Abstract:
A method and an apparatus for delaying parts of a coherent optical signal beam relative to each other, comprising: a first device for splitting the beam into a first part and a second part, a second device for delaying the second part relative to the first part, a third device for recombining the first and the second part, a fourth device for providing the recombined parts with different polarizations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical device, e.g. an element that can provide two different polarized optical signals delayed with respect to each other, e.g. a highly birefringent fiber, a so-called HiBi fiber. HiBi fibers are commonly used as polarization maintaining fibers. In addition, HiBi fibers do provide a delay of an optical signal traveling the fast axis of the HiBi fiber with respect to the optical signal traveling the slow axis of the HiBi fiber.  
           [0002]    However, the delay is about 1.7 ps/m, only. Therefore, to provide a delay of for example 170 ps it is necessary to use 100 m of the HiBi fiber which is costly and needs a lot of space.  
         SUMMARY OF THE INVENTION  
         [0003]    Therefore, it is an object of the invention to provide an improved optical device. The object is solved by the independent claims.  
           [0004]    An advantage of the present invention is the possibility of using the invention in a lot of different devices and for different purposes.  
           [0005]    First of all it can be used as a substitute for a HiBi fiber since it shows delays which are much longer than the above mentioned delays of HiBi fibers.  
           [0006]    Additionally, it can be used as a polarization delay step or unit in a method or an apparatus for determining optical properties, e.g. the polarization mode dispersion (PMD), of an optical device under test (DUT), e.g. as described in a patent application of the applicant called “Determination of properties of an optical device” (internal reference no. 200 04162-04) filed with the same application date as the present invention.  
           [0007]    The term “coherent” in this application means that the coherence length of the light beam is bigger than the difference of lengths of the different paths of the light beams to be superimposed in this invention.  
           [0008]    According to the inventive concept a polarization delay step generates two, preferably orthogonal, polarized signals which are delayed with respect to each other. By this in the polarization delay step there are provided two polarization states that are “coded” by different propagation delays. To delay the two signals with respect to each other the first light beam is split into a first undelayed light beam and a first delayed light beam in the polarization delay step. Preferably, the two parts of the first light beam are orthogonal polarized with respect to each other so that they do not interfere when being recombined.  
           [0009]    However, the two polarizations create interference patterns when being superimposed with another light beam. This effect is used in the aforementioned parallel patent application. However, when detecting the resulting superimposed light beam the two interference patterns can be separated in the electrical spectrum. It is therefore possible that polarization diversity receivers detect different spectral components that can be preferably separated by digital filters.  
           [0010]    In a respective polarization delay unit to perform the polarization delay step there are preferably used two polarization beam splitters, one to split the first light beam in the first delayed and in the first undelayed light beam, and the second to recombine the first delayed and the first undelayed light beam. The two polarization beam splitters each of them having at least three ports are connected with each other by two polarization maintaining fibers. In this embodiment, one of these two fibers is longer than the other to create the aforementioned delay of one part of the split first light beam. Alternatively, the recombining polarization beam splitter can be replaced by a polarization maintaining coupler (PMC). In this embodiment, one of the polarization maintaining fibers is connected directly to the PMC whereas the other polarization maintaining fiber is rotated by 90° before being connected to the PMC.  
           [0011]    It is clear for the skilled person that many other ways to realize the inventive concept, i.e. for splitting the first light beam, delaying one part with respect to the other part of the first light beam, and recombining them with different, preferably orthogonal, polarizations could be imagined.  
           [0012]    In some interferometric experiments, it is necessary to have a second or reference interferometer without an optical device in its measurement arm, e.g. a Mach-Zehnder interferometer parallel to the measuring interferometer. In the second interferometer, the same coherent laser beam of the laser source can be coupled in by a beam splitter before the two interferometers. With the help of the second interferometer any errors in the detected powers of the resulting beams of the first interferometer caused by a non-linearity in the scanning velocity when scanning the frequency of the laser can be eliminated.  
           [0013]    Instead of using a second interferometer it is possible according to the present invention to use the fourth port of the second polarization beam splitter as a reference signal, i.e. to recombine the two parts of the first light beam to couple out a signal to be used for a wavelength measurement. This can be done by connecting this port directly with a polarizer which causes interference between the two parts of the first light beam, so that an interference pattern is provided to evaluate the scanning velocity when scanning the frequency of the laser source. An advantage of the invention is that it does make the need for a reference interferometer superfluous, which reduces costs, necessary maintenance and the footprint of such an apparatus.  
           [0014]    Other preferred embodiments are shown by the dependent claims.  
           [0015]    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  
       [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. 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).  
         [0017]    [0017]FIG. 1 shows a schematic illustration of a first embodiment of the invention;  
         [0018]    [0018]FIG. 2 shows a schematic illustration of a second embodiment of the invention; and  
         [0019]    [0019]FIG. 3 shows a schematic illustration of a third embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Referring now in greater detail to the drawings, FIG. 1 shows a schematic illustration of a first preferred embodiment  100  according to the present invention. It shows a polarization delay unit (PDU)  102  in the path of a laser beam  18 . The PDU  102  is introduced in the path of the laser beam  18  between a beam splitter  14  and the DUT  2 . The PDU  102  contains a first polarization beam splitter (PBS)  104  and a second PBS  106 . The first PBS  104  splits the laser beam  18  into a first part  18   a  and a second part  18   b . The paths of both parts  18   a ,  18   b  are provided by polarization maintaining fibers (PMF) to preserve the polarization of each part  18   a ,  18   b , respectively. Subsequently, the first part  18   a  and the second part  18   b  are reunited by the second PBS  106 . It is assumed that both PBS  104 ,  106  are manufactured with two ports of PMF and two ports of single-mode fiber (SMF). If the PBSs  104 ,  106  are connected with standard-aligned connectors the light is guided completely to one port of the second PBS  106  and no light is coming out of the other port of the second PBS  106 . Therefore, the PMFs of each beam  18   a  and  18   b  are rotated by 45° to produce an output on both ports of the second PBS  106 .  
