DE2640417A1 - MULTIMODE OPTICAL FIBER - Google Patents

Description

BLUMBACH · WESER · BERGEN · KRAMER ZWIRNER - DEER

9 R U Π L 1

PATENT LAWYERS IN MUNICH AND WIESBADEN ^ ^D · '

Postal address Munich: Patentconsult 8 Munich 60 Radeckestrasse 43 Telephone (089) 883603/883604 Telex 05-212313 Postal address Wiesbaden: Patentconsult 62 Wiesbaden Sonnenberger Straße 43 Telephone (06121) 562943/561998 Telex 04-186237

Western Electric Company Incorporated

New York, N.Y., USA Cohen 3-2 / 3-4

Multimode optical fiber

The invention relates to optical fibers having an inner core region which has a radially decreasing refractive index and is surrounded by an outer cladding. In particular, the invention relates to fibers whose core areas consist primarily of SiO ₂ and at least one refractive index-modifying additive such as boron oxide, B ₃ O ₃ , the concentration of which increases radially and leads to a final composition at the core circumference that is essentially borosilicate, 10 to 20 Mol% B ₃ O ₃ contains.

Munich: Kramer ■ Dr. Weser Hirsch - Wiesbaden: Blumbach Dr. Bergen · Zwirr.er

709813/0688

The optical communication systems generally envisaged at present use a fiber optic transmission conductor. A special, currently widely investigated form that is used for Use over medium and long distances is intended, uses a conductor of sufficient size to additionally to be matched to many higher order modes in relation to the fundamental mode. These so-called "multimode transmission conductors" have the advantage of energy conservation, which is of particular interest for the multimode class of radiation sources that are now available. A disadvantage of multimode transmission that was recognized early on however, the energy dispersion is the result of the mode dependence of the group velocities of the propagating Waves occurs. One consequence of this mode dispersion phenomenon is a reduction in the usable system bandwidth. This can be seen most easily in a pulse system in which a pulse broadening during the transmission as a result different transmission times of the numerous modes occurs, which enter into the composition of the impulse. The eventual bandwidth limit occurs when the length of the Transmission line is such that a complete smearing of successive information pulses results.

It was recognized some time ago that mode dispersion can be reduced by changing the refractive index to minimize it mode-dependent group speed changes is provided. Put simply, this route simply delivers a lower refractive index and therefore a "faster"

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-f-

Medium in the outer sections of the core, through which "a statistically larger fraction of that present in higher modes Energy is transferred.

For some time it was thought that a parabolic changing the index of refraction would minimize the mode dispersion. This initially simple concept has grown into a somewhat more complex one Gradient function developed, which the previously neglected scattering effect (dispersive effect) as a result of the preservation of the desired refractive index profile introduced refractive index-modifying dopants taken into account. This development culminated in an article by Robert Olshansky and Donald B. Keck, published on page TuC5-1 ff. of the technical digest "Optical Fiber Transmission" (an overview of the technical articles presented at the Topical Meeting on Optical Fiber Transmission, January 7-9, 1975, Williamsburg Virginia).

It was recognized early that minimal mode dispersion is obtained with a refractive index gradient given by the following Equation is defined:

Xi = Ii ₁ J "i-2A (r / a) ° J ^1/2 · (1)

(see Bell System Technical Journal, Volume 52, (1973) page 1563), where η .. is the refractive index along the core axis, r the distance from the fiber axis, a the core radius, A the fractional refractive index difference between the refractive index along the core axis and the of the jacket as well as Cf a characterizing the profile

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- JT-

Mean exponent parameters.

According to Olshansky and Keck, op. Cit., CC is defined by

with y = ~ ²ⁿ 1 . _λ_.

(5 + 2y) (2)

(3)

Λ = ^η 1 " ^η 2 (4)

n.

_ _. dn ₁ .,. χ

- ^η -ΐ - λ 1 (5)

where Λ is the wavelength.

In all of these relationships, the parameters are calculated from measurements on massive, i.e. larger-volume, glass samples.

For the system specifically investigated by Olshansky and Keck (a TiO ₂ -doped SiO ₂ ~ fiber with gradually changing composition), the predicted deviation from the simple parabolic function was significant and improved transmission properties could be obtained.

