WO2009034413A1

WO2009034413A1 - Optical fiber and method for manufacturing

Info

Publication number: WO2009034413A1
Application number: PCT/IB2007/003740
Authority: WO
Inventors: Laurent Calvo; Ivo Flammer; Cédric Gonnet; Frans Gooijer; Roland Heuvelmans; Yves Lumineau; Bob J. Overton; Emmanuel Petitfrere; Elise Regnier
Original assignee: Draka Comteq B.V.
Priority date: 2007-09-14
Filing date: 2007-09-14
Publication date: 2009-03-19

Abstract

A method for manufacturing an optical fiber preform comprising a step of providing a primary preform forming a core and an inner cladding, and a step of overcladding said primary preform by projecting silica grain under a plasma torch on the primary preform and vitrifying said silica grain on the periphery of said primary preform in order to form an overcladding layer, wherein an alkali material is projected together with the silica grain during the overcladding step which alkali material is incorporated in the overcladding layer.

Description

OPTICAL FIBER AND METHOD FOR MANUFACTURING

The invention relates to the field of optical fibers and more particularly to a method for manufacturing low-loss optical fibers and a method for manufacturing an optical fiber preform from which such an optical fiber is drawn.

An optical fiber is typically composed of an optical core, having the function of transmitting and possibly amplifying an optical signal, an optical cladding, having the function of confining the optical signal in the core, and an outer cladding. For this reason, the refractive indexes of the core ri_c and of the outer cladding n_g are such that n_c>n_g. As commonly known, the propagation of an optical signal in a single-mode optical fiber is divided into a guided dominant mode in the core and into guided secondary modes over a certain distance throughout the core-cladding assembly, called cladding modes.

An optical fiber is made by drawing a preform on a drawing tower. A preform for example comprises a primary preform consisting of a very high quality glass tube, which forms the core and part of the optical cladding of the fiber. To manufacture the primary preform, a silica tube can be mounted horizontally and held in place on a glass working lathe; the silica tube is then rotated and locally heated to deposit components determining the composition of the preform on the inside of the silica tube. Deposition is conventionally made using any CVD technique (Chemical Vapor Deposition), for instance PCVD (Plasma Chemical Vapor Deposition). This primary preform is then overcladded or sleeved to increase its diameter and to form a preform, which can be used on a drawing tower. In this context, the term inner cladding is used for the cladding formed inside the tube during the deposition process, and the term outer cladding is used for the cladding formed outside the tube during the overcladding process. The homothetic fiber drawing operation consists of placing the preform vertically in a drawing tower and drawing a fiber strand from one end of the preform. For this purpose a high temperature is applied locally to one end of the preform until the silica is softened, the fiber drawing speed and temperature then being permanently controlled during the drawing operation since they determine the diameter of the fiber.

Attenuation is a principal limiting attribute of optical fibers. Optical fiber loss, for example, plays an important role in limiting the distance between optical fiber amplifiers. This is particularly important in long distance and ultra-long distance networks such as, for example, undersea applications, where such amplifiers represent a significant system cost, as well as a major factor in system reliability. Consequently there is tremendous commercial interest in reducing attenuation to the lowest possible level. Attenuation can be reduced by reducing the viscosity of the silica of the different layers of the optical fiber preform. For instance, for transmission fibers - single mode fibers (SMF) or multimode fibers (MMF)7 - attenuation is strongly linked to the fictive temperature of the core area material; the lower the fictive temperature, the lower the optical losses. An optical fiber drawn from a low viscosity silica preform will exhibit lower losses than an optical fiber drawn at the same temperature and speed conditions from a high viscosity silica preform. On the other hand, for dispersion compensating fibers (DCF), because of the strong index difference between the core and the inner cladding, losses are mainly due to small angles of light scattering (SALS). These defects are localized near interfaces between the preform layers and are due to stress during drawing of the fiber from the preform. By reducing the viscosity of the overcladding of the preform, the drawing temperature can be lowered and thus these defects can be reduced. US 6,532,775 discloses a method for manufacturing an optical fiber preform wherein the viscosity of the outer deposition layer (overcladding layer) is adjusted to be substantially identical to the viscosity of the outer peripheral portion of the primary preform by adding to the silica at least one compound selected from the group CaF₂, MgF₂, AlF₃, B₂O₃ and Al₂O₃. This document proposes to reduce the undesirable index step in the optical fiber resulting from hot drawing of the preform, said index step being located at the boundary between the outer peripheral portion of the primary preform and the silica-based outer deposition layer, said outer peripheral portion apparently not being subjected to the same compression stresses during hot drawing as is the outer deposition layer. This document also indicates that introducing particles of Al₂O₃, or AlF₃ or indeed B₂O₃ in the overcladding advantageously reduces attenuation losses due to the presence of hydrogen in the optical fiber. The dopant particles are supplied together with the silica grain to be vitrified by the plasma torch on the periphery of the primary preform. However, a homogeneous silica-dopant powder is very difficult to obtain and the overcladding is likely to be not homogeneous. In addition, it has been observed that pollution can occur during the mixing of the dopant particles with the silica grain.

