Abstract:
An optical fiber that transforms light of undesired polarization into light of desired polarization. In principle, the signal power of the non-preferred polarization is transformed into signal power of the preferred polarization, thereby actually transforming the polarization. The fiber can have a circular end for input of light having randomly phased polarization, and an elongated end for output of light having a single principal orientation component. A generally smooth transition from the circular end to the elongated end causes substantially all the light entering the fiber to exit from the fiber without losing light due to reflection or radiation. Alternatively, the fiber can have a lobed end, for input of light having first and second mutually orthogonal polarization components, and an elongated end, for output of light having a single principal orientation component. Alternatively, the fiber can be a combination of the two forms described above. That is, the fiber can have a circular section that transforms first into a lobed section with increasing distance from the input port, with the lobed section then transforms into an elongated (e.g., elliptical or ovoid) section with increasing distance from the input port.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to light signal processing systems, and more particularly to arrangements for converting one polarization of light into another. 
     2. Description of the Related Art 
     Light sources tend to emit randomly oriented, unpolarized light. Many types of light processing devices are polarization-sensitive, requiring placement of a polarization filter between the light source and the device for operation in the intended mode. This inevitably results in loss of half of the power from the light source, namely that portion of the light power that is in the non-preferred polarization. The lost light power goes into heating losses and often reduces system signal-to-noise and/or efficiency. 
     One way to recover the energy in the non-preferred polarization is to pass the preferred polarization through a polarizing filter and reflect the non-preferred polarization. The reflected polarization is then rotated to the desired polarization using any type of rotator, which may be a crystal rotator, and summed with the preferred polarization component which passed through the polarizing filter. This may be difficult to achieve, since any phase shift will tend to result in cancellation of the signals being combined. 
     Thus, what is needed is an improved light polarization transformer. 
     SUMMARY OF THE INVENTION 
     According to the invention, the randomly phased light source is applied to one end of a polarization-converting optical fiber, and the preferred polarization is extracted from the other end of the fiber. The signal power at the output end of the polarization converter is ideally exactly equal to that at the input end, but of course the actual power will depend on the through loss of the device. In principle, the signal power of the non-preferred polarization is transformed into signal power of the preferred polarization, thereby actually transforming the polarization. 
     In a preferred embodiment, the fiber of the present invention has a circular end, for input of light having randomly phased polarization, and an elongated end, for output of light having a single principal orientation component. A generally smooth transition from the circular end to the elongated end causes substantially all the light entering the fiber to exit from the fiber without losing light due to reflection or radiation. Thus, light propagating through the fiber is transformed from randomly oriented, unpolarized light into light having a single principal polarization component. 
     Alternatively, the fiber can have a lobed end, for input of light having first and second mutually orthogonal polarization components, and an elongated end, for output of light having a single principal orientation component. As light propagates through the fiber, the light is transformed from light having mutually orthogonal components into light having a single principal polarization component. 
     Alternatively, the fiber can be a combination of the two forms described above. That is, the fiber can have a circular section that transforms first into a lobed section with increasing distance from the input port, with the lobed section then transforming into an elongated (e.g., elliptical or ovoid) section with increasing distance from the input port. 
     Such fibers provide a low-cost and effective device for transforming undesired light polarization into one or more desired polarizations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described with reference to the accompanying drawings, in which like reference characters reference like elements, and wherein: 
     FIGS. 1 a ,  1   b  and  1   c  are illustrations of the principle behind the optical fiber of the present invention; 
     FIG. 2 shows an embodiment of the optical fiber of the present invention; 
     FIGS. 2 a ,  2   b ,  2   c  and  2   d  show cross-sections of the fiber of FIG. 2 at various lengths along the longitudinal axis; 
     FIG. 2 e  shows an alternate embodiment of the input port of the fiber of FIG. 2; 
     FIG. 3 shows another embodiment of the optical fiber of the present invention; 
     FIGS. 3 a ,  3   b ,  3   c  and  3   d  show cross-sections of the fiber of FIG. 3 at various lengths along the longitudinal axis; 
     FIG. 4 shows a microscopic view of a portion of the fiber of FIG. 3; 
     FIG. 5 shows a microscopic view of a portion of the fiber of FIG. 3; 
     FIG. 6 illustrates a method of making the optical fiber of the present invention; and 
     FIG. 7 shows another embodiment of the optical fiber of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention can be understood by noting the properties of the optical fibers of FIG. 1 a . In FIG. 1 a , an optical fiber  10  is elongated along an axis of elongation  8 , and has a constant cross-section at all points along its length. The cross-section defines a generally elongated oval or ellipse. The principal or main diameter of the elliptical or oval cross-section of fiber  10  is illustrated as being vertically oriented along its entire length, as illustrated in FIG. 1 b , and the fiber  10  thus preferentially propagates light applied to its proximal end  10   p , which is vertically oriented, as suggested by arrow  6 . Vertically polarized light introduced into the proximal end  10   p  of fiber  10  propagates in known fashion to the remote or distal end  10   d  of the fiber, and leaves fiber  10  at distal end  10   d . The light leaving distal end  10   d  is illustrated as forming an illuminated dot on a hypothetical “screen”  15 . 
