Optical fiber for optical amplifier

Abstract: Fluoride glass-based optical fiber for an optical amplifier which contains rare earth metal ions in the core glass has a relative refractive index difference Δn between the core and the cladding of 1.4% or more. The core glass contains PbF₂ in a proportion of 25 mol% or less based on the total composition of the core glass. The fluoride glass is doped with rare earth metal ions, and part of the fluorine in the glass may be substituted by at least one halogen. Pr³⁺, Pr³⁺ -Yb³⁺, Pr³⁺ -Nd³⁺, or Pr³⁺ -Er³⁺ can be doped as the rare earth metal ions. Chlorine, bromine or iodine may be used as the halogen.

Description: The present invention relates to an optical fiber for use in a 1.3 µm band optical fiber amplifier to be used in optical communication systems, and more specifically, to a rare earth metal ion-doped fluoride-based optical fiber having a high gain coefficient for optical amplifiers. In recent years, studies have been energetically performed on optical fiber amplifiers which involve doping of the core of an optical fiber with rare earth metal ions, especially Er 3+ ions, to induce stimulated emission as a result of transition within the 4f shell, so that they may be applied to 1.5 µm optical communication systems. Rare earth metal ion-doped optical fiber amplifiers are high in gain and have gain characteristics not dependent on polarization of beam. They also have a low noise index and broad-band wavelength characteristics. Therefore, their application to optical communication system is very attractive. The 1.3 µm band, in which wavelength dispersion in a quartz-based optical fiber becomes zero, is an important wavelength band for optical communication along with the 1.5 µm band. Researches into optical fiber amplifiers operating at 1.3 µm have been carried out using Nd 3+ ion doped silica-based optical fibers or fluoride optical fibers. In both optical fibers, however, the excited state absorption of Nd 3+ ions is great at a wavelength of 1.31 µm which is used in optical communication. Therefore, no amplification has been confirmed as described, for example, in W. J. Miniscalco, L. J. Andrews, B. A. Thompson, R. S. Quimby, L. J. B. Vacha and M. G. Drexhage, "Electron. Lett." (vol. 24, 1988, p. 28), or Y. Miyajima, T. Komukai, Y. Sugawa and Y. Katsuyama, "Technical Digest Optical Fiber Communication Conference '90 San Francisco" (1990, PD16). Under these circumstances, there is a keen demand for optical fiber amplifiers which have an amplifying effect at 1.31 µm. One of the candidates is an optical fiber amplifier using an optical fiber comprising a ZrF 4 -based fluoride glass as a host material and being doped with Pr 3+ as laser-activating ions, as proposed in Y. Ohishi, T. Kanamori, T. Kitagawa, S. Takanashi, E. Snitzer and G. H. Sigel "Technical Digest Optical Fiber Communication Conference '91 San Diego" (1991, PD2). This optical fiber amplifier utilizes the stimulated emission of the 1 G 4 - 3 H 5 transition of Pr 3+ ions, as seen from the energy level diagram of Pr 3+ ions in Fig. 1. That is, light emission at the 1.3 µm band is due to the transition from 1 G 4 to 3 H 5 . As shown in Fig. 1, the lower level 3 H 5 is higher than the ground state 3 H 4 , thus forming a four level system. The central wavelength of emission is 1.322 µm. The wavelength used for pumping is 1.017 µm which causes excitation from the ground level 3 H 4 directly to the upper level 1 G 4 . The above-mentioned optical fiber amplifier, however, is defective in that the energy difference between the 1 G 4 level and the 3 F 4 level is as small as about 3,000 cm -1 . In detail, the phonon energy of the ZrF 4 -based fluoride glass as the host material is 500 cm -1 . Hence, phonon relaxation from the 1 G 4 level to the 3 F 4 level easily occurs, and the quantum efficiency of the 1 G 4 - 3 H 5 transition is as low as 3% in the ZrF 4 -based fluoride glass, thus making the degree of amplification per unit pump power, i.e. gain coefficient, remain at about 0.2 dB/mW (Y. Ohishi, T. Kanamori, J. Temmyo, M. Wada, M. Yamada, M. Shimizu, K. Yoshino, H. Hanafusa, M. Horiguchi and S. Takahashi, Electronics Letters, vol. 27, no. 22, pp. 1995-1996, 1991). Thus, a high pump power of 25 mW is required for obtaining a gain of, for example, 5 dB. Since the output power of a laser diode has its own limitations, however, an increase in the gain coefficient of a rare earth metal ion-doped optical fiber for an optical amplifier is necessary in order to obtain a sufficiently high gain for practical use by pumping the laser diode. One measure for realizing the high efficiency of an optical fiber amplifier is to incorporate PbF 2 into zirconium glass, thereby increasing the refractive index of the core. With a conventional method in which BaF 2 is substituted by PbF 2 , however, the difference (ΔT) between the crystallization temperature (Tx) and the glass transition temperature (Tg) of the core glass (ΔT is generally used as an indicator of the heat stability of glass) decreases with the increase in the PbF 2 content. Hence, the core crystallizes upon heating during optical fiber fabrication, resulting in an increased transmission loss, thereby decreasing an effective gain. Therefore, the relative refractive index difference Δn between the core and the cladding of the fluoride optical fiber is at most 1.2%. The object of the present invention, which is defined in the appended claims, is to solve the above-described problems and provide an optical fiber for an optical amplifier capable of operating with sufficient gains at the 1.3 µm band. The present inventors conducted extensive studies and found that the object could be attained by increasing the core-cladding relative refractive index difference Δn, thereby increasing the pump power density in the core of the optical fiber. The present invention has been accomplished based on this discovery. The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. Fig. 1 is an energy level diagram illustrating energy levels of Pr 3+ ; Fig. 2A is a graph illustrating a gain characteristic of an optical fiber, plotting signal gain per pump power of 100 mW vs. core radius; Fig. 2B is a graph illustrating a gain characteristic of an optical fiber, plotting signal gain per pump power of 100 mW vs. core radius; Fig. 3 is a block diagram illustrating an arrangement of an optical amplifier to which the optical fiber for an optical amplifier in accordance with an embodiment of the present invention is applicable; Fig. 4 is a graph illustrating changes in the relative refractive index difference with the PbF 2 content in the optical fiber according to Example 1 of the present invention in which the composition of the core glass is 49ZrF 4 -(25-x)BaF 2 -xPbF 2 -3.5LaF 3 -2YF 3 -18LiF (mol%), and the composition of the cladding glass is 23.7ZrF 4 -23.8HfF 4 -23.5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -4.5NaF (mol%); Fig. 5 is a graph illustrating changes in the glass transition temperature, crystallization temperature, and refractive index with the LiF concentration (y mol%) in the optical fiber according to Example 1 of the present invention in which the composition of the core glass is (59.5-0.5y)ZrF 4 -2yBaF 2 -(32.5-2.5y)PbF 2 -3.5LaF 3 -2YF 3 -2.5AlF 3 -yLiF (mol%); Fig. 