With a waveguide alternative to TiO _? -doped silicon oxide fiber, the refractive index in a glass fiber with a high silicon oxide content is given a radially decreasing curve by adding boron oxide B ₃ O ₃ . This is for example in the US-PS

709813/0688 Continued on page

-Jf-

3 778 132. However, preliminary studies have shown that mode dispersion is not improved if the solution developed by Olshansky and Keck is followed, according to which measurements on solid borosilicate glass samples be used. Rather, the mode dispersion actually appears to be getting worse compared to fibers where the simple parabolic refractive index gradient is used.

According to the invention, this problem has now been solved for a fiber in that the fiber used to calculate the exponent parameter Size ^ is given by

"_ N" - ( _no - 0.007)

where n .. and n _{2 are} the refraction index values, measured on solid glass samples of the composition prevailing in the core center or on the core circumference including the cladding.

As can be seen by comparing equations (5) and (6), the expression for Λ is modified by a correction factor of magnitude 0.007 for fibers that use BO ₃ as a refractive index modifying additive.

In the simplest case, in which boron oxide is the only dopant, the fiber simply consists of a central one

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Core part made of unmodified silicon oxide and the boron oxide concentration increases radially to a certain maximum value in accordance with the profile according to the invention. The inventive But teaching is more general and also applies to ternary or quaternary or even more complicated compositions.

In addition to minimizing the mode dispersion, it can be advantageous, for example, to increase the numerical aperture (na) of the fiber at the same time. This can be achieved by adding a second, refractive index increasing material, the concentration of which is a maximum in the center and decreases in the radial direction. In this case, the second material serves to increase the refractive index along the fiber axis (i.e. increases), while the B ₃ O ₃ serves to decrease the refractive index at the fiber core periphery (i.e. n ₂ decreases). The resulting refractive index profile is then a function of the concentrations of the two refractive index modifying materials.

In a first embodiment, a fiber with two refractive indices Modifying additives, the central area of the fiber core consists essentially of germanium silicate. With a second Embodiment consists of the central area of the core in essentially made of phosphorus silicate.

The invention is illustrated below with reference to the drawing Embodiments explained in detail; show it:

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Fig. 1 is an oblique view of an optical fiber to which the invention relates.

Fig. 2 shows the change of QL versus wavelength \ for a multimode Borsilicatfaser known or inventive design and

3 shows the change in (X as a function of the wavelength Λ for three fibers made up of the ternary phosphorosilicate system according to the invention.

The refractive index profile is conveniently described in accordance with equation (1) using the exponent (X :

r \ = τ »ι t_'j / ι i - λ ι '/ ^

where η .. the refractive index along the fiber axis, r the radial distance from the fiber axis, a the core radius, J \ the fractional refractive index difference between the refractive index along the core axis and that of the cladding, and Ot an exponent parameter characterizing the profile.

The above equation is derived in detail from Bell System Technical Journal, Volume 52, page 1563 (1973). Of simplicity for the sake of this, reference is made to this. This equation is currently widely accepted as the optimal refractive index profile. The present invention is now concerned with the value of Of in this equation.

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Fig. 1 shows an optical waveguide 1 having a core portion 2, a cladding area 3 and a sometimes provided cladding 4. The invention is largely concerned with the core area 2, which has a gradient in the index of refraction. As indicated by equation (1), the refractive index gradient increases symmetrically from a maximum value in the center of the core 2 to a minimum at the circumference of the core 2. The invention is concerned largely with the value for 0 (in fibers where the peripheral Part (which is at or immediately in the interface between core 2 and cladding 3) is primarily borosilicate.

A ternary glass system can be made according to the invention Germanium silicate in the central part of the core and from borosilicate exist in the peripheral region of the core according to a preferred embodiment.

The cladding area 3 is mainly for the optical waveguide responsible. It generally, although not necessarily, has a constant index of refraction and exists for the present Purposes often from borosilicate of a refractive index equal to the low end value, like this one with the gradient is obtained within the core. In general, such a borosilicate shell made of 14 mol% boron oxide, the remainder silicon oxide, exist. A more general range can be considered between 10 and 20 mol% boron oxide, the remainder silicon oxide, lying, can be defined. In this example, a composition of 14 mol% boron

709813/08 8 8

oxide. Balance silicon oxide, at the low refractive index end of the core gradient for a binary glass system to a value of Δ in equation (1) of about 0.005.

The envelope shown in dashed lines can, if desired, be provided and generally has no optical function. It can consist of a silicon oxide layer and / or an organic layer intended to prevent scratching during to avoid processing.

