Abstract:
The polarization state of a light wave is changed by a fiber polarization retardation device constructed by a method including employment of a pair of clamping fixtures, and a series of steps including clamping, cleaving, splicing, releasing, clamping, and cleaving, so as to achieve precise optical fiber lengths to achieve a desired polarization retarding device.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to current sensors. More particularly, the present invention relates to a method for polarization state conversion of an electromagnetic wave.  
         BACKGROUND OF THE INVENTION  
         [0002]    Magnetic fields interact with circularly-polarized light waves propagating through optical fiber under a principle known as the magneto-optic Faraday effect. Under this principle, a magnetic field will rotate the plane of polarization of circularly-polarized light waves traveling in opposite directions and thereby cause a phase shift to occur between the relative phases of the two light waves. This phase shift is known as a non-reciprocal phase shift. For example, in a fiber optic current sensing coil affected by a magnetic field, where a first light wave, having a circular polarization state, travels in one direction in the coil and a second light wave having a circular polarization state travels in the opposite direction, there will be a phase shift between the two waves called non-reciprocal phase shift. The non-reciprocal phase shift experienced by a light wave will vary depending on whether the light wave is propagating in the same direction as the magnetic field or against the magnetic field. Measurements of the non-reciprocal phase shift may then be made to determine current or magnetic fields affecting the sensing coil.  
           [0003]    Because the non-reciprocal phase shift occurs between light waves in a circular polarization state and because the light waves are initially in a linear polarization state a method for converting the polarization state of a light wave is needed. To convert a light wave, e.g., vector E, having an x-axis component, Ex, and a y-axis component, Ey, from a linear polarization state to circular polarization state, the wave is passed through a highly birefringent medium. A birefringent medium is a medium that has two different indices of refraction, e.g., nx and ny. Each index of refraction corresponds to a different polarization axis where the axes are orthogonal to each other. For example, nx may correspond to an x-axis and ny may correspond to a y-axis. Because of the different refraction indices, Ex will travel at a different speed than Ey. Assuming that Ex and Ey enter the birefringent medium in phase with respect to one another, the phase difference between the components, φ, at the output of the birefringent medium is as follows:  
           φ=2 ( nx−ny )* d rad= 360( nx−ny )* d  degrees  
           [0004]    where d is the length of the birefringent medium and is the wavelength of the light wave. Thus, the phase difference between an x-axis component and a y-axis component of a light wave traveling through a birefringent medium equals the difference between the indices of refraction multiplied by the length of the birefringent medium and divided by the wavelength of the light wave.  
           [0005]    As shown in the formula, the change of polarization state is periodic through the birefringent medium. When the phase difference between Ex and Ey changes from 0 rad (0°) to /4 rad (90°), the polarization state of E changes from linear to circular when Ex and Ey are equal in magnitude.  
           [0006]    In addition, the change of polarization state is directly proportional to the length of the birefringent medium. With all other variables being constant, the length of the birefringement medium dictates the phase difference. The relationship between one wavelength of a given frequency of light and the length of the birefringent medium is referred to as the birefringence beat length, where  
       =       nx        -        ny     _                           
 
           [0007]    Thus, the beat length equals the wavelength divided by the difference between the indices of refraction of the birefringent medium. In other words, the physical length corresponding to one beat length of a birefringent medium corresponds to 2 of phase shift of the light passing through that medium.  
           [0008]    One type of birefringement medium that is typically used is know as a quarter-wave plate. One of the effects of a quarter-wave plate is to change the polarization state of a light wave from a linear polarization state to a circular polarization state. The length of a quarter-wave plate is such that components of a light wave are 90° out of phase with respect to one another upon exiting the quarter-wave plate. In particular, when a light wave is in a linear state of polarization being oriented at 45° from its principal axes, i.e., having equal components on its principal axes, and is input into a quarter-wave plate, the output is the light wave in a circular state of polarization. In a quarter-wave plate, d can be determined as follows:  
             d =(2 m+ 1)/4  
           [0009]    where m is an integer, including zero. Therefore, the length d of a birefringent quarter-wave plate is one quarter or three quarters of beat length longer than an integral number of beat lengths.  
