WO2004028992A1

WO2004028992A1 - Tellurite glass, optical fibre, optical amplifier and light source

Info

Publication number: WO2004028992A1
Application number: PCT/BR2002/000135
Authority: WO
Inventors: Luiz Carlos Barbosa; Carlos Lenz Cesar
Original assignee: Ericsson Telecomunicações S.A.
Priority date: 2002-09-27
Filing date: 2002-09-27
Publication date: 2004-04-08
Also published as: AU2002333008A1

Abstract

The present invention relates to a tellurite glass composition suitable for use in an optical fiber. A glass composition including at least 4 mole % of at least one alkalihalide XY for increasing its thermal stability range is provided, X being selected from the group of K, Rb, Cs and Fr and Y being selected from the group of F, Cl, Br and I. The XY content of a preferred glass composition is about 10 mole %. The addition of alkalihalide increases the thermal stability range of the glass considerably without loosing other desirable properties. The glass composition may be doped with Er2O3 or another lanthanide oxide for amplifying purposes. The increased thermal stability results in more reliable optical fiber drawing and an improved manufacturing process. Appropriate control of the refractive index of the glass composition is preferably achieved with Nb2O5 or Bi2O3.

Description

TEL URITE GLASS, OPTICAL FIBRE, OPTICAL AMPLIFIER AND LIGHT SOURCE

TECHNICAL FIELD

The present invention relates to optical fibers and in particular to a tellurite glass composition suitable for use in an optical fiber, especially for amplification of optical signals.

BACKGROUND

Telecommunication networks of today generally employ optical fibers, such as silica optical fibers, for signal transmission. Optical signals are transported long distances on one or a plurality of optical carriers and features like long legs and power splitting necessitate amplification or regeneration of weakened signals. Optical amplification is often the most desirable option, since it offers direct amplification without problematic conversion between optical and electric signals.

Optical amplifiers typically comprise a comparatively short amplifier fiber doped with a rare-earth metal or another substance that is capable of fluorescing. Light of the same wavelength as the input signals is pumped into the amplifier fiber by a pump laser and absorbed photons cause electrons of the rare-earth atoms to jump to a temporary excited stage. As the electrons decay, photons are released and added to the input signal, increasing its gain.

Optical amplifiers doped with the rare-earth metal erbium (Erbium Doped Fiber Amplifiers, EDFA) are particularly advantageous since they operate near the important 1.5 μm region. Furthermore, an EDFA is not polarization sensitive and provides high gain, a broad bandwidth and high saturation power. The broad amplification waveband makes EDFA highly suitable for WDM (Wavelength Division Multiplexing) technology.

The above-described advantages have led to extensive studies of the dopant erbium as well as of glass hosts for erbium. Silica and fluoride EDFA have hereby been considered but fibers of these materials are only capable of holding a small amount of erbium. In the search for an appropriate glass host for rare earth metals like erbium, the focus has recently turned to tellurite glasses.

Tellurite glasses have been found to provide a broader erbium emission spectrum than other glasses. Therefore, tellurite glass optical fibers result in broader band optical amplifiers for WDM, enabling an increase of the number of wavelength optical channels compared to with erbium doped silica fibers. At the same time, the solubility of the rare earth element is comparatively high in tellurite glasses. They can in fact be doped with up to 70,000 ppm, providing a very high gain per unit length. Due to the high doping level, amplifiers comprising tellurite glass only require centimeter long optical fibers, instead of the tens of meter scale length used up to now.

The main problem of prior art tellurite glasses for optical fibers is associated with their thermal properties and more specifically with the thermal stability range. The thermal stability range of a glass forming material is the difference between the crystallization temperature, T_x, and the glass transition temperature, T_g. At T_g, a solid glass is transformed into a viscous liquid and T_x is the temperature above which the viscous liquid can assume a crystalline structure instead of forming a glass when cooled. A larger value of T_x-T_g implies a more stable glass composition. Moreover, to draw glass fibers, the viscosity of the glass melt must be around 10⁵-⁵ Poise. Defining the drawing temperature as the temperature where the viscosity is 10⁵-⁵ Poise, successful production of an optical fiber requires that the drawing temperature is within the thermal stability range. The otherwise excellent previous tellurite glasses generally have a comparatively low and thus non- satisfactory thermal stability range. Accordingly, the resulting optical fibers are associated with crystallization problems deleterious to their optical quality as well as with a complicated and expensive optical drawing procedure. A glass composition with a large thermal stability range would evidently be very desirable in optical fiber production.

