WO2001006287A1

WO2001006287A1 - Compound cladded rod for transmission of optical power

Info

Publication number: WO2001006287A1
Application number: PCT/US2000/019588
Authority: WO
Inventors: Glenn S. Baker
Original assignee: Cogent Light Technologies, Inc.
Priority date: 1999-07-19
Filing date: 2000-07-19
Publication date: 2001-01-25
Also published as: AU6352900A

Abstract

An optical coupling element comprises a compound cladded rod (10, 20) having at least two core (core 1, core 2) layers separated by a cladding layer (clad 1) with an outer cladding layer (clad 2) surrounding the outermost periphery of the compound cladded rod (10, 20). Alternating core and cladding layers are preferably concentric, and the thicknessess of the cladding layers are negligible compared to the thicknesses of the core layers. The outer diameters of the core layers correspond to the outer diameters of output waveguides of varying diameters. The compound cladded rod (10, 20) may be tapered. An illumination system (40, 72, 92) employing the compound cladded rod optical (46, 76, 96) coupling element includes a light source (42, 72, 92) and an optical system for coupling the light source to a focus, which may comprise either an on-axis or an off-axis system. An input end of the compound cladded rod is placed at the focus of the optical coupling system, and an optical connector (52, 82, 102) capable of accomodating output waveguides of varying diameters is optically coupled to the output end (50, 80, 100) of the compound cladded rod (46, 76, 96).

Description

COMPOUND CLADDED ROD FOR TRANSMISSION OF OPTICAL POWER

Field of the Invention

The invention relates to an optical coupling element including a compound cladded rod for preserving radial optical power distribution.

Background of the Invention

In the field of fiber optic illumination, it has been a goal to couple light from the light source to the input end of an output fiber optic waveguide in the most efficient manner so as maximize the amount of light at the output end of the fiber optic. Common coupling schemes involve the use of an ellipsoidal reflector or a parabolic reflector together with a focussing lens. Systems of these types have been around for many years and are classified as on-axis systems, in which the axes of the reflectors coincide with the lines joining the light sources to the output fiber optics. Another class of coupling system classified as off-axis systems, in which the axes of the reflectors are not the same as the lines joining the light sources to the output fiber optics, have been developed and used. Systems of this type are described in US patents 4,757,431 , 5,414,600, and 5,430,634. One of the major advantages of off-axis coupling systems is that the magnification of the system is approximately equal to 1, and the focused spot has a minimum degradation in brightness due to aberrations. As a result, the brightness of the focused spot is almost the same as the arc. Together with a retro-reflector in which the light is focussed back into through the source, the brightness can be increased.

For example, an arc lamp with a 1 mm arc gap can be focussed into a spot of the same order of magnitude. The light at the focus can be coupled into a small diameter fiber with enough light for many applications. One such application is for a headlight in the surgical field using fibers with diameters ranging from 0.5 mm to 1.5 mm. Due to the small size of the fiber, the headlight can be made small. On the other hand, for the other schemes of coupling, such as on-axis systems, to obtain enough light for such a purpose, the diameter of the fiber is about 5 mm, which is usually achieved with a bundle of smaller fibers. With such a bundle, the size of the headlight is also larger and heavier, which is less preferable to most surgeons. Before the invention and commercialization of off-axis systems which permitted applications to be made smaller, the market had been filled with applications using fiber bundles, and many of these large diameter applications are still in use today. As a result, a multilink port, as described in U.S. Provisional Patent Application Serial No. 60/183,146 - the disclosure of which is hereby incorporated by reference--, has been developed to permit both the new smaller fiber optics and the larger fiber bundles to be connected to the same illuminator. Due to the intrinsic differences in the coupling of light into various sizes of output fiber optics, a system optimized for use with small fibers can be inefficient for larger fibers, and vise versa. There remains a need for an optical coupling system that delivers high efficiency for both small and large fibers from the same illuminator.

In the coupling of light from a high intensity arc lamp into a fiber optic, either a single fiber optic and/or a fiber bundle, due to difference in sizes, mode matching, heat dissipation, and other mechanical constraints, it is often necessary to place an optical element between the focus of the light and the fiber optic. This optical element can be a fused fiber bundle or a cladded rod. When matching to different numerical aperture or area is needed, a tapered fused fiber bundle or a tapered cladded rod can be used

When using the fused fiber bundle, the advantage is that the intensity profile of the arc is more or less preserved especially when the cladding of the individual fibers are large to prevent cross-talk. On the other hand, due to the space taken by the cladding of the individual fiber, the efficiency will be reduced. When using a cladded rod, the loss is smaller than the fused fiber bundle. But due to the mixing of the light inside the rod, the output intensity profile is changed and intensity profile of the input is not preserved.

