Abstract:
Apparatus for making wavelength-resolved polarimetric measurements comprises an interferometric source ( 10,12 ), for example a broadband source ( 10 ) and an optical interferometer unit ( 12 ), and a polarization generator unit ( 16 ) for generating different states of polarization of light received from the interferometric source and applying same to a device-under-test ( 30 ). A polarimeter unit ( 20 ) receives and polarimetrically-analyzes light from the device-under-test, converts the polarimetrically-analyzed light into electrical signals, and, using Fast Fourier Transform numerical analysis, computes therefrom the wavelength-resolved polarimetric measurements. Placing the optical interferometer unit ( 10,12 ) “upstream” not only of the device-under-test ( 30 ), but also of the polarisation generator unit ( 16 ), means that the latter substantially eliminates polarization dependent effects introduced by the former. The polarimeter ( 20 ) can still perform the necessary transformation even though the interferogram has been passed through the polarisation generator unit and the device-under-test.

Description:
This application claims priority from U.S. Provisional patent application No. 60/330,528 filed Oct. 24, 2001. 
    
    
     TECHNICAL FIELD 
     This invention relates to a method and apparatus for making wavelength-resolved polarimetric measurements to determine, for example, wavelength-dependent transmission properties, such as polarization mode dispersion, polarization-dependent loss, and so on. The invention is especially applicable to a method and apparatus wherein the wavelength-resolved measurement is carried out with an optical spectrum analyzer based upon a Michelson interferometer design. 
     BACKGROUND ART 
     It is often necessary to test elements of optical networks to determine, for example, polarization mode dispersion (PMD), differential group delay (DGD), polarization-dependent loss (PDL) or other characteristics, It is known to do so by means of a polarimetric PMD analysis technique, wherein light having each of several known states of polarization is launched into the device-under-test, and the corresponding transmitted state of polarization (SOP) is then analyzed, This so-called polarimetric data is then entered into known algorithms, such as the so-called Jones Matrix Eigenanalysis (JME) method or Poincaré Sphere Analysis (PSA) method, in order to calculate the desired parameters describing the PMD-related behaviour of the test device. A key component in such a test system is a real-time Stokes analyzer to measure Stokes polarization parameters or their equivalents. Additionally, there must be a means for providing wavelength-resolved measurements, a means for launching light into the device-under-test (DUT) with several known states of polarization (SOP), and means for analyzing the SOP of the light beam leaving the DUT. 
     A polarimetric analysis technique disclosed by Normand Cyr, Bernard Ruchet, André Girard and Gregory W. Schinn in an article entitled “Poincaré Sphere Analysis: Application to PMD Measurement of DWDM Components and Fibers”, Proceedings of SubOptic 2001, Kyoto, Japan May 20-24, 2001 pp 571-574 uses a polarized broadband source, an SOP generator, and an element for spectrally resolving the light beam, followed by a polarimeter. Spectral resolution of the light beam can be effected using a scanning Michelson interferometer, which, when coupled with Fast-Fourier-Transform (FFT) numerical analysis, is functionally equivalent to an optical spectrum analyzer based on a more traditional grating-based scanning monochromator design. Unlike this latter design, wherein only a small spectral “slice” of the light is detected at any one time, a Michelson-interferometer-based spectrum analyzer detects the full spectrum of the light at all times, hence offering a much more efficient light-gathering capability and, consequently, a much faster measurement time. 
     A limitation of such a measurement technique, however, is that the Michelson interferometer itself usually exhibits undesirable PDL and a small, intrinsic PMD which limits the accuracy of the overall polarimetric PMD measurement system. 
     DISCLOSURE OF INVENTION 
     An object of the present invention is to mitigate this problem and, to this end, the placement of the interferometer unit in the overall polarimetric measurement system is changed. 
     According to the present invention, apparatus for making wavelength-resolved polarimetric measurements comprises an interferometric source means, for example a broadband source and an interferometer unit, a polarization generator unit for generating different states of polarization of light received from the interferometric source means and applying same to a device-under-test, and a polarimeter unit for receiving and polarimetrically-analyzing light from the device-under-test, converting the polarimetrically-analyzed light into electrical signals, and computing therefrom the wavelength-resolved polarimetric measurements. 
