Abstract:
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. The invention is disclosing an element that is at least partly transmissive and at least partly reflective.

Description:
BACKGROUND OF THE INVENTION 
     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. 
     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. 
     From the disclosure of work of Sandel et al (David Sandel, Reinhold Noé, “Optical Network Analyzer applied for Fiber Bragg Grating Characterization”, ECOC 97, 22-25 Sep. 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. 
     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 to use 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 distinguish between these signals without really knowing all impacts of this superposition of signals. 
     SUMMARY OF THE INVENTION 
     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. 
     The object is solved by the independent claims. 
     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. 
     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. 
     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. 
     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. 
     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. 
     Other preferred embodiments are shown by the dependent claims. 
     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 
     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 shows a principle of an embodiment of the inventive method; 
     FIG. 2 shows a first embodiment of the element of the present invention; 
     FIG. 3 shows a second embodiment of the element of the present invention; 
     FIG. 4 shows a third embodiment of the element of the present invention, 
     FIG. 5 shows a first measurement setup according to the present invention; and 
     FIG. 6 shows a second measurement setup according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With regard to propagation of light, a single device may often perform functions of both, or either of, beam splitting and coupling. For example, light entering a first port of such a device can be split into two paths such that a portion of the light exits via a second and a portion of the light exits via a third port. Conversely, light entering the device via the second port and light entering the device via the third port can be coupled together and exit the device via the first port. The term “coupler” is used herein for a device that performs either of coupling or beam splitting, or both of coupling and beam splitting. 
     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, there is shown a reference arm  2  of a measurement setup  400  (see FIG. 5) for determination of optical properties of a DUT  6  (FIG. 5) 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, as disclosed in U.S. patent application Ser. No. 10/179,347, filed Jun. 25, 2002, which is incorporated herein by reference. 
     Reference arm  2  has two connectors  4   a  and  4   b . Between connectors  4   a  and  4   b , a patch-cord  7  is inserted. By releasing a connection at connectors  4   a  and  4   b  (indicated by an arrow  8 ) it is possible to disconnect patch-cord  7  from reference arm  2 . This, as shown in step B of FIG. 1, opens a gap  10  between connectors  4   a  and  4   b . This makes it possible to insert an element  12  into gap  10  (indicated by an arrow  14 ). For this purpose, element  12  is prepared with two short patch-cords  16   a  and  16   b  having connectors  18   a  and  18   b , which can be connected to connectors  4   a  and  4   b  of reference arm  2 , respectively. As shown in step C of FIG. 1, as a result, element  12  is inserted in reference arm  2  and has replaced patch-cord  7 . 
     FIG. 2 shows a first embodiment  100  of element  12 . In embodiment  100 , element  12  comprises a semi-transparent mirror, i.e., mirror  20 . Light  22  propagates along patch-cord  16   a  toward mirror  20 . Mirror  20  reflects 50% of light  22  back into patch-cord  16   a  as light  24 , and lets 50% of light  22  travel through mirror  20  as light  26 , which light travels along patch-cord  16   b  to connector  18   b . Therefore, element  12 , according to FIG. 2, provides transmission and reflection of incoming light, i.e., light  22 . However, different ratios of transmission and reflection can be used. 
     FIG. 3 shows a second embodiment  200  of element  12 . Element  12  of embodiment  200  comprises a first coupler, i.e., coupler  28 , which is preferably a 3 dB coupler, but other couplers, such as 10 dB couplers, can be used instead. Coupler  28  lies in an initial path provided by patch-cord  16   a  of light  22 . Coupler  28  couples out 50% of light  22  into a first path  30  as light  32 . The other 50% travels along the initial path as light  34 . Furthermore, element  12  comprises a second coupler, i.e., coupler  36 , which couples light  32  partly back into the Initial path in reverse direction as light  38 . Additionally, coupler  36  couples light  34  into first path  30 , as light  40 . Light  40  is partly coupled back into the initial path in reverse direction via coupler  28  as light  42 . The part of light  34  not coupled out of the initial path by coupler  36  travels along patch-cord  16   b  to connector  18   b  as light  44 . Therefore, element  12  in embodiment  200  provides light  44 , which is a portion of light  22 , in transmission at connector  18   b , and provides light  42 , which is also a portion of light  22 , in reflection at connector  18   a.    