6 is a graph illustrating the relationship between the LiF concentration and the relative refractive index difference (Δn) between the core and the cladding of the optical fiber according to Example 1 of the invention in which the composition of the cladding glass is 47.5ZrF 4 -23.5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%); Fig. 7 is a graph illustrating the amplification characteristics of an optical fiber amplifier having the optical fiber of Example 1 of the invention in which the core is doped with 500 ppm of Pr 3+ ; Fig. 8 is a graph illustrating changes in the difference (ΔT) between the crystallization temperature and the glass transition temperature and in refractive index with PbF 2 concentration (x mol%) in the optical fiber according to Example 7 of the present invention in which the composition of the core glass is 35InF 3 -25ZnF 2 -(25-x)BaF 2 -10SrF 2 -xPbF 2 -4AlF 3 -LaF 3 (mol%); Fig. 9 is a graph illustrating the relative refractive index difference with the PbF 2 concentration (x mol%) in the optical fiber according to Example 7 of the present invention in which the composition of the core glass is 35InF 3 -25ZnF 2 -(25-x)BaF 2 -10SrF 2 -xPbF 2 -4AlF 3 -LaF 3 (mol%), and the composition of the cladding glass is 35InF 3 -25ZnF 2 -15BaF 2 -10SrF 2 -5CdF 2 -5AlF 3 -5LaF 3 (n D =1.495); Fig. 10 is a graph illustrating changes in the difference (ΔT) between the crystallization temperature and the glass transition temperature and in the refractive index with the PbF 2 concentration (x mol%) in the optical fiber according to Example 12 of the present invention in which the composition of the core glass is 30InF 3 -25ZnF 2 -(25-x)BaF 2 -9SrF 2 -xPbF 2 -4GaF 3 -4YF 3 -3NaF (mol%); Fig. 11 is a graph illustrating the relative refractive index difference with the PbF 2 concentration (x mol%) in the optical fiber according to Example 12 of the invention in which the composition of the core glass is 30InF 3 -25ZnF 2 -(25-x)BaF 2 -9SrF 2 -xPbF 2 -4GaF 3 -4YF 3 -3NaF (mol%), and the composition of the cladding glass is 30InF 3 -25ZnF 2 -17BaF 2 -11SrF 2 -5CdF 2 -4AlF 3 -4YF 3 -4NaF (n D =1.495); Fig. 12 is a graph illustrating changes in the refractive index with the GaF 3 content in the optical fiber according to Example 12 of the present invention in which the composition of the core glass is 30InF 3 -25ZnF 2 -15BaF 2 -9SrF 2 -9PbF 2 -xGaF 3 -(5-x)AlF 3 -4YF 3 -3NaF (mol%); and Fig. 13 is a graph illustrating dependence, on the pump power, of gain in the optical fiber amplifier of the present invention using a mixed halide glass optical fiber in which the composition of the core glass is 49ZrF 4 -17BaF 2 -8BaCl 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -18.0LiF (mol%), and the composition of the cladding glass is 49ZrF 4 -10BaF 2 -15BeF 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -18LiF (mol%), along with the characteristics of a comparison optical fiber amplifier in which the composition of the core glass is 53ZrF 4 -16BaF 2 -15PbF 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -8LiF (mol%), and that of the cladding glass is 47.5ZrF 4 -23.5BaF 2 -2.5LaF 3 -2.0YF 3 -4.5AlF 3 -20NaF (mol%). Examples of the fluoride glass constituting the optical fiber for an optical amplifier in accordance with the present invention are ZrF 4 glass, InF 3 glass, and AlF 3 glass (Tomozawa and Poremus, Treatise on Materials Science and Technology, volume 26, Chapter 4, Academic Press, Inc., 1985). It is also possible to use a mixed halide glass prepared by substituting part of the fluorine in the composition of the above-mentioned fluoride glasses by one or more other halogens, such as chlorine (Cl), bromine (Br) or iodine (I). A known method for obtaining high An intended to increase the power density of pump light in the core of the optical fiber is to incorporate PbF 2 to thereby increase the refractive index of the core. The substitution of BaF 2 by PbF 2 , however, lowers the glass transition temperature and the crystallization temperature of the core glass. Therefore, the core crystallizes due to heat generated during fiber fabrication, resulting in an increased transmission loss, which causes a decrease in effective gain. In the light of these drawbacks, the present invention increases the relative refractive index difference between the core and the cladding, and incorporates no more than a specific amount (25mol%) of PbF 2 in the core glass, thereby raising the power density of pump light in the core. According to one embodiment of the present invention, PbF 2 (x mol%) is incorporated into the core glass, together with LiF (y mol%) which latter is in an amount of 3 to 12 mol% and within the range (11 - 0.4x) < y < (15 - 0.4x). Owing to this incorporation, the glass transition temperature of the core glass does not depend on the PbF 2 concentration, and the crystallization temperature does not decrease. Thus, the optical fiber can be made to have a high Δn (>1.4%), using the same cladding glass. At the same time, crystallization ascribed to heating during drawing of the optical fiber is also inhibited, so that the transmission loss of the optical fiber can be minimized. Accordingly, effective gain is increased when the optical fiber is applied to an optical fiber amplifier. According to another embodiment of the present invention, an indium-based fluoride glass having no high absorption in the infrared wavelength region is used as the core glass, whereby the fluorescence lifetime of the active ions in the host glass is prolonged compared with the use of a zirconium-based fluoride glass, and optical amplification is performed with a higher efficiency. PbF 2 incorporated for increasing the refractive index acts to prevent the formation of a stable glass network in zirconium-based fluoride glass, while it works to stabilize glass as network-modifying ions along with BaF 2 and SrF 2 in indium-based fluoride glass. Consequently, there is no decrease in ΔT according to the PbF 2 content. Thus, the use of a conventional zirconium type fluoride glass or an indium type fluoride glass as the cladding glass permits a high Δn of the optical fiber, and suppresses crystallization due to heating during drawing of the optical fiber, whereby the transmission loss of the optical fiber can be minimized. If this optical fiber is applied to an optical fiber amplifier, gain coefficient will increase. According to yet another embodiment of the present invention, an indium-based fluoride glass doped with GaF 3 is used, aiming at an even higher efficiency in optical amplification. That is, GaF 3 incorporated as a modifier for achieving stable glass network formation has an infrared absorption on the longer wavelength side than does AlF 3 . Therefore, it is effective for prolonging the lifetime of the active ions, i.e. for increasing the efficiency of optical amplification. According to still another embodiment of the present invention, part of the fluorine contained in the fluoride glass is substituted by at least one other halogen, thereby making it possible to improve the quantum efficiency of the 1 G 4 - 3 H 5 transition of Pr 3+ , and achieve a high efficiency in optical amplification. A halide glass such as chloride glass, bromide glass or iodide glass is known as a glass with a smaller phonon