           [0010]    Previously, other methods have been used to convert between linear polarization states and circular polarization states. One such method uses a bulk optic quarter-wave retarder. In the case of a bulk optic device or crystal, a linearly-polarized light wave travels from a first optical fiber through a lens to collimate the light wave. The light wave then travels through a bulk optic crystal having principal axes oriented orthogonally with respect to each other and oriented at  45 ° with respect to the principal axes of the optical fiber. The wave then travels through a second lens and into a second optical fiber. This method is relatively costly, complex and its components occupy a relatively large amount of space. In addition, bulk optic devices are not reliable over time and temperature.  
           [0011]    An alternate method includes the use of a single mode non-polarization maintaining fiber loop. The size and orientation of the loop converts a linear polarization state of a light wave into a circular polarization state. However, the single mode fiber loop may be hard to manipulate in achieving a desired orientation and its performance tends to degrade with temperature changes.  
           [0012]    Thus, there is a need for a method which converts the polarization state of a light wave which eliminates or substantially reduces the disadvantages associated with prior methods. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, wherein like reference numbers refer to like items throughout the Figures, and:  
         [0014]    [0014]FIG. 1 shows a schematic of a fiber optic current sensor having an all fiber polarization retardation device;  
         [0015]    [0015]FIG. 2 shows a partial cross section of a polarization-maintaining retardation fiber with its principal axes oriented at 45° (relative to the principal axes of the polarization-maintaining input fiber);  
         [0016]    [0016]FIG. 3 shows an exemplary embodiment of a polarization-maintaining fiber having a polarization-maintaining retardation fiber spliced therein;  
         [0017]    [0017]FIG. 4 shows a flow chart of an exemplary method for converting a wave from a linear polarization state to a circular polarization state via a polarization-maintaining fiber having a long beat length;  
         [0018]    [0018]FIG. 5 shows a flow chart of an alternate exemplary method for converting a wave from a linear polarization state to a circular polarization state by heating a polarization-maintaining retardation fiber; and  
         [0019]    [0019]FIG. 6 shows a flow chart of an alternate exemplary method for converting light from a linear polarization state to a circular polarization state by cleaving a long beat length polarization-maintaining retardation fiber to a desired length.  
     
    
     DETAILED DESCRIPTION  
       [0020]    The present invention is useful in all optics applications in which a change of polarization state of a light wave is desirable. The present invention utilizes an all-fiber polarization retarder which uses a high birefringent polarization-maintaining fiber to retard the polarization of a component of a light wave, or in other words, to change the polarization state of a light wave. An all-fiber polarization retarder is advantageous over prior devices because it is smaller, simpler, less expensive and more reliable. For exemplary purposes, the present invention is being described in a fiber optic current sensor application. In a fiber optic current sensor, the present invention provides an inexpensive and simple method to change the polarization state of a light wave, thereby allowing current and magnetic fields experienced by a sensing coil to be measured as a result of the magneto-optic Faraday effect.  
         [0021]    [0021]FIG. 1 shows a general fiber optic current sensor schematic. Fiber optic current sensor  100  includes a light source  102 , a coupler  106 , an integrated optics chip  108  having a polarizer  110  and a beam splitter  112 , a sensing coil  126 , a detector  128  and signal processing circuitry  130 . In operation, an electromagnetic wave, or light wave, travels from light source  102  along an optical fiber  104  into a coupler  106 . Coupler  106  serves as a light wave splitter or beam splitter. The light wave entering coupler  106  from light source  102  is split into two light waves traveling in the same direction but exiting through separate ports. The light wave then travels into an integrated optics chip  108  where the light wave is linearly polarized by a polarizer  110  and split by a light wave splitter or beam splitter  112 . Beam splitter  112  splits the light wave into two light waves  114   a  and  114   b . Light waves  114   a  and  114   b  exit integrated optics chip  108  through fibers  116   a  and  116   b  and propagate through retardation fibers  120   a  and  120   b  which are connected to fibers  116   a  and  116   b  at splices  118   a  and  118   b  respectively. Light waves  114   a  and  114   b  change polarization states from linear to circular while propagating through retardation fibers  120   a  and  120   b . Light waves  114   a  and  114   b  then propagate into fiber ends  124   a  and  124   b  of a tightly wound cylindrical structure of optical fiber known as a sensing coil  126  which are connected to retardation fibers  120   a  and  120   b  via splices  122   a  and  122   b  respectively. Light waves  114   a  and  114   b , which are circularly polarized, counterpropagate through coil  126  and exit coil  126  through opposite ends  124   b  and  124   a  respectively from which they entered. If coil  126  is effected by a magnetic field, a phase shift will occur between the phases of light waves  114   a  and  114   b  as they travel through coil  126 .  
