Abstract:
An optical communication apparatus includes: an erbium-doped optical fiber amplifier for amplifying a modulated optical signal, wherein a DC (direct current) light which is not modulated is inputted to the erbium-doped optical fiber amplifier together with the optical signal.

Description:
[0001]     This application claims foreign priority based on Japanese Patent application No. 2005-316180, filed Oct. 31, 2005, the content of which is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical communication apparatus, and more particularly to an optical communication apparatus in which an output waveform of an optical fiber amplifier (refer it simply to as an EDFA, hereinafter) is improved.  
         [0004]     2. Description of the Related Art  
         [0005]     Meeting the age of multi-media including an Internet, the optical communication network technique of a main communication system in which more improved service is desired over a wide area has been rapidly and progressively developed for an information society. Further, as the capacity of an optical communication line becomes large and the speed of the optical communication line becomes high, a receiving section of an optical transmitting system has been also requested to have a receiving function of a higher performance.  
         [0006]     As one of components of the optical communication apparatus, an EDFA (erbium-doped optical fiber amplifier) obtained by adding an erbium ion to an optical fiber has been known.  
         [0007]      FIG. 7  is a diagram showing a basic structure of an optical communication apparatus using the EDFA. The optical communication apparatus  1  includes the EDFA 2 , a light receiving element  3 , an amplifier  4  and a signal reproduction section  5 .  
         [0008]     The EDFA 2  receives and amplifies a signal from a modulation light source  10  that is transmitted through an optical fiber cable  6 . The light receiving element  3  converts an amplified optical signal to an electric signal. The amplifier  4  amplifies the electric signal. In the signal reproduction section  5 , identification of “1” or “0” is performed based on a set threshold value to reproduce data.  
         [0009]     The EDFA has excellent features such as high gain, high output, wide band, low noise and independence of a polarized wave. Therefore, the EDFA is indispensable for a present optical fiber communication system. In the present optical communication, the EDFA is employed in a structure in which a light that is modulated by a constant rapid period is inputted to the EDFA.  
         [0010]     Related arts using such EDFA are disclosed in JP-A-2003-179552 and JP-A-2002-026874.  
         [0011]     However, in the EDFA, when a light is suddenly inputted from a state that the light is not inputted or a pulse-shaped light having a long period equal to or above the order of μs is inputted, a light surge appears that has an output several times to several ten times as large as an output that can be obtained from an ordinary gain or a saturated output. This phenomenon results from a fact that a relaxation time of excitation and emission in the EDFA is in the order of μs. Accordingly, there is a possibility that optical components arranged in a rear stage may be broken due to the light surge.  
         [0012]      FIG. 8  shows an input light inputted to the EDFA 2  from the modulation light source  10  of the optical communication apparatus  1  shown in  FIG. 7 .  FIG. 9  shows a waveform of an output light from the EDFA 2  and a state that the light inputted from the modulation light source  10  is inputted to the EDFA 2  and outputted as an amplified and modulated light. In these drawings, an axis of ordinate designates a value (mv) obtained by inputting a light power to a PD (Photo Diode) and converting it to an electric signal, and an axis of abscissa designates a time (μs).  
         [0013]     When a rectangular optical signal having a long period (1 kHz, −10 dBmp-p) ΔY=2.0 mv as shown in  FIG. 8  is inputted, the output waveform is ΔY=40 mv as shown in  FIG. 9 , so that a large gain can be obtained. However, in a head portion of the rectangular signal, the light surge having an extremely large output exceeding 100 mv appears as shown in a part A.  
         [0014]      FIGS. 10A-10D  show output waveforms when the power of the input light is changed from 0 dBmp-p to −3 dBmp-p, to −7 dBmp-p and to −10 dBmp-p. Also in this drawing, an axis of ordinate designates a value (mv) obtained by inputting a light power to a PD and converting it to an electric signal and an axis of abscissa designates a time (μs). The frequency of a signal light at this time is set to 1 kHz. A large protrusion (surge voltage) is generated at the beginning of the waveform, which is different from a case of 10 GHz. This protrusion shows that as the power of the input light (0 dBmp-p to −3 dBmp-p, to −7 dBmp-p and to −10 dBmp-p) is large, the protrusion of the surge voltage is also large.  
