|Publication number||USRE41438 E1|
|Application number||US 09/105,572|
|Publication date||Jul 13, 2010|
|Filing date||Jun 26, 1998|
|Priority date||Aug 12, 1991|
|Also published as||CA2057535A1, CA2057535C, DE69109672D1, DE69109672T2, EP0527265A1, EP0527265B1, US5131069|
|Publication number||09105572, 105572, US RE41438 E1, US RE41438E1, US-E1-RE41438, USRE41438 E1, USRE41438E1|
|Inventors||Douglas W. Hall, Mark A. Newhouse|
|Original Assignee||Oclaro Technology, Plc|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (4), Non-Patent Citations (2), Referenced by (1), Classifications (22), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to fiber amplifiers having means for selectively attenuating or removing unwanted wavelengths to modify or control the amplifier gain spectrum.
Doped optical fiber amplifiers consist of an optical fiber the core of which contains a dopant such as rare earth ions. Such an amplifier receives an optical signal of wavelength λs and a pump signal of wavelength λp which are combined by means such as one or more couplers located at one or both ends of the amplifier. The spectral gain of a fiber amplifier is not uniform through the entire emission band.
The ability to modify the gain spectrum of a fiber amplifier is useful. Three modifications are of interest: (1) gain flattening, (2) changing the gain slope, and (3) gain narrowing. Gain flattening is of interest for such applications as wavelength division multiplexing. A change in the gain slope can be used to reduce harmonic distortion in AM modulated optical systems (see A. Lidgard et al. “Generation and Cancellation of Second-Order Harmonic Distortion in Analog Optical Systems by Interferometric FM-AM Conversion” IEEE Phot. Tech. Lett., vol. 2, 1990, pp. 519-521) Gain narrowing is of interest because although the amplifier can be operated at wavelengths away from the peak gain without gain narrowing, disadvantages occur due to: increased spontaneous-spontaneous beat noise, a reduction in gain at the signal wavelength because of amplified spontaneous emission at a second wavelength (such as at 1050 nm in a Nd fiber amplifier designed to amplify at 1300 nm), and possible laser action at the peak gain wavelength.
Various techniques have been used for flattening the gain spectrum. An optical notch filter having a Lorentzian spectrum can be placed at the output of the erbium doped gain fiber to attenuate the narrow peak. A smooth gain spectrum can be obtained, but with no increase in gain at longer wavelengths.
Another filter arrangement is disclosed in the publication, M. Tachibana et al. “Gain-Shaped Erbium-Doped Fibre Amplifier (EDFA) with Broad Spectral Bandwidth”, Topical Meeting on Amplifiers and Their Applications, Optical Society of America, 1990 Technical Digest Series, Vol. 13, Aug. 6-8, 1990, pp. 44-47. An optical notch filter is incorporated in the middle of the amplifier by sandwiching a short length of amplifier fiber between a mechanical grating and a flat plate. This induces a resonant coupling at a particular wavelength between core mode and cladding leaky modes which are subsequently lost. Both the center wavelength and the strength of the filter can be tuned. The overall gain spectrum and saturation characteristics are modified to be nearly uniform over the entire 1530-1560 nm band. By incorporating the optical filter in the middle of the erbium doped fiber amplifier, the amplifier efficiency is improved for longer signal wavelengths.
An object of the present invention is to further improve the efficiency of a fiber amplifier and/or tailor the spectral output of a fiber amplifier.
The present invention relates to a fiber amplifier having spectral gain altering means. Fiber amplifiers conventionally comprise a gain optical fiber having a single-mode core containing gain ions capable of producing stimulated emission of light within a predetermined band of wavelengths including a wavelength λs when pumped with light of wavelength λp. Means are provided for introducing a signal of wavelength λs and pump light of wavelength λp into the gain fiber. In accordance with this invention, the fiber amplifier is provided with absorbing ion filtering means for attenuating light at at least some of the wavelengths within the predetermined band of wavelengths including the wavelength λs.
In accordance with a first aspect of the invention, the absorbing ion filtering means comprises unpumped gain ions; this embodiment requires means for preventing the excitation of the unpumped gain ions by light of wavelength λp. In accordance with a further aspect of the invention, the absorbing ions are different from the rare earth gain ions of gain fiber.