         [0022]    Light waves  114   a  and  114   b  then re-travel part of the path from which they came. Light waves II  4   a  and  114   b  interfere with each other at beam splitter  112  resulting in a light interference wave in accordance with well-known principles of optics. Some of the returning light interference wave is diverted by coupler  106  into detector  128 . Detector  128  converts the light interference wave into the electrical domain where it can be analyzed to determine current, magnetic field strength, etc.  
         [0023]    A phase shift between light waves  114   a  and  114   b  which is induced by a magnetic field will occur if light waves  114   a  and  114   b  are in circular polarization states. Thus, to enable the measurement of a phase shift between light waves  114   a  and  114   b , they may need to be converted from a linear polarization state to a circular polarization state prior to entering coil  126 . In a typical fiber optic current sensor, light waves  1114   a  and  114   b  are linearly polarized by polarizer  110  as discussed above. The propagation of light waves  114   a  and  114   b  through retardation fibers  120   a  and  120   b  having a high birefringence converts the polarization state of light waves  114   a  and  114   b . A high birefringent fiber has two dominant indices of refraction, suitably largely differing in value, which affect the propagation of a light wave through the fiber. The birefringent medium causes one component of the light wave to propagate more slowly, corresponding to the higher refractive index, than the other component of the light wave, corresponding to the lower refractive index.  
         [0024]    [0024]FIG. 1 shows high birefringent polarization-maintaining retardation fibers  120   a  and  120   b  connected to linearly polarization-maintaining input fibers  116   a  and  116   b  via splices  118   a  and  18   b  respectively. An exemplary splice in accordance with the present invention is shown in FIG. 2. A first or input fiber  202  is a linear polarization-maintaining fiber and is spliced with a high birefringent polarization-maintaining second or retardation fiber  204  such that the polarization axes  208  of retardation fiber  204  are oriented at 45° with respect to the polarization axes  206  of input fiber  202 .  
         [0025]    An exemplary embodiment and method in accordance with the present invention are shown in FIGS. 3 &amp; 4. A first or input fiber  302  and a second or retardation fiber  304  are linear polarization-maintaining fibers. Input fiber  302  has a short beat length which is typically less than three millimeters per wavelength of retardation. Retardation fiber  304  has a longer beat length, suitably at least four millimeters per wavelength of retardation.  
         [0026]    Beat length is the length of fiber which corresponds to one wavelength of retardation between two light waves, each traveling along a different polarization axis of the fiber. In a typical polarization-maintaining fiber, one millimeter corresponds to one wavelength of retardation. In general, the applications for the fiber are such that the goal is to minimize the length of the fiber to achieve a desired retardation. This corresponds to the difference, n x -n y , being as large as possible. However, in accordance with one aspect of the present invention, the retardation fiber suitably has an increased beat length so that a retardation fiber is more manageable to manipulate. A retardation fiber that has one wavelength of retardation per four or five millimeters of fiber is much easier to manipulate than a retardation fiber that has one wavelength of retardation over one millimeter of fiber. For example, assume it is desirable to retard a component of a light wave by 90°, i.e., one-quarter of a wavelength. If a retardation fiber having a beat length of one millimeter was used, it would be necessary to cut the fiber to a length equal to one quarter of one millimeter. On the other hand, if a four millimeter beat length retardation fiber was used, it would be necessary to cut the fiber to a length equal to one millimeter, which is a much more manageable operation. A retardation fiber is suitably selected to be a length which is short enough to maintain its polarization-maintaining characteristics and long enough to make it practical to handle and cleave.  
