Fluorinated rare earth doped glass and glass-ceramic articles

Details Fluorinated rare earth doped glass and glass-ceramic articles Fluorinated rare earth doped glass and glass-ceramic articles

1998-05-19

2001-03-20

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Having particular optical characteristic modifying chemical...

Of waveguide core

C385S144000, C501S037000, C501S040000

Reexamination Certificate

active

06205281

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a fluorinated, rare earth doped glass composition that exhibits reduced rare earth ion clustering and fluorescence quenching in the presence of relatively high concentrations of rare earth dopants, and to ultratransparent glass-ceramic articles, particularly active fiber waveguides and devices, e.g., fiber amplifiers and lasers, that utilize such waveguides. As used herein, the term “ultratransparent” refers to high optical clarity; i.e., the transparency of the glass-ceramic of the invention compares to the transparency of glass over the spectrum from about 350 nm to 2.5&mgr;.
BACKGROUND OF THE INVENTION
There is widespread interest in, and industrial need for, optical compositions and articles made therefrom having potential applications in the 1300 nm and 1550 nm telecommunications windows. Promising candidates for an efficient 1300 nm fiber optical amplifier material, for example, have included the rare earth ions Pr
3+
and Dy
3+
doped in fluoride, mixed halide, and sulfide glass hosts, while 1550 nm amplifier materials are suitably doped with Er
3+
. A recent publication by Borrelli et al.,
Transparent glass ceramics for
1300
nm amplifier applications
, J. Appl. Phys. 78 (11), (Sept. 1995), reported an alternative host for the Pr
3+
ion which combines some of the advantages of both fluoride and oxide glasses. The new material is described in the '505 patent and consists of an oxyfluoride glass which has been appropriately heat treated to form a transparent glass-ceramic. This glass-ceramic contained 5-40 volume % fluoride nanocrystals having diameters ranging between about 6-15 nm, embedded in a primarily oxide glass matrix. As described in detail in the '505 patent, optically active fluoride based glass-ceramic articles were produced from Yb-free compositions that included between about 50 to 900 ppmw Pr
3+
. The glass-ceramic was shown to perform as an active device in the 1300 nm spectral window over this dopant concentration range. For the Pr
3+
doped glass-ceramics, fluorescence lifetimes greater than 120 microseconds were observed in the base glasses of the '505 patent at Pr
3+
concentrations up to about 500 ppmw. Concentration quenching was observed at Pr
3+
concentrations slightly above 500 ppmw, and fluorescence lifetimes were observed to decrease approximately linearly to about 70 microseconds at 900 ppmw. It was reported that a best case balance between fluorescence lifetime and concentration was achieved with Pr
3+
in the range of about 200 to 550 ppmw; however, functional active devices were reported with Pr
3+
concentrations in the range of about 50 to 650 ppmw. Since both longer fluorescence lifetimes and higher dopant concentrations are desirable for the production of active devices as described herein, the inventors recognized a need to improve upon the compositional ranges of the new glass-ceramic material described in the '505 patent, and to devise glass-ceramic compositions having similar advantages suitable for 1550 nm applications.
The radiative quantum efficiency is a key parameter in evaluating transparent glass-ceramics as a potential gain medium for fiber lasers and amplifiers. Quimby and Tick, in an article entitled
Quantum efficiency of Pr
3+
doped transparent glass
-
ceramics
(to be published) report on the quantum efficiency of the 1300 nm emission in Pr
3+
doped transparent glass-ceramics using a direct measurement technique based upon relative fluorescence measurements. Fluorescence was observed by exciting the Pr
3+
1
D
2
level, peaking at around 1460 nm (the “A” transition), and the fluorescence from the
1
G
4
level, peaking at about 1300 nm (the “B” transition), when the
1
D
2
level was directly excited with 595 nm dye laser radiation. Following the analysis described by Quimby et al.,
Opt. Lett.,
20, 2021 (1995), the quantum efficiency of the
1
G
4
1300 nm emission was determined by taking the ratio of the total B transition rate to the total A transition rate. The data in
FIG. 1
, to be described in more detail below, shows the measured B/A ratio for exemplary embodiments of the two base composition glass-ceramics of the '505 patent having Pr
3+
concentrations ranging from about 25 ppmw to 1000 ppmw. As expected by the inventors, the B/A ratio increases with increasing concentration which they believe to be due to the effect of cross-relaxation resulting from increased Pr
3+
ion clustering.
It is known that when a trivalent rare earth, e.g., Pr
3+
, is incorporated into these glass-ceramics, the rare earth is segregated into the second phase crystals which are formed during the ceraming process. These crystals have a cubic lattice structure and are thought to be comprised of mostly divalent cadmium- and lead-fluoride. The inventors believe that clustering arises from local strains that are established within the lattice because of the substitution of trivalent rare earth fluorides for the divalent fluorides. When direct substitution of a rare earth into the crystal lattice occurs, charge balance can be maintained by incorporating an interstitial fluorine into the crystal structure near the rare earth. In bulk crystals this is the source of the local strain, which is observed to decrease when these defects can cluster. The inventors believe that a similar mechanism occurs in the nanocrystals of the glass-ceramic. This, however, results in a decrease in the quantum efficiency at higher concentrations of Pr, observed by the authors to appear at concentrations of about 500 ppmw.
The inventors have therefore recognized a need for transparent rare earth doped glass and glass-ceramic compositions and articles made therefrom in which rare earth ion clustering and concentration quenching are reduced notwithstanding high rare earth dopant concentrations, which have a relatively high quantum efficiency, and a wider spectral gain band.
SUMMARY OF THE INVENTION
Accordingly, the invention is directed to a glass-ceramic optical article having a glass composition providing such features, and to a method for making glass-ceramic optical fiber waveguide articles having core and cladding compositions as described herein.
An embodiment of the invention is directed to a glass-ceramic optical article. The compositional structure of the glass-ceramic article is a second phase cubic lattice substantially including either divalent cadmium fluoride or divalent lead fluoride having a trivalent rare earth ion incorporated therein, and including a monovalent silver or monovalent thallium ion to charge balance the crystal.
Another embodiment of the present invention that is particularly suitable for applications in the 1300 nm telecommunications window describes a glass-ceramic optical article including an active core that is a transparent glass-ceramic having substantially only one crystal phase, consisting essentially, in terms of cation percent, of: SiO
2
20-40; AlO
1.5
10-20; CdF
2
19-34; PbF
2
19-23; wherein up to 5 mole % of CdS or 3 mole % of CdCl
2
can be substituted for an equivalent amount of CdF
2
, or an equivalent amount of an oxide can be substituted for the fluoride; and including at least one of the rare earth fluorides YF
3
(3-7), GdF
3
(3-7), and LuF
3
(4-15) wherein the total amount of these rare earth fluorides is (3-15); including at least one of Pr
3+
and Dy
3+
at a concentration in the range of about 300 to 2,000 ppmw; and including Ag
+
at a concentration in the range of about 500 to 2,000 ppmw; and a cladding that is a transparent glass, consisting essentially, in weight percent on an oxide basis, of: SiO
2
25-35; Al
2
O
3
3-5; CdF
2
12-16; PbF
2
40-50; ZnF
2
4-8; and Bi
2
O
3
0-10.
Another embodiment of the present invention that is particularly suitable for applications in the 1300 nm telecommunications window describes a glass-ceramic optical article including an active core that is a transparent glass-ceramic, having substantial

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