U.S. patent application number 11/138265 was filed with the patent office on 2005-09-29 for low phonon energy gain medium and related active devices.
This patent application is currently assigned to University of Highfield. Invention is credited to Brambilla, Gilberto, Chiodini, Norberto, Morazzoni, Franca, Paleari, Alberto, Scotti, Roberto, Spinolo, Giorgio, Taylor, Elizabeth.
Application Number | 20050213624 11/138265 |
Document ID | / |
Family ID | 27224377 |
Filed Date | 2005-09-29 |
United States Patent
Application |
20050213624 |
Kind Code |
A1 |
Taylor, Elizabeth ; et
al. |
September 29, 2005 |
Low phonon energy gain medium and related active devices
Abstract
Using sol-gel techniques, an optical gain medium has been
fabricated comprising a glass ceramic host material that includes
clusters of crystalline oxide material, especially tin oxide, and
that is doped with active ions concentrated at the clusters. The
active ions are preferentially located at the nanoclusters so that
they experience the relatively low phonon energy of the oxide and
are insensitive to the phonon energy of the host. A host with a
high phonon energy, such as silica, can therefore be used without
the usual drawback of reduced carrier lifetimes through enhanced
nonradiative decay rates.
Inventors: |
Taylor, Elizabeth;
(Southampton, GB) ; Brambilla, Gilberto;
(Southampton, GB) ; Chiodini, Norberto;
(Southampton, GB) ; Paleari, Alberto;
(Southampton, GB) ; Spinolo, Giorgio;
(Southampton, GB) ; Morazzoni, Franca;
(Southampton, GB) ; Scotti, Roberto; (Southampton,
GB) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER
LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
University of Highfield
Southampton
GB
|
Family ID: |
27224377 |
Appl. No.: |
11/138265 |
Filed: |
May 27, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11138265 |
May 27, 2005 |
|
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10260414 |
Oct 1, 2002 |
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60327768 |
Oct 10, 2001 |
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Current U.S.
Class: |
372/40 |
Current CPC
Class: |
C03C 13/006 20130101;
H01S 3/169 20130101; C03B 2201/30 20130101; H01S 3/0637 20130101;
C03B 2201/34 20130101; C03B 19/12 20130101; H01S 3/06716 20130101;
H01S 3/17 20130101; H01S 3/1603 20130101; C03C 13/046 20130101;
C03B 37/016 20130101; Y10S 65/901 20130101 |
Class at
Publication: |
372/040 |
International
Class: |
H01S 003/17 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2001 |
EP |
01308392.8 |
Claims
1. An optical gain medium comprising a glass ceramic host material
that includes clusters of crystalline oxide material and that is
doped with active ions concentrated at the clusters.
2. An optical gain medium according to claim 1, wherein the
clusters contain tin oxide.
3. An optical gain medium according to claim 2, wherein the tin
oxide is substantially pure.
4. An optical gain medium according to claim 2, wherein the tin
oxide is present in a concentration of greater than 0.5 mol %.
5. An optical gain medium according to claim 2, wherein the tin
oxide is present in concentration of greater than 0.5 mol % and up
to 30 mol %.
6. An optical gain medium according to claim 1, wherein the
crystalline oxide material includes an oxide of at least one of
zirconium, scandium, yttrium, lutetium, titanium and hafnium.
7. An optical gain medium according to claim 1, wherein the active
ions are rare earth ions.
8. An optical gain medium according to claim 7, wherein the rare
earth ions are erbium ions.
9. An optical gain medium according to claim 7, wherein the rare
earth ions are thulium ions.
10. An optical gain medium according to claim 1, wherein the host
material is silica or a silicate glass.
11. A laser comprising an optical gain medium comprising a glass
ceramic host material that includes clusters of crystalline oxide
material and that is doped with active ions concentrated at the
clusters.
12. An optical amplifier comprising an optical gain medium
comprising a glass ceramic host material that includes clusters of
crystalline oxide material and that is doped with active ions
concentrated at the cluster.
13-17. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a low phonon energy gain medium and
to active devices comprising such a gain medium.
[0002] Optical gain media are well-known as forming the basis of
both lasers and optical amplifiers. Many gain media are solid-state
and comprise a host material doped with active dopant ions, such as
rare earth ions. Silica is a particularly common host material for
amplifiers, partly because the material is widely used in optical
fibers. A silica amplifier is highly compatible with silica
transmission fiber they can be coupled together with low
losses.
