Abstract:
The object of the present invention is to provide an apparatus capable of measuring wavelength dispersion characteristic and other characteristics by using only a single fiber pair.  
     In order to achieve said object, the apparatus according to the present invention includes a variable wavelength light source  12  for generating a variable wavelength light, the wavelength of which is variable, a first light modulator  15  for inputting into the first optical fiber transmission line  32  the first incident light obtained by modulating the variable wavelength light by the frequency of the electrical signals inputted, a first optical/electrical converter  22  for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical fiber transmission line  32 , a fixed wavelength light source  21  for generating a fixed wavelength light, the wavelength of which is fixed, a power source (signal source)  25  for generating reference electrical signals of given frequencies, a second light modulator  23  for inputting in the second optical fiber transmission line  34  the second incident light obtained by modulating the fixed wavelength light by the frequency fin of the reference electrical signal and a second optical/electrical converter  16  for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical fiber transmission line  34  and for outputting into the first light modulator  15 . When the result of optical/electrical conversion of the first outgoing light and the reference electrical signals are available, it is possible to compute wavelength dispersion characteristic and other characteristics by comparing their phases.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The present invention relates to the measurement of the wavelength dispersion characteristic of devices under test (DUT) such as fiber pair, and in particular to the measurement of the wavelength dispersion characteristic by connecting separate measuring methodes on both ends of the DUT.  
           [0003]    2. Description of the Related Art  
           [0004]    In case of light being transmitted over a long distance, the transmission of light only through an optical fiber will involve considerable losses. Therefore, optical fiber transmission lines combined with optical amplifiers (EDFA) for amplifying optical signals are used for the optical fiber to prevent any possible losses. The optical amplifiers let light through only in one direction. Therefore, a bi-directional communication requires a cable integrating an optical fiber transmission line transmitting light in one direction and another optical fiber transmission line transmitting light in the direction opposite to the one direction. This cable is called a fiber pair.  
           [0005]    The configuration of a fiber pair is shown in FIG. 6( a ). An optical fiber  112  combined with an optical amplifier  114  constitute an optical fiber transmission line  110 . The optical fiber transmission line  110  lets light through to the right. An optical fiber  122  combined with an optical amplifier  124  constitutes an optical fiber transmission line  120 . The optical fiber transmission line  120  lets light through to the left. An optical fiber transmission line  110  and a optical fiber transmission line  120  constitutes an optical fiber pair  100   a . Incidentally, two sets of fiber pairs are called two fiber pairs as shown in FIG. 6( b ). Fiber pairs  100   a  and  10   b  constitute two fiber pairs  100 .  
           [0006]    The configuration of the measurement system for measuring the wavelength characteristic of two fiber pairs is shown in FIG. 7. At one end of a fiber pair  100   a , which is one of two fiber pairs  100 , a variable wavelength light source  202  is connected and at another end an O/E (optical/electric) converter  302  is connected. At one end of a fiber pair  10   b , which is one of the two fiber pairs  100 , a fixed wavelength light source  204  is connected, and at another end an O/E (optical/electric) converter  304  is connected. Incidentally, between the variable wavelength light sources  202 ,  204  and single fiber pairs  100   a ,  100   b , a light modulator may be installed.  
           [0007]    To measure wavelength dispersion characteristics, the wavelength λx of the variable wavelength light source  202  is swept, while the wavelength λ0 of the fixed wavelength light source  204  is fixed. The phase difference between the output signals of the O/E converter  302  and the output signals of the O/E converter  304  is measured by the phase comparator  306 , to measure the wavelength dispersion characteristic of the two fiber pairs.  
           [0008]    Here, in a bulk transmission line constituting a trunk line, two fiber pairs may be secured. In most lines already laid out, often only one fiber pair can be secured. Therefore, it is necessary to measure the wavelength dispersion characteristic of a single fiber pair.  
         SUMMARY OF INVENTION  
         [0009]    Such a measuring method of the wavelength dispersion characteristic, however, cannot be applied to a single fiber pair. The reason is that two lines consisting of a line for letting a fixed wavelength light through and another line for letting a variable wavelength light through cannot be secured by a single fiber pair.  
           [0010]    Further, even if such a measuring method of wavelength dispersion characteristic is applied to two fiber pairs  100 , the measurements may involve errors. In other words, due to physical quantitative variations including variations in the temperature, stress, etc. of the transmission line, the phase difference of light penetrating a fiber pair  100   a  and another fiber pair  100   b  may vary due to factors independent of wavelength. In such a case, the measurements may involve errors. Therefore, it is desirable that wavelength dispersion characteristics may be measured only by a single fiber pair without using two fiber pairs.  
           [0011]    Therefore, the present invention has an object of providing an apparatus capable of measuring wavelength dispersion characteristic and other characteristics only through a single fiber pair.  
           [0012]    According to the present invention as described in claim  1 , an optical characteristic measuring apparatus for measuring the characteristics of devices under test having the first optical transmission line letting light through in one direction only and the second optical transmission line letting light through only on the direction opposite to the aforementioned direction includes: a variable wavelength light source for generating a variable wavelength light, the wavelength of which is variable; a first light modulating unit for introducing into the first optical transmission line the first incident light obtained by modulating the variable wavelength light by the frequency of the electrical signal inputted; a first optical/electrical converting unit for converting by the optical/electrical conversion process the first incident light having penetrated the first optical transmission line; a fixed wavelength light source for generating a fixed wavelength light, the wavelength of which is fixed; a signal source for generating reference electrical signals of given frequencies; a second light modulating unit for injecting into the second optical transmission line the second incident light obtained by modulating the fixed wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting unit for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line; and for outputting the converted second outgoing light onto the first light modulating unit.  
