- Email: [email protected]
Fabrication of fluoride single-mode fibers for optical amplifiers
Journal of Non-Crystalline Solids 213 & 214 Ž1997. 121–125
Section 3. Applications
Fabrication of fluoride single-mode fibers for optical amplifiers T. Kanamori
a,)
, Y. Terunuma b, Y. Nishida a , K. Hoshino a , K. Nakagawa a , Y. Ohishi a , S. Sudo a b
a NTT Opto-Electronics Laboratories, Tokai, Ibaraki, 319-11, Japan NTT Electronic Technology Corporation, Tokai, Ibaraki, 319-11, Japan
Abstract A modified jacketing method, which uses a preform produced by suction casting and a tapered jacketing tube for fiber drawing, was developed to fabricate low-loss, high-D n fluoride single-mode fibers for optical amplifiers. A fabricated 500 m long ZrF4-based fiber had a constant core diameter and an almost constant mean strength of ; 435 MPa over its whole length. Low transmission losses below 20 dBrkm were achieved in rare earth-doped ZrF4-based single-mode fibers with a D n of 3.7%. An InF3-based single-mode fiber with a D n of 3.6%, a loss of 114 dBrkm at 1.23 mm and a signal gain coefficient of 0.18 dBrmW at 1.30 mm was fabricated using this jacketing method.
1. Introduction Fluoride glass fiber is a promising candidate for use in fabricating optical amplifiers because of the low phonon frequency properties of fluoride glasses w1x. It has already been confirmed that a Pr 3q-doped fluoride fiber amplifier, operating at 1.3 mm, has the potential for high gain, a high saturated output power, and a large bandwidth w2x, and that an Er 3q-doped fluoride fiber amplifier operating at 1.5 mm has a larger bandwidth than a silica fiber amplifier w3x. These gain characteristics have been achieved by developing low-loss high-D n fluoride single-mode fibers. The tapered-elongation and jacketing drawing method w4x shows promise as a method for fabricating low-loss high-D n fibers. In this method the
)
Corresponding author. Tel.: q81-29 287 7519; fax: q81-29 287 7193; e-mail: [email protected].
preforms are made by suction casting, which rapidly quenches the core and cladding liquids into glass. This rapid quenching is essential for producing high quality preforms with few scattering centers from fluoride glasses for high-D n fibers, because these glasses have a tendency to crystallize easily. Indeed, the high-D n fibers with the lowest reported loss were produced using this jacketing method w5x. With this method, two special processes are necessary to fabricate single-mode fibers. The tapered core of a preform, produced by suction casting, is reshaped into a smaller straight core in an elongation process. This core is achieved by programming the elongation speed as a function of time. The elongated preform is then polished into a cylindrical shape. This paper reports a modified jacketing method for fabricating single-mode fiber, which has been developed by using a preform produced by suction casting and a tapered jacketing tube for fiber drawing. In this method, the two reshaping processes required in the previously developed jacketing
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 7 . 0 0 0 5 2 - 5
T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
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method are eliminated. Moreover, we describe the properties of high-D n ZrF4-based fibers and high-D n InF3-based fibers.
2. Experimental
tapered core. The appropriate section of the secondary preform was inserted into a jacketing tube with a corresponding taper in its outer diameter and placed in the drawing furnace. A 500 m long singlemode optical fiber coated with UV-curable acrylate was drawn at a constant tension. This was achieved by regulating the temperature of the furnace.