The intensity profile of the arc imaged onto the input of this optical element is usually not uniform, but instead, is gaussian in its distribution with the highest intensity at the center and decreasing intensity radially out towards the edge. When the output fiber optic has a large diameter, all the light will be collected. But, if the output fiber optic has a diameter small compared to the focused spot size, it would be advantageous to couple only the high intensity portion of the light at the point of peak intensity profile. If the optical element is a fused fiber bundle, the output fiber optic can be aligned to the peak of the intensity at the output end of the optical element which has a similar profile as the input. On the other hand, when the optical element is a cladded rod, the output profile is scrambled and the peak intensity is spread out, resulting in a lower coupled power into a small output fiber optic.

Summary of the Invention It is an object of this invention to provide efficient coupling from a light source to small fibers.

It is another object of this invention to provide efficient coupling from a light source to a larger diameter fiber bundle. It is another object of this invention to provide efficient coupling from a light source to small and large fiber with the same output port without any change in configuration.

It is another object of this invention to provide efficient coupling from a light source to small and large fibers with various numerical apertures. According to one aspect of the invention, an optical coupling element has an input end with an input diameter and an output end with an output diameter. The coupling comprises a first core extending from the input end to the output end and having a first core index of refraction, a first cladding layer disposed radially outwardly of the first core and having a first cladding index of refraction, a second core layer extending from the input end to the output end and disposed radially outwardly of the first cladding layer and having a second core index of refraction, and a second cladding layer disposed radially outwardly of said second core layer and having a second cladding index of refraction.

According to another aspect of the invention, an illumination system optimized for use with output fibers of various sizes comprises a light source, an optical system for coupling the light source to a focus, and a compound cladded rod with an input end placed at the focus of the optical system.

According to another aspect of the invention, an optical coupling element for coupling light from a light source to a target comprises alternating, concentric core and cladding layers, the core layers having an index of refraction n_core larger than an index of refraction of the cladding layers n_clad and the core layers being thicker than the cladding layers.

Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of the specification.

Description of the Drawings

Embodiments of the present invention will be described with reference to the attached drawings in which corresponding components or features in the various figures are designated by like reference numbers.

Figure 1 is a schematic diagram presenting an end view of a compound cladded rod with two cores and two cladding layers.

Figure 2 is a schematic diagram presenting a longitudinal cross-section of a tapered compound cladded rod with two cores and two cladding layers in which the numerical aperture of the input light will be transformed to a different numerical aperture depending of the indices of the materials and the diameters of the input and output ends.

Figure 3 is a schematic diagram presenting an end view of a compound cladded rod with three cores and three cladding layers designed for use with output waveguides of three different diameters, for example, 1.5 mm, 3 mm, and 5 mm fibers and bundles.

Figure 4 is a schematic diagram of an illumination system including an arc lamp, an off-axis system coupling including a concave primary mirror (e.g., a spherical, toroidal, or elliptical reflector), a compound cladded rod, and a multilink connector system for coupling into output waveguides with various diameters. Figure 5 is a schematic diagram of an illumination system including a light source, an on-axis coupling system including an ellipsoidal primary mirror, a compound cladded rod, and a multilink connector system for coupling into output waveguides having various diameters.

Figure 6 is a schematic diagram of an illumination system including a light source, an on-axis coupling system including a parabolic primary mirror and a focusing lens, a compound cladded rod, and a multilink connector system for coupling into output waveguides having various diameters.

Figure 7 is a schematic diagram of a compound cladded rod with repeating layers of core and cladding layers.

Description of the Preferred Embodiments

Figure 1 is a schematic end view of a compound cladded rod 10 according to the present invention which provides high transmission efficiency while at the same time substantially preserving the radial intensity profile of the input light in two layered sections. Extension of this inventive feature can include various sections, or layers, the number and size of which can be determined by the intended application, ideally matching the size and number of layers to the various diameters of the output waveguides to be used in conjunction with the compound cladded rod.