Fiber amplifiers typically include a gain fiber 10 (FIG. 1), the core of which is doped with gain ions that are capable of producing stimulated emission of light within a predetermined band of wavelengths including a wavelength λs when pumped with light of wavelength λp that is outside the predetermined band. A wavelength division multiplexer (WDM) fiber optic coupler 11 can be used for coupling pump energy of wavelength λp from laser diode 15 and the signal of wavelength λs from input telecommunication fiber 14 to gain fiber 10. Such devices are disclosed in U.S. Pat. Nos. 4,938,556, 4,941,726, 4,955,025 and 4,959,837. Fusion spheres are represented by large dots in the drawings. Input fiber 14 is spliced to coupler fiber 13, and gain fiber 10 is spliced to coupler fiber 12. Splice losses are minimized when coupler 11 is formed in accordance with the teachings of copending U.S. Patent Application Ser. No. 671,075 filed Mar. 18, 1991.
Various fiber fabrication techniques have been employed in the formation of rare earth-doped amplifying and absorbing optical fibers. A preferred process, which is described in copending U.S. Patent Application Ser. No. 07/715,348 filed June 14, 1991, is a modification of a process for forming standard telecommunication fiber preforms. In accordance with the teachings of that patent application, a porous core preform is immersed in a solution of a salt of the dopant dissolved in an organic solvent having no OH groups. The solvent is removed, and the porous glass preform is heat treated to consolidate it into a non-porous glassy body containing the dopant. The glassy body is provided with cladding glass to form a draw preform or blank that is drawn into an optical fiber. The process can be tailored so that it results in the formation of a fiber having the desired MFD. The porous core preform could consist soley of core glass, or it could consist of core glass to which some cladding glass has been added. By core glass is meant a relatively high refractive index glass, e.g. germania silicate glass, that will form the core of the resultant optical fiber.
If the rare earth ions are to extend to a region of the resultant fiber beyond the core, then the porous core preform that is immersed in dopant containing solvent must contain a central core glass region and a sufficiently thick layer of cladding glass. After the resultant doped, cladding-covered core preform has been consolidated, it is provided with additional cladding glass and drawn into a fiber.
If too much rare earth dopant is added to a GeO2-doped silica core, the core can crystallize. Such higher rare earth dopant levels can be achieved without crystallization of the core glass by adding Al2O3 to the core.
As indicated above, it is sometimes desirable to modify the gain spectrum of a fiber amplifier. Since the erbium-doped fiber amplifier has utility in communication systems operating at 1550 nm, that fiber amplifier is specifically discussed herein by way of example. The invention also applies to fiber amplifiers containing gain ions other than erbium, since the gain spectrum of such other fiber amplifiers can also be advantageously modified. As shown by curve 23 of
In accordance with the present invention, the amplifier spectral gain curve is altered by providing the fiber amplifier with filtering means 17 which includes absorbing ions that modify the gain spectrum by attenuating the amplified signal at various wavelengths in the gain spectrum. In accordance with a first aspect of the invention the absorbing ions are the same rare earth “gain ions” as the active gains ions in gain fiber 10; however, these absorbing gain ions must remain unpumped by light at wavelength [p. Such unpumped “gain ions” can be located in a fiber that is in series with gain fiber 10, or they can be distributed along the pumped gain fiber ions of gain fiber 10 but be located at a radius that is sufficiently greater than that of the pumped gain ions that they are substantially unpumped and yet influence the propagation of light of wavelength λs. This first aspect is further discussed in conjunction with
In accordance with a further aspect of the invention, the absorbing ions are different from the rare earth gain ions of gain fiber 10; such absorbing ions remain unexcited when subjected to light at wavelength λp. The absorbing ions can be positioned as follows: (a) they can be used to co-dope the gain fiber such that they are distributed along with the gain ions (optionally at the same radius as the gain ions), or (b) they can be incorporated into the core of a fiber that is connected in series with gain fiber 10. This further aspect is further discussed in conjunction with
In the figures discussed below, elements similar to those of
If fiber 10′ of
If gain ion-doped fiber 32 of
The performance of the gain-ion doped filtering fiber may be improved by quenching the Er fluorescence to minimize signal induced bleaching of the absorption. The Er fluorescence can be quenched by adding dopants such as B or OH to the fiber or by increasing the doping density of Er in the absorbing fiber, for example, to levels above 500 ppm in SiO2-GeO2 fibers.
