Note: Descriptions are shown in the official language in which they were submitted.
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GLASS FOR HIGH AND FLAT GAIN 1.55~m OPTICAL AMPLIFIERS
TECHNICAL FIELD OF INVENTION
The present invention relates generally to the field of optical signal
amplifiers and in particular, to fluorophosphate glass compositions for use in
optical signal amplifiers operating at wavelengths around 1.55 Vim.
BACKGROUND OF THE INVENTION
Optical signal amplifiers have quickly found use in optical
telecommunication networks, particularly in those networks using optical fiber
over long distances. Although modern silica-based optical fibers general
exhibit
relatively low loss in the 1.55 ~m window, they are lossy to some extent and
the
loss accumulates over distance. To reduce this attenuation, opto-electronic
devices have been used to boost signal power. These devices require that the
optical signal be converted to an electronic signal. The electronic signal is
then
amplified using commonly known amplification techniques and is reconverted
back to an optical signal for re-transmission.
Optical signal amplifiers amplify optical signals without requiring an
opto-electronic conversion of the signal. In optical amplifiers, the weakened
fight
signal is directed through a section of an amplifying medium that has been
doped
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with ions from a rare earth element. Light from an external light source,
typically a
semi-conductor laser, is pumped into the amplifying medium stimulating the
rare
earth atoms to a higher energy level. Light entering the amplifying medium at
the
signal wavelength further stimulates the excited rare earth ions to emit their
excess photon energy as light at the signal wavelength in phase with the
signal
pulses, thereby amplifying the light signal. One type of optical amplifier
uses a
length of erbium-doped optical fiber. Erbium-doped fiber amplifiers (EDFA) are
usually doped on the order of 100-500 ppm of erbium ion. Typical EDFA fiber
lengths are on the order of 10-30 meters, depending on the final gain
requirements necessary for a particular application. In some applications, it
is
impractical to use a 10-30 meter length of fiber. Planar-type optical
amplifiers
have been developed for use in more confined spaces. The useful length of a
planar amplifying device is generally no more than 10 centimeters. To achieve
the
same amplification levels as a 10 to 30 meter length EDFA, a planar amplifier
requires an amplifying medium with a higher concentration of erbium ions, on
the order of up to 4 to 7 percent by weight.
However, in known types of optical amplifying medium, several gain loss
mechanisms occur at high erbium ion concentration levels, including ion
clustering and cooperative homogenous upconversion (concentration quenching).
Because erbium ions do not dissolve well in a silica matrix, erbium ions will
cluster, allowing energy transfer in the clustered region. In addition, at
higher
erbium concentrations, ion-to-ion interaction becomes more significant. The
resulting energy upconversion quenches the inverted population. Erbium ion
energy is used in the clustering and quenching processes and is therefore
unavailable for the required amplifying phonon process. As a result, quantum
efficiency of the amplifying medium decreases rapidly with higher erbium ion
concentration, with a concomitant decrease in amplifier gain.
Yet further, known silica-based erbium-doped amplifiers exhibit a distinct
spectral nonuniformity of gain. The lack of a flat gain spectrum over a wide
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bandwidth causes several problems. For instance, extremely short optical
pulses
have a relatively wide power spectrum and are not accurately amplified if the
gain
spectrum is not flat. In addition, in larger bandwidth applications, such as
wavelength division multiplexing (WDM), the fiber receives data-modulated
optics
signals from several optical transmitters, each using a different optical
carrier
frequency. If the gain spectrum from the optical amplifier is not flat over
the
operating wavelength, the carrier frequencies at gain peaks might saturate
while
the carrier frequencies at the skirts and valleys may not be sufficiently
amplified.
Past efforts to address gain flattening have primarily relied on passive or
active
filtering of the high gain features of the gain spectrum. However, this
requires a
close matching of the particular amplifier and filter and must account for
temporal
variations in the gain spectrum.