     Traditionally, an interferometer comprises an optical interferometer unit, which uses autocorrelation to produce an interferogram, and a processor unit which, following conversion of the interferogram to an electrical signal, uses FFT numerical analysis to transform the electrical signal into an optical spectrum of the light incident upon the optical, interferometer unit. The present invention is predicated upon the rather surprising realisation that, for measurement of the wavelength-dependent transmission properties of a DUT, the optical interferometer unit need not be “downstream” of the device-under-test, along with the processor, but can be located not only “upstream” of the device-under-test, but also “upstream” of the polarisation selection unit. The processor can still perform the necessary transformation even though the interferogram has been passed through the polarisation selection unit and the device-under-test. The FFT numerical analysis performed by the processor need not be changed. 
     Locating the optical interferometer unit upstream of the polarization generator unit means that the latter eliminates polarization dependent effects inherently introduced by the optical interferometer unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: 
         FIG. 1  is a block schematic diagram of an optical spectrum analyzer (OSA); 
         FIG. 2  is a perspective schematic representation of a polarimetric analyzer unit of the OSA having three waveplates and a plain glass plate; and 
         FIG. 3  is a detail end view illustrating respective axes of the three waveplates. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawing, apparatus for making wave-length resolved polarimetric measurements comprises an interferometric source, specifically a broadband light source  10  and an optical spectrum analyzer, specifically a Michelson interferometer  12 , interconnected by an optical fiber  14 , a polarization state generator  16 , for example a rotating polarizer, coupled to the output of the interferometer  12  by an optical fiber  18 , and a polarimeter  20 , i.e., an instrument for measuring SOP of a light beam. The polarimeter  20  comprises a polarimetric analyzer  22  having four output ports connected by optical fibers  24 A- 24 D, respectively, to a bank of detectors  26  which are connected to a processor  28 . Preferably, the fibers  24 A- 24 D are multimode fibers. A device-under-test  30  is connected by an optical fiber  32  to an output of the polarization state generator  16  and by an optical fiber  34  to an input of the polarimetric analyzer  22 . 
     Within the polarimetric analyzer  22 , an input lens  36  collimates light from the optical fiber  34  and applies it to a beam splitter and analyzer unit  38 , and a bank of four output lenses  40 A- 40 D converge light from the analyzers into the output optical fibers  24 A- 24 D, respectively. Preferably, the output lenses  40 A- 40 D are attached directly to the beam splitter and analyzer unit&#39;s output ports, so that only the coupling between the lenses  40 A- 40 D and the optical fibers  24 A- 24 D, respectively, is a free space arrangement. The polarimetric analyzer unit  22  comprises a device for decomposing the light beam from fiber  34  into components with predetermined SOPs enabling computation of the Stokes parameters. A suitable polarimetric analyzer unit having an advantageous composite construction is the subject of U.S. patent application No. 10/278,081 filed contemporaneously herewith, and will now be described. 
     Referring to  FIGS. 2 and 3 , the polarimetric analyzer unit  22  comprises a parallelepiped linear polarizer  42 , specifically a Glan-Taylor prism, having three square waveplates  44 A,  44 B and  44 C each having a retardation of λ/3, i.e., with phase retardation of 2π/3, and a transparent plate (i.e., a window with zero retardation)  46  adhered to its input face  48  using suitable optical cement. The polarizer  42  conveniently is made of calcite and is of an air-gap design, and the plate  46 , which has the same thickness as the waveplates  44 A- 44 C, conveniently is made of glass. The waveplates  44 A,  44 B and  44 C and the glass plate  46  comprise quadrants which together cover the input face  48  of the polarizer  42 . The are placed into respective openings of a cruciform opaque divider  50  which has one limb (shown vertical in  FIGS. 2 and 3 ) aligned in the same sense as the polarizer axis P, and attached to adjacent limbs of the divider by adhesive. The cruciform divider  50  “slices” the input light beam cleanly into four portions. A wedge-shaped plate  52  is adhered, using index matching glue, to the front of the waveplates  44 A,  44 B and  44 C and the glass plate  46  and serves to reduce reflections that could lead to undesirable Fabry-Perot-type interferometric noise. Input means comprises collimating lens  36  which collimates light received from the single mode input optical fiber  34  and directs the collimated light beam onto the waveplates  44 A,  44 B and  44 C and the glass plate  46 , so that each receives an equal portion of the light beam. 
