Abstract:
It is an object of the invention to provide a level-flattening circuit for WDM optical signals which can be used in an optical signal repeating station. A level flattening circuit for WDM optical signals is supplied with WDM optical signal and demultiplexs them into individual optical signals having different wavelengths, levels of which are separately feedback controlled to provide flattened optical signal levels.

Description:
The invention relates to a wavelength division multiplexed (MDW, hereinafter) optical communication system, and especially to a level-flattening circuit for WDM optical signals for maintaining optical powers of repeating optical signals at a constant level throughout whole wavelengths. 
     BACKGROUND OF THE INVENTION 
     In a conventional technology disclosed in Japanese Patent Kokai 7-30520, n (n is an integer) optical transmitters generate n optical signals having n different wavelengths λ 1  to λ n  and these optical signals are respectively attenuated by n optical attenuators corresponding wavelengths of λ 1  to λ n , multiplexed by a wavelength division multiplexer, amplified by an Er-doped optical fiber, and transmitted therefrom. In most cases, the optical signals are plus code modulated. 
     A part of optical energy of the WDM optical signals is split by an optical splitter, and demultiplexed by a wavelength division demultiplexer into n optical signals having the wavelengths of λ 1  to λ n , which are respectively detected by optical to electrical converters (O/E converters, hereinafter) corresponding to the wavelengths of λ 1  to λ n . The aforementioned optical attenuators corresponding to the n optical signals are respectively feedback controlled based on the outputs of the n O/E convertors. 
     In this way, powers of the WDM optical signals amplified by the Er-doped optical fiber amplifier are maintained at a constant level throughout the whole wavelengths. 
     However, the aforementioned level-flattening circuit for the WDM optical signals is designed as an apparatus to be used in an optical signal transmitting station, which is provided with plural optical transmitters for generating plural optical signals having the wavelengths of λ 1  to λ n , and cannot be used in an optical signal repeating station. 
     If the principle of the aforementioned level-flattening circuit for the WDM optical signals is directly applied to the same used in the optical signal repeating station, the individual optical signal levels in the optical signal repeating station must be transmitted to the optical signal transmitting station in order to constitute a feedback loop, the system is magnified, and a statistactry result cannot be obtained. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to solve problems in the aforementioned level-flattening circuit for WDM optical signals, and provide a level-flattening circuit for WDM optical signals which can be applied to a optical signal repeating station. 
     According to a feature of the invention, a level-flattening circuit for WDM optical signals comprises; 
     A wavelength division demultiplexer, which is supplied with the WDM optical signals and demultiplexing them into individual optical signals having different wavelengths, 
     optical attenuators provided for the individual optical signals supplied from the wavelength division demultiplexer, 
     optical splitters, which split the individual optical signals supplied from the optical attenuators at a predetermined rate, 
     photodiodes for converting the split optical signals supplied from the optical splitters into electrical signals, 
     a control circuit, which feedback controls attenuations of the optical attenuators corresponding to the photodiodes so that the electrical signals outputted from the photodiodes are maintained at respective predetermined levels, and 
     a wavelength division multiplexer for multiplexing the individual optical signals passed through the optical splitters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in conjunction with appended drawings, wherein: 
     FIG. 1A shows a block diagram of a preferred embodiment of the invention. 
     FIG. 1B shows a detail of a feature of the preferred embodiment of the invention of FIG.  1 A. 
     FIG. 2 shows a spectrum of WDM optical signals at an input port of a circuit according to the invention, and 
     FIG. 3 shows a spectrum of WDM optical signals at an output port of a circuit according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Thereafter, preferred embodiments of the invention will be explained referring to the amended drawings. 
     FIG. 1A is a block diagram for showing a preferred embodiment of the invention. In FIG. 1A, optical signals having wavelengths of λ 1 , λ 2 , . . . λ i , . . . λ n  are multiplexed and supplied to an input port  101 , where n is a natural number, and i is a natural number arbitrarily selected from a series, 1 to n. 
     The inputted optical signals are demultiplexed into individual optical signals having wavelengths of λ 1 , λ 2 , . . . λ i , . . . λ n  by a wavelength division demultiplexer  111 , and they are respectively supplied to optical attenuators  121 ,  122 , . . .  12 i, . . .  12 n, which respectively correspond to the wavelength λ 1  to λ n . 
