OPTICAL FIBER POLARIZER

Abstract: A fiber-optic polarizer comprises an optical fiber (25) having a core (26) and cladding (27) with different refractive indices and forming a single-mode guiding region, the core (26) having a non-circular cross-section defining two trans­ verse orthogonal axes which, in combination with the different refractive indices, permit the de-coupling of waves polarized along the axes. The guiding region is located close to the surface (29) of the fiber (25), and the outer surface (29) of the fiber (25) has a non-circular cross-section so that the location of the guiding region and the orientation of the axes can be ascertained from the geometry of the outer surface 29). A polymeric film (40) is coupled to the fiber surface (29) that is closest to the core, the film (40) having different indices of refraction along axes aligned with the transverse orthogonal axes of the core (26), one of the indices of refraction of the film (40) being less than the effective index of refraction of a desired wave in the guiding region of the fiber (25), and the other index of refraction of the film (40) being greater than the effective index of refraction of an unwanted wave in the guiding region of the fiber (25).

Description: The present invention relates generally to the field of fiber optics and, more particularly, to fiber-optic polarizers. Fiber optics is generally concerned with the transmission of light along a transparent fiber structure which has a higher refractive index than its surroundings. Currently it is possible to manufacture long, continuous strands of optical fiber which may propagate signals without substantial attenuation over long distances. It is also possible to manufacture the fiber structure as an optical waveguide wherein only preselected modes of light propagate in the fiber. By limiting wave propagation through the fiber to a single-mode, the bandwidth of the optical fiber may be exceedingly high to provide a high information-transfer capacity. Moreover, optical-fiber transmission equipment is compact, lightweight, and potentially inexpensive. Transmission over optical fibers does not generate interference and is unaffected by external interference. While the development of optical fibers for telecommunications systems is becoming rather highly advanced, the use of fiber optics for sensing and control systems is still in its early development. In sensing and control systems a fiber-optic transducer is used that exploits either multimode or single-mode light propagation in an optical fiber. While multimode sensors use amplitude variations in the optical signals to sense and transmit the desired information, single-mode sensors use phase variations rather than amplitude variations. The single-mode sensors usually involve mechanisms for altering such properties of the fiber as path length or index of refraction to effect the desired phase variations in the optical signal. In the case of the fiber-optic gyroscope, the single-mode sensor measures acceleration which inherently alters the propagation of light even though the fiber is not affected. Thus, in contrast to multimode sensors, in single-mode sensors the uniformity and mechanism of light propagation are especially critical. Single mode sensors are also sensitive to the state of polarization of light in the fiber since, if the fiber is not significantly polarization-holding or preserving, waves of different polarization may have different phase velocities. Thus, for single-mode transducers, it is desirable to use elliptical-core or other kinds of polarization-holding fiber. See, e.g., McMahon et al., "Fiber-Optic Transducers," IEEE Spectrum, December 1981, pages 24-27. Even in an optical fiber that has good polarization-holding properties, however, unwanted guided waves can be present. For example, in an optical fiber having a core with an elliptical cross-section, in which the desired wave is usually polarized along the major axis of the ellipse, an unwanted wave can be present and will be polarized along the minor axis of the ellipse. Such an unwanted wave can originate, for instance, from a laser source which launches imperfectly polarized light into the fiber or from coupling between polarizations in the fiber itself. Although such unwanted waves can be tolerated during transmission, they become extremely undesirable at the output end of the fiber where the phase of the desired wave is sensed and used for various purposes. A self-aligning optical fiber with accessible guiding region has already been proposed (Electronic Letters, Vol 19, No 16, page 601) comprising a core and cladding having different refractive indices and forming a guiding region located close to the surface of the fiber along a selected length of the fiber. The core of the fiber has an elliptical cross-section, and the outer surface of the fiber has a non-circular (preferably D-shaped) cross-section with a predetermined geometric relationship to the elliptical core so that the orientation of the major and minor axes of the elliptical core can be ascertained