Abstract:
This invention relates to fluoride glass with a specific composition having wide infrared transmission. A fluoride optical fiber using this fluoride glass can give high efficiency with a low loss. The fluoride optical fiber having a second cladding on the outer periphery of a first cladding can adjust the refractive index of the first cladding suitably.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to fluoride glass having satisfactory wide infrared transmission, and a fluoride optical fiber capable of achieving highly efficient amplification using the fluoride glass. 
     2. Description of the Related Art 
     Fluoride glass has an extended transparency in regions ranging from visible to infrared wavelengths. Thus, it finds use in lenses, prisms or filters of various optical instruments, or in optical fibers for optical communication, optical measurement or power transmission. A fluoride optical fiber amplifier having a core doped with praseodymium as rare earth ions permits optical amplification in a 1.3 μm wavelength band, an important wavelength band for optical communication systems. The application of fluoride glass to an optical fiber amplifier, therefore, has drawn increased attention in recent years. 
     This optical amplification in the 1.3 μm wavelength band is induced by stimulated emission due to the transition from the  1  G 4  level to the  3  H 5  level in praseodymium. This stimulated emission is observed in a glass with low lattice vibration energy (phonon energy), such as fluoride glass, but not in a glass with high phonon energy, such as silicate glass, because of the existence of nonradiative transition attributed to multiphonon relaxation. 
     Therefore, it is essential to use a glass with low phonon energy such as fluoride glass to obtain such praseodymium-based optical amplification at a wavelength of 1.3 μm. 
     However, the quantum efficiency of amplification in the 1.3 μm band remains low, at 3.4% with ZrF 4  -based fluoride glass which currently used as a host material of a praseodymium-doped fiber. To improve this low quantum efficiency, it is important to develop fluoride glass with lower phonon energy than that of ZrF 4  -based fluoride glass. 
     The fluoride glass with low phonon energy known thus far is fluoride glass comprising 4 to 48 mol % of ZnF 2 , 32 to 63 mol % of PbF 2 , 0 to 34 mol % of GaF 3 , and 0 to 43 mol % of InF 3 , provided GaF 3  +InF 3  =17 to 53 mol %, and ZnF 2  +PbF 2  +GaF 3  +InF 3  ≧70 mol %, as Japanese Patent Application Laid-open No. 60-155549 describes. 
     This fluoride glass has satisfactory wide infrared transmission because of its low phonon energy. If doped with praseodymium, this fluoride glass can provide a higher quantum efficiency than does ZrF 4  -based fluoride glass. 
     Owing to its insufficient thermal stability against crystallization, however, this fluoride glass cannot give a jacketing tube indispensable for the production of a single-mode fiber as will be described later herein. Thus, the use of this fluoride glass has not enabled a single-mode fiber to be produced. 
     We have developed InF 3  -based fluoride glass as fluoride glass which is clearly different in the proportions of the components from the aforementioned PbF 2  -based fluoride glass, and which has low phonon energy, as Japanese Patent Application No. 6-172499 discloses. We have also developed a fluoride fiber comprising this fluoride glass. 
     With a Pr 3+  -doped single-mode fiber using the InF 3  -based fluoride glass described in Japanese Patent Application No. 6-172499, the resulting gain coefficient has exceeded a gain coefficient of 0.2 dB/mW attained by the Pr 3+  -doped ZrF 4  -based fluoride fiber. However, the glass composition has not been optimized, and the optical loss of the fiber has not been sufficiently decreased. This fiber, therefore, has not achieved the maximum gain coefficient of 0.4 dB/mW expected from the spectroscopic properties of Pr 3+  contained in the InF 3  -based fluoride glass. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished under these circumstances. Its object is to provide fluoride glass having satisfactory wide infrared transmission, and an optical fiber for optical amplification with low loss and high efficiency. 
     In the first aspect of the present invention, there is provided a fluoride glass comprising 10 to 30 mol % of InF 3 , 7 to 30 mol % of GaF 3 , 10 to 19 mol % of ZnF 2 , 4 to 30 mol % of BaF 2 , 0 to 24 mol % of SrF 2 , 0 to 30 mol % of PbF 2 , and 1.5 to 10 mol % of at least one member selected from the group consisting of LaF 3 , YF 3 , GdF 3  and LuF 3 , 1.5 to 30 mol % of LiF, 0 to 30 mol % of NaF, and 0 to 15 mol % of an additive, with the total amount of all components being 100 mol %. 
     In the second aspect of the present invention, there is provided fluoride optical fiber having a core and a cladding, wherein the matrix of said cladding comprises 10 to 30 mol % of InF 3 , 7 to 30 mol % of GaF 3 , 10 to 19 mol % of ZnF 2 , 4 to 30 mol % of BaF 2 , 0 to 24 mol % of SrF 2 , 0 to 30 mol % of PbF 2 , and 1.5 to 10 mol % of at least one member selected from the group consisting of LaF 3 , YF 3 , GdF 3  and LuF 3 , 1.5 to 30 mol % of LiF, 0 to 30 mol % of NaF, and 0 to 15 mol % of an additive, with the total amount of all components being 100 mol %. 
     Here, the matrix of the core comprises 5 to 25 mol % of InF 3 , 13 to 40 mol % of GaF 3 , 4 to 25 mol % of ZnF 2 , 30 to 46 mol % of PbF 2 , 0 to 20 mol % of CdF 2 , and 1.5 to 12 mol % of at least one member selected from the group consisting of LaF 3 , YF 3 , GdF 3  and LuF 3 , and 0 to 15 mol % of an additive, with the total amount of all components being 100 mol %. 
     The matrix of the core may comprise 10 to 30 mol % of InF 3 , 7 to 30 mol % of GaF 3 , 10 to 19 mol % of ZnF 2 , 4 to 30 mol % of BaF 2 , 0 to 24 mol % of SrF 2 , 0 to 30 mol % of PbF 2 , and 1.5 to 10 mol % of at least one member selected from the group consisting of LaF 3 , YF 3 , GdF 3  and LuF 3 , 1.5 to 30 mol % of LiF, 0 to 30 mol % of NaF, and 0 to 15 mol % of an additive, with the total amount of all components being 100 mol %. 
     Transition metal ions or rare earth ions may be contained in the core, and the relative refractive index difference Δn between the core and the cladding is more than 1.0%. 
     At least one type selected from Ce 3+ , Pr 3+ , Nd 3+ , Pm 3+  Sm 3+ , Eu 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+  and Yb 3+  may be contained as the rare earth ions. 
     In the third aspect of the present invention, there is provided a fluoride optical fiber having a core, a first cladding, and a second cladding on the outer periphery of the first cladding, wherein the matrix of the first cladding comprises 10 to 30 mol % of InF 3 , 7 to 30 mol % of GaF 3 , 10 to 19 mol % of ZnF 2 , 4 to 30 mol % of BaF 2 , 0 to 24 mol % of SrF 2 , 0 to 30 mol % of PbF 2 , and 1.5 to 10 mol % of at least one member selected from the group consisting of LaF 3 , YF 3 , GdF 3  and LuF 3 , 1.5 to 30 mol % of LiF, 0 to 30 mol % of NaF, and 0 to 15 mol % of an additive, with the total amount of all components being 100 mol %; and the matrix of the second cladding comprises 10 to 30 mol % of InF 3 , 7 to 30 mol % of GaF 3 , 10 to 19 mol % of ZnF 2 , 4 to 30 mol % of BaF 2 , 0 to 24 mol % of SrF 2 , 0 to 30 mol % of PbF 2 , and 1.5 to 10 mol % of at least one member selected from the group consisting of LaF 3 , YF 3 , GdF 3  and LuF 3 , 1.5 to 30 mol % of LiF, 0 to 30 mol % of NaF, and 0 to 15 mol % of an additive, with the total amount of all components being 100 mol %. 
     Here, the matrix of the second cladding comprises fluoride glass comprising at least one fluoride from ZrF 4  and HfF 4  and at least one member of the group consisting of BaF 2 , LaF 3 , GdF 3 , YF 3 , LiF, NaF, PbF 2  and AlF 3 . 
     The refractive index of the first cladding may be adjusted such that the refractive index of the first cladding is consistent with the refractive index of the second cladding, or the refractive index of the first cladding is larger than the refractive index of the second cladding but smaller than the refractive index of the core. 
     Here, the adjustment of the refractive index of the first cladding may be performed by substituting part of the PbF 2  in the matrix of the first cladding by NaF. 
     The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a characteristic chart showing the BaF 2  concentration dependence of the glass transition temperature (Tg) and (the crystallization temperature Tx-the glass transition temperature Tg) in the fluoride glass of the present invention; 
     FIG. 2 is a characteristic chart showing the PbF 2  concentration dependence of Tg, (Tx-Tg), and the refractive index (nD) in the fluoride glass of the invention; 
     FIG. 3 is a characteristic chart showing the CdF 2  concentration dependence of (Tx-Tg) in the fluoride glass of the invention; 
     FIG. 4 is a characteristic chart showing the wavelength dependence of transmittance of glass material; 
     FIGS. 5A and 5B are schematic sectional views showing the steps in the production of a fiber perform by the suction casting method; 
     FIGS. 6A and 6B are schematic sectional views showing the steps in the production of a fiber perform by the rotational casting method; 
     FIGS. 7A to 7C are schematic sectional views showing the steps in the production of an optical fiber according to the invention; 
     FIG. 8 is a characteristic chart showing the wavelength dependence of transmission loss in an embodiment of the optical fiber of the invention; and 
     FIG. 9 is a characteristic chart showing the wavelength dependence of transmission loss in another embodiment of the optical fiber of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     We have conducted extensive studies on the glass forming region of fluoride glass comprising InF 3 , GaF 3 , ZnF 2 , BaF 2 , SrF 2 , PbF 2 , LaF 3 , YF 3 , GdF 3 , LuF 3 , LiF and NaF. These studies have led us to discover fluoride glass having a glass forming region as described in claim 1, the fluoride glass having a glass transition temperature (about 260° C.) close to that of ZrF 4  -based fluoride glass and high thermal stability against crystallization. 
     In the fluoride glass of the present invention, InF 3 , GaF 3 , and ZnF 2  are essential components constituting the network former. Desirably, the glass contains 10 to 30 mol % of InF 3  and 7 to 30 mol % of GaF 3 , and preferably, 20 to 30 mol % of InF 3  and 7 to 20 mol % of GaF 3 . In regions where the InF 3  concentration is more than 30 mol % and the GaF 3  concentration is less than 7 mol % in the composition of the fluoride glass of the invention, there will be obtained glass thermally stable to crystallization. However, its glass transition temperature rises to approximately 300° C., and thus a glass transition temperature close to that of ZrF 4  -based fluoride glass cannot be achieved. In regions where the InF 3  concentration is less than 10 mol % and the GaF 3  concentration is more than 30 mol %, on the other hand, the resulting glass will have poor thermal stability against crystallization, and easy to crystallize. 
     The concentration of ZnF 2  is desirably 10 to 19 mol %. In the composition of the fluoride glass claimed in the invention, if more than 19 mol % of ZnF 2  is contained, single crystals of ZnF 2  will tend to form in the resulting glass. If its concentration is less than 10 mol %, crystals composed of InF 3  and GaF 3  will develop in the glass. 
     BaF 2  and SrF 2  are essential components for modifying the network former. Desirably, the glass contains 4 to 30 mol % of BaF 2  and 0 to 24 mol % of SrF 2 . The BaF 2  concentration of 10 to 24 mol % and the SrF 2  concentration of 0 to 14 mol % would make it possible to obtain glass with excellent thermal stability against crystallization. 
     FIG. 1 is a characteristic chart showing the dependence of the glass transition temperature (Tg) and (the crystallization temperature Tx-the glass transition temperature Tg) on the BaF 2  concentration (x mol %) in the claimed fluoride glass having the composition 28 InF 3  -9 GaF 3  -17 ZnF 2  -12 PbF 2  -xBaF 2  -(24-x) SrF 2  -5 YF 3  -5 LiF (mol %). Generally, (Tx-Tg) is used as an indicator of the thermal stability of glass. In FIG. 1, the left vertical axis shows the glass transition temperature (Tg), and the right vertical axis shows (Tx-Tg), in which the value of Tg (closed circle, ) is read from the left vertical axis, and the value of (Tx-Tg) (closed squared ▪) from the right vertical axis. FIG. 1 shows that the values of Tg are close to 260° C. in the entire region with the BaF 2  concentration being varied from 0 to up to 24 mol %. Tx-Tg, on the other hand, shows an upwardly convex BaF 2  dependence. The values of Tx-Tg are high values not less than 90° C. in the region 0≦BaF 2  ≦24 mol %, especially, high values of not less than 100° C. in the region 10≦BaF 2  ≦24 mol %, namely, in the region 0&lt;SrF 2  ≦14 mol %. This is proof of possession of high thermal stability against crystallization. 
     LiF and NaF are also essential components for modifying the network former in the claimed fluoride glass. Incorporation of these components lowers the melting temperature of glass melt, and gives uniform glass melt even at low temperatures. Thus, their incorporation enhances the glass forming ability. 
     NaF is desirably contained in an amount of 0 to 30 mol %. If its concentration is more than 30 mol %, stable glass is not obtained because of a marked tendency toward crystallization. LiF is a particularly important component for ensuring the thermal stability of glass. Desirably, its concentration is 1.5 to 30 mol %, and a concentration of 5 to 10 mol %, in particular, markedly improves thermal stability against crystallization. With the concentration of more than 30 mol %, however, there will be a considerable tendency to crystallization, resulting in the failure to obtain stable glass. 
     In the claimed fluoride glass, LaF 3 , YF 3 , GdF 3  and LuF 3  are also essential components for increasing the thermal stability of the glass. Desirably, at least one of these components is contained in an amount of 1.5 to 15 mol %, preferably 1.5 to 10 mol %. If the concentration is less than 1.5 mol %, the increase in thermal stability cannot be confirmed. With the concentration of more than 15 mol %, on the other hand, there will be a marked tendency toward crystallization, resulting in the failure to obtain stable glass. 
     In the claimed fluoride glass, PbF 2  is an essential component for controlling the refractive index, and preferably, is contained within the range from 0 to 30 mol %. However, PbF 2  may be replaced partially, in the range of from 0 to 20 mol %, by NaF without impairing the thermal stability of the glass, whereby the refractive index of the claimed fluoride glass can be controlled. 
     FIG. 2 shows the dependence of the glass transition temperature (Tg), the difference (Tx-Tg), and the refractive index nD on the PbF 2  concentration (x mol %) in the claimed fluoride glass having the composition 25 InF 3  -10 GaF 3  -14 ZnF 2  -xPbF 2  -18 BaF 2  -8 SrF 2  -2.5 YF 3  -2.5 LaF 3  -(20-x) (LiF+NaF) (mol %). In FIG. 2, the left vertical axis shows the temperature, and the right vertical axis shows the refractive index, in which the value of Tg (closed circle, ) and the value of (Tx-Tg) (closed square, ▪) are read from the left vertical axis, and the value of the refractive index (open circle, ◯) from the right vertical axis. 
     FIG. 2 shows that the glass transition temperature has values close to 260° C. in the entire region with the PbF 2  concentration being varied from 0 to up to 20 mol %. The values of (Tx-Tg) are not less than 90° C., showing high thermal stability. The refractive index, on the other hand, increases linearly from 1.46 to 1.54 as the PbF 2  concentration is increased. In the claimed fluoride glass, therefore, the refractive index can be controlled, with the glass transition temperature being maintained close to 260° C. and thermal stability maintained high, by replacing part of or all of a suitable amount of PbF 2  by NaF. 
     The claimed fluoride glass and the claimed fluoride optical fiber also contain 0 to 15 mol % of an additive. As the additive, there may be contained at least one member of the group consisting of 0 to 10 mol % of BeF 2 , 0 to 10 mol % of MgF 2 , 0 to 10 mol % of CaF 2 , 0 to 4 mol % of CdF 2 , 0 to 5 mol % of TlF 4 , 0 to 5 mol % of MnF 2 , 0 to 5 mol % of SmF 3 , 0 to 5 mol % of ScF 3 , 0 to 5 mol % of HfF 4 , 0 to 5 mol % of ZrF 4 , 0 to 10 mol % of KF, 0 to 10 mol % of RbF, 0 to 10 mol % of CsF, 0 to 15 mol % of BiF 3  and 0 to 15 mol % of AlF 3 . 
     In this case, CdF 2  may replace part of the aforementioned essential component ZnF 2  or PbF 2 , i.e. in the range of from 0 to 4 mol %. FIG. 3 shows the dependence of the value of (Tx-Tg) on the CdF 2  concentration (x) in the glass 28 InF 3  -9 GaF 3  -(15-x) ZnF 2  -xCdF 2  -12 PbF 2  -18 BaF 2  -8 SrF 2  -5 YF 3  -5 LiF (mol %). From FIG. 3, one can see that CdF 2  can be contained in the glass to a concentration of up to 4 mol % without impairing its thermal stability. With the concentration of more than 4 mol %, however, the value of (Tx-Tg) sharply decreases. In 5 mol % or more, the thermal stability of the glass is considerably impaired. Likewise, the respective elements BeF 2 , MgF 2 , CaF 2  and MnF 2  can replace part of each of the essential components ZnF 2 , PbF 2 , BaF 2  and SrF 2 . Furthermore, the elements TlF 4 , SmF 3 , ScF 3 , HfF 4  and ZrF 4  can replace part of each of the essential components InF 3 , GaF 3 , YF 3  and LaF 3 . If they are contained as replacements beyond the aforementioned ranges, however, the thermal stability of the glass will be considerably deteriorated. 
     The fluoride optical fiber of the invention achieves highly efficient optical amplification by possessing a structure with high Δn (Δn≧1.0%). This is because the quantum efficiency of the 1.3 μm transition of praseodymium is improved by using the low phonon energy glass as a host glass and increasing the Δn of the fiber which can achieve the high light intensity in the core. 
     For the production of a high Δn optical fiber using the PbF 2  -based fluoride glass described in the previously cited Japanese Patent Application No. 60-155549 as the core, the use of the known ZrF 4  -based fluoride glass as the cladding, however, crystals consisting essentially of PbF 2  and ZrF 4  grow at the interface between the core and the cladding, making it difficult to obtain a satisfactory optical fiber. 
     In the fluoride fiber of the invention, by contrast, the use of the fluoride glass described in claim 1 as the cladding caused no crystallization at the interface between the core and the cladding, and enabled a satisfactory optical fiber to be obtained even when the PbF 2  -based fluoride glass was used as the core glass. The reason is that InF 3  is also contained in the composition of the PbF 2  -based fluoride glass, thus there is no rapid crystal growth having PbF 2  and InF 3  as the main components. 
     Additionally, it is possible, needless to say, to produce the claimed optical fiber using the fluoride glass of claim 1 as the core glass. 
     A single mode fluoride optical fiber has been produced by the following fabrication steps: A fluoride fiber perform having a structure comprising a core and a cladding was fabricated by suction casting. Then, the resulting fluoride fiber perform was inserted into a first jacketing tube, and elongated under heat to form a second fiber perform. The second fiber perform was reinserted into a second jacketing tube, and drawn into a single-mode optical fiber. 
     To produce a single-mode optical fiber, having a core capable of well confining light, by the suction casting method, the core/cladding external diameter ratio of not less than 5 is generally said to be necessary. With the above-described method of producing a single-mode fiber, however, it is impossible to prepare a fiber perform with the core/cladding ratio of not less than 5 only by the fiber perform fabrication step using suction casting. 
     To produce a single mode fiber with the core/cladding ratio of more than 5, therefore, a first jacketing tube, i.e. a second cladding, should desirably have the same refractive index as does the cladding (first cladding). If the second cladding has a higher refractive index than the cladding, the field of propagating light in the core extends into the cladding, which results in weakened light intensity in the core. Thus, highly efficient amplification cannot be attained. 
     The invented optical fiber, on the other hand, uses the claimed fluoride glass as the cladding glass. Thus, the cladding and the second cladding are consistent in terms of the refractive index, the core/cladding ratio is not less than 5, and a single mode fiber with high intensity light confined in the core can be produced. 
     In case no suction casting method is employed, the core/cladding ratio is not restricted to not less than 5 as far as the ratio can depress the cladding mode. 
     As is apparent from the cross section of a single mode fluoride fiber, the area of the core and the cladding is at most 1/16 of the total cross sectional area of the optical fiber. Most of the cross section is composed of a first jacketing tube and a second jacketing tube. Thus, the jacketing tubes account for most of the weight of the optical fiber. This means that the price of the jacketing tube determines the price of the single mode fiber. 
     Fluoride raw materials such as InF 3  and GaF 3  are expensive compared with ZrF 4 . The use of InF 3  -based fluoride glass as the jacketing tube will necessarily raise the unit price of the resulting optical fiber. 
     In the fluoride fiber of the invention, using ZrF 4  -based fluoride glass as the first jacketing tube, accordingly the second cladding, the price of the resulting single mode fiber can be cut down. 
     When a jacketing tube of ZrF 4  -based fluoride glass is used as the second cladding, the preparation of a satisfactory fiber with high intensity light confined in the core requires that there be agreement between the refractive index of the cladding and that of the second cladding. In the claimed fluoride fiber, the concentration of PbF 2 , a component of the cladding glass, is partially substituted by NaF, whereby the refractive index can be varied within the range of from 1.46 to 1.54 without changing the glass transition temperature of the cladding glass. Furthermore, the glass transition temperature of the cladding glass is close to the glass transition temperature of ZrF 4  -based fluoride glass. Consequently, even when ZrF 4  -based fluoride glass is used as the second cladding, the fiber fabrication steps such as elongation and fiber drawing can be performed without problems. In addition, the refractive index of the cladding glass can be controlled to be equal to or greater than the refractive index of the jacketing tube, and smaller than that of the core. Thus, a satisfactory single mode fiber with high intensity light confined in the core can be produced. 
     In preparing an optical fiber with a relative refractive index difference (Δn) of not less than 4%, it is necessary to use as the core glass a fluoride glass with a higher refractive index than that of the claimed fluoride glass. The relative refractive index difference (Δn) is defined as follows: 
     Δn=(refractive index of core-refractive index of cladding)/refractive index of core (%) 
     Generally, fluoride glass with a high concentration of PbF 2  is known to be high in refractive index. Our extensive studies have led to discovery of a fluoride glass composition containing 30 to 46 mol % of PbF 2  and having a high refractive index close to 1.6. In the present invention, the preferred fluoride glass is one in which a core matrix comprises 5 to 25 mol % of InF 3 , 13 to 40 mol % of GaF 3 , 4 to 25 mol % of ZnF 2 , 30 to 46 mol % of PbF 2 , 0 to 20 mol % of CdF 2 , and 1.5 to 12 mol % of at least one member selected from LaF 3 , YF 3 , GdF 3  and LuF 3 , and 0 to 15 mol % of an additive, with the total amount of the components being 100 mol %. 
     In the fluoride glass for the core, InF 3  and GaF 3  are essential components as the network former. InF 3  is desirably contained in a concentration of 5 to 25 mol %. If its concentration is less than 5 mol %, transparent glass cannot be obtained. If the InF 3  concentration is more than 25 mol % in the core fluoride glass composition, a rather high crystallization rate occurs, giving no satisfactory glass. GaF 3  is desirably contained in a concentration of 13 to 40 mol %. If the GaF 3  concentration is less than 13 mol % in the core fluoride glass composition, transparent glass cannot be obtained because of crystallization. If its concentration is more than 40 mol %, the glass melt devitrifies yellow, so that transparent glass cannot be obtained, either. In the core fluoride glass, moreover, PbF 2  and ZnF 2  are essential components as network modifiers. Incorporation of these ions lowers the melting temperature of the glass melt, so that a uniform melt can be obtained at low temperatures, thus enhancing the glass forming ability. With this core fluoride glass composition, the desirable concentration of PbF 2  is 30 to 46 mol %. The concentration of less than 30 mol % poses difficulty in obtaining transparent glass because of crystallization. At the concentration of more than 46 mol %, the glass melt becomes volatile, and cannot provide stable glass. ZnF 2  is desirably contained in a concentration of 4 to 25 mol %. At a concentration of less than 4 mol %, crystallization makes it impossible to obtain transparent glass. At a concentration of more than 25 mol %, a rather high crystallization rate occurs, and does not give transparent glass, either. CdF 2  may be contained as substituting PbF 2  or ZnF 2  in the range of from 0 to 20 mol %. Preferably, it is contained in the range 0 to 7 mol %, thus taking effect in increasing the glass forming ability and obtaining stable glass. Furthermore, LaF 3 , YF 3 , GdF 3  or LuF 3  is an essential component for improving thermal stability against crystallization in the claimed fluoride glass. With the core fluoride glass composition, at least one of them is contained in a concentration of 1.5 to 12 mol %, whereby thermal stability to reheating can be improved. 
     Such fluoride glass is used as the core, and the invented fluoride glass claimed in claim 1 is used as the cladding, thereby making it possible to produce an optical fiber with Δn=not less than 4% that was unattainable with a conventional ZrF 4  -based fluoride fiber. 
     Additionally, in preparing a single mode optical fiber with Δn=not more than 4.0%, comparable to the relative refractive index difference Δn of an optical fiber produced using conventional ZrF 4  -based fluoride glass, the use of the claimed fluoride glass as the core and the cladding permits the production of exactly the same optical fiber as the ZrF 4  -based glass fiber. Moreover, the claimed fluoride glass has better infrared transmission than the ZrF 4  -based fluoride glass. Thus, the doping of its core with rare earth ions for optical amplification enables higher amplification efficiency than that of the ZrF 4  -based fluoride glass. 
     The fluoride fiber of the invention, when doped in the core with transition metal ions or rare earth ions, can be used as an optical fiber laser or an optical fiber amplifier. Examples of the doping transition metal ions are Cr, Ti, Fe, Co, Ni and Cu, whereas examples of the doping rare earth ions are Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb. These doping elements can be contained in the range of from 0.001 to 10% by weight. The concentration in excess of 10 wt. % is undesirable, since this will deteriorate the thermal stability of the core glass. At the concentration of less than 0.001 wt. %, sufficient emission cannot be attained because of the intrinsic loss of the fiber, such as scattering loss. 
     The present invention will be described in detail with reference to the Examples, which are presented to illustrate, not limit, the invention. 
     EXAMPLE 1 
     InF 3 , GaF 3 , ZnF 2 , PbF 2 , BaF 2 , SrF 2 , YF 3 , LaF 3 , GdF 3 , LuF 3 , LiF and NaF, all in anhydrous form, were weighed and mixed in the proportions shown in Table 1. To each of the mixtures, 4 g of ammonium bifluoride was added, and placed in a crucible. The crucible was set in a resistivity-heated furnace, and heated for 1 hour at 900° C. in an argon atmosphere to melt the contents of the crucible. The temperature of the furnace was lowered to 700° C., and the crucible was withdrawn. The melt inside was poured into a brass mold with an outside diameter of 8 mm that had been preheated to 200° C., to quench the melt, obtaining a glass rod. 
     A part of the glass rod was crushed, and measured for the glass transition temperature (Tg) and the crystallization temperature (Tx) using a differential scanning calorimeter. In all of the samples, the values of Tg were close to 260° C. The value of (Tx-Tg) is known as an indicator of the thermal stability against crystallization of glass. The measurements of (Tx-Tg) for all glass samples showed values 90° C. or higher, ascertaining that the resulting glasses had high thermal stability. 
     A 10 mm long cylindrical rod was cut out of the glass rod, and its opposite end surfaces were polished, followed by measuring the transmission spectrum of the rod. The rod was found to have good transmission at up to 10 μm. Some of the rod samples were measured for Raman spectrum. Peaks of all measurements were noted around 500 cm -1 , showing that the resulting glass had small phonon energy. 
     