           [0013]    According to an optical characteristic measuring apparatus thus configured, once the wavelength of the fixed wavelength light is set in such a way that wavelength dispersion may be small in the second optical transmission line, the result of optical/electrical conversion of the second outgoing light produces a small phase difference than that of the second incident light. Thus, it is possible to consider that the result of optical/electrical conversion of the second outgoing light and the reference electrical signals may have the identical frequencies and phases. Thus, it is possible to consider that the first incident light may be same as the result of modulation of the variable wavelength light by the reference electrical signals. Thus, once the result of optical/electrical conversion of the first outgoing light and the reference electrical signals are obtained, the comparison of their phases can lead to the discovery of phase differences related to the first optical transmission line. And from the phase difference, wavelength dispersion characteristic and other factors can be computed.  
           [0014]    According to the present invention as described in claim  2 , an optical characteristic measuring apparatus for measuring the characteristics of devices under test having the first optical transmission line for letting light through only in one direction and the second optical transmission line for letting light through only in the direction opposite to the one direction includes: a fixed wavelength light source for generating a fixed wavelength light, the wavelength of which is fixed; a first light modulating unit for introducing into the first optical transmission line the first incident light obtained by modulating the fixed wavelength light by the frequency of the electrical signals inputted; a first optical/electrical converting unit for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a variable wavelength light source for generating a variable wavelength light, the wavelength of which is variable; a signal source for generating reference electrical signals of given frequencies; a second light modulating unit for introducing onto the second optical transmission line the second incident light obtained by modulating the variable wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting unit for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating unit.  
           [0015]    According to an optical characteristic measuring apparatus thus configured, the result of optical/electrical conversion of the second outgoing light will be electrical signals containing phase differences related to the second optical transmission line. Therefore, once the wavelength of the fixed wavelength light is set in such a way that wavelength dispersion may be small in the first optical transmission line, the first outgoing light containing phase differences related to the second optical transmission line and yet free of errors related to the first optical transmission line can be obtained. Thus, once the result of optical/electrical conversion of the first outgoing light and the reference electrical signals are obtained, the comparison of their phases can lead to the discovery of phase difference related to the second optical transmission line. And from the phase difference, wavelength dispersion characteristic and other factors can be computed.  
           [0016]    According to the present invention as described in claim  3 , an optical characteristic measuring apparatus for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a first variable wavelength light source for generating the first variable wavelength light, the wavelength of which is variable; a first light modulating unit for introducing onto the first optical transmission line the first incident light obtained by modulating the first variable wavelength light by the frequency of electrical signals inputted; a first optical/electrical converting unit for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a second variable wavelength light source for generating the second variable wavelength light, the wavelength of which is variable; a signal source for generating reference electrical signals of given frequencies; a second light modulating unit for introducing into the second optical transmission line the second incident light obtained by modulating the second variable wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting unit for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating unit.  
           [0017]    According to an optical characteristic measuring apparatus thus configured, by using a first variable wavelength light source and a second variable wavelength light source, the wavelength dispersion characteristic and other factors of the first optical transmission line and the second optical transmission line can be computed.  
           [0018]    According to the present invention as described in claim  4 , the optical characteristic measuring apparatus according to claim  2  includes a third optical/electrical converting unit for converting by the optical/electrical conversion process the reflected light generated when the second light modulating unit introduces the second incident light into the second optical transmission line.  
           [0019]    According to the present invention as described in claim  5 , the optical characteristic measuring apparatus according to claim  1  includes: a phase comparing unit for measuring the phase difference between the electrical signals for measurement outputted by the first optical/electrical converting unit and the reference electrical signals; and a characteristic computing unit for computing the group delay characteristic or the dispersion characteristic of the devices under test by using the phase difference.  
           [0020]    According to the present invention as described in claim  6 , the optical characteristic measuring apparatus according to claim  4  includes: a phase comparing unit for measuring the phase difference between the electrical signals for reflection measurement outputted by the third optical/electrical converting unit and the reference electrical signals; and a characteristic computing unit for computing the group delay characteristic or the dispersion characteristic of the devices under test.  
           [0021]    According to the present invention as described in claim  7 , a light generating apparatus used in an apparatus for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only on the direction opposite to the one direction includes: a variable wavelength light source for generating a variable wavelength light, the wavelength of which is variable; a first light modulating unit for introducing into the first optical transmission line the first incident light obtained by modulating the variable wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting unit for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating unit.  
           [0022]    According to the present invention as described in claim  8 , an optical characteristic measuring apparatus for measuring the characteristics of devices under test having a first optical transmission line letting light through only in one direction and a second optical transmission line letting light through only in the direction opposite to the one direction includes: a first optical/electrical converting unit for converting by the optical/electrical conversion process the first incident light having penetrated the first optical transmission line; a fixed wavelength light source for generating a fixed wavelength light, the wavelength of which is fixed; a signal source for generating reference electrical signals of given frequencies; and a second light modulating unit for introducing into the second optical transmission line the second incident light obtained by modulating the fixed wavelength light by the frequency of the reference electrical signals.  