energy than that of a fluoride glass. If one of these halide glasses is used as the host material of an optical fiber for an optical amplifier, it is expected that a Pr 3+ -doped optical fiber amplifier with a high efficiency can be provided. These halide glasses, however, are highly deliquescent, and unsuitable for practice use. So-called mixed halide glasses comprising fluoride glass mixed with at least one of chloride glass, bromide glass, and iodide glass, on the other hand, are highly durable and suitable for practical use. The use of the mixed halide glass as the host material of an optical fiber doped with rare earth metal ions such as Pr 3+ , therefore, can provide a highly practical Pr 3+ -doped optical fiber for an optical amplifier. In the case of ZrF 4 glass as the host material, for example, the phonon energy of the Zr-F bond is about 500 cm -1 . This phonon energy contributes mainly to multiphonon relaxation from 1 G 4 to 3 F 4 . When Cl, Br or I is doped into the ZrF 4 glass, the fluorine of some Zr-F bonds is substituted by Cl, Br or I. This substitution results in the formation of Zr-Cl, Zr-Br or Zr-I bonds with a lower phonon energy than the phonon energy of the Zr-F bonds. As a result, the probability of multiphonon relaxation from 1 G 4 to 3 F 4 lowers. This increases the quantum efficiency of the 1 G 4 - 3 F 5 transition, and also improves gain coefficient. The fluoride glass as the core glass that can be used for the optical fiber for an optical amplifier in accordance with the present invention includes, for example, the following: (a) Fluoride glass with a core matrix containing 50 to 60 mol% of ZrF 4 , 7 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , 1.5 to 6 mol% of LaF 3 , 0 to 4 mol% of at least one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of AlF 3 , and 3 to 12 mol% of LiF, with the total amount of the components being 100 mol%. In the case of deviation from the above range, i.e. 50 to 60 mol% of ZrF 4 , 7 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , 1.5 to 6 mol% of LaF 3 , 1.5 to 5 mol% of AlF 3 , and 3 to 12 mol% of LiF, with the total amount of these components being 100 mol%, no glass will be formed. Therefore, it is necessary that the amounts of the respective components be within the above-described range. If at least one of ScF 3 , YF 3 , GdF 3 and LuF 3 is added in an amount of 4 mol% or less to the above-mentioned fundamental components so that the total amount of the components may be 100 mol%, there will be a favorable effect that the glass is stabilized. (b) Fluoride glass with a core matrix containing 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 5 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 7 mol% of CdF 2 , 0 to 10 mol% of LaF 3 , 0 to 4 mol% of at least one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 0 to 5 mol% of AlF 3 , and 0 to 15 mol% of LiF or NaF, with the total amount of the components being 100 mol%. In the case of deviation from the above range, i.e. 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 5 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , and 5 to 15 mol% of SrF 2 (the total amount: 100 mol%), no glass will be formed. Therefore, it is necessary that the amounts of the respective components be within the above-described range. If 7 mol% or less of CdF 2 , 10 mol% or less of LaF 3 , 0 to 4 mol% of at least one of ScF 3 , YF 3 , GdF 3 and LuF 3 , 5 mol% or less of AlF 3 , 15 mol% or less of LiF or NaF are added to the above mentioned fundamental components in such amounts that the total amount of the components may be 100 mol%, there will be a favorable effect that the glass is stabilized. In this case, if these additional components are incorporated in amounts exceeding the above-mentioned amounts, the system will become microcrystalline and no glass will be formed. It is necessary, therefore, that the amounts of these additional components be within the above range. (c) Fluoride glass with a core matrix containing 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF2, 6 to 30 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 25 mol% of PbF 2 , 0 to 7 mol% of CdF 2 , 1.5 to 6 mol% of at least one member selected from the group consisting of LaF 3 , ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of GaF 3 , and 2 to 10 mol% of LiF or NaF, with the total amount of the components being 100 mol%. In the case of deviation from the above range, i.e. 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF2, 6 to 30 mol% of BaF 2 , and 5 to 15 mol% of SrF 2 , 1.5 to 6 mol% of at least one of LaF 3 , ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of GaF 3 , and 2 to 10 mol% of LiF or NaF (the total amount: 100 mol%), no glass will be formed. Therefore, it is necessary that the amounts of the respective components be within the above-described range. If 25 mol% or less of PbF 2 or 7 mol% or less of CdF 2 are added to the above-mentioned fundamental components in such amounts that the total amount of the components may be 100 mol%, there will be a favorable effect that the glass is stabilized. In this case, if these additional components are incorporated in amounts exceeding the above-mentioned amounts, the system will become microcrystalline and no glass will be formed. It is necessary , therefore, that the amounts of these additional components be within the above range. The matrix composition of the cladding glass that can be used for the optical fiber for an optical amplifier in accordance with the present invention includes, for example, the following: (d) Fluoride glass containing 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 5 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 7 mol% of CdF 2 , 0 to 10 mol% of LaF 3 , 0 to 4 mol% of at least one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 0 to 5 mol% of AlF 3 , and 0 to 15 mol% of LiF or NaF, with the total amount of the components being 100 mol%. (e) Fluoride glass containing 43 to 55 mol% of at least one member selected from the group consisting of ZrF 4 and HfF 4 , 18 to 28 mol% of BaF 2 , 0 to 10 mol% of PbF 2 , 1.5 to 6 mol% of LaF 3 , 0 to 4 mol% of one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of AlF 3 , and 15 to 25 mol% of at least one member selected from the group consisting of LiF and NaF, with the total amount of the components being 100 mol%. (f) Fluoride glass containing 43 to 55 mol% of at least one member selected from the group consisting of ZrF 4 and HfF 4 , 18 to 28 mol% of BaF 2 , 1.5 to 6 mol% of LaF 3 , 0 to 4 mol% of one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of AlF 3 , and 15 to 25 mol% of at least one member selected from the group consisting of LiF and NaF, with the total amount of the components being 100 mol%. (g) Fluoride glass containing 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 6 to 30 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 25 mol% of PbF 2 , 0 to 7 mol% of CdF 2 , 1.5 to 6 mol% of at least one member selected from the group consisting of LaF 3 , ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of AlF 3 or GaF 3 and 2 to 10 mol% of LiF or NaF, with the total amount of the components being 100 mol%. A combination of the core (b) and the cladding (d) or (f), and a combination of the core (c) and the cladding (f) or (g) are