US 7,088,900 discloses an optical fiber having a silica core doped with fluorine and including an alkali metal oxide dopant. Such fiber exhibits attenuation at 1550 nm of less than or equal to 0,178 dB/km. The cladding is also doped with fluorine and may include alkali metal oxide as well. US 7,088,900 refers to document WO 2005/021455 to disclose various method for producing an alkali doped preform and fiber. A silica glass tube is prepared and an alkali source compound is introduced into the tube in a vapour phase while the tube is simultaneously rotated and heated. An alkali-doped glass rod is therefore produced and an overcladding is deposited thereon to form an optical fiber preform.

WO 2006/068940 also discloses a method of making an alkali metal silicate glass rod. An alkali metal is mixed with silicate to form a precursor, which is melted to form molten glass shaped into a glass rod. US 2005/0129376 discloses an alkali-doped optical fiber preform and method for making the same. An optical fiber preform is heated in a furnace below the softening point of the glass rod in an atmosphere containing alkali metal vapour to form an alkali metal oxide doped glass rod. This alkali-doped glass rod may then be overcladded with additional glass to form an optical fiber preform ready for drawing into an optical fiber.

US 2005/0201699 discloses a method of forming an alkali metal oxide-doped optical fiber. Alkali metals are being diffused through the surface of the silica rod preform by heating said surface, for instance with a plasma burner. The diffusion surface is then etched to remove impurities that may have been unintentionally introduced into the glass rod. The silica glass rod may be further processed to form a complete optical fiber preform, i.e. sleeved or overcladded.

US 6,970,630 discloses a method for producing an optical fiber preform. Alkali and/or alkali earth fluxing agents are introduced with aluminium into the silica glass core by chemical vapour deposition CVD. The optical fiber thus fabricated exhibits law scattering losses.

US 2005/0144986 discloses a method for manufacturing an optical fiber preform. A first glass rod is formed and inserted into a glass sleeve tube while alkali metal vapour is passed between the sleeve tube and the first glass rod. The sleeve may then be collapsed to form a second glass rod doped with alkali metal oxide. EP-A-O 915 065 discloses an optical fiber formed of ultralow-loss glass which is composed of silica glass containing at least one network modifying oxide selected from the group consisting Of Na₂O, K₂O, Li₂O, MgO, CaO, and/or PbO. The ultralow-loss glass was obtained by ion implantation into a high purity silica glass specimen.

US 5,146,534 discloses a low intrinsic loss optical fiber having a silica core doped with fluorine and an alkali metal oxide dopant. JP63040744 and JP62283845 disclose low-loss optical fibers with improved hydrogen sensitivity due to alkali and/or earth alkali material doping.

The prior art methods for making alkali-doped optical fiber preforms are complex to implement because a specific alkali-doped glass rod must be produced to form the primary preform. Many constraints already affect the primary preform manufacture and introducing alkali dopant in the primary preform adds further complexity to the manufacturing process.

There is therefore a need for a low cost and reliable one step process to produce an alkali-doped optical fiber preform.

Accordingly, the invention proposes to carry out the alkali material deposition and the silica overcladding during the same process step. Depending on the alkali material that is added to the overcladding and depending on the drawing conditions of the optical fiber, alkali material may diffuse towards the core of the fiber or may remain in the outer cladding. Accordingly, a low attenuation optical fiber can be produced.

