U.S. patent application number 12/399407 was filed with the patent office on 2009-06-25 for process for making a glass fiber with a core and two glass cladding layers and glass fiber made thereby.
Invention is credited to Frank Buellesfeld, Martin Letz, Ulrich Peuchert, Ruediger Sprenhard.
Application Number | 20090158778 12/399407 |
Document ID | / |
Family ID | 26010103 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090158778 |
Kind Code |
A1 |
Peuchert; Ulrich ; et
al. |
June 25, 2009 |
Process for making a glass fiber with a core and two glass cladding
layers and glass fiber made thereby
Abstract
The glass fiber for an optical amplifier has a glass core, a
first glass cladding, and a second glass cladding. The core has a
composition, in mol %, of Bi.sub.2O.sub.3, 30-60; SiO.sub.2,
0.5-40; B.sub.2O.sub.3, 0.5-40; Al.sub.2O.sub.3, 0-30;
Ga.sub.2O.sub.3, 0-20; Ge.sub.2O.sub.3, 0-25; La.sub.2O.sub.3,
0-15; Nb.sub.2O.sub.5, 0-10; SnO.sub.2, 0-30; alkali metal oxides,
0-40; and Er.sub.2O.sub.3, 0.05-8. The process for making the glass
fiber includes first making a preform consisting of the core and
the first glass cladding by drawing from a double crucible. Then
the second glass cladding is formed around the preform by a
rod-in-tube process. The glass claddings have a composition that
includes a transition metal compound as an absorbent.
Inventors: |
Peuchert; Ulrich;
(Bodenheim, DE) ; Sprenhard; Ruediger; (Mainz,
DE) ; Letz; Martin; (Klein-Winternheim, DE) ;
Buellesfeld; Frank; (Frankfurt, DE) |
Correspondence
Address: |
STRIKER, STRIKER & STENBY
103 East Neck Road
Huntington
NY
11743
US
|
Family ID: |
26010103 |
Appl. No.: |
12/399407 |
Filed: |
March 6, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10489020 |
Aug 16, 2004 |
7515802 |
|
|
12399407 |
|
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Current U.S.
Class: |
65/435 |
Current CPC
Class: |
C03B 2201/30 20130101;
C03B 2203/23 20130101; C03B 2201/32 20130101; C03C 13/046 20130101;
G02B 6/03627 20130101; C03B 2201/10 20130101; C03B 2201/36
20130101; C03C 3/068 20130101; C03B 37/01211 20130101; C03B 37/023
20130101; C03B 37/01274 20130101; C03C 3/253 20130101; G02B 6/0365
20130101 |
Class at
Publication: |
65/435 |
International
Class: |
C03B 37/02 20060101
C03B037/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 10, 2001 |
DE |
101 44 475.3 |
Mar 13, 2002 |
DE |
102 11 247.9 |
Claims
1. A process for producing a glass fiber, said glass fiber
comprising a core and at least two glass claddings surrounding said
core, said core comprising a matrix glass containing at least one
heavy metal oxide and at least one rare earth compound, wherein the
matrix glass has a refractive index of greater than about 1.85, a
refractive index change .DELTA.n from a first of the at least two
glass claddings to the core is in a range from 0.001 to 0.08, and
the refractive index of the first glass cladding is lower than that
of the core; said process comprising the steps of: a) making a
preform, said preform comprising said core and the first glass
cladding, by drawing from a double crucible; and b) forming at
least one further glass cladding around the preform by a
rod-in-tube process.
2. The process as defined in claim 1, wherein said rod-in-tube
process comprises drilling a through-going hole into a second
cladding glass rod to form a tube of said second cladding glass,
introducing said perform comprising said core and said first glass
cladding into said tube of said second cladding glass, and drawing
said tube of said second cladding glass to form said glass
fiber.
3. The process as defined in claim 1, wherein said at least one
heavy metal oxide is at least one oxide of Bi, Te, Se, Sb, Pb, Cd,
Ga and/or As, or a mixture thereof.
4. The process as defined in claim 1, wherein the core comprises at
least Bi.sub.2O.sub.3 and/or TeO.sub.2 and/or Sb.sub.2O.sub.3.
5. The process as defined in claim 1, wherein the at least one rare
earth compound contains at least one element selected from the
group consisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,
Tm, Yb and Lu, or a mixture thereof.
6. The process as defined in claim 1, wherein the first glass
cladding contains said at least one rare earth compound.
7. The process as defined in claim 1, wherein said matrix glass
contains an amount of each of said at least one rare earth compound
equal to 0.05 to 5 mol %, but the first glass cladding contains
only up to one half of said amount of each of said at least one
rare earth compound in said matrix glass.
8. The process as defined in claim 1, wherein an outermost cladding
of the at least two glass claddings contains from 5 to 5000 ppm of
at least one absorbent component.
9. The process as defined in claim 8, wherein said at least one
absorbent component is at least one transition metal compound.
10. The process as defined in claim 9, wherein said at least one
transition metal compound comprises Co.sup.+2.
11. The process as defined in claim 1, wherein the index of
refraction of the outermost cladding of the at least two glass
claddings is greater than the index of refraction of the first
glass cladding.