[0003] An integrated fiber laser may be fabricated from such a gain
medium by additionally including a photosensitive dopant such as
tin. The photosensitivity of tin allows optical gratings to be
written directly into the fibers to provide the necessary cavity
mirrors. Co-doping of silica fibers with the rare earth ions erbium
or ytterbium for gain, and tin for photosensitivity, has been used
to produce fiber lasers [1]. Tin-oxide-doped silica glass has been
produced by modified chemical vapor deposition (MCVD) and the
sol-gel technique [2-4), and tin-oxide-doped silica glass ceramics
by the sol-gel technique alone [3, 4].
[0004] An important characteristic of a host material for a gain
medium is its principal optical phonon energy. A large phonon
energy is associated with short carrier lifetimes arising from
large non-radiative recombination rates. For a host material having
a maximum phonon energy E.sub.p and an active dopant having a
lasing energy transition .DELTA.E, the nonradiative recombination
rate is a very strong function of the ratio R=.DELTA.E/E.sub.p. The
lower the value of R, the higher is the probability of undesirable
nonradiative recombination. Therefore, host materials having low
phonon energies E.sub.p are required, especially for small lasing
energy transitions .DELTA.E.
[0005] Unfortunately, silica has a high principal optical phonon
energy, which arises from contributions from vibrations of the
Si--O bond, in the vicinity of 1000 cm.sup.-1 [5]. Therefore,
although silica is a useful host material as regards its
compatibility with commonly used fibers, it is disadvantageous as
far as phonon energy is concerned
[0006] Fluoride glasses have been proposed as an alternative to
silica [5, 6]. The phonon energy is 500 cm.sup.-1, which makes
these materials more suitable for hosting active dopants with a low
.DELTA.E. However, fluoride glasses have low compatibility with
silica optical fibers, and it is difficult to fabricate low loss
fibers directly from fluoride glasses.
SUMMARY OF THE INVENTION
[0007] According to the invention there is provided an optical gain
medium comprising a glass ceramic host material that includes
clusters of crystalline oxide material and that is doped with
active ions concentrated at the clusters. In one embodiment, the
clusters contain tin oxide.
[0008] It has been discovered that in the presence of tin oxide
clusters, doping with active ions results in the active ions being
concentrated at the tin oxide clusters, rather than being evenly
distributed throughout the host material, as might be expected. The
active ions thus predominantly experience the low phonon energy
environment of the tin oxide clusters, rather than the phonon
environment of the host The host material can thus be chosen
without regard to its phonon energy, allowing high phonon energy
host materials such as silica to be used without the usual
drawbacks. It is thus possible to fabricate a gain medium based on
a silica host material that has an improved carrier lifetime (i.e.
reduced nonradiative recombination rate) in comparison to
conventional active ion doped silica in which the active ion
dopants are distributed evenly throughout the silica host
[0009] The clusters may be of substantially pure tin oxide.
However, embodiments of the invention use other oxides, so that the
crystalline oxide material includes an oxide of at least one of
zirconium, scandium, yttrium, lutetium, titanium and hafnium. Like
tin, these materials have low phonon energies so that the active
ions experience a phonon energy less than that of the host
material.
[0010] In embodiments of the invention, the host material is silica
or a silicate glass so as to be compatible with other silica or
silicate glass components, thus providing compatibility in terms of
refractive index and other optical, physical and chemical
properties. Therefore, fiber components can be joined together with
low splicing losses and low Fresnel losses. Similarly planar
devices can be more readily integrated. Optical mode distribution
can also be maintained constant from one component to the next. The
gain medium of the invention is thus superior to the previously
proposed low phonon energy host materials based on fluoride glasses
which are silica incompatible, and to silica glasses doped directly
with active ions.
[0011] In embodiments of the invention, the active ions are rare
earth ions, such as Er, Pr, Nd or Tm. The invention may however be
applicable to other active ions. In one example, the are earth
element Tm is used as the active ions in an amplifier for
amplification in the range 1450-1520 nm, and especially at around
1470 nm, which has application in dense wavelength division
multiplexed (DWDM) systems operating between the second and third
telecommunications windows. By contrast, devices made from silica
doped directly with Tm.sup.3+ ions are known to have an undesirably
high level of nonradiative decay at the 1470 nm lasing transition,
owing to the high phonon energy of silica.
[0012] At least from the experiments performed to date, it appears
that the tin oxide needs to be present above a certain
concentration to induce cluster formation. In the examples, a tin
oxide concentration of greater than about 0.5 mol % is needed for
the desired glass ceramic formation with clusters. It is thought
that the tin oxide may be present up to concentrations of 30 mol %
or above.
[0013] The tin oxide clusters may be nanoclusters, typically with
cluster diameters of between 5 and 10 nm. Cluster diameters of up
to 50 nm may be acceptable, although smaller cluster sizes in the 1
to 10 nm range are preferred. The cluster diameter should be small
enough to avoid significant scattering losses at the operating
wavelength, for example at 1.3 .mu.m or 1.5 .mu.m, from the
refractive index difference between tin oxide and the host
material.
