Abstract:
A filter processing system making it possible to be able to set a frequency pass-band automatically and to provide the optimum filter to an input signal. A filter processing method of an output signal of an Optical Time Domain Reflectometer (OTDR) in a chromatic dispersion distribution measuring apparatus is disclosed. The filter processing method includes establishing measuring-condition parameters, generating an ideal signal waveform based on previously established chromatic dispersion values and the measuring-condition parameters, and providing correlation results between the ideal signal waveform and a filter input signal. The method also includes comparing the correlation results to a threshold value to generate a minimum chromatic dispersion value and a maximum chromatic dispersion value, and performing filter processing for the output of the OTDR based on the minimum chromatic dispersion value and the maximum chromatic dispersion value.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates that a filter processing is performed in the input signal having the fluctuated intensity, and belonged to measurement of the chromatic dispersion distribution characteristics in the characteristics of the optical fiber used for ultrahigh speed optical communication field. 
   2. Description of the Prior Art 
   Presently, in the ultra high-speed optical communications field, in order to realize and maintain high quality communication, various researches about communication quality control and compensation technology of an optical fiber lines are advanced. 
   Based upon this, the demand from a market to characteristic evaluation of optical fiber became higher than former, and also in it, the chromatic dispersion characteristics attracts attention as an important item which discerns transmission restrictions by the wavelength band and the transmission speed. 
   And as for an example of this kind of chromatic dispersion distribution measurement of the optical fiber, Japanese Patent Publication No. Hei 10-83006 (corresponding to the U.S. Pat. No. 5,956,131 and the European Patent Application No. 0819926A2) is well known as shown in FIG.  6 . 
   The principle of chromatic dispersion distribution measurement for the optical fiber is explained using  FIG. 6 , which showed a configuration of the conventional chromatic dispersion distribution measurement apparatus for the optical fiber. 
   In  FIG. 6 , a Laser Source  1  (LS 1 ) generates coherent light having wavelength λ 1 , a Laser Source  2 (LS 2 ) generates coherent light having wavelength λ 2 , and these 2 lights are synthesized at a Coupler  3 . 
   The synthesized light at the Coupler  3  is transformed to a pulse-like light synchronized to the clock signal (It is not illustrating) in A 0  Switch  4 , and amplified by an Erbium Doped Fiber Amplifier  5  (EDFA 5 ). 
   An amplified light from EDFA  5  is supplied to a target Optical fiber  7  through an Optical Circulator  6 . 
   Additionally, said Optical Circulator  6  branches the total backscattered light generated by incident light coming into the optical fiber  7 . 
   A terminator  8  is repressing the Fresnel Reflection in the extreme of the optical fiber  7 . 
   Besides, an Optical Band Pass Filter  9  operates to extract the one side of wavelength element of a four-wave mixing light generated by the interaction between each two wavelength in the total backscattered light generated by incident lights coming into the optical fiber  7 . 
   An Optical Time Domain Reflectometer  10  (OTDR 10 ) calculates a data pointing out the fluctuation of intensity based on a light of specific wavelength passing through the Optical Band Pass Filter  9  as a one side of wavelength element of four-wave mixing light generated by the interaction between a couple wavelengths of an incident light in the total backscattered light. 
   The data calculated in the OTDR 10  accumulates to a RAM (Random Access Memory) of a Personal Computer  11  (PC 11 ), and uses to a various computing. 
