Abstract:
An optical amplifier including a pumping source for supplying pump light to an optical fiber transmission line is provided so that at least a part of the optical fiber transmission line produces Raman amplification to an optical signal. An optical filter unit for selectively switching between a first condition where the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed is connected between the optical fiber transmission line and the optical amplifier. The power of the optical signal in the first condition is compared with that in the second condition, and the characteristics of the optical fiber transmission line, such as a splice loss therein, is evaluated according to the result of this comparison. In the second condition, the residual pump light is not supplied to the optical fiber transmission line, so that a measurement error due to Raman amplification can be eliminated to thereby allow accurate evaluation of the characteristics of the optical fiber transmission line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method, device, and system for evaluating the characteristics of an optical fiber transmission line, such as a loss at an optical fiber connection point.  
           [0003]    2. Description of the Related Art  
           [0004]    A manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber (e.g., silica fiber) have been established, and an optical fiber transmission system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber transmission line, one or more optical amplifiers for amplifying an optical signal are arranged along the optical fiber transmission line.  
           [0005]    An optical amplifier known in the art includes an optical amplifying medium for propagating an optical signal, and a pumping source for pumping the optical amplifying medium so that the optical amplifying medium gives a gain to the optical signal. For example, an erbium doped fiber amplifier (EDFA) has an erbium doped fiber (EDF) as the optical amplifying medium, and the EDF is pumped by pump light having a wavelength included in a 0.98-μm band or a 1.48-μm band.  
           [0006]    In recent years, Raman amplification is sometimes utilized to obtain an additional gain in a system having an optical amplifier in the middle of an optical fiber transmission line. In general, when light having large power is supplied to an optical fiber, a relatively wide gain band is generated by the effect of Raman amplification at wavelengths longer than the wavelength of the light. For example, when light having a wavelength included in a 1.45 to 1.48 μm band and having a power larger than +0 dBm is supplied to a silica fiber, a gain band providing a gain of about 0.1 to 8 dB is generated at wavelengths longer by 0.09 to 0.10 μm than the wavelength (1.55-μm band) of input light.  
           [0007]    In improving the efficiency of generation of Raman amplification, it is effective to increase the power of light input into an optical fiber or to use an optical fiber having a small mode field diameter. Conversely, the gain by Raman amplification changes with a change in either the power of input light or the mode field diameter of an optical fiber.  
           [0008]    It is difficult to accurately grasp the mode field diameter of an optical fiber used as an optical fiber transmission line from the viewpoint of manufacturing technique. Further, the power and wavelength of light input to an optical fiber transmission line to produce Raman amplification differ according to systems. Accordingly, in the case of evaluating the characteristics of an optical fiber transmission line, such as a loss at a splice connection point on the optical fiber transmission line, there is a possibility of large errors due to variations in gain by Raman amplification.  
           [0009]    In the case that the loss by splice connection is high, the connection of optical fibers may be imperfect, causing a possibility of breaking of the transmission line due to aged deterioration or shock against a cable.  
           [0010]    Such imperfect connection of optical fibers causes a fatal defect in a system required to have the long-term reliability of a cable, such as in a submarine communication system.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an object of the present invention to provide a method, device, and system for accurately evaluating the characteristics of an optical fiber transmission line.  
           [0012]    In accordance with the present invention, there is provided a first method for evaluating the characteristics of an optical fiber transmission line. In this method, an optical amplifier comprising a pumping source for supplying pump light to the optical fiber transmission line is provided so that at least a part of the optical fiber transmission line produces Raman amplification to an optical signal. An optical filter unit for selectively switching between a first condition where the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed is connected between the optical fiber transmission line and the optical amplifier. The power of the optical signal in the first condition is compared with that in the second condition, and the characteristics of the optical fiber transmission line are evaluated according to the result of this comparison.  
           [0013]    According to this method, Raman amplification does not occur in the second condition, so that a measurement error due to variations in gain by Raman amplification can be eliminated to thereby attain the object of the present invention.  
