# Patent

## Title
Directional light beam generators

## Abstract

Directional light beam generators are implemented in waveguides with tilted core structures. The waveguides have cores, cladding and jacket layers. A directional light beam with small divergent beam angle is generated after propagating through the waveguides. Divergent light beams with large beam sizes can be converted into beams with small sizes.

## Background
FIELD OF THE INVENTION
This invention relates to a method to generate a directional light beam with small divergent beam angle from light sources.
BACKGROUND OF THE INVENTION
A directional light beam with small divergent angle or a parallel light beam such as laser beam can theoretically maintain its original energy level indefinitely, no matter how far it travels from the source. Parallel beams also can be efficiently focused into a point with lens or mirrors. Therefore, a parallel light beam is desirable for use in many optical systems since it is easily manipulated by optical components such as lens or mirrors.
It is difficult to efficiently convert all of light into a parallel beam by using conventional optical components such as lens or mirrors when a light source has finite size and light beam emitted with finite angle range. Optical lens or mirrors can only partially convert light beam emitted from light source with finite size into a parallel beam when the light source is located at their focus points.
There are enormous efforts to obtain a parallel beam from light source with a finite size such as lamps. In U.S. Pat. No. 5,613,768, two arrayed mirrors were used to generate parallel light. Non-parallel lights from light source were blocked by plurality of holes. Thus, part of light energy was wasted and collection of lights is not efficient.
Present invention discloses a technique which can efficiently convert divergent light beams from light source with finite size into a directional light beam with small divergent angle or parallel light beam.
There are number of applications for the invented directional beam generator. The brightness of light can be greatly increased when the given light are coupled into smaller area from large area by employing invented directional beam generators; It can be used as a beam shaper for diode laser coupling to fiber with small core size; It can be used generate point light sources from lamps with finite size for projector; A white “laser”—a directional visible light beam can be generated using the invented directional beam generator.
SUMMARY OF THE INVENTION
According to the invention, a parallel beam or directional beam with smaller divergent angles can be obtained after divergent light passed through the invented devices. The divergent angle of incident light beam can be reduced to as small as diffraction-limited. The devices have fiber waveguides geometry with core, cladding layers and protect jacket. Cone structures with tilted surfaces between core and cladding layers are used to direct light into beam which possesses smaller propagation angle or even is parallel to the axis of devices. Lightwave with most of input energy and all of wavelength can be directed into a directional beam with small divergent angle or parallel beams.



BRIEF INTRODUCTION TO THE DRAWINGS
A better understanding of the invention will obtained by reference to the detailed description below, in conjunction with the following drawings, in which:
FIG. 1 shows cross-sectional side views of waveguide geometry with tilted wall surface.
FIG. 2 shows cross-sectional side view of simplified waveguide geometry with tilted wall surface.
FIG. 3 shows cross-sectional side view of waveguide with array of cone core structure. 3a) side view; 3b) top view.
FIG. 4 shows cross-sectional side views of waveguide geometry with double cores with tilted wall surfaces.
FIG. 5 shows cross-sectional side views of fiber waveguide geometry with multi-section of cone surface structure with varied tilted angels.
FIG. 6 shows a system setup to couple lights from a light source to a fiber with a small core.



DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, referring to FIG. 1, divergent lights were converted into light beams with smaller divergent angle or even parallel beam with waveguide structure 10. The waveguide structure typically is similar to optical fiber with round circle cross-section shape. However, its cross section can be other shapes such as rectangle, square and other polygons. It has first core 11, cone structure 12 with tilted surface 13, second core 14, cladding layer 16 and protect jacket 17. The materials for core, the cone structure and cladding layer are either transparent glasses or polymers or their combinations for the interested wavelength. The absorption loss of the medium materials should be low for the interested wavelength of light. The refractive index of first core with cone structure, second core, cladding and protect jacket materials are n1, n2, n3, and n4 respectively.
There are numbers of identical cone structures on the interface of core and cladding layer. The tilted angle of cone structure surface 13 is α relative to axis 18 of the waveguide. Its value can be either positive or negative. α is positive when the tilted surface is tilted toward the medium with lower index relative to axis of waveguides. α is negative when the tilted surface is tilted toward the medium with higher index. The pitch of cone structure, P, is ranged from wavelength level to whole device length. However, diffraction effect is negligible when the pitch of cones is much larger than the interested wavelength, λ, of incident light.
The distance from the outside surface of cladding layer to the edge of tilted surface, t, is ranged from on the order of wavelength of incident light to as large as core size waveguide, D. The range of t is given by

λ≦t≦(D−2P tan α)/2

However, the diameter of waveguide structure, D, should be compatible to the size of light source or its image, which can be up to multi-centimeter level or even bigger, depending on applications. The whole length of devices is

L≧D/tan βout

The outside surface wall of the second core layer is parallel to the axis of the device.