     FIG. 7  corresponding to  FIG. 6  minutely shows a conventional procedure of measuring the chromatic dispersion distribution characteristics. Like  FIG. 7 , a target optical fiber connected to a measuring apparatus, and on starting the measurement of chromatic dispersion distribution characteristics for a target optical fiber;
         Firstly, sets up measurement condition that two light sources having different wavelength each other (light source  1  &amp;  2 ), OTDR and EDFA etc. (STEP S 11 ).   After the setting up a measurement condition corresponding to STEP S 11 , executes measurement about a fluctuated intensity data of the light having a specific wavelength of the target optical fiber using OTDR. (STEP S 12 )   A data obtained in the measurement of STEP S 12  send to a Personal Computer. (STEP S 13 )   A Personal Computer executes calculation of a chromatic dispersion distribution value using the data from OTDR. (STEP S 14 )   A chromatic dispersion distribution value, total dispersion value and a waive obtained by the processing in STEP S 114  respectively are displayed in an indicator being inexistent in the figure. (STEP S 15 )       
   In the conventional measurement of chromatic dispersion distribution showing in  FIG. 6 , the inputted signal to the personal computer PC 11  disposed the filter processing, which is inexistent in the figure. And the suchlike filter processing installed externally a frequency pass-band being arbitrary value or fixed value. 
   Hereinafter, a configuration of the filter processing system of the conventional chromatic dispersion distribution measuring apparatus being implemented in the PC 11  for example in  FIG. 6  will be explained with reference to FIG.  2 . 
   A filter processing of the conventional chromatic dispersion distribution apparatus comprises an input signal S 1 , a minimum chromatic dispersion value S 24 A, a maximum chromatic dispersion value S 24 B, a measuring condition parameter S 25 , a frequency converting section  21 , a minimum frequency value S 21 A, a maximum frequency value  21 B, filter coefficient generating section  22 , filter coefficient S 22 , a filter processing section  23  and an output signal from the filter S 23  (a signal passed through the filter). 
   Deriving a signal frequency from a minimum chromatic dispersion value S 24 A, a maximum chromatic dispersion value S 24 B and a measurement condition parameter S 25  which are having a fixed value or an externally arranged arbitrary value, above mentioned frequency converting section  21  outputs a minimum frequency value S 21 A and a minimum frequency value  21 B. 
   The filter coefficient generating section  22  generates the filter coefficient  22 S using the minimum frequency value S 21 A and the maximum frequency value  21 B assigned by the inputted pass range, and outputs them. 
   The filter processing section  23  using the filter coefficient S 22  being assigned in the filter coefficient generating section  22 , provides a filter function for the supplied input signal S 1 , and outputs an output signal from the filter S 23  as the output. At that time, a kind of the filter depends on the filter coefficient generating section  22 . 
   A processing flow of the conventional system shown in next  FIG. 2  will be explained using a flowchart of FIG.  3 . 
   For a start, a minimum chromatic dispersion value S 24 A, a maximum chromatic dispersion value S 24 B and the measuring condition parameter S 25  are set and measurement is started. 
   Next, the minimum frequency value S 21 A and the maximum frequency value S 21 B are derived from the minimum chromatic dispersion value S 24 A, the maximum chromatic dispersion value S 24 B and the measuring condition parameter S 25  at the frequency converting section  21 . 
   Subsequently, the coefficient generating section  22  generates the filter coefficient S 22  from the minimum frequency value S 21 A and the maximum frequency value S 21 B, which were calculated by the frequency converting section  21 . 
   Moreover, the filter processing section  23  performs filter operation using the filter coefficient S 22  provided from the coefficient generating section  22 , and outputs an output signal from the filter S 23 . 
   After that, the chromatic dispersion distribution measurement results are accomplished by performing operation processing of the chromatic dispersion distribution to the output signal from the filter S 23 . 
   However, such a system described in the  FIG. 2 , there is a difficulty to providing consistently a best suited filter for the input signals having an intensity fluctuation, since the frequency pass-band of the filter is set to an arbitrary value from the outside or is fixed to a value derived from the frequency converting section  21 . 
   A problem (goal) of the present invention is to provide the filter processing system configuring constantly optimum filter by setting up a frequency pass-band automatically. 