from the geometry of the outer surface. The elliptical cross-section of the core in combination with the different refractive indices of the core and cladding permit the decoupling of waves polarized along such axes. While an optical fiber of the type proposed permits the de-coupling of waves polarized along two transverse orthogonal axes, a separate polarizing device or polarizer is required to suppress the waveguide's ability to guide one of the two orthogonally polarized modes (e.g., the e HE₁₁ mode) and to cause the waveguide to guide only the other mode (e.g., the ₀HE₁₁ mode) and thereby limit the waveguide to true single-mode operation. A fiber optic polarizer is also known from U.S. 4386822 wherein a fiber optic strand of circular cross-section is secured in an arcuate recess of a mounting block and a portion of the cladding of the strand is removed evenly with the surface of the mounting block. A crystal of birefringent material is then mounted on the block in close proximity to the core of the strand in the region where the cladding is removed. It is a principal object of the present invention to provide a fiber-optic polarizer which is easy and inexpensive to fabricate and yet has excellent performance characteristics. In this connection, one specific object of the invention is to provide such a polarizer that greatly reduces the amplitude of unwanted waves in the polarization-holding optical fiber. A further object of the present invention is to provide an improved fiber-optic polarizer which provides stable operation under widely varying environmental conditions. In accordance with the present invention, there is provided a fiber-optic polarizer, comprising: an optical fiber having a core and cladding with different refractive indices and forming a single-mode guiding region capable of supporting the two orthogonally polarized waves of the fundamental mode, said guiding region being located sufficiently close to the outer surface of the fiber, on one side thereof, along a selected length of the fiber, to expose the evanescent field of the guiding region at the outer surface of said one side to allow coupling to a contiguous medium, the outer surface of the fiber having a non circular cross-section with a predetermined geometric relationship to said guiding region so that the location of said guiding region can be ascertained from the geometry of said outer surface, and birefringent means extending longitudinally along a selected length of said fiber on said one side thereof and coupled to the evanescent field exposed at the fiber outer surface on said one side, said birefringent means having different indices of refraction along orthogonal axes having predetermined geometric relationships to said non-circular cross-section of the outer surface of said fiber, one of said indices of refraction of said means being less than the effective index of refraction of a desired one of said two orthogonally polarized waves in said guiding region of said fiber, and the other index of refraction of said means being greater than the effective index of refraction of the other, unwanted wave in said guiding region of said fiber, characterised by the fact that said core of said fiber has a non-circular cross-section defining two transverse orthogonal axes along which said two orthogonal waves are respectively polarized, said non-circular cross-section of said core being such, in combination with said refractive indices of said core and said cladding as to permit the de-coupling of said two orthogonal waves polarized along said axes, and thus to confer good polarization-holding properties to said optical fiber; that said non-circular cross-section of said outer surface of said fiber has furthermore a predetermined geometric relationship to said orthogonal transverse axes of said fiber core so that the orientation of said axes can be ascertained from the geometry of said outer surface; and that said birefringent means is constituted by a polymeric film, the different indices of refraction of which extend along axes which are aligned with said transverse orthogonal axes of said core, the index of refraction of said film that is aligned with that transverse axis of said core along which the polarization of said desired wave in said fiber is aligned being less than the effective index of refraction of said desired wave in said guiding region of said fiber, and the other index of refraction of said film which is aligned with the other transverse axis of said core along which the polarization of said unwanted wave in said fiber is aligned being greater than the effective index of refraction of said unwanted wave in said guiding region of said fiber. Other objects and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings. Fig. 1, labelled PRIOR ART, is a diagrammatic perspective view, in partial section, illustrating the electric and magnetic fields in their preferred directions of polarization in the elliptical core of a single-mode optical fiber waveguide; Fig. 2 is a perspective view of an optical fiber having an elliptical