In addition, it is desired to have optical fibers presenting good mechanical strength under stress condition. Optical fibers can experience significant mechanical stress when being set up in a telecommunication cable or when being picked up during a by-pass operation. A good metric for mechanical reliability for an optical fiber design is the stress corrosion parameter, n. This parameter, a component of brittle fracture mechanics, describes the rate at which a flaw on the surface of an optical fiber grows under a defined applied stress. A flaw, or crack, on the surface acts as a stress concentrator, magnifying the stress at the tip of the crack. The basic equation for the stress concentration effect is

K₁ = Yσa^/X/l

where K₁ is the stress intensity factor, Y a constant for a given crack shape, σ the stress applied to the fiber, and a is the dimension of the flaw in the direction normal to the applied stress. The flaw size changes, grows, with time under stress, so all factors except the applied stress are functions of time. The rate at which the crack grows under stress, or crack growth velocity V, is dα/dt, described by

where A is a material scaling factor and n is the stress corrosion parameter. Thus the rate of crack growth under stress is sensitive to the magnitude of the stress corrosion parameter. The parameter n may be estimated for an optical fiber by a dynamic fatigue test, where the fiber undergoes a strain, increasing at a constant rate, to failure. Samples of a population of optical fibers are tested in this way at several different rates of strain covering several orders of magnitude of strain rate. The median stress at failure for the samples of the optical fiber is plotted versus the strain rate on a log-log scale. The relationship of log(failure stress) to log(strain rate) is linear, expressed as

where nj is indicating the stress corrosion parameter estimated by this dynamic fatigue method. This stress corrosion parameter na is then used to predict lifetime of the fiber corresponding to other applied stresses. To increase the stress corrosion parameter, it has been proposed to add a silicon or carbon-based coating layer on the fiber; one can refer to US 4,512,629 or the publication by Lu K.E. et al, "Hermetically coated optical fiber", IWCS, pp 241-244 (1987) for such prior art solutions. However, this solution is costly. A high stress corrosion parameter can also be reached when the silica glass of the fiber shows a high fictive temperature, i.e. when the fiber is drawn from a high viscosity glass preform or at high speed; one can refer to the publication by Tomozawa M. et al., "Effect of Fictive Temperature on Mechanical Strength of Soda-Lime Glass", Journal of non-Crist. Solids, 241 , 3100-3108 (1998) for such prior art solution. However, a high fictive temperature will also increase the fiber losses, notably the Rayleigh scattering.

Documents US 5,033,815, US 3,673,049 and US 5,140,665 propose to increase the stress corrosion parameter by providing outer layers with a lower thermal expansion coefficient than the fiber core. However, a high dopant concentration in the outer cladding is required which is expensive and increase the risk of core pollution.

There is therefore a need for increasing the stress corrosion parameter of a fiber without altering any of the fiber optical characteristics and without expensive manufacturing process.

Accordingly, the invention proposes to carry out an alkali material deposition in the silica during the overcladding process step. When drawing an optical fiber from such an alkali doped optical preform, the alkali material reduces the glass viscosity and prevents crack growth of silica. Accordingly, an optical fiber with increased stress corrosion parameter can be produced.

More specifically, the invention relates to a method for manufacturing an optical fiber preform comprising a step of providing a primary preform forming a core and an inner cladding, and a step of overcladding said primary preform by projecting silica grain under a plasma torch on the primary preform and vitrifying said silica grain on the periphery of the said primary preform in order to form an overcladding layer, which overcladding layer forms an outer cladding, wherein an alkali material is projected together with the silica grain during the overcladding step which alkali material is incorporated in the overcladding layer.

The alkali material can be selected from materials comprising at least one alkali metal from the group consisting of Na, Li, K, Rb and Cs, and combinations thereof.

According to an embodiment, the alkali material is dissolved in an aqueous solution, which can be mixed with air, for projection on the primary preform. The concentration of alkali material in the aqueous solution can be in the range of 0.1 ppm to 100 ppm. The aqueous solution can be a solvent that is free of Hydrogen (H) According to an embodiment, the rate of projection of the alkali material is monitored during the overcladding step.

The invention also relates to a method for manufacturing an optical fiber, the method comprising a step of providing an optical fiber preform manufactured according to the invention, and a step of drawing said preform under conditions of controlled speed and temperature during which drawing at least part of the preform is softened, and which conditions are chosen in such a manner as to allow for diffusion of the alkali material from the overcladding towards the core of the optical fiber being drawn.

According to embodiments, the alkali material is selected from materials comprising at least one alkali metal from the group consisting of Na and Li, and combinations thereof, and the drawing temperature is preferably comprised between 1600 and 2200°C and the drawing speed is preferably comprised between 100 and 1500 m/min.