   SUMMARY OF THE INVENTION 
   In order to solve the above described problem, according to a first aspect of the present invention, a filter processing system for an output signal of an OTDR in a chromatic dispersion distribution apparatus measuring a chromatic dispersion distribution characteristics of an optical fiber;
         Comprises: an ideal signal generating method that generates the ideal signal waveforms being corresponding to the chromatic dispersion values coming from a chromatic dispersion setting method sequentially, and the said ideal signal generating method that generates the ideal signal waveforms based on the measuring condition parameters setup beforehand;   a correlation processing method to output results of the correlation between the input signal waveform and the ideal signal waveform; a correlation result judging method to compare the correlation results with a threshold value;   And: depending on a minimum chromatic dispersion value and a maximum chromatic dispersion value coming each from the correlation result judging method, with performs filter processing for the output signals of the OTDR.       

   Besides, the chromatic dispersion values are used for calculating the minimum chromatic dispersion value and maximum chromatic dispersion value by repeated processing to calculate serially the ideal signal waveform with given configured interval in a possible setting range. (Second aspect of the present invention) 
   Additionally, the correlation result judging method sets a chromatic dispersion value having exceeded threshold ranges in the correlation results to the minimum chromatic dispersion value and the maximum chromatic dispersion value respectively. (Third aspect of the present invention) 
   Additionally, the measuring condition parameter includes first and second light signal wavelengths and a measurement range at the least. (Fourth aspect of the present invention) 
   Additionally, a filter processing system for the OTDR output signals in the chromatic dispersion distribution measuring apparatus comprises:
         an ideal signal generating method that generates the ideal signal waveforms being corresponding to the signal frequency values coming from a signal frequency settings method sequentially;   and the said ideal signal generating method that generates the ideal signal waveforms based on the measuring condition parameters set up beforehand;   a correlation processing method to output a correlation result of the input signal waveform and the ideal signal waveform;   a correlation result judging method to compare the correlation results with the threshold, and performs filter processing for the OTDR output signal with responding to a minimum frequency value and a maximum frequency value coming respectively from the correlation result judging method. (Fifth aspect of the present invention)       

   Additionally, the configured signal frequency value is characterized by calculating the minimum frequency and the maximum frequency, and is characterized by performing repeated calculation to obtain the ideal signal waveform using given setting intervals continuously in an available setting range. (Sixth aspect of the present invention) 
   Additionally, the correlation result judging method outputs “a signal frequency value which has a correlation result exceeding a threshold” as a minimum frequency value and/or a maximum frequency value. (Seventh aspect of the present invention) 
   Additionally, the measuring condition parameter includes at the least a first light signal wavelength, a second light signal wavelength and a measurement range. (Eighth aspect of the present invention) 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a configuration of a filter processing section in the chromatic dispersion distribution measuring apparatus according to the present invention. 
       FIG. 2  shows a configuration of a filter processing section in the chromatic dispersion distribution measuring apparatus of conventional art. 
       FIG. 3  is the flow chart of the conventional system. 
     FIGS.  4 ( a ),  4 ( b ) are the flow chart of the system of the present invention. 
       FIG. 5  is showing the setting relation between a correlation result, a minimum chromatic dispersion value, and a maximum chromatic dispersion value. 
       FIG. 6  is the figure showing a conventional chromatic dispersion measuring apparatus for an optical fiber. 
       FIG. 7  shows a chromatic dispersion measuring procedure of conventional art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereafter, one of an embodiment of the present invention will be explained in detail with reference to the drawing. 
     FIG. 1  is a block diagram showing the structure of the filter processing system of the chromatic dispersion distribution measurement apparatus to which the present invention applied. 
   In  FIG. 1 , which is showing the filter processing system of the chromatic dispersion distribution measurement apparatus implementing the present invention, symbol S 1  denotes an input signal, reference number  10  denotes a detection section, symbol  14 A denotes a minimum chromatic dispersion value, symbol  14 B denotes a maximum chromatic dispersion value, reference number  21  denotes a frequency converting section, symbol  21 A denotes a minimum frequency value, symbol  21 B denotes a maximum frequency value, symbol S 25  denotes a measuring condition parameter, reference number  22  denotes a filter coefficient generating section, symbol S 22  denotes a filtering coefficient, reference number  23  denotes a filter processing section, and symbol S 23  denotes an output signal from the filter. 
