Abstract:
The present invention relates determination of optical properties, e.g. polarization dependent loss (PDL), polarization mode dispersion (PMD), differential group delay (DGD), insertion loss, return loss and/or chromatic dispersion (CD), of a device under test (DUT) in transmission and in reflection of an optical beam. The invention is disclosing an element that is at least partly transmissive and at least partly reflective.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to determination of optical properties, e.g. polarization dependent loss (PDL), polarization mode dispersion (PMD), differential group delay (DGD), insertion loss, return loss and/or chromatic dispersion (CD), of a device under test (DUT) in transmission and in reflection of an optical beam.  
           [0002]    Measurement setups for the above-mentioned purpose shall be as easy to handle as possible and shall reveal all optical properties of the DUT as fast as possible and with as little handling as possible. This means that the DUT should be fully characterized to all parameters required when it is once connected to the measurement setup. For a full characterization it is required to measure all parameters both in transmission and in reflection as fast as possible.  
           [0003]    From the disclosure of work of Sandel et al (David Sandel, Reinhold Noé, “Optical Network Analyzer applied for Fiber Bragg Grating Characterization”, ECOC 97, Sep. 22-25, 1997, Conference Publication No. 448, © IEE, 1997, pp. 186-189; David Sandel et al, “Optical Network Analysis and Longitudinal Structure Characterization of Fiber Bragg Grating”, Journal of Lightwave Technology, Vol. 16, No. 12, December 1998, pp. 2435-2442) it is known a method for polarization-resolved optical fiber Bragg grating characterization. However, in these disclosures only the reflection of the DUT is measured.  
           [0004]    From a work of Froggatt at al (Froggatt et al, “Full Complex Transmission and Reflection Characterization of a Bragg Grating in a Single Laser Sweep”,) it is known a measurement setup to measure the group delay of a DUT in transmission and in reflection in both directions. However, with the disclosed measurement setup it is not possible to measure PMD or PDL. Moreover, the measurement setup disclosed in this article causes problems because the detectors used to detect the signals of reflection and transmission receive the signals of both directions simultaneously, i.e. the reflected signal of one direction is superimposed with the transmitted signal of the other direction and the transmitted signal of one direction is superimposed with the reflected signal of the other direction. Therefore, complex measures are necessary to distinct between these signals without really knowing all impacts of this superposition of signals.  
         SUMMARY OF THE INVENTION  
         [0005]    Therefore, it is an object of the invention to provide improved determination of optical properties of a DUT in one direction in transmission and in reflection of an optical beam.  
           [0006]    The object is solved by the independent claims.  
           [0007]    An advantage of the present invention is the provision of a fast way to convert a measurement setup of the above-mentioned art for measuring in transmission into a measurement setup which is able to measure the DUT in one direction in transmission and in reflection, simultaneously. In a preferred embodiment of the invention the inventive element comprises a semi-transparent mirror. This embodiment is easy to fabricate, easy to handle and cheap in production costs.  
           [0008]    In a further preferred embodiment of the invention the element has a known proportion of transmission and reflection, more preferred also known optical properties, e.g. PDL, PMD, DGD, insertion loss, return loss, CD. It is preferred to have an element with substantially no PMD, DGD, insertion loss, return loss, PDL, and CD in the relevant wavelength range.  
           [0009]    It is further preferred that the element is prepared in such a way that the optical properties can be adjusted. This embodiment guarantees more flexibility when using the inventive element.  
           [0010]    In another preferred embodiment of the invention the element comprises a first beam splitter or coupler in an initial path of the beam for coupling out at least a part of the beam into a first path, an optical guide for guiding the part of the beam partly back into the initial path in reverse direction, the guide preferably comprising a second beam splitter or coupler in the first path for coupling the part of the beam back into the initial path. This embodiment realizes the invention without the necessity of using a semi-transparent mirror.  
           [0011]    In another preferred embodiment of the invention the element comprises a first beam splitter or coupler in an initial part of the beam for coupling out at least part of the beam into a first path, a mirror in the first path for reflecting back the part of the beam to the first beam splitter so that the first beam splitter partly guides the part back into the initial path in reverse direction and partly into a second path guiding the reflected signal in the initial direction.  
           [0012]    Other preferred embodiments are shown by the dependent claims.  
           [0013]    It is clear that 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  
       [0014]    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).  
