Abstract:
For measuring a polarization dependent parameter of an optical device under test —DUT—( 30 ), an optical source ( 10 ) provides an optical stimulus signal at variable wavelengths, and a polarization translator ( 20 ) translates the polarization state of the optical stimulus signal applied from the optical source ( 10 ) at its input to its output in a deterministic way dependent on the wavelength of the optical stimulus signal. A receiving unit ( 40 ) receives an optical response signal from the DUT ( 30 ) to the applied optical stimulus signal, and an analyzing unit ( 60 ) analyzes received optical response signals for different wavelengths for determining values of the polarization dependent parameter of the DUT ( 30 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to testing of optical components in particular for communication systems.  
           [0002]    Measurement of polarization dependent parameters like polarization dependent loss (PDL) and polarization dependent group delay PDGD (covering Differential Group Delay DGD and Polarization Mode Dispersion PMD) is of increased importance for advanced communication systems and generally described in ‘Fiber Optic Test and Measurement’ by Dennis Derickson, ISBN 0-13-534330-5, 1998, pages 354ff. Especially long-haul high-speed systems require that polarization properties of its components fulfill certain requirements. In general, component manufacturers address this by 100% testing of components for critical parameters. PDL nowadays in many cases is already measured 100%, PDGD may also develop to be a 100% test in manufacturing.  
           [0003]    Today&#39;s solutions for measuring polarization dependent loss parameters are the scrambling method (applying a random variation of polarization states and comparing maximum with minimum determined loss) or the Mueller method, whereby 4 defined polarization states are measured for each wavelength point and analyzed together. The latter requires multiple measurement sweeps at predefined polarization states. These methods are either slow, if testing at multiple wavelengths is required (PDL measurement using the scrambling method), or require multiple measurement sweeps at predefined polarization states (Mueller method). Multiple sweeps are disadvantageous because measurement time is increased and require very high stability of the measurement setup because no change of polarization properties of the whole setup (between laser and DUT) is allowed between the sweeps.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the invention to provide an improved measurement of polarization dependent parameters. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.  
           [0005]    For measuring polarization dependent parameters of an optical device under test (DUT), an optical source (preferably a tunable laser) provides an optical signal through an optical polarization translator to the DUT. The polarization translator translates the polarization of the optical signal from its input to its output in a deterministic way dependent on the wavelength of the optical signal.  
           [0006]    The polarization translator provides the translation of the polarization dependent on the wavelength preferably using birefringent properties. Accordingly, the optical source may also provide a variation of the wavelength over the time, and polarization translator provides a variation of the polarization over the time, so that effectively the translator also provides a ‘translation’ of the polarization dependent on the wavelength. The parameters wavelength and frequency shall be regarded here as equivalents (related by the general equation f=c/λ).  
           [0007]    When varying the wavelength of the optical source, the polarization translator changes the polarization of the signal launched into the DUT. Tuning the wavelength of the optical source in a way that measurement points with different polarization states are covered thus allows determining polarization dependent parameters of the DUT in that particular wavelength range.  
           [0008]    Typical polarization dependent parameters that can be analyzed by the invention are polarization dependent loss (PDL) or polarization dependent group delay PDGD (also referred to as Differential Group Delay (DGD) or Polarization Mode Dispersion (PMD)).  
           [0009]    The uncertainty of the polarization state of the output signal may be reduced by tapping off some fraction of the signal in an appropriate way and analyzing its polarization state at each wavelength with a polarimeter or a reduced polarization analysis device like an Analyzer.  
           [0010]    The polarization translator may be purely passive. The optical signal is preferably provided that it does not hit a Principle State of the Polarization (PSP) of the polarization translator, so that the output signal will follow a trajectory (e.g. a circle) on the Poincare Sphere in a deterministic way.  
           [0011]    The same principle of scanning the polarization can be applied to various PMD measurement techniques: For example the Jones Matrix Eigenanalysis (JME) or a novel method which is outlined in the European Patent Application No. 125089.3 (EP 1113250). In case of PMD measurements in general only two polarization states are combined to a measurement value.  
           [0012]    In case that several measurement points (defined by the wavelength and the polarization state of the optical signal applied to the DUT) are to be analyzed together for determining a value of a polarization dependent parameter, the wavelength range for those measurement points is preferably selected that a value of the polarization dependent parameter of the DUT can be considered as substantially constant in that wavelength range.  
           [0013]    Preferred algorithms for analyzing together several such measurement points are interpolation of neighboring measurement points, combining 4 measurement points using the Mueller Matrix analysis, or combining 2 measurement points using e.g. the Jones Matrix analysis.  
           [0014]    The invention has various advantages compared to today&#39;s standard methods (polarization scrambling and Mueller Matrix analysis). The polarization transformation device may be purely passive, the number of measurement points can be chosen to be much smaller compared to the scrambling method, and, most important, the complete measurement can be performed within one sweep (instead of four for the Mueller Matrix Analysis). Thus, the invention allows fast measurements and is also less sensitive against e.g. environmental or mechanical disturbances.  