     As shown in  FIG. 2 , the lenses  40 A,  40 B,  40 C and  40 D are rectangular, specifically square. They are adhered to the output face  54  of the linear polarizer  42  and receive the corresponding four light components from the waveplates  44 A,  44 B,  44 C and glass plate  46 , respectively and focus them into the four multimode output optical fibers  24 A,  24 B,  24 C and  24 D, respectively, which are coupled to the set of four photodetectors  26 A,  26 B,  26 C and  26 D, respectively. The photodetectors  26 A,  26 B,  26 C and  26 D convert the optical signals into electrical signals and convey them to the processor  28  which uses their intensities to compute the Stokes parameters. 
     The three waveplates  44 A,  44 B and  44 C are identical and each has a fast axis at an angle of about 27.5 degrees to one edge. As shown in  FIG. 3 , however, each of the waveplates  44 A,  44 B. and  44 C is disposed with its fast axis at a different angle relative to the polarizer axis P which, in  FIG. 3 , is shown as extending vertically in the plane of the drawing. Thus, assuming clockwise rotation from the vertical, the fast axes of the waveplates  44 A,  44 B and  44 C are at angles of 27.5 degrees, 117.5 degrees and 332.5 degrees, respectively. 
     The measured intensity or power of the signal received by way of glass plate  46  and detector  26 D represents the degree of polarization and is used with the intensities measured by way of the three waveplates  44 A,  44 B and  44 C and the detectors  26 A,  26 B and  26 C to calculate the Stokes parameters. 
     It is instructive to consider the operation of the device as if the linear polarizer  42  were in front of the waveplates  44 A,  44 B and  44 C. Thus, the linear polarizer  42  exhibits high transmission for one linear SOP and extinguishes the orthogonal linear SOP (at 180 degrees on the Poincaré sphere). The preferred Glan-Taylor polarizer is recognized as having a high degree of extinction. On leaving the polarizer  42 , therefore, the SOP of the light is along the polarizer axis P. Each waveplate rotates the SOP about the sphere, the resultant polarizations corresponding to the equivalent axis of the analyzers. 
     It should be noted that, in contrast to known methods for analysing the SOP of a light beam, all four beams pass through a, preferably common, linear polarizer used as an analyser. Hence, no one light beam permits a direct determination of the Stokes parameter SO. Once the system has been suitably calibrated, the signals from the four detectors permit the determination of the four Stokes parameters. 
     It should also be noted that optical spectrum analyzers which use analyzers permitting measurement of Stokes parameters S 0 , S 1 , S 2  and S 3 , or a linear combination thereof, and having alignments based upon the mathematics used to compute the Stokes vectors, are optimized to square with the “first” mathematical solution to the detriment of hardware optimization. Embodiments of the present invention using four polarization analyzers with their axes linearly independent, so that a nonsingular matrix describing the transformation relating the intensities measured at the four detectors to the four Stokes parameters can be constructed, allow more freedom for the hardware to be optimized. While the transformation matrix may be based upon the design, it is preferred to produce it by measurement, i.e., calibration, which gives better precision and reliability. Moreover, the calibration changes little with time or temperature and yet changes smoothly with wavelength, which is desirable. 
     Thus, the calibration produces, for each wavelength, a calibration transformation matrix that relates the measured intensities to the Stokes vectors. This calibration procedure can be described as follows. 
     First one generates four known SOPs, each having a DOP of 100%. Each of these states is, in turn, sent to the polarimeter, where one measures the resulting electrical currents. 
             Generated  SOPs:           [           S   01               S   11               S   21               S   31           ]           [           S   02               S   12               S   22               S   32           ]           [           S   03               S   13               S   23               S   33           ]           [           S   04               S   14               S   24               S   34           ]             
               Measured  currents:     ⁢                   [           I   11               I   21               I   31               I   41           ]           [           I   12               I   22               I   32               I   42           ]           [           I   13               I   23               I   33               I   43           ]           [           I   14               S   24               I   34               I   44           ]             
 