12. The process as defined in claim 1, wherein the matrix glass has
a composition, in mol % on an oxide basis, of: TABLE-US-00005
Bi.sub.2O.sub.3 30-60 SiO.sub.2 0.5-40 B.sub.2O.sub.3 0.5-40
Al.sub.2O.sub.3 0-30 Ga.sub.2O.sub.3 0-20 Ge.sub.2O.sub.3 0-25
La.sub.2O.sub.3 0-15 Nb.sub.2O.sub.5 0-10 SnO.sub.2 0-30 MI.sub.2O
0-40 Rare 0.5-8, earths
wherein MI is at least one of Li, Na, K, Rb and Cs.
13. The process as defined in claim 1, wherein the core has a
diameter of from 1 to 15 .mu.m.
14. The process as defined in claim 1, wherein the first glass
cladding has a thickness d.sub.m1 in a range from 5 to 100
.mu.m.
15. The process as defined in claim 14, in which a second glass
cladding of the at least two glass claddings has a thickness
(d.sub.m2) in a range from 10 to 300 .mu.m.
16. The process as defined in claim 1, wherein the glass fiber has
a total thickness of 125 .mu.m.
Description
CROSS-REFERENCE
[0001] This is a divisional of U.S. patent application Ser. No.
10/489,020, filed on Aug. 16, 2004. The aforesaid U.S. patent
application, whose disclosures are expressly incorporated herein by
reference thereto, describes the same invention as described and
claimed herein below. In accordance with 35 U.S.C. 120 the benefit
of the filing date of the aforesaid U.S. patent application is
claimed for the claims presented herein below.
BACKGROUND OF THE INVENTION
[0002] 1. The Field of the Invention
[0003] The present invention relates to a glass fiber which
comprises a core, the matrix glass of which contains at least one
heavy metal oxide and at least one rare earth compound, the core
being surrounded by at least two glass claddings. Furthermore, the
present invention relates to a process for producing a glass fiber
according to the invention, to an optical amplifier which comprises
at least one glass fiber according to the invention, and to the use
of the glass fiber according to the invention.
[0004] 2. The Related Art
[0005] Optical amplifiers are one of the most important key
components of optical communication technology. If a purely optical
telecommunications signal is transmitted in a glass fiber, it is
inevitable that intrinsic signal attenuation will occur. To
compensate for this attenuation, it is necessary to use highly
efficient optical amplifiers which are able to amplify a signal
without the optical signal having to be converted into an
electronic signal and then back into an optical signal. Optical
amplifiers can also increase the speed of amplification, and the
deterioration in the signal/noise ratio is significantly lower on
account of the elimination of the conversion, into electronic
signals and back.
[0006] In this context, the technical demands imposed on optical
amplifiers are increasing in particular on account of the
continuously rising demand for ever greater bandwidths. Currently,
broadband data transmission is realized using WDM (WDM "wavelength
division multiplexing") technology. Most amplifiers of the prior
art operate in the C band (approx. 1528 nm to 1560 nm) and have
only a limited broadband capacity, since optical amplifiers of this
type have hitherto been based on Er.sup.3+-doped SiO.sub.2 glasses.
Therefore, the demand for greater bandwidths has required the
development of multicomponent glasses, for example heavy metal
oxide glasses (HMO glasses). As manifested by their intrinsically
very high refractive index (at 1.3 .mu.m) of n>approx. 1.85,
heavy metal oxide glasses have high internal electrical fields and
therefore, on account of greater Stark splitting, lead to
broad-band emission from the rare earth ions. However, the high
refractive index of HMO glasses also leads to new problems which
have to be overcome.
[0007] Various mechanisms in optical amplifier fibers can give rise
to scattered light, which can lead to a deterioration in the
signal/noise ratio and should therefore be removed or avoided as
fully as possible.
[0008] In amplifier fibers based on SiO.sub.2, scattered light is
removed by a polymer coating applied to the glass fiber. Since
absorbent polymer coatings with a refractive index of n>1.4 are
available, it is readily possible for noise which is caused by
reflected signals and/or scattered light from outside the fiber to
be absorbed by a polymer coating of this type on the SiO.sub.2
glass fiber.
[0009] Heavy metal oxide glasses which are suitable for use as
fiber amplifiers usually have a refractive index of approximately
n=1.9. Polymer coatings which have hitherto been available have
always had a lower refractive index than heavy metal oxide glasses.
Therefore, coating with polymers of this type for absorption of
scattered light causes problems, since it is only possible to
provide a polymer cladding with a lower refractive index. Any
coating with a cladding made from a material with a lower
refractive index then leads to strong, undesired reflection at the
interface between this material and the core regions or an inner
cladding.
[0010] Furthermore, in conventional SiO.sub.2 amplifier fibers,
there is substantially no change in refractive index at a contact
location between a standard telecommunications fiber and a glass
fiber of an optical amplifier, and consequently the reflection
which occurs at the transition from a SiO.sub.2 glass fiber
amplifier to a standard communications glass fiber is
negligible.