core as illustrated in Fig. 1; Fig. 3 is a perspective view of a fiber-optic polarizer according to one preferred embodiment of the present invention; Fig. 4 illustrates a practical device incorporating the polarizer of Fig. 3; and Fig. 5 is an enlarged fragmentary view of the central portion of Fig. 4. While the invention is susceptible to various alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the invention as defined by the appended claims. Turning now to Fig. 1, there is shown a dielectric core 20 for supporting the propagation of electromagnetic fields E , H in the axial direction. This particular core 20 has an elliptical cross-section with a major diameter 2 a and a minor diameter 2 b where a and b are the respective radii. An optical fiber having such a core 20 with a relatively high dielectric constant/index of refraction tends to confine and guide electromagnetic energy (i.e., light) along the axis of the core. It is known that if the index of refraction of the core 20 is properly chosen in relation to the index of refraction of the surrounding medium, the core dimensions a , b , and the wavelength of the light, the distribution of the fields E and H will tend to occur in a well-defined pattern, or mode. Shown in Fig. 1 is the field pattern for the ₀HE₁₁ mode. Well-defined field patterns have the advantage of facilitating the coupling of the fiber to optical devices. Another advantage is that the attributes of the light propagation, such as phase velocity and group velocity, are relatively constant as the light propagates down the fiber. The group velocity specifies how fast modulation or information travels down the fiber. Thus, for transmitting information over long distances it is important that the group velocity be relatively constant and in particular independent of frequency so that the information will be localised at a specific region rather than becoming "smeared out" as the information travels down the fiber. A single phase velocity is important in fiber-optic sensor applications where the phase of a wave in a sensor fiber is compared to the phase of a reference wave in a reference fiber. Single-mode propagation does not, however, guarantee that the polarization of a wave is fixed in any definite or constant angular relationship with respect to the core 20. Polarization is defined as the direction of the electric field vector E . Thus, as shown in Fig. 1, the light is polarized in a vertical direction, along the major axis of the ellipse. To maintain or preserve the polarization of a signal in an optical fiber, the optical properties of the fiber must be anisotropic, or in other words a function of the angle of polarization with respect to the fiber. One method of making the optical fiber anisotropic is to make the core 20 have a cross-section which is elliptical or some other non-circular shape which defines two transverse orthogonal axes permitting the de-coupling of waves polarized along those axes. A wave which is launched into such a fiber in alignment with one of the transverse axes tends to remain aligned with that axis as the wave is propagated through the fiber, thereby preserving the polarization of the wave. In the illustrative embodiment of Fig. 2, an optical fiber 25 has an elliptical core 26 with a relatively high index of refraction n₁ surrounded by a cladding 27 with a lower index of refraction n₂. The dimensions and the refractive indices of the core 26 and the cladding 27 are selected to provide a region which will guide only the two fundamental ₀HE₁₁ and e HE₁₁ modes. Because of its elliptical shape, this guiding region will also hold the polarization of waves propagated therethrough in alignment with either axis of the ellipse. That is, the major and minor axes of the elliptical cross-section represent two transverse orthogonal axes which permit the de-coupling of waves polarized along those axes. Surrounding the guiding region formed by the core 26 and cladding 27 is a support layer 28 which provides the fiber with increased mechanical strength and ease of manipulation. Since this support layer 28 is not a part of the guiding region, its optical properties are not nearly as critical as those of the core 26 and the cladding 27. To prevent light from being trapped in the cladding 27, the support layers 28 has an index of refraction n₃ higher than that of the cladding 27. As can be seen in Fig. 2, by removing a thin portion of the support layer 28, and also a portion of the cladding 27 (if necessary) to achieve the desired degree of coupling (e.g., by etching to the dashed contour in Fig. 2), the guiding region formed by the core 26 and cladding 27 can be located sufficiently close to a surface of the fiber to allow coupling to a contiguous medium by evanescent wave coupling. As an alternative, a selected segment of the fiber can be drawn to reduce the fiber diameter within that segment and thereby expand the field of the guiding region to permit the coupling of waves from the guiding region in that segment of the fiber. The outer surface of the fiber as defined by the