As shown in Figure 1, the compound cladded rod 10 includes four layers of waveguide materials, preferably arranged concentrically with each other, and designated as core,, clad,, core₂, and clad₂ having respective refractive indices of n_core), n_C]_adl, n_core2, and n _ad2. The layers n_corel and n_core2 have respective diameters of d, and d₂, assuming that the cladding layers clad, and clad₂ have negligible thicknesses compared to the overall waveguide diameters. One or more of the layers may be formed from optical materials such as quartz or borosilicate glass, or any other

- A - suitable material. To provide light guiding through the compound cladded rod 10, it is necessary that: i i n adi; and _COre2 ^{> n} _ci_adi nd n_core2 > n_clad2. The numerical apertures NA, and NA₂ of the respective cores, core, and core₂, can be calculated according to known waveguide equations from the respective indices of refraction. For a particular case:

"corel

~ n_core, ⁿciadi ^{= n}ciad2 ^{= n} _clad; and n_core > n_clad, the numerical apertures of the two cores will be the same and the light guided from each core will not leak from one to the other and will remain independent of each other. The losses incurred by this compound cladded rod will be much smaller those incurred by an equivalent fused fiber optic bundle, because the amount of clad materials for the compound cladded rod is much smaller than that of the fused bundle.

For example, a typical fused fiber bundle has fibers with 3 μm spacing of clad and 30 μm of core, which results in 20% of wasted area for any size bundle. On the other hand, for a cladded rod according to the present invention, such as the cladded rod shown in Figure 1 having two core layers and two clad layers, and having the same clad thickness as described above, and wherein d, = 3 mm and d₂ = 5 mm, the rod has a wasted area of 0.4%.

An exemplary application of a four-layer compound cladded rod such as rod 10 in Figure 1 , is for coupling light from an arc lamp to either a 1.5 mm fiber or a 5 mm fiber. In this case, d, = 1.5 mm and d₂ = 5 mm, and the cladding layers have negligible thicknesses compared to the size of the cores. Slight variations of these diameters can be employed to accommodate mechanical tolerances of the various parts. When the output of the arc lamp is imaged onto the input end of this compound cladded rod, the high intensity peak is coupled into core, and the rest of the arc is coupled into core₂. Given the fact that light in core, is scrambled along the length of the optical element, the total power does not change within core,. As a result, when the output is coupled into the 1.5 mm output fiber, the output from the compound cladded rod is similar to coupling from the image of the arc lamp itself. When a 5 mm fiber is used, all of the light from core, and core₂ will be captured, meaning that substantially all of the light entering the input end of the compound cladded rod is coupled into the 5 mm optical fiber. In another embodiment, the numerical apertures of the two core sections are different from one another. In one embodiment, NA, is smaller than NA₂. In this case, the inner core, core,, would couple only the low NA light from the input end to the output end of the compound cladded rod. The higher NA light between NA, and NA₂ will be lost from the inner waveguide, core,, into the outer waveguide, core₂, and will continue to be guided by the outer waveguide, core₂.

In yet another embodiment, NA, is larger than NA₂. In this case, the inner waveguide, core,, is optimized to couple high NA light into a small diameter fiber, while the overall waveguide will be optimized to couple light into a smaller NA and larger diameter fiber. When further optimization and matching is required for a wider range of numerical aperture and fiber size, a tapered compound cladded rod, such as rod 20 shown in Figure 2, can be employed. The numerical aperture and area considerations of a regular tapered cladded rod apply. The tapered compound cladded rod 20 shown in Figure 2 has four layers, namely core,, clad,, core₂, and clad₂. The input diameter of a first end 22 of the tapered compound cladded rod 20 is smaller than that of a second end 24. If end 22 is the input end of the rod 20 and end 24 is the output end of the rod 20, the output numerical aperture is smaller than the input numerical aperture. On the other hand, if large diameter end 24 is the input end and small diameter end 22 is the output end, the output numerical aperture will be larger than the input numerical aperture. The particular arrangement and configuration of the tapered compound cladded rod 20 can be defined and optimized based on the intended application. In addition, while the tapered compound cladded rod 20 shown in Figure 2 has a linear longitudinal taper, the rod may, alternatively, have a non-linear longitudinal taper.

It is also conceivable that a process can also be developed such that the taper ratio of the inner waveguide, core,, will be different from that of the outer waveguide, core₂. Such a capability will provide a wider range of output diameters and numerical apertures.

