SG182818A1

SG182818A1 - A waveguide structure and a method of coupling light

Info

Publication number: SG182818A1
Application number: SG2012056727A
Authority: SG
Inventors: Kazuya Takayama
Original assignee: Nitto Denko Corp
Priority date: 2010-06-28
Filing date: 2010-06-28
Publication date: 2012-09-27
Also published as: WO2012002905A1

Abstract

A planar waveguide structure, a method of coupling light into or out of a planar waveguide structure, a method of forming a planar waveguide structure and a method of coupling light out of a planar waveguide structure are provided. The planar waveguide structure comprises an array of first light reflective structures formed in a first medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle θ1 relative to a longitudinal plane of the planar waveguide structure for reflecting light into a second medium of the planar waveguide structure; an array of second light reflective structures formed in the second medium, each second light reflective structure having a second surface with a second angle θ2 relative to the longitudinal plane of the planar waveguide structure; wherein the second angle θ2 and the first angle θ1 are inter-related for propagating light via internal reflection in the second medium.

Description

A Waveguide Structure And A Method of Coupling Light

FIELD OF INVENTION

:

The present invention relates broadly to a planar waveguide structure and to a method of coupling light into or out of a planar waveguide structure.

BACKGROUND )

To couple light between a light emitting element or a light receiving element and an optical waveguide, there is typically a problem of low light coupling efficiency.

A number of current solutions for light coupling are discussed below. p Diffraction gratings have been used to change the light direction by diffraction and allow changing the ‘incident light angle to a desired direction. However, diffraction gratings are limited to specific wavelengths of light. Also, the coupling : efficiency between the light source and the waveguide is typically low in applications utilizing diffraction gratings. 45-degree mirrors have also been used to change the light direction by 90 degrees via reflection. However, the coupling efficiency between the light source and the waveguide is typically low in applications utilizing such mirrors. Furthermore, as the accuracy of mirror angle, position and surface flatness can affect the coupling efficiency, it is typically more difficuit to use 45-degree mirrors. :

It is appreciated that alignment of mirrors or reflectors is typically a problem.

In US 6473220, JP2008216936 and EP1102096, various. reflectors are used to change light direction. However, in these documents, mirror angle and therefore mirror alignment is crucial in achieving the results desired.

Prism coupling has been used for phase matching between a propagation ~ - constant and incident light using high index prism. However, prisms are typically expensive and a degree of alignment is still required for light coupling. Further, an evanescent wave method has also been used whereby a propagation mode is excited to facilitate light coupling. However, such a method uses a very thin waveguide and is typically not efficient. a ] . + In view of the above, there exists a need for a planar waveguide structure and "a method of coupling light into or out of a planar waveguide structure that seek to address at least one of the above problems. ]

SUMMARY oo

In accordance with an aspect of the present invention, there is provided a - planar waveguide structure, the structure comprising an array of first light reflective -structures formed in a first medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure for reflecting light into a second medium of the planar waveguide structure; an array of second light reflective structures formed in the second medium, each second light reflective structure having a second surface with a second angle 6, relative to the longitudinal plane of the planar waveguide structure; wherein the second angle 8, and the first angle 6, are inter- related for propagating light via internal reflection in the second medium. :

The array of the second light reflective structures may be formed as a textured surface of the second medium.

The textured surface may be disposed at an interface between the second medium and a third medium of the planar waveguide structure.

The textured surface may comprise a micromirror array.

The array of first reflective structures may be disposed at an interface between the first medium and the second medium.

A filler medium of each first reflective structure may have a lower refractive index than the first medium. =

The filler medium of each first reflective structure may comprise air.

A filler medium of each second reflective structure may have a lower refractive index than the second medium. :

The filler medium of each second reflective structure may comprise air. 0, may be based on . a first constraint 90 sin” ("i Jsin(180-26, +6,) |-tan™ LL 226, : \ n, L/ 1 h, /tan6, where ny is the refractive index of the first medium, n; is the refractive index of the second medium, d,is a light incident angle for light incident on the first surface, L, is a length of a base of each second light reflective structure and h, is a height of each second light reflective structure. &, may be based on a second constraint 26, 2 sin”(", (4 JsinG180-24 +0) oo n, n, where n, is the refractive index of the first medium, n, is the refractive index of the second medium, 6,is a light incident angle for light incident on the first surface, L, is a length of a base of each second light reflective structure and h; is a height of each ~ second light reflective structure.

