Patent Publication Number: US-6907157-B2

Title: Method and system for optical fiber transmission using raman amplification

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and system for optical fiber transmission using Raman amplification. 
     2. Description of the Related Art 
     In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying an optical signal or signal light has been put to practical use. 
     An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. 
     For example, an erbium doped fiber amplifier (EDFA) has already been developed to amplify signal light in a 1.55 μm band where the loss in a silica fiber is low. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 μm band or a 1.48 μm band, a gain band including a wavelength of 1.55 μm can be obtained. 
     As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels. 
     By using an optical amplifier as a linear repeater, the number of parts in the repeater can be greatly reduced as compared with the case of using a conventional regenerative repeater, thereby ensuring reliability and allowing a substantial cost reduction. 
     Recently, there has been extensively examined the application of an optical repeater using Raman amplification capable of further reducing noise and broadening the band in addition to or in place of an EDFA. In the Raman amplification, an optical fiber generally used as an optical fiber transmission line is used as an optical amplifying medium, and pump light is supplied to the optical fiber. As a pumping source used in the Raman amplification, a high-power pumping source is required. Accordingly, it is expected that a recent tendency of a laser diode (LD) to have a high power and a high efficiency can accelerate practical utilization of the optical repeater using Raman amplification. Further, also in a remote amplifying method such that pumping is performed from an end of an optical fiber transmission line without using an optical repeater, the Raman amplification using a general optical fiber as an optical amplifying medium is useful in providing a distributed amplification system. 
     In a Raman amplification process using a single pumping source, an obtainable gain band (a band where gain can be provided) is relatively narrow. Accordingly, a plurality of pumping sources for outputting pump lights having different wavelengths are practically used to thereby extend the gain band. However, since the Raman amplification process utilizes conversion of the power of pump light into the power of signal light, the gain that can be obtained varies according to the power of signal light included in the gain band. 
     For example, in the case of using Raman amplification for repeating of WDM signal light, the total power changes with a change in number of wavelength channels of the WDM signal light, causing a change in the gain that can be obtained. Specifically, in the case that two pumping sources are used, the gain given to optical signals having wavelengths included in a gain band obtained by one of the two pumping sources becomes different from the gain given to optical signals having wavelengths included in a gain band obtained by the other pumping source. As a result, there is a possibility that the transmission characteristics of optical signals obtaining a high gain may be degraded by nonlinear phenomena. 
     It has been proposed to prevent such a degradation in transmission characteristics by transmitting a supervisory signal to a Raman repeater by means of signal light to adjust an operational balance among a plurality of pumping sources. In this case, however, there is a limit to the number of signals transmittable because signal light is used for the transmission of the supervisory signal. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and system which can control a Raman amplification process without using signal light to be transmitted. 
     Other objects of the present invention will become apparent from the following description. 
     In the method according to the present invention as an aspect thereof, an optical fiber transmission line for transmitting signal light is first provided. A Raman repeater having a pumping source for supplying pump light propagating in a direction opposite to the direction of propagation of the signal light to the optical fiber transmission line is provided in the middle of the optical fiber transmission line. Similarly, a terminal device having a pumping source for supplying pump light propagating in the opposite direction to the propagation direction of the signal light is provided at one end of the optical fiber transmission line. A control signal is transmitted to the Raman repeater by the pump light from the terminal device. Then, the Raman repeater is controlled by the control signal transmitted. 
     According to this method, the Raman repeater is controlled by the pump light from the terminal device provided at the receiving end, thereby eliminating a problem arising in the case of control by signal light as in the prior art. 
