The present invention relates to optical fibers and devices incorporating the same.
An optical fiber is an elongated structure which typically includes a transparent solid core extending along the axis of elongation of the fiber and a transparent cladding layer surrounding and contacting the core. The cladding has an index of refraction lower than the index of refraction of the core. The “index of refraction” of a material is a measure of the speed of light in the material relative to the speed of light in a vacuum; the higher the index, the lower the speed of light in the material. Light can travel along the fiber in directions parallel to the axis of the fiber and within a certain range from perfectly parallel to the axis. Light which is not perfectly parallel to the axis will eventually encounter the interface between the core and the cladding. As further explained below, light passing along the fiber at a relatively small angle to the axis will encounter the interface between the core and cladding and will be directed back into the core by a phenomenon referred to as “total internal reflection.” Thus, light can be transmitted along the length of the fiber with minimal loss of light into the cladding or the surroundings. For example, common optical fibers can be used to transmit pulses of light over distances of kilometers or more in optical communication systems.
In other applications, optical fibers are used to provide illumination, most typically by directing light from a source at one end of the fiber so that the light exits from the opposite end of the fiber and passes out of the fiber at the opposite end.
Various proposals have been advanced for making optical fibers which will scatter some of the light passing along the fiber in radial directions, transverse to the axis of the fiber, so as to provide illumination along the length of the fiber for utilitarian or decorative purposes. Such a fiber is commonly referred to as “radially scattering” or “radially dispersive” optical fiber. The commercially available radially scattering optical fibers typically are former with a glass core and glass cladding. The brittle nature of such fibers limits their applicability. Moreover, because the processes commonly used to make radially scattering optical fibers are applied during formation of the fiber itself, such fibers are only available as a continuous length of radially scattering fiber. To provide illumination over a short length of radially dispersive fiber disposed remote from the light source, a separate length of transmission optical fiber typically is required. The two fibers must be coupled to one another, which adds to the cost, complexity, and bulk of the device.
One aspect of the invention provides an optical fiber having an elongated transparent core having an axis of elongation, a cladding surrounding the core, the cladding having an index of refraction lower than an index of refraction of the core, and one or more indentations extending from outside of the cladding into the core so that the indentations define one or more surfaces extending towards and away from the axis of elongation and extending into the core.
A further aspect of the invention provides an optical fiber having an elongated transparent core, a cladding surrounding the core, the cladding having an index of refraction lower than an index of refraction of the core, one or more indentations extending from outside of the cladding at least to the core, and an optically transmissive solid filler disposed in the indentations.
As further explained below, the indentations facilitate extraction of light from the core. The solid filler structurally reinforces the fiber and also facilitates extraction of light from the core. Thus, particularly preferred fibers incorporate both aspects of the invention discussed above.
Most desirably, the core and cladding of the fiber are formed from polymeric materials. The fiber may have an emission region incorporating the indentations, and may also incorporate a transmission region devoid of the indentations.
Further aspects of the invention provide optical devices incorporating fibers as discussed above. The emission region may be curved as, for example, to form a loop. The optical device may also incorporate a solid coupling material surrounding the emission region of the fiber.
