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TeX-glass infrared optical fibers delivering medium power from a CO2 laser
Optical Materials 13 (1999) 271±276
TeX-glass infrared optical ®bers delivering medium power from a CO2 laser F. Smektala *, K. Le Foulgoc, L. Le Neindre, C. Blanchetiere, X.H. Zhang, J. Lucas Laboratoire des Verres et C eramiques, UMR-CNRS 6512, Universit e de Rennes 1, Campus de Beaulieu, 35042 Rennes C edex, France Received 15 July 1998; accepted 30 October 1998
Abstract Infrared glasses based on tellurium halides (TeX-glasses) present a wide transmission window ranging from 1 lm to more than 18 lm, depending on the compositions, for few millimeters thick samples. They present very good chemical durability against water or usual atmospheric conditions. These glasses can be drawn into optical ®bers transmitting CO2 laser radiations. The ®bers can be monoindex ones or present a core-clad structure. A polymer that increases their ¯exibility can coat them. The minimum attenuation obtained for these infrared ®bers is 0.5 dB/m in the 7±9.5 lm range. Radiation from a tunable CO2 laser, operating around 9.3 lm and delivering a maximum output power of 7 W, has been injected in a 1 m long, 600 lm outer diameter, TeX-Glass ®ber. The obtained output power of the ®ber is 2.6 W. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The CO2 laser is of wide utilization in various ®elds of industry, for marking, cutting, welding etc. The disposability of ¯exible infrared ®bers transmitting this laser power would greatly simplify practical use in such applications [1]. Also medical surgery would appreciate such ®bers (a 1 J/cm2 intensity from a CO2 laser is sucient to destroy the human tissues [2]) and especially dental surgery in replacement of the diamond drill. Indeed, bone, enamel and dentine show absorptions in the 9±11 lm range, corresponding to the emission of the CO2 laser. This makes CO2 laser more interesting than excimer or Er:Yag la-
* Corresponding author. Tel.: +33 299281610; fax: +33 299281600; e-mail: [email protected]
ser for example. The interest of CO2 laser is also that laser ablation is not followed by thermal damage since temperature increases slightly [3]. At that time, there are mainly three types of ®bers able to transmit CO2 laser power: hollow core ®bers, silver halide polycrystalline ®bers, and chalcogenide ®bers. Each of them presents advantages and disadvantages. Indeed, hollow core ®bers can deliver high power, but bending induces very high losses. Silver halide polycrystalline ®bers (AgCl± AgBr) exhibit low losses around 0.5 dB/m at 10.6 lm and can deliver 10 W of CO2 laser power, but show bad aging [4]. Chalcogenide glass ®bers are easier to manufacture but their attenuation at 10.6 lm is relatively high and seems to be intrinsic, prescribed by multiphonon absorption [5,6]. The interest of TeX-glass ®bers that we present here is that they exhibit potential lower losses for CO2 laser power transmission than classical chalcog-
0925-3467/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 7 9 - 2
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F. Smektala et al. / Optical Materials 13 (1999) 271±276
enide ®bers. Indeed, TeX-glasses, based on tellurium halides, show lower phonon energy than chalcogenide glasses, mainly based on germanium, arsenic, sulfur and selenium. Their intrinsic multiphonon absorption is thus shifted to higher wavelengths. 2. Tellurium halide glasses TeX-glasses are obtained in various binary, ternary or quaternary systems, with large glass forming regions. Tellurium in association with an halogen (X Cl, Br, I) leads to glass formation. Other chalcogen such as selenium for example or pnictogen such as arsenic can be added to this binary system and also lead to glass formation [7]. Binary TeX-glasses are essentially one dimensional and present low glass transition temperature (Tg ). Addition of arsenic for example increases the dimensionally of the vitreous network and increases by the way this transition temperature and the working range temperature. Since these glasses are based on heavy elements, they exhibit low phonon energy and extended infrared transmission. It is when the halogen is iodine that the infrared transmission presents the biggest extension to the high wavelengths (Table 1). These glasses are very stable against devitri®cation and some compositions exhibit no crystallization peak when recording DSC curves. Essentially covalent, they present also very good chemical durability against water and normal atmospheric conditions. There is a big ¯exibility in the choice of compositions. So a variation in the amount of the dierent elements allows the determination of two compositions presenting suitable refractive index variation for the design of core-clad ®bers. Indeed, the refractive index of the core glass must be higher than the refractive index of the clad glass to obtain light propagation in the core of the ®ber by total re¯ection.
