Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-06-29
2001-11-20
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200, C359S341430, C372S006000, C372S034000
Reexamination Certificate
active
06320693
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical amplifiers used in lightwave transmission systems. More particularly, the invention relates to controlling variations in the gain spectrum of an optical amplifier as a result of changes in the amplifier's operating conditions.
OPTICAL AMPLIFIERS
The basic elements of a communication system are a transmitter, a receiver, and a transmission medium. Optical fibers are today the transmission medium of choice for sending voice, video, and data signals over long distances. Although modern fibers have very low losses per unit length, long fiber spans, e.g., cables extending from one city to another, require periodic amplification of the transmitted signal to ensure accurate reception at the receiver.
Erbium doped fiber amplifiers have been developed to satisfy this need for signal amplification. Such amplifiers consist of a length of optical waveguide fiber, e.g., 5 to 30 meters of fiber, which has been doped with erbium. The quantum mechanical structure of erbium ions in a glass matrix allows for stimulated emission in the ~1500 to ~1600 nanometer range, which is one of the ranges in which optical waveguide fibers composed of silica exhibit low loss. As a result of such stimulated emission, a weak input signal can achieve more than a hundred fold amplification as it passes through a fiber amplifier.
To achieve such stimulated emission, the erbium ions must be pumped into an excited electronic state. Such pumping can take place in various pump bands, the most effective of which include those having midpoint wavelengths of ~980 nanometers and ~1480 nanometers. Efficient semiconductor laser sources are available for both of these pump bands. As would be expected, trade-offs exist between these pump bands, with the 980 band providing lower noise in the amplified signal and the 1480 band providing a lower propagation loss for the pump light, which is of value when remote pumping is to be performed.
Although stimulated emission occurs throughout the 1500 to 1600 nanometer range, the amount of amplification achieved is not uniform throughout this range. As a result, optical amplifiers have a “gain spectrum,” representative examples of which are shown in FIG.
4
. These variations in gain as a function of signal wavelength produce problems in wavelength division multiplexed (WDM) systems where a group of wavelengths are used to simultaneously transmit multiple signals down an optical fiber. Such multiplexing is of great commercial value since it allows significantly increased transmission capacity per fiber. Indeed, a current priority in the telecommunications industry is to upgrade existing one wavelength transmission systems to a multi-wavelength environment in a cost effective manner so as to address the ever increasing demand for greater signal carrying capacity.
In a typical application, a multi-wavelength signal carried on an optical fiber will be subjected to repeated rounds of amplification as it passes from the transmitter to the receiver. At each such stage, any differences in amplification which may exist at the various wavelengths will compound, with the wavelengths subject to more amplification becoming ever stronger at the expense of those subject to less amplification. Various approaches have been used in the art to address this non-uniform amplification problem.
One of the most basic approaches involves the selection of the wavelengths used to transmit the multiple signals. As is well known in the art, the gain spectrum of an erbium doped fiber amplifier is flatter in the “red band,” i.e., in the longer wavelength region from about 1540-1545 nanometers to about 1565 nanometers, than in the “blue band,” i.e., in the shorter wavelength region from about 1525 nanometers to about 1535-1545 nanometers. In particular, a very flat gain in the red band can be achieved by adjusting the fraction of erbium ions in the excited (“inverted”) state through the selection of the length of the fiber amplifier and the level of pumping applied to the fiber.
To take advantage of this flatness, wavelength multiplexed systems employing erbium doped fiber amplifiers have had their signal channels in the red band. In addition, to address residual non-uniform gain, the signal input powers at the transmitter have been adjusted to take account in advance of the differential amplification which will occur as the signal is repeatedly amplified during its passage to the receiver.
To expand the useable wavelength range provided by erbium doped fiber amplifiers into the blue band, filters have been proposed to flatten the amplifier's gain spectrum. The standard assumption which is made in designing a practical filter for this purpose is that the gain of the amplifier is essentially “homogeneous” in character, i.e., that the gain can be described by the homogeneous model discussed in, for example, C R Giles, et al., “Modeling erbium-doped fiber amplifiers”,
J Lightwave Tech,
vol. 9, pp. 271-283, 1991, and C R Giles, et al., “Optical amplifiers transform long-distance lightwave telecommunications”,
Proc IEEE,
vol. 84, pp. 870-883, 1996. The essence of this assumption is that the gain of an amplifier is determined by the average inversion of the active species, e.g., the erbium ions in an erbium doped fiber amplifier, irrespective of the particular signal wavelengths, signal powers, pump wavelength, and pump power which produced that average inversion. Looked at another way, the assumption of homogeneous broadening means that if the gain at any one wavelength is by some means stabilized to a particular value then a gain at the other wavelengths is similarly stabilized (the stabilized value of the gain being different at different wavelengths).
By means of this assumption, a gain spectrum for an amplifier is calculated for a given average inversion and that gain spectrum is used to design a filter which can flatten the spectrum. A set of signal wavelengths when applied to the amplifier will then see a flattened gain spectrum provided that the average inversion in the presence of those signal wavelengths is the average inversion used in the design of the filter. The degree of flattening will, of course, depend on how well a manufactured filter actually has the desired attenuation spectrum.
Rather than calculating the gain spectrum using the homogeneous model, one could, for example, measure the gain spectrum of an actual amplifier and use that measured gain spectrum to design the filter. This empirical approach, however, also implicitly adopts the homogeneous model in that it is assumed that the gain spectrum will be flattened for any set of signal wavelengths and powers within the amplifier's operating range that has the same average inversion as that which existed when the empirical gain spectrum was measured.
The above approaches for implementing a gain flattening filter work well for signal wavelengths in the red band. However, as discussed in detail in the above-referenced Pump Wavelength Tuning Application, it has been found that the homogeneous model does not work well in the blue band. Rather, this band exhibits substantial inhomogeneous behavior. Specifically, when at least one signal wavelength is in this band, the gain spectrum can no longer be described by a single average inversion which applies to all active species.
The Pump Wavelength Tuning Application discloses various techniques for adjusting the gain spectrum of an optical amplifier, including the portion of the spectrum which lies in the blue band. Those techniques involve controlling the wavelength at which pump power is applied to the amplifier's amplifying medium, e.g., the amplifier's erbium doped fiber.
The present invention provides additional techniques for adjusting an amplifier's gain spectrum. These techniques involve controlling the temperature of the amplifying medium based on the inversion level of the medium. This additional variable in combination with those previously known allows for even better control of the shape of the gain spectr
Cereo Richard J.
Culverhouse David O.
McNamara Thomas W.
Sheih Shou-Jong
Corning Incorporated
Moskowitz Nelson
Short Svetlana Z.
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