         [0021]    However, the first part  18   a  and the second part  18   b  travel a different optical distance between the first PBS  104  and the second PBS  106  since beam  18   b  travels a longer distance symbolized by loops  108 . This means that the second part  18   b  is delayed with respect to the first part  18   a . Moreover, since both parts  18   a ,  18   b  are orthogonal to each other they do not interfere after being reunited by the second PBS  106 .  
         [0022]    The second PBS  106  has two outgoing ports  112  and  114 . At both outgoing ports  112  and  114  the two parts  18   a ,  18   b  are present since they do not interfere with each other.  
         [0023]    Instead of a reference interferometer the second port  114  of the PBS  106  can be used as a reference interferometer by detecting the outcoming light  18   a ,  18   b  through a PMF-connected polarizer  136  which makes the parts  18   a  and  18   b  interfere with each other so that there is produced a superimposed light beam  138  which shows a interference pattern which can be detected by a detector  140  connected to the polarizer  136 . Therefore, the present invention provides a device that can produce delayed signals and which serves at the same time as a reference interferometer.  
         [0024]    In embodiment  100 , the input polarization of the system is critically influencing the measurement performance. Preferably, the input polarization should be chosen in a way that the light of the local oscillator path  6  is split equally onto the two parts  18   a ,  18   b  leaving the PDU  102 . Therefore, the light hitting the PBS  104  has to be appropriately polarized to achieve a splitting ratio of 50%, preferably. The valid polarization states are located on a great circle on the Poincaré-sphere. Also for the PBS  32 ,  126  of the polarization diversity receivers a splitting ratio of 50% is preferred. Their valid input polarizations are also located on a great circle of the Poincaré-sphere. Generally, the orientation of these three circles is different. Two great circles always intersect at two points. Thus, it is always possible to choose an input polarization providing 50% power splitting at two PBS. In most cases, there is no intersection of all three circles. Therefore, a splitting ratio of 50% at all PBS  104 ,  32  and  126  cannot be guaranteed. It turns out that even for the worst-case condition an acceptable compromise with splitting ratios unequal to 50% can be found. The worst-case condition corresponds to a 22.5° misaligned linear polarization state of the input polarization. In this worst-case a minimum splitting ratio of sin 2  (25.5°)=15% after each PBS  104 ,  32  and  126  occurs which still leads to a well acceptable contrast of the interference patterns. The optimum input polarization of the PDU  102  has to be found during an initialization procedure.  
         [0025]    [0025]FIG. 2 shows a second embodiment  300  of the present invention.  
         [0026]    As a difference to the embodiment  100  of FIG. 1 in the embodiment  300  of FIG. 2, the PBS  106  is replaced by a polarization maintaining coupler  302 . Therefore, both output ports  112  and  114  emit the same signal. The use of the output signals of the output ports  112 ,  114  is the same as in embodiment  100  of FIG. 1. In the PDU  102 , only the longer path  18   b  has to be rotated by 90° as symbolized by symbol  304  because of the use of the polarization maintaining coupler  302 .  
         [0027]    [0027]FIG. 3 shows a schematic illustration of a third embodiment  500  of the present invention. Embodiment  500  is similar to the embodiment  100  of FIG. 1. However, in embodiment  500  of FIG. 3 the PDU  102  shows a different set-up. Instead of the second PBS  106  there are provided two Faraday mirrors  502  and  504 . Using the Faraday mirrors  502 ,  504  avoids the need of long pieces of PMF inside the PDU  102  that can cause problems if the polarization is not properly aligned to the axis of the PMF. The incoming light beam  18  is split into two linear polarization states (SOP) by the PBS  104  and travels along the SMF before it is reflected by the Faraday mirrors  502 ,  504 , respectively. In contrast to regular mirrors the Faraday mirrors  502 ,  504  transform each linear incoming SOP into an orthogonal, reflected polarization state. Thus, the light is emitted through the fourth port  506  of the PBS  104  without the need of a circulator.  
         [0028]    Using the PBS  104  also to recombine the reflected light reflected by the Faraday mirrors  502 ,  504  guarantees that the two delayed components are orthogonally polarized and do not interfere. If the Faraday mirrors  502 ,  504  do not generate perfectly orthogonal polarization states a small fraction of the light is reflected back to the laser source  4  and does not disturb the signal path.  
         [0029]    The output port  506  of the PBS  104  can be connected to a PMF  508  to provide the parts  18   a ,  18   b.    
         [0030]    It is possible to introduce a power detector (not shown in FIG. 3) in each path  18   a ,  18   b  in the PDU  102  of the embodiment  500  of FIG. 3 to measure if the power is distributed in each part  18   a ,  18   b  equally.