Attenuating means 31 of
In the embodiment of
TABLE 1 Gain Wavelength Absorbing Ion Ion Signal Pump or Center Er 1.52-1.6 μm 980 nm Yb, Dy, Pr, V, CdSe Er 1.52-1.6 μm 1480 nm Pr, Sm Er 1.52-1.6 μm 800 nm Nd, Dy, Tm, V, CdSe Nd 1.25-1.35 μm 800 nm Dy, Er, Tm, V, CdSe Pr 1.25-1.35 μm 1000 nm Dy, Er, Yb, V,
Curves of absorptivity v. wavelength were used in selecting the rare earth ions and the transition metal (vanadium) ion. The CdSe should be present in the absorbing fiber in the form of micro crystallites.
The light attenuating fiber means of this invention is also useful in fiber amplifiers employing alternate pumping schemes. In the counter-pumping device of
In the dual-ended device of
The signal is first introduced into section 46a where it gradually increases in amplitude due to amplification in that section. The amplitude of the original that is introduced into section 46b is therefore much greater that that which was introduced into section 46a. The pump power is therefore absorbed at a greater rate per unit length in section 46b, and section 46b can be shorter than section 46a.
In the embodiment of
Gain fiber 62 of
In the embodiment of
The embodiment of
That aspect of the invention wherein the signal absorbing ions are different from the rare earth gain ions of the gain fiber is illustrated in
TABLE 2 Gain Ion Gain Wavelength Range Absorbing Ion Er 1.52-1.61 μm Pr, Sm Nd 1.25-1.35 μm (undesired Sm, Dy, Pr gain at 1050 nm) Pr 1.25-1.35 μm Sm, Dy, Nd
Curves of absorptivity v. wavelength were used in selecting the absorbing ions of Table 2.
During the fabrication of a preform for drawing a gain fiber that is co-doped with absorbing ions as well as active gain ions, the central region of the fiber is provided with a sufficient concentration of active gain ions to provide the desired amplification; it is also provided with a sufficient concentration of absorbing ions to attenuate the undesired portion or modify the gain spectrum. Such a fiber could be formed in accordance with the aforementioned U.S. Pat. Application Ser. No. 07/715,348 by immersing the porous core preform in a dopant solution containing salts of both the active dopant ion and the absorbing ion.
That embodiment wherein the absorbing ions are incorporated into the core of a fiber that is connected in series with gain fiber is shown in
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US4778237 *||Jun 7, 1984||Oct 18, 1988||The Board Of Trustees Of The Leland Stanford Junior University||Single-mode fiber optic saturable absorber|
|US5050949||Jun 22, 1990||Sep 24, 1991||At&T Bell Laboratories||Multi-stage optical fiber amplifier|
|US5245467 *||May 28, 1992||Sep 14, 1993||Pirelli Cavi S.P.A.||Amplifier with a samarium-erbium doped active fiber|
|US5257273 *||Feb 28, 1992||Oct 26, 1993||Gec-Marconi Limited||Amplifier/filter combination|
|1||"Introduction to Materials Science for Engineers" Shackelford, MacMillan Publishing, ©1985, pp. 340-342.|
|2||"Optics Guide 4", Melles Griot pp. 11-2-11-3.|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US20120248287 *||Mar 21, 2012||Oct 4, 2012||Fujitsu Limited||Optical amplification apparatus, method for controlling same, optical receiver station, and optical transmission system|
|U.S. Classification||385/142, 385/141, 372/9, 385/30, 372/6, 385/123, 372/19, 372/68|
|International Classification||H01S3/07, H01S3/067, H01S3/06, H01S3/094, H01S3/23, H01S3/17, H01S3/10, G02B6/00|
|Cooperative Classification||H01S2301/04, H01S3/10023, H01S3/06766, H01S3/1608, H01S3/06787|
|May 27, 2005||AS||Assignment|
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Effective date: 20050519
|Mar 20, 2007||AS||Assignment|
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|Jun 5, 2012||AS||Assignment|
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