2.1. Single-mode fiber fabrication 2.2. Measurements Fig. 1 shows the fabrication processes of the modified jacketing method using a preform produced by suction-casting and a tapered jacketing tube for fiber drawing. The preforms and jacketing tubes for elongation, which were cylindrical, were prepared by conventional suction-casting and rotation casting, respectively w6,7x. The jacketing tubes for fiber drawing were produced by the rotation casting method using a mold with a tapered hole. Each stage of the melting and casting processes was carried out in a clean dry gas atmosphere w8x. Fluorides of better than 99.99% purity were used. Two glass systems were synthesized for high-D n fibers. One was ZrF4 –HfF4 – PbF2 –BaF2 –LaF3 –YF3 –AlF3 –LiF–NaF w5x and the other was InF3 –GaF3 –PbF2 –BaF2 –SrF2 –LaF3 – YF3 –LiF–NaF w9x. The core and cladding materials with NH 4 F P HF in crucibles were melted in an Ar q HF gas flow at 8508C for 2 h, and metal molds were preheated at 2508C. The liquids were cast in the molds to produce a preform and a jacketing tube by means of suction-casting and rotation casting, respectively. The straight preform with a tapered core was then elongated together with the straight jacketing tube into a secondary straight preform with a smaller
The core diameters of the fabricated fibers were measured by using SEM observations of fracture surfaces. We performed tensile strength tests on the fibers at a strain rate of 0.03 miny1 , a gauge length of 350 mm and at room temperature. The transmission loss spectra in the 0.5–2.1 mm wavelength region were measured using the cutback technique with a halogen lamp as the light source. Monochromatic light selected using step-scanning grating monochromators and filters was launched into a fiber sample at the same numerical aperture as the fiber. We detected the fiber output signals with a silicon– germanium photodiode and an In–Sb detector. The fiber lengths for the loss measurements were 70–100 m. The signal gain dependence on pump power was measured using the fabricated Pr 3q-doped InF3-based fiber. The fiber was butt-jointed to wavelength-division-multiplexing couplers and forward pumped using a Ti: sapphire laser. The pump wavelength was 1.015 mm, and the signal wavelength was 1.30 mm. The signals were detected with an optical spectrum analyzer.
3. Results and discussion 3.1. Longitudinal fluctuations in core diameter and tensile strength of high-D n ZrF4-based fiber
Fig. 1. Modified jacketing method.
Fig. 2 shows the longitudinal fluctuations in the core diameter of a drawn fiber 500 m in length and with a cladding diameter of 120 mm. These fluctuations are less than 3% over the whole fiber length. This indicates that the taper of an elongated core is easily compensated for by using a tapered jacketing tube and that a small core can be uniformly formed for high-D n fiber using our modified jacketing method.
T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
Fig. 2. Core diameter fluctuation for a fiber fabricated by the modified jacketing method. The line is drawn as a guide for the eye.
Fig. 3 shows Weibull plots of tensile strengths for three sections of a fiber. These data were sampled at the beginning, the middle, and the end of a 500 m long fiber, which was drawn from sections of a tapered jacketing tube with small, medium, and large outer diameters, respectively. The strength values are distributed over a narrow range of 350–500 MPa and the mean values are 438 MPa at the beginning, 411 MPa at the middle, and 463 MPa at the end of the fiber. SEM observations of the fractured surfaces revealed that all the fractures were caused by surface flaws. We observed no fractures resulting from inner
Fig. 3. Weibull plots of tensile strengths for three sections of a fiber. Solid line: the beginning section of a drawn fiber; dotted line: the middle section of a drawn fiber; broken line: the end section of a drawn fiber.
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flaws. These SEM results, which are also represented by the high minimum strength values and the narrow strength distribution in Fig. 3, indicate that an interface is successfully formed between the preform or the secondary preform and the jacketing tube. In jacketing drawing with a tapered tube, the furnace temperature is regulated to keep the drawing tension constant. This means that the furnace temperature is increased as the outer diameter of the jacketing tube increases. From Fig. 3, it is clear that the change in furnace temperature resulting from the use of a tapered jacketing tube has no influence on fiber strength. The strength characteristics we obtained indicate that single-mode fibers with high uniform mechanical strength over their whole length can be fabricated using this modified jacketing method. 3.2. Losses of rare earth-doped ZrF4-based fibers Fig. 4 shows a typical transmission loss spectrum for Er 3q-doped ZrF4-based fiber. The core diameter, cladding diameter, D n, and Er 3q concentration of the fabricated fiber were 2.2 mm, 120 mm, 3.7%, and 1000 ppm, respectively. The large peaks in the 0.6–2.0 mm wavelength range are caused by Er 3q. The small peak observed at 1.13 mm is due to the cutoff of the first higher-order mode ŽLP11 mode. and the cutoff wavelength Ž lc . can be determined as 1.20 mm from the cutoff peak. This lc is coincident with l c calculated from the observed values of the core diameter, D n and the refractive index of core glass. This clear cutoff peak indicates that a singlemode waveguide structure is successfully formed by
Fig. 4. Transmission loss spectrum of Er 3q-doped ZrF4-based fiber.
