Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-08-20
2004-07-27
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337000, C359S337100, C359S337110
Reexamination Certificate
active
06768579
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical amplifier arrangement having at least one optical amplifier stage and a variably settable attenuator.
Wavelength-division multiplexing (WDM) of optical transmission systems makes it possible to increase the transmission capacity of the optical transmission system, or WDM transmission system, by adding further WDM channels without any increase of the respective channel data rate. Optical amplifiers, or fiber amplifiers, arranged along the optical transmission path, are normally used to amplify simultaneously all the WDM channels, or WDM signals, transmitted via the optical transmission fiber. In order to avoid the problematic quality differences which occur in this case for the optical WDM channels at the end of the optical transmission path, it is necessary in particular that the WDM channels, or the WDM signals, have virtually the same signal level in each case at the amplifier inputs and outputs.
In optical WDM transmission systems, a variety of parameters which are responsible for, or give rise to, channel level differences in WDM signals are known. These include the wavelength dependency of the attenuation by passive components, which does not vary, or varies only slightly, during operation of the optical transmission path. Other effects are dependent on the operating state of the optical WDM transmission system, for example on the number of active WDM channels or on the ambient temperature, and hence result in variations during operation of the WDM transmission system.
In optical WDM transmission systems with large WDM channel numbers, which occupy a wide wavelength range and, at the same time, require a high total power at the input of the optical transmission fiber, a significant tilt of the channel level spectrum in each optical transmission path section is caused by the non-linear effect of stimulated Raman scattering (SRS) in the optical transmission fiber. In this case, the degree of tilt, or the level difference, of the respective WDM channel having the shortest and longest wavelength is dependent on the number of active WDM channels and is altered by switching further WDM channels on or off.
Furthermore, the channel level differences are produced, within the optical WDM signal to be transmitted, owing to the temperature dependency of the gain spectrum of the active fiber of optical fiber amplifiers. Especially in the case of erbium-doped fiber amplifiers, which are operated in the L transmission band (wavelength range of approximately 1570 to 1605 nm), any change in the ambient temperature leads to a significant tilt of the gain spectrum of the active fiber of the optical fiber amplifier.
Both SRS and the temperature dependency of the active fiber cause, at first approximation, a virtually dB-linear tilt of the channel level spectrum of the optical WDM signal. In the case of a dB-linear tilt, the level difference (in dB) between a WDM channel and its respectively neighbouring WDM channel—assuming a constant wavelength spacing of the WDM channels—has the same value in each case for all WDM channels.
In WDM transmission systems produced to date, the wavelength dependency of the gain spectrum of the active fiber of optical fiber amplifiers is usually leveled by using an optical filter having inverse attenuation response, so as to reduce the channel level differences at the end of the optical transmission path which are due to the tilt of the gain spectrum. Such an optical filter is configured, for a given working point, in such a way that the remaining gain variation is limited to less than 1 dB over all the WDM channels. If the working point of the optical fiber amplifier is altered, i.e. if the gain of the active fiber is altered, then the gain spectrum tilts—dynamic gain tilt.
One approach to compensating for the gain tilt involves splitting the optical amplifier into several optical amplifier stages. For instance, in the case of a 2-stage optical fiber amplifier, an attenuator which is preset to a fixed value and has a flat attenuation spectrum is interconnected between the first and second amplifier stages, and is used to compensate at least partially for the gain tilts in total due to the first and second amplifier stages. To that end, the individual amplifier stages are in each case regulated to a constant total output power, and the attenuator with a flat attenuation spectrum, which is interconnected between the amplifier stages, is therefore preset so that the gain of the two optical amplifier stages remains virtually constant.
It is furthermore possible to modify the regulation of the individual amplifier stages by regulating the individual amplifier stages not to a constant total output power, but rather to a constant gain. If the level at the input of the first amplifier stage rises, then its output level is also increased. The level at the input of the subsequent amplifier stage is kept constant since the attenuator with a flat attenuation spectrum, which is inserted between the amplifier stages, avoids passing on the level increase from its input to its output by increasing the attenuation value which is set. However, any increase in the attenuation between the amplifier stages impairs the noise properties of the overall two-stage amplifier arrangement.
When using a variable attenuator with a flat attenuation spectrum, only minor compensation for the tilt of the channel spectrum of the optical WDM signals, due to SRS or the temperature dependency of the active fiber of the optical fiber amplifier, is therefore possible.
A virtually dB-linear tilt of the channel level spectrum is also caused by a change in the gain of the active fiber of an optical fiber amplifier. The gain, or the gain spectrum, of the optical fiber amplifier can be adjusted, for constant input and output signal levels of the optical fiber amplifier, by changing the setting of a variable attenuator with a flat attenuation spectrum, so as to set a suitable tilt of the gain spectrum which virtually compensates for the constant part of the dB-linear tilt due to SRS or a temperature change. The disadvantage of this method is that it is only possible to reduce tilts, due to the active fiber of the optical fiber amplifier, which have a small amplitude. See, in particular, U.S. Pat. No. 5,530,583. To balance a strong tilt of, for example, 4 dB, it is therefore sometimes necessary to increase the inserted attenuation of the variable attenuator by more than 10 dB. Such a high additional inserted attenuation contributes to a significant increase in the noise factor of the optical fiber amplifier.
SUMMARY OF THE INVENTION
An advantage of the invention is to provide an optical amplifier arrangement with which it is possible to reduce channel level differences due to changes of the operating state of the optical WDM transmission system, for example as a result of switching channels on and off or temperature changes.
In an embodiment, on optical amplifier arrangement is provided having at least one optical amplifier stage (V
1
, V
2
) and a variably settable attenuator (VDL). The variably settable attenuator (VDL) has an attenuation spectrum (DV
2
, DV
3
, DV
5
, DV
6
) that increases or decreases proportionally to wavelength. The profile of the attenuation spectrum is variably settable in order to reduce any tilt of the channel level spectrum.
An aspect of the method according to the invention is that the variably settable attenuator has an attenuation spectrum that increases or decreases proportionally to wavelength, the profile of the attenuation spectrum being variably settable in order to reduce any tilt of the channel level spectrum. Advantageously, the channel level tilt of the optical transmission signal can be reduced with substantially less inserted attenuation, compared with compensation when using a settable attenuator with a flat attenuation spectrum, which leads to an improvement in the noise ratio of the optical amplifier arrangement. Furthermore, the attenuation spectrum with an inverse attenuation profile with respect t
Bell Boyd & Lloyd LLC
Black Thomas G.
Cunningham Stephen
Siemens Aktiengesellschaft
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