The invention also relates to a single mode fiber manufactured according to the invention; such a single mode fiber may have an attenuation at 1550 nm being less than or equal to 0,18 dB/km.

The invention also relates to a multimode fiber manufactured according to the invention. In preferred embodiments such a multimode fiber may have an attenuation at 850 nm being less than or equal to 2,2 dB/km, a draw peak at 630 nm being less or equal to than 1 dB/km and added losses at 850 nm being less than or equal to 0.1 dB/km. The invention further relates to a method for manufacturing an optical fiber having a core, an inner cladding and an outer cladding, the method comprising a step of providing an optical fiber preform manufactured according to the invention, and a step of drawing said preform under conditions of controlled speed and temperature during which drawing at least part of the preform is softened, and which conditions are chosen in such a manner as to maintain the alkali material in the overcladding.

According to embodiments, the alkali material is selected from materials comprising at least one alkali metal from the group consisting of K, Rb and Cs, and combinations thereof, and the drawing temperature is preferably comprised between 1600 and 2200°C and the drawing speed is preferably comprised between 100 and 600 m/min. The invention also relates to a dispersion compensating fiber manufactured according to the invention. In a preferred embodiment such a dispersion compensating fiber may have an attenuation at 1550 nm being less than or equal to 0,6 dB/km.

The optical fibers of the invention may have a stress corrosion parameter n being more than 28. Other characteristics and advantages of the invention will become clearer upon reading the description that follows the embodiments of the invention, given by way of example and in reference to the annexed drawings, which illustrate: figure 1 , a schematic representation of an installation for overcladding a primary preform according to the invention; figure 2, a graph showing the correlation between the 630 ran draw peak and added attenuation in the multimode fiber at 850 ran.

The invention proposes a method for manufacturing an alkali-doped optical fiber preform. The invention proposes to carry out the alkali material deposition during the process step of overcladding the primary preform. Thus, no additional process step is needed according to the invention. The invention also proposes a method for manufacturing an alkali-doped optical fiber, said optical fiber being drawn from an optical fiber preform having an alkali-doped overcladding. Optical fibers with reduced attenuation and increased stress corrosion parameter can thus be manufactured.

Figure 1 shows an installation for overcladding a primary preform according to the invention. A primary preform 100 is provided comprising a silica glass rod forming the core and inner cladding, which silica glass rod was prepared according to any prior art method; for instance, by PCVD deposition inside a silica glass tube. The overcladding is carried out by Advanced Plasma Vapor Deposition APVD where silica grain is projected from a silica grain injector 210 towards the periphery of the primary preform 100 and fused by a plasma torch 200 at a temperature of about 2300° C so that the silica grain vitrifies on the periphery of the primary preform 100. The primary preform 100 is caused to rotate around itself and either the plasma torch 200 or the primary preform 100 moves longitudinally with respect to each other to ensure uniform depositing of silica around the entire periphery of the primary preform 100. It is of course also possible that both the primary preform 100 and the plasma torch 200 each move longitudinally in opposite directions.

According to the invention, alkali material is projected together with the silica grain during the overcladding step of the primary preform 100. The alkali material can be delivered from a tank 300 where alkali material is dissolved in an aqueous solution. A pump 350 may withdraw doses of the aqueous alkali solution from the tank and provide it to an air/water mixer 360 feeding an alkali material injector 310. Alkali metal oxide materials could also be blown from a dried power mixed with the silica grain.

The use of an aqueous solution in which the alkali material is dissolved reduces the pollution risk and the mixture of such an aqueous alkali solution with air before projection with the silica grain provides a very homogeneous mixture of the alkali material with the silica grain projected on the primary preform periphery resulting in a homogeneous overcladding layer. Targeted concentrations of alkali material are very low, usually in the range of 0.1 ppm to 100 ppm. With very steady and small liquid flows, the concentration of alkali material in the glass is easily controlled. This is not the case when the overclad is manufactured using a mixture of two solid powders or when the doping material is introduced as particles in the main silica grain flow. In addition, to avoid unwanted introduction of OH groups in the preform, the solution in which the alkali material is dissolved can be a solvent free of any hydrogen (H) species instead of water, like a heavy water solution D₂O.

The invention makes it possible that some lattice modifying materials are introduced in the silica glass network during the APVD process step. Alkali material can substantially modify the glass viscosity, even at low concentration. The rate of projection or the flow rate of the alkali material projection is monitored during the overcladding step, just as the flow rate of silica grain is monitored. It can be constant or variable throughout the overcladding process step. For instance, the very outer layer can be manufactured using a pure silica flow free of alkali material or, on the contrary, the flow of alkali material, preferably the aqueous flow, can be increased substantially, resulting in layer having a higher alkali concentration than inside layers.

