![[US Patent & Trademark Office, Patent Full Text and Image Database]](/netaicon/PTO/patfthdr.gif)
| United States Patent |
6,037,285
|
|
Jha
, et al.
|
March 14, 2000
|
Infrared transmitting optical fiber materials
Abstract
An optical fiber amplifier is formed from glass doped with praseodymium.
The glass may include one or more of cadmium mixed halide, hafnium
halides, geranium and silicon disulphide based vitreous materials or
fluorozirconate glass fibers. It is possible to provide an optical fiber
amplifier which operates at a 1300 nm window for passive optical networks
| Inventors:
|
Jha; Animesh (Uxbridge, GB);
Jordery; Sophie (Tregunc, FR)
|
| Assignee:
|
BTG International Limited (London, GB)
|
| Appl. No.:
|
025106 |
| Filed:
|
February 17, 1998 |
| Current U.S. Class: |
501/37; 65/385; 65/397; 428/373; 428/378; 428/379; 501/35; 501/40 |
| Intern'l Class: |
C03C 013/00; C03C 013/04; C03C 003/32 |
| Field of Search: |
501/37,40,35
65/397,385,426,430,432,431,444
428/373,375,378,379
|
References Cited [Referenced By]
U.S. Patent Documents
| 4647545 | Mar., 1987 | Lucas et al. | 501/40.
|
| 5185847 | Feb., 1993 | Fevrier et al. | 385/141.
|
| 5278107 | Jan., 1994 | Tick et al. | 501/40.
|
| 5338607 | Aug., 1994 | Kawamoto et al. | 428/373.
|
| 5346865 | Sep., 1994 | Aitken et al. | 501/40.
|
| 5631194 | May., 1997 | Akella et al. | 501/37.
|
| Foreign Patent Documents |
| 0470612 A1 | Feb., 1992 | EP.
| |
| 2286390 | Aug., 1995 | GB.
| |
Other References
Chemical Abstracts, vol. 99, No. 10, Columbus, Ohio, U.S., abstract No.
75526d, p. 283; M. Matecki et al., & J. Noncryst. Solids, vol. 56, No.
1-3, 81-86, 1983. (no month).
Patent Abstracts, vol. 14, No. 270 (C-727); & JP, A, 02 080 349 (Hoya
Corp.) Mar. 1990.
Derwent WPI Abstract 93-080176110 & JP, A, 05 024 883 Feb. 1993.
Derwent WPI Abstract 93-121096115 & JP 050058674 A Mar. 1993.
Derwent WPI Abstract 93-031403104 & JP 040358131A Dec. 1992.
Derwent WPI Abstract 93-022289103 & JP 040349151A Dec. 1992.
Derwent WPI Abstract 92-327387140 & JP 040234021 Aug. 1992.
Hu et al., "preparation of a Nd doped Fluorozirconate glass laser fiber",
Journal of Non-Crystalline Solids, vol. 184 pp. 218-221, May 1995.
Bartholomew et al., "Praseodymium doped cadmium mixed halide glasses for
1,3, micron amplification", Journal of Non-Crystalline solids, vol. 184,
pp. 229-235, May 1995.
|
Primary Examiner: Marcheschi; Michael
Attorney, Agent or Firm: Pillsbury Madison and Sutro LLP
Parent Case Text
This application is a continuation of PCT/GB95/01923 filed Aug. 15, 1995.
Claims
We claim:
1. A method of producing a glass fiber comprising:
drying and fluorinating cadmium fluoride or gallium indium fluoride to form
a dried fluorinated material;
melting the dried fluorinated material to form a melted material; and
casting the melted material to form a cast preform.
2. The method of claim 1, said method further comprising adding a
praseodymium dopant during said melting of the dried material.
3. The method of claim 1, wherein said drying of the cadmium fluoride or
gallium indium fluoride is performed in an atmosphere comprising a gas
mixture of N.sub.2, SF.sub.6, and HF.
4. The method of claim 1, wherein said melting of the dried material is
performed in a gas mixture comprising HF/HCl.
5. The method of claim 1, wherein said melting of the dried material is
performed in a reduced pressure atmosphere.
6. The method of claim 1, further comprising annealing the cast preform in
a controlled atmosphere furnace.
7. The method of claim 1, further comprising drawing the cast preform into
a glass fiber in a 1000 to 2000 ppm mixture of HCl and HF with nitrogen
gas.
8. The method of claim 7, wherein said drawing of the cast preform is
conducted in a surrounding case.
9. The method of claim 7, further comprising coating the glass fiber with
at least one member selected from the group consisting of a polymer and a
metal.