         [0015]    [0015]FIG. 1 shows a principle of an embodiment of the inventive;  
         [0016]    [0016]FIG. 2 shows a first embodiment of the element of the present inventions;  
         [0017]    [0017]FIG. 3 shows a second embodiment of the element of the present inventions;  
         [0018]    [0018]FIG. 4 shows a third embodiment of the element of the present invention,  
         [0019]    [0019]FIG. 5 shows a first measurement setup according to the present invention; and  
         [0020]    [0020]FIG. 6 shows a second measurement setup according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring now in greater detail to the drawings, FIG. 1 shows schematically a principle of an embodiment of the inventive method. In step A of FIG. 1 it is shown the reference arm  2  of a measurement setup  400  (see FIG. 5) for determination of optical properties of a DUT  6  in transmission and in reflection in one direction. Such a measurement setup  400  can be calibrated and/or verified by a calibration and/or verification element disclosed in a parallel patent application of the applicant of the same day. Therefore, the description of the measurement setup calibration and/or verification given in the parallel application is incorporated herein by reference.  
         [0022]    The reference arm  2  has two connectors  4   a  and  4   b.  Between the two connectors  4   a  and  4   b  a patch-cord is inserted. By releasing the connection at the connectors  4   a  and  4   b  (indicated by the arrow  8 ) it is possible to disconnect the patch-cord  7  from the reference arm  2 . This, as shown in step B of FIG. 1, opens a gap  10  between the connectors  4   a  and  4   b.  This makes it possible to insert an inventive element  12  into the gap  10  (indicated by arrow  14 ). For this purpose the element  12  is prepared with two short patch-cords  16   a  and  16   b  which patch-cords have connectors  18   a  and  18   b  which connectors can be connected to the connectors  4   a  and  4   b  of the reference arm  2 , respectively. As shown in step C of FIG. 1, as a result the inventive element  12  is inserted in the reference arm  2  and has replaced the patch-cord  7 .  
         [0023]    [0023]FIG. 2 shows a first embodiment  100  of the inventive element  12 . In embodiment  100  the inventive element  12 , comprises a semi-transparent mirror  20 . The semi-transparent mirror  20  reflects 50% of light as indicated by a triangle  22  and guided by the patch-cord  16   a  to the mirror  20  back into the patch-cord  16   a  as indicated by  24  and lets 50% of the light  22  travel through the mirror  20  as indicated by triangle  26  which light travels along the patch-cord  16   b  to the connector  18   b.  Therefore, the element according to FIG. 2 provides transmission and reflection of the incoming light  22 . However, different ratios of transmission and reflection can be used.  
         [0024]    [0024]FIG. 3 shows a second embodiment  200  of the inventive element  12  of the present invention. Element  12  of embodiment  200  comprises a first coupler  28 , which is preferably a 3 dB coupler. However, other couplers, as 10 dB couplers, can be used, also. Coupler  28  lies in the initial path provided by patch-cord  16   a  of the incoming light  22 . The coupler  28  couples out 50% of the light  22  into a first path  30 , the part coupled out being indicated by a triangle  32 . The other 50% part as indicated by triangle  34  travels along the initial path  16   a.  Furthermore, element  12  comprises a second beam splitter  36  which couples part  32  partly back into the initial path  16   a  in reverse direction as indicated by triangle  38 . Additionally, the second coupler  36  couples the light  34  into the first path  30  as indicated by triangle  40 . Light  40  is partly coupled back into the initial path  16 a in reverse direction with the first coupler  28  as indicated by triangle  42 . The part of the light  34  not coupled out of the initial path  16   a  by the second coupler  36  travels along the patch-cord  16   b  to the connector  18   b  as indicated by triangle  44 . Therefore, the element  12  in the embodiment  200  of FIG. 3 provides a part  44  of the incoming light  22  in transmission at the connector  18   b  and a part  42  of the incoming light  22  in reflection at the connector  18   a.    
         [0025]    Furthermore, by adjusting the couplers  28  and  36 , e.g. by using 10 dB couplers or other couplers, it is possible to adjust the ratio of reflected light  42  to transmitted light  44 .  