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T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
Fig. 5. Transmission loss spectrum of Pr 3q-doped ZrF4 -based fiber.
Fig. 6. Transmission loss spectrum of Pr 3q-doped InF3 -based fiber.
the modified jacketing method. Fig. 5 shows a typical transmission loss spectrum for Pr 3q-doped ZrF4based fiber with a core diameter of 1.9 mm, a cladding diameter of 120 mm, a D n of 3.7%, and a Pr 3q concentration of 500 ppm. The large peaks at wavelengths of 0.6 and 1.0 mm are caused by Pr 3q. As shown in Figs. 4 and 5, losses of 19 and 20 dBrkm are obtained at 1.78 mm for the Er 3q-doped fiber and at 1.20 mm for the Pr 3q-doped fiber, respectively, while a Pr 3q-doped high-D n fiber fabricated by the tapered-elongation and jacketing drawing method had a loss of 44 dBrkm at 1.20 mm w5x. The low-loss characteristics we obtained indicate that the modified jacketing method has great potential for suppressing crystallization during single-mode fiber fabrication processes in the same way as the tapered-elongation and jacketing drawing method.
approach a practical level for use in optical amplifiers. The difference between the losses of the InF3based fiber and the ZrF4-based fiber is attributed to a scattering loss due to crystallization. In order to reduce the loss to the level of ZrF4-based fibers, the thermal stability of InF3-based glasses must be improved and impurities such as oxide must be eliminated from the raw materials. Fig. 7 shows the signal gain dependence on pump power at 1.30 mm for Pr 3q-doped InF3-based fiber. A gain coefficient of 0.18 dBrmW at 1.30 mm is achieved in the fiber with a D n of 3.6% and a loss of 114 dBrm at 1.20 mm. This gain coefficient is comparable to that for ZrF4-based fiber w5x but it is low compared with the expected value for InF3-based fiber. This low measured value is mainly the result
3.3. Loss and gain of Pr 3 q-doped InF3-based fiber A typical transmission loss spectrum for Pr 3qdoped InF3-based fiber is shown in Fig. 6. The length, core diameter, cladding diameter, D n, lc , and Pr 3q concentration of the fabricated fiber were 500 m, 1.9 mm, 120 mm, 3.6%, 1.0 mm, and 500 ppm, respectively. The large peaks around 0.6 mm and 1 mm are caused by Pr 3q. A loss of 114 dBrkm, which is the lowest for high-D n InF3-based fibers w10x, is achieved at 1.23 mm. These loss properties show that the losses of InF3-based fibers, which have higher potential as an amplification medium because of their lower phonon energies,
Fig. 7. Signal gain dependence on pump power at 1.30 mm for Pr 3q-doped InF3-based fiber. The line is drawn to show the departure of gain from a linear dependence on pump power.
T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
of the loss of the fiber and so it is expected that a high gain coefficient far above 0.2 dBrmW will be realized when this loss is reduced to the level of ZrF4-based fibers.
4. Conclusions We demonstrated that low-loss, high-D n, ZrF4based fluoride single-mode fibers with high mechanical strength could be fabricated by a modified jacketing method. We also showed that a low-loss, highD n, InF3-based fluoride single-mode fiber could be provided by this method. This study confirmed that the modified jacketing method using a preform produced by suction casting and a tapered jacketing tube for fiber drawing offered advantages as regards fabricating fluoride fiber for optical amplifiers.