10. An optical fiber comprising:
a core comprising about 50 mol % CdF.sub.2, about 10 mol % BaX.sub.2 and
about 40 mol % of a combined amount of NaX and KBr; and
a cladding layer comprising about 50 mol % CdF.sub.2, about 10 mol %
BaX.sub.2, about 40 mol % of a combined amount of NaX and NaPO.sub.3,
wherein X is selected from the group consisting of F, Cl, Br and I.
11. The optical fiber of claim 10, wherein in said cladding layer NaX is
present in an amount between about 10% and about 38 mol % and NaPO.sub.3
is present in an amount between about 2 and about 30 mol %.
12. An optical fiber amplifier comprising an optical fiber of claim 10.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to infrared transmitting optical fiber
materials, to a method of producing an optical fiber and to an optical
fiber amplifier.
2. Description of the Related Art
The development of silica glass single-mode optical fibers has led to the
possibility of broad-band communication at second and third transmission
windows situated at 1.3 .mu.m and 1.5 .mu.m respectively. In 1985, an
erbium-doped silica optical fiber amplifier, known as EDFA, was developed
for the third transmission window, with almost 97% quantum efficiency and
a large signal gain of 50 dB. The 1.5 .mu.m optical fiber amplifiers are
planned for use in the transoceanic submarine cable networks. EDFA will
play a key role in the high-speed data transmission networks.
However, globally the terrestrial networks utilize 1.3 .mu.m window and
currently electronic repeaters are used at the signal wavelength. The
electronic repeaters, used in the networks, are prohibitively high cost
items. Also, the electronic repeaters inherently introduce incompatibility
between optical and electronic components at high bit rate transmission
(>2.5 Gbit/sec). Hence there is a need for providing distortion-free
amplification without converting the optical signals into an array of
electrical pulses and vice-a-versa. Optical fiber amplifiers operating at
1300 nm window for passive optical networks (PONs) are therefore required.
SUMMARY OF THE INVENTION
According to an aspect of the invention, there is provided an optical fiber
comprising glass including one or more of cadmium mixed halide, hafnium
halides, and gallium-indium fluoride based vitreous materials or
fluorozirconate glass fibers.
In embodiments in which the glass is doped with praseodymium, the
rare-earth ions used as dopants are suitable for fluorescence in the 1300
nm wavelength-domain.
According to another aspect of the invention, there is provided a method of
producing a glass fiber comprising the steps of providing starting
compounds including cadmium fluoride and other halide ingredients; drying
and fluorinating the cadmium fluoride material and drying and halogenating
the halide ingredients; melting the dried material; and casting the melted
material.
According to another aspect of the present invention, there is provided an
optical fiber amplifier including an optical fiber as herein specified.
DETAILED DESCRIPTION OF THE INVENTION
A critical factor in designing an efficient 1.3 .mu.m fiber amplifier based
on Pr.sup.+3 as the dopant ion is the selection of suitable glass hosts
that would permit an efficient quantum yield of light at signal
wavelength, i.e. hosts of adequate purity. We have developed a number of
glass systems that are potentially suitable for fiber drawing trials and
hence could be useful for fabricating fiber lasers and amplifiers.
These glasses with Pr-ions as the dopant are primarily cadmium mixed
halide, hafnium halides and gallium-indium fluoride based vitreous
materials and their derivatives and modifications. These are all
potentially low phonon energy glasses which can be cast into 10-15 mm
diameter rods of 10-15 cm in length for fiber drawing. All the above
glasses also have excellent infrared transmission properties for other
applications such as in sensors, detectors and medical devices.
As specified above, the land-based telecommunicaticn network will utilize
1300 nm optical fiber amplifiers. The first possible category of 1300 nm
optical fiber amplifiers is based on the Pr-doped glasses which has been
developed by HP & BT&D. Currently fluorozirconate glass (ZBLAN) fibers are
being developed for 1300 nm Pr-doped optical fiber amplifiers and the
measured quantum efficiency is around 4 percent. The low quantum
efficiency of ZBLAN fibers is limited due to large non-radiative decay
process which determines the metastable lifetimes of .sup.1 G.sub.4 level
to .sup.3 F.sub.4 level in Pr-doped glasses. The non-radiative lifetime is
dependent on the phonon energy (580 cm.sup.-1) of the host glass, which is
significantly lower than the silica (1200 cm.sup.-1) glass, and permits
the depletion of the pump energy (1010 nm) via multiphonon relaxation
process. The larger phonon energy of the glass host increases the
probability of the non-radiative decay process because the number of
phonons required to provide relaxation of Pr-ions from metastable .sup.1
G.sub.4 level is small, and hence the process becomes energetically more
favorable than the glass hosts having phonon energies lower than 580
cm.sup.-1.