The composition under study in this paper is Te2 I0:5 Se4:5 As3 . This composition presents a good thermal stability since no crystallization peak can be observed during a DSC experiment at the heating rate of 10°C/mn. The Tg is 137°C and the dilatation coecient is 2.17 ´ 10ÿ5 Kÿ1 . The glasses are prepared by reaction of high purity (5± 6 N) raw materials. These raw materials are further separately puri®ed from surface oxide impurities and other impurities such as carbon for example by appropriate physical and chemical techniques before melting the batch in a silica ampoule sealed under vacuum [8]. Several centimeters long glass rods are obtained by cooling the melt to room temperature. Tubes are obtained by rotating the silica ampoule during cooling. These rods and tubes are used as rod-in-tube preforms for ®ber drawing (Fig. 1). 3. TeX-glass ®bers Monoindex ®bers are obtained by pulling TeXglass rods on a drawing tower designed in our laboratory [9] (Fig. 2). Typically, the diameter of the rod is 9 mm and the length is 7 cm. Fibers having a core-clad structure are obtained by drawing rod-in-tube preforms. The thermal gradient of the drawing furnace is specially designed for softening the preform just above its lower extremity. A drop appears and falls down under gravitational attraction pulling the ®ber in its wake. The ®ber is ®xed on the drum in rotary motion. In the same time the preform is moved down in the drawing furnace and is by this way pulled into ®ber. For a given moving speed of the preform, the diameter of the ®ber is controlled by the drum speed. The furnace temperature for both monoindex and core-cladding structure ®bers preparation is 240°C. The drawing speed is 1 m/ min. The ®bers are coated by a UV curable polymer or a thermal polymer increasing their me-
Table 1 Physical characteristics of some TeX-glasses Vitreous system
Glass temperature (Tg °C)
Transmission window (5 mm thickness)
Te±I±Se Te±I±Se±As
80±100 130±160
1±20 lm 2±18 lm
F. Smektala et al. / Optical Materials 13 (1999) 271±276
Fig. 1. Synthesis of a preform from TeX-glasses.
273
can also be used (Fig. 3). In this technique, no preform is realized but the core and clad glasses are directly melted in two separated crucibles presenting a calibrated aperture at their bottom. Typically, the volume of glasses used is 10 cm3 . An inert gas pressure controls their out¯ow. The two glasses enter in contact in a viscous state, cool rapidly out of the furnace and are drawn into ®bers on the same drawing tower as previously. The drawing temperature and the drawing speed are, respectively, 240°C and 1 m/min. The diameter of the aperture is the diameter of the ®ber. Fiber attenuation is measured by the classical cut-back method (Fig. 4). A large band HgCdTe detector cooled by liquid nitrogen and connected to a FTIR spectrometer detects the infrared light transmitted by the ®ber. A typical attenuation spectrum for a core-clad ®ber prepared by the double crucible method is given in Fig. 5. The numerical aperture of the ®ber is 0.15. The core diameter is 200 lm for a ®ber diameter of 350 lm. Losses are below 1 dB/m in the 7±9 lm range and a minimum of 0.5 dB/m is observed between 8.5 and 9 lm. The absorption peak at 6.3 lm is due to molecular water vibration, whereas the peak at 2.8 lm corresponds to O±H link vibration. Vibration of Se±H link is responsible for the absorption at 4.6 lm. 4. Working wavelength of the CO2 laser beam
Fig. 2. Drawing tower.
chanical properties. The minimum bending radius for a 200 lm outer diameter ®ber is less than 1 cm. Preforms are not the only way to manufacture optical ®bers. The double crucible drawing method
In the case of dentistry application, the tissues of the tooth are under consideration. Mains are enamel at the surface of the tooth, cement and dentine inside the tooth. The transmission spectra of these three dental tissues are represented in Fig. 6 [10]. As can be seen in the ®gure, the human tooth absorbs strongly between 9.2 and 9.8 lm, with a maximum of absorption near 9.5 lm. The emission spectrum of a CO2 laser is a complex set of rotational transitions superimposed on the vibration transitions. Fig. 7 shows the four groups of emissions which are respectively centered on 9.3, 9.6, 10.3 and 10.6 lm. About 100 distinct lines can be obtained by using tuning elements such as diraction gratings for example [11]. The laser used for the experiment is a tunable C7
274
F. Smektala et al. / Optical Materials 13 (1999) 271±276
Fig. 5. Typical attenuation spectrum of a TeX-glass ®ber with a core clad structure and coated by a UV curable polymer.