                                           TABLE 1__________________________________________________________________________No  InF3GaF3   ZnF2      PbF2         BaF2            SrF2               LaF3                  YF3                     GdF3                        LuF3                           NaF                              LiF                                 Tg Tx Tx - Tg__________________________________________________________________________ 1  25.511.5   14    19 8  2.5                  2.5      12 5  264.2                                    365.9                                       101.7 2  25.511.5   14  3 19 8  2.5                  2.5       9 5  259.9                                    354.2                                       94.3 3  25.511.5   14  6 19 8  2.5                  2.5       6 5  258.3                                    355.1                                       96.8 4  25.511.5   14  9 19 8  2.5                  2.5       3 5  261.5                                    354                                       92.5 5  25.511.5   14 12 19 8  2.5                  2.5         5  260.3                                    358                                       97.7 6  25.511.5   14 12 19 8     2.5                     2.5      5  259.6                                    357.6                                       98 7  25.511.5   14 12 19 8     2.5   2.5   5  262.3                                    360.1                                       97.8 8  25.511.5   14 12 19 8     5           5  257.3                                    356.5                                       99.2 9  25.511.5   14    19 8  2.5                  2.5      10 7  258.6                                    356                                       97.410  25.511.5   14  2 19 8  2.5                  2.5       8 7  258.2                                    355.1                                       96.911  25.511.5   14  4 19 8  2.5                  2.5       6 7  258.4                                    355.5                                       97.112  25.511.5   14  6 19 8  2.5                  2.5       4 7  256.4                                    355.3                                       98.913  25.511.5   14  8 19 8  2.5                  2.5       2 7  258.2                                    353.1                                       94.914  25.511.5   14 10 19 8  2.5                  2.5         7  262.1                                    358.4                                       96.315  25.511.5   15 12 18 8  2.5                  2.5         5  260.3                                    359.3                                       9916  28 9  17    10 14 2.5                  2.5      10 7  245.5                                    363.3                                       117.817  28 9  17    12 12 2.5                  2.5      10 7  252.6                                    378.5                                       125.918  28 9  17    18 6  2.5                  2.5      10 7  250.1                                    366                                       115.919  28 9  17    20 4  2.5                  2.5      10 7  248                                    367                                       11920  28 9  17    24    2.5                  2.5      10 7  249                                    361                                       11221  28 10 10    30    2.5                  2.5      10 7  252                                    342.3                                       90.322  28 10 12     4 24 2.5                  2.5      10 7  268                                    358.2                                       90.223  28 9  17 12  4 20 2.5                  2.5         5  256.8                                    351.8                                       9524  28 9  17 12  8 16 2.5                  2.5         5  258.5                                    357.9                                       99.425  28 9  17 12 12 12 2.5                  2.5         5  256                                    358.1                                       102.126  28 9  17 12 16 8  2.5                  2.5         5  260.2                                    353.3                                       93.127  28 9  17 12 20 4  2.5                  2.5         5  257                                    348.8                                       91.828  22 16 14 19 13 6  3  3           4  256.3                                    346.5                                       90.229  20 20 13.5      25  9 4  3  3           2.5                                 252                                    342.3                                       90.330  13 29 12 30  6.5  4  4           1.5                                 253.6                                    343.6                                       9031  10 30 10 30 10.5  4  4           1.5                                 251                                    345                                       94__________________________________________________________________________ 
    