     Furthermore, by adjusting couplers  28  and  36 , e.g., by using 10 dB couplers or other couplers, it is possible to adjust a ratio of reflected light, i.e., light  42 , to transmitted light, i.e., light  44 . 
     FIG. 4 shows a third embodiment  300  of element  12 . In embodiment  300  light  22  is partly coupled out by a coupler  46  into a first path  48  as light  52 . At the end of first path  48  there is provided a mirror  50 . Mirror  50  reflects light  52  in total as light  54 . Subsequently, coupler  46  couples light  54  into the initial path, in reverse direction,as light  56  and into patch-cord  16   b  in a direction to connector  18   b  as light  58 . Therefore, element  12  according to embodiment  300  provides a portion of light  22  in transmission at connector  18   b , i.e., light  58 , and a portion of light  22  in reflection at connector  18   a , i.e., light  56 . 
     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). Polarization controller  74  provides a polarization controlled coherent light beam  76  to an isolator  78 . Optically connected with isolator  78  and receiving a coherent light beam, i.e., beam  80 , from isolator  78  is a coupler  82 , e.g., a 3 dB coupler. Also optically connected with isolator  78  and receiving beam  80  is a wavelength reference unit  84  (see also FIG. 6) to detect a wavelength of beam  80 . 
     Reference arm  2  and a measurement arm  86  are connected to coupler  82 . A switch  88  is provided in measurement arm  86  to cut measurement arm  86  for calibration purposes. Additionally, measurement arm  86  contains a seat  90  to receive DUT  6 . Seat  90  has two connectors  92  and  94  to enable DUT  6  to be connected to measurement arm  86 . 
     Between coupler  82  and seat  90  there is a detector  96  for measuring signal strength of a portion of beam  80  that is split by coupler  82  into measurement arm  86 . Additionally, there is a detector  98  for measuring a signal strength of light being reflected by DUT  6 . 
     Furthermore, measurement arm  86  is connected to a coupler  102 , e.g., a 3 dB coupler. Between seat  90  and coupler  102  there is a detector  104  for measuring a signal strength of light transmitted through DUT  6 . 
     A polarization diversity receiver  106  is connected to coupler  102  to detect a superimposed signal, that is a superposition o a signal transmitted by DUT  6  and a reference signal coupled in by coupler  102  from reference arm  2 . The reference signal is coupled into reference arm  2  by coupler  82 . 
     A polarization diversity receiver  108  is connected to coupler  82 . Polarization diversity receiver  108  detects a superimposed signal, that is a superposition of a reflected signal from DUT  6  coupled in by coupler  82  from measurement arm  86  and a reflected reference signal coupled in from reference arm  2  coming from element  12 . 
     For further details may be found in European Patent Application 00125089.3, which issued as European Patent No. EP 1 113 250 A1, the disclosure of which is incorporated herein by reference. 
     FIG. 6 shows a second embodiment  500  of a measurement setup according to the present invention. FIG. 6 also shows further details of wavelength reference unit  84 . Wavelength reference unit  84  contains a six port coupler  110 , which splits a beam  112  coupled out from beam  80  into three beams  114 ,  116  and  118 . Beams  114  and  116  are directed onto Faraday mirrors  120  and  122 . Faraday mirror  120  can be shifted to change a length of the path of beam  114 . Furthermore, wavelength reference unit  84  contains a gas cell  124  connected with a power detector  126 . A gas in gas cell  124  has a known absorption spectrum. With the help of power detector  126  and the known absorption spectrum of the gas in gas cell  124 , it is possible to determine a wavelength of beam  80  very precisely. 
     Additionally, embodiment  500  shows polarization diversity receivers  106  and  108  in further detail. Both have polarization beam splitters  128  and  130  that are connected to power detectors  132 ,  134 ,  136  and  138 . 
     Contrary to embodiment  400 , in embodiment  500 , element  12  is not connected as shown in embodiment  200 . In embodiment  500 , path  30  is not coupled into a reference arm directly as shown in embodiment  200 . In embodiment  500 , path  30  is coupled with a coupler  140  to superimpose a reference signal guided by path  30  with a reflected signal of path  160  directly in front of polarization diversity receiver  108 . This advantageously avoids introduction of a reference signal on path  30  into the initial path of beam  80 .