   Moreover, the detection section  10  added in the present invention comprises a chromatic dispersion value setting section  11 , a chromatic dispersion value S 11 , an ideal signal generating section  12 , an ideal signal S 12 , a correlation section  13 , the correlation result S 13  and the correlation result judging section  14 . 
   In  FIG. 1 , the chromatic dispersion value setting section  11  sets the chromatic dispersion value S 11  for generating an ideal signal, and outputs the value thereof. 
   Additionally, the ideal signal generating section  12  calculates the ideal signal S 12  suitable for the characteristic of current input signal from a known formula, the measuring condition parameter S 25  and the chromatic dispersion value S 11 . 
   Additionally, the correlation section  13  calculates the correlation on the time-axis of the ideal signal S 12  and the input signal S 1  by using correlation operational-formula such as the Schwarz&#39;s inequality, and outputs the correlation results S 13  to the correlated result judging section  14 . 
   Additionally, the correlated result judging section  14  generates and outputs a minimum chromatic dispersion value S 14 A and a maximum chromatic dispersion value S 14 B among the chromatic dispersion values (S 11 ) having exceeded threshold. 
   Additionally, the frequency converting section  21  derives the signal frequency from the minimum chromatic dispersion value S 14 A, maximum chromatic dispersion value Sl 4 B and the measuring condition parameter S 25  which were determined by taking correlation the ideal signal S 12  and the input signal S 1  on the time-axis. And the frequency converting section  21  outputs the minimum frequency value S 21 A and the maximum frequency value S 21 B. 
   Additionally, the filter coefficient generating section  22  derives and outputs the filer coefficient S 22 , using the inputted minimum frequency value S 21 A and a maximum frequency value S 21 B. 
   Additionally, the filter processing section  23  performs a filter processing to the input signal S 1  using the filter coefficient S 22  appointed. And the filter processing section is outputting the output signal from the filter  23 . 
   At that time, a kind of the filter depends on the filter coefficient generating section  22 . 
   Additionally, by replacing the chromatic dispersion value setting section  11  with a signal frequency setting section  11 , and by replacing the chromatic dispersion value S 11  with a signal frequency value S 11 , it becomes unnecessary to calculate a signal frequency from the chromatic dispersion value by the frequency converting section  21 , and the signal frequency value S 11  would be determined as a maximum value or a minimum value of frequency pass-band directly. 
   Next, a processing flow of the method of the present invention shown in the  FIG. 1  will be explained using FIG.  4 . 
   Firstly, a measuring condition parameter S 25  is inputted, and then it starts the measurement. 
   Next, a chromatic dispersion value setting section  11  sets and outputs a chromatic dispersion value S 11 . 
   In the chromatic dispersion value setting section  11 , an available setting range and interval of the chromatic dispersion value S 11  are decided in advance, for example a setting range is 100˜300 ps/nm/km, a setting interval is 1 ps/nm/km etc. That is, 100 ps/nm/km will be assigned firstly as the chromatic dispersion value S 11  in the above-mentioned example. 
   Since the chromatic dispersion value S 11  is set, an ideal signal generating section  12  generates a waveform corresponding to the chromatic dispersion value S 11 . 
   Next, a correlation section  13  calculates a correlation results S 13  which correlated an inputted ideal signal S 12  and an input signal S 1 . 
   In the case of applying Schwarz&#39; inequality as a correlation formula, the correlation calculation result is a value of 0 or 1. 
   The correlation results S 13  is compared with a threshold value in a correlation result judging section  14 . 