     COMPARATIVE EXAMPLE 1 
     ZrF 4 , BaF 2 , LaF 3 , AlF 3 , YF 3  and NaF, all in anhydrous form, were used as raw materials, and weighed to form a batch with the composition 46.5 ZrF 4  -23.5 BaF 2  -2.5 LaF 3  -2.5 YF 3  -4.5 AlF 3  -20 NaF (mol %). 20 Grams of this batch was mixed, and 4 g of ammonium bifluoride was added, followed by placing the mixture in a crucible. The crucible was heated for 1 hour at 900° C. in a resistivity-heated furnace to melt the contents of the crucible. Then, the temperature of the furnace was lowered to 700° C., and the melt inside the crucible was poured into a brass mold preheated to 240° C., to quench the melt, obtaining a glass rod. 
     A 10 mm long cylindrical rod was cut out of the resulting glass rod, and its opposite end surfaces were polished, followed by measuring the transmission spectrum of the rod in the same manner as in Example 1. The results are indicated in FIG. 4 by a dashed line. As shown there, the absorption increased, starting at a wavelength of about 5 μm, and the transmission at a longer wavelength than 8 μm was small. 
     Some samples were measured for Raman spectrum in the same way as in Example 1. Peaks representing phonon energy appeared around 550 cm -1 , confirming the resulting glass to have greater phonon energy than the glass of Example 1. 
     Preparation of Core Glass Sample No. 1 
     Core glass of the optical fiber according to the present invention was prepared in the following manner: 
     InF 3 , GaF 3 , ZnF 2 , PbF 2 , YF 3  and LaF 3 , all in anhydrous form, were used as raw materials, and weighed and mixed to form a batch with the glass composition 13 InF 3  -29 GaF 3  -12 ZnF 2  -38 PbF 2  -4 YF 3  -4 LaF 3  (mol %). To 20 g of this batch, 4 g of ammonium bifluoride was added, followed by placing the mixture in a crucible. The crucible was set in a resistivity-heated furnace, where it was heated for 1 hour at 900° C. in an argon atmosphere to melt the contents of the crucible. Then, the temperature of the furnace was lowered to 700° C., and the crucible was withdrawn. The melt inside was poured into a brass mold with an outside diameter of 8 mm that had been preheated to 200° C., to quench the melt, obtaining a glass rod. 
     The resulting glass was measured for the glass transition temperature (Tg) and the crystallization temperature (Tx) using a differential scanning calorimeter. The results were Tg=258° C. and Tx=336° C. From these results, the value of (Tx-Tg) was 78° C., ascertaining that the resulting glass was thermally stable. 
     Preparation of Core Glass Samples Nos. 2 to 147 
     As in the preparation of core glass sample No. 1, InF 3 , GaF 3 , ZnF 2 , CdF 2 , PbF 2 , YF 3 , LaF 3 , GdF 3  and LuF 3 , all in anhydrous form, were weighed and mixed in the proportions shown in Tables 2 to 5. To each of the mixtures, 4 g of ammonium bifluoride was added, and placed in a crucible. The crucible was heated in a resistivity-heated furnace in the same manner as in Example 1 to melt the contents of the crucible. The melt inside the crucible was poured into a preheated mold for quenching, thereby obtaining a glass rod. 
     A 10 mm long cylindrical rod was cut out of each glass rod, and its opposite end surfaces were polished, followed by measuring the transmission spectrum of the rod. The rod was found to have good transmission at a wavelength of up to 10 μm. Some of the glass samples were measured for Raman spectrum. Peaks of all measurements were noted around 500 cm -1 , showing that the resulting glasses had small phonon energy. 
     