     In accordance with another aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting signal light; a Raman repeater provided in the middle of said optical fiber transmission line, said Raman repeater having a pumping source for supplying pump light propagating in a direction opposite to the direction of propagation of said signal light to said optical fiber transmission line; and a terminal device provided at one end of said optical fiber transmission line, said terminal device having a pumping source for supplying pump light propagating in the opposite direction to the propagation direction of said signal light to said optical fiber transmission line. The terminal device comprises means for transmitting a control signal to said Raman repeater by said pump light from said terminal device. The Raman repeater comprises means for controlling said Raman repeater by said control signal transmitted. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a Raman amplifier applicable to the present invention; 
         FIG. 2  is a graph for illustrating gain bands obtained by the Raman amplifier shown in  FIG. 1 ; 
         FIGS. 3A and 3B  are diagrams for illustrating a change in gain when the number of wavelength channels is changed; 
         FIG. 4  is a block diagram showing a preferred embodiment of the system according to the present invention; 
         FIGS. 5A and 5B  are waveform charts for illustrating frequency components to be superimposed on pump lights; 
         FIG. 6  is a diagram for illustrating the bands of the frequency components to be superimposed on the pump lights; 
         FIG. 7  is a block diagram of a Raman repeater applicable to the present invention; 
         FIGS. 8A and 8B  are graphs for illustrating the manner of control in the Raman repeater shown in  FIG. 7 ; 
         FIG. 9  is a block diagram of a system for illustrating an example of the control in the case that a plurality of Raman repeaters are provided; 
         FIGS. 10A ,  10 B, and  10 C are graphs for illustrating the control of the gain bands; 
         FIG. 11  is a flowchart showing the control of the gain bands; 
         FIG. 12  is a block diagram of another Raman repeater applicable to the present invention; and 
         FIG. 13  is a block diagram of still another Raman repeater applicable to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. 
     Referring to  FIG. 1 , there is shown a block diagram of a Raman amplifier applicable to the present invention. An optical coupler  4  is provided in the middle of an optical fiber transmission line  2 . Two laser diodes (LD)  8 (# 1 ) and  8 (# 2 ) are connected through another optical coupler  6  to the optical coupler  4 . Each of the laser diodes  8 (# 1 ) and  8 (# 2 ) functions as a pumping source for outputting pump light, and it is controlled by a control circuit  10  so that pump light having a constant power is output. 
     The pump light output from the laser diode  8 (# 1 ) and the pump light output from the laser diode  8 (# 2 ) are combined by the optical coupler  6  to enter the same optical path. The pump light thus obtained is introduced through the optical coupler  4  into the optical fiber transmission line  2  in a direction opposite to the direction of propagation of signal light. When the pump light is supplied to the optical fiber transmission line  2  along which the signal is propagating, gain is provided according to a mutual relation between the wavelength of the pump light and the wavelength of the signal light, so that the power of the pump light is converted into the power of the signal light to thereby amplify the signal light. 
     Referring to  FIG. 2 , there are shown gain bands obtained by the Raman amplifier shown in FIG.  1 . In  FIG. 2 , the vertical axis represents gain and the horizontal axis represents optical wavelength. The oscillation wavelengths of the laser diodes  8 (# 1 ) and  8 (# 2 ) are different from each other, so that different gain bands GB 1  and GB 2  are obtained. The gain band GB 1  corresponding to the laser diode  8 (# 1 ) appears at shorter wavelengths, and the gain band GB 2  corresponding to the laser diode  8 (# 2 ) appears at longer wavelengths. The gain that can be obtained changes according to the power of the pump light. 
     It is assumed that in the case that the number of optical signals S 1  included in the gain band GB 1  shown in  FIG. 2  is equal to the number of optical signals S 2  included in the gain band GB 2  shown in  FIG. 2  as shown in  FIG. 3A , the output powers of the laser diodes  8 (# 1 ) and  8 (# 2 ) are adjusted so that substantially the same gain is given to the optical signals S 1  and the optical signals S 2 . When the optical signals S 2  included in the gain band GB 2  decrease in number to optical signals S 2 ′ as shown in  FIG. 3B , the gain given to the optical signals S 2 ′ becomes relatively high as far as the output powers of the laser diodes  8 (# 1 ) and  8 (# 2 ) are constant. 
