The present invention relates to a pumping source having a number of regulated pump lasers for broadband optical amplification of a WDM (Wavelength Division Multiplex) signal.
It is known that an optical signal is attenuated when being transmitted through an optical fiber. It is, therefore, necessary to reamplify optical signals after their passage through a specific fiber link. In general, two physical criteria of the optical transmission of WDM signals are important: the gain spectrum and the optical signal-to-noise ratio (OSNR). In WDM systems, the gain spectrum or the amplification must be approximately constant for all the channels (individual signals) of the WDM signal, and the OSNR must be sufficiently large. In the ideal case, the profiles of the signal levels and of the OSNRs will be flat for all the channels. In this regard, Raman amplifiers offer clear advantages, since they amplify the signals in the transmission fiber. Further, the transmission characteristics are also decisively improved. All the channels at the end of the fiber should have at least approximately equal amplitudes. Specifically, when cascading a number of Raman amplifiers, a flat gain spectrum ensures that all the channels fall into the dynamic range of the receivers.
Consequently, in an optimal system the aim is to achieve a gain spectrum that is as flat as possible or equal output levels of the individual channels at the end of the transmission fiber section. A very flat gain spectrum over a wide wavelength range can be achieved by using a pumping source with a broadband spectrum. Previously known broadband Raman amplifiers for WDM transmission systems operate with a number of pump lasers whose output signals exhibit different wavelengths.
Pump lasers with low output powers, for example semiconductor laser diodes, are frequently provided as pumping sources. The power of the pump lasers can change in the event of aging and incalculable defects, or owing to external influences (e.g., temperature). Thus, one or more pump lasers may fail completely, thereby impairing the gain spectrum of the amplification of the WDM signal.
A method for avoiding the saturation of the pump laser signals from pump lasers (marked by LD 81, 82) is disclosed in U.S. Pat. No. 5,479,423. The saturation can be caused by aging or temperature fluctuations. According to FIG. 1 of U.S. Pat. No. 5,479,423, the laser diodes 81, 82 are driven via a control module 4 and, in each case, a laser current source (i.e., xe2x80x9cdrive circuitxe2x80x9d 71, 72). A xe2x80x9cfeedbackxe2x80x9d signal SV impressed by modulation performs the regulation. However, a disadvantage of this method is that it offers no compensation for an optimal amplification of the transmission signal if a laser diode fails.
A device and a method for automatically controlling the total power of a number of pumping sources of an optical amplifier are disclosed in European Patent Application No. EP 0932229 A2. The goal of this method is to keep a predefined total gain constant in the event of a change in the pumping power or a failure of a pumping source. For an amplifier stage 100 (see FIG. 1 of European Patent Application No. EP 0932229 A2), the signals of a number of pumping modules 200, 300 are fed into the transmission fiber. The pumping modules 200, 300 each have a pumping source 210 and a circuit for controlling the pumping power. The output signal of the amplifier is supplied to a control module 270 of the pumping source 210 via a splitter 120 and a power detector 230, in a fashion converted into a first reference signal 234. A second reference signal 254 (error signal) is obtained from the difference between the current measured pumping power 210 and the average power of the pumping sources of the pump modules and supplied to the control module 270. Maintaining the average power of the pumping source of all the pumping modules requires a network in order to supply information concerning the output powers of each pumping source to all of the other pumping modules. A third reference signal 264 in each pumping module corresponds to the pumping power for a required gain of the amplifier stage 100. Like the two other reference signals 234, 254, the third reference signal 264 is supplied to the control module 270. The control module 270 uses the three reference signals 234, 254, 264 to determine the power being delivered by the pumping source 210. Thus, during operation, the powers of all the pumping modules are supervised as a function of a predefined gain. In the event of a failure of a pumping module, additional pumping modules are activated and their pumping powers are regulated anew so that the desired total gain of the amplifier stage 100 reaches its original value. When amplifying WDM signals, it is possible to extend the pumping modules with a number of pumping sources that exhibit different wavelengths (see column 9, paragraph 0034, lines 31-35). However, a disadvantage of this device and method is that they do not provide regulation of the pumping powers in the event of a failure or aging of one of the pumping sources in a multiwavelength pumping module. A further disadvantage is that they do not address the flattening of the gain spectrum in WDM transmissions systems. Moreover, a software module is used for regulating the pump power of one or more pump modules in addition to the failing module and not for regulating the output powers of non-failed lasers in a failing module.
An advantage of the present invention is, therefore, to provide a multiwavelength pumping source which keeps the changes in gain over the entire bandwidth of the WDM signal within a minimal tolerance range in the event of a partial failure or a complete failure of a pump laser of the pumping source.
The output powers of the pump lasers of the pumping source according to an embodiment of the present invention are supervised individually and set such that the gain is constant for the entire bandwidth of the transmitted WDM signal. An advantage of the present invention is that in the event of a failure of a pump laser (or, more rarely, of another element of the closed loop), the intact pump lasers are controlled such that the gain remains approximately constant over the entire bandwidth of the transmission signal. Accordingly, there is no need for standby lasers.
In an embodiment, depending on the type of defect, the output powers of the intact pump lasers are reset in accordance with new predefined parameters that are stored as sets in a database, for example, in the form of a table.
In an advantageous embodiment, the database can be changed by reprogramming, such that further operational failures, for example partial failures of a number of pump lasers in the event of a slow deterioration, can be compensated for.
In an embodiment, it is also possible to reconfigure the pump lasers via an external database.
An appropriate alarm signal is output, in an embodiment, in the event of defective operation in order to identify a failure and to replace the defective module.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.