-Each first light reflective structure may be formed with a base length of between about 2 um to about 20 pm.

Each second light reflective structure may be formed with a base length between a range of about 4 pm to about 200um. ~The second medium may be formed as a waveguide with a thickness ina range of about 50um to about 200pm.

In accordance with another aspect of the present invention, there is provided a method of coupling light into or out of a planar waveguide structure, the method . comprising using an array of first light reflective structures formed in a first medium of the planar waveguide structure to reflect light into a second medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure; using ~ an array of second light reflective structures formed in the second medium to “propagate light via internal reflection in the second medium, each second light reflective structure having a second surface with a second angle 0, relative to the longitudinal plane of the planar waveguide structure; and wherein the second angle 6,and the first angle 6, are inter-related.

The array of the second light reflective structures may be formed as a textured surface of the second medium. : | The textured surface may be disposed at an interface between the second medium and a third medium of the planar waveguide structure.

The textured surface may comprise a micromirror array.

: A filler medium of each first reflective structure may have a lower refractive index than the first medium. oo

The filler medium of each first reflective structure may comprise air.

A filler medium of each second reflective structure may have a lower refractive index than the second medium.

The filler medium of each second reflective structure may comprise air. 0, may be based on a first constraint 90 -sin™ ", Jsin(180-24, +6) tan >26, nh, L, — 1 . h, /tan@, where n; is the refractive index of the first medium, n, is the refractive index of the second medium, &.is a light incident angle for light incident on the first surface, Lois a length of a base of each second light reflective structure and h; is a height of each second light reflective structure. 0, - may be based on a second constraint 26, 2sin”(", (4 Jsin@180-26, +6) n, hn, where ng is the refractive index of the first medium, ns is the refractive index of the second medium, 6,is a light incident angle for light incident on the first surface, L, is a + length of a base of each second light reflective structure and h, is a height of each second light reflective structure. :

Each first light reflective structure may be formed with a base length of between about 2 ym to about 20 pm.

oo : 6

Each second light reflective structure may be formed with a base length between a range of about 4 pm to about 200um. ~The second medium may be formed as a waveguide with a thickness in a range of about S0um to about 200pm. So :

In accordance with yet another aspect of the present invention, there is provided a method of forming a planar waveguide structure, the method comprising forming an array of first light reflective structures in a first medium of the planar } waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure for reflecting light into a second medium of the planar waveguide structure; forming an array of second light reflective structures in the second medium, each second light reflective structure having a second surface with a second angle 0, relative to the longitudinal plane of the planar waveguide structure; wherein the second angle 6, and the first angle 6, are inter-related for propagating light via internal reflection in the second medium. oo The method may comprise forming the array of the second light reflective structures as a textured surface of the second medium.

The method may further comprise forming the textured surface at an interface between the second medium and a third medium of the planar waveguide structure. oo oo

The method may further comprise forming the textured surface comprising a micromirror array. oo

The method may comprise forming the array of first reflective structures at an interface between the first medium and the second medium.

A filler medium of each first reflective structure may have a lower refractive index than the first medium.

The filler medium of each first reflective structure may comprise air. : : Co

The filler medium of each second reflective structure may comprise air. | Co 0, may be based on a first constraint ~ 90—sin”! yA Jsints0-25, +6,)|-tan™ _ 1 |s29, oo h, L/ _1 : h, /tan6, where n, is the refractive index of the first medium, n; is the refractive index of the second medium, ,is a light incident angle for light incident on the first surface, Lisa length of a base of each second light reflective structure and h; is a height of each second light reflective structure.

CA may be based on a second constraint 20, > sin”(", J-sin((, Jsin(180-26, +6) | oo n, n, ‘where n, is the refractive index of the first medium, ns is the refractive index of the : second medium, 6,is a light incident angle for light incident on the first surface, L is a - length of a base of each second light reflective structure and h; is a height of each second light reflective structure.

The method may further comprise forming the first light reflective structure with a base length of between about 2 um to about 20 pm.

- The method may further comprise forming the second light reflective structure with a base length between a range of about 4 um to about 200um.