           [0023]    According to the present invention as described in claim  9 , a light generating apparatus used in a measuring apparatus of the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a fixed wavelength light source for generating a fixed wavelength light, the wavelength of which is fixed; a first light modulating unit for introducing into the first optical transmission line the first incident light obtained by modulating the fixed wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting unit for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating unit.  
           [0024]    According to the present invention as described in claim  10 , an optical characteristic measuring apparatus for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a first optical/electrical converting unit for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a variable wavelength light source for generating a variable wavelength light, the wavelength of which is variable; a signal source for generating reference electrical signals of given frequencies; a second light modulating unit for introducing into the second optical transmission line the second incident light obtained by modulating the variable wavelength light by the frequency of the reference electrical signals.  
           [0025]    According to the present invention as described in claim  11 , a light generating apparatus used in a measuring apparatus of the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a first variable wavelength light source for generating the first variable wavelength light, the wavelength of which is variable; a first light modulating unit for introducing into the first optical transmission line the first incident light obtained by modulating the first variable wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting unit for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating unit.  
           [0026]    According to the present invention as described in claim  12 , an optical characteristic measuring apparatus for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a first optical/electrical converting unit for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a second variable wavelength light source for generating the second variable wavelength light, the wavelength of which is variable; a signal source for generating reference electrical signals of given frequencies; a second light modulating unit for introducing into the second optical transmission line the second incident light obtained by modulating the second variable wavelength light by the frequency of the reference electrical signals.  
           [0027]    According to the present invention as described in claim  13 , an optical characteristic measuring method for measuring the characteristics of devices under test having the first optical transmission line letting light through in one direction only and the second optical transmission line letting light through only on the direction opposite to the aforementioned direction includes: a variable wavelength light generating step for generating a variable wavelength light, the wavelength of which is variable; a first light modulating step for introducing into the first optical transmission line the first incident light obtained by modulating the variable wavelength light by the frequency of the electrical signal inputted; a first optical/electrical converting step for converting by the optical/electrical conversion process the first incident light having penetrated the first optical transmission line; a fixed wavelength light generating step for generating a fixed wavelength light, the wavelength of which is fixed; a signal generating step for generating reference electrical signals of given frequencies; a second light modulating step for injecting into the second optical transmission line the second incident light obtained by modulating the fixed wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting step for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line; and for outputting the converted second outgoing light onto the first light modulating step.  
           [0028]    According to the present invention as described in claim  14 , an optical characteristic measuring method for measuring the characteristics of devices under test having the first optical transmission line for letting light through only in one direction and the second optical transmission line for letting light through only in the direction opposite to the one direction includes: a fixed wavelength light generating step for generating a fixed wavelength light, the wavelength of which is fixed; a first light modulating step for introducing into the first optical transmission line the first incident light obtained by modulating the fixed wavelength light by the frequency of the electrical signals inputted; a first optical/electrical converting step for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a variable wavelength light generating step for generating a variable wavelength light, the wavelength of which is variable; a signal generating step for generating reference electrical signals of given frequencies; a second light modulating step for introducing onto the second optical transmission line the second incident light obtained by modulating the variable wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting step for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating step.  
           [0029]    According to the present invention as described in claim  15 , an optical characteristic measuring method for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a first variable wavelength light generating step for generating the first variable wavelength light, the wavelength of which is variable; a first light modulating step for introducing onto the first optical transmission line the first incident light obtained by modulating the first variable wavelength light by the frequency of electrical signals inputted; a first optical/electrical converting step for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a second variable wavelength light generating step for generating the second variable wavelength light, the wavelength of which is variable; a signal generating step for generating reference electrical signals of given frequencies; a second light modulating step for introducing into the second optical transmission line the second incident light obtained by modulating the second variable wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting step for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating step.  
           [0030]    According to the present invention as described in claim  16 , a light generating method used in a method for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only on the direction opposite to the one direction includes: a variable wavelength light generating step for generating a variable wavelength light, the wavelength of which is variable; a first light modulating step for introducing into the first optical transmission line the first incident light obtained by modulating the variable wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting step for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating step.  
           [0031]    According to the present invention as described in claim  17 , an optical characteristic measuring method for measuring the characteristics of devices under test having a first optical transmission line letting light through only in one direction and a second optical transmission line letting light through only in the direction opposite to the one direction includes: a first optical/electrical converting step for converting by the optical/electrical conversion process the first incident light having penetrated the first optical transmission line; a fixed wavelength light generating step for generating a fixed wavelength light, the wavelength of which is fixed; a signal generating step for generating reference electrical signals of given frequencies; and a second light modulating step for introducing into the second optical transmission line the second incident light obtained by modulating the fixed wavelength light by the frequency of the reference electrical signals.  
           [0032]    According to the present invention as described in claim  18 , a light generating method used in a measuring method of the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a fixed wavelength light generating step for generating a fixed wavelength light, the wavelength of which is fixed; a first light modulating step for introducing into the first optical transmission line the first incident light obtained by modulating the fixed wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting step for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating step.  
           [0033]    According to the present invention as described in claim  19 , an optical characteristic measuring method for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a first optical/electrical converting step for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a variable wavelength light generating step for generating a variable wavelength light, the wavelength of which is variable; a signal generating step for generating reference electrical signals of given frequencies; a second light modulating step for introducing into the second optical transmission line the second incident light obtained by modulating the variable wavelength light by the frequency of the reference electrical signals.  