[0014] The glass ceramic host with tin oxide clusters has good
miscibility with rare earth ions, so that high concentrations of
rare earth ions can be accommodated. Rare earth doping at
concentrations up to 2 mol % is possible, with doping preferably in
the range 0.1-2 mol %. High doping concentrations of the active
ions above 1 mol % allows for the manufacture of smaller optical
devices, since a sufficiently high level of active ions to provide
a certain amount of gain can be contained within a smaller volume
of gain medium. However, the concentration of the active ions needs
to be kept below the level at which the active ions precipitate out
and crystallize within the host, because this can undesirably
reduce the excited state lifetime of the active ions.
[0015] In the examples described below, the host material is silica
glass. Silicate glasses such as germanosilicate, phosphosilicate or
borosilicate glass may be used in other examples.
[0016] The gain medium may be utilized in lasers and optical
amplifiers, in fibers, planar waveguides or other waveguide
technology.
[0017] Gain media according to the present invention may be
manaufactured by sol-gel techniques, which offer good control over
the tin oxide cluster sizes. However, other techniques may be
suitable for fabricating the gain medium.
[0018] More specifically, a further aspect of the present invention
is directed to a method of manufacturing an optical gain medium
comprising: preparing a solution containing tin and an active ion;
allowing the solution to form a gel; evaporating the gel to form a
sample of xerogel; and sintering the sample of xerogel to form a
glass ceramic material containing clusters containing tin oxide
clusters by subjecting it to a heating cycle.
[0019] In an embodiment of this method, the heating cycle
comprises: heating the sample from a first temperature to a second
temperature in an oxygenated atmosphere; maintaining the sample at
the second temperature; heating the sample from the second
temperature to a third temperature in an oxygenated atmosphere or
in a vacuum; heating the sample from the third temperature to a
fourth temperature in an oxygenated atmosphere; maintaining the
sample at the fourth temperature in an oxygenated atmosphere; and
cooling the sample from the fourth temperature to the first
temperature in an oxygenated atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] For a better understanding of the invention and to show how
the same may be carried into effect reference is now made by way of
example to the accompanying drawings in which:
[0021] FIG. 1 shows a thermal cycle used in the manufacture of
samples of gain media by a sol-gel technique;
[0022] FIG. 2 shows measured ultraviolet absorption spectra from a
sample of gain medium according to the present invention and from a
comparison sample;
[0023] FIG. 3 shows measured Raman spectra from a sample of gain
medium according to the present invention and from two comparison
samples;
[0024] FIG. 4 shows measured photoluminescence spectra from two
samples of gain media according to the present invention and three
comparison samples;
[0025] FIG. 5 shows the measured optical absorption owing to the
.sup.4I.sub.15/2.fwdarw..sup.4I.sub.13/2 transition of erbium ions
of five samples of gain media according to the present
invention;
[0026] FIG. 6 shows an optical fiber amplifier according to an
embodiment of the invention;
[0027] FIG. 7 shows an optical fiber laser according to an
embodiment of the invention;
[0028] FIG. 8 shows a fiber Bragg grating reflector as used in the
laser of FIG. 7; and
[0029] FIG. 9 shows an optical amplifier implemented in planar
waveguides according to an embodiment of the invention.
DETAILED DESCRIPTION
[0030] Manufacture of the Gain Media
[0031] Optical gain media described in the following embodiments
and examples are of the glass ceramic type comprising a silica host
material which contains clusters of tin oxide at which active ions
are concentrated.
[0032] Samples of the gain medium have been made by a sol-gel
technique. Sol-gel processing techniques are known for producing
glasses and glass ceramics. The techniques involve first rib a
solution (sol) of precursor molecules in a solvent The molecules
react together to form a wet gel, from which a dryer xerogel is
created by evaporation. The xerogel is then heated, or sintered,
according to a thermal cycle, to produce the end glass or glass
ceramic material.
[0033] In the present case, the sol was formed by co-gelling a
number of precursors in a solvent. Tetraethoxysilane (TEOS,
Si(OCH.sub.2CH.sub.3).s- ub.4) provided silicon for silica, dibutyl
tin-diacetate
(Sn(CH.sub.2CH.sub.2CH.sub.2CH.sub.3).sub.2(OOCCH.sub.3).sub.2)
provided tin, and active ions, in this case of the rare earth metal
erbium, came from erbium nitrate ((Er(NO.sub.3).sub.3).5H.sub.2O).
It will be understood that Er can be substituted with any other
rare earth element as desired.