           [0014]    In accordance with the present invention, there is provided a device suitable for carrying out the first method. This device comprises an optical amplifier comprising a pumping source for supplying pump light to an optical fiber transmission line so that at least a part of the optical fiber transmission line produces Raman amplification to an optical signal; an optical filter unit connected between the optical fiber transmission line and the optical amplifier for selectively switching between a first condition where the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed; and a control circuit for controlling the optical filter unit so as to switch between the first condition and the second condition.  
           [0015]    In accordance with the present invention, there is provided a system suitable for carrying out the first method. This system comprises an optical fiber transmission line for propagating an optical signal; an optical amplifier comprising a pumping source for supplying pump light to the optical fiber transmission line so that at least a part of the optical fiber transmission line produces Raman amplification to the optical signal; an optical filter unit connected between the optical fiber transmission line and the optical amplifier for selectively switching between a first condition where the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed; and a control circuit for controlling the optical filter unit so as to switch between the first condition and the second condition.  
           [0016]    In accordance with the present invention, there is provided a second method for evaluating the characteristics of an optical fiber transmission line. In this method, first and second optical amplifiers are connected to a first end and a second end of the optical fiber transmission line, each of the first and second optical amplifiers comprising a doped fiber (e.g., EDF) doped with a rare earth element, a first pumping source connected to a first end of the doped fiber for outputting first pump light, and a second pumping source connected to a second end of the doped fiber for outputting second pump light. Switching is performed between a first condition where the first and second pumping sources of the first optical amplifier are turned off and on, respectively, and the first and second pumping sources of the second optical amplifier are turned on and off, respectively, and a second condition where the first and second pumping sources of the first optical amplifier are turned on, and the first and second pumping sources of the second optical amplifier are turned on. The power of the optical signal in the first condition is measured, and the characteristics of the optical fiber transmission line are evaluated according to the result of this measurement.  
           [0017]    According to this method, a measurement error due to variations in gain by Raman amplification can be eliminated in the first condition, thereby attaining the object of the present invention.  
           [0018]    In accordance with the present invention, there is provided a device suitable for carrying out the second method. This device comprises first and second optical amplifiers each comprising a doped fiber doped with a rare earth element, a first pumping source connected to a first end of the doped fiber for outputting first pump light, and a second pumping source connected to a second end of the doped fiber for outputting second pump light; and a control circuit for switching between a first condition where the first and second pumping sources of the first optical amplifier are turned off and on, respectively, and the first and second pumping sources of the second optical amplifier are turned on and off, respectively, and a second condition where the first and second pumping sources of the first optical amplifier are turned on, and the first and second pumping sources of the second optical amplifier are turned on, according to a supervisory signal.  
           [0019]    In accordance with the present invention, there is provided a system suitable for carrying out the second method. This system comprises first and second optical amplifiers each comprising a doped fiber doped with a rare earth element, a first pumping source connected to a first end of the doped fiber for outputting first pump light, and a second pumping source connected to a second end of the doped fiber for outputting second pump light; an optical fiber transmission line having a first end and a second end respectively connected to the first and second optical amplifiers; and a control circuit for switching between a first condition where the first and second pumping sources of the first optical amplifier are turned off and on, respectively, and the first and second pumping sources of the second optical amplifier are turned on and off, respectively, and a second condition where the first and second pumping sources of the first optical amplifier are turned on, and the first and second pumping sources of the second optical amplifier are turned on, according to a supervisory signal.  