A light beam 20 with maximum propagation angle, βin, relative to axis 18 of devices is incident on waveguide 10. The lights are limited and propagated in the first 11 and second core 12 regions by total internal reflection on interface 15 of second core 12 and cladding layer 16. The refractive indexes n2, n3 of second core and cladding layer are given by

n3=n2 cos βin

For the output light beam 21 with maximum desired output propagation angle, βout, the refractive indexes n1, n2 of first core and second core are given by

n2=n1 cos(βout−α)

The lights will either be reflected by the tilted surface 13 or transmit through the interface of first core and second core. For the lights reflected the tilted surface 13 once, the propagation angle will be reduced by

Δβr=2α

For the lights transmitting through the first core 11 once, the propagation angle will be reduced by

Δβt≧2(n1−n2 )α

After multi-reflections and transparent through the first core, all of lights will be converted into a beam with maximum propagation angle βout, which can be as small as diffraction limited.

When the pitch of cone structure 12, P, is same as the length of devices, L, the device is simplified to the one 22 which has a cone structure 23 with index n1, as shown in FIG. 2.
An array of cone structures can be located in core regions, as shown in FIG. 3. The cone 24 has start diameter φ1 and end diameter φ2 with tilted angle α. The cones have refractive index n1. The refractive indexes of second core 25, cladding 26 and jacket 27 are n2, n3, n4, respectively. The cones can be arrayed as hexagonal, square or other patterns. The pitch of cones is PA. The lights both transmitted through and reflected by the cone have reduced propagation angle in the same manners described above. Propagation angles of lights are continually decreased when lights propagate through or are reflected by following cones. The lights are guided in core regions by cladding layer 26. The devices length, LA, should be large enough that all of lights will be converted into a beam with maximum propagation angle βout. As a thumb of rules, the device length, LA, is given by

LA≧0.1N ln NPA/tan βout

N=(βin−βout)/k(n1−n2)α

where k is factor number related to parameters such as refractive indexs n1, n2, tilted angle α, input propagation angle βin and output beam angle βout.

Double or even more cone structure layers can be used to convert more energy of incident beam into beam with smaller divergent angle. The cross section structure of waveguide is shown in FIG. 4. This waveguide structure is fundamentally the same as one in FIG. 1 except double or even more cone structure 28 and 29 with tilted surfaces are used. These structures will provide more flexible design parameters. For the lights transmitting through the cores once, the propagation angle will be reduced by

Δβt≧2ΣΔni·αi

Other parameters such as refractive index of cores and cladding, device length can follow the way described above. After multi-reflections and transparent through the first core, all of lights will be converted into a beam with maximum propagation angle βout, which can be as small as diffraction limited.

To more efficiently reduce the divergent angle of incident light beams, multi-section of waveguide structure as shown in FIG. 5 can be used. The waveguide 30 has i section of structure from first section 31 to ith section 37. Each section, for example section 31, has first core 32, cone structure 33, second core 34, cladding 35 and protect jacket 36, which is similar to the structure shown in FIG. 1. The refractive index of core, cone structure and cladding of ith section 35 are n1i, n2i and n3i, respectively. The refractive index of jacket is n4. The tilted angle of cone structure is αi. The pitch of cone structure of ith section 35 is pi. The maximum divergent angle of input beam for ith section is βini and the divergent angle of output beam for ith section is βouti. The refractive index of cores for ith section is given by

n2i=n1i cos(βouti−αi)

The refractive index of cladding for ith section is given by

n3i=n2i cos βini

The length of ith section for the diameter of waveguides Di is given by

Li≧Di/tan βouti

The divergent angle of output beam, βouti, corresponded to ith section is given by

βouti=ηiβini

where η is referred as angle squeezed constant of devices, which is related to refractive index of medium, the titled angle.

