Abstract:
An exemplary aspect of the invention is an optical transmission system including a Raman amplifier, wherein the Raman amplifier corrects the gain of a light signal by excitation light including light of at least one wavelength that reduces difference between a minimum value and a maximum value of the power spectral distribution of the input light signal.

Description:
[0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-342027 filed on Dec. 20, 2006, the contents of which are incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical transmission system and its signal spectrum correction method, particularly relates to a signal spectrum correction method in an optical transmission system using a Raman amplifier. 
         [0004]    2. Description of the Related Art 
         [0005]    To correspond to a demand of communication that rapidly increases according to the population of the Internet, the transmission capacity of an optical transmission system configuring a basic communication network recently increases at surprising speed. To correspond to such rapid increase of the capacity, optical wavelength division multiplexing technology, that is, wavelength division multiplexing (WDM) technology is established and the increase of the capacity of transmitted data is realized. 
         [0006]    However, in a long-distance transmission system, the extension of a relay interval is a difficult objective together with the increase of transmission capacity. Then, to secure signal-to-noise power ratio (SN ratio) per one wave and to reduce the distortion of a transmission waveform by optical fiber non-linear effect, a transmission method that is called Raman amplification and that negates the loss of a transmission line has been proposed (for example, refer to Japanese Patent Laid-open Application Publication No. 2002-344054). Referring to  FIG. 11 , the related art using a Raman amplifier will be described below. 
         [0007]    As shown in  FIG. 11 , the output of an optical transmitter  10  is amplified by a light amplifier (an erbium doped fiber amplifier (EDFA)) on the transmitting side  20  and is inputted to a Raman amplifier  40  via an optical fiber transmission line  30 . The output of amplification by the Raman amplifier  40  is amplified by a light amplifier (EDFA) on the receiving side  50  and is received by an optical receiver  60 . 
         [0008]      FIG. 12  shows a signal spectrum at an output point “a” of the optical transmitter  10 ; and in this example, a wavelength division multiplexed wave of 40 waves in a signal band of 1574 to 1610 nm is shown. The output of the transmitter is made incident into the light amplifier on the transmitting side  20  and is amplified there.  FIG. 13  shows a signal spectrum at an output point “b” of the light amplifier on the transmitting side  20 . As the light amplifier  20  is provided with a gain equalization function, the flat signal spectrum is acquired at the output point “b”. 
         [0009]    The output of the light amplifier  20  is inputted to the optical fiber transmission line  30 , however, the transmission line  30  is a dispersion shifted fiber (DSF) including the transmission loss of 35 dB. A reference numeral  141  in  FIG. 14  shows a signal spectrum at a point “c” of the output of amplification by the Raman amplifier  40 , and the signal spectrum is amplified by 6.5 dB by Raman excitation light output from the Raman amplifier  40 . The Raman amplifier  40  is provided with a flat gain characteristic because the signal spectrum at the output point “b” of the light amplifier on the transmitting side  20  has a flat characteristic as shown in  FIG. 13 . A reference numeral  142  in  FIG. 14  shows a signal spectrum without Raman amplification. 
         [0010]    The output of the Raman amplifier is inputted to the light amplifier on the receiving side  50  and is amplified there.  FIG. 15  shows a signal spectrum at an output point “d” of the light amplifier  50 . As the light amplifier  50  is provided with a gain equalization function, the flat signal spectrum in a signal band of 1574 to 1610 nm is acquired at the output point “d” and is inputted to the optical receiver  60 . The light amplifier  50  is also an auto-level control (ALC) amplifier that keeps output power fixed. 
         [0011]      FIG. 16  shows optical characteristics (a gain flatness characteristic and a noise figure (NF) characteristic) of the light amplifier  50 . A reference numeral  161  shows the gain characteristic,  162  shows the NF characteristic, the characteristics are those in the case of long-distance transmission in configuration shown in  FIG. 11 , gain flatness is 0 dB, and an NF value is 11.2 dB. 
         [0012]    In the configuration shown in  FIG. 11  and using a Raman amplification method used for a long-distance transmission system according to the related art, to remove the dependency on a wavelength of a signal spectrum, a high-priced gain equalizer is required to be used for both the light amplifier on the transmitting side  20  and the light amplifier on the receiving side  50 . When the gain equalizer is used for only the light amplifier on the receiving side  50 , the dependency on a wavelength of a spectrum of a signal input to the light amplifier on the receiving side  50  increases. Therefore, there is caused a problem that the increase of the loss of the gain equalizer, the deterioration of the NF characteristic and the difficulty of the manufacture of the gain equalizer are caused and its price rises. 