[0034] Active ions in the form of rare earth metals may also be
provided by other inorganic salts (for example nitrates, acetates,
halides) and also by soluble complexes or alkoxides such as
Er(OR).sub.3, where R is a generic alkyl group. Similarly, other
tin precursors are expected to give comparable results, such as
those having the general formula SnR.sub.nX.sub.m-n with m=4 and
0.ltoreq.n.ltoreq.3, where R is a generic alkoxide group and X is a
halide group, a carboxyl anion or alkoxide group. Silicon may be
provided by pure or mixed compounds described by the general
formula SiR.sub.nX.sub.m(OR).sub.w where 0.ltoreq.n,m,w.ltoreq.4
and n+m+w=4, and R is an alkyl group, X is a halide or carboxyl
anion group and (OR) is an alkoxide group. In particular, the
addition of alkyl silane (such as trimethylsilylacetate or
trimethylmethoxysilane) may be used to obtain sols suitable for
spin- or dip-coating deposition of thin films for planar device
fabrication. Furthermore, the sol-gel reaction among these
precursors may be modified by a suitable change of solvent to
obtain xerogels with different mechanical properties and porosity.
For example, a solvent such as ethanol may be partially or totally
substituted by dimethylformamide, chetons, methoxyethanol and other
solvents generally known as drying-control-chemical-additives
(DCCA). In addition, other dopant elements (such as Ge, P and B)
may be introduced by suitable precursors (such as Ge alkoxide, B
alkoxide, and P ester).
[0035] Various proportions of the precursors were used to make a
number of samples having different compositions, including samples
lacking rare earth ions that were made for the purposes of
comparison. Gelation via evaporation was achieved by leaving the
samples in a sealed container in a thermostatic chamber at
35.degree. C., resulting in xerogels. Evaporation times of between
two hours and two weeks were used; two weeks were required to
produce bulk samples of material.
[0036] FIG. 1 shows the thermal cycle applied to the xerogel
samples by heating them in a furnace. The temperature T is plotted
as a function of time t in hours, the sintering process taking in
excess of 400 hours. The thermal cycle had seven phases, A, B, C,
D, E, F and G.
[0037] During Phase A, which lasted for approximately 70 hours, the
samples were kept in an oxygen atmosphere and heated steadily from
0.degree. C. to about 450.degree. C. During Phase B, this
temperature was maintained for about 48 hours, as was the oxygen
atmosphere.
[0038] In Phase C the samples were kept in a vacuum (about 0.1 Pa)
for about 24 hours, following which, during Phase D, in which the
vacuum was maintained, the temperature was increased at a constant
rate of 4.degree. C./hour up to about 750.degree. C. During Phase E
a 1% He:O.sub.2 atmosphere was provided, and the heating continued
for 150 hours up to 1050.degree. C. In Phase F this temperature was
maintained for 10 hours or so. Finally, during Phase G the samples
rapidly cooled back to room temperature in the course of about 10
hours.
[0039] Numerous variants of this heating cycle have been tested.
Faster cycles may be used, but this can increase the probability of
sample cracking. Insertion of fierier steps at constant
temperature, and substitution of the vacuum by a pure oxygen
atmosphere in the Phases D and E may be used to modify the sample
structure. Further processing steps in drying agents were also
tested, with the aim of improving the optical properties of the
rare earth ions.
[0040] After the heating cycle, the samples appeared to be
completely sintered.
[0041] Samples made by the above-described sol-gel technique may be
subjected to a few minutes of thermal annealing at a higher
temperature near to their softening temperature range. Such
treatment may improve the emission properties of the rare earth
ions in the glass ceramic gain medium, allowing higher rare earth
doping levels.
STRUCTURE OF THE GAIN MEDIA
[0042] Table 1 lists the characteristics of nine samples produced
by this technique. Samples 1, 2 and 3 contained no erbium ions, so
that comparative tests with erbium doped material could be carried
out.
1TABLE 1 Sample No. SnO.sub.2 (mol %) Er (mol %) Physical state 1
0.2 0 Glassy 2 0.4 0 Glassy 3 3.2 0 Glass ceramic 4 0.4 0.4 Glassy
5 3.2 0.34 Glass ceramic 6 3.2 0.67 Glass ceramic 7 3.2 1 Glass
ceramic 8 3.2 1.34 Glass ceramic 9 3.2 1.67 Glass ceramic
[0043] As can be seen from the final column of the table, which
gives the physical state of the samples, the samples can be
classified into one of two groups, depending on their level of
tin-dioxide (SnO.sub.2). The dividing point between the two groups
lies at approximately 0.5 mol % of SnO.sub.2, with glasses being
formed at SnO.sub.2 concentrations below about 0.5 mol % and glass
ceramics being formed at SnO.sub.2 concentrations above about 0.5
mol %. The glass ceramics are materials having a glass network or
matrix containing very small crystals or clusters of SnO.sub.2.