The final divergent angle of light beam after the devices is given by

βout=βinη1η2 . . . ni

The divergent angle of output light beam can be as small as desired in the manner as described above after passing multi-sections of the invented devices. All of wavelength and most of input energy will be converted into the beam with small divergent angle.
As an example of applications, it is shown in FIG. 6 that lights from conventional light source lamp are coupled into a fiber with small core size. Lights from a conventional light source 41 are reflected on a elliptic mirror 42. Lights 43 are focused on invented directional beam generator 44 with incident angle βin related to axis 45 of the system. The divergent angle of output beam 46 from directional beam generator 44 was reduced to βout, which is substantially smaller than the incident angle βin. The output beam 46 is efficiently focused to a fiber 48 with small core diameter Φ by a lens 47. The focus length of the lens is f. The core diameter, Φ, of the fiber is given by

Φ=2f·tan βout

## Claims

1. A waveguide device reducing divergent angle of input beam from βin to βout with fiber waveguide geometry comprising:
a first core with cone structure with tilted angle α; a second core surrounding said first core with said cone structure; a cladding surround second core with lower index than said second core materials and a protect jacket wherein cone arrays as said first core are located inside said second core with array pitch PA.

2. Fiber in claim 1, wherein said first core with said cone structure, said second core and cladding are comprised of transparent materials with low absorption losses for the interested wavelength including glasses or polymers or their combination with refractive index n1, n2, and n3.

3. The refractive index of said first core and said second core in claim 1 for desired divergent angle βout of output beam with relationship of n2=n1 cos(βout−α).

4. The refractive index of said second core and said cladding in claim 1 for said divergent angle βin of input beam with relationship of n3=n1 cos βin.

5. The fiber in claim 1, wherein said divergent angle of output light beam from said fiber is reduced to βout=ηβin with 0<η<1.

6. The fiber in claim 1, wherein the length of said fiber device is given by L≧D/tan βout with diameter D of said second core.

7. The fiber in claim 1, wherein said distance from the outside surface of said cladding layer to the edge of said tilted surface, t, is ranged by λ≦t≦(D−2P tan α)/2.

8. The fiber in claim 1, wherein said propagation angle of lights reflected once on said tilted surface is reduced by Δβr=2α.

9. The fiber in claim 1, wherein said propagation angle of lights transmitting once on said first core is reduced by Δβt≧2(n1−n2)α.

10. The fiber in claim 1, wherein said cross section shape of said waveguide structure is circle, square, rectangle or other polygons.

11. The fiber in claim 1, wherein said first core has a cone structure with said tilted angle α.

12. The fiber in claim 1, wherein the length of devices is given by LA=0.1PAN ln N/tan βout.

13. The fiber in claim 1, wherein double or even more said cone core structures with titled surface are used.

14. The fiber in claim 1, wherein said propagation angle of lights transmitting once on said cores is reduced by Δβt≧2ΣΔniαi.

15. Said fiber in claim 1 comprising multi-sections waveguide structure, wherein said each section of waveguides has first core, second core with cone structure, cladding and protect jacket.

16. Multi-sections waveguide structure in claim 15, wherein said refractive index of said core and cladding for ith section waveguide with said desired divergent angle βini of input beam are given by n3i=n2i cos βini.

17. Multi-sections waveguide structure in claim 16, wherein said refractive index of said first core and said second core for ith section waveguide with said desired divergent angle βini of output beam are given by n2i=n1i cos(βouti−αi).

18. The fiber of claim 16, wherein said divergent angle of output light beam from said ith fiber section is βouti=ηiβini with 0<ηi<17.

19. Multi-sections waveguide structure in claim 16, wherein said divergent angle of output beam from (i−1)th waveguide is said divergent angle of input beam to said ith waveguide section (βout(i−1)=βini).

20. The fiber of claim 16, wherein the length of said fiber section is Li≧Di/tan βouti with diameter of said cladding Di.

21. Multi-sections waveguide structure in claim 16, wherein said final divergent angle of output light beam is given by βout=βinη1·η2 . . . ηi.