           [0020]    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  
       [0021]    [0021]FIG. 1 is a block diagram showing a conventional optical repeater;  
         [0022]    [0022]FIG. 2 is a block diagram showing a general optical fiber transmission system in relation to its power diagram;  
         [0023]    [0023]FIG. 3 is a block diagram showing a first preferred embodiment of the device according to the present invention;  
         [0024]    [0024]FIG. 4 is a block diagram showing a specific configuration of a control circuit in the first preferred embodiment;  
         [0025]    [0025]FIG. 5 is a flowchart of supervisory control in the first preferred embodiment;  
         [0026]    [0026]FIG. 6 is a block diagram showing a second preferred embodiment of the device according to the present invention;  
         [0027]    [0027]FIG. 7 is a block diagram showing a specific configuration of a control circuit in the second preferred embodiment;  
         [0028]    [0028]FIG. 8 is a flowchart of supervisory control in the second preferred embodiment;  
         [0029]    [0029]FIG. 9 is a block diagram showing a third preferred embodiment of the device according to the present invention;  
         [0030]    [0030]FIG. 10 is a block diagram showing a specific configuration of a control circuit in the third preferred embodiment;  
         [0031]    [0031]FIG. 11 is a flowchart of supervisory control in the third preferred embodiment;  
         [0032]    [0032]FIG. 12 is a block diagram showing a fourth preferred embodiment of the device according to the present invention;  
         [0033]    [0033]FIG. 13 is a block diagram showing a specific configuration of a control circuit in the fourth preferred embodiment;  
         [0034]    [0034]FIG. 14 is a flowchart of supervisory control in the fourth preferred embodiment;  
         [0035]    [0035]FIG. 15 is a block diagram showing a first preferred embodiment of the system according to the present invention;  
         [0036]    [0036]FIG. 16 is a graph for illustrating a transmitting method for a supervisory signal; and  
         [0037]    [0037]FIG. 17 is a block diagram showing a second preferred embodiment of the system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    Some preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. Throughout the drawings, substantially the same parts are denoted by the same reference numerals.  
         [0039]    Referring to FIG. 1, there is shown the configuration of a conventional optical amplifier usable as an optical repeater. The optical amplifier shown in FIG. 1 has an input port  2  and an output port  4 . An optical coupler  6 , an erbium doped fiber (EDF)  8  as an optical amplifying medium, and an optical coupler  10  are connected between the input port  2  and the output port  4 . The EDF  8  has a first end  8 A and a second end  8 B.  
         [0040]    A laser diode (LD)  12  as a pumping source is connected to the optical coupler  10 , so that pump light output from the laser diode  12  is supplied through the optical coupler  10  into the EDF  8  from its second end  8 B. An optical signal to be amplified is supplied from the input port  2  through the optical coupler  6  into the EDF  8  from its first end  8 A. When the optical signal is supplied into the EDF  8  being pumped by the pump light, the optical signal is amplified on the principle of stimulated emission. The optical signal thus amplified is passed through the optical coupler  10  and output from the output port  4 .  
         [0041]    A photodetector (PD)  14  is connected to the optical coupler  6 , so as to monitor the power of the input optical signal. The photodetector  14  outputs an electrical signal reflecting the power of the input optical signal. This electrical signal is supplied to a control circuit  16 . The control circuit  16  controls the power of the pump light to be output from the laser diode  12  so that a proper gain according to the input power of the optical signal is obtained in this optical amplifier.  
         [0042]    Residual pump light having not contributed to the optical amplification in the EDF  8  is passed through the first end  8 A and the optical coupler  6  and output from the input port  2  in a direction opposite to the propagation direction of the optical signal. This residual pump light output from the input port  2  caused Raman amplification in an optical fiber transmission line connected to the input port  2 .  
         [0043]    The wavelength of the optical signal is included in a 1.55-μm band (1.50 to 1.60 μm), and the wavelength of the pump light is included in a 1.48-μm band (1.46 to 1.50 μm). By such wavelength setting, an effective gain for the optical signal can be generated in the EDF  8 , and the above-mentioned Raman amplification can also be generated in the optical fiber transmission line.  
         [0044]    Referring to FIG. 2, there are shown the configuration of a general optical fiber transmission system and a power diagram in this system. This system includes an optical fiber transmission line  20  and a plurality of (e.g., two as shown) optical repeaters  18 (# 1 ) and  18 (# 2 ) arranged along the optical fiber transmission line  20 , thereby compensating for losses in the optical fiber transmission line  20 . In the power diagram representing the relation between optical power (dBm) and transmission distance (km), the optical power of an optical signal propagating in the optical fiber transmission line  20  linearly attenuates with the transmission distance in the case that Raman amplification is not considered. When residual pump light is supplied from the optical repeater  18 (# 2 ) to the optical fiber transmission line  20  toward the optical repeater  18 (# 1 ), a gain by Raman amplification is obtained near the input of the optical repeater  18 (# 2 ) as shown by reference numeral  22  in the power diagram.  