As mentioned, the refractive index profile shape of the cylindrical waveguide designed for multimode operation so as to produce minimum mode dispersion can be described by equation (1) above. Previous analysis suggested a parabolic index, that is, Oi = 2 . In more recent work, as also mentioned, it was assumed that this value was too much of a simplification and that dispersive refractive index differences for the various components of the glass equally led to a wavelength dependence (profile dispersion). Specifically, it was shown in equation (2) that

(4 + y) (3 + y)

(5 + 2y) holds, where

^ ₁ A dA
axial refractive index,
On ₁

= Wavelength and

ⁿ 1 ~ ⁿ 2
= mean.

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- yc -

According to this finding, as has been reported by Olshans ^ y and Keck, op. Cit., Values for A could be determined by direct measurement on solid glass of the compositions which correspond to the axial and peripheral core compositions.

With the invention, however, it was found that a measurement on solid glass is not able to form a reliable criterion, when boron oxide is present in the glass compositions. In particular, it was found that the dispersion curve for each Borosilicate fiberglass composition only from the dispersion curve resulting from direct measurements on solid glass Correction of the refractive index values can be determined to account for the refractive index change, which in turn depends on the Difference in the heat treatment history of the solid and fiber samples depends. In detail, studies have shown that a difference of 0.007 must be subtracted from the refractive indices measured on solid glass to make the fiber case approximate. As indicated, this correction factor is suitable for current and future fiber dimensions that are based on both are based on the current as well as expected future process parameters. While Δ is defined in the above equations

ⁿ 1 " ⁿ 2
xst as (where n _{1 is} the axial refractive index and n, the

nl ' *

low refractive index at the core periphery, and both values are measured on solid glass) according to the invention Ji ₁ - (n ₀ - 0.007)

ⁿ 1

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Essentially, the value of 0.007 represents that correction between the refractive index of a solid sample and that of an actual fiber.

A second aspect of the invention is that the value of & , which determines the coordinates of the profile curve, is determined only by the indices present at the end points of the gradient. Accordingly, a modification of the fiber by substituting the silicon oxide in the core axis, for example with germanium silicon oxide, simply means a change in the value for n ... Refractive index. Final value applicable. Any desired profile can therefore be calculated directly from the equations given in order to obtain the data applicable to such multicomponent fibers.

Fig. 2 shows the change in Cf as a function of wavelength for three different cases. Curve 10, a horizontal line at (Χ = 2, represents the parabolic gradient that was initially generally considered to be optimal. Curve 11 represents the Oi values as derived from the above-mentioned article by Olshansky and Keck. Curve 12 represents the O (values as obtained according to the invention for the exemplary binary borosilicate glass system. As will be discussed in detail below, curve 12 defines a nearly optimal gradient for a multimode fiber with an outer dimension of approximately

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80 to 130 μπι (ie the entire glass fiber structure, as it is obtained after drawing). It will be seen that although the curve can shift somewhat for significantly different outer dimensions, this curve is not influenced by organic or additional coatings that are applied after the drawing process, for example as mechanical protection or for cabling purposes. A comparison of the curve 12 with the known curves 10 and 11 shows a pronounced difference corresponding to a greatly improved multimode waveguide. While curve 12 shows optimal (X values, a noticeably improved waveguide (reduction of smearing as a result of mode dispersion) is still important even for non-optimal values for

(X. Accordingly, (X values within a tolerance of _ + 10% are acceptable. It is noteworthy that the 0 (values of curve 12 are approximately constant in the wavelength range from 0.5 to 1.1 μm ( Οι approximately equal to 1 , 77 under the specified conditions).

As shown, the invention is based on borosilicate glass, at least within that region which represents the low refractive index end of the core gradient. The calculations of of in the literature are believed to be reasonably accurate for glass fibers not containing boron oxide. The invention is largely based on the fact that a reduced index of refraction in a borosilicate system is based on structural considerations which in turn depend on the thermal history of the glass. Curve 12 in FIG. 2 shows Cf values for common multimode fiber arrangements. Such a fiber has after drawing

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-Ur-

Process an outer diameter of approximately 80 to 130 μm and a core area of a diameter of approximately 30 to 80 μm. The values given are also applicable to fibers that are inherently deterred by direct air cooling. The temperature drop from the softening point is sufficiently great that increased cooling or, equally, possible heat treatment will have little consequence. The correction of the Of value compared to the values that can be taken from the literature is greater for smaller dimensions and smaller for larger dimensions. However, changes introduced in such (X values for the affected changes in the diameter of the drawn fiber, ie + _ 25% of the above range, do not result in so great a change in the quenching conditions as that revised