Acknowledgements The authors would like to thank K. Oikawa for transmission loss measurements. They also thank M.
125
Yamada for useful discussions, and Dr H. Hiratsuka for his encouragement. References w1x Y. Ohishi, T. Kanamori, T. Kitagawa, S. Takahashi, E. Snitzer, G.H. Sigel Jr., Opt. Lett. 16 Ž1991. 1747. w2x Y. Ohishi, T. Kanamori, T. Nishi, S. Takahashi, IEEE Photo Technol. Lett. 3 Ž1991. 715. w3x J.F. Marcerou, S. Artigaud, J. Hervo, H, Fevrier, Tech. Dig. ´ of ECOC’92, Berlin, 1992, MoA2.3. w4x T. Kanamori, Y. Terunuma, K. Fujiura, K. Oikawa, S. Takahashi, NTT R&D 39 Ž1990. 1353. w5x T. Kanamori, Y. Terunuma, K. Fujiura, Y. Ohishi, S. Sudo, Proc. 9th Int. Symp. Non-Oxide Glasses, Hangzhou, 1994, p. 74. w6x Y. Ohishi, S. Sakaguchi, S. Takahashi, Electron Lett. 22 Ž1986. 1034. w7x D.C. Tran, C.F. Fisher, G.H. Sigel Jr., Electron Lett. 18 Ž1982. 657. w8x T. Kanamori, K. Oikawa, Y. Terunuma, K. Kobayashi, S. Takahashi, Jpn. J. Appl. Phys. 28 Ž1989. L1979. w9x Y. Nishida, Y. Ohishi, T. Kanamori, Y. Terunuma, K. Kobayashi, S. Sudo, Tech. Dig. of ECOC’93, Montreux, 1993, TuC3.1. w10x H. Yanagita, K. Itoh, E. Ishikawa, H. Aoki, H. Toratani, Tech. Dig. of OFC’95, San Diego, 1995, PD2.
Section 3. Applications
Fabrication of fluoride single-mode fibers for optical amplifiers T. Kanamori
a,)
, Y. Terunuma b, Y. Nishida a , K. Hoshino a , K. Nakagawa a , Y. Ohishi a , S. Sudo a b
a NTT Opto-Electronics Laboratories, Tokai, Ibaraki, 319-11, Japan NTT Electronic Technology Corporation, Tokai, Ibaraki, 319-11, Japan
Abstract A modified jacketing method, which uses a preform produced by suction casting and a tapered jacketing tube for fiber drawing, was developed to fabricate low-loss, high-D n fluoride single-mode fibers for optical amplifiers. A fabricated 500 m long ZrF4-based fiber had a constant core diameter and an almost constant mean strength of ; 435 MPa over its whole length. Low transmission losses below 20 dBrkm were achieved in rare earth-doped ZrF4-based single-mode fibers with a D n of 3.7%. An InF3-based single-mode fiber with a D n of 3.6%, a loss of 114 dBrkm at 1.23 mm and a signal gain coefficient of 0.18 dBrmW at 1.30 mm was fabricated using this jacketing method.
1. Introduction Fluoride glass fiber is a promising candidate for use in fabricating optical amplifiers because of the low phonon frequency properties of fluoride glasses w1x. It has already been confirmed that a Pr 3q-doped fluoride fiber amplifier, operating at 1.3 mm, has the potential for high gain, a high saturated output power, and a large bandwidth w2x, and that an Er 3q-doped fluoride fiber amplifier operating at 1.5 mm has a larger bandwidth than a silica fiber amplifier w3x. These gain characteristics have been achieved by developing low-loss high-D n fluoride single-mode fibers. The tapered-elongation and jacketing drawing method w4x shows promise as a method for fabricating low-loss high-D n fibers. In this method the
)
Corresponding author. Tel.: q81-29 287 7519; fax: q81-29 287 7193; e-mail: [email protected].