preferred. In the case of the optical fiber having the combination of the core (b) and the cladding (f), the use of a zirconium-based fluoride glass for the cladding will enable the glass transition temperature and crystallization temperature comparable to those of the core to be achieved with the cladding, and also permit Δn to become great. The rare earth metal ions to be doped into the core of the optical fiber for an optical amplifier in accordance with the invention may be one member selected from the group consisting of Pr 3+ , Pr 3+ -Yb 3+ , Pr 3+ -Nd 3+ , and Pr 3+ -Er 3+ . Typically, Pr 3+ is used, but its codoping with other ions will be effective for optical amplification, because it will result in the stimulated emission due to transition within the 4f shell as in the case of Pr 3+ . The doping of rare earth metal active ions such as Pr, Yb, Nd or Er makes it possible to achieve optical amplification at various wavelengths, thus permitting high-performance optical communication systems. The diameter of the optical fiber is generally 2 µm or less, preferable 1.95 µm or less. EXAMPLES Working examples of the present invention will be described in detail with reference to the appended drawings, but the invention is in no way restricted to the Examples. Example 1 Figs. 2A and 2B are each a graph illustrating an optical characteristic of an optical fiber, plotting signal gain per pump power of 100 mW vs. core radius. Signal gain (G) expressed in dB is calculated from the following equation (M. J. F. Digonnet, IEEE Journal of Quantum Electronics, vol. 26, No. 10, P. 1788, 1990): Equation 1: G = 4.3 στ hν p P A F η p where σ is the stimulated emission cross-section of the 1 G 4 - 3 H 5 transition (3.5X10 -22 cm 2 ), τ is the lifetime of the 1 G 4 (110 µsec), h is Planck's constant, ν p is the frequency of pump wavelength, P is pump power, A is the cross-sectional area of the core, and F/η p is a value calculated from the equation: Equation 2: F/η p = a 2 (W s 2 + W p 2 ) 1 1 - exp(-(a/W p 2 )) W s,p = a(0.46 + 1.145Vs ,p -1.5 + 2.036V s,p -6 ) where V s and V p are V values at signal and pump wavelengths, respectively. As shown in Figs. 2A and 2B, when the relative refractive index difference is 1.4 % or more, gain per 100 mW is 10 dB or more. Thus, semiconductor laser pumping can achieve such a practically acceptable gain. Fig. 3 is a block diagram illustrating an arrangement of an optical fiber amplifier using the optical fiber of the present invention. The reference numeral 1 denotes a pump light source (a Ti-sapphire laser with a pump wavelength of 1.017 µm), 2 is a lens, 3 is a photocoupler, 4 is a single source (a distributed feedback (DFB) laser lasing at 1.31 µm), and 5 is a 500 ppm Pr 3+ doped fluoride glass optical fiber of the present invention with a core diameter of 2 µm. The optical fiber used in this Example had a relative refractive index difference of 1.8 %, and a length of 20 m. The composition of the core glass was 49ZrF 4 -22BaF 2 -3PbF 2 -3.5LaF 3 -2YF 3 -2.5AlF 3 -18LiF (mol%), while the composition of the cladding glass was 23.7ZrF 4 -23.8HfF 4 -23.5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%). An output from the optical fiber 5 was guided to an optical spectrum analyzer 7 via an optical fiber pigtail 6 to monitor signal light intensities when the pump light source was turned on and off. Gain, determined by the ratio of these intensities, was found to be 13 dB per pump power of 100 mW. Fig. 4 is a graph illustrating changes in the relative refractive index difference between the core and the cladding in the optical fiber of this Example when the composition of the core glass was 49ZrF 4 -(25-x)BaF 2 -xPbF 2 -3.5LaF 3 -2YF 3 -18LiF (0≦x≦25) was used, with the composition of the cladding glass being the same as described above. By using this core-cladding combination, the relative refractive index difference can be varied from 1.36 % to up to 4.95 %. Therefore, the use of these core and cladding glasses makes it possible to set the relative refractive index difference at 1.4 % or more and to prepare a Pr 3+ -doped optical fiber amplifier with a gain of 10 dB or more per pump power of 100 mW. Table 1 below shows examples of the core radius of the optical fiber which gives a gain of 10 dB or more per pump power of 100 mW when the relative refractive index difference is 1.4 % or more. Relative Refractive Index Difference Core Radius (r) (µm) 1.4 1.02 ≦ r ≦ 1.3 1.6 0.72 ≦ r ≦ 1.5 1.8 0.71 ≦ r ≦ 1.63 2.0 0.62 ≦ r ≦ 1.7 2.5 0.48 ≦ r ≦ 1.84 3.0 0.38 ≦ r ≦ 1.95 ZrF 4 glass was used in this Example, but other fluoride glasses such as InF 3 glass, or AlF 3 glass (Tomozawa and Poremus, Treatise on Materials Science and Technology, volume 26, Chapter 4, Academic Press, Inc., 1985) may be used. Example 2 The amplification characteristics of an optical amplifier with the arrangement shown in Fig. 3 were measured using a 500 ppm Pr 3+ -1,000 ppm Yb 3+ -codoped fluoride optical fiber with a core diameter of 2 µm, a relative refractive index difference of 1.8 % and a length of 5 m as an optical fiber for amplification. Pump wavelength was 980 nm. As a result, a gain of 13 dB per pump power of 100 mW was obtained. With a Pr 3+ -Yb 3+ -codoped optical fiber as shown in this example, Yb 3+ is excited with pump light at 980 nm, and Pr 3+ is excited upon energy transfer from the excited Yb 3+ to Pr 3+ . Since a high-output laser diode operating at the 980 nm band is present, the use of the optical fiber of this Example enables a laser diode-pumped optical amplifier to be constructed, thus facilitating its application to optical communication systems. Example 3 Fig. 5 is a graph illustrating changes in the glass transition temperature, crystallization temperature, and refractive index of the core glass in the optical fiber of the present invention having a core glass composition of (59.5-0.5y)ZrF 4 -2yBaF 2 -(32.5-2.5y)PbF 2 -3.5LaF 3 -2YF 3 -2.5AlF 3 -yLiF (mol%) with the LiF content being y mol%. The glass transition temperature did not depend on the LiF concentration, i.e. PbF 2 concentration, but was almost constant. No decrease in the crystallization temperature was noted at y>3 mol%. The refractive index n D took a value of 1.58 to 1.52 in response to the LiF concentration ranging from 3 to 12 mol%. Fig. 6 is a graph illustrating Δ n when glass with a glass transition temperature comparable to that of the core and a composition of 47.5ZrF 4 -23 .5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%) (n D =1.4952) was used as the cladding. An optical fiber with a high Δ n of 5.4 to 1.6 % relative to the LiF concentration of 3 to 12 mol% was obtained. With an optical amplifier having the same arrangement as shown in Fig. 3 and using 20 m of the optical fiber with Δ n =3.7 % (y=7) having a 500 ppm Pr 3+ -doped core, a high gain coefficient of 0.2 dB/mW was obtained (Fig. 7). Pump wavelength was 1.017 nm. The loss of the resulting optical fiber was as low as 50 dB/km at a wavelength of 1.28 µm. Example 4 The glass transition temperature and crystallization temperature of glass with a core glass composition of (59.5-0.5y)ZrF 4 -(2y-z)BaF 2 -(32.5-2.5y+z)PbF 2 -3.5LaF 3 -2YF 3 -2.5AlF 3 -yLiF (mol%) containing y mol% of LiF in the optical fiber of the present invention were both within ±7°C when