         [0015]     As described above, in the case of a wavelength of about 1 kHz, the light surge appears, however, when a modulated light in unit of GHz employed in the present optical communication is continuously inputted, the EDFA can be used as an extremely good optical amplifier.  
         [0016]     However, in an optical switch using a label switching high in its availability as an optical communication apparatus in future, an optical burst switching network of a next generation or an optical packet switching network, a signal is intermittently inputted. Accordingly, during a time of disconnection, there is a state that the light is not inputted to the EDFA. Therefore, in an ordinary using method, the light surge is undesirably generated as shown in  FIG. 9 .  
         [0017]     When the modulated light of several GHz that is used in the present optical communication is constantly and continuously inputted to the EDFA, the EDFA can be used as a very preferable amplifier. However, when a light is suddenly inputted from a state that the light is not inputted or a pulse-shaped light having a long period equal to or more than the order of μs as shown in  FIG. 8  is inputted, a light surge appears that has an output several times to several ten times as large as an output obtained from an ordinary gain or a saturated output.  
         [0018]     Accordingly, since in the optical switch using the label switching in future, the optical burst switching network of a next generation or the optical packet switching network, there is a state that the light is not inputted to the EDFA, the light surge is undesirably generated by a current using method.  
       SUMMARY OF THE INVENTION  
       [0019]     The present invention has been made in view of the above circumstances, and provides a light receiving device that can greatly reduce a light surge outputted from an EDFA even when a long-pulse-shaped light is inputted.  
         [0020]     In some implementations, an optical communication apparatus of the invention comprising:  
         [0021]     an erbium-doped optical fiber amplifier for amplifying a modulated optical signal,  
         [0000]     wherein a DC (direct current) light which is not modulated is inputted to the erbium-doped optical fiber amplifier together with the optical signal.  
         [0022]     The optical communication apparatus of the invention further comprising:  
         [0023]     a wave filter for removing the DC light from an amplified optical signal being outputted from the erbium-doped optical fiber amplifier.  
         [0024]     In the optical communication apparatus of the invention, a band-pass filter is used as the wave filter.  
         [0025]     In the optical communication apparatus of the invention, an isolator and a Fiber Bragg Grating (FBG) are used as the wave filter.  
         [0026]     In the optical communication apparatus of the invention, a wavelength of the DC light is from about 1550 nm to 1568 nm.  
         [0027]     In the optical communication apparatus of the invention, a wavelength of the DC light is variable.  
         [0028]     In the optical communication apparatus of the invention, the wavelength of the DC light is variable with respect to a wavelength of the modulated optical signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIGS. 1A, 1B  and  1 C are block diagrams showing an embodiment of an optical communication apparatus according to the present invention.  
         [0030]      FIG. 2  shows an output waveform when an idle light having a wavelength of 1560 nm is inputted to an EDFA 2  together with a modulated light.  
         [0031]      FIGS. 3A  to  3 H show relations between the wavelength of the idle light and the output of a signal light.  
         [0032]      FIGS. 4A  to  4 D show dependence of an output waveform on the power of the signal light.  
         [0033]      FIGS. 5A  to  5 D show dependence of the output waveform on the power of the idle light.  
         [0034]      FIG. 6  shows that an output of an EDFA is stabilized by measuring a light power of a signal light by using an optical amplifier shown in  FIG. 1  to increase or decrease a power of an idle light correspondingly to the measured value.  
         [0035]      FIG. 7  is a diagram showing a basic structure of an optical communication apparatus using an EDFA.  
         [0036]      FIG. 8  is a diagram showing an input light inputted to the EDFA from a modulation light source of the optical communication apparatus shown in  FIG. 7 .  
         [0037]      FIG. 9  shows a waveform of an output light from the EDFA of the optical communication apparatus shown in  FIG. 7 .  
         [0038]      FIGS. 10A  to  10 D show output waveforms when the power of the input light is changed. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]     Now, by referring to the drawings, the present invention will be described below in detail.  