Some attempts have been made in the prior art to improve the tellurite glass compositions intended for optical fibers. In US Patent 5,251,062, the fact that a tellurite glass host allows large amplification bandwidth was recognized. The document concerns the vitreous system Teθ2-ZnO- Na2θ, where Zn and Na can be replaced with other divalent and monovalent metals, respectively. The glass composition may be doped with a rare-earth metal and used for optical waveguides, amplifiers and oscillators. The tellurite glass can incorporate a comparatively large amount of excitable rare-earth metal ions, such as Er³⁺. According to US Patent 5,251,062, a higher Er-content of the glass implies an increased thermal stability.

As the interest in WDM technology increased, NT&T researchers used the glass family of US Patent 5,251,062 to demonstrate a broad band optical amplifier, disclosed in EP 0 858 976. Bi2θ₃ was added to the glass composition in order to achieve better control of the refractive index and viscosity of the glass. In particular, addition of up to 5 mole % Bi2O₃ was reported to affect the thermal stability range favorably.

US Patent 6, 194,334 presents a tellurite glass family containing tungsten (in the form of WO₃) as well as R O, where R is Li, Na, K, Rb, Cs and/ or Tl. These glasses allow a comparatively broad range of Teθ2 content, resulting in high solution of erbium ions as well as broad emission spectra. The importance of a high thermal stability range when optical fibers are drawn is discussed. The disclosed alkali- tungsten-tellurite glasses have thermal stabilities in the range of 100 to 150 degrees.

Even though there has been some progress, problems associated with tellurite glasses intended for optical fibers remain. No tellurite glass composition with a satisfactory thermal stability range has been disclosed in the prior art. The need for a glass composition which has further improved thermal stability and still presents excellent spectral properties is considerable.

SUMMARY

A general object of the present invention is to provide an improved glass composition suitable for use in an optical fiber. A specific object of the invention is to provide a tellurite glass composition having a large thermal stability range. Another object is to provide a tellurite glass composition presenting improved stability as well as preserved amplifying characteristics. Still other objects are to provide optical fibers suitable for broadband amplification and associated with a reliable manufacturing process.

These objects are achieved in accordance with the attached claims.

Briefly, the present invention achieves an improved optical fiber tellurite glass by adding a certain amount of alkalihalide, such as CsCl, to the glass composition. The alkalihalide addition increases the thermal stability range of the glass considerably without loosing other desirable properties such as a high solubility for rare earth metals and a broad amplification bandwidth. The increased thermal stability results in an improved fiber manufacturing process with more reliable optical fiber drawing and better optical fiber products.

A tellurite glass composition suitable for optical fibers is thus provided. The glass composition includes at least 4 mole % of at least one alkalihalide XY for increasing its thermal stability range, X being selected from the group of K, Rb, Cs and Fr and Y being selected from the group of F, CI, Br and I. The XY content of a preferred glass composition is about 10 mole %. The glass composition may be doped with Er2θ₃ or another lanthanide oxide for amplifying purposes. Appropriate control of the refractive index of the glass composition may be achieved by using either Nb θs or Bi2θ3.

The following advantages are obtained with the glass composition of the invention:

• considerably increased thermal stability

• preserved amplifying characteristics

• better optical fibers for broadband optical amplifiers

• excellent refractive index control • improved drawing process

According to other aspects of the present invention, an optical fiber, an optical amplifier, a laser device and methods of manufacturing an optical fiber are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

Fig. 1 is a schematic cross-sectional view of an exemplary embodiment of an optical fiber in accordance with the present invention;

Fig. 2 is a diagram illustrating the thermal stability of tellurite glass compositions with different alkalihalide content;

Fig. 3 is an erbium emission spectrum for an exemplary glass composition in accordance with the present invention;

Fig. 4 is a schematic block diagram of an exemplary embodiment of an optical amplifier in accordance with the present invention;

Fig. 5 is a schematic block diagram of an exemplary embodiment of a laser device in accordance with the present invention; and

Fig. 6 is a flow chart of an exemplary embodiment of a method of manufacturing an optical fiber in accordance with the present invention.