the y value was the same as the LiF concentration of 3 to 12 mol% (y mol%) at -5<z<5. The transmission losses of the optical fibers fabricated using the compositions with y=7, z=-4 and y=7, z=4 for the core and the glass described in Example 1 for the cladding were both as low as 100 dB/km at a wavelength of 1.28 µm. This Example indicates that if x mol% of PbF 2 is incorporated, with the LiF concentration = 3 to 12 mol% and (11-0.4x)<y<(15-0.4x), the glass transition temperature and crystallization temperature are almost unchanged within the fluctuation of ±7°C, and an optical fiber with a minimal transmission loss is obtained if the same cladding glass is used. Example 5 Optical fibers were prepared using glass with a composition of 47.5ZrF 4 -23.5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%) for the cladding and using the core glass with compositions shown in Table 2. The resulting optical fibers had low transmission losses (50 to 200 dB/km at a wavelength of 1.28 µm) and high Δ n (>1.4%). Example 6 Optical fibers were prepared in the same manner as in Example 5 except that the fluoride glass with the compositions shown in Table 3 were used as the core glass. The resulting optical fibers had low transmission losses (50 to 200 dB/km at a wavelength of 1.28 µm) and high Δ n (>1.4%). Component (mol %) ZrF 4 BaF 2 PbF 2 LaF 3 ScF 3 YF 3 GdF 3 LuF 3 AlF 3 LiF 60 6 23 6 0 0 0 0 2 3 59 6 24 3 0 0 0 0 5 3 58 6 25 3.5 0 2 0 0 2.5 3 57 10 20 3 0 2.5 0 0 2.5 5 56 9 20 3.5 0 2 0 0 2.5 7 52 17 15 1.5 0.5 0.5 0.5 0.5 2.5 10 52 15 15 1.5 4 0 0 0 2.5 10 52 15 15 1.5 0 0 4 0 2.5 10 52 15 15 1.5 0 0 0 4 2.5 10 52 15 15 2.5 2 1 0 0 2.5 10 52 15 15 2.5 0 1 2 0 2.5 10 52 15 15 2.5 0 1 0 2 2.5 10 52 15 15 1.5 0 4 0 0 2.5 10 Component (mol %) ZrF 4 BaF 2 PbF 2 LaF 3 ScF 3 YF 3 GdF 3 LuF 3 AlF 3 LiF 50 19 15 2 0 2 0 0 2 10 50 19 15 2 2 0 0 0 2 10 50 19 15 2 0 0 2 0 2 10 50 19 15 2 0 0 0 2 2 10 60 6 22 6 0 1.5 0 0 1.5 3 60 6 25 3 1.5 0 0 0 1.5 3 60 13 15 3 0 0 1.5 3 1.5 3 60 13 15 1.5 3 3 0 0 1.5 3 60 17 7 4 0 3 0 0 5 4 60 17 7 4 0 4 0 0 4 4 60 17 7 3 4 0 0 0 5 4 60 17 7 3 0 0 4 0 4 5 60 17 7 3 0 0 0 4 4 5 56 17 7 3 0 0 0 0 5 12 53 17 7 6 0 0 0 0 5 12 Example 7 Fig. 8 illustrates changes in the difference (ΔT) between the crystallization temperature (Tx) and the glass transition temperature (Tg), as well as in the refractive index, with the PbF 2 content (x mol%) when the core glass composition was 35InF 3 -25ZnF 2 -(25-x)BaF 2 -10SrF 2 -xPbF 2 -4AlF 3 -LaF 3 (mol%) in the optical fiber of the present invention. The white circles (o) represent the refractive index, and the black circles (•) the glass transition temperature. As clear from Fig. 8, ΔT was almost constant without depending on the PbF 2 concentration. The refractive index (n D ) was 1.51 to 1.56 for the PbF 2 concentration of 5 to 20 mol%. Therefore, a high Δn optical fiber with Δn of 1.0 to 3.7% is obtained by using as the cladding an indium-based fluoride glass (n D =1.4950 with the composition 35InF 3 -25ZnF 2 -15BaF 2 -10SrF 2 -5CdF 2 -5AlF 3 -5LaF 3 and a glass transition temperature comparable to that of the core (Fig. 9). With an optical amplifier using 20 m of the optical fiber with a Δn of 3.7 % (x=20) and having a core doped with 500 ppm of Pr 3+ , a high gain coefficient of 0.2 dB/mW was obtained. The core diameter of the optical fiber was 2 µm, and pump wavelength was 1,017 nm. The transmission loss of the resulting optical fiber was low (100 dB/km) at a wavelength of 1.25 µm. Example 8 High Δn optical fiber with a Δn of 1.0 to 3.7 % was obtained by using, as the cladding, a zirconium-based fluoride glass (n D =1,4952) with a composition of 47.5ZrF 4 -23.5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%) and a glass transition temperature comparable to that of the core glass used in Example 7. With an optical amplifier having the same arrangement as in Fig. 3 and using 20 m of the optical fiber (core diameter: 2 µm) with a Δn of 3.3 % (x=15) and having a core doped with 500 ppm of Pr, a high gain coefficient of 0.2 dB/mW was obtained. Pump wavelength was 1,017 nm. The transmission loss of the resulting optical fiber was low (50 dB/km) at a wavelength of 1.25 µm. Example 9 With an optical amplifier having the same arrangement as in Fig. 3 and using 20 m of the optical fiber (core diameter: 2 µm) of Example 1 with a Δn of 3.7 % (x=20) and having a core codoped with 3,000 ppm of Yb and 500 ppm of Pr, a gain of 10 dB at a pump power of 100 mW was obtained when a laser diode operating at 980 nm was used as a pump light source. Example 10 With an optical amplifier having the same arrangement as in Fig. 3 and using 20 m of the optical fiber (core diameter: 2 µm) of Example 1 with a Δn of 3.3 % (x=15) and having a core codoped with 3,000 ppm of Er and 500 ppm of Pr, a gain of 10 dB at a pump power of 100 mW was obtained when a laser diode operating at 980 nm was used as a pump light source. Example 11 An optical fiber with a low transmission loss (50 to 200 dB/km at a wavelength of 1.3 µm) and a high Δn (>2 %) was obtained when glass with a composition of 47.5ZrF 4 -23.5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%) was used for the cladding and indium-based fluoride glass with one of the compositions shown in Table 4 was used as the core glass. Example 12 High Δn optical fiber with a Δn of 1 to 4.0 % was obtained in the same manner as in Example 7, except that the composition of the core glass and that of the cladding glass in the optical fiber of the present invention were selected from those shown in Table 5. An optical amplifier was constructed in the same manner as in Example 7 using the resulting optical fiber. It gave a high gain coefficient and a low transmission loss comparable to those in Example 7. Example 13 Fig. 10 illustrates the difference (ΔT) between the crystallization temperature (Tx) and the glass transition temperature (Tg), as well as in the refractive index, with the PbF 2 content (x mol%; 5≦x≦20) when the core glass composition was 30InF 3 -25ZnF 2 -(25-x)BaF 2 -9SrF 2 -xPbF 2 -4GaF 3 -4YF 3 -3NaF (mol%) in the optical fiber of the present invention. The white circles (o) represent the refractive index, and the black circles (•) the difference (ΔT) between the crystallization temperature and the glass transition temperature. As clear from Fig. 10, ΔT was almost constant without depending on the PbF 2 concentration. The refractive index (n D ) was 1.515 to 1.57 with respect to the PbF 2 concentration of 5 to 20 mol%. Therefore, a high Δn optical fiber with a Δn of 1.3 to 4.7 % is obtained by using as the cladding an indium-based fluoride glass (n D =1.495) with a composition of 30InF 3 -25ZnF 2 -17BaF 2 -11SrF 2 -5CdF 2 -4AlF 3 -4YF 3 -4NaF and a glass transition temperature comparable to that of the core (Fig. 11). With an optical amplifier having the same arrangement as in Fig. 3 and using 20 m of the optical fiber with a Δn of 3.7 % (x=15) and having a core doped with 500 ppm of Pr, a high gain coefficient of 0.2 dB/mW was obtained. The core diameter of the optical fiber was 2 µm, and pump wavelength (λ ex ) was 1,017 nm. The transmission loss of the resulting optical fiber was low (100 dB/km) at a wavelength of 1.25 µm. Example 14 Fig. 12 is a graph illustrating changes in the refractive index with the GaF 3 content (x