         [0040]      FIGS. 1A-1C  are block diagrams showing an embodiment of an optical communication apparatus of the present invention. In  FIG. 1A , reference numeral  10  designates a modulation light source, reference numeral  11  designates a DC light source, reference numeral  12  designate a coupler to which a signal light and an idle light are inputted. Reference numeral  2  designates an EDFA to which the signal light and the idle light outputted from the coupler are inputted. Reference numeral  13  designates a wave filter for blocking the idle light in an output from the EDFA 2  so as to output the light to a light receiving element (not shown) arranged in a rear stage.  
         [0041]      FIGS. 1B and 1C  show specific examples of the wave filter  13 .  FIG. 1B  shows the example using a band-pass filter  13   a , and  FIG. 1C  shows the example using an isolator  14  and an FBG (Fiber Bragg Grating)  15 .  
         [0042]     In  FIG. 1A , a light having different wavelength is inputted to the EDFA 2  together with a modulated light. The output of the light having the different wavelength is cut by the wave filter  13 . Here, an input modulated light desired to be amplified is referred to as a signal light and a DC light having the different wavelength is referred to as an idle light. The idle light is a laser beam that is constantly inputted to the coupler  12  as a light of a prescribed wavelength.  
         [0043]     Since the idle light is constantly inputted, even when a light is suddenly inputted from a state that the signal light is not inputted, the generation of a light surge can be prevented.  
         [0044]      FIG. 2  shows a waveform of an output when the idle light having the wavelength of 1560 nm is inputted to the EDFA 2  together with the modulated light and indicates a case that an output signal after the idle light is transmitted to the wave filter  13  shown in  FIG. 1  is measured. When  FIG. 2  is compared with  FIG. 9  in which the idle light is not inputted to the EDFA 2 , it is recognized that the light surge is greatly reduced and the waveform of the output is more greatly improved.  
         [0045]      FIGS. 3A-3H  show relations between the wavelength of the idle light and the output of the signal light. Here, results are shown that are obtained by inputting as the signal light the rectangular optical signal having a long period (1 kHz, −10 dBmp-p) ΔY=2.0 mv as shown in  FIG. 8  to intermittently change the wavelength of the idle light to 1526 to 1572 nm.  
         [0046]     That is, as shown in  FIG. 3A , when the idle light having the wavelength of 1526 nm is inputted, an output voltage is high as large as ΔY=51 mv, however, it is found that a strong surge voltage (about 20 mv) is generated at the beginning of the waveform.  
         [0047]     Then, when the input wavelength of the idle light is 1534 nm, the output voltage is ΔY=29 mv or so as shown in  FIG. 3B  and is lower than the output voltage when the wavelength of 1526 nm shown in  FIG. 3A  is inputted, however, it is obviously understood that the surge voltage is about 5 mv and considerable improvement is recognized.  
         [0048]     Then, as shown in  FIG. 3C , when the idle light having the wavelength of 1542 nm is inputted, the output voltage is ΔY=26 mv or so. However, it is found that the surge voltage (about 5 mv) is generated at the beginning of the waveform, though the surge voltage is not so strong as shown in  FIG. 3A .  
         [0049]     Then, when the input wavelength of the idle light is 1550 nm, the output voltage is ΔY=18 mv or so as shown in  FIG. 3D  and it is obviously understood that considerable improvement can be recognized on the surge voltage (about 3 mv).  
         [0050]     Then, when the input wavelength of the idle light is 1555 nm, the output voltage is low as small as ΔY=15 mv or so as shown in  FIG. 3E , however, it is obviously understood that considerable improvement can be recognized on the surge voltage (about 3 mv).  
         [0051]     Then, when the input wavelength of the idle light is 1560 nm, the output voltage is ΔY=14 mv or so as shown in  FIG. 3F , however, it is obviously understood that further improvement can be recognized on the surge voltage is about 2 mv.  
         [0052]     Then, as shown in  FIG. 3G , when the idle light having the wavelength of 1568 nm is inputted, the output voltage is ΔY=28 mv or so and it is found that the surge voltage (about 5 mv) is generated at the beginning of the waveform.  