                                           TABLE 2__________________________________________________________________________No  InF3GaF3   ZnF2      PbF2         CdF2            YF3               LaF3                  GdF3                     LuF3                        *   Tg Tx Tx - Tg__________________________________________________________________________ 2  17.017.0   19.0      43.0     4.0      1.6157                            234.0                               296.9                                  62.9 3  17.017.0   19.0      38.0         5.0   4.0      1.6109                            234.0                               281.0                                  47.0 4  17.017.0   19.0      33.0         10.0      4.0             0.0 5  17.017.0   19.0      39.0     8.0          235.4                               304.0                                  68.6 6  17.017.0   19.0      35.0     12.0         233.8                               302.7                                  68.9 7  17.017.0   16.0      43.0         3.0   4.0      1.6181                            231.8                               297.4                                  65.6 8  17.017.0   16.0      39.0         3.0   8.0          232.6                               307.7                                  75.1 9  17.017.0   16.0      35.0         3.0      12.0      235.6                               303.6                                  68.010  17.017.0   13.0      43.0         6.0   4.0          232.3                               300.3                                  68.011  17.017.0   13.0      39.0         6.0   8.0          235.6                               308.7                                  73.112  17.017.0   13.0      35.0         6.0   12.0         237.1                               293.4                                  56.313  13.033.0   12.0      38.0     4.0      1.5983                            256.8                               332.1                                  75.314  17.017.0   16.0      39.0         3.0            8.0         1.6031                            242.2                               340.1                                  97.915  15.015.0   16.0      39.0         3.0            12.0            248.0                               299.1                                  51.116  17.017.0    9.0      39.0         10.0            8.0             243.3                               318.7                                  75.417  17.017.0    9.0      35.0         10.0            12.0                   0.018  17.033.0    8.0      38.0           4.0           0.019  10.040.0    8.0      38.0     4.0          271.9                               344.7                                  72.820  17.017.0   12.0      39.0         3.0            12.0        1.603                            252.5                               348.7                                  96.221  17.017.0    8.0      39.0         3.0            16.0                   0.022  17.017.0   16.0      43.0         3.0            4.0         1.6165                            231.7                               319.6                                  87.923  17.017.0   16.0      35.0         3.0            12.0        1.598                            254.1                               355.4                                  101.324   6.040.0   12.0      38.0     4.0                 0.025   9.033.0   16.0      38.0     4.0                 0.026  17.033.0   12.0      34.0     4.0      1.5867                            263.0                               351.7                                  88.727   9.033.0   12.0      42.0     4.0          254.9                               332.4                                  77.528  16.026.0   16.0      38.0     4.0          248.1                               337.0                                  88.929  20.026.0   12.0      38.0     4.0                 0.030   9.033.0   12.0      42.0     4.0      1.6031                            254.4                               328.4                                  74.031  13.029.0   12.0      42.0     4.0      1.6042                            249.8                               330.6                                  80.832  13.037.0   12.0      34.0     4.0          270.3                               356.4                                  86.133  19.019.0   16.0      39.0         3.0            4.0         1.5984                            237.3                               319.4                                  82.134  17.017.0   20.0      39.0         3.0            4.0         1.6 237.7                               291.7                                  54.035  21.029.0   12.0      34.0     4.0          257.7                               348.1                                  90.436  13.033.0   16.0      34.0     4.0      1.587                            261.4                               353.5                                  92.137  21.033.0    8.0      34.0     4.0                 0.038   9.029.0   16.0      42.0     4.0          246.6                               325.4                                  78.839  17.029.0    8.0      42.0     4.0      1.6116                            246.4                               327.0                                  80.640  13.033.0    8.0      42.0     4.0      1.5941                            257.2                               332.5                                  75.341  17.025.0   12.0      42.0     4.0      1.6034                            245.4                               324.4                                  79.042  15.015.0   20.0      43.0         3.0            4.0             231.5                               299.1                                  67.643  13.013.0   20.0      43.0         3.0            8.0             237.3                               286.6                                  49.344  15.015.0   16.0      43.0         3.0            8.0             236.3                               301.0                                  64.745  17.017.0   12.0      43.0         3.0            8.0             237.8                               327.9                                  90.146  13.013.0   16.0      43.0         3.0            12.0                   0.0__________________________________________________________________________ * Refractive Index 
    
     
                                           TABLE 3__________________________________________________________________________No  InF3GaF3   ZnF2      PbF2         CdF2            YF3               LaF3                  GdF3                     LuF3                        *   Tg Tx Tx - Tg__________________________________________________________________________47  15.015.0   12.0      43.0         3.0            12.0                  0.048  13.037.0    4.0      42.0     4.0                0.049  17.033.0    4.0      42.0     4.0                0.050  21.029.0    4.0      42.0     4.0                0.051  21.025.0    8.0      42.0     4.0                0.052   9.037.0    8.0      42.0     4.0                0.053  13.025.0   16.0      42.0     4.0          240.5                               308.4                                  67.954  13.029.0    8.0      46.0     4.0                0.055   9.033.0    8.0      46.0     4.0                0.056  19.019.0   12.0      43.0         3.0            4.0         1.6066                            234.3                               313.8                                  79.557  13.025.0   12.0      46.0     4.0                0.058   9.029.0   12.0      46.0     4.0                0.059  21.021.0    8.0      43.0         3.0            4.0                   0.060  13.029.0   12.0      42.0  4.0                   0.061   9.029.0   12.0      42.0  4.0               4.0                0.062  13.025.0   12.0      42.0  4.0               4.0          253.5                               330.5                                  77.063  13.029.0    8.0      42.0  4.0               4.0      1.6014                            259.3                               340.7                                  81.464  11.029.0   10.0      42.0  8.0                   0.065  11.029.0   10.0      42.0  4.0               4.0          258.1                               337.1                                  79.066  13.029.0   12.0      39.0         3.0   4.0      1.5742                            251.6                               327.1                                  75.567  13.029.0   12.0      42.0  2.0               2.0                0.068  13.029.0   12.0      38.0  4.0               4.0      1.578                            258.4                               336.7                                  78.369  17.017.0   16.0      42.0  8.0                   0.070  17.017.0   16.0      36.0         6.0            8.0                   0.071  17.017.0   16.0      33.0         9.0            8.0                   0.072  17.017.0   16.0      30.0         12.0            8.0                   0.073  13.029.0   12.0      36.0         6.0   4.0      1.597                            248.7                               321.1                                  72.474  13.029.0   12.0      33.0         9.0   4.0      1.5779                            252.5                               303.8                                  51.375  13.029.0   12.0      30.0         12.0  4.0                0.076  21.013.0   16.0      39.0         3.0            8.0             238.8                               324.2                                  85.477  13.021.0   16.0      39.0         3.0            8.0         1.5821                            247.8                               340.2                                  92.478  13.029.0   16.0      38.0     4.0          249.5                               315.8                                  66.379  13.029.0   12.0      38.0     8.0                0.080  13.029.0    8.0      42.0     8.0                0.081  13.029.0    8.0      46.0     4.0                0.082  13.029.0   16.0      34.0  4.0               4.0      1.5914                            271.6                               353.1                                  81.583  13.029.0   12.0      34.0  8.0               4.0          272.8                               363.2                                  90.484  13.029.0    8.0      38.0  8.0               4.0          267.7                               353.7                                  86.085  13.029.0    4.0      42.0  8.0               4.0          267.7                               342.0                                  74.386  13.029.0    8.0      39.0         3.0            4.0               4.0                0.087  13.029.0    5.0      42.0         3.0            4.0               4.0                0.088  13.029.0   20.0      30.0  4.0   4.0             0.089  13.029.0    4.0      46.0  4.0      4.0          0.090  13.025.0    8.0      42.0  8.0               4.0          258.8                               317.3                                  58.591  13.021.0   12.0      42.0  8.0               4.0                0.0__________________________________________________________________________ * Refractive Index 
    