   If the correlation result S 13  is not exceeding the threshold value, it performs nothing and returns to the chromatic dispersion value setting section  11 . 
   If the correlation result S 13  is exceeding the threshold value, it assigns the chromatic dispersion value S 11  in the current round to a minimum chromatic dispersion value S 14 A and maximum chromatic dispersion value S 14 B respectively, and returns to the chromatic dispersion value setting section  11 . 
   Here, the processing is returned to the chromatic dispersion value setting section  11 . 
   Then a next chromatic dispersion value S 11  being incremented by specified interval; for example, 101 ps/nm/km is replaced with a previous chromatic dispersion value S 11 . 
   Subsequently, a sequences procedure is repeated with in the ideal signal generating section  12  and the correlation result judging section  14 , until the maximum value of setting range of the chromatic dispersion value S 12 . 
   Additionally in the correlation result judging section  14 , if a correlation results S 113  is not exceeding the threshold in the first step of the processing and is exceeding the threshold in the second step subsequently, then the current chromatic dispersion value S 11  is assigned as minimum chromatic dispersion value S 14 A and maximum chromatic dispersion value S 14 B respectively, and returns to the chromatic dispersion value setting section  11 . 
   Additionally, if the correlation result S 13  is exceeding the threshold after a minimum chromatic dispersion value S 14 A, the current assigned chromatic dispersion value S 11  is assigning newly as a maximum chromatic dispersion value S 14 B, and returns to the chromatic dispersion value setting section  11 . 
   That is, the minimum chromatic dispersion value S 14 A is the chromatic dispersion value S 11  which exceeded first the threshold, and the maximum chromatic dispersion value S 14 B is the chromatic dispersion value S 11  which exceeded last the threshold. 
   If the minimum chromatic dispersion value S 14 A and the maximum chromatic dispersion value S 14 B were obtained by the repeated sequence processing until maximum value for example 300 ps/nm/km, in the setting range of the chromatic dispersion value S 11 . The processing goes to a frequency converting section  21  through out the finish check. 
   An explanation of the processing sequence from the frequency converting section  21  will be omitted because of the same as  FIG. 3  in the related art. 
   The present invention given in claims  1 - 4 , a filter processing system for the outgoing signals of OTDR in the chromatic dispersion distribution measurement apparatus comprises: measuring condition parameters set beforehand; an ideal signal generating method generating ideal signal waveforms based on the chromatic dispersion values setting up sequentially from a chromatic dispersion value setting method; a correlated processing method outputting results of correlation between said ideal signal waveforms and an input signal waveform; and a correlation result judging method comparing said correlation results with a threshold. 
   Since the architecture is that the filter processing for an output of OTDR is performed in response to a minimum chromatic dispersion value and a maximum chromatic dispersion value which were obtained by the correlation result judging method, the frequency pass-band of the filter to the output of OTDR can set an optimum value automatically, and it can be providing always an optimum filter for the input signals of the chromatic dispersion distribution measurement apparatus. 
   The present invention given in claims  5 - 8 , the filer processing system for outgoing signals of OTDR in the chromatic dispersion distribution measurement apparatus measuring a chromatic dispersion distribution characteristics of the target optical fiber comprises: the ideal signal waveforms being corresponding to the chromatic dispersion values coming from a chromatic dispersion setting method sequentially, and the said ideal signal generating method that generates the ideal signal waveforms based on the measuring condition parameters setup beforehand; a correlated processing method outputting a result of correlation between the ideal signal waveform to an input signal waveform. 
   Since the architecture is that the filter processing for an output of OTDR is performed in response to a minimum chromatic dispersion value and a maximum chromatic dispersion value which were obtained by said correlation result judging section, it becomes unnecessary to calculate a signal frequency from the chromatic distributed value and the frequency pass-band of the filter to the output of OTDR can set an optimum value automatically, and it can be providing always optimum filter for the input signals of the chromatic dispersion distribution measurement apparatus.