Erbium and ytterbium co-doped phosphate glass optical fiber...

Details Erbium and ytterbium co-doped phosphate glass optical fiber... Erbium and ytterbium co-doped phosphate glass optical fiber...

2000-06-09

2003-08-26

Black, Thomas G. (Department: 3663)

Optical: systems and elements

Optical amplifier

Optical fiber

Reexamination Certificate

active

06611372

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention is related to optical amplifiers, more particularly, to rare-earth doped phosphate optical fibers for amplifiers pumped by diode or solid state laser sources. The rare-earth ions, erbium and ytterbium, are co-doped at high levels to provide for extremely high gain. Erbium ion concentration is increased to provide very high active ion content. Ytterbium ion concentration is increased to provide efficient absorption of pump power. The phosphate glass fiber provides for a low-loss and stable fiber host. Phosphate glass fibers can be produced with a temperature coefficient of refractive index close to zero.
BACKGROUND OF THE INVENTION
Deregulation, long distance price declines, bandwidth stockpiling, and internet usage are driving bandwidth demand in telecom and datacom networks. Data traffic is now growing at 100 percent or more per annum, straining global fiber capacity. Dense Wavelength Division Multiplexing (DWDM), where multiple wavelength channels propagate within a single fiber multiplies fiber capacity by 2-128 times or more, is an approach for extending carrier capacity without the need of deploying new fiber. Systems being deployed today generally can transmit from 8 up to 128 channels in the 1550 nm low-dispersion window. Channel spacing ranges from 1.6 nm (200 GHz) to 0.4 nm (50 GHz).
Optical amplifiers are considered enabling components for bandwidth expansion in DWDM fiber optic communications systems. In particular, silica glass Erbium Doped Fiber Amplifiers (EDFA) exhibit many desirable attributes including high gain, low noise, negligible crosstalk and intermodulation distortion, bit-rate transparency, and polarization insensitive gain. These properties make optical fiber amplifiers superior to semiconductor devices as amplifiers in fiber optic systems. Moreover, fiber-based amplifiers do not require conversion from electrical energy to photon energy as do semiconductor devices. In a communications system of any significant size, there is typically a distribution network that includes long communication paths and nodes where the network branches. In such a network, amplifiers are required in order to maintain the amplitude of the signal and the integrity of any data in route between a source and destination. For these amplifiers to function properly, the amplifiers must exhibit high small signal gains and/or high output saturation powers.
Application of erbium-doped optical fibers as amplifiers has received considerable attention recently because the characteristic gain bandwidth of these fibers is within the telecommunications window of 1.5 &mgr;m commonly used in fiber optic communications systems. Since the announcement of a single mode Er
3+
doped fiber amplifier (EDFA) in 1987 at the University of Southampton, enormous research has been performed, and more than 400 U.S. patents have been issued in fiber amplifiers. To date, all erbium fiber amplifiers use erbium doped silica fibers more than one meter long to achieve greater than 20 dB gain near the 1.54 &mgr;m range. More commonly, the length of the erbium doped silica fiber is approximately 10 to 20 meters. Such lengths are not practical for assembly into integrated optical components. There is a compelling need for amplifiers that can introduce high gain into an integrated, compact package.
To shorten length in fiber amplifiers, high gain must be achieved. In order to enable fiber amplifiers of only a few centimeters in length, magnitudes of doping two orders higher than what is commercially achievable (~10
18
cm
−3
) in silica fiber amplifiers is required. However, in silica fiber, cooperative upconversion and ion clustering effects develop from the interactions between nearby ions in silica glass, and electrons depopulate from the erbium metastable level (
4