[0011] By contrast, the high refractive index of HMO fibers means
that any contact location with a standard SiO.sub.2
telecommunications glass fiber leads to strong reflection at the
interface between SiO.sub.2 standard fiber and heavy metal oxide
glass fiber of the optical amplifier. Since an optical amplifier is
at both outputs connected to SiO.sub.2 telecommunications glass
fibers or transition fibers based on SiO.sub.2 with a high
numerical aperture, there is a considerable tendency for a laser
resonator with standing light waves to form in the optical
amplifier. To prevent the latter phenomenon, it is recommended for
the contact locations in relation to the glass fibers to be
designed at a defined or finite angle. However, this in turn leads
to considerable or significant reflection which is scattered into
the cladding of the fiber. Therefore, scattered light which
migrates through the cladding of the fiber is reflected back and
forth and it is impossible to prevent scattered light from reaching
the central core region and penetrating into the latter. This
scattered light will influence the inversion of the state of the
rare earth ions and leads to amplification of the noise and a drop
in the signal power(s) of the amplifier.
[0012] Outer absorbent claddings for various glass systems are
known from the prior art (for example K. Itoh, et al., J.
Non-Cryst. Sol, 1, pp. 256-257 (1999)).
[0013] EP 1127858 describes a light-amplifying glass, the matrix
glass of which is doped with 0.01 to 10 mol % of Er, with the
matrix glass necessarily containing 20 to 80 mol % of
Bi.sub.2O.sub.3, 0.01 to 10 mol % of CeO.sub.2, and at least one of
B.sub.2O.sub.3 or SiO.sub.2. However, the glass fibers described in
this document are only provided with standard polymer coatings. The
same is true of the glasses with a high antimony oxide content
described in WO 99/51537.
[0014] JP 11274613 A describes a glass fiber comprising glasses
with a high refractive index, which has two glass claddings.
According to this document 10000 ppm of absorbent material are
required. However, such high levels of absorbent material influence
the properties of the glass and are therefore disadvantageous.
SUMMARY OF THE INVENTION
[0015] Therefore, the object of the present invention was to
provide a glass fiber comprising a matrix glass with at least one
heavy metal oxide, for an optical amplifier, which allows the
problems of the prior art described above to be avoided. In
particular, this glass fiber should allow the noise caused by
scattered light to be minimized and therefore the signal power of
the amplifier to be increased.
[0016] This object is achieved by the embodiments of the present
invention which are described in the claims.
[0017] In particular, the present invention relates to a glass
fiber comprising a core, the matrix glass of which contains at
least one heavy metal oxide and at least one rare earth compound,
and at least two glass claddings surrounding the core. The matrix
glass has a refractive index greater than about 1.85, the change in
the refractive index .DELTA.n from the core to the first cladding
is in a range from 0.001 to 0.08, and the first cladding has a
lower refractive index than the core.
BRIEF DESCRIPTION OF THE DRAWING
[0018] FIG. 1 is a diagrammatic cross-sectional view through a
particularly preferred embodiment of the glass fiber according to
the invention.
[0019] FIGS. 2, 5 and 7 are respective photographic images of the
cross section through glass fibers according to the invention with
two glass claddings.
[0020] FIGS. 3 and 4 diagrammatically depict preferred designs of
double-clad fibers according to the invention with two or three
claddings.
[0021] FIG. 6 shows a comparison of the absorbing action of iron
oxide and cobalt oxide as absorbing material in a bismuth
oxide-containing glass which has been melted under strongly
oxidizing conditions.
[0022] FIGS. 8a and 8b show the maximum gain, calculated from Giles
parameters, for a fixed number of channels as a function of the
wavelength, as well as the change in the noise as a function of the
wavelength.
[0023] FIGS. 9a and 9b show the energy which is transmitted in each
case in the core region, in the region of the first cladding and in
the region of the second cladding, for various fiber lengths as a
function of the doping of the outer cladding.
DETAILED DESCRIPTION OF THE INVENTION
[0024] It is preferable for the core of the glass fiber according
to the invention to contain at least one heavy metal oxide which is
selected from oxides of Bi, Te, Se, Sb, Pb, Cd, Ga, As and/or mixed
oxides and/or mixtures thereof. The matrix glass of the core
particularly preferably contains heavy metal oxides which are
selected from oxides of Bi, Te, Sb and/or mixtures thereof.
[0025] Furthermore, the matrix glass of the core comprises at least
one dopant which can be excited by light. According to the
invention, the matrix glass of the core contains rare earth ions as
dopant. In this context, a dopant is to be understood as meaning a
component which is only added to the glass in small quantities and
which therefore has very little influence on most of the physical
properties of the glass, such as Tg, the refractive index or the
softening point. However, a dopant of this type may have a
significant influence on certain properties, in particular optical
properties, such as for example the capacity for optical
stimulation.
[0026] It is preferable for the matrix glass of the core to
comprise at least one rare earth compound which is selected from
compounds of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb
and/or Lu. Oxides of the elements Er, Pr, Tm, Nd and/or Dy are
particularly preferred, and oxides of Er are most preferred.
[0027] If appropriate, it is also possible for Sc and/or Y
compounds to be present in the glass according to the invention in
addition to one or more rare earth compound(s).