         [0045]    Such a gain by Raman amplification varies with the power of the residual pump light leaking from the optical repeater  18 (# 2 ) and the mode field diameter of an optical fiber used as the optical fiber transmission line  20 , so that there is a case that a loss at a splice connection point SC of the optical fiber transmission line  20  on the input side of the optical repeater  18 (# 2 ) cannot be accurately measured, for example. In this case, it is difficult to accurately determine whether or not the splicing work has been well done, causing a possibility of serious trouble from the viewpoint of quality control. To cope with this problem, it may be proposed to suppress the residual pump light leaking from the optical repeater  18 (# 2 ). In this case, however, the gain to be obtained by Raman amplification comes to abandonment, and this method is therefore useless in view of the operation of the system as a whole.  
         [0046]    Letting Lsp denote the loss at the splice connection point SC, Pincabl denote the power of an optical signal output from the optical repeater  18 (# 1 ) to the optical fiber transmission line  20 , Lcabl denote the loss in the optical fiber transmission line  20  between the optical repeaters  18 (# 1 ) and  18 (# 2 ), Gram denote the gain by Raman amplification, and Pinrep denote the input power to the optical repeater  18 (# 2 ), the following equation holds.  
         
       Pincabl−Lcabl+Gram−Lsp=Pinrep  
     
         [0047]    In the above equation, Pincabl, Lcabl, and Pinrep can be easily grasped in advance by measurement, and Lsp is to be measured in the splicing work. The measurement accuracy of Lsp is reduced for the reason that Gram cannot be accurately grasped because of its variations.  
         [0048]    [0048]FIG. 3 is a block diagram showing a first preferred embodiment of the device according to the present invention. The device according to the present invention is usable as an optical repeater in an optical fiber transmission system (this applies also to the following preferred embodiments). In contrast to the optical repeater shown in FIG. 1, the device shown in FIG. 3 is characterized in that an optical filter unit  24  is connected between the optical coupler  6  and the first end  8 A of the EDF  8 . The optical filter unit  24  includes an optical switch  26  connected to the optical coupler  6 , an optical switch  28  connected to the first end  8 A of the EDF  8 , first and second optical paths  30  and  32  connected in parallel between the optical switches  26  and  28 , and an optical filter  34  provided in the middle of the optical path  30 . The optical filter  34  has a function of passing an optical signal to be supplied from the input port  2  to the EDF  8  and to be amplified in the EDF  8 , and not passing residual pump light to be supplied from the EDF  8  to the input port  2 . Accordingly, by interlockingly operating the optical switches  26  and  28 , the optical filter unit  24  can selectively switch between a first condition where both the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed. An optical bandpass filter or an optical bandstop filter may be used as the optical filter  34 .  
         [0049]    The control circuit  16  in this preferred embodiment has an additional function in relation to the optical filter unit  24 , in addition to the above-mentioned function. As will be hereinafter described, a supervisory signal is superimposed on a main signal of the optical signal to be amplified, and the control circuit  16  can regenerate the supervisory signal according to the electrical signal supplied from the photodetector  14 . By operating the optical switches  26  and  28  according to the regenerated supervisory signal, remote control of the optical filter unit  24  can be performed. Further, the supervisory signal may be updated according to the result of monitoring of the input optical power by the photodetector  14 , and the updated supervisory signal may be transmitted to a downstream device (e.g., another optical repeater or a receiving terminal device). The transmission of the updated supervisory signal may be effected, for example, by a method of intensity-modulating the pump light to be output from the laser diode  12  according to the updated supervisory signal to thereby modulate the gain generated in the EDF  8  and superimpose the updated supervisory signal on the main signal.  