When a particular lighting application requires more than two different output waveguide diameters, more sections, or layers, of core and clad can be implemented according to the sizes of the output waveguides. Figure 3 shows a compound cladded rod 30 having three cores, core,, core₂, and core₃, alternating with three cladding layers, clad,, clad₂, and clad₃. The compound cladded rod 30 may have core diameters d, = 1.5 mm, d₂ = 3 mm, and d₃ = 5 mm for accommodating these three common waveguide diameters. Again, when other area and numerical aperture requirements dictate, such a compound cladded rod can also be tapered. Figure 4 shows a preferred arrangement of a illumination system generally indicated by reference number 40. The system 40 comprises a light source 42 coupled to a target using an off-axis concave primary mirror 44. The light source 42 is preferably a xenon arc lamp, but may be a metal halide or mercury arc lamp or a halogen lamp. The concave primary mirror 44 may be spherical, toroidal, or elliptical, and preferably has a numerical aperture of approximately 0.7. The light source 42 is placed at a first focal point 58 of the primary concave mirror 44, and the input end 48 of a compound cladded rod 46 is placed at a second focal point 60 of the concave primary mirror 44. For further output intensity, a retro-reflector 56 may be placed on a side of the light source 42 opposite the primary mirror 44 to reflect light back through the focal point 58 and toward the primary concave mirror 44. In the illustrated embodiment, the compound cladded rod 46 has four layers — two core and two cladding layers — and is tapered along at least a portion of its length. An output end 50 of the cladded rod 46 is optically coupled to a connector 52, which is preferably a multilink connector for connecting output waveguides 54 of varying diameters to the illumination system 40. In a preferred arrangement, the output fibers 54 include a 1.5 mm plastic fiber and a

5 mm fiber bundle with a numerical aperture of about 0.5. For efficient coupling, the diameters of the cores of the compound cladded rod 46 at the output end 50 are 1.5 mm and 5 mm with appropriate cladding thicknesses which are on the order of a few microns and are negligible compared to the diameters of the cores. For practical implementation, the cladding layers can be made thicker than what is needed optically, especially for the outermost cladding which also needs structural strength. The compound cladded rod is preferably tapered such that the diameter at the input end 48 of the rod 46 is smaller than the diameter at the output end 50, and the input numerical aperture of 0.7 is transformed to a numerical aperture of 0.5 at the output end 50 to match the output fibers. The multilink connector system 52 placed at the output end 50 of the tapered compound cladded rod 46 is constructed and arranged to accept the two different sized output fibers and may comprise a connector such as that described in previously incorporated U.S. Provisional Patent Application Serial No. 60/183,146.

Alternate illumination systems are shown in Figures 5 and 6. The illumination systems in Figures 5 and 6 include on-axis light coupling systems. For example, in Figure 5 the illumination system 70 includes a light source 72 placed at a first focal 86 of an ellipsoidal primary reflector 74. The light is refocused at a second focal point 88 at which is located the input end 78 of a compound cladded rod 76. The output end 80 of the compound cladded rod 76 is coupled to a connector 82, which is preferably a multilink connector capable of connecting output waveguides 84 of varying diameters to the light coupling system. The focal points 86 and

88 as well as the compound cladded rod 76, and the connector 82 are arranged along the optical axis 75 of the primary mirror 74.

Figure 6 shows an on-axis illumination system 90 that includes a light source 92 placed at the focal point 108 of a parabolic primary mirror 94. Light reflected by the primary parabolic mirror 94 is collimated and thereafter refocused by a focusing lens 106 toward a focal point 1 10 at which is located the input end 98 of a compound cladded rod 46. The output end 100 of the compound cladded rod 96 is optically coupled to a connector 102. The coupling system is arranged along the common optical axis 95 of the primary mirror 94 and the focusing lens 106. Although the embodiments of the compound cladded rods shown and described thus far have either two or three core and cladding layers, in reality, there is no limit as to the number of cores and cladding layers that a compound cladded rod may have. It is mostly dependent on how many types of output fibers the system must accommodate.

Figure 5 shows another embodiment of this invention in which a compound cladded rod 120 has multiple layers of core, core,, core₂,. . ., core_j5. . ., core, and cladding, clad,, clad₂,. . ., clad,,. . ., clad_n. The thicknesses of the core layers can be on the order of 30 to 100 μm, and the thicknesses of the cladding layers can be on the order of 3 to 5 μ . With such an arrangement, the input radial distribution profile of light will be preserved through these multiple layers. Since each core/cladding layer thickness is small compared to the diameter of the output waveguide, it becomes unnecessary to customize the diameters of the various layers. Instead, this compound cladded rod will be applicable to fibers with all sizes larger than the size of the center core, core,. While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but, on the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Thus, it is to be understood that variations in the particular parameters used in defining the present invention can be made without departing from the novel aspects of this invention as defined in the following claims.