SUMMARY OF THE INVENTION
The present invention is concerned with a family of glasses that find
particular utility in production of optical signal amplifiers. These glasses
are
doped with high concentrations {up to 10 wt. %) of erbium oxide while
exhibiting
weak concentration quenching behavior. These glasses also provide higher
fluorescence efficiency an more uniform gain characteristics than known
silicate
and fluorozirconate glass medium. These glasses provide high and flat gain
characteristics that are particularly useful for optical amplification in the
1.55~,m
optical wavelength window, and are particularly well suited for use in
wavelength
division multiplexing (WDM) systems.
One aspect of the present invention is directed to a family of glasses,
particularly fiuorophosphate glasses, that are particularly well suited for
high rare
earth ion concentration levels. It is an object of the present invention to
provide a
fluorophosphate glass medium doped with erbium oxide ions for use in an
optical
amplifier for providing flat and high gain in an optical wavelength window
around
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1.55~m. The fluorophosphate glasses of the present invention comprise high
concentrations of erbium ions (i.e. close to 10% by weight) and provide a more
spectrally uniform gain, similar to ZBLAN <->, and significantly improved over
typical silicate and phosphate glass compositions.
The present invention is directed to a family of glass for optical
amplification comprising a substantially silica free fluorophosphate glass
medium,
doped for 100 parts by weight constituted of:
<fluorinated glasses (that is, the fluorinated glasses for optical fibers
containing
the fluorides ZrF4, BaFz, LaF3, AlF3, and NaF)>
PZOs 15-40 MgFz 0-10
AI2030-5 CaFz 0-25
Mg0 0-9 SrF2 0-25
Ca0 0-9 BaF2 0-20
Sr0 0-9 KHFz 0-2
Ba(J 0-45 KZTiFs 0-2
AIF3 5-25
with up to 10, preferably between 0.01 and 10, parts by weight of erbium
oxide.
Preferably, the fluorophosphate glass according to the present invention has
a composition, comprising in parts by weight:
PzOs 16.9-24.0 MgFz 0-7.5
AIz031.6-3.2 CaFz 0-18.7
Mg0 0-5.0 SrFz 0-19.7
Ca0 0-5.1 BaFz 1.5-11.3
Sr0 0-8.5 KHFz 0-1.3
Ba0 2.7-43.2 K2TiF6 0-0.6
AIF3 9.5-19.3
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The fluorophosphate glass according to the present invention may also be
co-doped with up to 15 parts by weight of Yb203 as a sensitizes to increase
pump efficiency at around 980 nm. The fluorophosphate glass according to the
5 invention preferably has an index of refraction between about 1.48 and 1.58.
According to another aspect, the present invention is directed to an
erbium-doped optical amplifier for a wavelength band of approximately 1.55~m,
having a medium for optical amplification comprising a substantially silica
free
fiuorophosphate glass composition that further comprises in addition to 100
parts
by weight of other components, about 0.01 to 10 parts by weight of Erz03. The
optical amplifier according to the present invention may be either a planar-
type
optical amplifier or a single mode fiber type optical amplifier.
The optical amplifier according to the present invention includes
fluorophosphate glass compri ing, for 100 parts by weight constituted by:
PZOs 15-40 MgFz 0-10
AI2030-5 CaF2 0-25
Mg0 0-9 SrF2 0-25
Ca0 0-9 BaF2 0-20
Sr0 0-9 KH F2 0-2
Ba0 0-45 K2TiFs 0-2
AIF3 5-25
up to 10, and preferably between 0.01 and 10 parts by weight of erbium oxide.
The fluorophosphate glass used for the optical amplifier according to the
present invention may also be doped with up to 15 parts by weight of Yb203 as
a
sensitizes to increase pump efficiency at around 980 nm, and preferably has an
index of refraction between about 1.48 and 1.58. Optical amplifiers according
to
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the present invention are particularly useful in wavelength division
multiplexing
(WDM) systems.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the invention will be seen from the
following detailed description and referring to the annexed drawings, given by
way
of example only, and in which:
FIGS. 1 and 2 are graphs illustrating typical behavior of concentration
quenching on fluorescence lifetime and efficiency of binary silicate glass.
FIGS. 3 and 4 are graphs illustrating fluorescence lifetime and efficiency of
fluorophosphate glasses according to the present invention.