           [0015]    The invention can be partly or entirely 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  
       [0016]    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. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).  
         [0017]    [0017]FIG. 1 shows a measurement setup according to the invention for measuring polarization dependent parameters.  
         [0018]    [0018]FIG. 2 shows a representation of polarization transformation on Poincare Sphere.  
         [0019]    [0019]FIGS. 3 and 4 show embodiments of the polarization translator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    In FIG. 1, a tunable laser  10  as an optical source provides an optical signal through an optical polarization translator  20  to an optical device under test (DUT)  30 . A power meter  40  receives and detects the optical signal after passing the DUT  30 . The polarization translator  20  translates the polarization of the optical signal from its input to its output in a deterministic way dependent on the wavelength of the optical signal.  
         [0021]    A polarization analyzer  50  might be optionally coupled to the output of the polarization translator  20  in order to determine the polarization state of the output signal of the polarization translator  20 .  
         [0022]    A controller  60  is coupled to the power meter  40  for analyzing polarization dependent parameters. Preferably, the controller  60  is further coupled to the tunable laser  10  for controlling the application and variation of the provided an optical signal and to the polarization analyzer  50  for receiving information about the actual polarization state of the output signal from the polarization translator  20 .  
         [0023]    When varying the wavelength of the tunable laser  10 , the polarization translator  20  changes the polarization of the optical signal launched into the DUT  30 . In operation, the wavelength of the tunable laser  10  is tuned in a way that optical signals with different polarization states are provided to the DUT  30 . For each measuring point (defined by the wavelength and the polarization state of optical signal applied to the DUT  30 ), the controller  60  receives a value of power intensity determined by the power meter  40 . Analyzing the power intensity values for a plurality of different measuring points thus allows determining polarization dependent parameters of the DUT  30  such as polarization dependent loss (PDL).  
         [0024]    In a preferred embodiment, several measurement points will be analyzed together for determining a value of a polarization dependent parameter. The wavelength range for those measurement points is selected that a value of the determined polarization dependent parameter of the DUT can be considered as substantially constant in that wavelength range.  
         [0025]    Preferably, a set of four measurement points with different polarization states is analyzed together resulting e.g. in one PDL value for the DUT. The wavelength range of the set of measurement points is thereby preferably selected to be smaller than the wavelength resolution for that measurement. In an example wherein PDL measurements are desired with a spectral resolution of 1 pm, a set of measurement points with 4 different polarizations should have a spectral distance of 0.25 pm. Within such wavelength range (even going further up to say 10 pm) it can be seen as sufficiently ensured with typical DUTs of present optical networks that constant PDL properties are maintained in that range.  
         [0026]    [0026]FIG. 2 shows a representation of polarization transformation on Poincare Sphere. The polarization state Pin of the optical signal applied to the polarization translator  20  will be transformed to polarization states P i   out  (with i=1, 2, 3, . . . ) at the output of the polarization translator  20 . In case of a waveplate (which shows purely linear birefringence) as polarization translator  20  with an orientation O, the polarization states P i   out  are all located on a circle on the Poincare Sphere dependent on the wavelength λ of the optical signal.  
         [0027]    The polarization translator  20  can be implemented in several ways depending on requirements of the particular measurement to be conducted. If a PDL measurement is to be performed with a Mueller Matrix type of implementation (see for example on pages 356 ff in ‘Fiber Optic Test and Measurement’ by Dennis Derickson, ISBN 0-13-534330-5, 1998), a set of at least four measurements has to be made with polarization states fulfilling certain requirements: they have to be significantly different, must not be located on a great circle of the Poincare Sphere, and preferably should not be located on any circle on the Poincare Sphere. A very high order waveplate that is excited by the linearly polarized signal of the tunable laser  10  can be used. The angle between the polarization of the optical signal and the optical axis of the waveplate is defined to φ 1 . However, in this configuration the states of polarization P i   out  are located on a circle that under certain circumstances won&#39;t allow conducting the Mueller Matrix type of calculation. This problem can be avoided if at least two waveplates are concatenated with their principle state of polarization PSP not aligned. This configuration would provide second order PMD, which means that the trajectory of the polarization translation function on the Poincare sphere won&#39;t be a circle anymore.  
         [0028]    In contrast to the Mueller Matrix based PDL measurement the PMD measurement techniques mentioned before only require measurement on 2 states of polarization. There are no requirements for these States of Polarization as long as they are sufficiently different.  
         [0029]    In a preferred implementation, a high order waveplate as the polarization translator  20  creates a phase difference between the propagating modes by its birefringence. An angle α 1  represents the phase difference of the two optical signals propagating in the two Eigenmodes of the waveplate device. When entering the device the phase difference is α 1 =0. When exiting the angle is given by:  
               α   1     =         2      π     λ        Δ                   n   ·   L               (     Equation                 1     )                               
 