Although Table 1 lists samples having SnO.sub.2 concentrations of
0.2, 0.4 and 3.2 mol %, samples containing other amounts including
0.5, 1 and 15 mol % of SnO.sub.2 have been fabricated It is
believed that tin dioxide can be incorporated at levels up to at
least 30 mol %.
[0044] It has previously been shown [3] that in tin-doped silica
(tin silicate), the physical state depends on the concentration of
SnO.sub.2. Tin ions are bigger than silicon ions, but at low
concentrations the tin is included in the silica matrix at
substitutional positions. At these low concentrations, no
crystallization of SnO.sub.2 occurs, and the material is glassy. At
higher concentrations, oxygen coordinated Sn sites cluster together
and form SnO.sub.2 clusters, which have an average diameter
somewhat less than 10 nm. A cluster size in the nanometer range is
preferable because it means that the crystal clusters do not induce
significant scattering losses at the wavelengths commonly used in
telecommunications applications. In other words, the cluster size
is much less than the intended guiding wavelength, typically at
least an order of magnitude less. Scattering arises from mismatch
between the refractive index values of the clusters and matrix. Tin
oxide has a refractive index of around 2, whereas the index of
silica is nearer to 1.5, so that the difference between the two is
relatively large. Manufacture by the sol-gel technique offers
sufficient control for the cluster size to be kept to less than 10
nm. The cluster size can be controlled in the sol-gel heating
cycle, by the use of fewer or more phases in which the sample is
maintained at a constant temperature, and/or by the use of either a
vacuum or an atmosphere of substantially pure oxygen during the
final heating phases (for example, Phases D and E described above
with reference to FIG. 1).
[0045] Measured Raman and UV-visible spectra obtained from the
present samples containing erbium show similar behavior. At low
concentrations of tin ions Sn.sup.4+ (for example, Sample 4), tin
is inserted in a substitutional position in the silica network,
while at high concentrations (Samples 5 to 9), SnO.sub.2
nanocrystals or nanoclusters occur.
[0046] FIG. 2 shows ultraviolet absorption spectra (optical
absorbance A against photon energy E) measured for Samples 4 and 8.
The spectrum for Sample 8, which has a high Sn.sup.4+
concentration, shows a strong absorption edge at 3.7 eV. This
results from electronic transitions between valence and conduction
bands in SnO.sub.2, and its presence is therefore a sensitive
indication of SnO.sub.2 clustering [3]. The spectrum for Sample 4,
which has a low Sn.sup.4+ concentration, does not show this
absorption edge, indicating that the tin is substituted in the
silica network instead of being present in clusters. Therefore it
is concluded that the presence of the erbium ions does not affect
the overall tin-silicate structure.
[0047] In the high tin concentration samples, the results show
that, as a result of the fabrication process, the erbium ions are
concentrated in or immediately around the SnO.sub.2 clusters, most
likely distributed within the clusters, or perhaps distributed on
or adjacent to the cluster surfaces.
[0048] It is this unexpected effect that results in the
advantageous properties of the material. Since the active ions are
concentrated at the tin clusters, the effective phonon energy for
the gain medium is that of the tin clusters, not the host material.
Consequently, non-radiative decay rates are lower. If the active
ions were not preferentially attracted to the metal clusters, there
would be no significant difference between host material with tin
clusters and host material without. At present, the precise
mechanism by which the erbium ions are attracted to the SnO.sub.2
clusters is not known. However, the fact that the surprising effect
is occurring is borne out by the experimental results.
[0049] FIG. 3 shows Raman spectra (Raman intensity I against
wavenumber .nu.) which demonstrate that the erbium ions are held in
the SnO.sub.2 clusters. The spectra were measured by exciting
various samples (including some not shown in Table 1) with 633 nm
light from a helium-neon laser. Three spectra are shown, labeled
10, 12 and 14. Each of them shows a broad photoluminescence
emission on which other features are superimposed. Spectrum 10 is
from a sample of silica doped with 0.5 mol % of erbium, and
containing no tin. The spectrum is relatively smooth, but includes
peaks at 490 and 800 cm, which are Raman scattering features of
silica.