     In FIG. 1 a , a further optical fiber  20  elongated along an axis of elongation  18  defines a proximal end  20   p  and a distal end  20   d . Optical fiber  20  also has a generally oval or elliptical cross-section similar to that of fiber  10 . Unlike fiber  10 , however, fiber  20  is twisted along its length so that the axis of elongation of the cross-section is vertical at the proximal end  20   p  and horizontal at the distal end  20   d . Simplistic analysis might assume that vertically oriented light illustrated as  22  introduced into proximal end  20   p  of fiber  20  would not propagate through the fiber. Such an analysis might be based upon the distal end view of FIG. 1 c , in which it can be observed that the cross-sectional dimensions of the optical fiber are D at proximal end  20   p , but only d at distal end  20   d . It might be thought that this reduction in principal dimension from D to d would cause the light introduced at proximal end  20   p  to be reflected, or possibly radiated along the length of optical fiber  20 , so that little or no light would arrive at distal end  20   d . However, those skilled in the art know that the light “follows” the optical fiber notwithstanding that the fiber may twist and turn. But it is true that a sharp turn or discontinuity will cause light to reflect or radiate at the location of the discontinuity. 
     In considering why the light “follows” optical fiber  20 , one may be led to consider that a known effect in optics is that of the antireflection coating. An antireflection coating includes one or more layers of transparent material over the surface of an optical element, such as a camera lens, which aids in reducing reflections at the surface that arise due to the differences between the indices of refraction of the materials on either side of the surface. A simple version of such a coating may take the form of a layer having a thickness of one-quarter wavelength at the frequency of the light traversing the surface. The index of refraction of the coating is selected so that the magnitude of the reflection between the coating and free space is of generally the same magnitude as the magnitude of the reflection between the lens and the coating. The thickness of the coating, namely one-quarter wavelength, is such that, for light propagating in a particular direction, the reflections are out-of-phase and therefore tend to cancel. 
     In the art of transmission lines for electromagnetic signals, a similar reflection occurs at locations where the impedance of the transmission line changes. In this art, there is a known type of “impedance transformer” which performs the same type of cancellation of reflections at the frequency of operation. Such an impedance transformer includes one or more additional step changes in the transmission line impedance. Such steps are spaced apart by a distance, and are of such a magnitude, that the various spaced-apart reflections tend to cancel, at least over a band of frequencies. Among the types of impedance transformers used for transmission lines is the “tapered” transmission-line transformer. These may take several forms, one of which is discrete capacitors and inductors formed into a transmission line, with the values selected so that the characteristic impedance of the transmission line changes along its length to match the impedances of the source and load. Another form that the tapered transmission line transformer may take is that of a coaxial cable (coax), in which the center conductor extends from one end of the transformer to the other, but in which the outer conductor is cut away progressively toward the higher-impedance end so that at the higher-impedance end the transmission line takes the form of a two-wire line rather than that of a coax. The tapered forms of transmission-line transformers tend to have much greater bandwidth than those based on a discrete number of step transitions. That is to say, the reflections from such a tapered transmission line transformer tend to be smaller or reduced, over a greater bandwidth, than the reflections of a discrete transformer. 
     Analogizing the propagation of polarized light in an oval or elliptical optical fiber to the propagation of electromagnetic energy in a transmission line, one might conclude that the reason that the light follows the fiber is that, so long as the curvature or twist of the fiber is very small per unit wavelength, reflections do in fact occur that are attributable to the incremental change in dimension of the fiber in the direction of the polarization vector of the light propagating therein, but that these reflections tend to cancel. The same type of action may be attributable to radiation of light along the length of the fiber; it does occur, but the incremental radiation at any location is cancelled by out-of-phase radiation from other locations on the fiber. The case of the reflections is easier to understand. Imagine a location at the wall of an optical fiber, where the wall is curved, but in which the curvature is such as to occur over thousands of wavelengths of the light. A reflection having a magnitude and a phase will occur at that selected location. However, for any such location, there is an essentially identical second location spaced one-quarter wave away, which has a reflection of the same magnitude (since the curvature is essentially constant), but which is of a phase that tends to cancel the reflection at the first location. Since each and every point along the curve is associated with another such point that produces a canceling reflection, essentially all reflections are canceled. If all reflections cancel, no energy can be returned to the source, so the light must either propagate through the fiber or be lost by radiation. Analysis of radiation is performed in a similar manner, although it is conceptually a bit more difficult to understand, because velocities of propagation of light inside the fiber and outside the fiber differ. Nevertheless, for each radiating location on the outer surface of the curved fiber, there is another virtually identical location spaced one-quarter wave away along the fiber. The “far-field” or radiation pattern of such a “line array” of randomly phased “point” sources is equivalent to the summation of all possible phases of unit magnitude, and tends toward zero. Thus, the optical fiber neither radiates nor reflects, so long as the curve is gentle. Since there is neither reflection nor radiation, the light remains within the fiber or “follows” the fiber. 