DETAILED DESCRIPTION

In the following description, the "lanthanide series" refers to the group of rare earth metals containing the elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

Fig. 1 illustrates the basic structure of a typical fiber-optic cable. An optical fiber 10 comprising a core 12 and a cladding 14 is shown. The core 12 is a transparent glass material through which a light beam travels. It is surrounded by another glass sheet, the cladding 14, which generally has a refractive index lower than that of the core. The cladding acts like a mirror, reflecting light back into the core, and the light beam is thus transmitted through the optical fiber 10 by means of internal reflection. The outer side of the optical fiber 10 is covered with a protective coating 16 of an insulating material.

As outlined in the background section, the major problem concerning tellurite glasses used for cores and/ or claddings in optical fibers is the small thermal stability range of all prior art glasses. The optical drawing procedure becomes complicated and expensive and the resulting optical fibers have crystallization problems devastating to their optical quality, which in turn leads to poor reliability in the overall telecommunication system. The present invention offers glass compositions with significantly improved thermal stability. It has been discovered that addition of alkalihalides like CsCl, KI, Csl to tellurite glass compositions results in an unexpectedly high increase of the thermal stability range by as much as 200°C. As will be evident from the following text, glass compositions based on this knowledge are very advantageous and highly suitable for broadband optical fiber amplifiers.

Fig. 2 is a diagram illustrating the thermal stability of tellurite glass compositions, including glasses in accordance with the present invention, with different alkalihalide content. Differential Thermal Analysis (DTA) measurements were performed on three Teθ2-ZnO-Na2θ-B_2θ₃ glass compositions. The DTA-signal (arbitrary units) versus temperature (°C) is in Fig. 2 shown for glasses with (A) 0 mole % CsCl; (B) 5 mole % CsCl and (C) 10 mole % CsCl. The first feature of the curves is the Tg-point and the second is the T_x-point. The thermal stability range, T_x-T_g, is indicated by arrows for curves (A) and (C). From Fig. 2, it is evident that the CsCl-addition results in a considerable improvement of the thermal stability range. Glass composition (C) with 10 % CsCl has about 200°C higher thermal stability range than the glass composition (A) without CsCl. A very similar effect has been observed for addition of the alkalihalides Csl and KI. The improvement is obtained for both glass compositions TeO₂-ZnO-R₂0-Bi₂O3 and TeO₂-Wθ3-R2θ-Nb₂θ5 (R = Li, Na, K, Rb or Cs). The addition of alkalihalide is the key idea of the invention and its merit is above all to increase the thermal stability range of the glass without loosing other desirable properties such as solubility and broadening of the amplification bandwidth.

Fig. 3 contains an erbium emission spectrum for an exemplary glass composition in accordance with the present invention. Photoluminescence (PL, arbitrary units) versus wavelength (nm) is shown for glass composition (C) above, doped with 70,000 ppm Er2θ3. The favorably broad amplification bandwidth characteristics of tellurite glasses are clearly preserved in the glass composition according to the invention. Accordingly, the invention offers a tellurite glass with improved stability, which still presents equally good amplifying characteristics as previous glasses. The glass composition of the invention with the alkalihalide salt can therefore be used to produce better optical fibers for large bandwidth optical amplifiers.

The improved thermal stability range obtained according to the invention is related to glass structure changes. The advantageous structure is achieved by combining a high electronegativity element, such as one of the halides F, CI, Br and I, with a heavy element, such as one of the alkalimetals K, Rb, Cs and Fr. The highly electronegative element should be strong enough to replace oxygen bonds in eθ2. This function has partly been confirmed by experimental results, which show that there are Te-Cl bonds in the CsCl-Teθ2-glass. However, fluor has shown a tendency to crystallization and might be too strong and therefore use of CI, Br or I is preferred. The heavy element, on the other hand, seems to prevent crystallization by keeping the glass atoms apart. It does not allow the glass network to close in small units, whereby the amorphous character of the glass is preserved.

Any alkalihalide XY, where X is K, Rb, Cs, Fr and Y is F, CI, Br and I, can be used to improve the thermal stability of glass compositions in accordance with the invention. However, glasses including in particular CsCl but also Csl and KI are preferred.

As for the amount of alkalihalide XY that has to be added to a glass in order to obtain a significantly improved thermal stability, experimental results show that there is a rather sharp change in the thermal stability around 5 mole %. Too much alkalihalide would nevertheless result in inferior glass properties. When more than 12 mole % alkalihalide is used, onset of glass crystallization and a sudden drop in the thermal stability range can be observed. Consequently, the amount of alkalihalide XY in a glass composition in accordance with the invention should be at least 4 mole % but preferably no more than 12 mole %. The most preferred glasses contain about 10 mole % (9- 11 mole %) alkalihalide XY.