[0028] The rare earth compounds used as dopants are preferably what
are known as "optically active compounds"; the term "optically
active compounds" is to be understood in particular as meaning
compounds which lead to the glass according to the invention being
capable of stimulated emission when the glass is excited by a
suitable pumping source.
[0029] It is also possible for at least two rare earth compounds to
be used, in a total quantity of from 0.01 to 15 mol %. Glasses
containing optically active rare earth ions can be co-doped with
optically inactive rare earth elements in order, for example, to
increase the emission lifetimes. For example, Er can be co-doped
with La and/or Y. To increase the pumping efficiency of the
amplifier, it is also possible, for example, for Er to be co-doped
with further optically active rare earth compounds, such as for
example Yb. Co-doping with Gd may also be effected in order to
provide stability against crystallization.
[0030] Doping with other rare earth ions, such as for example Tm,
makes it possible to open up other wavelength regions, for example,
in the case of Tm, what is known as the S band between 1420 and
1520 nm.
[0031] Furthermore, to make more effective use of the excitation
light, it is possible to add sensitizers, such as Yb, Ho and Nd in
a suitable quantity, for example 0.005 to 8 mol %.
[0032] The level of each individual rare earth compound is, for
example, from 0.005 to 8 mol %, preferably 0.05 to 5 mol %, based
on oxide.
[0033] According to one embodiment, the matrix glass comprises both
Ce and Er.
[0034] According to a further embodiment, the matrix glass contains
no cerium.
[0035] According to a preferred embodiment of the present
invention, the glass fiber according to the invention contains at
least one Bi.sub.2O.sub.3 glass in the core and/or in one or more
claddings. The following compositions are particularly
preferred:
TABLE-US-00001 Particularly Preferred preferred components and
components and Component ranges [mol %] ranges [mol %]
Bi.sub.2O.sub.3 10-80 30-60 SiO.sub.2 0-60 0.5-40 B.sub.2O.sub.3
0-60 0.5-40 Al.sub.2O.sub.3 0-50 0-30 Ga.sub.2O.sub.3 0-50 0-20
GeO.sub.2 0-30 0-25 Ln.sub.2O.sub.3 0-30 WO.sub.3 0-30 MoO.sub.3
0-30 La.sub.2O.sub.3 0-30 0-15 Nb.sub.2O.sub.5 0-30 0-10
Ta.sub.2O.sub.5 0-15 ZrO.sub.2 0-30 TiO.sub.2 0-30 SnO.sub.2 0-40
0-30 M.sup.I.sub.2O 0-40 0-40 M.sup.IIO 0-30 F and/or Cl 0-10 Rare
earths 0.005-8 0.05-5 (based on oxide) (based on oxide)
[0036] In the above table, M' is at least one of Li, Na, K, Rb and
Cs and M'' is at least one of Be, Mg, Ca, Sr, Ba and/or Zn. It is
particularly preferable to use Li and/or Na as M'.
[0037] FIGS. 8a and 8b show the gain and the noise of a doped HMO
double-cladding fiber in accordance with the invention compared to
SiO.sub.2 amplifier fibers as a function of the wavelength and the
number of channels. To produce these diagrams, methods which are
known from the prior art are used to determine the so-called Giles
parameters for the amplifier fibers, and the maximum gain and the
noise at a defined wavelength are then determined from the Giles
parameters for a defined channel number. It can be seen from FIG.
8a firstly that with a set number of 120 channels [ch], a maximum
gain of approx. 25 dB is achieved with an amplifier fiber according
to the invention, while with the same number of channels only a
maximum gain of just below 20 dB is achieved for a silicate-based
amplifier fiber. To achieve a similar gain of 25 dB with a
silicate-based amplifier fiber, the number of channels has to be
reduced from 120 to 80 channels. At the same time, with the same
number of channels the noise for the glass fiber according to the
invention is significantly lower than the noise for a
silicate-based fiber. The same picture emerges even with a further
increase to 180 channels (FIG. 8b): the fiber according to the
invention has a higher maximum gain with a lower noise. These FIGS.
8a and 8b show that broader-band transmission at low noise is
possible with the HMO glass fiber according to the invention.
[0038] The glass fiber according to the invention, in addition to
the core, also comprises at least two glass claddings which
surround the core.
[0039] The cladding glasses are not subject to any particular
restriction. They preferably have similar physical properties to
the matrix glass of the core and/or the glass of the other
claddings, in particular a similar refractive index, a similar Tg
and a similar softening point. It is preferable for the claddings
to comprise substantially the same composition as the core, but
with the compositions being modified in such a way that the
required shifts in refractive index from the core to the first
cladding and, if appropriate, from one cladding to a further
cladding are fulfilled. Furthermore, the optical properties of the
core and cladding glasses preferably differ. It is also preferable
for the various cladding glasses to have different optical
properties.
[0040] According to the invention, the term "first cladding" is to
be understood as meaning the cladding which surrounds the core. The
claddings are numbered in ascending order from the first cladding
outward.