         [0053]     Subsequently, as shown in  FIG. 3H , when the idle light having the wavelength of 1572 nm is inputted, the output voltage is ΔY=56 mv or so and it is found that the strong surge voltage (about 25 mv) is generated at the beginning of the waveform.  
         [0054]     From the above-described results, it can be decided that when the wavelength of the idle light inputted to the coupler  12  ranges from about 1550 nm to 1568 nm or so, an influence of a surge voltage is hardly received.  
         [0055]     Accordingly, in the present invention, the wavelength of the idle light inputted to the coupler is set to about 1550 nm to 1568 nm, and desirably set to about 1560 nm.  
         [0056]      FIGS. 4A-4D  show dependence of the output waveform on the power of the signal light. In this case, the light power of the idle light is set to −3 dBmp-p and the wavelength is set to 1542 nm. According to the drawings, it is recognized that as the signal light is decreased, the surge (a protrusion) becomes smaller and a gain is increased.  
         [0057]      FIGS. 5A-5D  show dependence of the output waveform on the power of the idle light. In this case, the power of the signal light is set to −3 dBmp-p and the wavelength is set to 1542 nm. According to the drawings, it is recognized that as the power of the idle light is increased, the surge (a protrusion) becomes smaller and a gain is more decreased.  
         [0058]     As apparent from  FIGS. 4A-4D  and  5 A- 5 D, when the power of the idle light is increased and the power of the signal light is decreased, the waveform is more improved.  
         [0059]     Specifically, in the structure shown in  FIG. 1 , when the input power of the idle light is changed (for instance, decreased), the gain relative to the signal light can be changed (for instance, increased). Then, such an operation is used so that the EDFA 2  can be allowed to function as a variable gain amplifier.  
         [0060]      FIG. 6  shows that the output of an EDFA is stabilized by measuring the light power of a signal light by the use of an optical amplifier shown in  FIG. 1  to increase or decrease the power of an idle light correspondingly to the measured value.  
         [0061]     The output stabilizing operation functions even when the signal light becomes a burst or a packet signal.  
         [0062]     In  FIG. 6 , a splitter  16  is arranged in a rear stage of a modulation light source (a signal light)  10  to split the signal light to two directions. One optical signal split by the splitter  16  is inputted to a coupler  12  through a delay section  17 . Further, the other optical signal is inputted to a control circuit  18  to control the power of a DC light source (an idle light) in association with the output of the control circuit. Here, the delay section  17  functions as a delay element to meet the delay of the signal until the other split optical signal passes through the control circuit  18  and is inputted to the coupler  12  via the DC light source.  
         [0063]     In the structure of  FIG. 6 , the signal light is inputted to the control circuit  18  through the splitter  16 . The control circuit  18  measures the power of the signal light. Then, since the light power of the idle light is controlled in accordance with a measurement result, the idle light having an optimum power can be inputted to an EDFA 2  in association with the signal light to stabilize the output of the EDFA 2 .  
         [0064]     According to the optical communication apparatus of the invention, since the DC (non-modulated) light is inputted to the EDFA together with the optical signal, the light surge of an output can be reduced.  
         [0065]     According to the optical communication apparatus of the invention, since the DC (non-modulated) light is removed from the optical signal being amplified by the EDFA by using a wave filter, only the amplified optical signal can be detected.  
         [0066]     According to the optical communication apparatus of the invention, since the wavelength of about 1550 nm to 1568 nm is used, the light surge of the output can be reduced.  
         [0067]     According to the optical communication apparatus of the invention, since the wavelength of the DC light can be varied in association with the wavelength of the modulated optical signal, even when the wavelength of the optical signal is changed, the DC light having a proper wavelength can be inputted to effectively suppress a surge.  
         [0068]     The above description merely presents a specific preferred embodiment for the purpose of explaining the present invention and exemplification.  
         [0069]     Accordingly, the present invention is not limited to the above-described embodiment and further includes many changes and modifications within a scope without departing from its essence.  
         [0070]     It will be apparent to those skilled in the art that various modifications and variations can be made to the described preferred embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover all modifications and variations of this invention consistent with the scope of the appended claims and their equivalents.