[0050] Spectrum 12 is from a sample of SnO.sub.2 doped with 1 mol %
of erbium, and containing no silica. The spectrum, which has an
overall broad bell shape, shows an intense peak at 630 cm.sup.-1,
indicated by the asterisk in FIG. 3. This is the A.sub.1g Raman
scattering mode of SnO.sub.2, which clearly has a much lower energy
than the principal silica mode at around 1000 cm.sup.-1. Three much
smaller peaks at 540, 670 and 750 cm.sup.-1 are also seen, as
indicated by the arrows in FIG. 3. These peaks are not believed to
be due to Raman modes, as they are absent from anti-Stokes Raman
spectra of the same material (not shown). These peaks have an
intensity proportional to the erbium content, but are clearly not
present in the spectrum of the tin-free sample (Spectrum 10). This
suggests that these peaks arise from some interaction between the
tin and the erbium.
[0051] Spectrum 14 is from a sample of silica containing 15 mol %
SnO.sub.2 and 1 mol % erbium. This SnO.sub.2 concentration is high
enough to be in the range where clusters are formed. The spectrum
shows the silica Raman scattering peaks seen in Spectrum 10 in
conjunction with the SnO.sub.2 peak seen in Spectrum 12. Also, the
three peaks at 540, 670 and 750 cm.sup.-1 seen in Spectrum 12 are
present, indicating that the same SnO.sub.2-erbium structure is
present as in the erbium-doped SnO.sub.2 sample. This implies that
the erbium ions are held in, or adjacent to, the SnO.sub.2 clusters
within the silica matrix, rather than being dispersed within the
silica matrix itself.
[0052] This Conclusion is supported by the spectrum inset in FIG. 3
and labeled 16, which is the Raman spectrum of clustered erbium
oxide (Er.sub.2O.sub.3). The spectrum shows entirely different
peaks to those of Spectrum 14. This indicates that the features of
Spectrum 14 do not arise from erbium ions forming oxide clusters in
the silica matrix, which suggests that the ions are somehow trapped
by the SnO.sub.2 clusters.
[0053] FIG. 4 shows photoluminescence spectra (photoluminescence
intensity I against wavelength .lambda.) of a range of samples
excited at 488 nm. These spectra provide further evidence that the
erbium ions are preferentially located in or around the SnO.sub.2
nanoclusters.
[0054] Five spectra are shown in FIG. 4, labeled 18, 20, 22, 24 and
26. Spectrum 18 is from a sample of erbium doped silica (0.5 mol %
erbium) containing no tin and therefore having a glassy structure.
Spectrum 20 is from a sample of erbium doped silica (0.3 mol %
erbium) containing 0.5 mol % SnO.sub.2, so that the sample has a
glassy structure rather than being a glass ceramic. The two spectra
are similar, showing the same features. Therefore, the tin ions do
not greatly affect the structure of the erbium-doped silica, and
can be assumed to be in substitutional positions in the silica
matrix. In both cases, the erbium is dispersed within the silica
matrix. The presence of tin does not affect this, as the tin
concentration is too low for cluster formation.
[0055] Spectrum 22 is from a sample of SnO.sub.2 doped with 1 mol %
of erbium. A series of peaks is present around 550 nm. These peaks
are not observed in the glassy samples.
[0056] Spectra 24 and 26 are both from glass-ceramic samples of
erbium-doped silica containing concentrations of SnO.sub.2
sufficient to form nanoclusters. Spectrum 24 has 1 mol % SnO.sub.2
and 0.3 mol % erbium; Spectrum 26 has 15 mol % SnO.sub.2 and 0.5
mol % erbium. Both spectra show the same peaks observed in Spectrum
22 from the erbium-doped SnO.sub.2, indicating that the erbium ions
are present within the SnO.sub.2 nanoclusters rather than dispersed
in the silica matrix.
[0057] The absence of erbium clustering is another advantageous
feature of the material. It is known that in silica, rare earth
ions, including erbium, tend to cluster when present in sufficient
concentrations [7]. The clustering reduces the excited state
lifetime of the ions, which is detrimental to the performance of a
gain medium component. Significant interaction between Er.sup.3+
ions has been observed in silica containing erbium concentrations
as low as 100 parts per million [8]. The undesirable reduction in
lifetime caused by the interactions is known as concentration
quenching. An effect of this is that the useful level of rare earth
doping in gain media is limited to below that at which clustering
occurs. In turn, this limits the minimum size of components to that
at which the desired quantity of rare earth ions can be
accommodated at a concentration that does not cause clustering.
[0058] However, in the examples of the present invention, high
concentrations of erbium are used, without clustering. Table 1
shows that erbium concentrations up to 1.67 mol % were used in the
samples. Our results indicate that concentrations of rare earth
elements up to at least 0.5 mol % can be incorporated without
evidence of detrimental clustering. The reason for this is not
fully understood. However, as a consequence, smaller devices can be
made. This has great potential for the production of
multifunctional integrated optical components in planar waveguide
technology, and for reducing the lengths of fiber need in
fiber-based devices. Rare earth concentrations above about 0.5 mol
% begin to give concentration quenching, so that device efficiency
is reduced. It is likely that concentrations of up to between 2 and
3 mol % can be tolerated before the lifetime is reduced to an
unsatisfactory level.