mol %; 0≦x≦5) when the core glass 30InF 3 -25ZnF 2 -15BaF 2 -9SrF 2 -9PbF 2 -xGaF 3 -(5-x)AlF 3 -4YF 3 -3NaF (mol%) in the optical fiber of the present invention. As clear from Fig. 12, the refractive index (n D ) was 1.513 to 1.515 with respect to the GaF 3 concentration of 0 to 5 mol%. Therefore, a high Δn optical fiber with a Δn of 1.1 to 1.3 % is obtained by using as the cladding an indium-based fluoride glass (n D =1.495) with a composition of 30InF 3 -25ZnF 2 -17BaF 2 -11SrF 2 -5CdF 2 -4AlF 3 -4YF 3 -4NaF (mol%) and a glass transition temperature comparable to that of the core. The transmission loss of the resulting optical fiber was low (100 dB/km) at a wavelength of 1.25 µm. This Example indicates that a low transmission-loss optical fiber with a sufficient core-cladding relative refractive index difference andfree from instability of the glass can be prepared by varying the GaF 3 concentration even when the PbF 2 concentration is constant. Example 15 High Δn optical fiber with a Δn of 1.3 to 4.7 % was obtained by using, as the cladding, zirconium-based fluoride glass (n D =1.4952) with a composition of 47.5ZrF 4 -23.5BaF 2 -2.5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%) and a glass transition temperature comparable to that of the core glass used in Example 13. With an optical amplifier having the same arrangement as in Fig. 3 and using 20 m of the optical fiber (core diameter: 2 µm) with a Δn of 3.3 % (x=15) and having a core doped with 500 ppm of Pr, a high gain coefficient of 0.2 dB/mW was obtained. Pump wavelength was 1,017 nm. The transmission loss of the resulting optical fiber was low (50 dB/km) at a wavelength of 1.25 µm. Example 16 With an optical amplifier having the same arrangement as in Fig. 3 and using the optical fiber (core diameter: 2 µm) of Example 13 with a Δn of 3.7 % (x=20) and having a core codoped with 3,000 ppm of Yb and 500 ppm of Pr, a gain of 10 dB at a pump power of 100 mW was obtained when a laser diode operating at 980 nm was used as a pump light source. Example 17 With an optical amplifier having the same arrangement as in Fig. 3 and using the optical fiber (core diameter: 2 µm) of Example 13 with a Δn of 3.3 % (x=15) and having a core codoped with 3,000 ppm of Er and 500 ppm of Pr, a gain of 10 dB at a pump power of 100 mW was obtained when a laser diode operating at 980 nm was used as a pump light source. Example 18 With an optical amplifier having the same arrangement as in Fig. 3 and using the optical fiber (core diameter: 2 µm) of Example 13 with a Δn of 3.3 % (x=15) and having a core codoped with 3,000 ppm of Nd and 500 ppm of Pr, a gain of 10 dB at a pump power of 100 mW was obtained when a laser diode operating at 790 nm was used as a pump light source. Example 19 An optical fiber with a low transmission loss (50 to 200 dB/km at a wavelength of 1.3 µm) and a high Δn (>2 %) was obtained when glass with a composition of 47.5ZrF 4 (or 47.5HfF 4 )-23.5BaF 2 -2 .5LaF 3 -2YF 3 -4.5AlF 3 -20NaF (mol%) was used for the cladding and indium-based fluoride glass with the composition shown in Table 6 was used as the core glass. Example 20 An optical fiber with a low transmission loss (50 to 200 dB/km at a wavelength of 1.3 µm) and a high Δn (>2 %) was obtained when glass with a composition of 40InF 3 -20ZnF 2 -15BaF 2 -9SrF 2 -9PbF 2 -xGaF 3 -(3-x)AlF 3 -2YF 3 -2NaF (mol%), with the GaF 3 content being x mol% (0≦x≦3), was used for the core and ZrF 4 glass with one of the compositions shown in Table 7 was used as the cladding glass. Example 21 An optical fiber with a low transmission loss (50 to 200 dB/km at a wavelength of 1.3 µm) and a high Δn (>2 %) was obtained when glass with a composition of 30InF 3 -25ZnF 2 -15BaF 2 -9SrF 2 -9PbF 2 -xGaF 3 -(5-x)AlF 3 -4YF 3 -3NaF (mol%), with the GaF 3 content being x mol% (0≦x≦5), was used for the core and ZrF 4 -HfF 4 glass with one of the compositions shown in Table 8 was used as the cladding glass. Composition of Zr-Fluoride Glass for Cladding (mol %) ZrF 4 BaF 2 LaF 3 ScF 3 YF 3 GdF 3 LuF 3 AlF 3 NaF LiF 52 25 2.5 2.5 5 13 50 27 2.5 2.5 5 13 55 22 2.5 2.5 5 13 60 17 2.5 2.5 5 13 56 16 4 4 5 15 53.5 20 6 4 4.5 12 53.5 20 6 4 4.5 12 53.5 20 6 4 4.5 12 53.5 20 6 4 4.5 12 55.5 29 2.5 2.5 3.5 7 55.5 29 4.5 2.5 3.5 5 55.5 27 6 4 4.5 3 55.5 29 2.5 2.5 3.5 7 55.5 29 4.5 2.5 3.5 5 55.5 27 6 4 4.5 3 51.5 26 1.5 4 5 12 52 24 2.5 2 4.5 15 52 24 2.5 2 4.5 15 52 24 2.5 2 4.5 15 52 24 2.5 2 4.5 15 56.5 22 6 0 4.5 11 53.5 21 2.5 2.5 4.5 16 53.5 21 2.5 2.5 4.5 16 53.5 21 2.5 2.5 4.5 16 53.5 21 2.5 2.5 4.5 16 51.5 23 5 4 1.5 15 50 25 6 2.5 4.5 12 54.5 28 2.5 1.5 4.5 9 54.5 28 2.5 1.5 4.5 9 Composition of Zr-Hf-Fluoride Glass for Cladding (mol %) ZrF 4 HfF 4 BaF 2 LaF 3 ScF 3 YF 3 GdF 3 LuF 3 AlF 3 NaF LiF 47.5 23.5 2.5 2 4.5 20 31.5 16 23.5 2.5 2 4.5 20 31.5 16 23.5 2.5 2 4.5 20 31.5 16 23.5 2.5 2 4.5 20 31.5 16 23.5 2.5 2 4.5 20 22.5 25 23.5 2.5 2 4.5 20 15.5 32 23.5 2.5 2 4.5 20 47.5 23.5 2.5 2 4.5 20 37 12 25 3.5 2 2.5 18 21.7 25.8 23.5 2.5 2 4.5 20 23.7 23.8 23.5 2.5 2 4.5 20 43 28 2.5 4 2.5 20 43 28 2.5 4 2.5 20 31.5 11.5 28 2.5 4 2.5 20 43 28 2.5 4 2.5 20 55 18.5 4 2 2.5 18 52.5 18 3 2 4.5 20 55 18 2.5 2 4.5 18 52.5 23.5 2.5 2 4.5 15 43 26 3.5 2 2.5 23 41.5 24.5 2.5 2 4.5 25 44.5 22.5 3.5 2 2.5 25 48 25 1.5 1.5 5 19 48 25 1.5 1.5 5 19 48 25 1.5 1.5 5 19 48 25 1.5 1.5 5 19 48.5 23.5 4 0 5 19 46.5 23.5 6 2.5 1.5 20 46.5 22.5 1.5 4 4.5 21 Example 22 Optical fibers with a high Δn of 1.5 to 4.5 % were obtained in the same manner as in Example 13 except that the composition of the core glass was selected from one of those shown in Table 9. Optical amplifiers were fabricated in the same manner as in Example 13 except for the use of the optical fibers thus obtained. The resulting optical amplifiers had high gain coefficients and low transmission losses comparable to those of the amplifier of Example 13. Example 23 Optical fibers with high Δn of 1 to 4.5 % were obtained in the same manner as in Example 22 except that the composition of the core glass was selected from those shown in Table 10. Optical amplifiers were fabricated in the same manner as in Example 22 except for the use of the optical fibers thus obtained. The optical amplifiers had high gain coefficients and low transmission losses comparable to those of the amplifier of Example 13. Example 24 In the arrangement of the amplifier shown in Fig. 3, there was used a fluoride optical fiber with the core 5 doped with 500 ppm of Pr 3 . It had a core diameter of 2 µm, a relative refractive index difference of 3.7%, and a length of 20 m. The composition of the core glass was 49ZrF 4 -17BaF 2 -8BaCl 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -18.0LiF (mol%), while the composition of the cladding glass was 49ZrF 4 -10BaF 2 -15BeF 2 -3.5LaF 3 -2 .0YF 3 -2.5AlF 3 -18.0LiF (mol%). Fig. 13 is a graph illustrating dependence of gain on the pump power in the optical fiber for an optical amplifier obtained in this Example. In Fig. 13, the gain indicated by the solid line is that of the optical fiber for an optical amplifier of this Example, while the gain indicated by the dotted line is that of a comparison optical fiber for an optical amplifier. The construction of the comparison optical fiber is as described below. Pr 3+ concentration: 500 ppm Relative refractive index difference: 