An optical fiber in accordance with one aspect of the present invention includes a core 30 having a circular cross-section as best seen in
The fiber of
Kerf or indentation 42 is filled with a solid filler material 52. As depicted in
In operation, light from a source 60 such as a lamp, light emitting diode or laser is directed into the upstream end 39 of the fiber. The light includes light propagating within the core 30 at various angles to the axis 36 of the fiber. As best seen in
σcritical=arcsin(ηcladding/ηcore)
where σcladding is the index of refraction of the cladding 32 and ηcore the index of refraction of core 30. For any angle of incidence greater than σcritical, the light will be reflected at interface 31. For any angle of incidence less than σcritical, the light will pass into the cladding 32. For the particular fiber shown in
Typically, within the transmission region 38, cladding 32 of the fiber is surrounded by a coupling medium having an index of refraction close to that of the cladding, or by a light-absorbing medium such as an opaque jacket (not shown). In this case, the light passing into the cladding along will pass out of the fiber or will be absorbed. Therefore, essentially all of the light passing downstream along the fiber will be transmitted within the core. The light transmitted in this manner will consist of rays travelling within the core along paths within a range of angles less an angle referred to herein as the “maximum propagation angle” from parallel to axis 36 of the fiber and thus striking the core/cladding interface at angles of incidence greater than the critical angle. The maximum propagation angle is the complement of the critical angle, i.e., (90−σcritical) degrees. In the particular fiber shown, the maximum propagation angle is 20.02 degrees. This light is reflected repeatedly, so that the light travels along zizgzag paths as represented schematically by rays 64 and 66 in
If the cladding is surrounded by air, which has an index of refraction of approximately 1.0, or by another medium having an index of refraction lower than the index of refraction of the cladding, some of the light passing into the cladding will be reflected at the outer surface of the cladding, by total internal reflection at the interface between the cladding and the surrounding medium. In the particular fiber shown, the critical angle for the cladding-to-air interface is approximately 46 degrees. Light striking the cladding-to-air interface at the outer surface 33 of the cladding at an angle greater than this critical angle will be reflected. Total internal reflection of light passing along ray 62′ at the outside of the cladding is depicted by in indicated by ray 62″ in
Within emission region 40 (
As best seen in
Light travelling along other paths (not shown) more nearly parallel to the axis 36 than ray 10, also will be directed outwardly, into cladding 32, but at shallower angles to the axis than ray 10′. However, some light which is directed along paths very nearly parallel to axis 36, as shown by ray 11 in
If the emission region 40 is surrounded by an optical coupling material having an index of refraction close to or greater than the index of refraction of cladding 32, substantially all of the light diverted into the cladding will pass out of the cladding into the surrounding medium.
However, if the emission region 40 is surrounded by air or by another medium having an index of refraction substantially less than that of the cladding 32, the light directed into the cladding along ray 10′ will be reflected back into the cladding by total internal reflection at the outer surface of the cladding. For example, where the emission region is surrounded by air, the critical angle at the cladding-to-air interface is approximately 45.48 degrees. The angle of incidence σ10′-33 of ray 10′ is greater than this value, and hence total internal reflection will occur at the interface between the cladding and the air. However, the reflected ray 10″ will pass back to the core-to-cladding interface, and will be refracted to an angle of about 28.64 degrees to the axis 36 within the core as shown at 10′″ in
The fiber as discussed above can provide illumination effective illumination along the length of the emission region 40. Moreover, the fiber can be fabricated readily from commercially-available polymeric multimode optical fiber by forming the indentation 42 in the desired emission region 40, while leaving the desired transmission region 40 devoid of indentations. One of the most consistent and controllable methods to make a helical indentation 42 is to rotate the fiber against a cutting tool, such as a carbide, steel or diamond edge while moving the cutting tool or fiber in a direction parallel to the axis of the fiber. A machine tool such as a lathe that rotates both ends of the material to be worked (the fiber) may be used, as polymeric optical fiber cannot transfer torque applied at one end along the length of the fiber. For example, glassblowing lathes are designed for this type of application given the inability of the originally designed for working medium, molten glass, to support torque.
In the embodiment discussed above, the helical indentation has a uniform pitch, i.e., a uniform distance between turns of the indentation. Each turn of the indentation typically extracts a percentage of the light propagating in the core of the fiber per each circumferential cut. Because light is extracted from the fiber as the light propagates in the downstream or distal direction within the emission region 40, the optical power of the light propagating in the emission region diminishes progressively in the distal direction. Thus each sequentially distal turn of the indentation will extract less light, as there are simply fewer photons available for extraction at each more distal location. To achieve a uniform output of extracted light per unit length along the length of the emission region, the pitch of the indentation (rotations per unit length) may be adjusted along the length such that the pitch density increases in the distal direction. This increase can be linear, or the rate of increase can be varied along the length of the emission region. Indeed, the pitch can be varied as desired to achieve any desired pattern of light output along the length of the emission region.
Also, plural indentations can be provided, rather than a single helical indentation as discussed above. Two or more helical indentations can be formed as, for example, with turns of one indentation disposed between turns of another indentation. Also, the indentations can be provided as discrete indentations spaced apart from one another along the length of the emission region. For example, indentations may be formed as discrete cuts oriented transverse to the axis of the fiber. These discrete cuts may be distributed at uniform or non-uniform distances from one another in the axial or upstream-to-downstream directions. Also, discrete cuts may be formed in equal numbers around the circumference of the fiber to provide substantially equal emission in all directions transverse to the axis of the fiber, or may be provided only at certain locations around the circumference of the fiber to provide unequal emission.