     That is to say, the optical signal of the wavelength λ 1  is supplied to the optical attenuator  121 , the optical signal of the wavelength λ 2  to the optical attenuator  122 , and the optical signal of the wavelength λ i  to the optical attenuator  12 i. 
     The optical signals outputted from the optical attenuators  121 ,  122 , . . .  12 i, . . .  12 n respectively pass through corresponding optical splitters  131 ,  132 , . . .  13 i, . . .  13 n, are again multiplexed by awavelength division multiplexer  151 , and outputted from an output port  161 . 
     The wavelength division demultiplexer  111  and the wavelength division multiplexer  151  can be realized by arrayed waveguide gratings (AWGs). These structural elements can be realized by other optical circuit elements, such as optical couplers, optical filters, and fiber gratings also. 
     Optical power split by the optical splitter  13 i is converted into a voltage signal by a photodiode  14 i, and a control circuit  171  feedback controls an optical attenuator  12 i so that the aforementioned voltage signal is kept to be a predetermined value. Although  12 i,  13 i, and  14 i are not explicitly shown in FIGS. 1A and 1B, their meanings would be clear recalling the fact that i is an integer arbitrarily selected from a series, 1 to n. 
     For instance, the control circuit  171  is provided with a comparator  20  shown in FIG. 1B connected with the photodiode  14 i, and controls the attenuation of the optical attenuator  12 i in a way mentioned as follows. If the output voltage of the photodiode  14 i is higher than a predetermined value, the attenuation of the optical attenuator  12 i is increased. If the output voltage of the photodiode  14 i is lower than the predetermined value, the attenuation of the optical attenuator  12 i is reduced. In this way, the optical power inputted to the photodiode  14 i is kept to be constant. 
     Since the coupling coefficient of the optical splitter  13 i is constant, the optical power supplied to the photodiode being maintained at a constant level means the optical power supplied to the wavelength division multiplexer  151  being maintained at constant level, hence means the output power levels of the WDM optical signals at the output port  161  being made flattened. 
     FIG. 2 shows an example of a spectrum of the WDM optical signals at the input port  101  shown in FIG. 1A, and FIG. 3 shows a spectrum of the WDM optical signals at the output port  161 , which is flattened by a circuit according to the invention. 
     Now then, insertion losses of the optical signals in the wavelength division multiplexer  151  are different from each other in accordance with the wavelengths of the optical signals. Accordingly, if the wavelength division multiplexer  151  is interchanged, feedback control of the optical signal levels must be adjusted over again. 
     As mentioned in the above, in the control circuit  171  shown in detail in FIG. 1B, since the photodiode  14 i is provided with the comparator  20 i which compares the output voltage of the photodiode  14 i with a reference voltage and feedback controls, by means of optical attenuator control circuits  211  to  21 n, the attenuation of the optical attenuator  12 i so that the output voltage of the photodiode  14 i becomes equal to the reference voltage, the insertion loss  181  of the wavelength division multiplexer can be compensated by adjusting the reference voltage of the comparator  20 i by means of voltage converters  221  to  22 n. 
     Accordingly, the wavelength division multiplexer  151  is combined with information  181  on the insertion losses of the wavelength division multiplexer and completed as the wavelength division multiplexing block  191 . Since the wavelength division multiplexer  151  being interchanged means the wavelength division multiplexing block  191  being interchanged, information on the insertion losses of the wavelength division multiplxer  151  is transmitted to the control circuit  171 , and the control circuit  171  sets the reference voltages of the respective comparators based on information mentioned in the above. 
     For example, if information that the insertion loss of the optical signal of the wavelength λ i  in the multiplexer  151  is di dB is transmitted to the control circuit  171  by information  181  on the insertion losses of the wavelength division multiplexer, the control circuit  171  increases the reference voltage of the comparator  20 i by di dB. Accordingly, the output of the optical attenuator  12 i is controlled so that it is increases by di dB, hence the insertion loss of di dB in the wavelength division multiplexer  151  can be compensated and the output levels of the optical signals can be flattened. 
     As mentioned in the above, the intensities of the WDM optical signals in the repeater station can be flattened by the circuit according to the invention. Moreover, since the insertion losses of the wavelength division multiplexer can be compensated, the system can operate continuously, even when the wavelength division multiplexer for the WDM optical signals is interchanged.