Patent Publication Number: US-2005117159-A1

Title: Polarization diversity detection using a polarization multiplexed local oscillator

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to determination of an optical property of an optical device under test, in particular to optical polarization diversity detection.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide improved optical polarization diversity detection.  
      The object is solved by the independent claims.  
      The prior art measuring concept according to EP 1113250 acquired the Jones matrix elements of a DUT by using a laser signal which was swept in wavelength as stimulus. The output signal was analyzed by a coherent superposition with a LO signal. To provide polarization resolved measurement the output signal was split into two orthogonal components each of which were superimposed with the LO signal. To unambiguously acquire all 4 elements of the Jones matrix it was necessary to perform this measurement with two input polarization states. Therefore it was either necessary to perform an extra wavelength sweep or to insert a PDU into the DUT path allowing to stimulate the DUT with two polarization states ‘simultaneously’ according to U.S. Ser. No. 09/941,133.  
      In an embodiment of the present invention it is proposed to insert a PDU according to U.S. Ser. No. 09/941,133, the disclosure of which is incorporated herein by reference, into the LO path (PDU LO ) so that two orthogonal polarization states, a first state of polarization (SOP H ) and a second state of polarization (SOP V ), occur at the output of the PDU LO . The two orthogonal components have traveled different optical paths inside the PDU and thus produce interference signatures at different electrical frequencies (f H  and f V ) when combined with the DUT signal at the detector.  
      Therefore, in an embodiment of the present invention it is disclosed an enhancement of the interferometric measurement method of EP 1113250, the disclosure of which is incorporated herein by reference, which is able to measure the chromatic dispersion (CD) and polarization mode dispersion (PMD) of a device under test (DUT) with high accuracy. According to an embodiment of the present invention the use of a polarization delay unit (PDU) according to U.S. Ser. No. 09/941,133 in the local-oscillator (LO) path of the DUT interferometer allows to replace the polarization diversity detector (PDR) by a single detector to reduce complexity of the detection scheme. In this way, problems associated with detector symmetry and extinction ratio of the polarization beam splitter (PBS) can be solved since there is only one detector left so that the PBS can be omitted.  
      The interference signatures created by the interference of the DUT signal with SOP H  and SOP V  respectively can be distinguished preferably by applying band pass filters of appropriate center frequency and bandwidth.  
      Furthermore, amplitude and phase of the two spectral components only depend on the fraction of the DUT signal having the same polarization as the interfering LO signal. That is, the DUT signal is inherently decomposed into two polarizations SOP H  and SOP V  interfering with the corresponding two LO signals and producing signatures at f H  and f V  respectively. In an embodiment of the present invention the AC part of the detector signal can be written as follows:  
                 P   AC     ⁡     (   ω   )       =       ⁢         E   LO     ⁢       E   DUT     ⁡     (         sop   ⇀     DUT     ·       sop   ⇀     H       )       ⁢     cos   (       φ   H     +         (       τ   DUT     -     τ     LO   ,   H         )           1   ⁢           ⁢   4   ⁢           ⁢   4   ⁢           ⁢   2   ⁢           ⁢   4   ⁢           ⁢   43     ⁢               f   H         ⁢   ω       )       +                     ⁢       E   LO     ⁢       E   OUT     ⁡     (         sop   ⇀     DUT     ·       sop   ⇀     V       )       ⁢     cos   (       φ   V     +         (       τ   DUT     -     τ     LO   ,   V         )           1   ⁢           ⁢   4   ⁢           ⁢   4   ⁢           ⁢   2   ⁢           ⁢   4   ⁢           ⁢   43     ⁢               f   V         ⁢   ω       )                 
 
 Hence, the polarization dependence of the interference effect can be used to realize polarization diversity detection and the information contained in the two signatures is equivalent to a prior art PDR output according to EP 1113250. 
 