         [0050]    For example, in the case of monitoring a splice loss in the vicinity of the input port  2  by remote operation, the optical switches  26  and  28  are operated according to the supervisory signal to select the first optical path  30 . Accordingly, the optical signal supplied to the input port  2  and to be amplified in the EDF  8  is supplied through the optical filter  34  to the EDF  8 , while the pump light from the laser diode  12  is normally supplied to the EDF  8 , so that the operation of this device as an optical repeater is maintained. The residual pump light from the EDF  8  is stopped by the optical filter  34  and does not reach the input port  2 . Accordingly, the splice loss can be monitored with the occurrence of Raman amplification on the input side of this device being inhibited. The input optical power to this device can be measured by the photodetector  14 , and Gram in the above-mentioned equation is 0. Therefore, the splice loss can be obtained easily and accurately from the above-mentioned equation. After this monitoring, the optical switches  26  and  28  are operated according to the supervisory signal to select the second optical path  32 , so that the input port  2  and the EDF  8  are brought into direct coupling with each other, thus allowing the transmission with the occurrence of Raman amplification by the residual pump light.  
         [0051]    The supervisory control in the first preferred embodiment will now be described more specifically with reference to FIGS. 4 and 5. FIG. 4 is a block diagram showing a specific configuration of the control circuit  16  in the first preferred embodiment, and FIG. 5 is a flowchart of the supervisory control in the first preferred embodiment.  
         [0052]    As shown in FIG. 4, the control circuit  16  includes a level monitor circuit  161  and an SV signal extracting circuit  162  both receiving an output from the photodetector  14 . The level monitor circuit  161  detects the level of the input optical power according to the output from the photodetector  14  and supplies the result of this detection to a control section  163 . The SV signal extracting circuit  162  extracts a supervisory signal (SV signal) according to the output from the photodetector  14 , and supplies the result of this extraction to the control section  163 . For example, the SV signal is composed of a 10-bit digital signal indicating an address for identifying a repeater and a 4-bit digital signal determining a control command to the repeater. An output from the control section  163  is supplied to an LD driving circuit  164  and switch driving circuits  165  and  166 . The laser diode  12  for generating pump light is driven by the LD driving circuit  164 , and the optical switches  26  and  28  are driven by the switch driving circuits  165  and  166 , respectively.  
         [0053]    Referring to FIG. 5, a transmission system including repeaters is powered on in step  111 . In step  112 , a command for operating the optical switches  26  and  28  to select the optical path  30  having the optical filter  34  is transmitted from a transmitting device to a repeater to be monitored.  
         [0054]    In step  113 , an input level monitor command is transmitted from the transmitting device to the repeater. In step  114 , an input level monitor response (1) from the repeater is confirmed, and a command for operating the optical switches  26  and  28  to select the optical path  32  not having the optical filter  34  is then transmitted from the transmitting device to the repeater. In this stage, an input level not influenced by Raman amplification can be obtained.  
         [0055]    In step  115 , an input level monitor command is transmitted again from the transmitting device to the repeater. In step  116 , an input level monitor response (2) from the repeater is confirmed. In this stage, an input level with Raman amplification added can be obtained. In step  117 , a Raman amplification factor is calculated from the difference between the input level monitor responses (1) and (2). In step  118 , the operation is shifted to another monitoring operation.  
         [0056]    As an initial condition, the optical switches  26  and  28  are automatically reset by powering on the system to have the positions selecting the optical path  32 .  
         [0057]    [0057]FIG. 6 is a block diagram showing a second preferred embodiment of the device according to the present invention. In this preferred embodiment, an acousto-optic tunable filter (AOTF)  36  is used in place of the optical filter unit  24  shown in FIG. 3. The AOTF  36  may be obtained by forming an optical waveguide and a waveguide structure for surface acoustic waves (SAW) propagating in relation to this optical waveguide on a substrate. For example, an optical waveguide suitable for the AOTF  36  may be obtained by thermal diffusion of Ti on a LiNbO 3  substrate having birefringence of light. Further, to propagate surface acoustic waves in relation to the optical waveguide, an interdigital transducer (IDT) is formed on the substrate. By propagating surface acoustic waves in relation to the optical waveguide, mode conversion from a TE mode to a TM mode or vice versa is performed on light having a specific wavelength determined according to the power and frequency of the surface acoustic waves and the birefringence of the optical waveguide. Accordingly, by extracting the mode-converted light through specific means such as a polarization beam splitter, different wavelength components of light can be obtained. This selective operation depends on the frequency of the surface acoustic waves, so that the wavelength of light passing through or not passing through the AOTF  36  becomes tunable according to the frequency of the surface acoustic waves.  