Fig. 3. Double crucible drawing technique.
CO2 laser from SAT. Its output power, depending on the emission wavelength, is very stable with a variation under 1%. As shown in Fig. 8, this output power is adjustable in the range of 0 to almost 7 W with a maximum located at 9.3 lm. Since TeX glass ®bers present a minimum of attenuation under 1 dB/m located in the 9 lm region and since dental tissues exhibit a strong absorption in the 9.3±9.6 lm region, the working wavelength of 9.3 lm, at which the output power of the C7 CO2 laser is maximum, has been selected for laser power delivery experiments. 5. Power delivery results and discussion A schematic representation of the experimental set-up used for CO2 laser power delivery
Fig. 4. Set-up for optical ®ber attenuation measurements.
Fig. 6. Transmission spectrum of a human tooth. The three dental tissues are enamel, cement and dentine.
experiments is reported in Fig. 9. It is mainly composed of a tunable CO2 laser emitting at 9.3 lm, two focusing ZnSe lenses, a monoindex TeXglass optical ®ber and a power meter. The two lenses are used to focus the 2 mm diameter CO2 laser beam to a diameter of 300 lm, before injection in the TeX-glass ®ber. They induce losses of
Fig. 7. Emission spectrum of a tunable CO2 laser.
F. Smektala et al. / Optical Materials 13 (1999) 271±276
Fig. 8. C7 CO2 laser output power as a function of the emission wavelength.
Fig. 9. Set-up for CO2 laser power delivery experiments through a TeX-glass optical ®ber.
about 1 W in the high power region. Thus, the laser power injected into the ®ber is about 1 W below the laser output power in this region (Fig. 8). The light from the laser beam at the 9.3 lm speci®ed wavelength is propagated through the optical ®ber. The ®ber output signal is then detected by a power meter, which allows measurement of the output power. The characteristics of the TeX-glass optical ®ber used for these CO2 laser power delivery experiments are summarized in Table 2. One must note that the two extremities of the TeX-glass optical ®ber are just cleaved and that no antire¯ection coating has been applied on them.
275
The relationship between the input and the output CO2 laser power through the mono-index TeX-glass optical ®ber is illustrated in Fig. 10. The output power increases linearly with the input power. A maximum ®ber output power of 2.6 W at 9.3 lm is obtained and has successfully burned materials such as paper and wood. At this power, the TeX-glass optical ®ber shows no damage at the input or output surface. These results have to be compared to those of Inagawa [12] obtained with a Te30 Se25 I45 ®ber where a maximum ®ber output power of 0,8 W has been delivered though a 1 m long and 400 lm diameter ®ber. The maximum ®ber output power reported in this paper is limited by the maximum CO2 laser power available for injection in the ®ber (5.6 W). It is observed that the ®ber output power is nearly half of the input laser power. The major contribution to this dierence is due to Fresnel losses because of the high refractive index of the TeX-glass optical ®ber (n 2.8 at 9.3 lm). It is possible to drastically reduce these losses with the application of an antire¯ection coating on the two ends of the TeX-glass optical ®ber [13]. Higher CO2 laser powers delivered through silver halide polycrystalline ®bers or hollow core ®bers for example have been reported. Polycrystalline ®bers can deliver 20 W at 10.6 lm with low
Fig. 10. Relationship between the input and the output CO2 laser power for a TeX-glass optical ®ber.