     These four SOPs can be grouped into a single 4×4 matrix, and the measured currents grouped into another matrix; 
       Stokes   =           [           S   01               S   11               S   21               S   31                         S   02               S   12               S   23               S   33                       S   03               S   13               S   23               S   33                         S   4               S   14               S   24               S   34           ]               
       Intensity   =     [                 I   11               I   21               I   31               I   41                       I   12               I   22               I   32               I   43                       I   13               I   23               I   33               I   43                         I   14               I   24               I   34               I   44           ]                 
 
Now, the Stokes matrix is related to the intensity matrix via:
 
(Stokes)=M Calibration ·(Intensity)
 
The Calibration transformation matrix can then be directly calculated via:
 
M calibration =Stokes·(Intensity) −1 
 
     An advantageous and novel feature of the above-described invention is that all four portions of the light beam pass through a common polarizer serving as a linear analyzer element. This allows for a simple and compact design. The only real alignment of the polarizing elements (i.e., waveplates plus polarizer) is very straightforward as the three square waveplates can be “cut” from the same waveplate material, with the fast axis at 27.5 degrees from one edge. The three waveplates are then placed in the appropriate quadrant of the cruciform, whose “vertical” limb is aligned with the polarization axis P, and setting of the desired orientation of the respective fast axes then involves only “flipping” of one waveplate and rotation of another waveplate by ninety degrees. Of course, there still is alignment via four lenses into the four optical fibers, but this does not involve polarizing elements, as such. 
     An advantage of coupling the four outputs from the lenses  26 A,  26 B,  26 C and  26 D by means of the four optical fibers  30 A,  30 B,  30 C and  30 D, respectively, is that it eliminates, or at least significantly reduces, inaccuracies which are common in direct detection of a free-space beam by a detector, which can result in changes in the registration between the output beams and their respective detectors. As a general rule, when light is cut by sharp edges of any optical element, there is some diffraction causing the light beam to spread. Because there are four output light beams, any spreading could result in not only a change in registration but also in increased cross-talk. Launching the light beams into optical fibers for conveyance to the detector unit  26  permits better spatial filtering of all but the desired central portion of each light beam, i.e., less affected by edge effects of the waveplates and lenses, which may reduce cross-talk. 
     It should be appreciated that the waveplates  44 A,  44 B and  44 C and the glass plate  46  need not be square but could be circular, oval or any other suitable form. However, such a design would be less efficient at collecting the incident light, particularly due to loss of power in the centre of the beam, and would require additional alignment steps in fabrication to ensure that the angles of the fast axes of the waveplates were correctly aligned. 
     It should also be noted that, if DOP is not required, either, but not both, of the waveplates  44 A and  44 C could be omitted. 
     Various other alternatives and modifications are possible within the scope of the present invention. For example, the Michelson interferometer could be replaced by an alternative interferometer which detects the full spectrum of the light at all times. 
     An advantage of positioning the Michelson interferometer before the polarization generator is that polarization dependent effects produced in the Michelson interferometer, which generally are unavoidable, will be substantially removed by the polarization generator, leading to more accurate measurements. 
     Although single mode fiber provides excellent spatial filtering because of its small core size, launching of the light beams into single mode fibers would be inefficient. Multimode fiber is preferred, therefore, because it provides a good compromise between good spatial filtering and efficient light launching. 
     Advantageously, embodiments of this invention, in which the output beams from the polarimetric analyzer are supplied directly into the fibers  24 A- 24 D, greatly facilitate the measurement of very small (femtosecond) levels of PMD, which entails stringent requirements of precision and stability.