3.7% Core diameter: 2 µm Core glass composition: 53ZrF 4 -16BaF 2 -15PbF 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -8LiF (mol%) Cladding glass composition: 47.5ZrF 4 -23.5BaF 2 -2.5LaF 3 -2.0YF 3 -4.5AlF 3 -20NaF (mol%) The above-described comparison optical fiber for an optical amplifier has no chlorine incorporated in the core glass. Although the constructional elements of the optical fiber for an optical amplifier of this Example are the same as those of the comparison optical fiber for an optical amplifier, it is understood from Fig. 13 that a higher gain is obtained in this Example than in the comparison example. The reason for this difference therebetween may be as follows: The incorporation of BaCl 2 into the core glass results in the substitution of part of the fluorine in the Zr F bonds by Cl. As a result, Zr-Cl bonds with a low phonon energy are formed, and the ratio of multiphonon relaxation from the 1 G 4 level to the 3 F 4 level of Pr 3+ is decreased. Consequently, the quantum efficiency of the transition from 1 G 4 to 3 H 5 is increased. In this Example, experiments were conducted using BaCl 2 as a chlorine dopant. However, other chlorides such as BaFCl, LaCl 3 , PbCl 2 , AlCl 3 , LiCl or YCl 3 may be used as a chlorine dopant. Alternatively, the core glass may be melted in chlorine-containing gas such as Cl 2 , HCl or CCl 4 . When chlorine is incorporated into the core glass, the refractive index of the glass is increased. Thus, doping with chlorine is effective not only in improving the quantum efficiency of the transition from 1 G 4 to 3 H 5 , but also in increasing the numerical apertures of the optical fiber for an optical amplifier. Doping of chlorine as well as Pr 3+ into the core glass, therefore, enables the fabrication of an efficient optical fiber amplifier. In this Example, the rate of substitution of fluorine ions by chlorine ions was adjusted to about 5.5%, but the fabrication of an optical fiber was possible even if this rate was increased to 30%. Example 25 The dependence of gain on pump power at 1.31 µm was measured using an optical fiber for an optical amplifier which has the same arrangement as in Fig. 3 and which had a core glass composition of 49ZrF 4 -17BaF 2 -8BaBr 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -18LiF (mol%), a cladding glass composition of 49ZrF 4 -10BaF 2 -15BeF 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -18LiF (mol%), a Pr 3+ concentration of 500 ppm, a core diameter of 2µm, a relative refractive index difference of 3.7%, and a length of 20 m. As in the case of Example 24, it was confirmed that the amplification characteristics were improved by substituting part of the fluorine in the core glass by another halogen (bromine in this case). The fabrication of an optical fiber was possible even when the rate of substitution of fluorine by bromine was increased to 30%. In this Example, BaBr 2 was used as a bromine dopant. However, other bromides such as BaFBr, PbBr 2 , LaBr 3 , YBr 3 , or LiBr may be used as a bromine dopant. Alternatively, doping with bromine ions can be performed by melting the core glass in bromine-containing gas (e.g. Br 2 , HBr). Example 26 The dependence of gain on the pump power at the 1.31 µm band was measured in the same manner as in Example 24 using an optical fiber for an optical amplifier which had a core glass composition of 49ZrF 4 -17BaF 2 -8BaI 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -18.0LiF (mol%), a cladding glass composition of 49ZrF 4 -10BaF 2 -15BeF 2 -3.5LaF 3 -2.0YF 3 -2.5AlF 3 -18LiF (mol%), a Pr 3+ concentration of 500 ppm, a core diameter of 2 µm, a relative refractive index difference of 3.7%, and a length of 20 m. As in the case of Example 24, it was confirmed that the amplification characteristics of the optical fiber were improved by substituting part of the fluorine in the core glass by another halogen (iodine in this case). The fabrication of an optical fiber was possible even when the rate of substitution of fluorine by iodine was increased to 30%. In this Example, BaI 2 was used as an iodine dopant. However, other iodides such as PbI 2 , BaFI, LaI 3 , YI 3 , or LiI may be used as iodine dopant. Alternatively, doping with iodine can be performed by melting the core glass in iodine-containing gas. In the above-mentioned Examples 24 through 26, ZrF 4 -BaF 2 -LaF 3 -YF 3 -AlF 3 -LiF type glass in which part of BaF 2 , one of the components, was substituted by BeF 2 was used as the cladding glass. The role of this BeF 2 is to lower the refractive index of the glass, and this component is effective in increasing the relative refractive index difference between the core and the cladding. Moreover, BeF 2 may substitute ZrF 4 , as well as BaF 2 . The cladding glass may be a ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF type glass which contains BeF 2 , or ZrF 4 -HfF 4 -BaF 2 -LaF 3 -YF 3 -AlF 3 -LiF-NaF type glass which contains BeF 2 . The use of these glasses can increase the relative refractive index difference to 3.7 % or more. The core glass to be doped with a halogen other than fluorine may be ZrF 4 -BaF 2 -PbF 2 -LaF 3 -YF 3 -AlF 3 -LiF type glass (Table 11). Composition of Mixed-Halide Glass for Core (mol %) ZrF 4 HfF 4 BaF 2 BeF 2 LaF 3 YF 3 AlF 3 NaF LiF PbF 2 BaCl 2 49 10 15 3.5 2 2.5 18 25 10 24 3.5 2 2.5 18 31.5 16 13.5 10 2.5 2 4.5 20 51 20 15 4.5 4.5 20 53 14 3.5 2 2.5 8 2 10 Example 27 The dependence of gain on the pump power at the 1.31 µm band was measured in the same manner as in Example 24 using an optical fiber which had a core glass composition of 35InF 3 -25ZnF 2 -5BaF 2 -5BaCl 2 -10SrF 2 -15PbF 2 -4AlF 3 -LiF (mol%), a cladding glass composition of 35InF 3 -25ZnF 2 -15BaF 2 -10SrF 2 -5CdF 2 -5AlF 3 -5LaF 3 (mol%), a Pr 3+ concentration of 500 ppm, a core diameter of 2 µm, a relative refractive index difference of 3.7%, and a length of 20 m. As in the case of Example 24, it was confirmed that the amplification characteristics were improved by substituting part of the fluorine in the core glass by chlorine. While chlorine was doped in this example, it was confirmed that the amplification characteristics were improved when the core glass was doped with bromine or iodine by the use of a halide such as BaBr 2 or BaI 2 . A halide such as ZnCl 2 , SrCI 2 , ZnBr 2 , SrBr 2 , ZnI 2 or SrI 2 may be used to perform doping with chlorine, bromine or iodine. In the above Examples, experiments were conducted using ZrF 4 type and InF 3 type fluoride optical fibers. However, it may be permissible to use as the core glass other fluoride glass such as AlF 3 glass (Tomozawa and Doremus, "Treatise on Materials Science and Technology," Volume 25, Chapter 4, Academic Press, Inc., 1985; or Aggarwal and Lu, "Fluoride Glass Fiber Optics," chapter 1, Academic Press, Inc., 1991) which has been doped with chlorine, bromine or iodine. Thus, the invention is in no way limited to ZrF 4 type or InF 3 type fluoride glass. Furthermore, chlorine, bromine or iodine need not be doped singly into the core, but it may be doped as a combination like chlorine-bromine or bromine-idodine. Moreover, the rate of substitution by chlorine, bromine or iodine is in no way restricted to the values described in the above Examples. Example 28 The dependence of gain on the pump power dependence at the 1.31 