The method ‘may further comprise forming the second medium as a waveguide with a thickness in a range of about 50um to about 200um. ~ In accordance with another aspect of the present invention, there is provided a planar waveguide structure, the structure comprising an array of first light reflective structures formed in a first medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure for reflecting light propagating in the planar . waveguide structure out of the first medium into a second medium of the planar waveguide structure; an array of second light reflective structures formed in the second medium, each second light reflective structure having a second surface with a second angle 6, relative to the longitudinal plane of the planar waveguide structure for reflecting light out of the second medium. | :

In accordance with another aspect of the present invention, there is provided a method of coupling light out of a planar waveguide structure, the method - 20 comprising using an array of first light reflective structures formed in a first medium ofthe planar waveguide structure to reflect light propagating in the planar waveguide structure out of the first medium into a second medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure; using an array of second light reflective structures formed in the second medium to reflect light out of the second medium, each second light reflective structure having a second surface with a second angle 6, relative to the longitudinal plane of the planar waveguide structure.

BRIEF DESCRIPTION OF THE DRAWINGS | oo

Embodiments of the invention will be better understood and readily apparent ) to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1 is a schematic diagram illustrating a light coupling device in an example embodiment. oo

Figure 2 is an exploded view of a box in Figure 1.

Figure 3is a schematic diagram illustrating light reflection in a first medium in an example embodiment. So :

Figure 4 is a schematic diagram illustrating light reflection in the second medium in an example embodiment. : Figure 5 shows internal reflection of light at an interface between a first medium - and a second medium in an example embodiment. :

Figure 6 is a graph for constraints 1 and 2 in an example embodiment. " Figure 7(a) is a graph showing beam width and beam distance results as base size of a first reflective structure varies in an example embodiment.

Figure 7(b) is a graph showing reflection efficiency as a base size of a first reflective structure varies in an example embodiment.

Figure 8(a) is a graph showing reflection efficiency and relative shift results as base size of a second reflective structure varies in an example embodiment.

Figure 8(b) is a graph showing coupling efficiency results as the base size of a second reflective structure varies in an example embodiment.

Figure 9 is a graph showing constraints 1 and 2 results using exemplary values in an example embodiment.

Figure 10 is a graph showing coupling efficiency for different 6, and 0, values in an example embodiment. :

Figure 11 is a schematic flowchart illustrating a method of coupling light into or out of a planar waveguide structure in an example embodiment. : Figure 12 is a schematic flowchart illustrating a method of forming a planar waveguide structure in an example embodiment.

Figure 13 is a schematic flow diagram illustrating fabrication of a planar waveguide structure in an example embodiment. : }

Figure 14 is a schematic flowchart illustrating a method of coupling light out “of a planar waveguide structure in an example embodiment.

DETAILED DESCRIPTION

Figure 1 is a schematic diagram illustrating a light coupling device/waveguide structure in an example embodiment. The device 100 comprises an array of light reflective structures such as a microprism array 102, 114 disposed. in a first medium e.g. a cladding layer 104. The device 100 further comprises a textured surface structure 106 disposed in a second medium e.g. a guiding/core layer 108. The core layer is also referred to as a waveguide in the description. In the example -embodiment, a light emitting element 110 is positioned adjacent the microprism array 102 that is associated with the textured surface structure 106. A light receiving +30 element 112 is positioned adjacent a second microprism array 114 that is associated with a second textured surface structure 116. In the example embodiment, the © microprism arrays 102, 114 and textured surface structures 106, 116 are made up of an air gap or a material with a lower refractive index than the refractive index of the respective medium the arrays/textured surface structures are disposed in. A thin additional cladding layer (not shown) may be provided between the cladding layer 104 (including the microprism arrays 102, 114) and the waveguide 108. oo

Co Figure 2 is a detailed view of box 118 of Figure 1. In ‘the example embodiment, fight is coupled from a light source 110 to an optical waveguide (the guiding layer 108). Two arrays of light reflective structures (e.g. the microprism array 102 and the textured surface structure 106) comprise arrays of reflective planes 202, 204 respectively disposed at angles to the plane 206 of the optical waveguide 108.

