Tellurite glass, optical amplifier, and light source

Details Tellurite glass, optical amplifier, and light source Tellurite glass, optical amplifier, and light source

2000-11-14

2002-03-12

Hellner, Mark (Department: 3662)

Optical: systems and elements

Optical amplifier

Optical fiber

C372S006000

Reexamination Certificate

active

06356387

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tellurite glass as a glass material for an optical fiber and an optical waveguide, and in particular a broadband optical amplification medium using the tellurite glass which is capable of working even at wavelengths of 1.5 &mgr;m to 1.7 &mgr;m. The present invention also relates to a broadband optical amplifier and a laser device using the broad band optical amplification medium. Furthermore, the present invention relates to a method of splicing a non-silica-based optical fiber and a silica-based optical fiber reliably with the characteristics of low fiber-loss and low reflection.
2. Description of the Related Art
The technology of wavelength division multiplexing (WDM) has been studied and developed for expanding transmission volume of optical communication systems and functionally improving such systems. The WDM is responsible for combining a plurality of optical signals and transmitting a combined signal through a single optical fiber. In addition, the WDM is reversibly responsible for dividing a combined signal passing through a single optical fiber into a plurality of optical signals for every wavelength. This kind of transmitting technology requires a transit amplification just as is the case with the conventional one according to the distance of transmitting a plurality of optical signals of different wavelengths through a single optical fiber. Thus, the need for an optical amplifier having a broad amplification waveband arises from the demands for increasing the optical signal's wavelength and the transmission volume. The wavelengths of 1.61 &mgr;m to 1.66 &mgr;m have been considered as appropriate for conserving and monitoring an optical system, so that it is desirable to develop an optical source and an optical amplifier for that system.
In recent years, there has been considerable work devoted to research and development on optical fiber amplifiers that comprise optical fibers as optical amplification materials, such as erbium (Er) doped optical fiber amplifiers (EDFAs), with increasing applications to the field of optical communication system. The EDFA works at a wavelength of 1.5 &mgr;m where a loss of silica-based optical fiber decreases to a minimum, and also it is known for its excellent characteristics of high gain of 30 dB or more, low noise, broad gain-bandwidth, no dependence on polarized waves, and high saturation power.
As described above, one of the remarkable facts to be required of applying the above EDFA to the WDM transmission is that the amplification waveband is broad. Up to now, a fluoride EDFA using a fluoride glass as a host of an erbium-doped optical fiber amplifier has been developed as a broad amplification band EDFA.
In U.S. Pat. Nos. 3,836,868, 3,836,871, and 3,883,357, Cooley et al. discloses the possibility of laser oscillation to be caused by tellurite glass containing an rare earth element. In this case, however, Cooley et al. have no idea of forming tellurite glass into an optical fiber because there is no description concerned about the adjustment of refractive index and the thermal stability of tellurite glass to be required for that formation.
In U.S. Pat. No. 5,251,062, Snitzer et al. insists that tellurite glass play an important role in extending the EDFA's amplification band and it should be formed into a fiber which is absolutely essential to induction of an optical amplification. Thus, they disclose the allowable percent ranges of ingredients in tellurite-glass composition in a concretive manner. The tellurite-glass composition includes a rare earth element as an optically active element and can be formed into a fiber. More specifically, the tellurite-glass composition of Snitzer et al. is a ternary composition comprising TeO
2
, R
2
O, and QO where R denotes a monovalent metal except Li and Q denotes a divalent metal. The reason why Li is excluded as the monovalent metal is that Li depresses thermal stability of the tellurite-glass composition.
In U.S. Pat. No. 5,251,062, furthermore, Snitzer et al. make a comparative study of fluorescence erbium spectra of silica and tellurite glass and find that the tellurite glass shows a broader erbium spectrum compared with that of the silica glass. They conclude that the ternary tellurite glass composition may allow a broadband amplification of EDFA and an optically active material such as praseodymium or neodymium may be added in that composition for inducing an optical amplification. In this patent document, however, there is no concrete description of the properties of gain, pump wavelength, signal wavelength, and the like which is important evidence to show that the optical amplification was actually down. In other words, U.S. Pat. No. 5,251,062 merely indicate the percent ranges of ingredients of ternary tellurite glass composition that can be used in an optical fiber.
Furthermore, Snitzer et al. show that thermal and optical features of various kinds of tellurite glass except of those described in U.S. Pat. No. 5,251,062 in a technical literature (Wang et al., Optical Materials, vol. 3 pages 187-203, 1994; hereinafter simply referred as “Optical Materials”). In this literature, however, there is also no concrete description of optical amplification and laser oscillation.
In another technical literature (J. S. Wang et al., Optics Letters, vol. 19 pages 1448-1449, 1994; hereinafter simply referred as “Optics Letters”) published in right after the literature mentioned above, Snitzer et al. show the laser oscillation for the first time caused by using a single mode optical fiber of neodymium-doped tellurite glass. The single mode fiber comprises a core having the composition of 76.9% TeO
2
-6.0% Na
2
O-15.5% ZnO-1.5% Bi
2
O
3
-0.1% Nd
2
O
3
and a clad having the composition of 75% TeO
2
-5.0% Na
2
O-20.0% ZnO and allows 1,061 nm laser oscillation by 81 nm pumping. In this literature, there is no description of a fiber loss. In Optical Materials, on the other hand, there is a description of which the loss for an optical fiber having a core composition of Nd
2
O
3
-77% TeO
2
-6.0% Na
2
O-15.5% ZnO-1.5% Bi
2
O
3
and a clad composition of 75% TeO
2
-5.0% Na
2
O-20.0% ZnO (it is deemed to be almost the same composition as that of Optics Letters) is 1500 dB/km at a wavelength of 1.55 &mgr;m (see
FIG. 1
that illustrates a comparison between
4
I
13/2
to
4
I
15/2
Er
3+
emission in tellurite glass and
4
I
13/2
to
4
I
15/2
Er
3+
emission in fluoride glass). The core composition of this optical fiber is different from that of a ternary composition disclosed in U.S. Pat. No. 5,251,062 because the former includes Bi
2
O
3
. It is noted that there is no description or teach of thermal stability of Bi
2
O
3
-contained glass composition in the descriptions of Optics Letters, Optical Materials, and U.S. Pat. No. 5,251,062 mentioned above.
However, the fluoride based EDFA has an amplification band of about 30 nm which is not enough to extend an amplification band of optical fiber amplifier for the purpose of extending the band of WDM.
As described above, tellurite glass shows a comparatively broader fluorescence spectral band width, so that there is a possibility to extend the amplification band if the EDFA uses the tellurite glass as its host. In addition, the possibility of producing a ternary system optical fiber using the composition of TeO
2
, R
2
O, and QO (wherein R is a univalent metal except Li and Q is a divalent atom) has been realized , so that laser oscillation at a wavelength of 1061 nm by a neodymium-doped single mode optical fiber mainly comprising the above composition has been attained. In contrast, EDFA using tellurite glass is not yet realized. Therefore, we will describe the challenge to realize a tellurite-based EDFA in the following.
First, the difference between the objective EDFA and the neodymium-doped fiber laser (i.e., the difference between 1.5 &mgr;m band emission of erbium and 1.06 &mgr;m band emission of neodymium in glass) should be described in det

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