         [0013]    The art disclosed in Japanese Patent Laid-open Application Publication No. 2002-344054 has a defect that a signal spectrum is flattened in only the Raman amplifier, therefore, a high-performance characteristic is required for the Raman amplifier and its price rises. 
       SUMMARY 
       [0014]    An exemplary object of the invention is to provide an optical transmission system that can be low-priced and can avoid the deterioration of an NF characteristic and its signal spectrum correction method. 
         [0015]    An exemplary aspect of the invention is an optical transmission system including a Raman amplifier, wherein the Raman amplifier corrects the gain of a light signal by excitation light including light of at least one wavelength that reduces difference between a minimum value and a maximum value of the power spectral distribution of the input light signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram showing a first exemplary embodiment of the invention; 
           [0017]      FIG. 2  shows a signal spectrum at a point A; 
           [0018]      FIG. 3  shows a signal spectrum at a point B; 
           [0019]      FIG. 4  shows a signal spectrum at a point C; 
           [0020]      FIG. 5  shows a gain characteristic of a Raman amplifier in comparison with that in related art; 
           [0021]      FIG. 6  is a sequence diagram showing a signal spectrum at a point D; 
           [0022]      FIG. 7  shows again characteristic of a receiving light amplifier  5  in comparison with that in the related art; 
           [0023]      FIG. 8  shows an NF characteristic of the receiving light amplifier  5  in comparison with that in the related art; 
           [0024]      FIG. 9  is a block diagram showing a second exemplary embodiment of the invention; 
           [0025]      FIG. 10  is a block diagram showing an integrated light amplifier; 
           [0026]      FIG. 11  is a block diagram for explaining the related art; 
           [0027]      FIG. 12  shows a signal spectrum at a point a; 
           [0028]      FIG. 13  shows a signal spectrum at a point b; 
           [0029]      FIG. 14  shows a signal spectrum at a point c; 
           [0030]      FIG. 15  shows a signal spectrum at a point d; and 
           [0031]      FIG. 16  shows a gain characteristic and an NF characteristic of a light amplifier on the receiving side  50 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     1. First Exemplary Embodiment 
       [0032]    Referring to the drawings, an exemplary embodiment of the invention will be described below.  FIG. 1  shows an optical transmission system using Raman amplification as a first exemplary embodiment of the invention. As shown in  FIG. 1 , a light amplifier (a booster amplifier) on the transmitting side  2  is arranged at the back of an optical transmitter  1  and is used for amplifying transmitted light power. A Raman amplifier  4  brings about signal gain in a wavelength band by making intense excitation light incident into an optical fiber as a transmission line  3  and uses the transmission line fiber itself for an amplification medium. 
         [0033]    A light amplifier (a preamplifier) on the receiving side  5  is arranged in front of an optical receiver  6  and is used for improving receiving sensitivity. The light amplifiers  2  and  5  makes signal light and excitation light incident into the optical fiber into which a rare earth element is doped and amplify the signal light. However, the light amplifier  2  is not provided with a gain equalizer including a profile opposite to the dependency on a wavelength of the gain of the light amplifier and the light amplifier  5  is provided with a gain equalizer. 
         [0034]    As the light amplifiers  2  and  5  are well-known for persons skilled in the art and besides, are not directly related to the invention, their detailed configuration is omitted. Further, as the Raman amplifier  4  is also well-known for the persons skilled in the art and is not directly related to the invention except the selection described later of an excitation wavelength, its detailed configuration is omitted. 
         [0035]    In the configuration of the first exemplary embodiment, it is general that the light amplifiers  2  and  5  amplify signal light by making the signal light and excitation light incident into a core of the optical fiber including the rare earth element; however, they may also have configuration that light is amplified by multi-mode excitation for amplifying signal light by using a double-clad fiber for an optical fiber including a rare earth element and making excitation light incident into a second-clad part. 
         [0036]    Next, the operation of the first exemplary embodiment of the invention will be described in detail. In this exemplary embodiment, as in the case of the related art, 40 waves in a signal band of 1574 to 1610 nm are also multiplexed into a WDM wave. 