           [0034]    According to the present invention as described in claim  20 , a light generating method used in a measuring method of the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction includes: a first variable wavelength light generating step for generating the first variable wavelength light, the wavelength of which is variable; a first light modulating step for introducing into the first optical transmission line the first incident light obtained by modulating the first variable wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting step for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating step.  
           [0035]    The present invention as described in claim  21 , is an optical characteristic measuring method for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction including: a first optical/electrical converting step for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a second variable wavelength light generating step for generating the second variable wavelength light, the wavelength of which is variable; a signal generating step for generating reference electrical signals of given frequencies; a second light modulating step for introducing into the second optical transmission line the second incident light obtained by modulating the second variable wavelength light by the frequency of the reference electrical signals.  
           [0036]    The present invention as described in claim  22 , is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring the characteristics of devices under test having the first optical transmission line letting light through in one direction only and the second optical transmission line letting light through only on the direction opposite to the aforementioned direction, the optical characteristic measuring process including: a variable wavelength light generating processing for generating a variable wavelength light, the wavelength of which is variable; a first light modulating processing for introducing into the first optical transmission line the first incident light obtained by modulating the variable wavelength light by the frequency of the electrical signal inputted; a first optical/electrical converting processing for converting by the optical/electrical conversion process the first incident light having penetrated the first optical transmission line; a fixed wavelength light generating processing for generating a fixed wavelength light, the wavelength of which is fixed; a signal generating processing for generating reference electrical signals of given frequencies; a second light modulating processing for injecting into the second optical transmission line the second incident light obtained by modulating the fixed wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting processing for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line; and for outputting the converted second outgoing light onto the first light modulating processing.  
           [0037]    The present invention as described in claim  23 , is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring the characteristics of devices under test having the first optical transmission line for letting light through only in one direction and the second optical transmission line for letting light through only in the direction opposite to the one direction, the optical characteristic measuring process including: a fixed wavelength light generating processing for generating a fixed wavelength light, the wavelength of which is fixed; a first light modulating processing for introducing into the first optical transmission line the first incident light obtained by modulating the fixed wavelength light by the frequency of the electrical signals inputted; a first optical/electrical converting processing for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a variable wavelength light generating processing for generating a variable wavelength light, the wavelength of which is variable; a signal generating processing for generating reference electrical signals of given frequencies; a second light modulating processing for introducing onto the second optical transmission line the second incident light obtained by modulating the variable wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting processing for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating processing.  
           [0038]    The present invention as described in claim  24 , is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction, the optical characteristic measuring process including: a first variable wavelength light generating processing for generating the first variable wavelength light, the wavelength of which is variable; a first light modulating processing for introducing onto the first optical transmission line the first incident light obtained by modulating the first variable wavelength light by the frequency of electrical signals inputted; a first optical/electrical converting processing for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a second variable wavelength light generating processing for generating the second variable wavelength light, the wavelength of which is variable; a signal generating processing for generating reference electrical signals of given frequencies; a second light modulating processing for introducing into the second optical transmission line the second incident light obtained by modulating the second variable wavelength light by the frequency of the reference electrical signals; and a second optical/electrical converting processing for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating processing.  
           [0039]    The present invention as described in claim  25 , is a computer-readable medium having a program of instructions for execution by the computer to perform a light generating process used in a process for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only on the direction opposite to the one direction, the light generating process including: a variable wavelength light generating processing for generating a variable wavelength light, the wavelength of which is variable; a first light modulating processing for introducing into the first optical transmission line the first incident light obtained by modulating the variable wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting processing for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating processing.  
           [0040]    The present invention as described in claim  26 , is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring the characteristics of devices under test having a first optical transmission line letting light through only in one direction and a second optical transmission line letting light through only in the direction opposite to the one direction, the optical characteristic measuring process including: a first optical/electrical converting processing for converting by the optical/electrical conversion process the first incident light having penetrated the first optical transmission line; a fixed wavelength light generating processing for generating a fixed wavelength light, the wavelength of which is fixed; a signal generating processing for generating reference electrical signals of given frequencies; and a second light modulating processing for introducing into the second optical transmission line the second incident light obtained by modulating the fixed wavelength light by the frequency of the reference electrical signals.  
           [0041]    The present invention as described in claim  27 , is a computer-readable medium having a program of instructions for execution by the computer to perform a light generating process used in a measuring process of the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction, the light generating process including: a fixed wavelength light generating processing for generating a fixed wavelength light, the wavelength of which is fixed; a first light modulating processing for introducing into the first optical transmission line the first incident light obtained by modulating the fixed wavelength light by the, frequency of electrical signals inputted; and a second optical/electrical converting processing for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating processing.  
           [0042]    The present invention as described in claim  28 , is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction, the optical characteristic measuring process including: a first optical/electrical converting processing for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a variable wavelength light generating processing for generating a variable wavelength light, the wavelength of which is variable; a signal generating processing for generating reference electrical signals of given frequencies; a second light modulating processing for introducing into the second optical transmission line the second incident light obtained by modulating the variable wavelength light by the frequency of the reference electrical signals.  
           [0043]    The present invention as described in claim  29 , is a computer-readable medium having a program of instructions for execution by the computer to perform a light generating process used in a measuring process of the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction, the light generating process including: a first variable wavelength light generating processing for generating the first variable wavelength light, the wavelength of which is variable; a first light modulating processing for introducing into the first optical transmission line the first incident light obtained by modulating the first variable wavelength light by the frequency of electrical signals inputted; and a second optical/electrical converting processing for converting by the optical/electrical conversion process the second outgoing light having penetrated the second optical transmission line and for outputting the converted second outgoing light onto the first light modulating processing.  