     In a WDM system handling WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths, the operation is frequently carried out in the condition where the number of optical signals is unbalanced as mentioned above, and when the gain to be given changes according to the number of optical signals as mentioned above, the transmission characteristics of optical signals having relatively high powers may be sometimes degraded by nonlinear phenomena in the fiber. 
     To avoid such a problem, the laser diodes  8 (# 1 ) and  8 (# 2 ) shown in  FIG. 1  are remotely controlled. That is, the drive condition of the control circuit  10  is adjusted by the remote control from a terminal device connected to one end of the optical fiber transmission line  2  according to the operational conditions of the system. 
     Such remote control is conventionally performed by superimposing a tone signal on an optical signal and performing supervisory control with the frequency or a burst signal. However, as described above, the number of signals is sometimes limited because the tone signal is used in a gain band. 
     Referring to  FIG. 4 , there is shown a preferred embodiment of the system according to the present invention. This system is configured by connecting a terminal device  12  as a transmitting station to one end of the optical fiber transmission line  2  and connecting a terminal device  14  as a receiving station to the other end of the optical fiber transmission line  2 . One or more Raman repeaters  16  are arranged along the optical fiber transmission line  2 . 
     The terminal device  12  includes a plurality of optical transmitters for respectively outputting a plurality of optical signals having different wavelengths, and an optical multiplexer for wavelength division multiplexing the plurality of optical signals to output WDM signal light, which is launched into the optical fiber transmission line  2 . The WDM signal light propagating along the optical fiber transmission line  2  is attenuated during the propagation. The attenuated WDM signal light is amplified by the Raman repeaters  16  so that the power of the WDM signal light is maintained until it reaches the terminal device  14 . The terminal device  14  includes an optical receiver  18  for receiving the WDM signal light transmitted by the optical fiber transmission line  2 , demultiplexing the WDM signal light into a plurality of optical signals, and converting these optical signals into electrical signals. 
     A part of the WDM signal light transmitted by the optical fiber transmission line  2  is separated off by an optical coupler  19 . The spectrum of the WDM signal light separated by the optical coupler  19  is measured by an optical spectrum analyzer  20 , and the result of this measurement is supplied to a control circuit  22 . 
     In this preferred embodiment, the terminal device  14  also has a function as a Raman amplifier to compensate for the loss of the WDM signal light in the optical fiber transmission line  2  in the vicinity of the terminal device  14 . More specifically, pump light from a laser diode  24 (# 1 ) as a pumping source and pump light from a laser diode  24 (# 2 ) as another pumping source are combined by an optical coupler  26  and supplied through an optical coupler  28  to the optical fiber transmission line  2  in a direction opposite to the propagation direction of the WDM signal light. 
     Drive currents are supplied from DC control circuits  30 (# 1 ) and  30 (# 2 ) to the laser diodes  24 (# 1 ) and  24 (# 2 ), respectively. AC superimposing circuits  32 (# 1 ) and  32 (# 2 ) are interposed between the laser diodes  24 (# 1 ) and  24 (# 2 ) and the DC control circuits  30 (# 1 ) and  30 (# 2 ), respectively. In the AC superimposing circuits  32 (# 1 ) and  32 (# 2 ), frequency components f 1  and f 2  from the control circuit  22  are superimposed on the drive currents for the laser diodes  24 (# 1 ) and  24 (# 2 ), respectively. 
     The magnitudes of the DC drive currents to be supplied from the DC control circuits  30 (# 1 ) and  30 (# 2 ) to the laser diodes  24 (# 1 ) and  24 (# 2 ) are controlled by the control circuit  22  so that the spectrum of the WDM signal light measured by the optical spectrum analyzer  20  becomes uniform, for example. 
     The frequency components f 1  and f 2  to be superimposed on the drive currents are decided according to the levels of the optical signals included in the gain bands GB 1  and GB 2  shown in  FIG. 2 , respectively, for example. Accordingly, a control signal or a supervisory signal reflecting the spectrum of the WDM signal light received by the terminal device  14  can be transmitted to the upstream side of the terminal device  14  by the pump light. That is, since the power of pump light in a Raman amplification process is high, the pump light from the terminal device  14  can be used for the supervisory control of the plural Raman repeaters  16 . 