Claims

1. An optical coupling element including an input end having an input diameter and an output end having an output diameter, said coupling comprising: a first core extending from said input end to said output end and having a first core index of refraction; a first cladding layer disposed radially outwardly of said first core and having a first cladding index of refraction; a second core layer extending from said input end to said output end and disposed radially outwardly of said first cladding layer and having a second core index of refraction; and a second cladding layer disposed radially outwardly of said second core layer and having a second cladding index of refraction.

2. The optical coupling element of claim 1, wherein said first core index of refraction is larger than said first cladding index of refraction.

3. The optical coupling element of claim 1, wherein said second core index of refraction is larger than said first cladding index of refraction and said second cladding index of refraction.

4. The optical coupling element of claim 1, wherein said first core index of refraction is equal to said second core index of refraction.

5. The optical coupling element of claim 1 , wherein at least a portion thereof is made with borosilicate glass.

6. The optical coupling element of claim 1, wherein at least a portion thereof is made with quartz.

7. The optical coupling element of claim 1, wherein said first core, said first cladding layer, said second core layer, and said second cladding layer are concentric.

8. The optical coupling element of claim 1, wherein said input diameter is the same as said output diameter.

9. The optical coupling element of claim 1 , wherein said input diameter is not equal to said output diameter.

10. The optical coupling element of claim 1 , further comprising a light selected from a group consisting of a xenon arc lamp, a metal halide arc lamp, and a mercury arc lamp

11. The optical coupling element of claim 1 , further comprising: a third core layer extending from said input end to said output end and disposed radially outwardly of said second cladding layer and having a third core index of refraction; and a third cladding layer disposed radially outwardly of said third core layer and having a third cladding index of refraction.

12. The optical coupling element of claim 11, further comprising at least one more core layer disposed radially outwardly of said third cladding layer and at least one more cladding layer disposed radially outwardly of said at least one more core layer.

13. An illumination system optimized for use with output fibers of various sizes comprising: a light source; an optical system for coupling said light source to a focus; and a compound cladded rod with an input end placed at said focus of said optical system.

14. The illumination system of claim 13, further comprising a connector system placed at an output end of said compound cladded rod for accepting an output fiber optic.

15. The illumination system of claim 13, wherein said light source is selected from a group consisting of a xenon arc lamp, a metal halide arc lamp, and a mercury arc lamp.

16. The illumination system of claim 13, wherein said optical system comprises an off-axis coupling system.

17. The illumination system in claim 13, wherein said optical system comprises an on- axis elliptical reflector.

18. The illumination system in claim 13, wherein said optical system comprises a parabolic reflector with a focusing lens.

19. The illumination system in claim 13, wherein said compound cladded rod comprises alternate concentric layers of core and cladding materials.

20. The illumination system in claim 13, wherein said connector system is a multilink connector.

21. The illumination system in claim 13, wherein said compound cladded rod is tapered from said input end thereof to an output end thereof.

22. The illumination system in claim 16, wherein said off-axis coupling system includes a concave primary mirror.

23. The illumination system in claim 22, wherein said concave primary mirror is selected from the group including spherical, toroidal, and ellipsoidal mirrors.

24. An optical coupling element for coupling light from a light source to a target comprising: alternating, concentric core and cladding layers, said core layers having an index of refraction n_core larger than an index of refraction of said cladding layers n_c,_ad, and said core layers being thicker than said cladding layers.

25. The optical coupling element of claim 24, wherein at least a portion thereof is made with borosilicate glass.

26. The optical coupling element of claim 24, wherein at least a portion thereof is made with quartz.

27. The optical coupling element of claim 24, having an output end with an input diameter and an output end with an output diameter, wherein said input diameter is the same as said output diameter.

28. The optical coupling element of claim 24, having an output end with an input diameter and an output end with an output diameter, wherein said input diameter is not equal to said output diameter.

29. The optical coupling element of claim 24, further comprising a light source selected from a group consisting of a xenon arc lamp, a metal halide arc lamp, and a mercury arc lamp.

30. The optical coupling element of claim 24, further comprising a fiber optic target.

31. The optical coupling element of claim 30, wherein said fiber optic target is selected from a group consisting of plastic fiber, glass fiber, quartz fiber, fiber bundles, and large core plastic fibers.

32. The optical coupling element of claim 24, being tapered from one end thereof to an opposite end thereof.