support layer 28 in Fig. 2 has a D-shaped cross-section, with the flat surface 29 of the D extending parallel to the major axis of the elliptical guiding region on the side of the fiber closest to the guiding region. This D-shaped optical fiber is easily interfaced to a polarization-sensitive optical device by using the flat surface 29 of the D as an indexing surface. The guiding region 26,27 of the D-shaped fiber 25 of Fig. 2 is preferably offset or displaced from the geometric center 30 (i.e. centroid of mass or center of gravity of the transverse section) toward the flat 29 of the D along the perpendicular-bisector 31 of the flat. Preferably, the guiding region 26,27 is located within a few average core diameters of the flat surface 29 so that the outer surface of a portion of the fiber may be etched to expose the guiding region 26,27 at the surface 29, thereby permitting the transmission or gradual exchange of light between the guiding region and the fiber surface. For example, the guiding region can be located within about three average core diameters of the flat surface 29. For the elliptical core 26, the average core diameter is the sum ( a + b ) of the major and minor radii. The fact that the flat surface 29 of the D is within a few average core diameters of the guiding region does not affect the attenuation or loss of the fiber since the flat surface 29 is not within the cladding 27. Although there is some light propagated within the cladding 27, substantially no light reaches the support layer 28 which forms the flat 29 of the D. But if an etchant such as hydrofluoric acid is applied to the outer surface of the fiber along a selected length, the flat surface 29 will be moved inwardly (e.g., to the dashed contour shown in Fig. 2), thereby allowing light to be coupled between the core 26 and the flat surface 29 via the cladding 27. In accordance with one important aspect of the present invention, a polymeric film is coupled to the flat side of the D-shaped optical fiber, the film having different indices of refraction along axes aligned with the orthogonal transverse axes of the elliptical core of the fiber, one of the indices of refraction of the film being less than the effective index of refraction of a desired wave in the guiding region of the fiber, and the other index of refraction of the film being greater than the effective index of refraction of an unwanted wave in the guiding region of the fiber. Thus, in the illustrative embodiment of Fig. 3 a polymeric film 40 has one edge 41 thereof extending longitudinally along the fiber 25 and coupled to the flat surface 29. The desired coupling between the film edge 41 and the flat fiber surface 29 can be achieved by merely pressing the fiber against the film edge 41, i.e., without any intermediate coupling medium. On the other hand, an intermediate coupling medium, such as a liquid or gel with the proper index of refraction, may be utilised if desired. As indicated by the legends in Fig. 3, the polymeric film 40 is birefringent in that it has different indices of refraction n y and n z along orthogonal axes which are perpendicular to the major and minor axes of the elliptical cross section of the fiber core 26. The axis of the index n y lies within the plane of the film 40 and is perpendicular to the film edge 41 coupled to the flat fiber surface 29. The axis of the index n z extends across the thickness of the film, perpendicular to both the plane of the film 40 and the axis of the index n y . By coupling the plastic film 40 to the flat side 29 of the D-shaped fiber, the anisotropic indices of refraction of the film can be properly aligned with the orthogonal axes of polarization in the fiber core 26 so that only the wave with the desired polarization is retained in the fiber core. Normally the desired wave is polarized along the major axis of the elliptical cross-section of the fiber core 26. The wave with this desired polarization in the fiber core, e.g. the wave propagating in the ₀HE₁₁ mode, is retained in the fiber 25 by providing the plastic film 40 with an index of refraction n z , along an axis parallel to the major axis of the elliptical core 26 less than the effective index of refraction n e of the desired wave in the guiding region of the fiber. The unwanted waves in the optical fiber 25 are those polarized along the minor axis of the elliptical core 26. As is well known, the evanescent fields in the fiber cladding 27 can be used to couple waves from the fiber 25. In this particular case, the wave having the unwanted polarization, e.g. that propagating in the e HE₁₁ mode, is coupled from the fiber by providing the polymeric film 40 with an index of refraction n y , along an axis parallel to the minor axis of the elliptical core 26, greater than the effective index of refraction n e of the unwanted wave in the guiding region of the fiber. This value of the refractive index n y causes the unwanted wave to radiate into the film 40 and thereby escape from the fiber 25. As a practical matter, the values of the refractive indices n z and n y will generally