, J. Lightwave Technol. 7 (1989) I%. [ I9f US Patent application pending for “Monolithic hohow waveguide and method and apparatus for making the same”. [ 20 ] TFCalcTM,Software Spectra Inc., Thintilm Design software for the Macintosh Computer Version 2.3 (December 1988). I21 JSurgilase Engineering Research and Development project.