     
                                           TABLE 4__________________________________________________________________________No InF3 GaF3    ZnF2       PbF2          CdF2             YF3                LaF3                   GdF3                      LuF3                         *   Tg Tx Tx - Tg__________________________________________________________________________ 92   13.0 17.0    16.0       42.0  8.0                4.0                 0.0 93   13.0 21.0    16.0       42.0  4.0                4.0          248.4                                319.4                                   71.0 94   21.0 21.0     8.0       39.0          3.0             8.0                    0.0 95   17.0 25.0     8.0       39.0          3.0             8.0         1.602                             255.6                                349.7                                   94.1 96   17.0 21.0    12.0       39.0          3.0             8.0         1.604                             249.5                                343.9                                   94.4 97   21.0 17.0    12.0       39.0          3.0             8.0             245.8                                339.3                                   93.5 98   17.0 13.0    20.0       39.0          3.0             8.0             237.5                                303.6                                   66.1 99   17.0 13.0    16.0       39.0          3.0             12.0                   0.0100   17.0 21.0    16.0       39.0          3.0             4.0             240.9                                329.6                                   88.7101   17.0 21.0     8.0       39.0          3.0             12.0         1.6071                             258.6                                357.1                                   98.5102   17.0 25.0    12.0       39.0          3.0             4.0             244.3                                331.9                                   87.6103   17.0 25.0     4.0       39.0          3.0             12.0                   0.0104   13.0 33.0     4.0       42.0  4.0                4.0          264.1                                328.3                                   64.2105   13.0 25.0     4.0       42.0  12.0                4.0          270.8                                318.8                                   48.0106   15.5 21.0    20.0       39.0          3.0             1.5             235.8                                315.8                                   80.0107   15.5 25.0    16.0       39.0          3.0   1.5          234.1                                284.9                                   50.8108    5.0 25.0    25.0       30.0          13.5             1.5                    0.0109   17.0 23.0    10.0       39.0          3.0             8.0         1.597                             252.0                                355.6                                   103.6110   19.0 19.0    16.0       35.0          3.0             8.0                    0.0111   17.0 17.0    20.0       35.0          3.0             8.0                    0.0112   19.0 19.0    12.0       35.0          3.0             12.0                   0.0113   25.0 13.0    12.0       35.0          3.0             12.0                   0.0114   21.0 21.0    16.0       35.0          3.0             4.0                    0.0115   19.0 19.0    20.0       35.0          3.0             4.0                    0.0116   17.0 13.0     4.0       34.0          20.0  12.0                0.0117   17.0 17.0     4.0       30.0          20.0             12.0                   0.0118   15.0 15.0    24.0       39.0          3.0             4.0             232.5                                276.7                                   44.2119   15.0 15.0    20.0       39.0          3.0             8.0             237.6                                301.7                                   64.1120   19.0 19.0    12.0       39.0          3.0             8.0                    0.0121   17.0 17.0     8.0       39.0          7.0             12.0        1.586                             244.4                                347.4                                   103.0122   17.0 17.0    12.0       43.0          3.0             8.0                    0.0123   15.0 15.0    20.0       35.0          3.0             12.0                   0.0124   15.0 15.0    16.0       35.0          7.0             12.0                   0.0125   21.0 21.0     8.0       39.0          3.0             8.0                    0.0126   13.0 33.0     8.0       38.0  4.0                4.0          266.5                                347.6                                   81.0127   13.0 29.0     8.0       42.0  4.0                4.0                 0.0128   13.0 33.0    12.0       34.0  4.0                4.0      1.563                             269.1                                360.9                                   91.8129   13.0 25.0    12.0       42.0  4.0                4.0       1.6077                             254.3                                335.7                                   81.4130   13.0 29.0    16.0       34.0  4.0                4.0       1.5797                             264.3                                359.8                                   95.5131   13.0 25.0    16.0       38.0  4.0                4.0       1.5965                             251.6                                349.9                                   98.3132   13.0 37.0    12.0       30.0  4.0                4.0                 0.0133   13.0 37.0     8.0       34.0  4.0                4.0          276.8                                356.5                                   79.7134   13.0 29.0    20.0       30.0  4.0                4.0          263.4                                349.9                                   86.5135   13.0 33.0    16.0       30.0  4.0                4.0      1.568                             270.9                                373.3                                   102.4136   13.0 21.0    20.0       38.0  4.0                4.0       1.6032                             249.1                                328.5                                   79.4__________________________________________________________________________ * Refractive Index 
    
     
                                           TABLE 5__________________________________________________________________________No InF3 GaF3    ZnF2       PbF2          CdF2             YF3                LaF3                   GdF3                      LuF3                         *   Tg Tx Tx - Tg__________________________________________________________________________137   13.0 37.0    4.0       38.0  4.0                4.0      1.5895                             273.8                                350.1                                   76.3138   13.0 29.0    9.0       38.0          3.0             4.0                4.0          251.3                                333.4                                   82.1139   13.0 29.0    12.0       35.0          3.0             4.0                4.0      1.5778                             244.9                                357.6                                   112.7140   13.0 29.0    6.0       38.0          6.0             4.0                4.0          240.0                                322.1                                   82.1141   13.0 29.0    12.0       32.0          6.0             4.0                4.0          248.7                                346.9                                   98.2142   13.0 29.0    12.0       29.0          9.0             4.0                4.0143   13.0 29.0    3.0       38.0          9.0             4.0                4.0144   13.0 25.0    12.0       38.0          4.0             4.0                4.0145   13.0 25.0    12.0       35.0          7.0             4.0                4.0146   13.0 25.0    8.0       42.0          4.0             4.0                4.0147   13.0 25.0    4.0       42.0          8.0             4.0                4.0__________________________________________________________________________ * Refractive Index 
    
     Preparation of Optical Fiber 
     Core glass sample No. 1 was used as core glass, and fluoride glass with the composition 28 InF 3  -9 GaF 3  -17 ZnF 2  -18 BaF 2  -6 SrF 2  -5 YF 3  -10 NaF-7 LiF (mol %) was used as cladding glass to prepare a fluoride fiber perform by the suction casting method. 
     First, InF 3 , GaF 3 , ZnF 2 , PbF 2 , YF 3 , LaF 3 , BaF 2 , SrF 2 , NaF and LiF, all in anhydrous form, were weighed and mixed to have the compositions of the above core and cladding. Each of the mixtures was placed in a crucible, which was heated under an argon atmosphere in a resistivity-heated furnace to melt the contents of the crucible. 
     Then, a fiber perform was prepared by the suction casting method illustrated in FIG. 5. That is, the temperature of the glass melt formed by heating for 1 hour at 900° C. in the resistivity-heated furnace was lowered to 700° C. Then, the cladding glass melt 2 was poured into a brass mold 1, preheated to 220° C., up to its upper portion. Then, the core glass melt 3 was poured onto the cladding glass melt 2 when the cladding glass melt 2 began solidifying and its upper center began depressing, as shown in FIG. 5A. Significant volume contraction occurred during cooling and solidification. This volume contraction of the cladding glass 2a resulted in the suction of the core glass 3a into the depressed center of the cladding glass 2a. The sucked-in core glass 3a solidified in the center of the cylindrical cladding glass 2a to form a fiber perform 4, as shown in FIG. 5B. The resulting fiber perform 4 had a cladding outer diameter of 5 mm, a core outer diameter of 0.2 to 1.7 mm varying in a tapered manner, and a length of 30 mm. 
     Then, a jacketing tube having the same composition as the cladding glass was prepared by the rotational casting method illustrated in FIG. 6. That is, the raw materials weighed and mixed to have the composition of the cladding glass were put in a crucible, which was heated in a resistivity-heated furnace to melt the contents of the crucible. The resulting jacketing tube melt 12 was poured into a preheated brass mold 11 as shown in FIG. 6A. The mold 11 was laid horizontally and rotated at a high speed as shown in FIG. 6B. While rotated in this condition, the melt 12 was cooled and solidified to obtain a fluoride glass jacketing tube 13 with an outside diameter of 15 mm, an inside diameter of 5 mm, and a length of 140 mm. 
     Then, the glass fiber perform 4 was inserted into the jacketing tube 13 inside a glove box supplied with nitrogen gas with a dew point of -60° C. or lower. As shown in FIG. 7A, the jacketing tube 13 was held by a perform holder 22 via an O-ring 21. Then, with the inside being evacuated, the perform 4 along with the jacketing tube 13 was fed to a heating furnace 23 at a rate of 3 mm/min as shown in FIG. 7B. A lower part of the composite heated to the softening temperature was pulled downward to obtain a glass fiber perform 24 having an outside diameter of 5 mm. 
     Then, a portion having a core diameter of 0.2 mm was cut out of the perform 24, and housed in a heating vacuum chamber together with a jacketing tube 13&#39; prepared in the same manner as described above. Surface treatment was performed in an F 2  -HF mixed gas atmosphere. Inside a glove box (not shown) supplied with nitrogen gas at a dew point of -60° C. or lower, the perform 24 was inserted into the jacketing tube 13&#39;, and the jacketing tube 13&#39; was held by a perform holder 22 via an O-ring 21. Then, with the inside being evacuated, the perform 24 along with the jacketing tube 13&#39; was fed to a fiber drawing furnace 25 at a rate of 3 mm/min, as shown in FIG. 7C. The composite was heated to the softening temperature, and its lower part was pulled downward by a capstan driver 27 via a tensionmeter 26, whereby the composite was drawn into a fiber having an outside diameter of 125 μm. 
     The resulting optical fiber was a single mode fiber having Δn=8% and a core diameter of 1 μm, and its transmission loss at a wavelength of 1.3 μm was as low as 0.2 dB/m. 
     Fibers with low transmission losses were producible by the same method as described above, with the composition of the jacketing tube 13 and 13&#39; being 47.5 HfF 4  -23.5 BaF 2  -2.5 LaF 3  -2 YF 3  -4.5 AlF 3  -20 NaF, or 47.5 ZrF 4  -23.5 BaF 2  -2.5 LaF 3  -2 YF 3  -4.5 AlF 3  -20 NaF, or with the composition of the jacketing tube 13 being 47.5 HfF 4  -23.5 BaF 2  -2.5 LaF 3  -2 YF 3  -4.5 AlF 3  -20 NaF and that of the jacketing tube 13&#39; being 47.5 ZrF 4  -23.5 BaF 2  -2.5 LaF 3  -2 YF 3  -4.5 AlF 3  -20 NaF. 
     Optical fibers were prepared in the same manner as described above, except that the core glass-cladding glass combination was changed as in Table 6. The resulting optical fibers were single mode fibers having Δn=3 to 8%, and their transmission loss at a wavelength of 1.3 μm was as low as 0.2 dB/m. 
     