be selected to be less than and greater than, respectively, the refractive indices of both the fiber core 26 and the cladding 27. The use of a polymeric film is particularly advantageous because the axes of the anisotropic indexes of refraction are precisely known relative to the physical shape of the film, and the value of one or more of the anisotropic indices of refraction can be easily adjusted by simply stretching the film along the axis or axes of the index or indices that it is desired to change. Thus, it is relatively simple and inexpensive to arrive at exactly the desired values of the anisotropic indices of refraction along orthogonal axes in the plastic film, and to precisely align those axes with the orthogonal axes of the elliptical cross section of the fiber core. Consequently, polarizers with excellent performance characteristics can be fabricated easily and at a low cost. For example, referring to Fig. 3, a film 40 can be selected with an index of refraction n z smaller than the effective index n e of the desired wave. This film 40 is then stretched in a direction perpendicular to the major axis of the elliptical core so as to increase the index n y above the effective index n e of the unwanted wave. Then when the stretched film 40 is coupled to the flat side 29 of the D-shaped fiber 25, the desired polarizing effect is achieved. That is, any wave polarized along the minor axis of the elliptical core radiates out of the fiber 25 into the film 40, leaving only the desired wave polarized along the major axis of the elliptical core. The polymeric film 40 may have virtually any composition that is capable of providing the requisite birefringence. Because the film need not serve as a light transmission medium, its optical transmission properties are insignificant. One film that has been successfully used in the illustrative polarizer is "Tedlar 400/BG/20/SE", which is a polyvinylfluoride film made by the duPont Company of Wilmington, Delaware. Figs. 4 and 5 illustrate a practical device embodying the polarizer of Fig. 3. In this device the polymeric film 40 is rigidly supported between a pair of metal blocks or plates 50 and 51 which can be pressed toward each other by means of a screw 52 bearing on the right-hand side of the block 51. The screw 52 is threaded through a rectangular housing 53 which also serves to support the blocks 50 and 51. The flat surface 29 of the optical fiber 25 is placed against the top edge of the plastic film 40 held between the blocks 50,51. The fiber 25 is pressed downwardly by a resilient pad 54 on the bottom of a tongue 55 extending downwardly from a metal block 56. To apply downward pressure on the fiber 25, a screw 57 is threaded through the top of the housing 53 and coupled to the top of the block 56. The tongue 55 has about the same thickness as the fiber 25 and the film 40, which is typically about 70 microns. It will be appreciated that the tongue 55 depending from the metal block 56 not only serves to transfer the desired downward pressure from the screw 57 to the top of the fiber 25, but also serves as a spacer between the two lower blocks 50 and 51 to prevent any damage to the optical fiber 25. If it is desired to retain the optical wave which is polarized along the minor axis, rather than the major axis, of the elliptical cross-section of the fiber core, the relationships of the refractive indices of the polymeric film to the effective indices of refraction of the desired and unwanted waves, respectively, are simply reversed. That is, the refractive index n y of the film must be smaller than the effective index of refraction of the desired wave, which is now the wave polarized along the minor axis of the ellipse; and the index n z must be greater than the effective index of refraction of the unwanted wave, which is now the wave polarized along the major axis of the ellipse. In an alternative arrangement the flat surface of the D-shaped fiber is placed in contact with the plane surface of a birefringent plastic film which has a first refractive index in the plane of the film smaller than the effective refractive index of the desired guided wave, and a second refractive index also in the plane of the film but in a direction orthogonal to that of the first index and greater than the effective refractive index of the unwanted guided wave. In another arrangement the D fiber is constructed with the flat surface of the D-shaped fiber parallel to the minor axis of the elliptical core. As can be seen from the foregoing detailed description, this invention provides a fiber-optic polarizer which is easy and inexpensive to fabricate and yet has excellent performance characteristics. The polymeric film greatly reduces the signal level of unwanted signals in the single-mode polarization-holding optical fiber, and yet it has little or no attenuating effect on the desired signal in the optical fiber and can be easily coupled to the optical fiber at any desired location along the length of the fiber. It also provides stable operation under widely varying environmental conditions.

