Rare earth doped optical glasses

Details Rare earth doped optical glasses Rare earth doped optical glasses

1998-08-07

2002-03-12

Phan, James (Department: 2872)

Optical waveguides

Having particular optical characteristic modifying chemical...

Of waveguide core

C385S143000, C385S144000, C385S145000, C313S468000

Reexamination Certificate

active

06356699

ABSTRACT:

FIELD OF THE INVENTION
Glasses doped with rare earths, optical devices produced from such glasses, and methods of production.
BACKGROUND OF THE INVENTION
Optical fibers doped with a rare earth are used in producing lasers and amplifiers operative at various wavelengths. An optical signal can be amplified by ion fluorescence within the same operating wavelength region as the signal. Pump energy excites the rare earth metal causing it to fluoresce. This fluorescence, in a wavelength region in which an optical signal operates, amplifies the signal.
Different rare earth metals harmonize at different wavelengths. This makes it necessary to match a particular rare earth with a particular wavelength of interest. For example, excitation of erbium at 980 nm, or 1450 nm, provides pumped energy causing the erbium to fluoresce. Fluorescence in the 1520-1560 nm range allows a signal, operating in this wavelength region, to be amplified.
The significance of the 1550 nm wavelength in optical communication has led to extensive studies of erbium and its behavior as a rare earth dopant of glass. Other rare earth ions can be caused to fluoresce, and thereby amplify a signal, at their different characteristic wavelengths. Accordingly, while the present description is largely directed to erbium, it will be understood that the invention extends equally to other rare earth ions.
It is known that, in optical amplifiers, increased optical gain and decreased lifetime result from increasing the concentration of a fluorescing, rare earth ion. However, this is limited by clustering, a phenomenon that is occasioned by interactions between the same rare earth ions in close proximity to each other.
As the concentration of a rare earth ion, such as erbium, increases beyond a certain point in a glass, the fluorescence signal is quenched. This, in turn, decreases the lifetime of the important fluorescence transition, thereby decreasing the optical gain.
The dopant level of a rare earth ion, for example, erbium, in a glass is critical for controlling optical properties. The ions are raised from ground level to an excited state or level by energy pumped into the glass. The excited ions then undergo radiative decay while fluorescing. Optical signal amplification occurs by the stimulated emission of the signal photon in the excited state role of the ion.
The decay occurs in stages, referred to as fluorescing transitions, during which a proportion of the ion population decays to a given level. The initial level is at 36% of the maximum intensity, and is a reciprocal of the natural log “e.” The time required for this to occur is referred to as the lifetime. There are several levels of decay, and a lifetime for each may be measured, and reported, as e
1
, e
2
and e
3
. However, the lifetime of the first level is usually considered the important level.
Another undesirable effect of clustering is known as upconversion. This occurs when one rare earth ion is said to “steal” a signal from another ion. This is referred to as “sharing” fluorescence. The net result is that the thief goes to a higher energy level, but the victim goes to ground level. The desired population is thereby diminished.
It would, therefore, be highly desirable to provide a means of countering, or avoiding, this detrimental clustering of rare earth ions in glass. That would enable achieving the benefits obtainable with increased concentration of the rare earth in the glass.
It is a basic purpose of the invention to provide a solution to the problem of rare earth clustering in glass. Another purpose is to provide a method of increasing the concentration of a rare earth ion in a glass without quenching the fluorescence signal thereby obtainable. A further purpose is to provide an optical device with enhanced ability to amplify an optical signal and exhibit a longer fluorescence time. Still another purpose is to provide an improved amplifier for an optical signal. A still further purpose is to provide glasses having an enhanced ability to amplify an optical signal.
SUMMARY OF THE INVENTION
The article of the invention is a glass component in an optical system, the glass component comprising a silicate base glass doped with at least two oxides of Group III B elements, at least one of the elements being a rare earth.
The invention resides in part in a silicate glass containing at least two different rare earth/Group III B elements in its composition, at least one of the elements not having open shell “4f” orbitals.
The invention further resides in a method of decreasing clustering of a rare earth element in a silicate glass which comprises including in the glass composition at least one additional rare earth/Group III B element that does not have open shell “4f” orbitals.
The invention also resides in a method of producing a clad optical fiber which comprises forming the core of the fiber from a silicate glass which includes at least two rare earth/Group III B elements in it, composition, one of which does not have open shell “4f” orbitals.
PRIOR ART
Literature known to applicants, and deemed to have possible relevance, is provided in a separate document.
DESCRIPTION OF THE INVENTION
The present invention arose from studies directed at enhancing the amount of a rare earth element that could be included in a silicate glass while minimizing the tendency of such element to cluster in the glass. The problem of clustering is particularly severe in the production of clad optical fiber for devices such as optical amplifiers. However, it also finds application in planar devices, such as lasers and planar waveguides, as well as in MCVD, plasma, or axial vapor deposition processes.
The invention is founded on the discovery that the tendency of a rare earth to cluster can be counteracted by including a second rare earth in the glass composition. This expedient permits increasing the concentration of a desired rare earth metal in a glass while minimizing the loss of optical activity due to clustering of the rare earth in the glass.
The cause of the rare earth clustering is not known with certainty. However, a logical explanation, based on present evidence, is that rare earth ions tend to bond together, rather than dispersing in the glass. This could occur if there were a lack of bonding sites for the rare earth ions in the glass. It also suggests a glass structure such that the rare earth ions are incompatible with other glass ions due to differences in ionic size and charge. This is because the “4f” orbitals are so deeply buried that the electronic configuration is not likely to have any appreciable influence on mutual solubilities.
TABLE I sets forth the crystal ionic radii of several rare earths and transition metal ions of Group III B elements.
TABLE I
Ion
Ionic Radius
Ion
Ionic Radius
La
1.06
Dy
0.90
Ce
1.03
Ho
0.89
Pr
1.01
Y
0.89
Nd
1.00
Er
0.88
Sm
0.96
Tm
0.87
Eu
0.95
Yb
0.86
Gd
0.94
Lu
0.85
Tb
0.92
Sc
0.73
The crystal ionic radii of Si
+4
and Ge
+4
, principal ions employed in optical fiber production, are 0.41 and 0.53 Angstrom units. As shown in Table I, the crystal ionic radii of rare earth ions are in the range of 0.8-1.1 Angstroms. This substantial difference in ionic radii strongly suggests why rare earth ions do not disperse well in simple glasses, such as silica and silica-germania glasses.
It is believed that the addition of a second Group IIII B ion provides compatible bonding sites for the initial ion. This reduces the tendency for the initial ion to cluster in a glass. By providing equal concentrations of the two Group IIII B ions, clustering could be reduced by as much as 50%.
The second ion must not have open shell “4f” orbitals. Further, if the glass is being used for a fluorescing effect, as in an amplifier, the second ion must have no interaction with the pump wavelength, or the fluorescence, of the initial ion. For example, by applying these criteria, candidates for the second ion in a glass system doped with erbium, or another rare earth, are yttrium (Y), lanthanum (La), lutetium (Lu), gadolinium (Gd), and europium (Eu). Y, La,

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