A number of exemplary glass compositions in accordance with the invention will now be described. Two known glass families have been used as starting points. The first is the Teθ2-ZnO-Na2θ-Bi2θ3 family (hereafter referred to as TZNBi), as disclosed in EP 0 858 976, and the other is the Teθ2-Wθ3-Er2θ3- 2θ-Na2θ family (hereafter referred to as TWN), as described in US Patent 6, 194,334. The major modification of the original TZNBi and TWN compositions is addition of CsCl on behalf of the Teθ2-content.

Preferred clad (TWN (I) clad) and core (TWN (I) core) glass compositions based on the TWN glass system are disclosed in Table 1. The numbers are given in mole %. Unlike the clad glass, the core glass contains Nb2U5 in order to achieve an appropriate refractive index difference between the core and the cladding. Furthermore, if the glass is to be used for amplifying purposes, the core is doped with 1,000 to 40,000 ppm wt% Er₂O₃. An alternative core glass (TWN (II) core), which comprises larger portions of WO3 and Na2θ, is also disclosed in Table 1.

Table 1

Table 2 contains two other suitable optical fiber glasses, which are based on the TZNBi glass system. The first glass type (TZNBi (I)) includes Na₂0, while the other (TZNBi (II)) has been modified to include Li2θ instead. The refractive indexes are controlled by using different amounts of B12O3 in the core and cladding, respectively. Core glasses for optical amplifiers are preferably doped with 1,000 to 40,000 ppm wt% Er₂O₃.

The composition TZNBi (II) with Li2θ provides glasses with even higher thermal stability range than the TZNBi (I) -glasses with Na2θ. It is highly likely that this effect occurs when using Li2θ as replacement for Na2θ and/ or K2O in the TWN-glasses as well.

Table 2

Table 1 and 2 thus provide examples of representative TZNBi and TWN glass compositions. Table 3 and 4 contain comparatively broad mole % ranges for TZNBi- and TWN-glasses, respectively, in accordance with the present invention. The substances ZnO, R2O and B_2θ3 of Table 3 have ranges with zero as lower limit, but are positive numbers, not actually assuming the value zero. In other words, the TZNBi-glasses of the invention always contain a certain amount of these substances. Moreover, when R=Li in the TZNBi glass (Table 3), the preferred amount of R2O is in the range of 0-25 mole %. As previously mentioned, the XY- content in the glasses is preferably between 9 and 11 mole %. Table 3

It should be noted that the present invention can be applied to other tellurite glasses than TZNBi and TWN as well.

The preferred dopant in an amplifier fiber glass according to the invention is erbium. However, Er³⁺ may be replaced with any other rare earth metal ion in the lanthanide series. The important features of the dopant are its luminescence spectral range and lifetime, which affect the amplification efficiency. The inventors have successfully used erbium /ytterbium, thulium, holmium and thulium/ holmium for the 1500 nm window and neodymium, praseodymium and dysprosium for the 1300 nm window. As compared to undoped glasses, the doped glasses present a slightly larger thermal stability range which increases with Er-content.

An optical fiber according to the invention may with advantage present the basic structure, which was described above with reference to Fig. 1. In other words, Fig. 1 is a schematic cross-sectional view of an exemplary embodiment of an optical fiber in accordance with the present invention. The optical fiber 10 comprises the core 12, the cladding 14 and preferably also the protective coating 16. At least one of the core glass and the clad glass comprises a tellurite glass composition including 4-12 mole % alkalihalide XY for increasing its thermal stability range. It is important that the glass compositions of the core and cladding are about the same to avoid a significant expansion coefficient mismatch. Generally, the maximum tolerable difference between the core and clad glasses is in the range of 5 %. In a preferred embodiment, the same tellurite glass composition is used as a base for both the core 12 and the cladding 14 of the optical fiber 10. The core glass is then modified to contain a higher amount of a substance used for refractive index control. In an optical amplifier fiber, the core glass further includes a doping lanthanide oxide, such as Er2U3.