[0041] According to the invention, the refractive indices mentioned
are in each case the refractive indices of the glasses for
electromagnetic radiation in the near IR region, in particular at
approximately 1300 nm. The change in refractive index .DELTA.n from
the core to the first cladding is from 0.001 to 0.08, particularly
preferably from 0.003 to 0.04, even more preferably from 0.005 to
0.05, with the first cladding having a lower refractive index than
the core. The ratio of the refractive index of the various
claddings with respect to one another can be set as required using
methods which are known from the prior art. To set a refractive
index which is slightly higher than in the comparative glass, for
example, a proportion of at least one component with a lower
refractive index is swapped for at least one component with a
higher refractive index.
[0042] According to a first embodiment, the refractive index
n.sub.m2 of the second cladding is substantially equal to or
preferably higher than the refractive index n.sub.m1 of the first
cladding. According to other embodiments, however, it is also
possible for the refractive index of the second cladding to be
lower than that of the first cladding and for a third cladding,
which has a higher refractive index than the second cladding, to be
added. Particularly preferred embodiments will be dealt with in
more detail below.
[0043] According to a first embodiment, the glass of the claddings
also does not contain any rare earth doping, in particular any
doping with optically active rare earth compounds. According to
this embodiment, the amplification and guidance of the light
mode(s) preferably take place in the core.
[0044] According to another embodiment, however, the glass of the
first cladding contains small quantities of the rare earth
compound(s) used as doping in the core. It is preferable for the
first cladding to be doped with up to half the amount, particularly
preferably up to a third of the amount, used in the core.
Surprisingly, it has emerged that this measure makes it possible to
improve the signal/noise ratio of an amplifier fiber and that in
this way it is also possible to improve the coupling of the
amplifier fibers to SiO.sub.2 fibers. It is assumed that with large
core radii, a more effective overlap between the signal mode and
the pump mode is effected with the rare earth ions in the cladding
as well.
[0045] According to a preferred embodiment of the present
invention, the glass of at least one cladding, in particular of the
outermost cladding, contains at least one absorbent component or an
absorbent material. Absorbent components of this type which may be
used include transition metal compounds, for example compounds of
iron (in particular Fe.sup.2+ and Fe.sup.3+), nickel (in particular
Ni.sup.2+), cobalt (in particular Co.sup.2+), manganese (in
particular Mn.sup.2+), copper (in particular Cu.sup.+ and
Cu.sup.2+), vanadium (in particular V.sup.3+ and V.sup.4+),
titanium (in particular Ti.sup.3+) and/or chromium (in particular
Cr.sup.3+), and/or rare earth compounds. By way of example, the
doping with Fe.sup.2+ may amount to several 100 ppm (based on the
weight ratio). The composition of the second cladding may otherwise
correspond to that of the core glass.
[0046] The level of absorbent material to be added depends on the
absorption of the absorbent material. Levels of 5 ppm, preferably
10 ppm, may even be sufficient, for example in the case of
Co.sup.2+. It is preferable for the amount added to be at most 5000
ppm, more preferably 2000 ppm, most preferably at most 1000 ppm. If
greater quantities of absorbent material are added to the glass
composition, the properties of the glass, such as the
crystallization properties, may be adversely affected. This is
therefore not preferred.
[0047] It has been established that with certain glass compositions
iron oxides are unsuitable absorbent materials. It has been found
that in particular bismuth oxide in the molten state may be reduced
to form elemental bismuth, which leads to the precipitation of
black metallic Bi and therefore to a deterioration in the optical
properties of the glass. Therefore, glasses which contain
polyvalent heavy metal oxides, such as bismuth oxide, are
preferably melted under strongly oxidizing conditions. If the
glasses according to the invention are used as optical amplifiers
for the 1.5 .mu.m band, known as the C band, their absorption band
in the near infrared region could allow Fe.sup.2+ ions to serve as
suitable absorbers. However, experiments have shown that 99% of the
Fe.sup.2+ ions added were oxidized to form Fe.sup.3+ ions by the
oxidizing melting conditions. Since the absorption band of
Fe.sup.3+ is not in the required range, iron oxide cannot act as
absorbent material in glasses produced in this manner.
[0048] It has been found that Co.sup.2+ ions, which likewise have a
suitable absorption in the near infrared region, are surprisingly
not converted into a higher oxidation state even by relatively
strongly oxidizing conditions in the melt and are therefore
particularly suitable for use as absorbent material in glass of
this type. Therefore, it is preferable for the outermost cladding
to contain at least one preferably oxidic divalent cobalt compound
as absorbent material.
[0049] FIG. 6 compares the transmission spectrum of a bismuth oxide
glass containing iron oxide with that of a Co.sup.2+-containing
glass. Although iron has been added in the form of divalent iron
(added in a quantity of 1000 ppm) to the starting batch, the
transmission of the glass in the region of 1500 nm is scarcely
adversely affected. The absorbent action is therefore low. By
contrast, the transmission of a glass which contains just 250 ppm
of Co.sup.2+ in oxidic form has dropped to less than 50% in
particular in the region of 1500 nm. Therefore, cobalt oxide has an
excellent absorbent action compared to iron oxide in these
glasses.