           [0044]    The present invention as described in claim  30 , is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring the characteristics of devices under test having the first optical transmission line letting light through only in one direction and the second optical transmission line letting light through only in the direction opposite to the one direction, the optical characteristic measuring process including: a first optical/electrical converting processing for converting by the optical/electrical conversion process the first outgoing light having penetrated the first optical transmission line; a second variable wavelength light generating processing for generating the second variable wavelength light, the wavelength of which is variable; a signal generating processing for generating reference electrical signals of given frequencies; a second light modulating processing for introducing into the second optical transmission line the second incident light obtained by modulating the second variable wavelength light by the frequency of the reference electrical signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]    [0045]FIG. 1 is a block diagram showing the configuration of an optical characteristic measuring apparatus related to the first preferred embodiment of the present invention.  
         [0046]    [0046]FIG. 2 is a flowchart showing the operation of the first preferred embodiment of the present invention.  
         [0047]    [0047]FIG. 3 is a block diagram showing the configuration of an optical characteristic measuring apparatus related to the second and third preferred embodiments of the present invention.  
         [0048]    [0048]FIG. 4 is a flowchart showing the operation of the second preferred embodiment of the present invention.  
         [0049]    [0049]FIG. 5 is a flowchart showing the operation of the third preferred embodiment of the present invention.  
         [0050]    [0050]FIG. 6 is an illustration showing the structure of a fiber pair according to the prior art.  
         [0051]    [0051]FIG. 7 is an illustration showing the configuration of the measuring system used to measure the wavelength dispersion characteristic of two fiber pairs according to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0052]    The preferred embodiments of the present invention are described below with reference to the drawings.  
         [0053]    The First Preferred Embodiment  
         [0054]    [0054]FIG. 1 is a block diagram showing the configuration of an optical characteristic measuring apparatus related to the first preferred embodiment of the present invention. The optical characteristic measuring apparatus related to the first preferred embodiment includes a light source system  10  connected to an end of a fiber pair  30  and a characteristic measuring system  20  connected to another end of the fiber pair  30 .  
         [0055]    A fiber pair  30  includes a first optical fiber transmission line  32  and a second optical fiber transmission line  34 . The optical fiber transmission line  32  includes an optical fiber  32   a  and an optical amplifier  32   b  that amplifies light and is connected to the midway of the optical fiber  32   a . The optical fiber transmission line  32  lets light through to the right. The optical fiber transmission line  34  includes an optical fiber  34   a  and an optical amplifier  34   b  that amplifies light and is connected to the midway of the optical fiber  34   a . The optical fiber transmission line  34  lets light through to the left.  
         [0056]    In the first preferred embodiment, the measurement of the first optical fiber transmission line  32  is assumed, and the light source system  10  is connected to the input (left) side of the first optical fiber transmission line  32  and the characteristic measuring system  20  is connected to the output (right) side.  
         [0057]    The light source system  10  includes a variable wavelength light source  12 , a first light modulator  15 , a second optical/electrical converter  16  and an amplifier  18 . The variable wavelength light source  12  generates a variable wavelength light, the wavelength of which is variable. The variable wavelength light source  12  can sweep the wavelength Ax of the variable wavelength light. The first light modulator  15  modulates the variable wavelength light by the frequency of electrical signals outputted by the second optical/electrical converter  16 . The first light modulator  15  normally contains lithium niobate (LN), but it can dispense with LN provided that it can modulate. The light outputted by the first light modulator (the first incident light) is inputted into the first optical fiber transmission line  32 . The second optical/electrical converter  16  converts by the optical/electrical conversion process the second outgoing light outputted from the second optical fiber transmission line  34 . The amplifier  18  amplifies the electrical signals outputted by the second optical/electrical converter  16  and inputs them into the first light modulator  15 .  
         [0058]    The first incident light inputted into the first optical fiber transmission line  32  penetrates the first optical fiber transmission line  32 . The light having penetrated the first optical fiber transmission line  32  is called as the first outgoing light.  
         [0059]    The characteristic measuring system  20  includes a fixed wavelength light source  21 , a first optical/electrical converter  22 , a second light modulator  23 , an amplifier  24 , a power source (signal source)  25 , a phase comparator  26  and a characteristic computing section  28 .  
         [0060]    The fixed wavelength light source  21  generates a fixed wavelength light, the wavelength of which is fixed. It is desirable to fix the wavelength of the fixed wavelength light at a wavelength λ0 at which the wavelength dispersion will be reduced to the minimum in the second optical fiber transmission line  34 .  
         [0061]    The first optical/electrical converter  22  converts the first outgoing light by the optical/electrical conversion process. The power source (signal source)  25  generates electrical signals of a frequency fin (reference electrical signals). The second light modulator  23  modulates the fixed wavelength light by the frequency fin of the electrical signals outputted by the power source (signals source)  25 . The second light modulator  23  includes lithium niobate (LN). The light outputted by the second light modulator  23  (the second incident light) is inputted into the second optical fiber transmission line  34 . Incidentally, the second incident light penetrates the second optical fiber transmission line  34 . The light having penetrated the second optical fiber transmission line  34  is called as the second outgoing light. The amplifier  24  amplifies the output of the first optical/electrical converter  22 .  