Table 2 Characteristics of the TeX-glass infrared optical ®ber used for CO2 laser power delivery experiments Composition
Refractive index
Diameter
Minimum bending radius
Length
Attenuation at 9.3 lm
Glass transition temperature
Te2 Se3 As4:5 I0:5
2.8
640 lm
15 cm
1m
0.7 dB/m
140°C
276
F. Smektala et al. / Optical Materials 13 (1999) 271±276
propagation losses (0.5 dB/m at 10.6 lm) [14]. Hollow core ®bers can deliver CO2 laser powers up to 3 kW [15,16]. But, for these two types of ®bers, the bending induces very high losses and limits drastically their practical use. Polycrystalline ®bers have also to be protected against UV radiation [4,17]. 6. Conclusion TeX-glasses present an exceptionally wide transmission window, ranging from 1 lm to more than 18 lm. Low losses TeX-glass infrared optical ®bers have been drawn from the Te±Se±As±I vitreous system. These ®bers present a minimum of attenuation of 0.5 dB/m in the 9 lm region. They have been used in CO2 laser power delivery experiments. The obtained results show that 2.6 W at 9.3 lm can be transmitted through a 1 m long ®ber by injection of the maximum output power of the tunable CO2 laser emitting in the 9.2±9.4 lm range. No damage is observed on the ®ber. The chosen wavelength corresponds to a strong absorption for dental tissues. This delivered power has been successfully used to burn materials such as paper and wood. Thus CO2 laser power delivery through a ®ber may be an exceptional promise for providing rapid, secure and precise operations in dentistry applications.
References [1] G.M. Merberg, Lasers in Surgery and Medicine 13 (1993) 572. [2] S. Alimpiev, V. Artjuskenko and al., Proc. SPIE, 183 (1988) 906. [3] J. Melcer, Proc. 3rd Congress Int. Soc. Laser Dent. (1992) 3. [4] J.A. Harrington, Proc. SPIE 1591 (1991) 2. [5] J. Nishii, I. Inagawa, S. Morimoto, R. Iizuka, Proc. SPIE 1228 (1990) 846. [6] J. Nishii, S. Morimoto, I. Tanagawa, R. Izuka, T. Yamashita, T. Yamagishi, J. Non-Cryst. Solids 140 (1992) 199. [7] X.H. Zhang, H.L. Ma, G. Fonteneau, J. Lucas, J. NonCryst. Solids 140 (1992) 47. [8] X.H. Zhang, H.L. Ma, C. Blanchetiere, J. Lucas, J. NonCryst Solids 161 (1993) 327. [9] F. Smektala, Thesis, University of Rennes, France, (1992). [10] A. Nagasawa, Experta Medica, Amsterdam-OxfordPrinceton, (1983) 233. [11] J. Hecht, Laser Focus World 8 (9) (1992) 87. [12] I. Inagawa, T. Tamagishi, Y. Yamashita, J. Appl. Phys. 30 (11A) (1991) 2846. [13] X.H. Zhang, Thesis, University of Rennes, France, (1989). [14] J.A. Harrington, A. Katzir, Laser Focus World 27 (6) (1991) 139. [15] T. Abel, J. Hirsch, J.A. Harrington, Proc. SPIE. 2131 (1994). [16] A. Hongo, K. Morasawa, K. Matsumoto, T. Shiota, T. Hashimoto, Appl. Opt. 31 (24) (1992) 5114. [17] J.A. Wysocki, R.G. Wilson, A.G. Standlee, A.C. Pastor, R.N. Schwartz, A.R. Williams, G. Lei, L. Kevan, J. Appl. Phys. 63 (9) (1988) 2.