For an indentation or indentations of a given shape, the size of the target area on the upstream surface of the indentation, and thus the percentage of the light extracted can be adjusted by changing the depth of indentation; the deeper the cut, the larger the target area. In another variant, the depth of the indentation or indentations can vary along the length of the emission region. In one example, to maintain a uniform emission along the length of the emission region, the depth of the indentation or indentations can increase progressively in the downstream direction.
The shape of the indentations can be varied. For example, the orientations of the upstream and downstream surface relative to the axis may be varied from those shown above. In other variants, these surfaces may be curved as seen in a sectional view on a diametric plane, rather than straight as depicted in
In the embodiments discussed above, the filler material in the indentations is transparent. However, the filler material may be a light-scattering material. For example, the filler material may include particulates, voids or other discrete light-reflecting elements dispersed in a transparent matrix. With reflective elements in the filler material, light rays can be directly reflected out of the filler into the surrounding medium.
For example, the embodiment shown in
The concentration of these particulates in the kerf filler matrix may be adjusted to achieve the desired amount of light extraction at each indentation in the fiber. Alternatively, the particle size , the material constituting the particles, or both, can be varied along the length of the emission region to change the properties of the light most efficiently extracted from the core of the fiber. In the case where the fiber carries multiple colors, or frequencies of light, this technique can be used to extract these different colors at different point along the fiber. Or, this technique can be used as a filter, to preferentially pull out unwanted frequencies from the core of the fiber.
In the embodiments discussed above, the extension of the indentations into the core of the fiber provides enhanced light extraction from the core. However, some light extraction from the core into the cladding can occur even where the indentation extends to the core but not into the core. For example, in the embodiment of
Conversely, in those embodiments which include indentations extending into the core, the filler material may be omitted. In this case, the indentations will be filled with air or any other medium which surrounds the emission region of the fiber. The indentations will still permit extraction of at least some light from the core of the fiber.
In the foregoing discussion, it has been assumed that the axis of the fiber is a straight line. However, because the fiber is a polymeric fiber of relatively small diameter, it can be bent into a curved shape, such that the axis 36 of the fiber is curved. However, the fiber will work in substantially the same manner as discussed above, so as to emit light from emission section 40 into the surrounding medium. Although forming the fiber into a curved configuration will vary the exact angles of incidence of the light rays, the mode of operation will remain essentially the same as discussed above. In general, light emission from the emission region will increase when the fiber is in a curved configuration. Stated another way, in a fiber of infinite length, in a straight line, all but the propagating modes (with angles to the fiber axis less than the maximum propagation angle) will dissipate over some length. In practice of course fibers are not infinite, or straight. Bends can force certain propagating modes into incidence angles that lead the light out of the core and potentially out of the fiber. Where the emission region of the fiber is bent, this action supplements the action of the features discussed above. The ability to bend the fiber into a curved shape is highly advantageous in some applications. In this regard, the solid filler material in the indentations enhances the structural integrity of the fiber. The preferred polymeric optical fibers can be formed into curved configurations such as loops by annealing the fiber in the curved configuration at an elevated temperature as, for example, about 80 degrees C. Where the filler is a thermoset epoxy, the annealing process desirably is carried out either before application of the filler material or after curing of the filler material, so as to avoid a rapid exothermic curing of the epoxy which may overheat adjacent areas of the fiber.
In the embodiment discussed above with reference to
An optical device in accordance with a further aspect of the invention may include a fiber as discussed above, and may also include an optical coupling material surrounding the emission region of the fiber. The optical coupling material may have an index of refraction higher than that of air, and desirably approximately equal to or greater than the index of refraction of the cladding, to facilitate transmission of light out of the cladding.
One particularly useful application of the fibers and optical devices discussed herein is in application of ultraviolet or blue light for the purpose of corneal crosslinking as disclosed in US Published Patent Application US2014/0379054 (the “'054 Publication”), the disclosure of which is hereby incorporated by reference herein. An optical device suitable for such use is shown in
An optical device as shown in
As used in the present disclosure, the term “light” should be understood as including ultraviolet and infrared radiation, as well as light within the visible portion of the spectrum.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
The present application claims the benefit of U.S. Provisional Patent Application No. 62/518,971, filed Jun. 13, 2017, the disclosure of which is incorporated herein by reference herein.
Number | Date | Country | |
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62518971 | Jun 2017 | US |