                                           TABLE 6__________________________________________________________________________Composition of Glass for Core                      Compostion of Glass for Cladding  ΔNo  InF3GaF3   ZnF2      PbF2         CdF2            YF3               LaF3                  *   InF3                         GaF3                            ZnF2                               PbF2                                  BaF2                                     SrF2                                        YF3                                           LaF                                              NaF                                                 LiF                                                    *   n__________________________________________________________________________ 1  13.029.0   16.0      34.0  4.0               4.0                  1.5914                      25.0                         15.0                            16.0                               10.0                                  14.0                                     10.0                                        5.0      5.0                                                    1.5136                                                        4.9 2  13.029.0   12.0      38.0  4.0               4.0                  1.578                      30.0                         10.0                            16.0                               10.0                                  14.0                                     10.0                                        5.0      5.0     1.5195                                                 3.7 3  13.029.0   12.0      35.0         3.0            4.0               4.0                  1.5778                      25.0                         15.0                            16.0  18.0                                      6.0                                        5.0   10.0                                                 5.0     1.4705                                                 6.8 4  13.029.0   16.0      34.0  4.0               4.0                  1.5797                      25.0                         19.0                            12.0                               10.0                                  14.0                                     10.0                                        5.0      5.0     1.506                                                 4.7 5  13.029.0   12.0      35.0         3.0            4.0               4.0                  1.5778                      26.0                         11.0                            15.0                               12.0                                  16.0                                     10.0                                        5.0      5.0     1.5175                                                 3.8 6  13.029.0   12.0      35.0         3.0            4.0               4.0                  1.5778                      26.0                         11.0                            13.0                               12.0                                  18.0                                     10.0                                        5.0      5.0     1.5182                                                 3.8 7  17.017.0   16.0      35.0         3.0            12.0  1.598                      26.0                         11.0                            17.0                               14.0                                  12.0                                     10.0                                        5.0      5.0     1.5237                                                 4.6 8  13.033.0   12.0      34.0  4.0               4.0                  1.563                      26.0                         11.0                            17.0                               16.0                                  10.0                                     10.0                                        5.0      5.0     1.5306                                                 2.1 9  17.023.0   10.0      39.0         3.0            8.0   1.597                      26.0                         11.0                            17.0  18.0                                      6.0                                        5.0   12.0                                                 5.0     1.469                                                 8.010  13.029.0   12.0      35.0         3.0            4.0               4.0                  1.5778                      26.0                         11.0                            17.0  16.0                                      6.0                                        5.0   14.0                                                 5.0     1.4635                                                 7.211  13.025.0   16.0      38.0  4.0               4.0                  1.5965                      24.0                         13.0                            17.0                               14.0                                  12.0                                     10.0                                        5.0      5.0     1.5255                                                 4.412  13.033.0   16.0      30.0  4.0               4.0                  1.568                      28.0                          9.0                            17.0                               14.0                                  12.0                                     10.0                                        5.0      5.0     1.5294                                                 2.513  13.033.0   16.0      34.0     4.0                  1.587                      25.5                         10.5                            16.5                               14.5                                  12.5                                     10.5                                        4.5      5.5     1.5265                                                 3.814  13.029.0   12.0      35.0         3.0            4.0               4.0                  1.5778                      27.5                          8.5                            16.5                               14.5                                  12.5                                     10.5                                        4.5      5.5     1.5302                                                 3.015  17.017.0    8.0      39.0         7.0            12.0  1.586                      28.0                          9.0                            17.0                               12.0                                  14.0                                     10.0                                        5.0      5.0     1.5211                                                 4.116  13.025.0   12.0      42.0  4.0               4.0                  1.6077                      28.0                          9.0                            17.0                               10.0                                  16.0                                     10.0                                        5.0      5.0     1.5165                                                 5.717  17.021.0    8.0      39.0         3.0            12.0  1.6071                      28.0                          9.0                            17.0                               14.0                                  12.0                                     10.0                                        5.0      5.0     1.5275                                                 5.018  17.025.0    8.0      39.0         3.0            8.0   1.602                      28.0                          9.0                            17.0                               12.0                                  16.0                                      8.0                                        5.0      5.0     1.5225                                                 5.019  17.021.0   12.0      39.0         3.0            8.0   1.604                      28.0                          9.0                            17.0                               14.0                                  12.0                                     10.0                                        2.5                                           2.5   5.0     1.5305                                                 4.620  13.021.0   16.0      39.0         3.0            8.0   1.5821                      25.5                         11.5                            17.0                               14.0                                  12.0                                     10.0                                        2.5                                           2.5   5.0     1.5282                                                 3.421  13.029.0   12.0      35.0         3.0            4.0               4.0                  1.5778                      25.0                         15.0                            16.0                               10.0                                  14.0                                     10.0                                        5.0      5.0     1.514                                                 4.122  13.033.0   12.0      34.0  4.0               4.0                  1.563                      26.0                         11.0                            17.0                               20.0                                  10.0                                      6.0                                        5.0      5.0     1.548                                                 1.0__________________________________________________________________________ * Refractive Index 
    
     COMPARATIVE EXAMPLE 2 
     A comparative optical fiber was prepared in the same manner as in Example 2 using as the core glass fluoride glass with the composition 16 InF 3  -19 GaF 3  -15 ZnF 2  -22 CdF 2  -28 PbF 2  (mol %), and as the cladding glass fluoride glass with the composition 28 InF 3  -9 GaF 3  -12 ZnF 2  -18 BaF 2  -6 SrF 2  -5 YF 3  -10 NaF-7 LiF-5 CdF 2  (mol %). 
     The fluoride glass used here as the core glass was different from the fluoride glass used in Example 2 in that the CdF 2  concentration was more than 20 mol %, the PbF 2  concentration was less than 30 mol %, and at least one member selected from the group consisting of LaF 3 , YF 3 , GdF 3  and LuF 3  was not contained. The fluoride glass used here as the cladding glass was different from the fluoride glass used as the cladding glass in Example 2 in that 5 mol % of CdF 2  was contained. 
     The resulting optical fiber was a single mode fiber having a length of 100 m, a core diameter of 1.7 μm, and a cut-off wavelength of 0.95 μm, but its transmission loss at a wavelength of 1.3 μm was as high as 10 dB/m. 
     EXAMPLE 3 
     Optical fibers were prepared in the same manner as in Example 2, except that the combination of the core glass composition and the cladding glass composition was changed as in Table 7. The resulting optical fibers were all single mode fibers having Δn=1 to 4%, and their transmission loss at wavelength 1.3 μm was as low as 0.1 dB/m. FIG. 8 shows the transmission loss spectrum of the optical fibers at 1 to 4 μm. The transmission loss decreased as the wavelength became longer, and a minimum loss of 0.025 dB/m was obtained at wavelength 3.3 μm. 
     
                                           TABLE 7__________________________________________________________________________Composition of Glass for CoreNo  InF3  GaF3     ZnF2        PbF2           BaF2              SrF2                 LaF3                    YF3                       GdF3                          LuF3                             NaF                                LiF__________________________________________________________________________1   28.0   9.0     17.0        12.0           16.0              8.0                 2.5                    2.5         5.02   28.0   9.0     17.0        10.0           16.0              8.0                 2.5                    2.5      2.0                                5.03   28.0   9.0     17.0         8.0           16.0              8.0                 2.5                    2.5      4.0                                5.04   28.0   9.0     17.0         6.0           16.0              8.0                 2.5                    2.5      6.0                                5.05   28.0   9.0     17.0         4.0           16.0              8.0                 2.5                    2.5      8.0                                5.06   25.5  11.5     15.0        12.0           18.0              8.0                 2.5                    2.5         5.07   25.5  11.5     15.0        10.0           18.0              8.0                 2.5                    2.5      2.0                                5.08   25.5  11.5     15.0         8.0           18.0              8.0                 2.5                    2.5      4.0                                5.09   25.5  11.5     15.0         6.0           18.0              8.0                 2.5                    2.5      6.0                                5.010  25.5  11.5     15.0         4.0           18.0              8.0                 2.5                    2.5      8.0                                5.011  25.5  11.5     15.0        12.0           18.0              8.0                 2.5                    2.5         5.012  25.5  11.5     15.0        12.0           18.0              8.0                 2.5                    2.5         5.013  25.5  11.5     15.0        12.0           18.0              8.0                 2.5                    2.5         5.014  25.5  11.5     15.0        12.0           18.0              8.0   2.5                       2.5      5.015  25.5  11.5     15.0        12.0           18.0              8.0   2.5   2.5   5.0__________________________________________________________________________Composition of Glass for Cladding      ΔNo InF3 GaF3    ZnF2       PbF2          BaF2             SrF2                LaF3                   YF3                      GdF3                         LuF3                            NaF                               LiF                                  n__________________________________________________________________________1  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      12.0                               5.0                                  4.02  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      12.0                               5.0                                  3.53  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      12.0                               5.0                                  3.04  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      12.0                               5.0                                  2.55  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      12.0                               5.0                                  2.06  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      10.0                               7.0                                  3.77  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      10.0                               7.0                                  3.08  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      10.0                               7.0                                  2.59  25.5 11.5    14.0  19.0             8.0                2.5                   2.5      10.0                               7.0                                  2.010 25.5 11.5    14.0  19.0             8.0                2.5                   2.5      10.0                               7.0                                  1.511 25.5 11.5    14.0  19.0             8.0                2.5                   2.5      12.0                               5.0                                  4.012 25.5 11.5    14.0       2.5          19.0             8.0                2.5                   2.5       7.5                               7.0                                  2.513 25.5 11.5    14.0       5.0          19.0             8.0                2.5                   2.5       5.0                               7.0                                  1.014 25.5 11.5    14.0  19.0             8.0   2.5                      2.5   10.0                               7.0                                  3.715 25.5 11.5    14.0  19.0             8.0   2.5   2.5                            10.0                               7.0                                  3.7__________________________________________________________________________ 
    
     EXAMPLE 4 
     An optical fiber was prepared in the same manner as in Example 2, except that the core glass was doped with 500 ppm Pr 3+ . The resulting optical fiber had an outside diameter of 125 μm, Δn=8%, a core diameter of 1 μm, a cut-off wavelength of 1 μm. Its transmission loss at a wavelength of 1.3 μm was as low as 0.2 dB/m. 
     A large peak shown in FIG. 9 is an absorption due to Pr 3+ . An amplifier for amplifying signal light with a wavelength of 1.31 μm by pumping with light at a wavelength of 1.017 μm was constructed using the optical fiber obtained in Example 4. A gain coefficient of 0.5 dB/mW was obtained. 
     Single mode optical fibers of the formulation shown in Table 8 were prepared, whose core glass for the fiber with Δn=2.5% was doped with 1000 ppm Pr 3+ , or the core glass for the fiber with Δn=3.7%, 6.6% or 8% was doped with 500 ppm Pr 3+ . Amplifiers for amplifying the signal light at a wavelength of 1.31 μm, by pumping with a light at a wavelength of 1.017 μm, were constructed using the resulting optical fibers. The optical fiber with Δn=2.5% gave a gain coefficient of 0.25 dB/mW. The optical fiber with Δn=3.7% gave a gain coefficient of 0.3 dB/mW. The optical fiber with Δn=6.6% gave a gain coefficient of 0.4 dB/mW. The optical fiber with Δn=8% gave a gain coefficient of 0.5 dB/mW. 
     