CX values would be required. While multimode fibers from even smaller or even larger dimensions are currently not yet considered desirable, on the one hand for reasons a reduced mode capacitance and, on the other hand, for reasons of a reduced return of the total energy capacity an approximately linear process of correcting the ft value with the reciprocal value of the fiber diameter, the value being approximately zero for fibers with an outer diameter of 1000 μm.

example 1

A flow-through system was made within one of a quartz glass tube commercial purity existing casing, and the dimensions of the pipe were 14 mm inside diameter, 16 mm outside

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diameter and 90 cm length. A gas mixture of silicon tetrachloride (SiCl.), Boron trichloride (BCl ₃ ) and oxygen was applied to the tube, the

3

Throughputs of the individual components were 66 cm / min, 22.6 cm / min and 780 cm / min, respectively. The tube and its contents were heated by a 5 cm long hot zone moving at 17 cm / min, which was generated by a multi-jet oxyhydrogen burner. A formation of powdery substance was observed in the area of the burner, which moved downstream and was deposited as a white precipitate on the inner surface of the tube downstream of the hot zone. The precipitation was strongest on the downstream side over a length of about 10 cm and then decreased except for a still noticeable precipitation at the exit end of the pipe. As the hot zone moved downstream, the visible white precipitate was melted to form a much thinner transparent glassy film. The operation was 35 times repeated _next> with rapid return movement of the hot zone to any downstream movement. The resulting glass layer was about 0.3 mm thick and was intended to serve as the optical cladding. Thereafter, a core area with changing composition was knocked down using essentially the same general procedure with the same advance speed of the same oxyhydrogen blower, but the composition of the gas mixture flowing through was changed from the initial composition over a total of 31 passes to a final composition consisting exclusively of SiCl ₄ and oxygen with throughputs of about 66 cm and about 780 cm / min, respectively.

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The BClo throughput was determined according to the equation below reduced.

Τ -./- Π ^ -Uj- , 22.6 in / min. , ~. _Λ _.1,12 BCl ^ throughput = ( '- = -η— ' - ) (31-1) '

With I = passage number. The one generated by the 31 passes Total layer thickness was about 0.2 mm so that the total film thickness deposited was about 0.5 mm.

The pipe was then collapsed along with the deposited layers. The collapse was accomplished by switching off the gas flow and increasing the temperature by gradually reducing the migration speed of the oxyhydrogen burner. Collapse occurred after 10 passes in one Zone migration speed of about 1.2 cm / min. The collapse was seen as a transformation of the tube and its contents into one fixed rod observed in the direction of the torch movement.

The collapsed assembly, now known as the preform, was about 9 mm in diameter and about 35 cm in length. These The length of the zone of uniform precipitation was taken from the entire original pipe by local melting and peeling away.

The fiber drawing to obtain a fiber with a diameter of about 100 μm was carried out by passing the preform through a sleeve open, resulting in a local heating to about

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2000 ° C, and by pulling the heated zone at a uniform speed of about 1 m / sec. The final one Fiber length was 1100 m, this length being limited by the fiber drawing device.

Experiments were then carried out to empirically identify the optimal value for Qf. The method used was based on the fact that α values above the optimum lead to a comparative delay in the higher order modes, while CC values less than the optimum value lead to a delay in the fundamental mode. Circular diaphragms and ring diaphragms were used in the experiments. The light passing through these spatial beam filters was collected and focused on a detector. Spatially filtered pulse shapes were measured in fibers with changing (X after transmission over 1 km. The examined fibers had a numerical aperture of approximately 0.14 and an almost optimal profile, ie one obtained practically coincidence between the fundamental and higher mode peaks at A ⁼ 907.5 nm (corresponding to the specially used GaAs injection laser). This resulted in a fiber having an (X value of about 1.77, measured by interferometric methods. As can be seen, this value corresponded quite well with the corresponding point on the Curve 12 in FIG. 2.

From a practical standpoint, there is an even minor drawback to the otherwise excellent binary borosilicate system present with gradually changing composition. The possible refractive index change from the axial to the

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peripheral core composition leads to a maximum numerical Aperture of only about 0.15. While this aperture is sufficient, the mode number expected for the dimensions concerned of 10,000 under ideal conditions, for example, deviations from these conditions, such as inhomogeneity, Microbending etc. cause some of the higher order modes on the core / clad interface above the critical Hit the angle. About loss of energy under these conditions To avoid this, it is generally desirable to increase the numerical aperture.