         [0037]    Wavelength division multiplexed signal light output from the optical transmitter  1  is amplified in the light amplifier  2  without a gain equalizer and is outputted to the transmission line  3  with the signal light including the dependency on a wavelength of a few dB. On the transmission line  3 , amplification is acquired in a waveband longer by approximately 100 nm (13 THz) than the wavelength of excitation light in stimulated emission based upon Raman scattering by Raman excitation light output from the Raman amplifier  4 , and the gain of an output spectrum including the dependency on a wavelength of a few dB is corrected. Afterward, the signal is again amplified in the light amplifier  5  provided with the gain equalizer and is inputted to the optical receiver  6  in a condition of a flat signal spectrum. 
         [0038]      FIG. 2  shows a signal spectrum at an output point A of the optical transmitter  1 . An input signal including the signal spectrum shown in  FIG. 2  is incident into the light amplifier  2 .  FIG. 3  shows a signal spectrum at an output point B of the light amplifier on the transmitting side  2 . The signal spectrum shown in  FIG. 2  is amplified in the light amplifier  2  without a gain equalizer to be a signal spectrum including the dependency on a wavelength of 4.0 dB as shown in  FIG. 3  and is outputted. 
         [0039]    A DSF fiber  3  including the loss on a transmission line of 35 dB is estimated from the point B to a point C.  FIG. 4  shows a signal spectrum at the point C. For the wavelength of Raman excitation light output from the Raman amplifier  4 , 1462 nm and 1505 nm are selected and the signal spectrum  41  shown in  FIG. 4  is an input signal spectrum to the light amplifier  5  acquired when the gain is corrected in the Raman amplifier. A spectrum of Raman gain acquired on the transmission line  3  at this time is shown by a reference numeral  51  in  FIG. 5 . A reference numeral  52  in  FIG. 5  shows a spectrum of Raman gain in the related art described referring to  FIG. 11 . 
         [0040]    A signal spectrum  42  shown in  FIG. 4  is an input signal spectrum to the light amplifier  5  when the signal is amplified so that Raman gain is flat in the Raman amplifier  4  (when no gain is corrected in the Raman amplifier). Raman gain acquired by the Raman amplifier  4  is 6.5 dB on the average. Further, an input signal spectrum to the light amplifier  5  when no Raman amplification is made is shown as a signal spectrum  43  for reference data. 
         [0041]      FIG. 6  shows a signal spectrum at an output point D of the light amplifier on the receiving side  5 . The signal spectrum shown in  FIG. 6  is an output signal spectrum after the signal spectrum  41  shown in  FIG. 4  is amplified by the light amplifier  5 . The light amplifier  5  is an ALC control amplifier that keeps output power fixed and the flat signal spectrum in the signal band of 1574 to 1610 nm is inputted to the optical receiver  6 . 
         [0042]    Optical characteristics (a gain flatness characteristic and an NF characteristic) in the light amplifier  5  are shown in  FIGS. 7 and 8  in comparison with those in the related art shown in  FIG. 11 . A gain spectrum  72  and the NF characteristic  82  are optical characteristics in a light amplifier on the receiving side when no gain is corrected by a Raman amplifier in long-distance transmission in the related art shown in  FIG. 11 , gain flatness is 4.0 dB, and an NF value is 13.6 dB. 
         [0043]    In the meantime, a gain spectrum  71  and the NF characteristic  81  are optical characteristics in the light amplifier  5  on the receiving side when gain is corrected in the Raman amplifier  4  according to the invention in long-distance transmission in the configuration according to the invention shown in  FIG. 1 , gain flatness is 2.5 dB, and an NF value is 11.8 dB. It is known from this result that the flatness of a signal spectrum input to the light amplifier on the receiving side  5  after the transmission line is improved by correcting gain in the Raman amplifier, a maximum loss characteristic of the gain equalizer in the light amplifier on the receiving side  5  is reduced and the NF characteristic of the light amplifier  5  is improved by 1.8 dB. 
       2. Second Exemplary Embodiment 
       [0044]    The basic configuration of a second exemplary embodiment of the invention is similar to that of the first exemplary embodiment; however, the configuration of a light amplifier on the receiving side is further devised.  FIG. 9  shows the configuration. In  FIG. 9 , the same reference numeral is allocated to the similar part to that shown in  FIG. 1 . As shown in  FIG. 9 , the Raman amplifier  4  and the light amplifier on the receiving side  5  respectively shown in  FIG. 1  are integrated to be an integrated light amplifier  7 , and a signal light input monitor in the Raman amplifier and a signal light input monitor in the light amplifier on the receiving side are made common. Hereby, the price of the light amplifier on the receiving side can further be reduced. 