Preferably, the substance used for refractive index control is Bi2U3 or Nb2θδ and the refractive index of the core glass is about 2% higher than that of the clad glass. The difference in refractive indexes can easily assume any desired value between 0.2 and 6 % by addition of appropriate amounts of B12O3 or Nb2θs. The fact that a tellurite glass optical fiber in accordance with the invention allows refractive index control with Bi2θ3 and Nb2θs constitutes another advantage thereof. It should be noted that the optical fiber structure of Fig. 1 is rather simplified. Other optical fibers in accordance with the invention may present more complex structures with non- symmetrical components, graded-index cores, more than one cladding, etc. However, an optical fiber of the invention generally includes at least one fiber component that has been formed in a drawing procedure. Fig. 4 is a schematic block diagram of an exemplary embodiment of an optical amplifier in accordance with the present invention. The illustrated optical amplifier 20 comprises signal processing means 22, couplers 24, pump light sources 26 and an optical amplifier fiber 28. A weak optical signal that needs to be amplified is input to the amplifier 20. The input signal first passes the optional signal processing means 22a, which modifies the signal in an appropriate way. The amplifier fiber 28 is pumped at both ends with pump lasers or similar pump light sources 26a and 26b. The couplers 24a and 24b, embodied e.g. as mirrors, combine the excitation light provided by the pump light sources with the signal light. In the amplifier fiber 28, the excitation light then causes rare-earth atoms to attain a temporary excited state. As the electrons decay, light is released and the gain of the optical signal is thus increased. Finally, the amplified signal is further modified in the optional signal processing means 22b. A comparatively strong optical signal is output from the amplifier.

The optical amplifier fiber 28 of the optical amplifier 20 comprises a core of doped tellurite glass. The core and/ or cladding includes at least 4 mole % alkalihalide XY. The fiber may with advantage comprise one of the above-described glass compositions. In a preferred embodiment, the amplifier fiber 28 is adapted for operating in a wavelength region near 1.5 μm by being doped with Er. It is then appropriate to use pump light source(s) 26, which generates excitation light at 980 or 1480 nm. For amplifier fibers with other rare earth dopants, other pump light sources may be used. Oxides of Pr, Yb and Nb may for instance be used with pump light sources that emits at 1020 nm for operation in the 1.3 μm band. The signal processing means 22 preferably comprises isolators, the purposes of which are to prevent unwanted reflections and suppress the oscillations of the amplifier. The signal processing means 22 may also include further devices for modulation, filtering, polarization, absorption, attenuation, etc. The optical amplifier according to Fig. 4 may of course be subject to various modifications obvious to the skilled man. It would for instance be possible to use a single pump light source (and a single coupler), even though two pump light sources generally results in better amplifier efficiency. The number and position of the optional signal processing means units may vary and filters and the like can be either internal or external. There may further be more than one amplifier fiber in the optical amplifier. Besides the amplifier fiber(s), there are generally several undoped "ordinary" optical fibers in the optical amplifier, providing connections between components thereof. The above-described optical amplifier glass may also be used in a laser device. Fig. 5 is a schematic block diagram of an exemplary embodiment of a laser device in accordance with the present invention. Each laser device component is provided with the same reference number as the corresponding amplifier component (Fig. 4) plus 10. The main difference between the laser device and the optical amplifier is that the laser device does not receive a signal light input but has feedback means for signal generation. The laser device 30 of Fig. 5 accordingly comprises two reflectors 35, placed at opposite ends of the optical amplifier fiber 38. Excitation light from the pump light sources 36 give rise to photon emission in the amplifier fiber 38 in the same way as for the optical amplifier. The first reflector 35a is preferably a high reflector mirror, ideally reflecting all light, whereas the second reflector 35b is a partially transparent mirror. The relatively small fraction of light passing through the second reflector is the laser beam output of the laser device 30. Optional signal processing means 32 of the laser device may include internal or external devices for modulation, filtering, polarization, q- switching, absorption and the like.

In another embodiment (not shown) of the laser device according to the invention, feedback is instead achieved by a ring-shaped structure, where a part of the output signal basically is led back to the coupler 34a.

Lasers with lanthanide-doped tellurite glasses, both in the form of optical fibers or just a rod, offer a couple of important advantages as compared to conventional lasers. Firstly, the larger bandwidth allows shorter pulses for mode locked lasers. Secondly, the tellurite glass lasers have a larger tuning range for cw or pulsed lasers, which is particularly important in spectroscopic applications or even as signal source in a WDM communication system. These advantageous features are easily obtained by using the proposed glass for lasing optical fiber elements. Fig. 6 is a flow chart of an exemplary embodiment of a method of manufacturing an optical fiber in accordance with the present invention. The procedure is based on the so-called rod-in-tube method. In a step S I, a batch of glass-forming substances, including tellurite and at least 4 mole % alkalihalide XY, are mixed together. Tellurium, tungsten and lanthanide components are preferably introduced to the batch as oxides and alkali metal oxides as carbonate or nitrate. The alkalihalides can be introduced directly, i.e. as the salts CsCl, Csl, KI etc.