         [0062]    The phase comparator  26  receives the electrical signals generated by the power source (signal source)  25  at a terminal Ref In and the electrical signals outputted by the amplifier  24  at a terminal Prob_In. The phase comparator  26  takes the electrical signals received at the terminal Ref_In as a reference for computing the phase of the electrical signals received at the terminal Prob_In.  
         [0063]    The characteristic computing section  28  records the phases measured by the phase comparator  26  and computes the group delay characteristic and the wavelength dispersion characteristic of the first optical fiber transmission line  32  based on the phases recorded. The group delay characteristic can be computed from the relationship between the phases measured by the phase comparator  26  and the modulation frequency fm. The wavelength dispersion characteristic can be computed by differentiating the group delay characteristic by the wavelength.  
         [0064]    And now, the operation of the first preferred embodiment of the present invention will be described with reference to the flowchart in FIG. 2. On the left side the operation of the characteristic measuring system  20  is shown, and on the right side the operation of the light source system  10  is shown. Referring to the left side to begin with, the fixed wavelength light source  21  generates a fixed wavelength light (λ=λ0) (S 20 ). Then, the fixed wavelength light is modulated by the frequency fm of the reference electrical signals generated by the power source (signal source) (S 22 ). And the process returns to the generation of the fixed wavelength light source (S 20 ).  
         [0065]    The fixed wavelength light modulated by the frequency fm is the second incident light. The second incident light penetrates the second optical fiber transmission line  34  and is inputted into the light source system  10  as the second outgoing light.  
         [0066]    At this point, let us refer to the right side of FIG. 2. The wavelength Ax of the variable wavelength light is changed (S 10 ). Then, the variable wavelength light source  12  generates a variable wavelength light (λ=λx) (S 12 ). The second outgoing light is converted by the optical/electrical conversion process by the second optical/electrical converter  16  (S  14 ).  
         [0067]    Here, the wavelength λ0 of the fixed wavelength light is set in such a way that the wavelength dispersion may be reduced to the minimum in the second optical fiber transmission line  34 . Therefore, the result of the optical/electrical conversion of the second outgoing light has a smaller phase difference than that of the second incident light. Thus, the result of the optical/electrical conversion of the second outgoing light and the reference electrical signals can be considered to have the identical frequencies and phases.  
         [0068]    And the output of the second optical/electrical converter  16  is amplified by the amplifier  18  (S 16 ). Then, the variable wavelength light is modulated by the first light modulator  15  by the frequency of the electrical signals outputted by the second optical/electrical converter  16  (S 18 ). The frequency of the electrical signals outputted by the second optical/electrical converter  16  can be considered to be equal to the frequency fin of the reference electrical signals. In the meanwhile, the light modulated by the first light modulator  15  (the first incident light) is inputted into the first optical fiber transmission line  32 .  
         [0069]    And now, the process returns to the change (sweep) of the wavelength λx of the variable wavelength light (S 10 ). And the operation is terminated by switching off the power at any time (S 19 ).  
         [0070]    Then, let us refer to the left of FIG. 2. The first incident light penetrates the first optical fiber transmission line  32  and becomes the first outgoing light. The first outgoing light is converted by the optical/electrical conversion process by the first optical/electrical converter  22  (S 24 ). The electrical signals outputted by the first optical/electrical converter  22  is amplified by the amplifier  24  (S 26 ). Then, the phase comparator  26  receives the reference electrical signals generated by the power source (signal source)  25  at its terminal Ref_In and the electrical signals for measurement outputted by the amplifier  24  at its terminal Prob_In. The phase comparator  26  takes the electrical signals received at the terminal Ref_In as a reference for computing the phase of the electrical signals received at the terminal Prob_In (S 28 ). And the phases measured are recorded at the characteristic computing section  28 .  
         [0071]    And the phases of the electrical signals for measurement received at the terminal Prob_In are affected by wavelength dispersion by the first optical fiber transmission line  32 . But, the phase of the reference electrical signals received at the terminal Ref_In are not affected by the wavelength dispersion by the first optical fiber transmission line  32 . Thus, the measurement of the phases of the electrical signals for measurement received at the terminal Prob_In by taking the reference electrical signals received at the term Ref_In as references enables to compute the characteristics of the first optical fiber transmission line  32 .  
         [0072]    When the light source system  10  stops operating, the characteristic computing section  28  computes the group delay characteristic and the wavelength dispersion characteristic of the first optical fiber transmission line  32  (S 29 ). The group delay characteristic can be computed from the relationship between the phases measured by the phase comparator  26  and the modulation frequency fm. The wavelength dispersion characteristic can be computed by differentiating the group delay characteristic by the wavelength.  
         [0073]    According to the first preferred embodiment, it is possible to measure the wavelength dispersion of the first optical fiber transmission line  32  even if only one fiber pair can be secured.  
         [0074]    The Second Preferred Embodiment  
         [0075]    The optical characteristic measuring apparatus related to the second preferred embodiment is different from the first preferred embodiment in that the characteristic measuring system  20  has a variable wavelength light source and that the characteristic measuring system  20  converts by the optical/electrical conversion process and amplifies the reverberation of the second incident light and compares the phases with those of the reference electrical signals.  
         [0076]    [0076]FIG. 3 is a block diagram showing the summarized configuration of an optical characteristic measuring apparatus related to the second preferred embodiment. Hereafter, the portions similar to the first preferred embodiment will be marked by the codes of similarity and their descriptions will be omitted.  