         [0045]    Referring to  FIG. 10 , the details of the integrated light amplifier  7  will be described below. An exciting laser diode (LD)  73  is a Raman pumping source and amplifies signal light using Raman effect on the transmission line  3 . An exciting WDM coupler  71  multiplexes signal light and excitation light, the excitation light is multiplexed in a reverse direction to the signal light, and is outputted to the transmission line  3 . 
         [0046]    A demultiplexing coupler  72  demultiplexes input signal light at certain ratio and a photodiode (PD)  74  receives a light signal and converts it to an electric signal. As a light amplifier  5  is well-known for persons skilled in the art, its detailed configuration is omitted. Signal power input to the integrated light amplifier  7  is received by PD  74 , and according to received output, the exciting LD  73  as a Raman pumping source and a pumping source in the light amplifier  5  are controlled by a controller  75 . As a transmitted state of a wavelength division multiplexed signal in  FIG. 9  is described above, the description of the state is omitted. 
         [0047]    As described above, as the signal light input monitor in the Raman amplifier and the signal light input monitor in the light amplifier on the receiving side are made common in the second exemplary embodiment, the price of the light amplifier on the receiving side can be reduced. 
         [0048]    As described above, in the invention, loss in a gain equalizer used in the light amplifier on the receiving side is reduced by providing no gain equalization function to a light amplifier on the transmitting side, correcting a gain characteristic of a signal spectrum by providing no gain equalization function to the light amplifier on the transmitting side using a Raman amplifier, inputting the signal spectrum to the light amplifier on the receiving side with as a flat characteristic as possible and finally, also correcting gain in the light amplifier on the receiving side in comparison with a case that a signal spectrum is inputted to a light amplifier on the receiving side using a normal Raman amplifier (without a gain correction function), and an NF characteristic is also improved. 
         [0049]    As a gain characteristic of a signal spectrum by providing no gain equalization function to the light amplifier on the transmitting side is corrected in both the Raman amplifier and the light amplifier on the receiving side, the performance of the gain characteristic required for both can be moderated. 
         [0050]    In the above-mentioned exemplary embodiments, the light amplifier on the transmitting side  2  is used after the optical transmitter  1 ; however, this light amplifier does not have to be used. The light amplifier on the transmitting side  2  is arranged after the optical transmitter  1  and the light amplifier on the receiving side  5  is arranged before the optical receiver  6 , however, arrangement in the invention is not limited to this, for example, an optical multiplexer may be also arranged after the optical transmitter  1 , and an optical demultiplexer may be also arranged before the optical receiver  6 . 
         [0051]    Further, a method of controlling the light amplifiers  2  and  5  is also not limited and for example, the light amplifier may be also configured by one stage of amplifying elements (an EDFA part) (therefore, its NF characteristic is deteriorated), however, in the invention, the number of the amplifying elements in the light amplifier is not limited (the NF characteristic is further improved by arranging a gain equalizer between two amplifying elements). 
         [0052]    Besides, in the above-mentioned exemplary embodiments, signal light in 40 channels in an L-band is inputted to the light amplifier; however, a signal waveband and the number of signals (channels) are not limited. Further, in the exemplary embodiments, DSF is estimated for a transmission line fiber; however, a type of the transmission line fiber is not limited. 
         [0053]    Furthermore, in the exemplary embodiments, two waves of 1462 nm and 1505 nm are used for excitation light by the Raman amplifier, however, an excitation wavelength of Raman amplification and the number of excitation wavelengths are not limited, and they are suitably selected according to a frequency band of the system and an extent to which the variation caused by omitting a gain equalization function in the light amplifier on the transmitting side of gain characteristics of spectra of signals of each wavelength is corrected. That is, the excitation wavelength of the Raman amplification and the number of excitation wavelengths are selected so that a signal spectrum of light incident into the optical receiver  6  is finally flat by the correction of gain characteristics by the Raman amplifier  4  and the light amplifier on the receiving side  5 . 
         [0054]    According to the invention, as the gain of a signal spectrum output to the transmission line and including a wavelength characteristic of a few dB is corrected by Raman amplification, effect that the loss of the gain equalizer used in the light amplifier on the receiving side is reduced and the NF characteristic is improved is acquired. In addition, a gain equalizer in the light amplifier on the transmitting side can be removed, further, as the loss of the gain equalizer used in the light amplifier on the receiving side is reduced, its manufacture is facilitated, and the price of the light amplifier can be reduced. 
         [0055]    The previous description of these embodiments is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope as defined by the limitations of the claims and equivalents.