         [0058]    Also by using the AOTF  36 , the first condition where both the optical signal and the pump light are passed and the second condition where the optical signal is passed and the pump light is not passed can be selectively switched. Accordingly, by configuring the device so that the control circuit  16  controls the AOTF  36  according to the supervisory signal, the present invention can be carried out as in the preferred embodiment shown in FIG. 3.  
         [0059]    [0059]FIG. 7 is a block diagram showing a specific configuration of the control circuit  16  in the second preferred embodiment, and FIG. 8 is a flowchart of the supervisory control in the second preferred embodiment.  
         [0060]    In contrast to the preferred embodiment shown in FIG. 4, the control circuit  16  in the second preferred embodiment is characterized in that an output from the control section  163  is supplied to an AOTF driving circuit  167  as shown in FIG. 7. The AOTF driving circuit  167  turns on/off the driving of the AOTF  36  according to a signal from the control section  163 .  
         [0061]    Referring to FIG. 8, the flow of the supervisory control in this preferred embodiment is characterized in that the steps  112  and  114  shown in FIG. 5 are changed to steps  112 A and  114 A, respectively. In step  112 A, an AOTF drive command is transmitted from a transmitting device to a repeater to be monitored. In step  114 A, an input level monitor response (1) from the repeater is confirmed, and an AOTF drive stop command is then transmitted from the transmitting device to the repeater. The other steps are similar to those of the flowchart shown in FIG. 5, so the description thereof will be omitted herein.  
         [0062]    [0062]FIG. 9 is a block diagram showing a third preferred embodiment of the device according to the present invention. In contrast to the preferred embodiment shown in FIG. 3, the third preferred embodiment is characterized in that the optical filter unit  24  is omitted and a laser diode  38  as a second pumping source is additionally provided. Pump light output from the laser diode  38  is supplied through an optical coupler  40  connected between the optical coupler  6  and the first end  8 A of the EDF  8  into the EDF  8  from its first end  8 A. Accordingly, the EDF  8  can be subjected to forward pumping by the pump light from the laser diode  38  (the pump light and the optical signal propagate in the same direction in the EDF  8 ) and backward pumping by the pump light from the laser diode  12  (the pump light and the optical signal propagate in opposite directions in the EDF  8 ). The operations of the laser diodes  12  and  38  are controlled by the control circuit  16  according to the supervisory signal received by the control circuit  16 .  
         [0063]    There will now be described a method of evaluating the characteristics of the optical fiber transmission line  20  by using the device shown in FIG. 9 as each of the optical repeaters  18 (# 1 ) and  18 (# 2 ) shown in FIG. 2. In the case of monitoring a splice loss on the input side of the optical repeater  18 (# 2 ) by remote operation, the laser diode  38  in the optical repeater  18 (# 1 ) is turned off by a supervisory signal, and the laser diode  12  in the optical repeater  18 (# 2 ) is turned off by a supervisory signal. In this case, the EDF  8  in the optical repeater  18 (# 1 ) is being pumped by the laser diode  12  in the optical repeater  18 (# 1 ), and the EDF  8  in the optical repeater  18 (# 2 ) is being pumped by the laser diode  38  in the optical repeater  18 (# 2 ), thereby allowing the transmission of an optical signal. Further, no residual pump light leaks from the optical repeaters  18 (# 1 ) and  18 (# 2 ) to the optical fiber transmission line  20  between the optical repeaters  18 (# 1 ) and  18 (# 2 ), so that Raman amplification does not occur. Accordingly, the loss at the splice connection point SC can be measured easily and accurately in accordance with the above-mentioned equation, and the supervisory control relating to this monitoring and the transmission of the results of the supervisory control can be performed online.  
         [0064]    After monitoring the splice loss, the laser diode  38  in the optical repeater  18 (# 1 ) is turned on by a supervisory signal, and the laser diode  12  in the optical repeater  18 (# 2 ) is turned on by a supervisory signal. As a result, a predetermined gain in the EDF  8  in each of the optical repeaters  18 (# 1 ) and  18 (# 2 ) can be obtained.  
