Wavelength Multiplex Division (WDM) technology) offers the option of connecting transmitters and receivers in different places directly via optical paths of a network, without an electro-optical conversion being required at nodes. In the future it will also be possible to set up and clear down optical paths as required with the aid of optical switching matrices. Compared to the current prior art, major cost savings can be achieved without having to compromise the flexibility of the connections.
However the dropping and adding of transmitted signals in channels of a multiplex transmission system causes strong variations in power on individual link sections. To avoid bit errors at the end of the link the amplifier gain for the channels continuing to be transmitted or those being added may not change. FIG. 1 shows the average gain G of all channels over time for an individual amplifier stage and for two different cases, in which case it has been assumed that at time zero the input power reduces by 19 dB by dropping channels.
If the pump power is kept constant (see the solid curve), the gain G of 20 dB before the dropping process increases to a constant value of 30 dB after the dropping process. A further curve drawn in this Figure shows, by way of illustration, the timing curve when using a simple integral controller which ensures that the average gain after a synchronization process with a duration of over 20 ms again amounts to 20 dB. The overshoots and undershoots can be greatly reduced by more complex regulation, but cannot be completely eliminated. In a cascade of amplifiers the end result can thus be an accumulation of power variations and thus bit errors or even destruction of the receive diodes.
Overshoots and undershoots can be almost completely eliminated if the pump power required after a change in the input signal to maintain the gain under stable conditions is already known at the time of the change of load at the input. The actual difficulty lies in calculating this pump power in advance with the highest possible accuracy.
A simplest solution to this provides for choosing a linear approach for the required pump power as a function of the signal input power at the optical amplifier. This aspect is described in U.S. Pat No. 6,414,788 B1 and U.S. Pat. No. 6,341,034 B1, the contents of which are incorporated by reference in their entirety. In this case two constant parameters are employed. With this method however the following significant influencing variables are not taken into account:                The required pump power not only depends on the input power but also on the gain of the relevant amplifier stage. Since the stage of an amplifier can exhibit very different gain values, depending on use and channel occupation, marked variations emerge which adversely affect the determination of the correct pump power to be set.        No account is taken of the fact that the pump power required depends on the wavelength of the surviving channels in a drop process. This also applies to the so-called “gain ripple”.        “Fitting parameters” for determining the pump power values to be set are defined at the start of operation, so that aging effects lead to increasing variations as the operating life increases.        Non-linear effects in an amplification fiber of a fiber amplifier, such as “Excited State Absorption” in an Erbium-doped fiber of an EDFA (Erbium Doped Fiber Amplifier), continue to be ignored and thus lead to additional deviations.        
To take account of the spectral dependency it is proposed in document 6341034 that a spectral filter be fitted before an input monitor at the optical amplifier. The wavelength dependency of the method can thus be improved, if not eliminated entirely. Because of the high costs of components however this method is unlikely to be used.
In U.S. Pat. No. 6,366,393, which is also incorporated by reference in its entirety as in the document previously mentioned, a control unit for the gain of an optical amplifier is presented which opts for a linear approach for the required pump power as a function of the signal input power at the optical amplifier. The error implied in this approach is correct by means of a correction loop which is located after the amplifier and contains a microprocessor. This correction does not include wavelength dependency, making the method slow and imprecise.
In US Patent Publication 2003/0053200, which is also incorporated by reference in its entirety, the pump power of the optical amplifier is set using a feed forward control loop. In this case a small part of the WDM signal is routed through a filter of which the filter transfer function is adapted to the characteristics of the amplifier. The signal input power is weighted selectively by the filter as a function of the wavelength. The influence of the wavelengths with above-average effects on the decay rate of the amplifier energy level excited is increased or decreased in this case. The signal arrives at a photo detector after the filter which is connected to a control unit of the pump power of the amplifier.
In US Patent Publication 2001/0043389, which is incorporated by reference in its entirety, the amplifier gain is controlled by means of a forward and backward loop. The forward loop (feed forward loop) controls the amplifier by means of a fast photo diode, which measures the input power. The backward loop regulates the amplifier gain slowly depending on the output power of the amplifier. The two loops are connected to one another for checking the pump laser unit. The gain of the amplifier is essentially set by the backward loop, whereas the forward loop includes the compensation of offsets in the gain curve of the optical amplifier.
In U.S. Pat. No. 6,407,854, which is incorporated by reference in its entirety, a feed-forward control of an optical amplifier in a WDM system is presented. The pump power of the amplifier is set via a control unit which measures the input power of the amplifier and controls the current of the pump laser diodes as a function of the measurement level. In this case the electrical signal of the pump laser diodes can be changed by multiplication by a factor or by addition of an offset, to guarantee a gain curve of the amplifier which remains constant over the entire wavelength range. With this method synchronization processes of less than 200 μs are achieved.
In “Superior high-speed automatic gain controlled erbium-doped fiber amplifier”, Nakaji H., Nakai Y., Shigematsu M and Nishimura M., Optical Fiber Technology 9 (2003), pp. 25-35, a method for suppressing cyclic gain variations over time in a surviving channel of a WDM during the adding or dropping of further channels of the WDM signal is described. An EDFA is used for amplification of the WDM signal which operates with a pump source at 980 nm or with a pump source at 1480 nm. When a pump laser in the wavelength range of 1480 nm is used and with an optimum setting of the control parameters for a specific application case overshoots during dropping of a channel can be almost completely avoided. By contrast, when a pump laser in the range of 980 nm is used a small overshoot occurs after dropping of channels. If the pump power is now reduced or adapted to a new value, not as assumed above at the point of switching, but somewhat earlier, e.g. by a delay element connected upstream from the amplifier, the overshoot when using a pump source at 980 nm can be almost completely eliminated. This method is based on the fact that the reduction of the output power (effect) is detected later than the reduction of the input power (cause), so that the gain control is made to think for a period corresponding to the delay that there is a strong increase in gain to which it reacts by reducing the pump power. Experimental gain measurements can be verified from this literature reference. In any event a very short duration overshoot continues to occur.
This method, which is referred to below as the “feedback method” is well suited to laboratory experiments but can barely be used for commercial systems, since the optimal time delay depends on the number of surviving channels, no specification is known for predefining this optimum delay and the control parameters are only optimized for a specific event. In practice any given events, i.e. the dropping of a different number of channels for example, are taken into account. The time delay should be constantly recalculated and set for this, which would however be impossible or unrealizable in real time. Thus the wavelength-dependent gain curve experiences unavoidable variations for one or more surviving channels which adversely affect the broadband gain, in addition to the known timing variations of the channel-related gain. For these reasons this method is not suitable for current optical switching networks.
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