The number of phonon (p) involved in the non-radiative relaxation process
can be determined by the energy gap (.increment.E in cm.sup.-1) and the
phonon energy (hw) relationship: p=.increment.E/hw. The relationship
clearly explains the significance of hw of glass hosts for Pr-ions as
dopants. Here .increment.E is the energy gap between the .sup.1 G.sub.4
and .sup.3 F.sub.4 levels.
Cadmium fluoride glasses have been made suitable for 1300 nm optical fiber
amplifier application. In the bulk glass fabrication, the impurities must
be controlled in order to eliminate the contribution of high phonon energy
relaxation paths. The control of impurities and method of dopant addition
has been systematically studied. The glass compositions melted with
impurities dissolved are unsuitable for fiber fabrication, although the
measured lifetimes are more than two times longer than the ZBLAN
compositions. Table 1 summarizes the effect of impurities on the measured
fluorescence lifetimes from .sup.1 G.sub.4 level of Pr-ions in cadmium
mixed halide glasses.
TABLE 1
______________________________________
Relationship between the fluorescence lifetime
and impurities in mixed halide glasses.
Lifetime
.mu.sec Lifetime
Glass composition Impure .mu.sec
mole percent glass Purified
______________________________________
Clad glass 180-210 290-330
CdF.sub.2 - 50, BaX.sub.2 - 10, NaX = 40,
X = F, CI, Br, I.
Core glass 180-210 290-330
CdF.sub.2 - 50, BaX.sub.2 - 10, NaX = 40 - a, KBr = a,
X = F, Cl, Br, I.
______________________________________
The predicted fluorescence lifetimes in these glasses are expected to be of
the order of 500 .mu.sec. The shorter measured lifetimes in impure and
relatively purified materials are due to the presence of high phonon
energy impurities (>600 cm.sup.-1) in the glass which appear to cluster
around Pr-ions and provide fast non-radiative relaxation paths.
The preparation for bulk glass fabrication involves the following steps
which differ for each type of halide glass. In particular, the chemical
treatment of raw material halide powders such as cadmium fluoride and
zinc, gallium and indium fluorides, which are the major constituents of
cadmium mixed halide and gallium-indium fluoride glasses respectively,
enhances the glass-forming tendency and the optical quality of the halide
glasses. The optical, thermal and spectroscopic properties of these
glasses after chemical treatment improve significantly in favor of
realizing an optical fiber device. The metastable lifetimes of .sup.1
G.sub.4 level in these glasses are strongly dependent on the overall
impurity concentrations present in the glass.
a) Extensive drying and fluorination of cadmium, gallium, indium and zinc
fluorides, all of which contaminated by high-phonon energy
oxygen-associated impurities, must be carried out prior to melting the
charge for making glass. These oxygen-containing impurities are typically
oxides, oxyfluorides, nitrates, carbonates and sulphates. The minimum
concentration of the fluorinating gas such as HF is determined by the
chemical equilibrium.
b) Other halides such as sodium and barium chlorides should be dried under
the atmosphere of either chlorine or HCl gas. The minimum concentration of
the chlorinating gas e.g. HCl is determined by the chemical equilibrium.
c) CdF.sub.2 is dried under a controlled atmosphere of N.sub.2, SF.sub.6
and HF gas mixture. The residual proportions of SF.sub.6 and HF in the
carrier nitrogen gas vary with the relative stability of oxide to be
removed from the starting material. For example, the equilibrium
concentrations of HF and SF.sub.6 required in the nitrogen carrier gas is
less for CdO than for either ZnO or GaOF and Ga.sub.2 O.sub.3. Related
chemical reactions also participate and assist the removal of adsorbed
moisture. These impurity removal reactions, for example in cadmium
fluoride, are described below. Reactions (i) and (ii) describe the
oxide-to-fluoride conversion reactions whereas (iii) and (iv) are the
moisture removal reactions.