preforms are made by suction casting, which rapidly quenches the core and cladding liquids into glass. This rapid quenching is essential for producing high quality preforms with few scattering centers from fluoride glasses for high-D n fibers, because these glasses have a tendency to crystallize easily. Indeed, the high-D n fibers with the lowest reported loss were produced using this jacketing method w5x. With this method, two special processes are necessary to fabricate single-mode fibers. The tapered core of a preform, produced by suction casting, is reshaped into a smaller straight core in an elongation process. This core is achieved by programming the elongation speed as a function of time. The elongated preform is then polished into a cylindrical shape. This paper reports a modified jacketing method for fabricating single-mode fiber, which has been developed by using a preform produced by suction casting and a tapered jacketing tube for fiber drawing. In this method, the two reshaping processes required in the previously developed jacketing
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 7 . 0 0 0 5 2 - 5
T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
122
method are eliminated. Moreover, we describe the properties of high-D n ZrF4-based fibers and high-D n InF3-based fibers.
2. Experimental
tapered core. The appropriate section of the secondary preform was inserted into a jacketing tube with a corresponding taper in its outer diameter and placed in the drawing furnace. A 500 m long singlemode optical fiber coated with UV-curable acrylate was drawn at a constant tension. This was achieved by regulating the temperature of the furnace.
2.1. Single-mode fiber fabrication 2.2. Measurements Fig. 1 shows the fabrication processes of the modified jacketing method using a preform produced by suction-casting and a tapered jacketing tube for fiber drawing. The preforms and jacketing tubes for elongation, which were cylindrical, were prepared by conventional suction-casting and rotation casting, respectively w6,7x. The jacketing tubes for fiber drawing were produced by the rotation casting method using a mold with a tapered hole. Each stage of the melting and casting processes was carried out in a clean dry gas atmosphere w8x. Fluorides of better than 99.99% purity were used. Two glass systems were synthesized for high-D n fibers. One was ZrF4 –HfF4 – PbF2 –BaF2 –LaF3 –YF3 –AlF3 –LiF–NaF w5x and the other was InF3 –GaF3 –PbF2 –BaF2 –SrF2 –LaF3 – YF3 –LiF–NaF w9x. The core and cladding materials with NH 4 F P HF in crucibles were melted in an Ar q HF gas flow at 8508C for 2 h, and metal molds were preheated at 2508C. The liquids were cast in the molds to produce a preform and a jacketing tube by means of suction-casting and rotation casting, respectively. The straight preform with a tapered core was then elongated together with the straight jacketing tube into a secondary straight preform with a smaller
The core diameters of the fabricated fibers were measured by using SEM observations of fracture surfaces. We performed tensile strength tests on the fibers at a strain rate of 0.03 miny1 , a gauge length of 350 mm and at room temperature. The transmission loss spectra in the 0.5–2.1 mm wavelength region were measured using the cutback technique with a halogen lamp as the light source. Monochromatic light selected using step-scanning grating monochromators and filters was launched into a fiber sample at the same numerical aperture as the fiber. We detected the fiber output signals with a silicon– germanium photodiode and an In–Sb detector. The fiber lengths for the loss measurements were 70–100 m. The signal gain dependence on pump power was measured using the fabricated Pr 3q-doped InF3-based fiber. The fiber was butt-jointed to wavelength-division-multiplexing couplers and forward pumped using a Ti: sapphire laser. The pump wavelength was 1.015 mm, and the signal wavelength was 1.30 mm. The signals were detected with an optical spectrum analyzer.
3. Results and discussion 3.1. Longitudinal fluctuations in core diameter and tensile strength of high-D n ZrF4-based fiber
Fig. 1. Modified jacketing method.
Fig. 2 shows the longitudinal fluctuations in the core diameter of a drawn fiber 500 m in length and with a cladding diameter of 120 mm. These fluctuations are less than 3% over the whole fiber length. This indicates that the taper of an elongated core is easily compensated for by using a tapered jacketing tube and that a small core can be uniformly formed for high-D n fiber using our modified jacketing method.