FIG. 5 is a graph illustrating gain shape versus wavelength for a typical
silicate based glass.
FIG. 6 is a graph illustrating gain shape versus wavelength for a typical
ZBLAN glass.
FIGS. 7-9 are graphs illustrating gain shape versus wavelength for
fluorophosphate glasses according to' the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a family of glasses having utility
particularly in lighting, optical and electronic applications. The glasses
have
unique features that render them particularly useful in the production of
optical
signal amplifiers.
One feature of these glasses is their substantial freedom from SiOz. Erbium
ions do not dissolve well in a silica matrix, thus promoting ion clustering
and
degrading gain efficiency., such that the removal of the silica matrix enables
ion
clustering to be prevented, and excess ion photon energy to be preserved for
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amplification. The glasses according to the present invention contain a
relatively
high concentration of PZOs. The present invention is directed to a family of
glasses for optical amplification comprising a substantially silica free
fluorophosphate glass medium, doped for 100 parts by weight of other
components, with up to 10 parts by weight of erbium oxide. Table 1 sets forth
the
essential composition ranges for the fluorophosphate glass according to the
present invention.
TABLE 1 - (parts by weight)
PZOS 15-40 MgFz 0-10
AI2O30-5 CaF2 0-25
Mg0 0-9 SrF2 0-25
Ca0 0-9 BaFz 0-20
Sr0 0-9 KHFz 0-2
Ba0 0-45 K2TiFs 0-2
AIF3 5-25
The family of erbium-doped glasses according to the invention may further
comprise about .01 to 15 parts by weight of Yb203 to be used as a sensitizes
to
increase pump efficiency at around 980 nm. Table 2 defines narrower, preferred
ranges of oxide constituents of the present glasses. Optimum properties for
optical signal amplifiers, and their production, obtain within these narrower
ranges.
TABLE 2 - (parts by weight)
PZOs 16.9-24.0 MgFz 0-7.5
AI2031.6-3.2 CaF2 0-18.7
Mg0 0-5.0 SrFz 0-19.7
Ca0 0-5.1 BaFz 1.5-11.3
Sr0 0-8.5 KHF2 0-1.3
Ba0 2.7-4.3.2 KZTiFs 0-0.6
AIF3 9.5-19.3
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Another feature of the glasses according to the present invention is their
ability to be doped with relatively high concentrations of erbium oxide
(Er203). In
the absence of a silica-based glass, high concentrations of Er203 doping
provide
excellent fluorescent effects that are important for optical signal
amplification by
laser pumping due to the reduction of ion clustering and upconversion
quenching.
This property provides an excellent amplification medium for use in optical
amplifiers for the 1550 nm wavelength. According to another aspect, the
present
invention is directed to an erbium-doped optical amplifier comprising a medium
for
optical amplification comprising a fluorophosphate glass composition.
Preferably,
the fluorophosphate glass composition is doped for 100 parts by weight
constituted by:
TABLE 3 - (parts by weight)
P20s 15-40 MgF2 0-10
AIZOs0-5 CaF2 0-25
Mg0 0-9 SrF2 0-25
Ca0 0-9 BaF2 0-20
Sr0 0-9 KHFZ 0-2
Ba0 0-45 K2TiF6 0-2
AIF3 5-25
with about 0.01 to 10 parts by weight of Er203.
According to another embodiment of the present invention, the
erbium-doped optical amplifier medium comprises, with regard to components
other than erbium oxide, 100 parts by weight constituted as shown in Table 4
below.
