Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-09-20
2002-11-19
Tarcza, Thomas H. (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
Reexamination Certificate
active
06483632
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to optical amplifier modules in which homogeneous gain broadening is dominant, such as modules incorporating optically pumped rare-earth doped optical waveguide amplifiers.
Such optical amplifiers have an optical gain spectrum that is liable to depart significantly from flatness over the whole of its spectral range, not merely at the extremities of that range. For any particular example of such an amplifier, the shape of the gain spectrum is not uniquely defined, but changes significantly when the magnitude of the gain exhibited at any particular wavelength within its gain spectrum is changed. In an optical communication system using multiple wavelengths for the transmission of data, the gain spectrum of the amplifier results in different signal channels having a different optical gain. Whilst small differences in gain may be tolerable, significant differences in gain are common causing degradation to some of the optical signal channels. In principle these differences can be cancelled out to some degree using selective optical filtering. However, when the operating gain of the amplifier changes due to a change in the incoming optical signal power, or the demanded optical output power, the resulting change in amplifier gain causes a change in the gain spectrum shape.
Specifically, at low gain, the spectral gain characteristic shows a slight rounded peak at the short wavelength end of the characteristic spaced by a shallow trough from a much broader, and slightly higher, peak extending to the long wavelength end of the characteristic, this broader peak having a slight upward tilt towards its long wavelength end As the gain is increased, the amplitude of the broad peak increases unevenly, with the result that its tilt is first evened out, and end. As the gain is increased, the amplitude of the broad peak increases unevenly, with the result that its tilt is first evened out, and then reversed. At the same time the amplitude of the short wavelength peak increases at a faster rate, overtaking the gain of the broad peak, and also broadening slightly to longer wavelengths. This change in the spectral gain characteristic consequent upon change of gain level is referred to as dynamic gain tilt. Because the effect is ‘dynamic’ it can not be compensated by optical filtering that is entirely passive. If dynamic gain tilt is present ,and is not compensated, then different signal channels of a wavelength division multiplexed (WDM) transmission system will be subject to different differential gain under different conditions, with the result that some channels may suffer significant degradation limiting system performance.
SUMMARY OF THE INVENTION
The present invention is directed to the circumvention of the problem of dynamic gain tilt.
According to a first aspect of the present invention there is provided a variable gain amplifier module having a gain spectrum in which homogeneous broadening is dominant, which amplifier module includes at least two optical gain-providing regions and at least one loss-providing region, wherein the loss provided by said at least one loss-providing region is substantially uniform across the gain spectrum and is electrically variable, and wherein the amplifier module includes gain regulation means adapted to maintain substantially constant the spectral gain characteristic of the amplifier module by maintaining substantially constant, at a selected wavelength within the gain spectrum of the amplifier module, the aggregate gain of all the gain-providing regions of the amplifier module.
One preferred form of module has two gain-providing regions with a variable loss-providing region optically between them.
According to a second aspect of the present invention there is provided a method of operating a variable gain amplifier module having a gain spectrum in which homogeneous broadening is dominant, and which amplifier module includes at least two optical gain-providing regions and at least one loss-providing region, wherein the loss provided by said at least one loss-providing region is substantially uniform across the gain spectrum and is electrically variable, in which method of operating the amplifier module substantial invariance of the spectral gain characteristic of the amplifier module is maintained by maintaining substantially constant, at a selected wavelength within the gain spectrum of the amplifier module, the aggregate gain of all the gain-providing regions of the amplifier module.
According to a third aspect of the present invention there is provided, in a wavelength division multiplexed transmission system, a method of amplification by an amplifier module of optical wavelength division multiplexed traffic transmitted along a transmission path from a transmitter to a receiver, in which module the traffic is operated on by the module in succession first to amplify it for a first time using a first fixed gain optical waveguide optical amplifier, next to attenuate it by an amount that is variable, and then to amplify it for a second time using a second fixed gain rare-earth doped optical waveguide optical amplifier, wherein the first and second amplifiers are amplifiers that, under variable gain operating conditions, exhibit gain tilt.
According to a fourth aspect of the present invention there is provided a wavelength division multiplexed transmission system having a concatenation of optical amplifier modules in a transmission path between a transmitter and a receiver, each of which amplifier modules of the concatenation is a variable gain amplifier module having a gain spectrum in which homogeneous broadening is dominant, each containing at least one optical gain-providing region and at least one loss-providing region, wherein the loss provided by said at least one loss-providing region is substantially uniform across the gain spectrum and is electrically variable, wherein the concatenation includes gain regulation means adapted to maintain, under changes in the partitioning of the gain between the individual amplifier modules of the concatenation, substantially constant the spectral gain characteristic of the concatenation by maintaining substantially constant, at a selected wavelength within the gain spectrum of the amplifier modules, the aggregate gain of all the optical gain-providing regions of the concatenation.
According to a fifth aspect of the present invention there is provided, in a wavelength division multiplexed transmission system having an optical transmitter optically coupled with an optical receiver via a transmission path that includes a concatenation of optical amplifier modules having a gain spectrum in which homogeneous broadening is dominant, each containing at least one optical gain-providing region and at least one loss-providing region that provides a loss that is substantially uniform across the gain spectrum of the gain-providing regions and is electrically variable, a method of operating the concatenation to provide it with a substantially invariant spectral gain characteristic by regulating the gain, at a selected wavelength within the gain spectrum of the concatenation, of the amplifier modules of the concatenation so as to maintain, under changes in the partitioning of the gain between the individual amplifier modules of the concatenation, the aggregate gain of all the optical gain-providing regions of the concatenation.
REFERENCES:
patent: 5050949 (1991-09-01), DiGiovanni et al.
patent: 5115338 (1992-05-01), DiGiovanni et al.
patent: 5764406 (1998-06-01), Newhouse et al.
patent: 5900969 (1999-05-01), Srivastava et al.
patent: 6049413 (2000-04-01), Taylor et al.
patent: 6055092 (2000-04-01), Sugaya et al.
patent: 6157481 (2000-12-01), Sugaya et al.
patent: 6172803 (2001-01-01), Masuda et al.
patent: 6198572 (2001-03-01), Sugaya et al.
patent: 6215584 (2001-04-01), Yang et al.
patent: 2 310 094 (1997-08-01), None
patent: 08248455 (1996-09-01), None
patent: 09191303 (1997-07-01), None
Dentai et al., “Electrically Tunable Semiconductor Fabry-Perot Filter”, IEEE Photonics T
Davis Fiona
Epworth Richard Edward
Jolley Nigel Edward
Hughes Deandra M.
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Tarcza Thomas H.
LandOfFree
Multistage optical amplifier with gain flattening does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multistage optical amplifier with gain flattening, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multistage optical amplifier with gain flattening will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2933512