T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
Fig. 2. Core diameter fluctuation for a fiber fabricated by the modified jacketing method. The line is drawn as a guide for the eye.
Fig. 3 shows Weibull plots of tensile strengths for three sections of a fiber. These data were sampled at the beginning, the middle, and the end of a 500 m long fiber, which was drawn from sections of a tapered jacketing tube with small, medium, and large outer diameters, respectively. The strength values are distributed over a narrow range of 350–500 MPa and the mean values are 438 MPa at the beginning, 411 MPa at the middle, and 463 MPa at the end of the fiber. SEM observations of the fractured surfaces revealed that all the fractures were caused by surface flaws. We observed no fractures resulting from inner
Fig. 3. Weibull plots of tensile strengths for three sections of a fiber. Solid line: the beginning section of a drawn fiber; dotted line: the middle section of a drawn fiber; broken line: the end section of a drawn fiber.
123
flaws. These SEM results, which are also represented by the high minimum strength values and the narrow strength distribution in Fig. 3, indicate that an interface is successfully formed between the preform or the secondary preform and the jacketing tube. In jacketing drawing with a tapered tube, the furnace temperature is regulated to keep the drawing tension constant. This means that the furnace temperature is increased as the outer diameter of the jacketing tube increases. From Fig. 3, it is clear that the change in furnace temperature resulting from the use of a tapered jacketing tube has no influence on fiber strength. The strength characteristics we obtained indicate that single-mode fibers with high uniform mechanical strength over their whole length can be fabricated using this modified jacketing method. 3.2. Losses of rare earth-doped ZrF4-based fibers Fig. 4 shows a typical transmission loss spectrum for Er 3q-doped ZrF4-based fiber. The core diameter, cladding diameter, D n, and Er 3q concentration of the fabricated fiber were 2.2 mm, 120 mm, 3.7%, and 1000 ppm, respectively. The large peaks in the 0.6–2.0 mm wavelength range are caused by Er 3q. The small peak observed at 1.13 mm is due to the cutoff of the first higher-order mode ŽLP11 mode. and the cutoff wavelength Ž lc . can be determined as 1.20 mm from the cutoff peak. This lc is coincident with l c calculated from the observed values of the core diameter, D n and the refractive index of core glass. This clear cutoff peak indicates that a singlemode waveguide structure is successfully formed by
Fig. 4. Transmission loss spectrum of Er 3q-doped ZrF4-based fiber.
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T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
Fig. 5. Transmission loss spectrum of Pr 3q-doped ZrF4 -based fiber.
Fig. 6. Transmission loss spectrum of Pr 3q-doped InF3 -based fiber.
the modified jacketing method. Fig. 5 shows a typical transmission loss spectrum for Pr 3q-doped ZrF4based fiber with a core diameter of 1.9 mm, a cladding diameter of 120 mm, a D n of 3.7%, and a Pr 3q concentration of 500 ppm. The large peaks at wavelengths of 0.6 and 1.0 mm are caused by Pr 3q. As shown in Figs. 4 and 5, losses of 19 and 20 dBrkm are obtained at 1.78 mm for the Er 3q-doped fiber and at 1.20 mm for the Pr 3q-doped fiber, respectively, while a Pr 3q-doped high-D n fiber fabricated by the tapered-elongation and jacketing drawing method had a loss of 44 dBrkm at 1.20 mm w5x. The low-loss characteristics we obtained indicate that the modified jacketing method has great potential for suppressing crystallization during single-mode fiber fabrication processes in the same way as the tapered-elongation and jacketing drawing method.