[0050] FIGS. 9a and 9b show the energy transmitted in each case in
the core 40 and the claddings 42 and 44 for two types of glass
fibers according to the invention. FIG. 9a shows the energy
transmitted in a fiber according to the invention whose outer
cladding 44 is doped with iron as oxidizing material. The various
curves 30 to 36 correspond to different fiber lengths. FIG. 9a
shows that with longer fiber lengths the energy transmitted in the
second cladding 44 decreases in relation to the energy transmitted
in the core 40 and first cladding 42. FIG. 9b shows the
corresponding energy transmission as a function of the radius of a
glass fiber whose outer cladding 44 is doped with cobalt. The
absorption effect of the second cladding is significantly less
effective in this case. Scarcely any energy is transmitted in the
outer cladding. The absorption effect is in this case independent
of the fiber length.
[0051] FIGS. 3 and 4 show two particularly preferred designs of a
glass fiber according to the invention in schematic form. These
figures diagrammatically depict the refractive index as a function
of the radius of the glass fiber.
[0052] According to a preferred embodiment of the present
invention, the core of the glass fiber according to the invention
is surrounded by precisely two glass claddings.
[0053] FIG. 1 shows a sectional view through a preferred embodiment
of the glass fiber 1 according to the invention. The core 2 is
surrounded by an inner cladding 3, which is in turn surrounded by
an outer cladding 4. According to this embodiment, the outer
cladding also contains an absorbent material as described
above.
[0054] FIG. 3 shows a particularly preferred design of the
refractive indices of a double-clad fiber. The region 11 is the
core of the fiber, which is generally located approximately in the
center of the fiber and is doped with at least one rare earth
compound, the region 12 is the inner cladding and has a lower
refractive index than the core region 11, so that it is ensured
that the light propagating in the region of the core is guided. The
region 13 represents the second and in this case outer cladding,
which is primarily intended to absorb scattered light. As shown
here, the refractive index of the second cladding may be higher
than the refractive index of the core, but it is also possible for
the second cladding to have the same refractive index as the core
or a lower refractive index than the core. In general, an outermost
cladding of this type has a higher refractive index than the inner
cladding which adjoins it.
[0055] According to a further embodiment of the present invention,
the core of the glass fiber according to the invention is
surrounded by precisely three glass claddings.
[0056] FIG. 4 shows a particularly preferred design of a glass
fiber according to the invention with three glass claddings. The
region 21 represents the core of the fiber, which is generally
located in the center of the glass fiber, is doped with, for
example, Er.sup.3+ and guides the signal mode. The inner cladding
22 may be doped with Yb.sup.3+. Doping of the first cladding with,
for example, Yb.sup.3+ in this way allows the fiber to be used for
what is known as multimode pumping. Whereas in the case of
single-mode pumping light is radiated only into the core region of
the amplifier fiber, and only very small lasers, which are
therefore very expensive, can be used for this purpose, in the case
of multimode pumping, light is radiated into the wider
cross-sectional region of core and, in addition, the first
cladding. This radiation of light causes Yb.sup.3+ to be excited at
approx. 975 nm (.sup.2F.sub.7/2.fwdarw..sup.2F.sub.5/2). Since
Yb.sup.3+ is fluorescent at a similar wavelength, this fluorescence
pumps the .sup.4I.sub.11/12 level of the Er.sup.3+ ion at approx.
980 nm. The light sources which can be used for multimode pumping
are significantly less expensive. The region of the second cladding
23, which has a lower refractive index than the first cladding,
adjoining the first cladding 22 is responsible for guiding the
light which propagates in the region of the first cladding 22, and
the region of the third cladding 24 in turn serves as an outer
absorbent cladding.
[0057] The glass fiber according to the invention is preferably
substantially circular in cross section. However, the present
invention also encompasses glass fibers which have a cross section
which differs from a circular cross section.
[0058] The core of the glass fiber according to the invention
generally lies in the center of the glass fiber according to the
invention, with the claddings preferably arranged coaxially around
the core. However, the present invention also encompasses
embodiments in which the core does not lie in the center of the
glass fiber.
[0059] Furthermore, it is preferable for the glass fiber according
to the invention to comprise precisely one core. However, according
to other embodiments it is also possible for the glass fiber
according to the invention to include a plurality of core
fibers.
[0060] The glass fiber according to the invention preferably has an
overall thickness of 100 to 400 .mu.m, more preferably 100 to 200
.mu.m. An overall thickness of approximately 125 .mu.m is
particularly preferred.
[0061] For use as an optical amplifier fiber, the core of the glass
fiber according to the invention preferably has a diameter of from
1 to 15 .mu.m. The thickness d.sub.m1 of the first cladding is
preferably in the range from 5 to 100 .mu.m. The thickness d.sub.m2
of the second and further claddings is preferably in the range from
10 to 150 .mu.m. However, for other applications it is also
possible for the core and/or claddings to be of any other desired
thickness.
[0062] According to the invention, the term "core of a glass fiber"
is to be understood as meaning the region which has been produced
by the glass technology process and thereby differs from the
cladding. By contrast, a "core region" encompasses the region in
which the intensity of the optical signal has dropped to the
e.sup.th part of the input intensity.
