System for minimizing or compensating pmd-induced distortion in optical transmission systems and transmission fibres in particular

Abstract

What is described here is a system for compensating distortions induced by polarisation modulation dispersion (PMD) in optical transmission systems and in transmission fibres in particular, comprising a means for measuring PMD-induced distortions, an emulator unit for adjustable PMD levels, and a controller which the output signal of said measuring means is applied to and which serves to control said emulator unit. In accordance with the invention both the emulator unit and the measuring means are improved for the PMD-induced distortions as well as the controller and the applied control criterion (alone or in combination).

1 - 6 . (canceled)  
   
   
       7 . System for compensating distortions induced by polarization modulation dispersion (PMD) in optical transmission systems and in transmission fibres in particular, comprising 
 a measurement unit capable of measuring PMD-induced distortions,    a PMD emulator unit for adjustable PMD levels, and    a controller which the output signal of said measurement unit is applied to and which serves to control said PMD emulator unit,    characterized in that said PMD emulator unit comprises at least one basic emulator unit consisting of two DGD (differential group delay elements) elements having each a defined invariable time lag for an incoming signal, which DGD elements are connected to each other via a connecting element producing the effect of a transformation element, with all the three elements having a defined angle of the birefringence axes such that the birefringence axes of said connecting element will be distinguished in terms of their angular position from the birefringence axes of said two DGD elements, and    that at least one regulator element is provided for each basic emulation unit, which acts upon one of said elements of this basic emulator unit in such a way that the DGD level of the system can be completely adjusted by a slight variation of the time lag of the influenced element.    
   
   
       8 . System according to  claim 7 , 
 characterized in that at least one of said DGD elements or said connecting elements are PM fibres, and    that said regulator element exerts a mechanical effect upon at least one of said DGD elements for varying the time lag and hence polarization.    
   
   
       9 . System according to  claim 8 , 
 characterized in that said regulator element or elements, respectively, which produce a mechanical action, are fibre squeezers or stretchers with electrically controllable elements such as piezo elements creating a mechanical action upon the PM fibre.    
   
   
       10 . System according to  claim 9 , 
 characterized in that at least one of said regulator elements comprises a ring onto which said PM fibre is wound without being twisted, for distributing the mechanical action over the longest fibre length possible.    
   
   
       11 . System according to  claim 10 , 
 characterized in that at least one pressurizing element creates a pressure on a plurality of fibre segments of said wound fibre at least at one site.    
   
   
       12 . System according to  claim 11 , 
 characterized in that said pressurizing element is an elongating element such as a piezo element that acts upon at least one circle segment bearing against said ring, and    that counter-segments are provided for at least one part of said circle elements, which bear against said fibre segments and create pressure on said fibre.    
   
   
       13 . System according to  claim 7 , 
 characterized in that said two DGD elements are birefringent crystals having a birefringence adapted to be electrically influenced.    
   
   
       14 . (canceled)  
   
   
       15 . System according to  claim 7 , 
 characterized in that the angle of the birefringence axis of said first DGD element is selected to be 0° and that of the second DGD element to be 90° and that of said connecting element to be 45°, or to be 0°, 45°, 0°, or 90°, 45°, 0°.    
   
   
       16 . System according to  claim 7 , 
 characterized in that a further element is series-connected to said two DGD elements and said connecting element for setting an optional input PSP.    
   
   
       17 . System according to  claim 16 , 
 characterized in that said series-connected element comprises a further birefringent element such as a PM fibre, and    that the angle of said birefringence axes of said series-connected element and of said first DGD element are different from each other.    
   
   
       18 . System according to  claim 17 , 
 characterized in that said angular difference amounts to 45°.    
   
   
       19 . System according to  claim 16 , 
 characterized in that said series-connected element comprises a further birefringent element such as a PM fibre, and    that a regulator element produces an effect on said first DGD element for varying the time lag and hence the polarization.    
   
   
       20 . System according to  claim 19 , 
 characterized in that said series-connected element and/or said connecting element consists of two PM fibres or two birefringent crystals having each a different angular position of the birefringence axes.    
   
   
       21 . System according to  claim 19 , 
 characterized in that said regulator element acts upon one of said two fibres or on one of said crystals.    
   
   
       22 . System according to  claim 7 , 
 characterized in that at least two systems for setting a variable DGD are connected in tandem, whereof at least one comprises a basic emulator unit, if necessary with a PSP setting element.    
   