         [0026]    [0026]FIG. 4 shows a third embodiment  300  of the inventive element  12  of the present invention. In embodiment  300  the incoming light  22  is partly coupled out by a coupler  46  into a first path  48  as indicated by triangle  52 . At the end of the first path  48  there is provided a mirror  50 . Mirror  50  reflects the light  52  in total as indicated by triangle  54 . Subsequently, coupler  46  couples the reflected light  54  into the initial path  16   a  in reverse direction as indicated by triangle  56  and into the patch-cord  16   b  in a direction to the connector  18   b  as indicated by triangle  58 . Therefore, the element  12  according to the embodiment  300  of FIG. 4 provides a part  58  of the incoming light  22  in transmission at the connector  18   b  and a part  56  of the incoming light  22  in reflection at the connector  18   a.    
         [0027]    [0027]FIG. 5 shows a first embodiment  400  of a measurement setup according to the present invention. Measurement setup  400  contains a tunable light source  70  that provides a coherent laser beam  72  to a polarization controller  74  (which can be a Hewlett-Packard HP 8169A). The polarization controller  74  provides a polarization controlled coherent light beam  76  to an isolator  78 . Optically connected with the isolator  78  and receiving a coherent light beam  80  from the isolator  78  is a third beam splitter  82  that is a 3 dB coupler. Also optically connected with the isolator  78  and receiving the optical beam  80  is a wavelength reference unit  84  (see also FIG. 6) to detect the wavelength of the beam  80 .  
         [0028]    Connected to the coupler  82  is a reference arm  2  and a measurement arm  86 . In the measurement arm there is provided a switch  88  to cut the measurement arm  86  for calibration purposes. Additionally, the measurement arm  86  contains a seat  90  to receive the DUT  6 . The seat  90  has two connectors  92  and  94  to enable the DUT  6  to be connected to the measurement arm  86 .  
         [0029]    Between the third coupler  82  and the seat  90  there is provided a fifth power  96  for measuring the signal strength of the beam  80  split by the coupler  82  into the measurement arm  86 . Additionally, there is a provided a sixth detector  98  for measuring the signal strength of the light being reflected by the DUT  6 .  
         [0030]    Furthermore, the measurement arm  86  is connected to a fourth beam splitter  102  that is a 3 dB coupler. Between the seat  90  and the fourth beam splitter  102  there is provided a seventh power detector for measuring the signal strength of the light transmitted through the DUT  6 .  
         [0031]    Connected to the coupler  102  is a polarization diversity receiver  106  to detect a superimposed signal being the superposition of the transmitted signal by the DUT  6  and a reference signal coupled in by the fourth coupler  102  from the reference arm  2 . The reference signal is coupled into the reference arm  2  by the third coupler  82 .  
         [0032]    Connected to the third coupler  82  is also a polarization diversity receiver  108 . This polarization diversity receiver detects the superimposed signal of the reflected signal from the DUT  6  coupled in by the coupler  82  from the reference arm and the reflected reference signal coupled in from the reference arm coming from the element  12 .  
         [0033]    For further details it is referred to the European Patent Application 00125089.3 of the applicant the disclosure of which is incorporated herein by reference.  
         [0034]    [0034]FIG. 6 shows a second embodiment  500  of a measurement setup according to the present invention. FIG. 6 shows further details of the wavelength reference unit  84 . The wavelength reference unit  84  contains a six port coupler  110  which coupler  110  splits a part  112  coupled out from the beam  80  into three beams  114 , 116  and  118 . Beams  114  and  116  are directed onto Faraday mirrors  120  and  122 . The Faraday mirror  120  can be shifted to change the length of the path  114 . Furthermore, the wavelength reference unit  84  contains a gas cell  124  connected with a eighth power detector  126 . The gas in the gas cell  124  has a known absorption spectrum. With the help of the detector  126  and the known absorption spectrum of the gas in the gas cell  124  it is possible to determine the wavelength of the beam  80  very precisely.  
         [0035]    Additionally, embodiment  500  shows the polarization diversity receivers  106  and  108  in detail. Both have polarization beam splitters  128  and  130  that are connected to first  132 , second  134 , third  136  and fourth  138  power detectors.  
         [0036]    Contrary to the embodiment  400  of FIG. 5 embodiment  500  of FIG. 6 has the element  12  according to embodiment  200  of FIG. 3 not connected as shown in embodiment  200  of FIG. 3. In embodiment  500  of FIG. 6 the guide  30  is not coupled into the reference arm directly as shown in FIG. 3. In embodiment  500  the guide  30  is coupled with a coupler  140  to superimpose the reference signal guided by-guide  30  with the reflected signal of path  160  directly in front of the polarization diversity receiver  108 . This advantageously avoids introduction of the reference signal  30  into the initial path  80 .