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TABLE 4
P205 16.9-24.0MgF2 0-7.5
AI2031.6-3.2 CaF2 0-18.7
Mg0 0-5.0 SrFz 0-19.7
Ca0 0-5.1 BaF2 1.5-11.3
Sr0 0-8.5 KHFZ 0-1.3
Ba0 2.7-43.2 K2TiF6 0-0.6
AIFs 9.5-19.3
The erbium-doped optical amplifier according the invention may further
comprising about .01 to 15 parts by weight of Yb203, to be used as a
sensitizer to
increase pump efficiency at around 980 nm. The optical amplifier according to
the
present invention may take any number of forms, as long as the medium is
capable of being doped with erbium ions. The optical amplifier can be a single
mode fiber type optical amplifier. Alternatively, the optical amplifier could
also be
a planar-type optical amplifier. The effects of concentration quenching on
typical
binary silica-based glasses is illustrated in FIG. 1. At low concentration
levels of
Erz03, less than 5E19 ionslcc, (the equivalent of less than 0.5 parts by
weight),
the fluorescence lifetime is constant. Above this concentration level,
fluorescence
lifetime decreases rapidly as concentration increases. Two characteristic
concentrations can be defined in order to differentiate glasses. The
concentration
Cqb corresponds to the onset of concentration quenching. The concentration Cq
corresponds to the concentration level where the fluorescence lifetime is
divided
by 2. As -illustrated in FIG. 1, concentration quenching begins in typical
binary
silicate glasses when Cqb = 7E19 ionslcc (or approximately 0.9 parts by
weight).
At this point, fluorescence lifetime is approximately 13ms. When CQ =3E20
ions/cc
(or approximately 3 parts by weight. the fluorescence lifetime is
approximately 7.5
ms.
FIG. 2 is a graph illustrating the effects of concentration quenching on
fluorescence efficiency of typical silica based glasses. Fluorescence
efficiency is
*rB
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defined as the 1.55um fluorescence per Er ion versus Er concentration. For the
ion concentrations levels of interest, i.e., between 3 to 5 E20 Er ionslcc
(approximately 4-7 parts by weight}, the fluorescence efficiency of the silica-
based
glasses is between .5 to 2 E-19nW/ion.
5 FIG. 3 is a graph illustrating the effects of concentration quenching on the
fluorescence lifetime of three types of fluorophosphate-based glasses
according
to the present invention. FIG. 4 is a graph illustrating the effects of
concentration
quenching on fluorescence efficiency of three types of fluorophosphate-based
glasses according to the present invention. As shown in FIGS. 3 and 4, both
Cqb
10 and Cq are one order of magnitude higher than the corresponding values for
the
silica-based glasses. This indicates that the concentration quenching behavior
at
high concentration levels of Er ions is relatively weak in the fluorophosphate
glasses according to the invention, and that these glasses are very good
candidates for short length, high gain optical signal amplifiers. A comparison
of
composition with associated measured and calculated properties of three types
of
fluorophosphate glasses according to the present invention, a typical
borosiiicate-based glass and ZBLAN are provided in Table 5. The compositions
in
Table 5 are expressed as batch quantity. The actual ingredients of the batch
can
consist of any type of raw material, oxides, fluorides or phosphates, which
when
melted together, are converted into desired oxides and fluorides in the proper
proportions. Examples of raw materials (not exhaustive ) are: Ca(P03)Z,
Ba2Pz0~,
AI4(P20,)3, AI(POs)3, NaP03, K2TiF6, X20y, XFY where X is the metal ion of
valence
Y.
The values given in Table 5 (life the values given elsewhere in this text)
represent the theoretical quantities of the different components in the final
glass,
according to normal practice in this field. In the case of oxides, the
theoretical
quantities are very close to the natural quantities (that is, the "batch
yield" is very
close to 100% for the oxides}. In the case of the fluorides, which are more
slatile,
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the actual values are slightly lower than the theoretical values (the batch
yield is
about 90 to 95%).