approach a practical level for use in optical amplifiers. The difference between the losses of the InF3based fiber and the ZrF4-based fiber is attributed to a scattering loss due to crystallization. In order to reduce the loss to the level of ZrF4-based fibers, the thermal stability of InF3-based glasses must be improved and impurities such as oxide must be eliminated from the raw materials. Fig. 7 shows the signal gain dependence on pump power at 1.30 mm for Pr 3q-doped InF3-based fiber. A gain coefficient of 0.18 dBrmW at 1.30 mm is achieved in the fiber with a D n of 3.6% and a loss of 114 dBrm at 1.20 mm. This gain coefficient is comparable to that for ZrF4-based fiber w5x but it is low compared with the expected value for InF3-based fiber. This low measured value is mainly the result
3.3. Loss and gain of Pr 3 q-doped InF3-based fiber A typical transmission loss spectrum for Pr 3qdoped InF3-based fiber is shown in Fig. 6. The length, core diameter, cladding diameter, D n, lc , and Pr 3q concentration of the fabricated fiber were 500 m, 1.9 mm, 120 mm, 3.6%, 1.0 mm, and 500 ppm, respectively. The large peaks around 0.6 mm and 1 mm are caused by Pr 3q. A loss of 114 dBrkm, which is the lowest for high-D n InF3-based fibers w10x, is achieved at 1.23 mm. These loss properties show that the losses of InF3-based fibers, which have higher potential as an amplification medium because of their lower phonon energies,
Fig. 7. Signal gain dependence on pump power at 1.30 mm for Pr 3q-doped InF3-based fiber. The line is drawn to show the departure of gain from a linear dependence on pump power.
T. Kanamori et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 121–125
of the loss of the fiber and so it is expected that a high gain coefficient far above 0.2 dBrmW will be realized when this loss is reduced to the level of ZrF4-based fibers.
4. Conclusions We demonstrated that low-loss, high-D n, ZrF4based fluoride single-mode fibers with high mechanical strength could be fabricated by a modified jacketing method. We also showed that a low-loss, highD n, InF3-based fluoride single-mode fiber could be provided by this method. This study confirmed that the modified jacketing method using a preform produced by suction casting and a tapered jacketing tube for fiber drawing offered advantages as regards fabricating fluoride fiber for optical amplifiers.
Acknowledgements The authors would like to thank K. Oikawa for transmission loss measurements. They also thank M.
125
Yamada for useful discussions, and Dr H. Hiratsuka for his encouragement. References w1x Y. Ohishi, T. Kanamori, T. Kitagawa, S. Takahashi, E. Snitzer, G.H. Sigel Jr., Opt. Lett. 16 Ž1991. 1747. w2x Y. Ohishi, T. Kanamori, T. Nishi, S. Takahashi, IEEE Photo Technol. Lett. 3 Ž1991. 715. w3x J.F. Marcerou, S. Artigaud, J. Hervo, H, Fevrier, Tech. Dig. ´ of ECOC’92, Berlin, 1992, MoA2.3. w4x T. Kanamori, Y. Terunuma, K. Fujiura, K. Oikawa, S. Takahashi, NTT R&D 39 Ž1990. 1353. w5x T. Kanamori, Y. Terunuma, K. Fujiura, Y. Ohishi, S. Sudo, Proc. 9th Int. Symp. Non-Oxide Glasses, Hangzhou, 1994, p. 74. w6x Y. Ohishi, S. Sakaguchi, S. Takahashi, Electron Lett. 22 Ž1986. 1034. w7x D.C. Tran, C.F. Fisher, G.H. Sigel Jr., Electron Lett. 18 Ž1982. 657. w8x T. Kanamori, K. Oikawa, Y. Terunuma, K. Kobayashi, S. Takahashi, Jpn. J. Appl. Phys. 28 Ž1989. L1979. w9x Y. Nishida, Y. Ohishi, T. Kanamori, Y. Terunuma, K. Kobayashi, S. Sudo, Tech. Dig. of ECOC’93, Montreux, 1993, TuC3.1. w10x H. Yanagita, K. Itoh, E. Ishikawa, H. Aoki, H. Toratani, Tech. Dig. of OFC’95, San Diego, 1995, PD2.