For instance, the aqueous solution may contain about 0.1 to 1 mol/1 of NaCl. This solution is pumped at a rate of 1 to 10 ml/h and mixed with a small airflow. The mixed air/alkali solution is projected on the primary preform at a flow rate of about 1 1/min while silica powder is projected on the primary preform at a flow rate of about 5 Kg/h. This results in an overcladded preform in which the overcladding layer contains about 5 ppm of alkali material.

Depending on the alkali material that is added to the overcladding layer and depending on the drawing conditions of the optical fiber, the alkali material may diffuse towards the core of the fiber or may remain in the overcladding layer.

If it is desired to reduce the viscosity of the silica material of the core, in order to reduce the attenuation of a SMF or MMF for instance, small alkali materials with a high diffusion coefficient will be preferred, such as Na, Li and combinations thereof. During drawing of the fiber from a preform comprising these alkali materials, under conditions of controlled speed and temperature, these alkali materials will diffuse towards the core of the fiber. Since these alkali materials have a small van der Waals radius as well as a high diffusion coefficient, they easily move through the softened silica during the drawing process in order to move towards the core of the optical fiber being drawn. For instance, starting from an optical preform having an overcladding containing 0.01 to 100 ppm of the chosen alkali material, an optical fiber can be drawn with a drawing temperature comprised between 1600 and 2200⁰C and a drawing speed comprised between 100 to 1500 m/min resulting in an alkali-doped core optical fiber with about 0.001 to 10 ppm of alkali material in the fiber core. The invention makes it possible to produce an optical fiber having an alkali-doped core without the need to use high purity raw material because the alkali dopant will diffuse in the core through the glass lattice whereas other pollutants, such as transition metals for instance, will not diffuse towards the fiber core under the same drawing conditions because their diffusion coefficient is very small in the glass compared to the diffusion coefficient of the chosen alkali material and because their van der Waals radii are higher. A standard single mode fiber with attenuation at 1550 nm being lower or equal to 0.18 dB/km or a 50/125 multimode fiber (core diameter of 50 μm and total diameter of 125 μm) with an attenuation at 850 nm being lower or equal to 2.2 dB/km can be produced according to the method of the invention.

If it is desired to reduce the viscosity of the cladding only and not of the core, in order to reduce the attenuation of a DCF for instance, large alkali materials (i.e. alkali materials of which the positive species have large van der Waals radii) with a low diffusion coefficient will be preferred, such as K, Rb and Cs, and combinations thereof. During drawing of the fiber, under conditions of controlled speed and temperature, these alkali materials will remain in the overcladding, i.e. these alkali materials will not diffuse towards the core of the fiber. For instance, starting from an optical preform having an overcladding layer containing 50 ppm of the chosen alkali materials, an optical fiber can be drawn with a reduced drawing temperature comprised between 1600 and 2200°C and a drawing speed comprised between 100 and 600 m/min. The defects localized near the interface between the primary preform and the overcladding layer can thus be reduced because of the lower drawing temperature and a DCF with reduced SALS losses can be produced. A dispersion compensating fiber having an attenuation at 1550 nm being less than or equal to 0.6 dB/km and with figure of merit as high as 350 ps/(nm.dB) in the C band can be produced using this technique. The figure of merit FOM is defined as the opposite of the ratio between the chromatic dispersion D and the fiber attenuation A at a given wavelength: FOM = - D/A In addition, the invention also makes it possible to increase the stress corrosion parameter of the fiber. When drawing an optical fiber from an optical preform having an alkali-doped overcladding, the alkali material reduces the glass viscosity and prevents crack growth of silica.