      To produce a signal which can be processed in the established way by the Jones matrix eigenanalysis, the two spectral components can be separated using two bandpass filters (possibly FIR filters). Then the faster oscillating signal can be shifted in frequency so that it is aligned to the slower oscillating signal. This can easily be achieved if the differential group delay (DGD) of the PDU (DGD PDU =τ LO,H −τ LO,V ) is known. Then a linear phase term can be subtracted from the analytical signal by multiplying with exp(−(τ LO,H −τ LO,V )ω). Now the two generated signals are identical to those which would have been generated by a coherent prior art PDR detector.  
      Furthermore, the prior art concepts incorporating PDRs as detectors suffered from the fact that it is very difficult to choose an optimum LO polarization at more than two PDRs simultaneously. This problem can be solved by an embodiment of the present invention since the polarization of the LO signal does not enter the evaluation anymore. This property makes this embodiment particularly well suited for multiport device characterization where the LO is distributed among several detectors.  
      Using the prior art concept according to EP 1113250 for multiport device characterization requires distributing the LO signal among several PDR detectors. Unfortunately it is very difficult to provide an optimum input polarization of the LO signal at every PDR simultaneously simply by adjusting the input polarization of the whole setup. Therefore, with the cited prior art multiport devices can only be measured by performing several sweeps and adjusting the optimum input polarization for each sweep. Another solution would be to insert a one dimensional polarization controller (i.e. a wave plate of tunable retardation) in front of each PDR. This gives the opportunity to acquire all data in a single or at least in two scans.  
      According to an embodiment of the present invention a replacement of a PDR by a polarization multiplexed LO signal solves this problem since no special absolute orientation of the polarization states SOP H  and SOP V  are required.  
      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  is a schematic diagram displaying a way how to generate signals according to an embodiment of the present invention;  
       FIG. 2  is a graph showing two spectral components occurring in the electrical spectrum corresponding to two elements of the Jones matrix according to an embodiment of the present invention;  
       FIG. 3  shows a setup for multiport device characterization according to an embodiment of the present invention;  
       FIG. 4  shows a setup for single-scan multiport device characterization according to an embodiment of the present invention; and  
       FIG. 5  is a graph showing resulting spectral components when using the embodiment of  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Embodiment of  FIG. 3  shows a possible implementation of a measurement setup for multiport device characterization according to an embodiment of the present invention. A LO signal  3  coming from a tunable laser source (TLS)  4  is adjusted in its polarization to a defined polarization by a polarization setting tool  26  positioned in the path of the light beam  3  before a first beam splitter  14 . The resulting incoming light beam  6  is split by the first beam splitter  14  into a first light beam  18  and a second light beam  20 .  
      An optical device under test  2  is positioned in a first path of the first light beam  18  for coupling in the first light beam  18 . A LO polarization delay unit  5  is positioned in a second path of the second light beam  20  for coupling in the second light beam  20  for splitting the second light beam  20  into a first part and a second part, delaying the second part of the second light beam  20  relative to the first part of the second light beam  20 , and recombining the first and the second part of the second light beam  20  to a resulting recombined beam  7 .  
      A second beam splitter  28  in said first and in said second path for superimposing the first light beam  18  and the recombined beam  7  with the recombined parts of the second light beam  20  to produce interferences between the first light beam  18  and the recombined parts of the second light beam  20  in at least one resulting superimposed light beam  30  traveling a resulting path.  
      A detector P 1  in said resulting path is then detecting the power of the resulting superimposed light beam  30  traveling the resulting path as a function of frequency and polarization when tuning the frequency of the incoming light beam  6  over a given frequency range with the TLS  4 . Then, a (not shown) evaluation unit derives optical properties of the optical device under test  2  from the frequency dependency of the detected powers.  
      Since the PDU  5  is a multiport device the resulting beam  7  coming from the PDU  5  is distributed to four identical beam splitters  28 . Accordingly, beam  18  is distributed to the four beam splitters  28 . Leaving beam splitters  28  are four superimposed beams  30  which are detected by four receivers P 1 , P 2 , P 3 , P 4  in the above described manner.  
      The received detector signal can be processed using the filtering setup of  FIG. 1 .  FIG. 1  is a schematic diagram displaying a way how to generate signals according to an embodiment of the present invention. According to  FIG. 3  the polarization controller  26  at the input of the system can adjust the input polarization of the PDU  5  appropriately so that the two propagation paths are excited with the same optical power. For the two-scan method according to EP 1113250, preferably two scans with orthogonal polarizations are performed. The equal power splitting inside the PDU  5  is maintained even if the input polarization is adjusted to the orthogonal state. The setup displayed in  FIG. 3  has a significant reduced complexity and can be enhanced to more than four ports without any further considerations.  
      Because the PDU  5  is introduced into the LO path, two orthogonal polarization states, a first state of polarization SOP H  and a second state of polarization SOP V , occur at the output of the PDU  5 . The two orthogonal components have traveled different optical paths inside the PDU  5  and thus produce interference signatures at different electrical frequencies f H  and f V  according to  FIG. 2  when combined with the DUT signal  18  at the detectors P 1 -P 4 .  FIG. 2  is a graph showing two spectral components occurring in the electrical spectrum corresponding to two elements of the Jones matrix.  
      These spectral components or interference signatures created by the interference of the DUT signal  18  with signals SOP H  and SOP V  respectively can be distinguished preferably by applying band pass filters of appropriate center frequency and bandwidth.  
      Furthermore, amplitude and phase of the two spectral components only depend on the fraction of the DUT signal  18  having the same polarization as the interfering LO signal  7 . That is, the DUT signal  18  is inherently decomposed into two polarizations SOP H  and SOP V  interfering with the corresponding two LO signals  7  and producing signatures at f H  and f V  respectively. The AC part of the detector signal can be written as follows:  
                 P   AC     ⁡     (   ω   )       =       ⁢         E   LO     ⁢       E   DUT     ⁡     (         sop   ⇀     DUT     ·       sop   ⇀     H       )       ⁢     cos   (       φ   H     +         (       τ   DUT     -     τ     LO   ,   H         )           1   ⁢           ⁢   4   ⁢           ⁢   4   ⁢           ⁢   2   ⁢           ⁢   4   ⁢           ⁢   43     ⁢               f   H         ⁢   ω       )       +                     ⁢       E   LO     ⁢       E   OUT     ⁡     (         sop   ⇀     DUT     ·       sop   ⇀     V       )       ⁢     cos   (       φ   V     +         (       τ   DUT     -     τ     LO   ,   V         )           1   ⁢           ⁢   4   ⁢           ⁢   4   ⁢           ⁢   2   ⁢           ⁢   4   ⁢           ⁢   43     ⁢               f   V         ⁢   ω       )                 
 