     With the preceding analysis, we may now consider the structure of FIG.  2 . In FIG. 2, the two fibers  10  and  20  of FIG. 1 a  are superimposed or combined into a single optical fiber  110 . Fiber  110  has a horizontal lobe  30  and a rotational lobe  32 . Horizontal lobe  30  extends from proximal end  110   p  to distal end  110   d  with a constant cross-section. Rotational lobe  32 , like fiber  20  of FIG. 1, extends from a vertical orientation at proximal end  110   p  to a horizontal orientation at distal end  110   d . FIGS. 2 a ,  2   b ,  2   c  and  2   d  show cross-sections of fiber  110  taken at various lengths along axis  18 . Light  16  enters fiber  110  at input port  12 . It is seen that light  16  at input port  12  contains mutually orthogonal polarization components. Rotational lobe  32  is gradually rotated or twisted toward horizontal lobe  30  with increasing distance from proximal end  110   p . By rotating lobe  32  over the span of several wavelengths, as previously discussed, light  16  follows the rotation of lobe  32  and exits at output port  14  having a single principal polarization component, which is the horizontal component of light  16  at input port  12 , without reflection or radiation. That is, the electrical vector of light  16  exiting fiber  110  through output port  14  is parallel with the axis of elongation of outlet port  14 . Output port  14  can take any desired shape, with an elliptical or ovoid shape being preferred. As seen in FIG. 2 e , lobes  30 ,  32  can also be arranged asymmetrically. 
     FIG. 3 shows another embodiment of an optical fiber  200 . Here, randomly phased light  16  enters fiber  200  at input port  12 , which has a circular cross-section. The circular cross-section can be thought of as the lobed cross-section of input port  12  of fiber  110 , but with the spaces between lobes  30 ,  32  “filled in.” As light  16  propagates through fiber  200  in the direction indicated by arrow  202 , the light  16  is transformed into light having a single principal polarization component. This is achieved similarly as with respect to fiber  110  discussed above. The cross section of fiber  200  is gradually transformed from circular at input port  12  to an elongated form at exit port  14 . This gradual transformation is illustrated in FIGS. 3 a ,  3   b ,  3   c  and  3   d , which show cross-sections of fiber  200  at various lengths along the longitudinal axis. Light  16  follows the cross-sectional transformation and exits fiber  200  at output port  14  having a single principal polarization component. Although the exit orientation is illustrated as being horizontal in FIG. 3, any desired orientation may be achieved. 
     FIG. 4 represents an elevation view of a microscopic portion of fiber  200 , illustrating the taper in the transverse dimension as a plurality of discrete steps designated R 1 , R 2 , . . . R N , separated by infinitesimal incremental distances. As the vertical component of light  16  propagates from input port  12  to output port  14 , light  16  encounters a miniscule reduction in transverse dimension at each incremental distance. This reduction of the transverse dimension is equivalent to a small change in the impedance of a transmission line, and in turn results in a minuscule reflection from the step. Consequently, the gradually tapered dimension may be viewed as generating an infinitely large number of reflections, each having about the same dimension as the ones in its vicinity, and each somewhat delayed or phase shifted relative to the others. When the number of these steps is large, these reflections, with their random phases and equal amplitudes, tend to add together and cancel. The continuous taper of fiber  200  (see FIG. 3) corresponds with an infinite number of infinitely small steps. Thus, energy propagated into input port  12  in the vertical orientation is not lost by way of reflection. 
     FIG. 5 illustrates a small portion of the tapered-elliptical optical fiber  200 , in which the vertically polarized light component  16   v  propagates in a tapered dimension approximated by a series of steps. As illustrated, at the first step, a small component of the light signal represented by an arrow  510  is radiated in various directions. At any point P, the field strength is attributable to the radiation from all the steps along the length of the polarization transforming fiber  200 . The radiation patterns of each of the steps will tend to be the same as that of the steps in its vicinity, and thus each point in space will tend to receive the same amount of radiated energy from steps which are near each other. Since the energy will tend to be distributed in phase, cancellation of the radiated fields will occur. 
     FIG. 6 illustrates a method of making a tapered elliptical optical fiber  600 . For the purposes of explanation, a circular-to-elliptical transforming optical fiber is shown, but those of ordinary skill in the art will appreciate the same process, with minor modification, can be employed to create any desired form. Fiber  600  is maintained at a temperature at which it is soft or malleable. Fiber  600  sits on a flat plate  610 , and a second flat plate  612  is pressed down at an angle, in the direction of arrows  614 . Fiber  600  is then allowed to cool, either before or after removing plate  612 . 
     FIG. 7 shows another embodiment of optical fiber  700 . This embodiment has a circular end  710  and an orthogonal end  720 . Randomly phased light propagating through fiber  700  in the direction of arrow  730  is transformed as described above into light having two orthogonal components. Fiber  700  can be used in conjunction with fiber  110  of FIG. 2 by placing the fibers with the orthogonal ends facing each other. By adding or removing fiber  700 , one can alter the polarization of the emitted light as needed. 
     While the preferred embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.