         [0027]    As discussed above, retardation fiber  304  is designed to convert polarization states of a light wave. As shown in FIGS. 3 &amp; 4, in an exemplary method  400  of the present invention, retardation fiber  304  is connected at splice  306  using fusion splicing, or some other method now known or later discovered, to input fiber  302  (step  402 ) such that their polarization axes are oriented at 45° with respect to each other (as shown in FIG. 2). Retardation fiber  304  is then cleaved at end  314  to a length from splice  306  that is slightly longer than that which will give the desired retardation (step  404 ). An exemplary retardation fiber has a beat length of four millimeters, i.e., four millimeters of retardation fiber  304  corresponds to one wave length of retardation. To achieve 90° or quarter-wave retardation, the optimal length of retardation fiber  304  is one quarter of four millimeters or one millimeter. Thus, in this exemplary method, retardation fiber  304  is cleaved to slightly more than one millimeter, e.g., one-and-one-half millimeters. Once cleaved, the remaining retardation fiber  304  is secured in a glass capillary  308  (step  406 ). A substance  312 , such as wax, may be inserted between retardation fiber  304  and glass capillary  308  if it is desirable to remove glass capillary  308  later. More specifically, wax may be placed on input fiber  302  at an edge  310  of glass capillary  308  such that the wax wicks along input fiber  302  and retardation fiber  304  through capillary  308 . Alternatively, substance  312  may be a more permanent adhesive used to affix capillary  308  to input fiber  302  and retardation fiber  304  if it is desirable to have fiber  304  remain in capillary  308 . Polarized light is then transmitted into input fiber  302  (step  408 ). The output of retardation fiber  304  is input into a polarization detector and analyzer (not shown) to determine the polarization state of light emitted from retardation fiber  304  (step  410 ). If the polarization state of the emitted light is sufficiently close to circular, the method is complete (step  414 ). If the polarization state of the emitted light is not sufficiently close to circular, cleaved end  314  of retardation fiber  304  is lapped with an abrasive substance (step  412 ) commonly known in the art. The abrasive substance is used to slightly decrease the length of retardation fiber. The slight decrease may amount to a fraction of a beat length, for example.  
         [0028]    Polarized light is then again transmitted into input fiber  302  (step  408 ) and step  410  is repeated. If the polarization state of the emitted light is sufficiently close to circular, the method is complete (step  414 ). If the polarization state of the emitted light is not sufficiently close to circular, the retardation fiber  304  is again lapped with the abrasive substance (step  412 ). These steps are repeated until the desired result, i.e., a circular polarization state, is obtained. The cleaving and lapping steps need to be executed with extreme accuracy. If retardation fiber  304  is cleaved or lapped too short, a new retardation fiber may be necessary because the desired polarization state may no longer be obtained. Thus, precision is important in cleaving retardation fiber  304 .  
         [0029]    Once the desired polarization state is obtained, an output fiber  316  may then be joined to end  314  of retardation fiber  304  via a splice or some other suitable means. Typically, output fiber  316  is an end of a sensing coil. In addition, output fiber  316  suitably preserves circular polarization states.  
         [0030]    In an alternate exemplary method  500  in accordance with the present invention, as shown in FIGS. 2 &amp; 5, input fiber  202  is spliced to retardation fiber  204  such that their polarization axes are oriented at 45° with respect to each other (step  502 ). Input fiber  202  and retardation fiber  204  are suitably linear polarization-maintaining fibers. Retardation fiber  204  is then cleaved at a length from the splice that is slightly more than that which will give the desired retardation as discussed above (step  504 ). A third or output fiber  210  may then be spliced to an end of retardation fiber  204  (step  506 ). The above connection of input fiber  202 , retardation fiber  204  and output fiber  210  may be done in any sequence. Polarized light is then transmitted into input fiber  202  (step  508 ). The output of output fiber  210  is transmitted into a polarization analyzer and detector to determine the polarization state of light emitted from output fiber  210  (step  510 ). If the polarization state of emitted light is sufficiently close to circular, the method is complete (step  514 ). If the polarization state of emitted light is not sufficiently close to circular, either all of or a part of retardation fiber  204  is then heated (step  512 ). Retardation fiber  204  may be heated with a low power fusion splicer arc, a flame or any other suitable means. Heating retardation fiber  204  will reduce its internal stresses, thereby increasing the beat length of retardation fiber  204  and reducing the polarization retardation. Polarized light is then again transmitted into input fiber  202  (step  508 ) and the output of output fiber  210  is transmitted into a polarization analyzer and detector to determine the polarization state of light emitted from output fiber  210  (step  510 ). If the polarization state of emitted light is sufficiently close to circular, the method is complete (step  514 ). If the polarization state of emitted light is not sufficiently close to circular, retardation fiber  204  is again heated to change the polarization retardation (step  512 ), and the sequence is repeated.  