TABLE 5
Code Example 1 Example Example BorosilicateZBLAN2
2 3
type glass
parts weight molar
by
SiOz 66.6 i
11.6 j
E'z~s 16.9 24.0 30.9 ;
AI20s 3.2 2.7 1.6 ;
MgFz 5.8 7.5 0.0 ;
CaFz 18.7 0.5 0.0 ;
S rFz 19. 7 17. 9 0. 0 ;
BaFz 11.3 14.4 1.5 ~ 22
AI F3 19. 3 11. 3 9.5 ; 4
ZrF4 i 48
InF3
LaF3
3.2
NaF ~ 22
KH Fz 1. 3 0. 0 0. 0 ;
K2TiF6 0.6 0.5 0.0 ;
NazO 0.5 0.0 0.0 ;
K20
Ca0 0.0 5.1 0.0 ;
Sr0 0.0 2.4 8.5 ;
Ba0 2.7 13.7 43.2 ;
Mg0 0.0 0.0 4.9 ;
ErF3
i 0.8
Erz03 6.0 4.0 1.5 2
ErzOs 5.84+20 4.64+20 1.6+20 1.6E+20 1.5E+20
(ions/cm3
Index 1.49 1.54 1.59 1.52
Density 3.62 3.83 3.976 2.552
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Code Example Example Example Borosilicate ZBLAN2
1 2 3
type glass
parts by weight molar
Fluorescence 6.8 7.6 7 6.3
lifetime
Fluorescence 9.5 8 7 16
lifetime at
low
Er content
( ms)
QE (%) 68 95 100 39
Fluorescence 2.3 2.7 3.2 1.3
efficiency
(nWIEr ion)*1
E-
19
Cross-sections (cm2)*1 E-21
Absorption Pump
975 nm 1.9 2.3 (980 nm) 0.8 2.6
1480 nm 3.3 3.9 1.2 4.7
Absorption Signal (6ab(~t,))
1533 nm 4.9 5.9 (1527 nm) 5.6
FWHM (nm) 65 64 15
Emission Signal (aem(~.))
1522 nm 5.3 6.5 (1537 nm) 7.2 6
FWHM (nm) 51 49 17
Emission 1.1 1.1 1.3 1.1
Jabsorption
Radiative 10 8 16 8
lifetime (ms)
The batch ingredients are mixed together to provide homogeneity, placed
inside a platinum crucible, and Joule-heated at about 1000°C. When
melting is
completed, the temperature is raised to between 1050 to 1350°C to
obtain giass
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homogeneity and fining. The melt then is cooled and simultaneously formed into
the desired shape, and finally transferred into an annealing furnace operating
at
about 400°C. An alternative melting process consists of forming the
glass from
batch ingredients and remelting this glass together with the desired
proportion of
Er orland Yb raw materials. This procedure can in some cases increase
homogeneity of the glass.
As seen in Table 5, the Quantum Efficiency ~ob~h~d is in the range of 70 to
100% for the fluorophosphate glasses according to the present invention at the
desired Er concentration levels, whereas the Quantum Efficiency for the
silica-based glasses at the same concentration level is in the range of 20 to
35%.
One limitation to the full use of bandwidth in WDM systems is the spectral
nonuniformity of gain exhibited in silica-based glass EDFA. Another important
feature of the high concentration Er-doped fluorophosphate compositions
according to the present invention as compared to silica-based glasses is that
the
fluorophosphate glasses exhibit a very flat gain spectrum, over a range of
approximately 28 to 30 nm in the 1550nm bandwidth. This is comparable to Er
doped ZBLAN glass fibers. To obtain this gain flatness between 1528 and 1563
nm, the glass medium according to the present invention preferably has a
fluorine
content of at least 18 parts by weight. A good representation of gain spectral
shape versus wavelength can be obtained using the following formula:
9(~m i)-~6ernO)*N2 ' 6abU)*N1l ~~)
where: ae,"(~,) is the emission cross section in cm2;
aab(~,)* is the absorption cross section ion cm2;
NZ is the upper level (°I,~,Z) ion population (averaged over the
length);
N, is the ground state (° I ,~,2) ion population (averaged over the
length; and
Nt is the total Er ion concentration (in ions per cm3).
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If the inversion percentage is defined as D=(NZ-N,)IN,, then equation (1)
can be rewritten as:
G(dB/Cm) 2.15*Nt*~6em(~.)*(1'~~) - ~ab(~.)*1-~)~ (2)
where: D+-1: % inversion; and
D++1: 100% inversion.
Equation 2 was used to calculate the gain shape versus wavelength of
different glass compositions, the results of which are shown in FIGS. 5-9.
FIG. 5 illustrates the gain shape of a typical borosilicate type glass used in
optical signal amplifiers. It is clear that the gain spectrum around the 1550
nm
bandwidth which is used in WDM, is nonuniform in character. Amplification
between about 1535 nm and 15fi5 nm, a typical range used in WDM, is uneven.