LASING PROPERTIES OF THE GAIN MEDIA
[0059] Measurements were made on samples of the material to
evaluate the lasing properties.
[0060] The lasing efficiency of erbium-doped gain media depends on,
among other things, the spontaneous emission probability A, and the
quantum efficiency .eta. of the
.sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2 transition of erbium ions
Er.sup.3+. These parameters depend on the interaction of the rare
earth ions with the host material. Measured optical absorption and
time resolved emission spectra can be used to calculate the
parameters. In fact, .eta.=.tau..sub.PL/.tau..sub.rad, where
.tau..sub.PL is the experimental lifetime of photoluminescence
recorded in the emission spectra, and .tau..sub.rad is the
radiative lifetime of the .sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2
transition, equal to 1/A.
[0061] The emission probability A can be calculated from the
oscillator strength P of the absorption transition
.sup.4I.sub.15/2.fwdarw..sup.4I.s- ub.13/2, since
A=[2.pi.n.sup.2e.sup.2/(m.sub.ec.epsilon..sub.0.lambda..sup- .2)]P,
where .epsilon. is the charge on an electron, me is the mass of an
electron, c is the speed of light in a vacuum, .epsilon..sub.0 is
the vacuum dielectric constant, and n is the refractive index of
the material at the wavelength .lambda.. P may be estimated from
measured experimental absorption spectra, by integrating the
absorption band at the wavelength of interest, and using the
relation P=4.318.times.10.sup.-9.intg..epsilon- .(.nu.)d.nu., where
.epsilon.(.nu.) is the molar extinction coefficient (1 mol.sup.-1
cm.sup.-1) at the wavenumber .nu. (cm.sup.-1).
[0062] FIG. 5 shows the measured optical absorption A of a number
of samples owing to the .sup.4I.sub.15/2.fwdarw..sup.4I.sub.13/2
transition of the erbium ions. Samples 5, 6, 7, 8 and 9 are shown.
The samples had thicknesses of 1.3 mm. Values of P for each sample
were estimated by integrating under these curves, and using the
formula given in the previous paragraph. In addition, the
photoluminescence lifetime .tau..sub.PL at 1.5 .mu.m was measured
by exciting the samples with an argon ion laser delivering 15 mW at
a wavelength of 514.5 nm.
[0063] Table 2 shows the values of .tau..sub.PL, .tau..sub.rad, P
and .eta. for three samples. Sample 4 was a glassy sample having no
SnO.sub.2 clusters, and Samples 5 and 8 were glass ceramics having
SnO.sub.2 clusters. The results show that .tau..sub.PL is smaller
for the glass ceramic samples than for the glassy sample. However,
.tau..sub.rad is also smaller for the glass ceramic samples, so
that the quantum efficiency .eta. of the glass ceramic samples is
high, and is comparable with the quantum efficiency of the glassy
sample.
2TABLE 2 Sample No. .tau..sub.PL (ms) .tau..sub.rad (ms) P
(.times.10.sup.6) .eta. (%) 4 12.0 22 0.74 55 5 7.4 17 0.93 44 8
5.7 11 1.42 52
[0064] Therefore, the lasing properties of the low phonon energy
glass ceramic materials are comparable with those of the
conventional glassy materials, so that the provision of a low
phonon host material of this type is not detrimental to lasing
efficiency.
[0065] The reliability of the estimated P values was tested by
comparing them with calculated values. The calculations were made
by fitting the intensity of observed Er.sup.3+ absorption
transitions from 1.5 .mu.m to 300 nm for the samples within the
so-called Judd-Ofelt formalism and Carnell matrices. Comparison of
the calculated values with the estimated values indicated that the
latter have an uncertainty of less than 20%.
DEVICE APPLICATIONS
[0066] FIG. 6 shows a 1470 nm band rare-earth doped optical fiber
amplifier. Pump radiation and a 1470 nm input signal are supplied
to respective input arms 112 and 110 of a silica fiber coupler, and
mixed in a fused region 114 of the silica coupler. A number of
pumping schemes may be used For example, the pump wavelength may be
800 nm, 1064 nm (from a Nd:YAG laser), 1047 nm (from a Nd:FLF
laser) or in the range 1210.+-.80 nm. The pump may also comprise
two different wavelengths and/or be applied to an arm 117 of the
coupler in a reverse pumping configuration. A portion of the mixed
pump and signal light is supplied by an output arm 116 of the
silica coupler to a section of SnO.sub.2-clustered Tm.sup.3+-doped
silica fiber 118 where it is amplified and output. Other rare-earth
dopants such as Er, Nd or Dy could also be used with an appropriate
choice of pump wavelength.