                                           TABLE 8__________________________________________________________________________Δ n__________________________________________________________________________2.5%   Core       25.5InF3-11.5GaF3-15ZnF2-12PbF2-18BaF2-8SrF2-2.5YF3-2.5LaF3-         5LiF   Kladding   25.5InF3-11.5GaF3-14ZnF2-2.5PbF2-19BaF2-8SrF2-2.5YF3-2.5LaF3         -7.5NaF-7LiF   A First Jacketing Tube         47.5HfF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF   A second Jacketing Tube         47.5ZrF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF2.5%   Core       25.5InF3-11.5GaF3-15ZnF2-12PbF2-18BaF2-8SrF2-2.5YF3-2.5LaF3-         5LiF   Kladding   25.5InF3-11.5GaF3-14ZnF2-2.5PbF2-19BaF2-8SrF2-2.5YF3-2.5LaF3         -7.5NaF-7LiF   A First Jacketing Tube         25.5InF3-11.5GaF3-14ZnF2-2.5PbF2-19BaF2-8SrF2-2.5YF3-2.5LaF3         -7.5NaF-7LiF   A Second Jacketing Tube         47.5ZrF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF3.7%   Core       22InF3-16GaF3-14ZnF2-19PbF2-13BaF2-6SrF2-3YF3-3LaF3-4LiF   Kladding   25.5InF3-11.5GaF3-14ZnF2-2.5PbF2-19BaF2-8SrF2-2.5YF3-2.5LaF3         -7.5NaF-7LiF   A First Jacketing Tube         47.5HfF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF   A Second Jacketing Tube         47.5ZrF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF3.7%   Core       25.5InF3-11.5GaF3-15ZnF2-12PbF2-18BaF2-8SrF2-2.5YF3-2.5LaF3-         5LiF   Kladding   25.5InF3-11.5GaF3-14ZnF2-19BaF2-8SrF2-2.5YF3-2.5LaF3-10NaF-7         LiF   A First Jacketing Tube         25.5InF3-11.5GaF3-14ZnF2-19BaF2-8SrF2-2.5YF3-2.5LaF3-10NaF-7         LiF   A Second Jacketing Tube         47.5ZrF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF6.6%   Core       38PbF2-29GaF3-13InF3-12ZnF2-4YF3-4LaF3   Kladding   25.5InF3-11.5GaF3-14ZnF2-2.5PbF2-19BaF2-8SrF2-2.5YF3-2.5LaF3         -7.5NaF-7LiF   A First Jacketing Tube         47.5HfF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF   A Second Jacketing Tube         47.5ZrF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF8.0%   Core       38PbF2-29GaF3-13InF3-12ZnF2-4YF3-4LaF3   Kladding   25.5InF3-11.5GaF3-14ZnF2-19BaF2-8SrF2-2.5YF3-2.5LaF3-10NaF-7         LiF   A First Jacketing Tube         25.5InF3-11.5GaF3-14ZnF2-19BaF2-8SrF2-2.5YF3-2.5LaF3-10NaF-7         LiF   A Second Jacketing Tube         47.5ZrF4-23.5BaF2-2.5LaF3-2YF3-4.5AlF3-20NaF__________________________________________________________________________ 
    
     COMPARATIVE EXAMPLE 3 
     A comparative optical fiber was prepared in the same manner as in Example 2 using as the core glass 500 ppm PrF 3  -doped ZrF 4  -based fluoride glass with the composition 50 ZrF 4  -15 BaF 2  -3.5 LaF 3  -10 PbF 2  -2 YF 3  -2.5 AlF 3  -10 LiF-7 NaF (mol %), and as the cladding glass ZrF 4  -based fluoride glass with the composition 47.5 ZrF 4  -23.5 BaF 2  -2.5 LaF 3  -2 YF 3  -4.5 AlF 3  -20 NaF (mol %). 
     The resulting optical fiber was a single mode fiber having a Δn of 3.7%, a core diameter of 1.7 μm, and a cut-off wavelength of 0.95 μm, and its transmission loss at a wavelength of 1.3 μm was as low as 0.2 dB/m. An amplifier of signal light with a wavelength of 1.31 μm by pumping with light at a wavelength of 1.017 μm was constructed using the optical fiber obtained in Comparative Example 3. A gain coefficient of 0.2 dB/mW was obtained. 
     A gain coefficient obtained with the high Δn fiber (Δn=6.1%) according to the reported Japanese Patent Application No. 5-281112 was 0.25 dB/mW, which was inferior to the gain coefficient of the claimed fluoride fiber. 
     A comparison between Example 4 and Comparative Example 3 showed that the use of the claimed fluoride fiber suppressed the nonradiative relaxation of Pr 3+ , increasing the emission efficiency. 
     EXAMPLE 5 
     Optical fibers were prepared in the same manner as in Example 2, except that the combination of the core glass composition and the cladding glass composition was changed as in Table 9, and that each core glass was doped with the rare earth ions shown in Table 9. The transmission spectra of the resulting optical fibers were measured for optical loss. The results revealed the increase in transmission loss at the absorption wavelength of the doped rare earth ions. 
     
                                           TABLE 9__________________________________________________________________________Doper Rare Composition of Glass for CoreNo  Earth Ion      InF3          GaF3              ZnF2                  PbF2                      CdF2                          YF3 LaF3                                  *__________________________________________________________________________ 1  Pr     13.0          29.0              16.0                  34.0    4.0 4.0 1.5914 2  Pr, Yb 13.0          29.0              12.0                  38.0    4.0 4.0 1.578 3  Tm     13.0          29.0              12.0                  35.0                      3.0 4.0 4.0 1.5778 4  Tm, Ho, Eu      13.0          29.0              16.0                  34.0    4.0 4.0 1.5797 5  Nd     13.0          29.0              12.0                  35.0                      3.0 4.0 4.0 1.5778 6  Er     13.0          29.0              12.0                  35.0                      3.0 4.0 4.0 1.5778 7  Er, Yb 17.0          17.0              16.0                  35.0                      3.0 12.0    1.598 8  Pr, Nd 13.0          33.0              12.0                  34.0    4.0 4.0 1.563 9  Ce     17.0          23.0              10.0                  39.0                      3.0 8.0     1.59710  Pm     13.0          29.0              12.0                  35.0                      3.0 4.0 4.0 1.577811  Sm     13.0          25.0              16.0                  38.0    4.0 4.0 1.596512  Sm, Eu 13.0          33.0              16.0                  30.0    4.0 4.0 1.56813  Tb     13.0          33.0              16.0                  34.0        4.0 1.58714  Dy     13.0          29.0              12.0                  35.0                      3.0 4.0 4.0 1.577815  Dy, Yb 17.0          17.0               8.0                  39.0                      7.0 12.0    1.58616  Dy, Er 13.0          25.0              12.0                  42.0    4.0 4.0 1.607717  Yb     17.0          21.0               8.0                  39.0                      3.0 12.0    1.607118  Eu     17.0          25.0               8.0                  39.0                      3.0 8.0     1.60219  Dy, Pr 17.0          21.0              12.0                  39.0                      3.0 8.0     1.60420  Nd, Yb 13.0          21.0              16.0                  39.0                      3.0 8.0     1.582121  Er, Tm 13.0          29.0              12.0                  35.0                      3.0 4.0 4.0 1.577822  Pr     13.0          33.0              12.0                  34.0    4.0 4.0 1.563__________________________________________________________________________Doper Rare   Composition of Glass for Cladding ΔNo  Earth Ion   InF3      GaF3         ZnF2            PbF2               BaF2                  SrF2                     YF3                        LaF                           NaF                              LiF                                 *   n__________________________________________________________________________ 1  Pr    25.0      15.0         16.0            10.0               14.0                  10.0                     5.0      5.0                                 1.5136                                     4.9 2  Pr, Yb   30.0      10.0         16.0            10.0               14.0                  10.0                     5.0      5.0                                 1.5195                                     3.7 3  Tm    25.0      15.0         16.0  18.0                   6.0                     5.0   10.0                              5.0                                 1.4705                                     6.8 4  Tm, Ho, Eu   25.0      19.0         12.0            10.0               14.0                  10.0                     5.0      5.0                                 1.506                                     4.7 5  Nd    26.0      11.0         15.0            12.0               16.0                  10.0                     5.0      5.0                                 1.5175                                     3.8 6  Er    26.0      11.0         13.0            12.0               18.0                  10.0                     5.0      5.0                                 1.5182                                     3.8 7  Er, Yb   26.0      11.0         17.0            14.0               12.0                  10.0                     5.0      5.0                                 1.5237                                     4.6 8  Pr, Nd   26.0      11.0         17.0            16.0               10.0                  10.0                     5.0      5.0                                 1.5306                                     2.1 9  Ce    26.0      11.0         17.0  18.0                   6.0                     5.0   12.0                              5.0                                 1.469                                     8.010  Pm    26.0      11.0         17.0  16.0                   6.0                     5.0   14.0                              5.0                                 1.4635                                     7.211  Sm    24.0      13.0         17.0            14.0               12.0                  10.0                     5.0      5.0                                 1.5255                                     4.412  Sm, Eu   28.0       9.0         17.0            14.0               12.0                  10.0                     5.0      5.0                                 1.5294                                     2.513  Tb    25.5      10.5         16.5            14.5               12.5                  10.5                     4.5      5.5                                 1.5265                                     3.814  Dy    27.5       8.5         16.5            14.5               12.5                  10.5                     4.5      5.5                                 1.5302                                     3.015  Dy, Yb   28.0       9.0         17.0            12.0               14.0                  10.0                     5.0      5.0                                 1.5211                                     4.116  Dy, Er   28.0       9.0         17.0            10.0               16.0                  10.0                     5.0      5.0                                 1.5165                                     5.717  Yb    28.0       9.0         17.0            14.0               12.0                  10.0                     5.0      5.0                                 1.5275                                     5.018  Eu    28.0       9.0         17.0            12.0               16.0                   8.0                     5.0      5.0                                 1.5225                                     5.019  Dy, Pr   28.0       9.0         17.0            14.0               12.0                  10.0                     2.5                        2.5   5.0                                 1.5305                                     4.620  Nd, Yb   25.5      11.5         17.0            14.0               12.0                  10.0                     2.5                        2.5   5.0                                 1.5282                                     3.421  Er, Tm   25.0      15.0         16.0            10.0               14.0                  10.0                     5.0      5.0                                 1.514                                     4.122  Pr    26.0      11.0         17.0            20.0               10.0                   6.0                     5.0      5.0                                 1.548                                     1.0__________________________________________________________________________ * Refractive Index 
    
     EXAMPLE 6 
     Optical fibers were prepared in the same manner as in Example 2, except that the combination of the core glass composition and the cladding glass composition was changed as in Tables 10 and 11, and that each core glass was doped with the rare earth ions shown in Tables 10 and 11. The transmission spectra of the resulting optical fibers were measured for optical loss. The results revealed the increase in transmission loss at the absorption wavelength of the doped rare earth ions. 
     