[0063] According to a further embodiment of the present invention,
the glass fiber according to the invention comprises, on the
outermost glass cladding, at least one coating, which comprises at
least one plastic or polymer. This outer plastic coating is used in
particular to mechanically protect the glass fiber. The thickness
of this plastic coating is preferably from 2 to 400 .mu.m. A
coating thickness of less than 2 .mu.m cannot generally provide
sufficient protection to the glass fiber. It is particularly
preferable for the thickness to be at least 3 .mu.m, more
preferably at least 8 .mu.m. With thicknesses of over 400 .mu.m, it
becomes difficult to provide a uniform coating. The thickness is
particularly preferably at most 70 .mu.m.
[0064] Any type of polymer can be used for a plastic coating of
this type, so long as it bonds securely to the cladding glass.
Examples of plastics of this type include heat-curable silicone
resins, UV-curable silicone resins, acrylic resins, epoxy resins,
polyurethane resins and polyimide resins, as well as mixtures
and/or blends thereof. Furthermore, the present invention relates
to a process for producing the glass fiber according to the
invention, in which at least two cladding glasses are formed around
a core glass. This can be produced by-production processes such as
for example a "rod-in-tube" process, a multiple crucible process
and extrusion processes, as well as combinations of these
processes.
[0065] According to one embodiment, first of all a "preform"
comprising core and one or more claddings, is produced, this
preform already having the layer structure of the subsequent glass
fiber; it can be drawn out to form a glass fiber. The thickness of
a preform of this type is, for example, from 4 to 30 mm, and its
length is from 5 to 40 cm. This preform is drawn out to form a
fiber at a suitable temperature.
[0066] In the case of a "rod-in-tube" process, a hole is drilled
into a cladding glass which is in the form of a strand or rod, so
that a tubular cladding glass is obtained. A matching rod of the
core glass is introduced into this tubular cladding glass.
Furthermore, the cladding glass can also be drawn out as a tube by
means of suitable shaping processes. By way of example, a rod of a
core glass with a diameter of from 1.0 to 1.4 mm is introduced into
a tubular first cladding with a diameter of the internal hole of
1.5 mm and an external diameter of 7 mm. To obtain a core
surrounded with more than one cladding, it is possible for this
method to be repeated a number of times, i.e. for a second cladding
a hole is drilled into a second cladding glass in rod form, and the
preform comprising core and first cladding is introduced into the
tubular second cladding. To join the interfaces, this arrangement
of core and claddings is heated, preferably to above the
transformation temperature, in order to obtain a "preform". If
appropriate, a preform comprising core and at least a first
cladding, after it has been heated in this manner, can be drawn out
to a certain extent and introduced in this drawn-out form, as a
rod, into a second or further cladding. In the rod-in-tube process,
it is also possible for a hot-formed, drawn-out rod to be fitted
into a hot-formed, drawn tube.
[0067] Furthermore, a preform of this type can also be produced by
what is known as an extrusion process. In this case, a block of the
core glass is placed onto a block of a cladding glass and is then
heated linearly from below. Along the heated line, the core glass
slowly sinks into the cladding glass until it is completely
surrounded by the latter.
[0068] In the case of a multiple crucible process, such as a double
or triple crucible process, a "preform" comprising a core or one or
more claddings is produced directly from the melt using nested
crucibles.
[0069] According to a further embodiment of the process according
to the invention, it is also possible for a glass fiber with a
diameter of, for example, 125 .mu.m to be produced directly, i.e.
without prior production of a preform. Triple of multiple crucible
processes are used in particular for direct fiber production.
[0070] These processes for producing a preform can be combined with
one another in order to obtain the glass fibers according to the
invention with at least two claddings.
[0071] According to the present invention, it is particularly
preferred for a double crucible process to be used to produce a
"preform" comprising the core and the first cladding, and for the
preform obtained in this way, comprising core and one cladding, to
be introduced as a rod into a tubular second cladding using a
"rod-in-tube" process. It has emerged that this combination on the
one hand makes it possible to obtain a particularly good interface
between core and first cladding, and on the other hand allows a
second and/or further cladding to be added in an economic way.
[0072] Furthermore, the present invention relates to an optical
amplifier which comprises at least one glass fiber according to the
invention. By way of example, the optical amplifier has the
following structure. The incoming light signal is connected to a
coupler via an optical insulator for suppressing light reflections.
Signal and pumping light are combined in the coupler and are
together introduced into the optically active fiber. The other end
of the amplifier fiber is connected to the outgoing fiber. It is
also possible for a filter, if appropriate with a further optical
insulator, to be arranged here. Furthermore, it is possible for the
amplifier fibers to be pumped in both directions, in which case a
second coupler is required.
[0073] The signal light source is connected at the wave-mixing
optical coupler through the optical insulator. Furthermore, the
optical coupler is connected to the excitation light source. Then,
the optical coupler is connected to an end of the glass fiber. The
other end of the optical glass fiber is connected to the optical
insulator through the optical coupler for wave splitting. Each part
is connected to the optical fiber.
[0074] Furthermore, the present invention comprises the use of the
glass fiber according to the invention as optically active glass in
a laser arrangement.
[0075] The present invention is explained in more detail below by
means of examples. However, it is not restricted to these
examples.
EXAMPLES
Example 1
[0076] Glass compositions were produced for the core, the first
cladding and the second cladding. Table I shows the compositions of
the glasses in mol %.