TeX-glass infrared optical ®bers delivering medium power from a CO2 laser F. Smektala *, K. Le Foulgoc, L. Le Neindre, C. Blanchetiere, X.H. Zhang, J. Lucas Laboratoire des Verres et C eramiques, UMR-CNRS 6512, Universit e de Rennes 1, Campus de Beaulieu, 35042 Rennes C edex, France Received 15 July 1998; accepted 30 October 1998
Abstract Infrared glasses based on tellurium halides (TeX-glasses) present a wide transmission window ranging from 1 lm to more than 18 lm, depending on the compositions, for few millimeters thick samples. They present very good chemical durability against water or usual atmospheric conditions. These glasses can be drawn into optical ®bers transmitting CO2 laser radiations. The ®bers can be monoindex ones or present a core-clad structure. A polymer that increases their ¯exibility can coat them. The minimum attenuation obtained for these infrared ®bers is 0.5 dB/m in the 7±9.5 lm range. Radiation from a tunable CO2 laser, operating around 9.3 lm and delivering a maximum output power of 7 W, has been injected in a 1 m long, 600 lm outer diameter, TeX-Glass ®ber. The obtained output power of the ®ber is 2.6 W. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The CO2 laser is of wide utilization in various ®elds of industry, for marking, cutting, welding etc. The disposability of ¯exible infrared ®bers transmitting this laser power would greatly simplify practical use in such applications [1]. Also medical surgery would appreciate such ®bers (a 1 J/cm2 intensity from a CO2 laser is sucient to destroy the human tissues [2]) and especially dental surgery in replacement of the diamond drill. Indeed, bone, enamel and dentine show absorptions in the 9±11 lm range, corresponding to the emission of the CO2 laser. This makes CO2 laser more interesting than excimer or Er:Yag la-
* Corresponding author. Tel.: +33 299281610; fax: +33 299281600; e-mail: [email protected]
ser for example. The interest of CO2 laser is also that laser ablation is not followed by thermal damage since temperature increases slightly [3]. At that time, there are mainly three types of ®bers able to transmit CO2 laser power: hollow core ®bers, silver halide polycrystalline ®bers, and chalcogenide ®bers. Each of them presents advantages and disadvantages. Indeed, hollow core ®bers can deliver high power, but bending induces very high losses. Silver halide polycrystalline ®bers (AgCl± AgBr) exhibit low losses around 0.5 dB/m at 10.6 lm and can deliver 10 W of CO2 laser power, but show bad aging [4]. Chalcogenide glass ®bers are easier to manufacture but their attenuation at 10.6 lm is relatively high and seems to be intrinsic, prescribed by multiphonon absorption [5,6]. The interest of TeX-glass ®bers that we present here is that they exhibit potential lower losses for CO2 laser power transmission than classical chalcog-
0925-3467/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 7 9 - 2
272
F. Smektala et al. / Optical Materials 13 (1999) 271±276
enide ®bers. Indeed, TeX-glasses, based on tellurium halides, show lower phonon energy than chalcogenide glasses, mainly based on germanium, arsenic, sulfur and selenium. Their intrinsic multiphonon absorption is thus shifted to higher wavelengths. 2. Tellurium halide glasses TeX-glasses are obtained in various binary, ternary or quaternary systems, with large glass forming regions. Tellurium in association with an halogen (X Cl, Br, I) leads to glass formation. Other chalcogen such as selenium for example or pnictogen such as arsenic can be added to this binary system and also lead to glass formation [7]. Binary TeX-glasses are essentially one dimensional and present low glass transition temperature (Tg ). Addition of arsenic for example increases the dimensionally of the vitreous network and increases by the way this transition temperature and the working range temperature. Since these glasses are based on heavy elements, they exhibit low phonon energy and extended infrared transmission. It is when the halogen is iodine that the infrared transmission presents the biggest extension to the high wavelengths (Table 1). These glasses are very stable against devitri®cation and some compositions exhibit no crystallization peak when recording DSC curves. Essentially covalent, they present also very good chemical durability against water and normal atmospheric conditions. There is a big ¯exibility in the choice of compositions. So a variation in the amount of the dierent elements allows the determination of two compositions presenting suitable refractive index variation for the design of core-clad ®bers. Indeed, the refractive index of the core glass must be higher than the refractive index of the clad glass to obtain light propagation in the core of the ®ber by total re¯ection.
