Patent Publication Number: US-2005117840-A1

Title: Otdr with concurrently applied stimulus signals

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to determination of optical properties of an optical component, e.g. an optical fibre, more particularly to optical time domain reflectometry measurements and to optical time domain reflectometers (OTDRs) to perform these measurements.  
      In an OTDR for measurement of reflected optical signals, an optical signal with a defined measuring wavelength is coupled into the component under test, e.g. an optical fibre under test, and the reflected optical signal to be measured is detected with an optical detector connected to a computer for using the reflected optical signal for quantitative analysis and preferably visual representation.  
      Such OTDRs are known in the prior art. They are widely used during the installation of optical fibres to check for proper deployment and fibre integrity. U.S. Pat. No. 6,141,089 shows an OTDR of the aforementioned art for measurements in optical networks with currently applied traffic signals, for example.  
      To achieve a complete test coverage of the component under test the necessary measurements have to be conducted at different wavelengths which are similar to the wavelengths of the traffic signals that currently or later are routed through the fibres. A system for determination of such a wavelength dependent information is disclosed in EP 0,872,721 B1, the disclosure of which is incorporated herein by reference. With such an OTDR the measurements at different wavelengths are executed sequentially, i.e. a first measurement is taken at wavelengths λ 1 , then a second measurement is taken at wavelength λ 2 , and so on.  
      Since the receiver of such an OTDR of the prior art, a part of which is depicted in  FIG. 1 , cannot distinguish between different wavelengths of an optical input signal, the laser diodes have to be triggered one at a time to prevent measurement signals of different wavelengths to hit the receiver simultaneously. Because every measurement at each wavelength has to be repeated about 10000 times (or even more often) the whole measurement process takes a lot of time.  
      JP-A-05 281087 (Fujikura) discloses an OTDR with different wavelengths for light loss causing search. The so-called four-wave mixing for mapping chromatic dispersion in optical fibers is known from EP-A-819926 (Lucent).  
     SUMMARY OF THE INVENTION  
      Therefore, it is an object of the invention to provide improved determination of optical properties of an optical component.  
      The object is solved by the independent claims.  
      An advantage of the present invention is the possibility to execute OTDR measurements with a number of different wavelengths in parallel. This is important because it can be foreseen that in the new dense wavelength division multiplexing (DWDM) networks the number of different test wavelengths starts to become high. Thus, the sequential execution of the measurements as known from the prior art would cause the total test time to increase considerably. However, the present invention allows for a significant reduction in measurement time by conducting OTDR measurements with different test wavelengths all at the same time.  
      In a preferred embodiment of the invention the inventive method is performed with a number of laser diodes which are connected to a wavelength multiplexer. With the help of a wideband directional coupler the signal of the wavelength multiplexer is coupled into a fiber under test. Furthermore, there is a wavelength demultiplexer coupled to the wideband directional coupler to receive the reflected optical signals reflected from the fiber under test. Connected to the wavelength demultiplexer is a respective number of individual receivers so that a multiwavelength input signal to the wavelength demultiplexer can be separated by the wavelength demultiplexer and processed individually in a straightforward way by the respective receivers.  
      Further preferred embodiments process the receiver output signals further separately by a respective number of data acquisition units. Alternatively, the receiver output signals can be multiplexed to feed a common signal processing circuit.  
      Furthermore, in a preferred embodiment of the present invention it is possible to calibrate the demultiplexer before using it for the measurement to consider cross-talk between the different reflected optical signals. The calibration of the demultiplexing is done by repeating the following steps for a set of wavelengths to be used for the optical signals: demultiplexing optical calibration signals having defined calibration wavelengths to a number of N demultiplexing ports, detecting a leakage of the optical calibration signal into each port. This gives the leakage L pw  of a wavelength w into a port p for all wavelengths and ports  1 ,  2 , . . . , N. Therefore, it is possible to determine a matrix representation of the leakage L which correlates a matrix of the actual measurement signals S a  with a matrix of the ideal measurement signals S i  according to the following formula: 
 
 S   i   =L   −1   ×S   a . 
 