         [0077]    The light source system  10  includes a fixed wavelength light source  11 , a first light modulator  15 , a second optical/electrical converter  16  and an amplifier  18 . The fixed wavelength light source  11  generates a fixed wavelength light, the wavelength of which is fixed. It is preferable to set the wavelength of the fixed wavelength light at a wavelength λ0 at which the wavelength dispersion will be reduced to the minimum in the first optical fiber transmission line  32 .  
         [0078]    The characteristic measuring system  20  includes a variable wavelength light source  29 , a first optical/electrical converter  22   a , a third optical/electrical converter  22   b , a second light modulator  23 , amplifiers  24   a  and  b , a power source (signal source)  25 , a phase comparator  26  and a characteristic computing section  28 .  
         [0079]    The variable wavelength light source  29  generates a variable wavelength light, the wavelength of which is variable. The variable wavelength light source  21  can sweep the wavelength λy of the variable wavelength light. The third optical/electrical converter  22   b  converts by the optical/electrical conversion process the reverberations of the second incident light. The amplifier  24   b  amplifies the electrical signals outputted by the third optical/electrical converter  22   b.    
         [0080]    The phase comparator  26  receives the electrical signals generated by the power source (signal source)  25  at a terminal Ref In, the electrical signals outputted by the amplifier  24   a  at a terminal Prob_In 1 and the electrical signals for the measurement of reverberations outputted by the amplifier  24   b  at a terminal Prob_In 2. The phase comparator  26  takes the electrical signals received at the terminal Ref_In as a reference for computing the phase of the electrical signals received at the terminal Prob_In 1 and the terminal Prob_In 2.  
         [0081]    The operation of the second preferred embodiment will be described with reference to the flowchart in FIG. 4. On the left side the operation of the characteristic measuring system  20  is shown, while on the right side the operation of the light source system  10  is shown. Let us refer to the left side to begin with. The wavelength λy of the variable wavelength light is changed (S 20 ). Then, the variable wavelength light source  12  generates a variable wavelength light (λ=λy) (S 21 ). Then, the variable wavelength light is modulated by the frequency fm of the reference electrical signals generated by the power source (signal source) (S 22 ). And then the process returns to the generation of the variable wavelength light (S 20 ).  
         [0082]    The fixed wavelength light modulated by the frequency fm is the second incident light. The second incident light penetrates the second optical fiber transmission line  34  and is inputted into the light source system  10  as the second outgoing light.  
         [0083]    At this point, let us refer to the right side of FIG. 4. To begin with, the fixed wavelength light source  21  generates a fixed wavelength light (λ=λ0) (S  10 ). The second outgoing light is converted by the optical/electrical conversion process by the second optical/electrical converter  16  (S 14 ).  
         [0084]    Here, the result of optical/electrical conversion of the second outgoing light is affected by the wavelength dispersion of the second optical fiber transmission line  34 .  
         [0085]    And the output of the second optical/electrical converter  16  will be amplified (S 16 ). Then, the variable wavelength light will be modulated by the first optical/electrical converter  15  by the frequency of the electrical signals outputted by the second optical/electrical converter  16  (S 18 ). In the meanwhile, the light modulated by the first light modulator  15  (the first incident light) will be injected into the first optical fiber transmission line  32 .  
         [0086]    Here, the wavelength λ0 of the fixed wavelength light is set in such a way that the wavelength dispersion may be reduced to the minimum in the first optical fiber transmission line  32 . Thus, the result of the optical/electrical conversion of the first outgoing light is not affected by the wavelength dispersion of the first optical fiber transmission line  32  and is affected only by the wavelength dispersion of the second optical fiber transmission line  34 .  
         [0087]    And the process returns to the generation of the fixed wavelength light (S 10 ). In the meanwhile, the whole operation is terminated by switching off the power at any time (S 19 ).  
         [0088]    Then, let us refer to the left of FIG. 4. The first incident light penetrates the first optical fiber transmission line  32  and becomes the first outgoing light. The first outgoing light is converted by the optical/electrical conversion process by the first optical/electrical converter  22   a  (S 24 ). And the third optical/electrical converter  22   b  converts by the optical/electrical conversion process the reverberations of the second incident light (S 24 ). Then, the electrical signals outputted by the first optical/electrical converter  22   a  and the third optical/electrical converter  22   b  are respectively amplified by the amplifiers  24   a  and  b  (S 26 ). Then, the phase comparator  26  receives the reference electrical signals generated by the power source (signal source)  25  at its terminal Ref_In, the electrical signals for measurement outputted by the amplifier  24   a  at its terminal Prob_In 1 and the electrical signals for measurement of reverberations outputted by the amplifier  24   b  at its terminal Prob_In 2. The phase comparator  26  takes the electrical signals received at the terminal Ref_In as a reference for computing the phase of the electrical signals received at the terminals Prob_In 1 and Prob_In 2 (S 28 ). And the phases measured are recorded at the characteristic computing section  28 .  
         [0089]    And the phases of the electrical signals received at the terminals Prob_In 1 and Prob_In 2 are affected by wavelength dispersion by the second optical fiber transmission line  34 . But, the phase of the reference electrical signals received at the terminal Ref_In is not affected by wavelength dispersion by the second optical fiber transmission line  34 . Thus, the measurement of the phases of the electrical signals received at the terminals Prob_In 1 and Prob_In 2 by taking the reference electrical signals received at the term Ref_In as references enables to compute the characteristics of the second optical fiber transmission line  34 .  