The composition under study in this paper is Te2 I0:5 Se4:5 As3 . This composition presents a good thermal stability since no crystallization peak can be observed during a DSC experiment at the heating rate of 10°C/mn. The Tg is 137°C and the dilatation coecient is 2.17 ´ 10ÿ5 Kÿ1 . The glasses are prepared by reaction of high purity (5± 6 N) raw materials. These raw materials are further separately puri®ed from surface oxide impurities and other impurities such as carbon for example by appropriate physical and chemical techniques before melting the batch in a silica ampoule sealed under vacuum [8]. Several centimeters long glass rods are obtained by cooling the melt to room temperature. Tubes are obtained by rotating the silica ampoule during cooling. These rods and tubes are used as rod-in-tube preforms for ®ber drawing (Fig. 1). 3. TeX-glass ®bers Monoindex ®bers are obtained by pulling TeXglass rods on a drawing tower designed in our laboratory [9] (Fig. 2). Typically, the diameter of the rod is 9 mm and the length is 7 cm. Fibers having a core-clad structure are obtained by drawing rod-in-tube preforms. The thermal gradient of the drawing furnace is specially designed for softening the preform just above its lower extremity. A drop appears and falls down under gravitational attraction pulling the ®ber in its wake. The ®ber is ®xed on the drum in rotary motion. In the same time the preform is moved down in the drawing furnace and is by this way pulled into ®ber. For a given moving speed of the preform, the diameter of the ®ber is controlled by the drum speed. The furnace temperature for both monoindex and core-cladding structure ®bers preparation is 240°C. The drawing speed is 1 m/ min. The ®bers are coated by a UV curable polymer or a thermal polymer increasing their me-
Table 1 Physical characteristics of some TeX-glasses Vitreous system
Glass temperature (Tg °C)
Transmission window (5 mm thickness)
Te±I±Se Te±I±Se±As
80±100 130±160
1±20 lm 2±18 lm
F. Smektala et al. / Optical Materials 13 (1999) 271±276
Fig. 1. Synthesis of a preform from TeX-glasses.
273
can also be used (Fig. 3). In this technique, no preform is realized but the core and clad glasses are directly melted in two separated crucibles presenting a calibrated aperture at their bottom. Typically, the volume of glasses used is 10 cm3 . An inert gas pressure controls their out¯ow. The two glasses enter in contact in a viscous state, cool rapidly out of the furnace and are drawn into ®bers on the same drawing tower as previously. The drawing temperature and the drawing speed are, respectively, 240°C and 1 m/min. The diameter of the aperture is the diameter of the ®ber. Fiber attenuation is measured by the classical cut-back method (Fig. 4). A large band HgCdTe detector cooled by liquid nitrogen and connected to a FTIR spectrometer detects the infrared light transmitted by the ®ber. A typical attenuation spectrum for a core-clad ®ber prepared by the double crucible method is given in Fig. 5. The numerical aperture of the ®ber is 0.15. The core diameter is 200 lm for a ®ber diameter of 350 lm. Losses are below 1 dB/m in the 7±9 lm range and a minimum of 0.5 dB/m is observed between 8.5 and 9 lm. The absorption peak at 6.3 lm is due to molecular water vibration, whereas the peak at 2.8 lm corresponds to O±H link vibration. Vibration of Se±H link is responsible for the absorption at 4.6 lm. 4. Working wavelength of the CO2 laser beam
Fig. 2. Drawing tower.
chanical properties. The minimum bending radius for a 200 lm outer diameter ®ber is less than 1 cm. Preforms are not the only way to manufacture optical ®bers. The double crucible drawing method
In the case of dentistry application, the tissues of the tooth are under consideration. Mains are enamel at the surface of the tooth, cement and dentine inside the tooth. The transmission spectra of these three dental tissues are represented in Fig. 6 [10]. As can be seen in the ®gure, the human tooth absorbs strongly between 9.2 and 9.8 lm, with a maximum of absorption near 9.5 lm. The emission spectrum of a CO2 laser is a complex set of rotational transitions superimposed on the vibration transitions. Fig. 7 shows the four groups of emissions which are respectively centered on 9.3, 9.6, 10.3 and 10.6 lm. About 100 distinct lines can be obtained by using tuning elements such as diraction gratings for example [11]. The laser used for the experiment is a tunable C7
274
F. Smektala et al. / Optical Materials 13 (1999) 271±276
Fig. 5. Typical attenuation spectrum of a TeX-glass ®ber with a core clad structure and coated by a UV curable polymer.
Fig. 3. Double crucible drawing technique.
CO2 laser from SAT. Its output power, depending on the emission wavelength, is very stable with a variation under 1%. As shown in Fig. 8, this output power is adjustable in the range of 0 to almost 7 W with a maximum located at 9.3 lm. Since TeX glass ®bers present a minimum of attenuation under 1 dB/m located in the 9 lm region and since dental tissues exhibit a strong absorption in the 9.3±9.6 lm region, the working wavelength of 9.3 lm, at which the output power of the C7 CO2 laser is maximum, has been selected for laser power delivery experiments. 5. Power delivery results and discussion A schematic representation of the experimental set-up used for CO2 laser power delivery
Fig. 4. Set-up for optical ®ber attenuation measurements.