         [0031]    In this exemplary method, the beat length of retardation fiber  204  is less critical than in other methods because the heating of retardation fiber  204  provides for a greater tolerance in the cleaved length of retardation fiber  204 .  
         [0032]    In an alternate exemplary method  600  in accordance with the present invention shown in FIGS. 2 &amp; 6, input fiber  202  and retardation fiber  204  are suitably linear polarization-maintaining fibers. Input fiber  202  has a short beat length which is typically less than three millimeters per wavelength of retardation. Retardation fiber  204  has a longer beat length which is preferably at least four millimeters per one wavelength of retardation. Retardation fiber  204  is spliced to input fiber  202  (step  602 ) using fusion splicing techniques or some other suitable method now known or later discovered such that their polarization axes are oriented at 45° with respect to each other. Retardation fiber  204  is then cleaved at a length from the splice which will yield a desired retardation (step  604 ). Such precise cleaving may be performed using the following steps:  
         [0033]    a) clamp the input fiber into a holding fixture,  
         [0034]    b) cleave the input fiber a controlled distance, B, from the holding fixture,  
         [0035]    c) splice the input and retardation fibers together, and  
         [0036]    d) cleave the retardation fiber at a second distance, B+B, from the holding fixture.  
         [0037]    An alternate method for precise cleaving may be performed by the following steps: a) clamp the input fiber into a holding fixture,  
         [0038]    b) cleave the input fiber a controlled distance, B, from its holding fixture,  
         [0039]    c) splice the input and retardation fibers together,  
         [0040]    d) clamp the retardation fiber in a holding fixture,  
         [0041]    e) release the clamp on the input fiber,  
         [0042]    f) move the input fiber holding fixture the desired distance, d, closer to the retardation fiber holding fixture,  
         [0043]    g) again clamp the input fiber into its holding fixture, and  
         [0044]    h) cleave the retardation fiber at the controlled distance, B, from the input fiber holding fixture.  
         [0045]    Polarized light is then transmitted into input fiber  202  (step  606 ). The output of retardation fiber  204  is transmitted into a polarization analyzer and detector to determine the polarization state of the light emitted from retardation fiber  204  (step  608 ). In this exemplary method, because the initial cleave of retardation fiber  204  occurs at a length so near the length which will yield the desired retardation, no additional modifications to the length of retardation fiber  204  are made.  
         [0046]    Although the above exemplary methods are all oriented towards converting the polarization of light waves from linear to circular, the present invention is not so limited. The methods may be used to convert any polarization state to any other polarization state by the orientation of input fiber to retardation fiber, the length and beat length of retardation fiber and the techniques for adjusting the length of the retardation fiber. For example, if a desired retardation is 180° or one-half of a wavelength, the above techniques can be used to achieve the desired retardation.  
         [0047]    In addition, as stated above, the present invention is useful in all optics applications in which a change of polarization state of a light wave is desirable.  
         [0048]    Irrespective of which method disclosed herein is used to convert the polarization state of a light wave, the splices may be rejacketed or packaged in a rigid housing as is known in the industry.  
         [0049]    It will be understood that the foregoing description is of exemplary embodiments and methods of this invention and that this invention is not so limited. Various modifications may be made in the design, arrangement, and implementation of these embodiments and methods without departing from the spirit and scope of the present invention, as set forth in the claims below.