In a second embodiment of a two-dopant fiber system, the core region of the fiber is essentially phosphosilicate which is gradually changed in composition to a core peripheral composition which is essentially borosilicate. , During the boron oxide content in the peripheral core region range from 10 to 20 mol%, the phosphorus dopant-containing Po is ⁰ S '^ ^{m core} b ^ere i ^c h ^w ith a narrower fixed concentration of 10 to 15 mol% in the composition available.

Figure 3 shows the changes in cc as a function of λ for three different fibers. Curve 13 shows the data for a fiber in which the axial core composition was 10 mol% Po ⁰ S ', remainder SiO ₂ /. Curve 14 applies to a fiber in which the axial core composition was 12 mol% Ρο ° 5 ', ^{remainder s} i ° 2'. Curve 15 applies to a fiber in which the P ₃ O ₅ gel on the

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-> sr-

Core axis was 15 mol%. As indicated, the peripheral composition is essentially borosilicate and the level of modifying additive, in this case B ₂ O. ,, is somewhat less critical. For comparison purposes, a core peripheral composition of about 14 mol% B ₂ ⁰ B ', ^remainder SiO ₂ , has been selected for the fibers represented by the three curves. A change in the BO - ^ - content in the range from about 10 mol% to about 20 mol% has only a minimal effect on the size of the values plotted.

The teaching according to the invention is largely aimed at making the optimal Cf value essentially independent of the wavelength. It can be seen that the fiber of curve 14 (axial composition about 12 mol% P ₂ ⁰ S ' ^{remainder s} ^ - ⁰ 2 ^ accordingly as As a preferred embodiment, changes in Of dependency represented by non-optimal axial compositions for the fibers represented by curves 13 and 15 can be seen to be generally in the range of + 2%.

Significantly, however, from a more general design standpoint, the numerical aperture of the fiber for stronger P-O ^ doping, as is to be expected, is higher. Sp Specifically, the numerical aperture of the fiber represented by curve 15 is 0.3, while the corresponding values for the fibers corresponding to curves 14 and 13 are 0.28 and 0.26, respectively. As mentioned, a large numerical aperture is general advantageous for real (i.e. non-ideal) fibers where micro-bends or other deviations from the theoretical

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- 2fr -

retic structure can lead to energy losses in higher order modes. It follows from this that an even further increase in the PO ^ doping concentration on the core axis or an addition of a further refractive index modifier is desirable in those cases where an even greater dependence of the value CK on the wavelength can be permitted.

Example 2

A flow system was built up in an envelope consisting of a quartz glass tube of commercially available purity. The dimensions of the pipe were 14 mm in inner diameter, 16 mm in outer diameter and 90 cm in length. The tube was charged with a flowing gas mixture of silicon tetrachloride (SiCl ₄ ), boron trichloride (BCl ₃ ) and oxygen, the corresponding throughputs being 66 cm / min,

3 3

Were 22.6 cm / min and 780 cm / min, respectively. Tube and contents were moved by a moving at a speed of 17 cm per minute, 5 cm long hot zone, which was created by a multi-jet oxyhydrogen burner. An education was observed of powdery substance in the general area of the burner moving downstream and manifesting as a white precipitate deposited on the inner surface of the pipe downstream of the heating zone. The precipitation was strongest in a roughly 10 cm long downstream area and then decreased to a still noticeable precipitation at the exit end of the pipe. With the Moving down the hot zone, the white precipitate was melted to a much thinner transparent glassy To lead film. The process was repeated 35 times, with the hot zone rapidly after each downstream pass was retrieved. The resulting glass layer was about 0.3 mm

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23 2640A17

thick and should serve as an optical cladding. A core area with gradually changing composition was then precipitated by following the same general procedure with the same advance speed of the same oxyhydrogen burner, but changing the composition of the flowing gas mixture from the initial composition over a total of 31 passes to a final composition consisting entirely of POCl ₃ , SiCl . and oxygen, the

3 3

approximate flow rates were 66 cm / min SiCl., 12.8 cm / min POCl ₃ and 780 cm / min O ₃ . The POCl ₃ content was increased according to the equation

POCl -, - flow rate = (15.1 cm ³ / min) fj - (-J-qTf) (31-1) ^{Of 96} U · J 31 ^{U 'yb}

The BCl ₃ ~ throughput was reduced according to the equation

.τλ1Ί5

BCl ^ throughput = (

In the above equations, I means the number of the respective Passage. The total thickness created by the 31 passes was about 0.2 mm, resulting in a total film deposit thickness of about 0.5 mm.