(i) 2 CdO+SF.sub.6 (g)=2 CdF.sub.2 +SO.sub.2 (g)
(ii) CdO+HF(g)=CdF.sub.2 +H.sub.2 O(g)
(iii) 2[CdF.OH]+SF.sub.6 =2 CdF.sub.2 +2HF(g)+SO.sub.2 (g)+F.sub.2 (g)
(iv) CdF.OH+HF=CdF.sub.2 +H.sub.2 O (g)
The component halides for a particular type of glass e.g. cadmium mixed
halide should be similarly dried. After drying, the charge for cadmium
mixed halide should be melted under HF/HCl gas mixture diluted with
N.sub.2 gas. This gas mixture should be maintained throughout the entire
duration of the melting procedure and its composition is determined from
the thermodynamic equilibrium between CdF.sub.2 and HCl gas designated by
CdF.sub.2 +2HCl=CdCl.sub.2 +2HF. The concentration of HCl gas should not
exceed the thermodynamically prescribed value.
d) The glass melting procedure and the time for dopant addition in cadmium
mixed halide glasses were carefully selected. The short prescribed melting
time curtails the oxygen and hydroxyl ion pick-up. The addition of Pr-ions
in the halide melt was administered at the homogenization temperature
maintained at 775.degree. C. for the cadmium mixed halide glasses. After
the incorporation of Pr-ions, the melt was further homogenized for an
additional 15 minutes, the total time at 775.degree. C. being 1/2 hour.
The melting and homogenization of cadmium mixed halide glass above
800.degree. C. was found to be disadvantageous for the quality of the cast
glass. A higher melting and homogenization temperature often led to rapid
volatilization of cadmium salts as vapors from the melt.
e) Treatment of halide melts in a reduced pressure atmosphere is prescribed
for a rapid removal of the gas bubbles from the melt prior to casting
glass. The reduced pressure treatment of melt should not exceed longer
than 2-3 minutes during which the chamber pressure should not be less than
0.90 atm. The partial removal of gas bubbles can also be achieved by
stirring the melt with platinum wire.
f) Adopting the well-known rotational casting technique assists the removal
of gas bubbles from the cast core-clad glass preforms.
g) Annealing of cast preforms should be carried out inside a controlled
atmosphere furnace for relieving thermal stresses and for minimizing
surface contaminants e.g. moisture.
h) The use of 1000-2000 ppm mixture of HCl and HF with nitrogen gas is
recommended for fiber drawing This has been established from
thermodynamics calculations which was also verified by carrying out
complementary experiments.
i) Drawing fibers with an external case around drawing furnace to prevent
water attack on glass is also recommended for loss-less fiber fabrication.
j) The hermetic coating of cadmium mixed halide glass with a suitable
metallic film is recommended for improving the environmental sensitivity
of this glass. The best candidate for this purpose appears to be tin and
aluminum, both of which can be deposited by evaporation technique.
k) The core and clad gallium-indium fluoride glasses for preform
fabrication (see Table 2) were fabricated by adopting the procedure
described above for cadmium mixed halide glass. In particular, the
exclusion of reactive atmosphere drying and purification from the main
stream of glass preparation route was found to be unsuitable for making
high-quality glass samples. The glass preparation without reactive
atmosphere processing (RAP) often result into extensive crystallization on
cooling. The impurity level and optical quality of the cast glass was
therefore very much dependent upon the concentrations of SF.sub.6 and HF
gases in the nitrogen gas.
The overall drying and purification in RAP was very sensitive to
temperature and time. At higher temperatures than 650.degree. C. for RAP,
gallium fluoride glasses of sufficiently high optical quality were
difficult to produce. The fluorination time was usually longer than 12
hours. The purification process had a major influence on the
devitrification tendency of the gallium fluoride glass. These results are
summarized below in Table 2 by determining the value of T.sub.x -T.sub.g
gap which is a critical parameter for fiber drawing. The larger is the
value of this parameter, the more stable is the glass and resists
devitrification during fibre drawing.
A good quality glass preform usually yields better quality fibers. The
methods described above are recommended to achieve enhancement in the
fluorescence lifetimes.
It is also possible to manipulate the local phonon energy of Pr-ions in the
glass host by selecting the processing steps as well as chemistry of the
dopants.
For suppressing the moisture attack on core glass, a new form of cladding
glass with sodium phosphate (NaPO.sub.3) has been designed. In this glass
sodium chloride from standard halide composition, listed in Table 1, is
replaced by NaPO.sub.3. The range of substitution is between approximately
2 to 30 mole percent, with 10 mole percent being ideally suited for the
high refractive index core (1.615). The measured refractive indices of a
few phosphate compositions based on CdF.sub.2 : (50), BaX.sub.2 : (10) and
NaX: (40-y) are listed below where y is the mole percent of NaPO.sub.3.
TABLE 2
__________________________________________________________________________
Effect of the reactive atmosphere processing of GaF.sub.3 -based glasses
on their thermal characteristics.
Composition (mole percent)
Core glass: 22 GaF.sub.3, 13 InF.sub.3, 30 PbF.sub.2, 18 CdF.sub.2, 13
ZnF.sub.2, 2 GdF.sub.3, 1 NaF, 1 LiF,
Clad Glass: 25 GaF.sub.3, 13 InF.sub.3, 29 PbF.sub.2, 15 CdF.sub.2, 13
ZnF.sub.2, 2 GdF.sub.3, 3 NaF.