I
13/2
). Thus, increased doping in silica glass does not improve gain.
Other glasses such as for example phosphate glasses exhibit high solubility and large emission cross sections for many rare-earth ions. Phosphate glasses for optical components have been investigated. Y. L. Lu, Y. et al., in “Fluorescence and attenuation properties of Er
+3
-doped phosphate glass fibers and efficient infrared-to-visible up-conversion,”
Applied Physics B
, Vol. 62, pp.287-291 (1996) and Ya Lin Lu et al., in “Properties of Er
+3
doped phosphate glasses and glass fibers and efficient infrared to visible upconversion,”
Journal of Materials Science
, Vol. 30, No. 22, Nov. 15, 1995, pp.5705-10, (1995) discuss phosphate glass fiber for use in up-conversion schemes. S. Jiang et al., in “Er
+3
doped phosphate glasses and lasers,”
Journal of Non Crystalline Solids
, Vol.239, No. 1-3, October 1998, pp. 143-8, show phosphate glasses for application as bulk lasers. T. Nishi et al., in “The amplification properties of a highly Er
+3
doped phosphate fiber,”
Jpn. J Appl. Phys
., Vol. 31 (1992), Pt. 2, 2B, pp. L177-L179, show phosphate fiber with moderate erbium oxide doping. The maximum gain per unit length reported by Nishi et al. was only 1 dB/cm. S. Jiang, T. Luo et al. in “New Er 3+ doped phosphate glass for ion-exchanged waveguide amplifiers,”
Optical Engineering
, Vol. 37, No. 12, December 1998, pp. 3282-6, disclose phosphate glasses for application in ion-exchanged waveguide amplifiers.
In addition, a number of patents have addressed doped glasses in various optical applications. For example, Hsu et al. (U.S. Pat. No. 5,425,039), Myers (U.S. Pat. No. 4,962,067), Myers et al. (U.S. Pat. No. 4,333,848), Myers et al. (U.S. Pat. No. 4,248,732), Myers et al. (U.S. Pat. No. 4,075,120), each disclose doped fibers for application as fiber lasers. In addition, Myers et al. (U.S. Pat. No. 5,322,820) and Myers (U.S. Pat. No. 5,164,343) disclose various glass compositions for laser applications. Grubb et al. (U.S. Pat. No. 5,225,925) disclose silica fibers or phosphorous doped silica fiber. Andrews et al. (U.S. Pat. No. 4,962,995) disclose glasses that are optimized for pumping by 800 nm laser light.
Recently, Y. C. Yan et al., in “Net optical gain at 1.53 &mgr;m in an Er-doped phosphate glass waveguide on silicon,”
Optical Amplifiers and Their Applications, Topical Meeting. OSA Trends in Optics and Photonics Series
, Vol. 16. Opt. Soc. America, Washington, D.C., USA; 1997; xlv+526, pp.93-5, investigated doped phosphate glasses as a high gain medium for planar waveguide amplifiers at wavelength of 1.5 &mgr;m. Y. C. Yan et al., in “Erbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 &mgr;m,”
Applied Physics Letters
, Vol.71, No. 20, Nov. 17, 1997, pp. 2922-4 reported a gain of 4.1 dB in a 1 cm long phosphate glass waveguide prepared by an R-F sputtering technique. D. Barbier et al., in “Net gain of 27 dB with a 8.6-cm-long Er/Yb-doped glass-planar-amplifier,”
OFC
'98
Optical Fiber Communication Conference and Exhibit, Technical Digest
, Conference Edition 1998 OSA Technical Digest Series Vol.2 (IEEE Cat. No.98CH36177), Opt. Soc. America, Washington, D.C., USA; 1998; vii+421, pp.45-6, demonstrated a net gain of 27 dB in a 8.6 cm long ion-exchanged Er/Yb-doped phosphate glass waveguide.
Despite the high gains achieved for example in phosphate glass waveguides, planar waveguide amplifiers have significant disadvantages when compared with fiber amplifiers including polarization sensitivity, optical mode mismatch between waveguides and fiber networks, large propagation losses, and complicated fabrication processes.
SUMMARY OF THE INVENTION
One object of the invention is to provide a phosphate glass optical fiber amplifier with a gain per unit length, greater than 1.5 dB/cm and preferable over 3 dB/cm.
Another object of the invention is to provide a high gain per unit length doped phosphate glass fiber which can be utilized as a fiber amplifier in an optical communications system.
Another object of the invention is to provide an erbium and ytterbium codoped phosphate glass fiber with high (concentrations well above concentrations deeme

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