Fig. 6. Transmission spectrum of a human tooth. The three dental tissues are enamel, cement and dentine.
experiments is reported in Fig. 9. It is mainly composed of a tunable CO2 laser emitting at 9.3 lm, two focusing ZnSe lenses, a monoindex TeXglass optical ®ber and a power meter. The two lenses are used to focus the 2 mm diameter CO2 laser beam to a diameter of 300 lm, before injection in the TeX-glass ®ber. They induce losses of
Fig. 7. Emission spectrum of a tunable CO2 laser.
F. Smektala et al. / Optical Materials 13 (1999) 271±276
Fig. 8. C7 CO2 laser output power as a function of the emission wavelength.
Fig. 9. Set-up for CO2 laser power delivery experiments through a TeX-glass optical ®ber.
about 1 W in the high power region. Thus, the laser power injected into the ®ber is about 1 W below the laser output power in this region (Fig. 8). The light from the laser beam at the 9.3 lm speci®ed wavelength is propagated through the optical ®ber. The ®ber output signal is then detected by a power meter, which allows measurement of the output power. The characteristics of the TeX-glass optical ®ber used for these CO2 laser power delivery experiments are summarized in Table 2. One must note that the two extremities of the TeX-glass optical ®ber are just cleaved and that no antire¯ection coating has been applied on them.
275
The relationship between the input and the output CO2 laser power through the mono-index TeX-glass optical ®ber is illustrated in Fig. 10. The output power increases linearly with the input power. A maximum ®ber output power of 2.6 W at 9.3 lm is obtained and has successfully burned materials such as paper and wood. At this power, the TeX-glass optical ®ber shows no damage at the input or output surface. These results have to be compared to those of Inagawa [12] obtained with a Te30 Se25 I45 ®ber where a maximum ®ber output power of 0,8 W has been delivered though a 1 m long and 400 lm diameter ®ber. The maximum ®ber output power reported in this paper is limited by the maximum CO2 laser power available for injection in the ®ber (5.6 W). It is observed that the ®ber output power is nearly half of the input laser power. The major contribution to this dierence is due to Fresnel losses because of the high refractive index of the TeX-glass optical ®ber (n 2.8 at 9.3 lm). It is possible to drastically reduce these losses with the application of an antire¯ection coating on the two ends of the TeX-glass optical ®ber [13]. Higher CO2 laser powers delivered through silver halide polycrystalline ®bers or hollow core ®bers for example have been reported. Polycrystalline ®bers can deliver 20 W at 10.6 lm with low
Fig. 10. Relationship between the input and the output CO2 laser power for a TeX-glass optical ®ber.
Table 2 Characteristics of the TeX-glass infrared optical ®ber used for CO2 laser power delivery experiments Composition
Refractive index
Diameter
Minimum bending radius
Length
Attenuation at 9.3 lm
Glass transition temperature
Te2 Se3 As4:5 I0:5
2.8
640 lm
15 cm
1m
0.7 dB/m
140°C
276
F. Smektala et al. / Optical Materials 13 (1999) 271±276
propagation losses (0.5 dB/m at 10.6 lm) [14]. Hollow core ®bers can deliver CO2 laser powers up to 3 kW [15,16]. But, for these two types of ®bers, the bending induces very high losses and limits drastically their practical use. Polycrystalline ®bers have also to be protected against UV radiation [4,17]. 6. Conclusion TeX-glasses present an exceptionally wide transmission window, ranging from 1 lm to more than 18 lm. Low losses TeX-glass infrared optical ®bers have been drawn from the Te±Se±As±I vitreous system. These ®bers present a minimum of attenuation of 0.5 dB/m in the 9 lm region. They have been used in CO2 laser power delivery experiments. The obtained results show that 2.6 W at 9.3 lm can be transmitted through a 1 m long ®ber by injection of the maximum output power of the tunable CO2 laser emitting in the 9.2±9.4 lm range. No damage is observed on the ®ber. The chosen wavelength corresponds to a strong absorption for dental tissues. This delivered power has been successfully used to burn materials such as paper and wood. Thus CO2 laser power delivery through a ®ber may be an exceptional promise for providing rapid, secure and precise operations in dentistry applications.
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