         [0030]    with Δn, λ, L representing the difference of the refractive indices of the two propagating polarization modes, the optical wavelength and the length of the device, respectively. If the length L of the birefringent device  20  is kept fixed and dispersive effect are neglected (which means Δn is constant over wavelength), the wavelength increment to increase α 1  by a given amount, say Δα 1  is given by:  
             Δλ   ≈         Δα   1     ·     λ   2         2        π   ·   Δ                     n   ·   L                 (     Equation                 2     )                               
 
         [0031]    For example, if PMD measurement values are required with a spectral resolution of 1 nm (as it may be sufficient for fused couplers as DUT  30 ), measurement values should be taken with an interval of 0.5 nm. From Equation 2, a condition (at λ=1.5 μm) for the polarization transformer  20  can be derived:  
               Δ                   n   ·   L       ≡         Δα   1     ·     λ   2         2        π   ·   Δλ         ≅       2.3   ·     10     -   3                       m             (     Equation                 3     )                               
 
         [0032]    This requirement could be fulfilled with for example a LiNbO 3  waveguide or a birefringent fiber as the polarization translator  20 .  
         [0033]    In a LiNbO 3  based polarization transformer  20 , a Ti diffused waveguide can be implemented perpendicular to the c-axis (optical axis) of the LiNbO 3  crystal (typically selected: x- or y-cut). In this configuration, the waveguide  20  has a high birefringence of Δn≈0.079 with a beat length L B  of the two propagating modes of:  
               L   B     =             2      π     λ     ·   Δ                   n     ≈     21                 μm               (     Equation                 4     )                               
 
         [0034]    around λ=1.55 μm. Therefore the requirement mentioned in Equation 3 can be fulfilled with a LiNbO 3  waveguide  20  of a length of about 3 cm.  
         [0035]    A polarization transformer  20  based on Polarization Maintaining Fiber (PMF) has a typical birefringence of about 10 −3 . As this is much lower than in LiNbO 3 , a much longer length is required: 2.25 m. By increasing the length even further, a higher spectral resolution can be achieved. However, typical DWDM component test applications would require a spectral resolution of the PDL and PMD measurement around 1 . . . 3 pm. Therefore a PMF fiber length of more than 1000 m would be required, which might not be applicable for some applications e.g. for price, volume and possibly stability reasons.  
         [0036]    In a further preferred implementation, an ‘artificial birefringent device’  200  is used as polarization transformer  20  creating enough delay between the two propagating polarization modes for very high spectral resolution. The incoming (linearly polarized) light is split up in the artificial birefringent device  200  and guided along two different paths having different path lengths with a length difference ΔL. The artificial birefringent device  200  further provides the light returning from the two paths with orthogonal states of polarization. This can be done e.g. by splitting up the incoming light polarization dependent or by changing the state of polarization at least in one of the paths. After recombining the light returning from both paths, the state of polarization of the combined signal depends on the wavelength (or more accurately the frequency) of the optical signal in a deterministic, periodic way. By adjusting the length difference ΔL the periodicity can be broadly varied.  
         [0037]    [0037]FIG. 3 shows a first embodiment of the artificial birefringent device  200 . The incoming (linearly polarized) light is split up by a beam splitter or fiber coupler  210  and guided along the two different paths. One (typically short) path returns the signal with its original polarization. A second path, which has a geometric length difference of ΔL, returns the signal in its orthogonal state of polarization, e.g. by providing a Faraday Mirror  220 . After recombining the light of the first and the second path, the state of polarization of the combined signal depends on the wavelength of the optical signal.  
         [0038]    [0038]FIG. 4 shows a further embodiment of an artificial birefringent device  200 , preferably made of PMF components. The incoming (linearly polarized) light is split up by a polarization dependent beam splitter  250  into light beams having orthogonal states of polarization, guided along the two different paths with the length difference of ΔL, and recombined with the still orthogonal states of polarization. The state of polarization of the combined signal again depends on the wavelength of the optical signal. In order to provide both paths with substantially the same optical powers, a polarizer  260  might be inserted before the polarization dependent beam splitter  250  in order to polarize the incoming light to 45° with respect to polarization states provided by the polarization dependent beam splitter  250 .  
         [0039]    The artificial birefringent device  200  allows creating an almost arbitrarily selectable delay difference between two signal fractions. The delay difference is defined by the length difference of the fibers ΔL. For this setup Equation 2 changes to:  
             Δλ   ≈         Δα   1     ·     λ   2         2        π   ·   n   ·   Δ                   L               (     Equation                 5     )                               
 
         [0040]    where n represents the refractive index of the fiber. As an example, getting a PDL or PMD measurement resolution of 1 pm can be achieved by a length difference ΔL=1.5 m.