Claims: A fiber-optic polarizer, comprising: an optical fiber (25) having a core (26) and cladding (27) with different refractive indices and forming a single-mode guiding region capable of supporting the two orthogonally polarized waves of the fundamental mode, said guiding region being located sufficiently close to the outer surface (29) of the fiber (25), on one side thereof, along a selected length of the fiber (25), to expose the evanescent field of the guiding region at the outer surface of said one side to allow coupling to a contiguous medium, the outer surface (29) of the fiber (25) having a non circular cross-section with a predetermined geometric relationship to said guiding region so that the location of said guiding region can be ascertained from the geometry of said outer surface (29), and birefringent means (40) extending longitudinally along a selected length of said fiber (25) on said one side thereof and coupled to the evanescent field exposed at the fiber outer surface (29) on said one side, said birefringent means (40) having different indices of refraction along orthogonal axes having predetermined geometric relationships to said non-circular cross-section of the outer surface (29) of said fiber (25), one of said indices of refraction of said means (40) being less than the effective index of refraction of a desired one of said two orthogonally polarized waves in said guiding region of said fiber (25), and the other index of refraction of said means (40) being greater than the effective index of refraction of the other, unwanted wave in said guiding region of said fiber (25), characterised by the fact that said core (26) of said fiber (25) has a non-circular cross-section defining two transverse orthogonal axes along which said two orthogonal waves are respectively polarized, said non-circular cross-section of said core being such, in combination with said refractive indices of said core and said cladding, as to permit the de-coupling of said two orthogonal waves polarized along said axes, and thus to confer good polarization-holding properties to said optical fiber (25); that said non-circular cross-section of said outer surface (29) of said fiber (25) has furthermore a predetermined geometric relationship to said orthogonal transverse axes of said fiber core so that the orientation of said axes can be ascertained from the geometry of said outer surface (29); and that said birefringent means (40) is constituted by a polymeric film, the different indices of refraction of which extend along axes which are aligned with said transverse orthogonal axes of said core (26), the index of refraction of said film (40) that is aligned with that transverse axis of said core along which the polarization of said desired wave in said fiber (25) is aligned being less than the effective index of refraction of said desired wave in said guiding region of said fiber (25), and the other index of refraction of said film (40) which is aligned with the other transverse axis of said core along which the polarization of said unwanted wave in said fiber is aligned being greater than the effective index of refraction of said unwanted wave in said guiding region of said fiber (25). A fiber-optic polarizer as claimed in claim 1, characterised in that the index of refraction of the film (40) that is greater than the effective index of refraction of said unwanted wave extends along an axis lying in the plane of the film (40), and the index of refraction of the film (40) that is less than the effective index of refraction of said desired wave extends along an axis passing through the thickness of the film. A fiber-optic polarizer as claimed in either preceding claim, characterised in that said core (26) has an elliptical cross-section. A fiber-optic polarizer as claimed in claim 3, characterised in that the index of refraction of said film (40) along an axis parallel to the major axis of said elliptical core (26) is less than the effective index of refraction of said desired wave, and the index of refraction of said film along an axis parallel to the minor axis of said elliptical core (26) is greater than the effective index of refraction of said unwanted wave. A fiber-optic polarizer as claimed in claim 3 or claim 4, characterised in that the portion of said outer surface (29) of the fiber (25) that is closest to said guiding region is substantially parallel to the major transverse axis of said elliptical cross-section. A fiber-optic polarizer as claimed in any preceding claim, characterised in that said fiber (25) includes a support layer (28) surrounding said guiding region and the outer surface of which forms said non-circular outer surface (29) of the fiber (25). A fiber-optic polarizer as claimed in any preceding claim, characterised in that the higher index of refraction of said film (40) is greater than the refractive index of said cladding (27) and of said core (26). A fiber-optic polarizer as claimed in any preceding claim, characterised in that the outer surface (29) of said fiber (25) has a generally D-shaped cross-section. A fiber-optic polarizer as claimed in any preceding claim, characterised in that said film (40) is axially stretched to increase the index of refraction of the film (40) along such stretch axis above the effective index of refraction of said unwanted wave. A fiber-optic polarizer as claimed in any preceding claim, characterised by a pair of rigid plates (50, 51) disposed on opposite sides of said fiber (25) and said film (40) and rigidly supporting said film (40). A fiber-optic polarizer as claimed in any preceding claim, characterised by the outer surface of said fiber (25) having at least one part (29) thereof having a generally flat area. A fiber-optic polarizer as claimed in claim 11, characterised by said core (26) being located with one of said transverse axes extending generally perpendicular to said flat area (29), and being sufficiently close to said flat area to expose the evanescent field of the core (26) at said flat area. A fiber-optic polarizer as claimed in claim 12, characterised by a core (26) having an elliptical cross-section, said core having orthogonal major and minor transverse axes. A fiber-optic polarizer as claimed in claim 13, characterised in that said flat area (29) is substantially parallel to the major axis of said elliptical cross-section.

Inventors: DYOTT, RICHARD B., MEYER, JOHN D., ULRICH, REINHARD
Applicants: ANDREW A.G.
Application year: 1984
Main classification: G02B6
IPC codes: G02B6