The angles for the reflective planes 202, 204 in the example embodiment preferably satisfy a number of constraints to meet a wave propagation condition and for achieving an optimum reflection efficiency. | Co

In the description herein, although the light is described as being coupled between the microprism array 102 and its associated textured surface structure 106 (i.e. coupling of light from the light emitting element 110 into the waveguide 108), it will be appreciated that the same principles apply for coupling light between the second microprism array 114 and the associated second textured surface structure 116 (i.e. coupling of light from the waveguide 108 into the light receiving element 112). oo

In the example embodiment, an optimised-sized microprism array 102 can enhance light coupling efficiency into the waveguide/guiding layer 108 without using precise alignment operations with the textured surface structure 106. In the example embodiment, incident light 208 is reflected at the microprism array 102 in the cladding layer 104 such that the light is refracted and coupled (see numeral 210) into the core layer 108. The light is reflected at the textured surface structure 106 (see numeral 212) of the core layer 108. The light is reflected (see numeral 214) at the interface 216 between the cladding layer 104 and the core layer 108. It will be appreciated that the textured surface structure 106 can be in the form of, but not limited to, a micromirror ~ array to facilitate light reflection. In the example embodiment, an array of microprism is ‘used instead of a single prism or single mirror so that prior alignment with a light emitting source or a light receiving element is advantageously not needed.

Thus, the above example embodiment can provide a structure, and a method of designing such a structure, that can improve light coupling efficiency between a light : source and an optical waveguide and/or a light-receiving element.

Co ~ Figure 3 is a schematic diagram illustrating light reflection in a first medium 302 in an example embodiment. The first medium 302 corresponds to the cladding layer 104 in Figures 1 and 2. An array of first reflective structures 304, 306 are constructed in the first medium 302. The first reflective structures 304, 306 are - triangular in shape. For example, the structure 304 has an angle 4,, height hy, length

L4, and spacing W; to the structure 306 for light passage into a second medium 307. The second medium 307 corresponds to the core layer 108 in Figures 1 and 2. Cl in the example embodiment, an incident light (shown schematically at 308) emitted from a light source (compare 110 of Figure 1) having an incident angle 0. 310is reflected at an oblique side 312 of the triangle of the reflective structure 304. The angle between the other oblique side 314 and the interface 316 between the first medium 302 : and the second medium 307 can be more than about 90 degrees. Such a triangular shape can provide ease of production e.g. using a manufacturing process utilising a mold. In general, using a mold, a bigger apex angle is easier to press and remove.

However, it will be appreciated that a bigger apex angle translates into a longer L,, and the L, area’blocks the light passage into a second medium 307.

In the example embodiment, the ratio of reflected incident light (A) 318 to total incident light (A+B) 318,320 is calculated as follows:

A= ih ane, : ] tang, ) : 1

A (n/tan6)+h tans, :

A+B W +L : (3)

It will be appreciated that the reflected light A 318 may be interrupted by the adjacent reflective structure 306 (see numeral 322). The ratio of non-interrupted reflected light (C) 324 to total reflected fight (C+D) 324, 326 is calculated as follows:

C:C+D=W,:W,+L' @ where Lis the length of structure 306 that interrupts the reflected light, rf Nw _h oo

L -( tan(26,-6,-90)) ~~ Jiang, oo ©

Cc __m_ | Co

C+D W +L ) oo So ~ w, oo (A /tan(26, - 6, ~90))- A, /tan 6, (6)

Therefore, the ratio between the incident light (A+B) 318,320 and the light going through uninterrupted to the second medium 307 is calculated using equations (3) and (6) as follows: 4 C _(m/tan6,)+ hy tan 6, w,

A+B C+D w,+L (h/tan(26,-6,—90))—h /tan6,

Wi+L, (hy /tan(26, — 6, -90))~ hy /tan 6, __m tan 6, tang, +1

W,+L, (tan, /tan(26, —6,~90))-1 oo @

The first part of equation (7) comprises W, and L, and is referred to as a “receptive window opening ratio” and the second part comprises d,, 8, and h, and is referred to as an “angular efficiency”. .

The non-interrupted reflected light having an incident angle 90-(26,-6,- 90), which is equal to 180-2 8,+0,, is refracted at the interface 316 between the first medium 302 and the second medium 307. The refraction angle 6,328 is derived from Snell's

Law.

8, —sin”((, Jsin@180-26 +6) ? (8) : where n; is the refractive index of the first medium 302 and n, is the refractive index of the second medium 307. It will be appreciated that the refraction angle 0, 328 is wavelength dependent because both refractive indices ‘ny and n, typically have wavelength dispersion. In the example embodiment, the wavelength region for the light is visible light wavelength, such as from about 400 nm to about 800 nm.

Figure 4 is a schematic diagram illustrating light reflection in the second " medium 307 in the example embodiment. The light introduced into the second medium 307 having the angle &, 328 is reflected by an array of second reflective structures 402, 404 constructed in the second medium 307. In the example embodiment, : the array of second reflective structures 402, 404 is constructed at an interface 403 between the second medium 307 and a third medium 405. The second reflective . structures 402, 404 are triangular in shape. For example, the structure 402 has an angle 0, with a first oblique side, height h,, and length L,. The structure 404 shows an angle 0," with a second oblique side. In the following description, the structures 404, 406 are discussed interchangeably with the physical and angular characteristics.

