Abstract:
The present invention relates to calibration and verification of a measurement setup for 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 calibration and/or verification of a measurement setup for 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 in transmission and in reflection. This can be done for example by a measurement setup described in a parallel patent application of the applicant of the same day, which is incorporated herein by reference.  
           [0003]    However, the above requirements also mean that the calibration and/or the verification of the measurement setup should be as easy and as fast as the measurement itself.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the invention to provide improved calibration and/or verification of a measurement setup for determination of optical properties of a DUT in transmission and in reflection of an optical beam. The object is solved by the independent claims.  
           [0005]    An advantage of the present invention is the provision of a fast way to calibrate and/or verify a measurement setup of the above-mentioned art in transmission and in reflection, simultaneously. To the knowledge of the inventors, calibration and/or verification of a measurement setup of the above-mentioned art simultaneously in transmission and in reflection has not been reported so far. In other words the invention provides the first time a possibility to calibrate and/or verify a measurement setup of the above-mentioned art in one go.  
           [0006]    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.  
           [0007]    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. Especially for the measurement of PDL it is preferred to have an element with known PDL for calibration of the measurement setup. However, it is advantageous to have an element with known optical properties for verification, also, although this is not necessary for verification. Therefore, for verification of a measurement setup it is also possible to use an element that has substantially no PMD, DGD, insertion loss, return loss, PDL and CD in the relevant wavelength range.  
           [0008]    It is further preferred that the element is prepared in such a way that the optical properties can be adjusted. This embodiment guarantees a maximum of flexibility when using the inventive element.  
           [0009]    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.  
           [0010]    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.  
           [0011]    Other preferred embodiments are shown by the dependent claims.  
           [0012]    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  
       [0013]    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).  
         [0014]    [0014]FIG. 1 shows a principle of an embodiment of the inventive;  
         [0015]    [0015]FIG. 2 shows a first embodiment of the element of the present inventions;  
         [0016]    [0016]FIG. 3 shows a second embodiment of the element of the present inventions; and  
         [0017]    [0017]FIG. 4 shows a third embodiment of the element of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    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 measurement arm  2  of a (not shown) measurement setup for determination of optical properties of a DUT  6  in transmission and in reflection. Such a measurement setup is for example disclosed in the above-mentioned parallel patent application of the applicant of the same day. Therefore, the description of the measurement setup given in the parallel application is incorporated herein by reference. The measurement arm has two connectors  4   a  and  4   b . Between the two connectors  4   a  and  4   b  the DUT  6  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 DUT  6  from the measurement 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 measurement arm  2 , respectively. As shown in step c of FIG. 1, as a result the inventive element  12  is inserted in the measurement arm  2  and has replaced the DUT  6 .  
         [0019]    [0019]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.  
         [0020]    [0020]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 coupler, 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  for calibration and/or verification.  
         [0021]    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 .  
         [0022]    [0022]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.