[0067] FIG. 7 is a schematic drawing of an optical fiber laser. A
laser cavity is defined by first and second Bragg gratings 120 and
130 optically written into optical fibers having cores of
photosensitive glass joined to a section of fiber gain medium 125
made out of the SnO.sub.2 cluster-containing SiO.sub.2 glass
ceramic, as described above.
[0068] FIG. 8 shows the Bragg grating 120/130 in more detail in
axial cross-section. The fiber comprises a core 124 of
photosensitive glass in which is optically written a grating
structure comprising a periodically modulated refractive index
profile, indicated schematically by the periodic shading 126 of the
core 124. The core is surrounded by a clad 122.
[0069] The gain medium 125 may be doped with Tm, Er or another
suitable rare earth element such as Nd or Yb. In operation, a pump
beam P is used to excite stimulated emission in the cavity to
produce a laser output of wavelength .lambda..sub.L. The pump
source may be integrated as a fiber laser, or may be a separate
device, such as a semiconductor diode source.
[0070] FIG. 9 shows a planar waveguide amplifier structure written
into co-planar layers 130a and 130b of glass. The waveguiding
channel may be written into the layers 130a and 130b by a
lithography method. The co-planar layers 130a and 130b are
deposited on a silica substrate 132. Layer 130a comprises the gain
medium described above, and includes a rare earth dopant, such as
Er or Tm. In the layer 130b, there is no rare earth dopant, and
only a silicate, such as germanosilicate or tin silicate, with
conventional refractive index. The waveguide stroke has the form of
a Y-branch to provide a signal input I/P and pump input PUMP in the
layer 130b, these inputs combining in to a common channel that
leads to the active medium layer 130a where amplification
occurs.
SUMMARY
[0071] In summary, it has been discovered that erbium is
preferentially attracted to tin oxide clusters in silica glass, so
that a silica-based gain medium can be produced in which the active
ions experience the phonon environment of tin oxide rather than
silica, thus reducing nonradiative recombination rates, as is
generally desirable. The effect appears to follow from the
coexistence of tin oxide clusters and a rare earth element. Any
rare earn element is believed to be suitable, namely La, Ce, Pr,
Nd, Pm, Sm, Fu, Gd, Th, Dy, Ho, Er, Tm, Yb, or Lu. Selection of the
rare earth element will made on the basis of matching the desired
optical application to the optical properties of relevant
transitions of the rare earth elements, as is usual.
[0072] Since the effect appears primarily to follow from the
coexistence of tin oxide clusters and a rare earth element, it is
also to be expected that a wide variety of glass matrices may be
possible, for example silicate glasses such as germanosilicate,
phosphosilicate or other glass hosts. Furthermore, other
crystalline materials are expected to yield similar effects as a
low phonon energy host for rare earth ions, provided that the
maximum phonon energy is sufficiently low and that it is possible
to control the clustering process on a nanometer scale inside the
glass host.
[0073] Other possible crystalline materials are the sesquioxides
Sc.sub.2O.sub.3, Y.sub.2O.sub.3, and Lu.sub.2O.sub.3. They are
suitable hosts for rare earth ions since the effective phonon
energies are around 400 to 600 cm.sup.-1 [9]. Moreover, TiO.sub.2,
ZrO.sub.2 and HfO.sub.2 possess sufficiently low phonon energy
values below 800 cm.sup.-1 [10, 11, 12]. All these oxides show low
solubility in silica and are expected to be suitable for the
production of glass ceramics. Nanostructured glass ceramics can be
obtained from these oxides by sol-gel techniques, similarly to tin
oxide silica glass ceramics, provided a substitutional xerogel is
produced and a thermally activated clustering of the crystalline
phase takes place. For instance, sol-gel reactions with TAOS may be
carried out for TiO.sub.2 and ZrO.sub.2 by means of suitable
heterosilsesquioxanes such as titaniumsilsesquioxane and
zirconiumsilsesquioxane. Other Ti or Zr compounds useful for
producing substitutionally doped xerogels are
tetrakis(trimethylsiloxy)titanium C.sub.12H.sub.36O.sub.4Si.sub.4Ti
and tetrakis(trimethylsiloxy)zirconium
C.sub.12H.sub.36O.sub.4Si.sub.4Zr. Other more commonly used
reagents might be employed in the sol-gel synthesis, as
metal-alkoxides, provided that suitable conditions are chosen to
avoid uncontrolled phase separation during the sol-gel transition.
This is important as glass ceramics commonly obtained from
metal-alkoxide precursors typically do not show good optical
properties owing to uncontrolled segregation.
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