RAP Condition T.sub.g, .degree. C.
T.sub.g, .degree. C.
T.sub.x -T.sub.g, .degree. C.
Remarks
__________________________________________________________________________
a) Cast bulk clad glass treated in 2.5 vol % HF +
254 300 46 Clear glass rod with lots of gas
bubbles in
97.5 vol % N.sub.2 at 600.degree. C. for 20 hours. High
the centre along the axis. Some
microcrystals
content GaF.sub.3 powder (0 wt % = 0.4)
present. Multiple crystallization
peaks. T.sub.p - T.sub.x :
10.degree. C.
b) clad glass composition powder except SF.sub.6 gas
254 345 91 Clear glass. Gas bubbles present
along the
was used with N.sub.2 for 19 hours. Low oxygen content
axis of the glass rod. Two major
(0.07 wt % O) crystallization peaks,
T.sub.p - T.sub.x : 5.degree. C.
c) same composition as b) except NH.sub.4 HF.sub.2 was used
252 343 91 Along the axis of the cast rod,
crystals
at 350.degree. C. for 2 hours and slowly heated to melting
nucleated. Thermal analysis
performed on the
temperature of 800.degree. C. in 2 hours in N.sub.2 gas
clear glass on the periphery.
T.sub.p - T.sub.x : 13.degree. C.
d) same composition as in b). GaF.sub.3 and InF.sub.3
249 343 94 Clear glass rod with no bubbles and
crystals.
separately fluorinated in SF.sub.6 at 600.degree. C. for 19
T.sub.p - T.sub.x : 15.degree. C.
Prior to this, CdF.sub.2 and PbF.sub.2 fluorinated with HF at
375.degree. C. for 5 hours.
e) same composition as b) except fluorinated in
256 348 88 Clear glass and a few bubbles,
T.sub.p - T.sub.x : 19.degree. C.
SF.sub.6 + HF atmosphere at 600.degree. C. for 3 hours and
broad crystallisation peak
19 hours in SF.sub.6 at 600.degree. C.
f) HF + SF.sub.6 fluorination of clad powders at 400.degree. C.
250 333 83 Clear glass and no bubbles, T.sub.p
- T.sub.x = 4.degree. C.
for 19 hours.
g) same as e) plus zinc fluoride separately
254 354 100 Very clear glass, a few bubbles,
T.sub.p - T.sub.x : 11.degree. C.
fluorinated with SF.sub.6 + HF at 600.degree. C.
__________________________________________________________________________
All RAP treated powders were melted in the temperature range of
800.degree.-825.degree. C. and then cast in a preheated brass mould at
240.degree. C. All glasses were annealed for several hours at this
temperature.
______________________________________
y, refractive
Thermal Expansion
mol % index coefficient, .alpha./.degree.C.
______________________________________
5 1.5870 230 .times. 10.sup.-7
7 1.5861
10 1.5828
15 1.5804 225 .times. 10.sup.-7
______________________________________
The index and the coefficient of thermal expansion coefficient are matched
to yield the value of NA greater than 0.3 which is a requirement for
producing efficient Pr-doped fiber device for amplification.
The cladding glass of 7 and 10 mole percent containing phosphates have been
drawn into fibers of 65 .mu.m core diameter. The total measured loss in
these fibers varied between 10 dB/m and 7 dB/m at 700 nm and 1250 nm
respectively. For improving the durability of the phosphate fibers, it is
recommended that the fibers should be coated with either aluminum metal or
tin metal.
The addition of NaPO.sub.3 is carried out during the glovebox melting
stage.
Gallium-indium fluoride glass rods manufactured by employing RAP had an
enhanced stability against devitrification. The rods manufactured without
RAP yielded fibers with 40-60 dB/m loss. After RAP, the total loss was due
to surface defects such as uneven rod surface, solidification striations,
etc. The measured loss was 10-20 dB/m.
* * * * *
![[Image]](/netaicon/PTO/image.gif)
![[Home]](/netaicon/PTO/home.gif)
![[Boolean Search]](/netaicon/PTO/boolean.gif)
![[Manual Search]](/netaicon/PTO/manual.gif)
![[Number Search]](/netaicon/PTO/number.gif)
![[Help]](/netaicon/PTO/help.gif)
![[Bottom]](/netaicon/PTO/bottom.gif)
![[View Shopping Cart]](/netaicon/PTO/cart.gif)
![[Add to Shopping Cart]](/netaicon/PTO/order.gif)
![[Top]](/netaicon/PTO/top.gif)