       FIGS. 5A and 5B  are waveform charts of the pump lights to be introduced from the laser diodes  24 (# 1 ) and  24 (# 2 ) to the optical fiber transmission line  2 , respectively. As apparent from  FIGS. 5A and 5B , the pump lights from the laser diodes  24 (# 1 ) and  24 (# 2 ) are amplitude-modulated (intensity-modulated) by the frequency components f 1  and f 2 , respectively. The frequency components f 1  and f 2  are set to different values in different bands. 
     Referring to  FIG. 6 , there is shown an example of the ranges of presence of the frequency components f 1  and f 2 . In this example, the frequency component f 1  is set to a band of lower frequencies as shown by reference numeral  30 , and the frequency component f 2  is set to a band of higher frequencies as shown by reference numeral  32 . The reason why the frequency components f 1  and f 2  are set to such different bands is to easily separate the frequency components f 1  and f 2  in each Raman repeater  16  (see  FIG. 4 ) receiving the control signal transmitted by the pump light. 
       FIG. 7  is a block diagram showing a preferred embodiment of the Raman repeater according to the present invention. In contrast to the configuration shown in  FIG. 1 , this preferred embodiment is characterized in that a supervisory control unit  34  operating according to the control signal by the pump light from the terminal device  14  (see  FIG. 4 ) is additionally provided. 
     The pump light from the terminal device  14  contributes primarily to amplification of the WDM signal light in the optical fiber transmission line  2 , and a part of the remaining pump light is branched off by an optical coupler  36 . The pump light branched off by the optical coupler  36  is converted into an electrical signal by a photodetector (PD)  38 . The output from the photodetector  38  is supplied to filters  40 (# 1 ) and  40 (# 2 ). Each of the filters  40 (# 1 ) and  40 (# 2 ) is provided by a bandpass filter. The passbands of the filters  40 (# 1 ) and  40 (# 2 ) are set so as to respectively correspond to the bands  30  and  32  shown in  FIG. 6 , for example. 
     Accordingly, the signals passed through the filters  40 (# 1 ) and  40 (# 2 ) reflect the frequency components f 1  and f 2 , respectively. The frequency components f 1  and f 2  are converted into digital signals by comparators  42 (# 1 ) and  42 (# 2 ), respectively, and the values of f 1  and f 2  are measured by frequency counters  44 (# 1 ) and  44 (# 2 ), respectively. DC control circuits  46 (# 1 ) and  46 (# 2 ) control the drive currents for the laser diodes  8 (# 1 ) and  8 (# 2 ) as pumping sources according to the measured values f 1  and f 2 . 
     The manner of control of the drive currents for the laser diodes  8 (# 1 ) and  8 (# 2 ) will now be described with reference to  FIGS. 8A and 8B . As shown in  FIG. 8A , the output voltages from the frequency counters  44 (# 1 ) and  44 (# 2 ) increase with an increase in the frequencies output from the comparators  42 (# 1 ) and  42 (# 2 ). As shown in  FIG. 8B , the drive currents to be supplied from the DC control circuits  46 (# 1 ) and  46 (# 2 ) to the laser diodes  8 (# 1 ) and  8 (# 2 ) increase with an increase in the output voltages from the frequency counters  44 (# 1 ) and  44 (# 2 ). Accordingly, by predetermining the relation between the spectrum measured by the optical spectrum analyzer  20  (see  FIG. 4 ) and the frequency components f 1  and f 2  to be superimposed on the drive currents for the laser diodes  24 (# 1 ) and  24 (# 2 ) in consideration of the above relation shown in  FIGS. 8A and 8B , the gain characteristic of each Raman repeater  16  can be controlled so that the spectrum of the WDM signal light transmitted becomes constant, for example. 