The glass mixture is heated in a step S2, whereby a glass melt is formed. An electric furnace at oxygen atmosphere can for example be used for this. The temperature of the glass melt is brought down to the specific drawing temperature of the glass, which may be measured with the so-called wetability method. At the drawing temperature, a core rod (also referred to as a seed) is drawn in a step S3. The diameter of the core rod is typically in the range of 300 ?m and can be controlled through the drawing velocity and temperature. The core refractive index is preferably controlled with Nh_Os or B-2O3. The core glass may be doped with Er2θ3 or another rare earth oxide from the lanthanide series.

Thereafter, a clad tube is formed in a step S4. The clad tube is preferably accomplished in two sub-steps S4-1 and S4-2. First, clad glass melt is arranged into the interior of a silica tube in the step S4-1. This may be achieved by sucking undoped tellurite glass into the silica tube with a vacuum pump or the like. Then, the clad tube is separated from the silica tube in the step S4-2 by cooling the aggregate. Since the silica and tellurite glass expansion coefficients differ considerably from each other, the clad tube will fall off in the cool down process. This is a most advantageous way of forming the clad tube, since it is very simple and reliable.

The core rod is in a step S5 fitted into the clad tube, whereby a preform is achieved. From the preform, an optical fiber is successfully drawn at an appropriate drawing temperature in a step S6. Preferably, the drawing is performed in a drawing tower having a graphite furnace with a heat zone length less than or equal to the preform diameter to avoid that the initial glass drop becomes unstable. After the first glass drop falls down the pulling velocity controls the optical fiber diameter. The drawing process is facilitated by the large thermal stability range of the glass compositions, and the fibers drawn in accordance with the present invention hence disclose a minimum or erroneous features. Typically, an optical fiber is provided with a protective coating immediately after the drawing.

Although the invention has been described with reference to specific illustrated embodiments thereof, it should be understood that various modifications and changes obvious to the man skilled in the art also lie within the scope of the invention, as defined by the appended claims.

Claims

1. A tellurite glass composition suitable for optical fibers, characterized in that the glass composition includes at least 4 mole % of at least one alkalihalide XY for increasing its thermal stability range, X being selected from the group of K, Rb, Cs and Fr and Y being selected from the group of F, CI, Br and I.

2. The glass composition according to claim 1, characterized in that the alkalihalide XY is selected from the group of KI, Csl and CsCl.

3. The glass composition according to claim 2, characterized in that the alkalihalide XY is CsCl.

4. The glass composition according to any of previous claims, characterized in that the glass composition includes up to 12 mole % of the alkalihalide XY.

5. The glass composition according to claim 4, characterized in that the glass composition includes 9-11 mole % of the alkalihalide XY.

6. The glass composition according to any of previous claims, characterized in that the glass composition includes an oxide of a rare earth metal ion in the lanthanide series.

7. The glass composition according to claim 6, characterized in that the lanthanide oxide is Er θ3.

8. The glass composition according to any of previous claims, characterized in that the glass composition includes up to 40 mole % of Li₂O.

9. The glass composition according to any of previous claims, characterized in that the glass composition essentially consists of (in mole %):

55-90 % TeO₂; 4-12 % XY; 0-35 % ZnO; 0-35 % R2O, where R is at least one element selected from the group of Na, Li, K, Rb, Cs; and 0-20 % Bi2θ3, the amounts of ZnO, R2O and Bi2θ3 each being larger than zero.

10. The glass composition according to any of claims 1-8, characterized in that the glass composition essentially consists of (in mole %):

15-85 % Te0₂; 4-12 % XY; 5-55 % WO3;

0.5-40 % R2O, where R is at least one element selected from the group of Na, Li, K, Rb, Cs; and

0-15 % Nb₂O₅.

11. An optical fiber (10) comprising a core (12) surrounded by at least one cladding (14), characterized in that at least one of the core and the cladding is at least partly formed of a tellurite glass composition including at least 4 mole % of at least one alkalihalide XY for increasing its thermal stability range, X being selected from the group of K, Rb, Cs and Fr and Y being selected from the group of F, CI, Br and I.