The variation between the maximum and minimum gain can reach 250%.
FIG. 6 illustrates the gain shape of a ZBLAN glass for use in an optical
signal amplifier. In comparison to the borosilicate type glass, ZBLAN offers a
flattened gain shape over a range of wavelengths about 30 nm wide.
FIG. 7 illustrates the gain shape for a first fluorophosphate type glass
according to the present invention. This glass, identified as Example 1 has an
Er
concentration over 7 parts by weight for 100 parts by weight of other
components,
yet still exhibits a substantially flat gain shape over a 28nm spectrum in the
1530 -
1560 nm band.
FIG. 8 illustrates the gain shape for a second type of fluorophosphate glass
according to the present invention. This glass, identified as Example 2, has
an Er
concentration over 4 parts by weight for 100 parts by weight of other
components
and exhibits a flattened gain shape over about a 26nm spectrum.
FIG. 9 illustrates the gain shape a phosphate-based glass. This glass,
identified as Example 3 has an Er concentration slightly less than 3 parts by
weight, for 100 parts by weight of other components, and exhibits two
relatively
flat areas of gain; the first being about 10nm wide arid the second about 9nm
in
width.
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Another aspect of the present invention is the ability to efficiently pump the
amplifying medium at 980 nm while maintaining relatively low noise levels.
Optical
amplification requires excitation of erbium ions in the glass medium to a
higher
energy level, and then relaxation of the ions This process causes emission of
5 photons as the erbium ions relax to ground level. The photons emitted during
this
process are at a wavelength so as to amplify optical signals at the same
wavelength.
Considering the first three energy levels of erbium, useful emission occurs
between level 2 (the metastable level) to level 1 (the ground level}. To have
10 population inversion (population at level 2 higher or equal than 50%) and,
therefore, gain, the gain medium must be pumped with an external source.
Generally, with optical signal amplification, the gain medium is pumped with a
980
or 1480 nm diode laser. When a 980 nm diode laser is used, electrons move to
the third level (''I""z) and then relax to the second level and then to the
ground
15 level by emitting a 1.55~.m photon. When a 1480 nm diode laser is used,
electrons move directly to the lasing level (2) and then to the ground level
by
emitting a 1.55~m photon. The most efficient and reliable pump for optical
amplification is a 980 nm pump. However, because the 980 nm pumping process
moves the electrons to the third level, the lifetime at level (3) should be
very low,
preferably on the order of micro seconds, otherwise the electrons may be
excited
to upper levels and thereby decrease pump efficiency. This in fact happens
with
ZBLAN-like glass medium when pumped with a 980 nm pump due to its relatively
long lifetime at level (3) (around 9ms). Accordingly, a ZBLAN-like amplifying
medium cannot be pumped as efficiently as the inventive fiuorophosphate glass
medium with a 980 laser diode pump.
Er-doped fluorophosphate glass according to the present invention as an
amplification medium with a 980 nm pump, has advantages over other Er-doped
fluorides. ZBLAN-like {100% fluoride no oxygen) compositions, due to high
fluorescence lifetime (9 ms) at the °I""z pumping level, lose pumping
efficiency.
*rB
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ZBLAN-like compositions are, therefore, usually pumped at 1480 nm. However,
there are drawbacks to pumping at wavelengths this high. For example, the ion
population cannot be fully inverted at this level and noise in the amplifier
increases. To the contrary, the fluorophosphates glass medium according to the
present invention can be efficiently pumped at 980 nm since the 41""2 lifetime
time
is in the range of 10 to 70 yes.
FIGS. 5-9 illustrate that fluorophosphate glasses according to the present
invention show gain flattening characteristics using a 980 nm pump similar to
ZBLAN, and significantly improved in comparison with silicates and phosphates.
The glass compositions according to the present invention provide high and
flattened gain characteristics in short length optical amplifiers, and then
can be
used for the manufacturing of planar amplifiers andlor short length single
mode
fibers having ZBLAN-like gain that are useful in WDM and other similar
applications.