                                           TABLE 10__________________________________________________________________________Doped Rare     Glass Composition for Optical Fibre (mol %)No.   Earth Ion   InF3             GaF3                ZnF2                   PbF2                      BaF2                         SrF2                            CdF2                               YF3                                  LaF3                                     GdF3                                        LiF                                           NaF__________________________________________________________________________1  Pr, 1000 ppm     Core 13.0             29.0                16.0                   34.0        4.0                                  4.0     Cladding          29.0              8.0                15.0  14.0                         4.0   2.5                                  2.5   5.0                                           202  Nd, 1000 ppm     Core 13.0             29.0                12.0                   38.0        4.0                                  4.0     Cladding          29.0              8.0                15.0  14.0                         4.0   2.5                                  2.5   5.0                                           203  Tm, 1000 ppm     Core 13.0             29.0                12.0                   35.0     3.0                               4.0                                  4.0   Yb, 4000 ppm     Cladding          25.5             11.5                15.0  18.0                         8.0   2.5                                  2.5   5.0                                           12.04  Tm, 1000 ppm     Core 13.0             25.0                16.0                   38.0        4.0                                  4.0     Cladding          26.0              8.0                17.0  19.0                         8.0   2.5                                  2.5   5.0                                           12.05  Ho, 1000 ppm     Core 13.0             33.0                12.0                   34.0        4.0                                  4.0     Cladding          25.5             11.5                15.0  18.0                         8.0   2.5                                  2.5   5.0                                           12.06  Er, 2000 ppm     Core 13.0             29.0                12.0                   38.0        4.0   4.0     Cladding          25.5             11.5                15.0                    5.0                      18.0                         8.0   2.5                                  2.5   5.0                                            7.07  Tm, 0.5 wt %     Core 13.0             29.0                12.0                   38.0        4.0                                  4.0   Ho, 1 wt %     Cladding          25.5             11.5                15.0                    2.5                      18.0                         8.0   2.5                                  2.5   5.0                                            9.58  Tm, 2000 ppm     Core 13.0             29.0                12.0                   38.0        4.0                                  4.09  Tb, 4000 ppm     Cladding          25.5             11.5                15.0                   12.0                      18.0                         8.0   2.5                                  2.5   5.0   Pr, 500 ppm     Core 13.0             29.0                16.0                   34.0        4.0                                  4.0     Cladding          25.5             11.5                15.0  18.0                         8.0   2.5                                  2.5   5.0                                           12.010 Pr, 2000 ppm     Core 13.0             29.0                16.0                   34.0        4.0                                  4.0     Cladding          25.5             11.5                15.0  18.0                         8.0   2.5                                  2.5   5.0                                           12.011 Er, 1000 ppm     Core 13.0             29.0                15.0                   34.0        4.0                                  4.0     Cladding          25.5             11.5                16.0  18.0                         8.0   2.5                                  2.5   5.0                                           12.012 Nd, 2000 ppm     Core 13.0             29.0                12.0                   38.0        4.0                                  4.0     Cladding          29.0              8.0                15.0  18.0                         8.0   2.5                                  2.5   5.0                                           12__________________________________________________________________________ 
    
     
                                           TABLE 11__________________________________________________________________________Doped Rare     Glass Composition for Optical Fibre (mol %)No.   Earth Ion   InF3             GaF3                ZnF2                   PbF2                      BaF2                         SrF2                            LaF3                               YF3                                  NaF                                     LiF__________________________________________________________________________1  Pr, 1000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0                    2.5                      19.0                         8.0                            2.5                               2.5                                   7.5                                     7.02  Nd, 1000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0                    2.5                      19.0                         8.0                            2.5                               2.5                                   7.5                                     7.03  Tm, 1000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0   Yb, 4000 ppm     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.04  Tm, 1000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.05  Ho, 1000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.06  Er, 2000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.07  Tm, 0.5 wt %     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0   Ho, 1 wt %     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.08  Tm, 2000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0   Tb, 4000 ppm     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.09  Pr, 500 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.010 Pr, 2000 ppm     Clore          25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.011 Er, 1000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.012 Nd, 2000 ppm     Core 25.5             11.5                15.0                   12.0                      18.0                         8.0                            2.5                               2.5   5.0     Cladding          25.5             11.5                14.0  19.0                         8.0                            2.5                               2.5                                  10.0                                     7.0__________________________________________________________________________ 
    
     EXAMPLE 7 
     Table 8 shows combinations of the optimal glass compositions for the core, cladding and jacketing tubes of the claimed single mode optical fiber, the combinations provided for the relative refractive index differences of 2.5%, 3.7%, 6.6% and 8%. In Table 8, it should be noted that the amounts of the substituted PbF 2  and the substituting NaF were adjusted to bring the refractive index of the cladding into agreement with the refractive index of the first jacketing tube for which the composition of the cladding glass was used. 
     EXAMPLE 8 
     2 Meters of the optical fiber No. 3 shown in Tables 10 and 11, a fiber doped with 1000 ppm of Tm and 4000 ppm of Yb as rare earth ions, was used to construct an optical fiber laser. Each end of the rare earth ion-doped optical fiber was butted to a dielectric mirror to form a Fabry-Perot laser cavity. A Nd-YAG laser operating at 1.12 μm was used as a pump source. Light from this pump source was focused by a lens onto the fiber end. The dielectric mirror with the transparent to the pump wavelength and highly reflective to the lasing wavelength of 450 to 500 nm was used. The optical fiber laser with this arrangement gave blue laser oscillations at wavelengths 455 nm and 480 nm. 
     EXAMPLE 9 
     2 Meters of the optical fiber No. 4 shown in Tables 10 and 11, a fiber doped with 1000 ppm of Tm as rare earth ions, was used to construct the same laser cavity as in Example 8. The pump source was a krypton ion laser, and the pump wavelengths were 647 nm and 676 nm. The dielectric mirror with the transparent to the pump wavelengths and highly reflective to the lasing wavelength of 450 to 500 nm was used. The optical fiber laser in this configuration gave blue laser oscillations at wavelengths 455 nm and 480 nm similar to Example 8. Furthermore, a high output power LD, operating at 1.48 μm, was added to the pump source of Example 9, so that two wavelength pumping involving 647 nm and 1.48 μm was performed to increase a blue laser output power. 
     EXAMPLE 10 
     2 Meters of the optical fiber No. 6 shown in Tables 10 and 11, a fiber doped with 2000 ppm of Er as rare earth ions, was used to construct the same laser cavity as in Example 7. The pump source was a laser diode operating at 0.8 μm or 0.98 μm, and the mirror was one highly reflective to the lasing wavelength of 540 to 545 nm. The optical fiber laser in this configuration gave green laser oscillation at wavelength 540 nm. 
     Laser oscillations were also observed at wavelength 412 nm in the fiber (No. 2, 12) doped with Nd as rare earth ions, at wavelength 492 nm in the fiber (No. 1, 10) doped with Pr, and at wavelength 549 nm in the fiber (No. 5) doped with Ho. 
     EXAMPLE 11 
     10 Meters of the optical fiber No. 11 shown in Tables 10 and 11, a fiber doped with 1000 ppm of Er as rare earth ions, was used to construct a 1.5 μm-band optical amplifier. That is, signal light (wavelength 1.55 μm) and pump light (wavelength 1.48 μm) from LDs were combined by a WDM fiber coupler, and launched into the fiber end. An output signal was obtained from the output end via an optical isolator. A gain of more than 25 dB was obtained throughout the wavelength band of 1530 to 1560 nm with a pump power of 150 mW. 
     EXAMPLE 12 
     The optical fiber No. 7 shown in Tables 10 and 11, a fiber doped with 0.5 wt. % Tm and 1 wt. % Ho as rare earth ions, was used to construct a 1.4 μm-band optical amplifier. The pump source was a laser diode operating at 0.8 μm. A gain of 20 dB was obtained with a pump power of 100 mW. 
     EXAMPLE 13 
     The optical fiber No. 8 shown in Tables 10 and 11, a fiber doped with 2000 ppm of Tm and 4000 ppm of Tb as rare earth ions, was used to construct a 1.65 μm-band optical amplifier. The pump source was a laser diode operating at 1.2 μm. A gain of 20 dB was obtained with a pump power of 100 mW. 
     EXAMPLE 14 
     20 meters of optical fiber No. 9 shown in Tables 10 and 11 was used to construct a 1.3 μm-band optical amplifier. The pump source was a laser diode operating at 0.98 μm. A gain of 20 dB was obtained with a pump power of 200 mW. 
     The present invention has been described in detail with respect to various embodiments, and it will now be apparent from the foregoing to those skilled in the art tat changes and modifications may be made without departing from the invention in its broader aspects, and it is our intention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention. 
     As is apparent from the foregoing description, the present invention has succeeded in providing fluoride glass having satisfactory infrared transmission. The invention has also permitted the production of an optical fiber for optical amplification with low loss and high efficiency (Δn). Thus, the invention has made it possible to increase the gain coefficient and the effective gain, and to construct an optical amplifier for semiconductor laser pumping essential for practical use. Furthermore, the invention provides the advantages of lowering the cost of and raising the performance of optical communication systems.