         [0065]    In the case of evaluating a splice loss on the output side of the optical repeater  18 (# 1 ), the laser diode  38  in the optical repeater  18 (# 1 ) is turned off to thereby allow the measurement in the condition where Raman amplification does not contribute.  
         [0066]    The wavelength of the pump light to be output from the laser diode  38  for forward pumping the EDF  8  may be included in the 1.48-μm band which includes the wavelength of the pump light for backward pumping. Alternatively, the wavelength of the pump light to be output from the laser diode  38  may be included in a 0.98 μm band (0.96 to 1.0 μm). In this case, Raman amplification by this pump light to an optical signal having a wavelength included in the 1.55 μm band does not occur. Accordingly, it is not necessary to turn off the laser diode  38  in evaluating the characteristics of the optical fiber transmission line  20 .  
         [0067]    [0067]FIG. 10 is a block diagram showing a specific configuration of the control circuit  16  in the third preferred embodiment of the present invention, and FIG. 11 is a flowchart of the supervisory control in the third preferred embodiment.  
         [0068]    Referring to FIG. 10, the control circuit  16  in this preferred embodiment is characterized in that the control section  163  controls an LD driving circuit  168 , in contrast to the control circuit  16  shown in FIG. 4 wherein the control section  163  controls the switch driving circuits  165  and  166 . The LD driving circuit  168  drives the LD  38  for forward pumping.  
         [0069]    Referring to FIG. 11, the flow of the supervisory control in this preferred embodiment is characterized in that the steps  112  and  114  shown in FIG. 5 are changed to steps  112 B and  114 B, respectively. In step  112 B, an LD  12  drive stop command is transmitted from a transmitting device to a repeater to be monitored. In step  114 B, an input level monitor response (1) from the repeater is confirmed, and an LD  12  drive command is then transmitted from the transmitting device to the repeater. The other steps are similar to those of the flow shown in FIG. 5, so the description thereof will be omitted herein.  
         [0070]    [0070]FIG. 12 is a block diagram showing a fourth preferred embodiment of the device according to the present invention. In contrast to the preferred embodiment shown in FIG. 9, the fourth preferred embodiment is characterized in that a laser diode  42  as a pumping source for positively producing Raman amplification is additionally provided. Pump light having a wavelength included in the 1.48-μm band, for example, is output from the laser diode  42 , and this pump light is passed through an optical coupler  44 , the optical coupler  6 , and the input port  2  in this order and then output to an optical fiber transmission line. Accordingly, in the case of using this device as the optical repeater  18 (# 2 ) shown in FIG. 2 and measuring the loss at the splice connection point SC on the input side of the optical repeater  18 (# 2 ), the laser diodes  12  and  42  are turned off to thereby allow the measurement in the condition where Raman amplification does not occur. In this case, the EDF  8  is being forward pumped by the pump light from the laser diode  38 , so that the transmission of an optical signal through the optical repeater  18 (# 2 ) is allowed, and the monitoring by the supervisory control and the transmission of the monitoring result by a supervisory signal are also allowed.  
         [0071]    [0071]FIG. 13 is a block diagram showing a specific configuration of the control circuit  16  in the fourth preferred embodiment, and FIG. 14 is a flowchart of the supervisory control in the fourth preferred embodiment.  
         [0072]    Referring to FIG. 13, the control circuit  16  in this preferred embodiment is characterized in that an LD driving circuit  169  is additionally provided, in contrast to the control circuit  16  shown in FIG. 10. The LD driving circuit  169  drives the laser diode  42  as a pumping source for positively producing Raman amplification.  
         [0073]    Referring to FIG. 14, the flow of the supervisory control in this preferred embodiment is characterized in that the steps  112  and  114  shown in FIG. 5 are changed to steps  112 C and  114 C, respectively. In step  112 C, an LD  12  and LD  42  drive stop command is transmitted from a transmitting device to a repeater to be monitored. In step  114 C, an input level monitor response (1) from the repeater is confirmed, and an LD  12  and LD  42  drive command is then transmitted from the transmitting device to the repeater. The other steps are similar to those of the flow shown in FIG. 5, so the description thereof will be omitted herein.  