      In another preferred embodiment of the present invention there is provided coding an optical signal with an unique feature, and detecting the optical signal by detecting its unique feature. By this, it is possible to assign each detected signal to the original sent signal.  
      Other preferred embodiments are shown by the dependent claims.  
      It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).  
       FIG. 1  shows a schematic illustration of a part of the above mentioned OTDR setup of the prior art;  
       FIG. 2  shows an embodiment of the present invention; and  
       FIG. 3  shows the cross-talk problem that can occur with imperfect isolation between different ports of a demultiplexer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring now in greater detail to the drawings,  FIG. 2  shows an embodiment  1  of an OTDR of the present invention. In embodiment  1  a number N of laser diodes  2   1  to  2   N  emit optical signals  3   1  to  3   N  to a wavelength multiplexer  4  connected with the laser diodes  2   1  to  2   N . The wavelength multiplexer  4  multiplexes the N optical signals  3   1  to  3   N  and feeds a multiplexed signal  6  in a wideband directional coupler  8 . The wideband directional coupler  8  couples the multiplexed signal  6  into a fiber under test  10 . For further details of OTDRs refer to chapter 11 of “Dennis Derickson, Fiber Optic Test and Measurement, Prentice Hall PTR, Upper Saddle River, N.J. 07458, 1998”, the disclosure of which is incorporated herein be reference.  
      Reflected optical signals  12  reflected from the fiber under test  10  are coupled via the wideband directional coupler  8  into a wavelength demultiplexer  14 . The wavelength demultiplexer  14  is demultiplexing the reflected optical signals  12  into demultiplexed signals  16   1  to  16   N . The demultiplexed signals  16   1  to  16   N  are detected by N receivers  18   1  to  18   N . Each receiver  18   1  to  18   N  of the N receivers  18   1  to  18   N  is connected with a data acquisition module  20   1  to  20   N  of a computer  22  to analyze and preferably show the acquired results on a (not shown) monitor. In the illustrated embodiment  1  of  FIG. 2  the number N of measurement wavelengths can vary between 1 and any other reasonable natural number.  
      However, the demultiplexer  14  in this embodiment is an additional component in the test signal path, which inevitably affects the test results due to its non-ideal behavior. Therefore, the insertion-loss of the demultiplexer  14  should be minimized in order to not decrease the dynamic range of the OTDR measurement performed with the shown embodiment  1 .  
      However, what is more important, is the possible limited wavelength isolation between the individual output ports of the wavelength demultiplexer  14 .  
       FIG. 3  shows this cross-talk problem that can occur with imperfect isolation between the different ports of the wavelength demultiplexer  14 . Small spurious signals from any port superimposed to any other output port and can lead to useless test results. On the left hand side of  FIG. 3  there is shown a first detected reflected optical signal having a wavelength λ 1 . On the right hand side of  FIG. 3  there is shown a second detected reflected optical signal having a wavelength λ 2 . As can be seen from  FIG. 3 a  part L 12  of the signal detected at wavelength λ 1  adds to the signal detected at wavelength λ 2  and a part of the signal detected at wavelength λ 2  adds to the signal detected at the wavelength λ 1 .  
      The inventive method deals with this possible problem by determining the leakage L pw  of wavelength w into port p, for all wavelengths and ports  1 ,  2 , . . . , N, and to consider this in a post processing step. To derive the necessary formulas a general relation between the actual measurement signal s a  and the ideal measurement signal si is needed. For the actual signal s a1  at port  1  the following equation holds: 
 
 s   a1   =L   11   ×s   i1   +L   12   s   i2   +L   13   ×s   i3   + . . . +L   1N   ×s   iN  
 
      In a similar way the actual signals at the other ports can be given. A matrix representation is the preferred way to combine the complete set of equations:  
         S   a     =     L   ×     S   i     ⁢           ⁢   with         
         S   a     =         (           s   a1             ⋮             s   aN           )     ⁢           ⁢     S   i       =       (           s   i1             ⋮             s   iN           )     ⁢           ⁢   and           
       L   =     (           L   11         ⋯       ⋯         L     1   ⁢   N               ⋮       ⋯       ⋯       ⋮           ⋮       ⋯       ⋯       ⋮             L   N1         ⋯       ⋯         L   NN           )         
 
      Solving the matrix equation for the required set of signals S i  yields readily 
 
 S   i   =L   −1   ×S   a  
 
 with L −1  being the inverse of matrix L. 
 
      In a practical arrangement as in embodiment  1  of  FIG. 2 , the set of actual signals S a  depends on several factors like different laser output power of the laser diodes  2   1  to  2   N , fiber scatter factor of the optical fiber  10 , and coupling ratio of the directional coupler  8 . In addition, the leakage factors L pw  of the demultiplexer can be temperature dependent. Therefore, an automatic calibration step is proposed prior to a multi-wavelength measurement to determine the relation between S i  and S a . It can be implemented in a way, where fast single wavelength measurements were taken for each wavelength λ w , and each acquired signal s a  on port p (p=1 . . . N) is used to calculate the leakage factor L pw  with reference to L pp  which is always set to 1. After N single wavelength measurements, which take only a fraction of a second, matrix L is known and matrix L −1  can be calculated. During calibration, the optical component  10  is not connected to the coupler  8 , instead a reflection of a then open connector of the coupler  8  is used for calibration purposes. Alternatively, a mirror can be connected to the open connector of coupler  8 . After the calibration the multi-wavelength measurement can be conducted and the ideal signals s i  can be calculated.