     An example of the control for the plural Raman repeaters  16  will now be described with reference to FIG.  9 . Pump light P 1  including the frequency component f 1  and pump light P 2  including the frequency component f 2  both output from the terminal device  14  are transmitted toward the terminal device  12  by the optical fiber transmission line  2 . During this transmission, the frequency components f 1  and f 2  are used for the control of the plural Raman repeaters  16 . The manner of control of each Raman repeater  16  is similar to that described above. 
     In the case that the peak gains in the gain bands GB 1  and GB 2  are set equal to each other as shown in  FIG. 10A , for example, there is a case that the peak gain in the gain band GB 2  becomes higher than the peak gain in the gain band GB 1  because of a change in operational conditions or the like as shown in FIG.  10 B. In this case, the peak gain in the gain band GB 1  can be made relatively high by changing the frequency component f 1  toward lower frequencies or by changing the frequency component f 2  toward higher frequencies as shown in  FIG. 10C , thereby obtaining an optimum balance between the gain bands GB 1  and GB 2 . 
     The flow of this control will now be described more specifically with reference to FIG.  11 . In step  101 , spectrum data on the WDM signal light received by the terminal device  14  is read by the optical spectrum analyzer  20 . In step  102 , a required gain control amount is calculated. In step  103 , the gain control amount calculated is compared with a present gain control amount maintained at this time. If the present gain control amount is equal to the calculated gain control amount as the result of comparison, the program proceeds to step  104  to maintain the present frequency components (f 1  and f 2  in the above preferred embodiment), whereas if the present gain control amount is not equal to the calculated gain control amount, the program proceeds to step  105  to change the frequency component to be superimposed on the drive current for each laser diode by a required amount. 
     By performing such control, a constant gain characteristic can be always obtained in each Raman repeater  16  irrespective of a change in wavelength channels of WDM signal light, for example. 
       FIG. 12  is a block diagram showing another preferred embodiment of the Raman repeater according to the present invention. In this preferred embodiment, a plurality of laser diodes  8 (# 1 ) to  8 (#N) for outputting pump lights for pumping the optical fiber transmission line  2  are used, and the number of components included in a supervisory control unit  34 A is accordingly increased. The pump lights output from the laser diodes  8 (# 1 ) to  8 (#N) are wavelength division multiplexed by an optical multiplexer (MUX)  48  to obtain WDM signal light, which is in turn supplied through the optical coupler  4  to the optical fiber transmission line  2  in a direction opposite to the propagation direction of WDM signal light along the optical fiber transmission line  2 . 
     By using N frequency components corresponding to the number N of the laser diodes  8 (# 1 ) to  8 (#N) in the terminal device  14 , a constant gain characteristic can be always obtained in each Raman repeater  16  irrespective of a change in wavelength channels of WDM signal light as in the previous preferred embodiment. 
       FIG. 13  is a block diagram showing a further preferred embodiment of the Raman repeater according to the present invention. This preferred embodiment includes a modification for making each Raman repeater  16  receive an SV signal (supervisory signal) from the transmitting terminal device  12 . The SV signal may be generated by superimposing a tone signal on the whole or part of the WDM signal light to be transmitted. 
     A part of the WDM signal light transmitted by the optical fiber transmission line  2  is branched off by an optical coupler  36 A and converted into an electrical signal by a photodetector  50 . The electrical signal output from the photodetector  50  is supplied to an SV control circuit  52 . By using this SV signal, each Raman repeater  16  can be controlled. 
     For example, the SV control circuit  52  sends ON/OFF control signals to the DC control circuits  46 (# 1 ) and  46 (# 2 ) according to the SV signal, thereby allowing temporary stop of the Raman amplification as required. Accordingly, it is easy to cope with abnormality of the system, for example. 
     In the above preferred embodiments, a plurality of frequency components for controlling the drive currents for a plurality of laser diodes are separated in the electrical stage. Alternatively, the frequency components may be separated by using optical bandpass filters, for example, in the optical stage before converting the pump light branched from the optical fiber transmission line  2  into an electrical signal by the photodetector  38 . 
     According to the present invention as described above, it is possible to provide a method and system which can control a Raman amplification process without using signal light to be transmitted.