µm band was measured in the same manner as in Example 24 using an optical fiber for an optical amplifier which had a core glass composition of 23MgF 2 -10SrCl 2 -30SrF 2 -13YF 3 -23AlF 3 -1BeF 2 (mol%), a cladding glass composition of 20MgF 2 -50YF 3 -20AlF 3 -10BeF 2 (mol%), a Pr 3+ concentration of 500 ppm, a core diameter of 2 µm, a relative refractive index difference of 4 %, and a length of 20 m. The amplification characteristics were improved more highly than when the comparison glass shown in Example 24 was used as the cladding. In this Example, part of the fluorine was substituted by chlorine, but it was confirmed that substitution by bromine or iodine rather than chlorine was also effective, as demonstrated in other Examples. As in the case of Examples 24 to 27, MgBr 2 , Yl 3 , YBr 3 , etc. can be used to dope bromine or iodine. The glass used in this Example is called AlF 3 glass. Typical examples of AlF 3 glass are as shown in Tables 12 to 14. Similar results can be obtained using these glasses. Example 29 In Examples 24 to 27, the rare earth metal ions doped into the core were Pr 3+ only. In the optical fibers for an optical amplifier mentioned in these Examples, however, Pr 3+ (0.2 wt%)-Yb 3+ (0.3 wt%), Pr 3+ (0.2 wt%)-Nd 3+ (2 wt%), or Pr 3+ (0.2 wt%)-Er 3+ (1 wt%) may be doped instead of Pr 3+ . In the case of Pr 3+ (0.2 wt%)-Yb 3+ (0.3 wt%) and Pr 3+ (0.2 wt%)-E r3+ (1 wt%), pumping is performed with light at 0.98 µm. In the case of Pr 3+ (0.2 wt%)-Nd 3+ (2 wt%), pumping is performed with light at 0.8 µm. Comparison was made in terms of gain characteristics between the optical fiber with the core doped with chlorine, bromine or iodine and the optical fiber with the core not doped with halogen. It was confirmed that the optical fiber with the core doped with chlorine, bromine or iodine gave a higher gain at the 1.3 µm band at a lower pump power. In Examples 24 to 29, chlorine, bromine or iodine was doped into the core only, but may be simultaneously doped into the cladding. When the concentration of each of Yb 3+ , Nd 3+ or Er 3+ to be codoped with Pr 3+ was within the range larger than 0 wt% and up to 3 wt%, it was confirmed that the gain characteristics could be improved by also doping at least one halogen of chlorine, bromine and iodine. The concentration of the Yb 3+ , Nd 3+ or Er 3+ is in no way restricted to the above-mentioned value. Nor are the rare earth metal ions restricted to Pr 3+ , Yb 3+ , Nd 3+ or Er 3+ , but the quantum efficiency can be improved by effecting the above-described substitution in the presence of the transition of other rare earth metal ions.

Claims: A fluoride glass optical fibre for an optical amplifier having a core and a cladding, and containing rare earth metal ions in a core glass, wherein the relative refractive index difference (An) between the core and the cladding is 1.4% or more, and the core glass contains no more than 25 mol% PbF 2 based on the total composition of the core glass. An optical fibre as claimed in Claim 1, wherein said core glass contains LiF in a proportion of 3 to 12 mol% and within the range (11-0.4x)<y<(15-0.4x) in which x represents the concentration of PbF 2 (mol%) and y represents the concentration of LiF (mol%). An optical fibre as claimed in Claim 2, wherein said core contains a core matrix comprising a fluoride glass composed of 50 to 60 mol% of ZrF 4 , 7 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , 1.5 to 6 mol% of LaF 3 , 0 to 4 mol% of at least one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of AlF 3 , and 3 to 12 mol% of LiF, with the total amount of the components being 100 mol%. An optical fibre as claimed in Claim 1, wherein said core contains a core matrix comprising a fluoride glass composed of 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 6 to 30 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 25 mol% of PbF 2 , 0 to 7 mol% of CdF 2 , 1.5 to 6 mol% of at least one member selected from the group consisting of LaF 3 , ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of GaF 3 , and 2 to 10 mol% of LiF or NaF, with the total amount of the components being 100 mol%. An optical fibre as claimed in Claim 4, wherein said cladding contains a cladding matrix copmrising a fluoride glass composed of 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 6 to 30 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 25 mol% of PbF 2 , 0 to 7 mol% of CdF 2 , 1.5 to 6 mol% of at least one member selected from the group consisting of LaF 3 , ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of AlF 3 or GaF 3 , and 2 to 10 mol% of LiF or NaF, with the total amount of the components being 100 mol%. An optical fibre as claimed in Claim 1, wherein said core contains a core matrix comprising a fluoride glass composed of 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 5 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 7 mol% of CdF 2 , 0 to 10 mol% of LaF 3 , 0 to 4 mol% at least one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 0 to 5 mol% of AlF 3 , and 0 to 15 mol% of LiF or NaF, with the total amount of the components being 100 mol%. An optical fibre as claimed in Claim 6, wherein said cladding contains a cladding matrix compising a fluoride glass composed of 10 to 40 mol% of InF 3 , 20 to 50 mol% of ZnF 2 , 5 to 25 mol% of PbF 2 , 6 to 19 mol% of BaF 2 , 5 to 15 mol% of SrF 2 , 0 to 7 mol% of CdF 2 , 0 to 10 mol% of LaF 3 , 0 to 4 mol% of at least one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 0 to 5 mol% of AlF 3 , and 0 to 15 mol% of LiF or NaF, with the total amount of the components being 100 mol%. An optical fibre as claimed in Claim 4 or 6, wherein said fluoride glass constituting the cladding glass matrix comprises 43 to 55 mol% of at least one member selected from the group consisting of ZrF 4 and HfF 4 , 18 to 28 mol% of BaF 2 , 1.5 to 6 mol% of LaF 3 , 0 to 4 mol% of one member selected from the group consisting of ScF 3 , YF 3 , GdF 3 and LuF 3 , 1.5 to 5 mol% of AlF 3 , and 15 to 25 mol% of one member selected from the group consisting of LiF and NaF, with the total amount of the components being 100 mol%. An optical fibre as claimed in any preceding claim, wherein said rare earth metal ions are ones selected from a group consisting of Pr 3+ , Pr 3+ -Yb 3+ , Pr 3+ -Nd 3+ , and Pr 3+ -Er 3+ . An optical fibre as claimed in Claim 1, wherein said core has a core radius of 1.95 µm or less. An optical fibre as claimed in Claim 1 wherein said fluoride glass is a ZrF 4 glass, an InF 3 glass or an AlF 3 glass. An optical fibre as claimed in Claim 1 wherein said fluoride glass is a mixed halide glass consisting in a ZrF 4 glass, an InF 3 glass or an AlF 3 glass in which a part of the fluoride composition is replaced by one or more halogen substituents chosen from chlorine, bromine and iodine. A 1.3 µm band optical fibre amplifier comprising an optical fibre as claimed in any preceding claim. An optical communication system comprising an amplifier as claimed in the preceding claim.

Inventors: KANAMORI TERUTOSHI, MORI ATSUSHI, NISHIDA YOSHIKI, OHISHI YASUTAKE, SUDO SHOICHI, KANAMORI, TERUTOSHI, MORI, ATSUSHI, NISHIDA, YOSHIKI, OHISHI, YASUTAKE, SUDO, SHOICHI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1992
Main classification: C03C13
IPC codes: C03C13, H01S3, C03C4