         [0090]    When the light source system  10  stops operating, the characteristic computing section  28  computes the group delay characteristic and the wavelength dispersion characteristic of the first optical fiber transmission line  32  (S 29 ). The group delay characteristic can be computed from the relationship between the phases measured by the phase comparator  26  and the modulation frequency fm. The wavelength dispersion characteristic can be computed by differentiating the group delay characteristic by the wavelength.  
         [0091]    According to the second preferred embodiment, it is possible to measure the wavelength dispersion of the second optical fiber transmission line  34  even if only one fiber pair can be secured.  
         [0092]    The Third Preferred Embodiment  
         [0093]    The optical characteristic measuring apparatus related to the third preferred embodiment is different from the second preferred embodiment in that the light source system  10  has a variable wavelength light source.  
         [0094]    The configuration of the third preferred embodiment is described with reference to FIG. 3. The light source system  10  includes a variable wavelength light source  12 , a first light modulator  15 , a second optical/electrical converter  16  and an amplifier  18 . The first variable wavelength light source  12  generates the first variable wavelength light, the wavelength of which is variable. The first variable wavelength light source  12  enables to sweep the wavelength λx of the first variable wavelength light. The configuration of other parts is similar to that of the second preferred embodiment. Also the configuration of the characteristic measuring system  20  is similar to that of the second preferred embodiment. However, the variable wavelength light source  21  in the second preferred embodiment is replaced by the second variable wavelength light source  21  in the third preferred embodiment.  
         [0095]    The operation of the third preferred embodiment will be described with reference to the flowchart in FIG. 5. On the left side the operation of the characteristic measuring system  20  is shown, while on the right side the operation of the light source system  10  is shown. Let us refer to the left side to begin with. The wavelength λy of the second variable wavelength light is changed (S 20 ). Then, the variable wavelength light source  12  generates the second variable wavelength light (λ=λy) (S 21 ). Then, the second variable wavelength light is modulated by the frequency fm of the reference electrical signals generated by the power source (signal source) (S 22 ). And then the process returns to the generation of the second variable wavelength light (S 20 ).  
         [0096]    The fixed wavelength light modulated by the frequency fm is the second incident light. The second incident light penetrates the second optical fiber transmission line  34  and is inputted into the light source system  10  as the second outgoing light.  
         [0097]    At this point, let us refer to the right side of FIG. 5. The wavelength Ax of the first variable wavelength light is changed (S 10 ). Incidentally, the change (sweep) of λx and that of λy will be synchronized. Then, the first variable wavelength light source  12  generates the first variable wavelength light (λ=λx) (S 12 ). The second outgoing light is converted by the optical/electrical conversion process by the second optical/electrical converter  16  (S 14 ).  
         [0098]    And the output of the second optical/electrical converter  16  will be amplified (S 16 ). Then, the first variable wavelength light will be modulated by the first light modulator  15  by the frequency of the electrical signals outputted by the second optical/electrical converter  16  (S 18 ). In the meanwhile, the light modulated by the first light modulator  15  (the first incident light) will be inputted into the first optical fiber transmission line  32 .  
         [0099]    And the process returns to the generation of the first variable wavelength light (S 10 ). In the meanwhile, the whole operation is terminated by switching off the power at any time (S 19 ).  
         [0100]    Then, let us refer to the left of FIG. 5. The first incident light penetrates the first optical fiber transmission line  32  and becomes the first outgoing light. The first outgoing light is converted by the optical/electrical conversion process by the first optical/electrical converter  22   a  (S 24 ). And the third optical/electrical converter  22   b  converts by the optical/electrical conversion process the reverberations of the second incident light (S 24 ). Then, the electrical signals outputted by the first optical/electrical converter  22   a  and the third optical/electrical converter  22   b  are amplified by the amplifiers  24   a  and  b  (S 26 ). Then, the phase comparator  26  receives the reference electrical signals generated by the power source (signal source)  25  at its terminal Ref_In, the electrical signals for measurement outputted by the amplifier  24   a  at its terminal Prob_In 1 and the electrical signals for measurement of reverberations outputted by the amplifier  24   b  at its terminal Prob_In 2. The phase comparator  26  takes the electrical signals received at the terminal Ref_In as a reference for computing the phase of the electrical signals received at the terminals Prob_In 1 and Prob_In 2 (S 28 ). And the phases measured are recorded at the characteristic computing section  28 .  
         [0101]    When the light source system  10  stops operating, the characteristic computing section  28  computes the group delay characteristic and the wavelength dispersion characteristic of the first optical fiber transmission line  32  (S 29 ). The group delay characteristic can be computed from the relationship between the phases measured by the phase comparator  26  and the modulation frequency fm. The wavelength dispersion characteristic can be computed by differentiating the group delay characteristic by the wavelength.  
         [0102]    According to the third preferred embodiment, it is possible to measure the wavelength dispersion of the first optical fiber transmission line  32  and the second optical fiber transmission line  34  even if only one fiber pair can be secured.  
         [0103]    In the meanwhile, the embodiment described above can be realized by having a media reading apparatus of a computer provided with a CPU, a hard disk, memory media (a floppy disk, a CD-ROM, etc.) read a program executing various functions described above and installing the program on a hard disk. In this way, the functions described above can be performed.  
         [0104]    According to the present invention, it is possible to measure group delay characteristic and other characteristics even if the device under test is a single fiber pair.