For instance, an optical preform with a silica overcladding doped with 10 ppm of sodium was drawn into a 50/125 multimode fiber. The sodium doped silica overcladding results into a reduction of about 200-300K in the glass transition temperature compared to pure silica. Such a MMF fiber was measured with a high stress corrosion parameter n_d=28.5 after drawing. After 12 months of aging in 85°C water, the stress corrosion parameter of the fiber was still very high

In addition, it was noticed that alkali-doped multimode fibers manufactured according to the invention show reduced added loss at 850 ran due to a low 630 nm draw peak. A low draw peak in a fiber is a sign of relaxed silica structure. According to the invention, by reducing the core viscosity with an alkali dopant, we obtain a low UV absorbing defect concentration and a low fictive temperature in the core which both reduce losses at 850nm. The controlled temperature and speed drawing of the manufacturing method according to the invention also results in avoiding a strong tensional stress in the fiber core thus reducing silica defects and leading to a low 630 nm draw peak. Notably, according to the invention, it is possible to realize a 630 nm draw peak lower than 1 dB/km; this guarantees added loss at 850 nm to be lower than 0.1 dB/km. Figure 2 shows the correlation between the 630 nm draw peak and added attenuation in the multimode fiber at 850 nm. The lower the 630 nm draw peak, the more reduced is the added attenuation at 850 nm. A multimode fiber according to the invention has therefore reduced attenuation.

Claims

1. A method for manufacturing an optical fiber preform comprising a step of providing a primary preform forming a core and an inner cladding, and a step of overcladding said primary preform by projecting silica grain under a plasma torch on the primary preform and vitrifying said silica grain on the periphery of said primary preform in order to form an overcladding layer, which overcladding layer forms an outer cladding, wherein an alkali material is projected together with the silica grain during the overcladding step which alkali material is incorporated in the overcladding layer.

2. The method according to claim 1 , wherein the alkali material is selected from materials comprising at least one alkali metal from the group consisting of Na, Li, K, Rb and Cs, and combinations thereof.

3. The method according to claim 1 or 2, wherein the alkali material is dissolved in an aqueous solution for projection on the primary preform.

4. The method according to claim 3, wherein the concentration of alkali metal in the aqueous solution is in the range of 0.1 ppm to 100 ppm.

5. The method according to claim 3 or 4, wherein the aqueous solution is a solvent that is free of Hydrogen (H).

6. The method according to any one of claims 3 to 5, wherein the aqueous solution is mixed with air for projection on the primary preform.

7. The method according to any one of claims 1 to 6, wherein the rate of projection of the alkali material is monitored during the overcladding step.

8. A method for manufacturing an optical fiber, the method comprising a step of providing an optical fiber preform manufactured according to any one of claims 1 to 7, and a step of drawing said preform under conditions of controlled speed and temperature during which drawing at least part of the preform is softened, and which conditions are chosen in such a manner as to allow for diffusion of the alkali material from the overcladding layer towards the core of the optical fiber being drawn.

9. The method according to claim 8, wherein the alkali material is selected from materials comprising at least one alkali metal from the group consisting of Na and

Li, and combinations thereof.

10. The method according to claim 8 or 9, wherein the drawing temperature is comprised between 1600 and 2200°C.

1 1. The method according to any one of claims 8 to 10, wherein the drawing speed is comprised between 100 and 1500 m/min.

12. A method for manufacturing an optical fiber having a core, an inner cladding and an outer cladding, the method comprising a step of providing an optical fiber preform manufactured according to any one of claims 1 to 7, and a step of drawing said preform under conditions of controlled speed and temperature during which drawing at least part of the preform is softened, and which conditions are chosen in such a manner as to maintain the alkali material in the overcladding layer.

13. The method according to claim 12, wherein the alkali material is selected from materials comprising at least one alkali metal from the group consisting of K, Rb and Cs, and combinations thereof.

14. The method according to claim 12 or 13, wherein the drawing temperature is comprised between 1600 and 2200⁰C

15. The method according to any one of claim 12 to 14, wherein the drawing speed is comprised between 100 and 600 m/min.

16. A single mode fiber manufactured according to any one of claims 8 to 1 1.

17. The single mode fiber of claim 16, having an attenuation at 1550 nm being less than or equal to 0.18 dB/km.

18. A multimode fiber manufactured according to any one of claims 8 to 1 1.

19. The multimode fiber of claim 18, having an attenuation at 850 nm being less than or equal to 2.2 dB/km.

20. The multimode fiber of claim 18 or 19, having a draw peak at 630 nm being less or equal to than 1 dB/km.

21. The multimode fiber of claim 20, having added losses at 850 nm being less than or equal to 0.1 dB/km.

22. A dispersion compensating fiber manufactured according to any one of claims 12 to 15.

23. The dispersion compensating fiber of claim 22, having an attenuation at 1550 nm being less than or equal to 0.6 dB/km.

24. The optical fiber of any one of claims 16 to 23, having a stress corrosion parameter n being more than 28.