12. The optical fiber according to claim 11, characterized in that the alkalihalide XY is selected from the group of KI, Csl and CsCl.

13. The optical fiber according to claim 12, characterized in that the alkalihalide XY is CsCl.

14. The optical fiber according to any of claims 11-13, characterized in that the glass composition includes up to 12 mole % of the alkalihalide XY.

15. The optical fiber according to claim 14, characterized in that the glass composition includes 9-11 mole % of the alkalihalide XY.

16. The optical fiber according to any of claims 11-15, characterized in that the glass composition includes up to 40 mole % of L12O.

17. The optical fiber according to any of claims 11-16, characterized in that the refractive index of the core (12) and/ or the cladding (14) is controlled by a substance selected from the group of -2O5 and Bi2θ3.

18. The optical fiber according to any of claims 11-17, characterized in that the glass composition essentially consists of (in mole %):

55-90 % TeO_a; 4-12 % XY; 0-35 % ZnO;

0-35 % R2O, where R is at least one element selected from the group of Na, Li, K, Rb, Cs; and 0-20 % B_2θ3,the amounts of ZnO, R2O and B-2O3 each being larger than zero.

1 . The optical fiber according to any of claims 11-17, characterized in that the glass composition essentially consists of (in mole %):

15-85% Te0₂; 4-12% XY; 5-55% WO3; 0.5-40% R2O, where R is at least one element selected from the group of Na, Li, K, Rb, Cs; and 0-15% Nb₂O₅.

20. An optical amplifier (20) including an optical amplifier fiber (28) comprising a core surrounded by at least one cladding, characterized in that at least one of the core and the cladding is at least partly formed of a tellurite glass composition including at least 4 mole % of at least one alkalihalide XY for increasing its thermal stability range, X being selected from the group of K, Rb, Cs and Fr and Y being selected from the group of F, CI, Br and I.

21. The optical amplifier according to claim 20, characterized in that the alkalihalide XY is selected from the group of KI, Csl and CsCl.

22. The optical amplifier according to claim 21, characterized in that the alkalihalide XY is CsCl.

23. The optical amplifier according to any of claims 20-22, characterized in that the glass composition includes up to 12 mole % of the alkalihalide XY.

24. The optical amplifier according to claim 23, characterized in that the glass composition includes 9-11 mole % of the alkalihalide XY.

25. The optical amplifier according to any of claims 20-24, characterized in that the glass composition includes an oxide of a rare earth metal ion in the lanthanide series.

26. The optical amplifier according to claim 25, characterized in that the lanthanide oxide is Er₂O3.

27. The optical amplifier according to any of claims 20-26, characterized in that the glass composition includes up to 40 mole % of Li θ.

28. A laser device (30) including an optical amplifier fiber (38), characterized in that the optical amplifier fiber is at least partly formed of a tellurite glass composition including at least 4 mole % of at least one alkalihalide XY for increasing its thermal stability range, X being selected from the group of K, Rb, Cs and Fr and Y being selected from the group of F, CI, Br and I.

29. A method of manufacturing an optical fiber, characterized by the steps of:

mixing together glass-forming substances including TeO₂ and at least 4 mole % of at least one alkalihalide XY for increasing the thermal stability, X being selected from the group of K, Rb, Cs or Fr and Y being selected from the group of F, CI, Br or I; heating the glass-forming mixture, whereby a glass melt is formed; and drawing an optical fiber core rod from the melt at a predetermined drawing temperature.

30. The method according to claim 29, characterized by the further steps of: forming a clad tube from the glass melt; inserting the core rod into the clad tube, whereby a preform is formed; and drawing an optical fiber from the preform. 31. The method according to claim 30, characterized in that the step of forming a clad tube in turn comprises the steps of:

arranging the glass melt in the interior of a silica tube; and separating the clad tube from the silica tube by cooling.

32. The method according to any of claims 29-31, characterized by the further step of:

doping the glass melt used to draw the core rod with a predetermined amount of an oxide of a rare earth metal ion in the lanthanide series.

33. A method of manufacturing an optical fiber from a glass forming mixture including TeO2, characterized by the steps of:

adding at least 4 mole % of an alkalihalide XY to the mixture for increasing the thermal stability range of the glass, X being selected from the group of K, Rb, Cs and Fr and Y being selected from the group of F, CI, Br and I; and drawing, at a predetermined drawing temperature, an optical fiber element from the mixture.