[0077] The core glass which had been drawn out into a strand
(length 10 cm, diameter 1 mm) was sheathed with the first cladding
(external diameter 7 mm; internal hole diameter 1.5 mm) by means of
the rod-in-tube process. The preform comprising core and first
cladding was then drawn out to a diameter of 1 mm and sheathed with
the outer cladding (external diameter 7 mm; internal hole diameter
1.5 mm) by means of a further rod-in-tube step.
TABLE-US-00002 TABLE I COMPOSITIONS OF CORE GLASS AND CLADDINGS
First Second Core cladding cladding SiO.sub.2 14.3 14.3 14.4
B.sub.2O.sub.3 28.5 28.5 21.4 Bi.sub.2O.sub.3 42.4 42.8 50.0
Al.sub.2O.sub.3 7.2 10.7 14.1 Ga.sub.2O.sub.3 7.2 3.7 --
Er.sub.2O.sub.3 0.4 -- -- Fe.sub.2O.sub.3 -- -- 0.1
n.sub.1300.sup.1 1.9931 1.984 2.047 Note: .sup.1Refractive index at
the wavelength of 1300 nm, measured using total reflection method
on plane-parallel plates of 5 mm.
[0078] The preform obtained was drawn out to form a glass fiber
with a thickness of 125 .mu.m.
[0079] FIG. 2 shows a photographic image of a cross section through
a glass fiber according to the invention. Core 2 is surrounded by
the first cladding 3, which is in turn surrounded by the outer
cladding 4.
Example 2
[0080] The same compositions as in example 1 were used to produce a
double-clad fiber. In this case, the core was sheathed with the
first cladding by means of a double crucible process. The core
diameter and the dimensions of the first cladding in this case
corresponded to those of example 1. Then, the preform obtained in
this way, comprising core and first cladding, was drawn out to a
thickness of 1.5 mm. Then, the second cladding was formed around
the drawn-out preform comprising core and first cladding by means
of the rod-in-tube process.
[0081] The preform obtained was drawn out to form a glass fiber
with a thickness of 125 .mu.m.
[0082] Optical examination revealed that in example 2 a better
interface was obtained between core and first cladding. FIG. 7
shows a photographic image of the cross section through a fiber
obtained in accordance with Example 2.
Example 3
[0083] The process described in example 1 was used to produce a
double-clad fiber with core and cladding glasses based on tellurium
oxide.
[0084] The preform obtained was drawn out to form a glass fiber
with a thickness 4 of 325 .mu.m and a core diameter of 4.5
.mu.m.
[0085] FIG. 5 shows a cross section through the Te double-clad
fiber produced. In this case, the cross section has been etched, so
that the transitions from core to first cladding or second cladding
are more clearly shown.
Example 4
[0086] The glass compositions shown in Table II were used to
produce a double-clad fiber. In this case, first of all a preform
comprising core and first cladding was produced using a double
crucible. Then, this preform was provided with the second cladding
by means of the rod-in-tube process. Next, the preform obtained was
drawn out to form a glass fiber with a diameter of 125 .mu.m.
TABLE-US-00003 TABLE II COMPOSITIONS OF CORE GLASS AND CLADDINGS
First Second Core cladding cladding SiO.sub.2 [mol %] 14.3 18.3
17.1 B.sub.2O.sub.3 [mol %] 28.5 26.5 26.5 Bi.sub.2O.sub.3 [mol %]
42.6 41.0 42.0 Al.sub.2O.sub.3 [mol %] 10.6 10.6 10.6 GeO.sub.2
[mol %] 3.6 3.6 3.6 Er.sub.2O.sub.3 [mol %] 0.4 -- -- CoO [mol %]
-- -- 0.2 n.sub.1300.sup.1 1.982 1.964 1.978 Note: .sup.1Refractive
index at the wavelength of 1300 nm, measured using total reflection
method on plane-parallel plates of 5 mm.
Example 5
[0087] The glass compositions shown in Table III were used to
produce a double-clad fiber. In this case, first of all a preform
comprising core and first cladding was produced using a double
crucible. Then, this preform was provided with the second cladding
by means of the rod-in-tube process. Next, the preform obtained was
drawn out to form a glass fiber with a diameter of 125 .mu.m.
TABLE-US-00004 TABLE III COMPOSITIONS OF CORE GLASS AND CLADDINGS
First Second Core cladding cladding Bi.sub.2O.sub.3 [mol %] 40.5
39.3 40.1 B.sub.2O.sub.3 [mol %] 30.5 28.5 28.4 SiO.sub.2 [mol %]
10.3 13.9 13.0 GeO.sub.2 [mol %] 7.6 7.6 7.6 Al.sub.2O.sub.3 [mol
%] 10.6 10.6 10.6 La.sub.2O.sub.3 [mol %] 0.1 0.1 0.1
Er.sub.2O.sub.3 [mol %] 0.4 -- -- CoO [mol %] -- -- 0.2
n.sub.1300.sup.1 1.973 1.959 1.969 Note: .sup.1Refractive index at
the wavelength of 1300 nm, measured using total reflection method
on plane-parallel plates of 5 mm.
* * * * *