         [0074]    [0074]FIG. 15 is a block diagram showing a first preferred embodiment of the system according to the present invention. This system includes a transmitting station  46  and a receiving station  48  each serving as a terminal device, an optical fiber transmission line  20  laid between the transmitting station  46  and the receiving station  48 , and a plurality of (e.g., three as shown) optical repeaters  18 (# 1 ),  18 (# 2 ), and  18 (# 3 ) arranged along the optical fiber transmission line  20 . Each of the optical repeaters  18 (# 1 ) to  18 (# 3 ) may be provided by the device according to the present invention. The transmitting station  46  sends out an optical signal composed of a main signal as a service signal and a supervisory signal superimposed on the main signal to the optical fiber transmission line  20 . The optical signal is amplified by each of the optical repeaters  18 (# 1 ) to  18 (# 3 ), and finally received by the receiving station  48 .  
         [0075]    Referring to FIG. 16, a transmitting method for the supervisory signal is shown. The main signal is shown by reference numeral  50 , and the supervisory signal is shown by reference numeral  52 . By intensity modulating the main signal  50  by several % of the amplitude of the main signal  50 , the supervisory signal  52  lower in speed than the main signal  50  is superimposed thereon. As a modulation method using the supervisory signal as a carrier, ASK, FSK, PSK, etc. are adoptable. By using such a supervisory signal in the system shown in FIG. 15, for example, a remote control command can be transmitted from the transmitting station  46  to each of the optical repeaters  18 (# 1 ) to  18 (# 3 ) or to the receiving station  48 . The remote control command includes an identification address preliminarily set for each of the optical repeaters  18 (# 1 ) to  18 (# 3 ), thereby allowing individual supervisory control for the optical repeaters  18 (# 1 ) to  18 (# 3 ).  
         [0076]    [0076]FIG. 17 is a block diagram showing a second preferred embodiment of the system according to the present invention. This system includes first and second terminals  54  and  56 , a down optical fiber transmission line  20 A and an up optical fiber transmission line  20 B both laid between the first and second terminals  54  and  56 , a plurality of (e.g., three as shown) optical repeaters  18 A(# 1 ),  18 A(# 2 ), and  18 A(# 3 ) arranged along the down optical fiber transmission line  20 A, and a plurality of (e.g., three as shown) optical repeaters  18 B(# 1 ),  18 B(# 2 ), and  18 B(# 3 ) arranged along the up optical fiber transmission line  20 B. The optical repeaters  18 A(# 1 ) and the optical repeater  18 B(# 1 ) are provided in the same repeater housing. The optical repeater  18 A(# 2 ) and the optical repeater  18 B(# 2 ) are provided in the same repeater housing. The optical repeater  18 A(# 3 ) and the optical repeater  18 B(# 3 ) are provided in the same repeater housing. A supervisory signal can be exchanged between the optical repeaters provided in the same repeater housing.  
         [0077]    Accordingly, in the case of carrying out the method according to the present invention by using the system shown in FIG. 17, the optical repeater  18 A(# 1 ) receives a remote control command, for example, and performs monitoring according to this command. Then, the monitoring result may be transmitted to the second terminal  56  or may be transmitted to the first terminal  54  through the optical repeater  18 B(# 1 ) exchanging the supervisory signal. The transmission of the monitoring result may be effected by using the intensity modulation of pump light as mentioned previously.  
         [0078]    It should be noted that the above preferred embodiments are merely illustrative and not limitative. For example, while the loss at the splice connection point is adopted as the characteristics of the optical fiber transmission line, the loss characteristics of the optical fiber transmission line itself may be evaluated in accordance with the present invention.  
         [0079]    According to the present invention as described above, it is possible to provide a method, device, and system for accurately evaluating the characteristics of an optical fiber transmission line. The effects obtained by the specific preferred embodiments of the present invention have been described above, so the description thereof will be omitted herein.  
         [0080]    The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.