 To produce a signal which can be processed in the established way by the Jones matrix eigenanalysis, the two spectral components can be separated using two FIR filters according to the scheme displayed in  FIG. 1 . The faster oscillating signal can be shifted in frequency so that it is aligned to the slower oscillating signal. This can easily be achieved if the DGD of the PDU is known, with DGD PDU =τ LO,H −τ LO,V . Then a linear phase term can be subtracted from the analytical signal by multiplying with exp(−(τ LO,H −τ LO,V )ω). According to  FIG. 1  the two generated signals are identical to those which would have been generated by a coherent prior art PDR detector. 
 
      The method according to this embodiment is compatible to the single-scan measurement concept described in U.S. Ser. No. 09/940,741, the priority of which is claimed by the present application and the disclosure of which is incorporated herein by reference.  
      If an additional but identical PDU  102  is inserted into the DUT path according to an embodiment of  FIG. 4  showing a setup for characterizing a DUT  2  with four output ports P 1 -P 4  using the single-scan approach, four interference signatures are generated at the receivers P 1 -P 4 . However, the DGD values of the two PDUs  5  and  102  have to be chosen appropriately to ensure that the four spectral components do not intersect. Equidistant frequency components can be generated if the two DGD values, hereafter referred to as DGD PDULO  and DGD PDUDUT , differ by a factor of two. Furthermore, frequency components can be generated in the low-frequency range which can easily be removed by a high-pass filter.  
       FIG. 5 . shows the resulting spectral components of the electrical spectrum. The four components J 11 , J 21 , J 12 , J 22  correspond to the four elements of the Jones matrix. They can be separated in the same way as it is done according to  FIG. 1 , and three of the four components J 11 , J 21 , J 12 , J 22  can be shifted in frequency to be realigned with the first component J 11 . At this point the established Jones matrix eigenanalysis can be applied to the four signals to derive information on PMD of DUT  2 .