The pipe was then collapsed along with the deposited layers. The pipe collapse was accomplished by switching off the gas flow and increasing the temperature by increasing the advance speed of the oxyhydrogen burner gradually

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- HT-

^1H 2640A17

was reduced. Pipe collapse occurred after 10 passes a zone velocity of about 1.2 cm / min. The pipe collapse was visually observed as a transformation of the tube and its contents into a solid rod in the direction of the burner movement.

The collapsed assembly, now known as the preform, was about 9 mm in diameter and about 35 cm in length. This length, which represents the area of uniform precipitation, is taken from all the original pipe by local Melting and stripping separately.

The fiber drawing to obtain a fiber was an approximate outside diameter of 100 μπι by passing the preform through a sleeve furnace, resulting in a local heating at about 2000 ⁰ C, and by pulling the heated zone at a uniform rate of about 1 m / sec. The final fiber length was 1,100 m, the length being limited only by the specific fiber drawing device used.

The wavelength range of 0.5 to 1.1 μm discussed above results from an almost complete independence of Oi from the wavelength (ie approximately + ^ 2%). A usable range with a somewhat greater wavelength dependency (+ 5%) leads to an expansion of the operating range from 1.1 μm to at least 2 μm wavelength. Since this change is small enough to meet many demands, it is for the present

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Purposes a total operating range for the wavelength of 0.5 to 2 μπι provided.

70 9813/0688

Claims (7)

BLUMBACH WESER BERGEN KRAMER ZWIRNER - HIRSCH 264041? PATENT AGENCIES IN MUNICH AND WIESBADEN Postal address Munich: Paienlconsult 8 Munich 60 Radeckestraße 43 Telephone (089) 883603/883604 Telex 05-212313 Postal address Wiesbaden: Patentconsult 62 Wiesbaden Sonnenberger Straße 43 Telephone (06121) 562943/561998 Telex 04-186237 Western Electric Company Incorporated York, NY USA Cohen 3-2 / 3-4 claims 1. Optical fiber for the transmission of electromagnetic wave energy with an inner core area, which is surrounded by an outer cladding, wherein the core area consists primarily of SiO ,, and at least one refractive index modifying additive, a radially decreasing refractive index at least partially as a result of radially increasing boron oxide Concentration (B "O ^.) And reached at the core periphery a final composition essentially of borosilicate with 10 to 20 mol% B ₂ O ₃ , the refractive index gradient being approximately given by whereby n ₁ the refractive index in the core center, r the distance from the core center,
a is the core radius and α is an exponent characterizing the profile Munich: Kramer - Dr. Weser Hirsch - Wiesbaden: Blumbach Dr. Mountains ". ■ Z-.virner 7098 1 3/0688 parameter
mean,
characterized in that the size Δ is given by _{Λ =} Xi ₁ - (n ₂ - 0.007) ⁿ 1 where n ₁ and n ~ are the refractive index values, measured on solid glass samples of the composition prevailing in the core center and on the core periphery, respectively. 2. Fiber according to claim 1, characterized in that the peripheral core area consists essentially of borosilicate glass consists. 3. Fiber according to claim 2, characterized in that the core axis region consists essentially of germanium silicate . 4. Fiber according to claim 2, characterized in that the core axis region consists essentially of silicon oxide. 5. Fiber according to claim 4, characterized in that (K = 1.77 jf 10% for electromagnetic radiation in the wavelength range between 0.5 and 1.1 μπι. 6. Fiber according to claim 1, characterized in that the core region modifies a second refractive index Addition contains. 7 ü H 8 Ϊ 3/0 B 8 8 - 2-6- - 3 2 6 A O A 1 7. Fiber according to claim 6, characterized in that the second refractive index-modifying additive is phosphorus pentoxide, P ₃ O ₅ . 8. A fiber according to claim 7, characterized in that the P ₂ O _c concentration decreases radially from a core axis composition which contains 10 to 15 mol% Ρ ₂ ° ₅ . 9. A fiber according to claim 7, characterized in that the composition consists essentially of along the core axis Phosphosilicate glass and the composition along the core periphery consist essentially of borosilicate glass. 10. Fiber according to claim 7, characterized in that d is about 1.9 μΐη for electromagnetic radiation in the wavelength range from 0.5 to 1.1. 7 0 0 B 1 3/0 H 9th H.