   
       23 . System according to  claim 22 , 
 characterized in that the individual systems for higher-order PMD compensation consist of basic emulator units with DGD elements having different individual time lags.    
   
   
       24 . System for compensating distortions induced by polarization modulation dispersion (PMD) in optical transmission systems and in transmission fibres in particular, comprising 
 a measurement unit capable of measuring PMD-induced distortions,    a PMD emulator unit for adjustable PMD levels, and    a controller which the output signal of said measurement unit is applied to and which serves to control said PMD emulator unit,    characterized in that said measurement unit detects the polarization of all spectral fractions contained in the signal output from said PMD emulator unit, for detecting the PMD.    
   
   
       25 . System according to  claim 24 , 
 characterized in that said measurement unit comprises a polarizer and an opto-electrical converter such as a photo receiver that is disposed to join said polarizer, and    that a polarization matching unit is provided that matches the output polarization of said PMD emulator unit to that of said polarizer.    
   
   
       26 . System according to  claim 24 , 
 characterized in that said measurement unit comprises a polarization beam splitter, with opto-electrical converters such as photo receivers being provided on the output terminals of said splitter and issuing signals for generating an ACTUAL signal for said controller, which are subjected to quotient formation.    
   
   
       27 . System according to  claim 26 , 
 characterized in that for detection of the polarization a polarimeter array known per se is provided.    
   
   
       28 . System according to  claim 24 , 
 characterized in that said polarization matching unit comprises two birefringent elements having birefringence axes forming an angle different from 0°, preferably 45°, and    that at least one regulator element is provided for setting the output polarization, which acts upon at least one of said birefringent elements.    
   
   
       29 . System according to  claim 28 , 
 characterized in that said birefringent elements are birefringent crystals or PM fibres.    
   
   
       30 . System according to  claim 7 , 
 characterized in that a polarisation matching unit is disposed directly upstream of said polarisation measurement unit or directly downstream of said PMD emulator.    
   
   
       31 . System according to  claim 7 , 
 characterized in that a polarisation matching unit is integrated as additional element into said PMD emulator.    
   
   
       32 . System according to  claim 25 , 
 characterized in that said polarization matching unit is a series-connected up-stream or downstream DGD element having an angle of 45°, with a regulator element acting upon this series-connected element and upon the DGD connected upstream or downstream thereof.    
   
   
       33 - 34 . (canceled)  
   
   
       35 . System for compensating distortions induced by polarization modulation dispersion (PMD) in optical transmission systems and in transmission fibres in particular, comprising 
 a measurement unit capable of measuring PMD-induced distortions,    a PMD emulator unit for adjustable PMD levels, and    a controller which the output signal of said measurement unit is applied to and which serves to control said PMD emulator unit,    characterized in that said controller comprises several control loops in which it modulates regulator elements of said PMD emulator unit with different frequencies, that said controller derives from the output signal of said measurement unit information about the amount and the phase position of the signal output from said PMD emulator unit, and uses this information to perform a high-speed and direct control function.    
   
   
       36 . System according to  claim 35 , 
 characterized in that the bandwidth or limit frequency of said opto-electrical converter is matched with the modulation frequency, and    that said controller sets the individual control loops in such a way that the polarization will be constant for all spectral fractions contained in the signal.    
   
   
       37 . System according to  claim 36 , 
 characterized in that said controller uses a minimum photo current of said opto-electrical converter or converters as a control criterion for setting a constant polarization for all spectral fractions contained in the signal.    
   
   
       38 . System according to  claim 36 , 
 characterized in that said controller evaluates the output signal from said opto-electrical converter or converters selectively in terms of frequency and phase.    
   
   
       39 . System according to  claim 38 , 
 characterized in that said controller comprises analog automatic-control circuits for said regulator elements to which said frequency-selective and phase-selective signals are applied.    
   
   
       40 . System according to  claim 35 , 
 characterized in that said controller also controls the regulator elements of said polarization matching unit.    
   
   
       41 . System according to  claim 40 , 
 characterized in that said controller controls the regulator elements of said polarization matching unit with the same control algorithm as that used in said PMD emulator unit.    
   
   
       42 . System according to  claim 35 , 
 characterized in that said controller comprises at least one CPU or at least one DSP circuit for performing various functions such as for frequency-selective and phase-selective evaluation or for controlling the operational sequence within the system.    
   
   
       43 . System according to  claim 35 , 
 characterized in that said controller performs essential parts of said control algorithm by using analog circuits.    
   
   
       44 - 48 . (canceled)