The ratio of the light being reflected by an effective reflective plane (E) 406 of the reflective structure 404 to all of light introduced into the second medium 307 and contacting the structure 404, i.e. (E+F) 406, 408 is calculated as follows: - _ E=("amo )-tutang,

Stan) (©)

E+F=1L, n _h/ wh tan 6, tan e, (1 0) 1 — - :

E Jans, tang, +F 1/4 1/

E+E Jano, Jane, : (11) 3 - | Lo.

: 15 Co

The light entering the second medium 307 (shown schematically at 410) reflects © at a reflective plane 412 and has an incident angle 6, +6, . The light may be interrupted by the adjacent reflective structure 404. The angle 6,414 is between the incoming light © 410 and the reflective plane/surface 412.

IE

The angle a 416 is between reflected light 418 and the oblique side 420 of the adjacent triangle structure 404. The ratio of non-interrupted light (G) 422 to all light (G+H) 422, 424 reflected by the effective reflective plane 412 is calculated as follows:

G+H="/, | | . : sind, | - (12)

Also, the relationship among a, 6,, 8,, 6,’ can be shown as follows by considering angle theorem around the interface 316 and the formed triangle comprising the three sides which are the reflective surface 412, the reflected light line 418 and the . oblique side 420 of the adjacent triangle structure 404 : a+6,-6,=6, a=6,+6,-6, (13) : n=" Jesinax—— . sin 6, sin 8, _hysin(6, +6, -6,) sind, sin 6, (14)

Thus, the ratio of non-interrupted light (G) 422 to all light (G+H) 422, 424 reflected by the effective reflective plane 412 is _G__,_sing,sin(6,+6,-6,)

G+H sind, sin, (15)

Using equations (11) and (15), the ratio of the incident light to the reflective structure 402 in the second medium 307 to the non-interrupted light being reflected by the effective reflective plane 412 is calculated as follows:

EG _ Vang, = 200, [1-060] So

E+F G+H Vine, * Vane: sin &, sin 6, : © _l-un6, ung, {- sin 8, sin(6, 4-6.) hE TEA ,-\ sin@;siné, oo tnd; | (16)

The first part of equation (16) is referred to as “the ratio of effective reflective area” and the second part is referred to as “the structural efficiency”. Also, it will be appreciated that the equation (16) is wavelength dependent because it contains 6; as a parameter, 6; being wavelength dependent. oo oo : In order to maximize the structural efficiency, the angle a, which is equal to 0,+6,’-0, under equation (13), preferably satisfies the following condition, which means that all the reflected light of structure 402 is not interrupted by the oblique side 420 of adjacent triangle/structure 404. . a<0 6,+6,-6,<0 6,-6,>0, a7 where, ) 0; +6, +6,=90 6, =90-6,-6, oo (18) | | :

Figure 5 shows internal reflection of light 502 at the interface 316 between the first medium 302 and second medium 307. The relationship in equation (18) is shown in

Figure 5.

Further to the above, the second term of equation (16), such as G/(G+H), can be omitted using this condition since all the reflected light under the condition can transmit - to the interface 316 between the first medium 302 and the second medium 307.

Therefore, the condition for a can be rewritten in terms of 6, using equations (17) and (18) as follows: : 90-6, 6, 226, oo | | (19) - By using equation (10), an equation (20) for 8,’ can be obtained as follows: _h h, : b=" ne, Vane - ” 1 . . : . rr _ -1 .

For the purposes of description, equation (21) is also referred to as constraint 1.

In the example embodiment, the reflected light by the second reflective structure 402, having an incident angle 26, +6, to the interface 316 between the second medium 307 and the first medium 302 preferably satisfies a total internal reflection condition, ie.20,+6,26,

To derive the condition, let 8, be the critical angle at the interface 316 between the second medium 307 and the first medium 302. : 4 = sin”(", ) - : (22) : From Figure 5, for total internal reflection to occur,

90-(6, -6,)> 6, | | (23) ~ From equation 18, 6, =90-6,-6, : :

Therefore, using equations (18) and (23), 26,+6,26, | (24) ~ Therefore, using equations (8) and (22), another condition for the angle 6, is derived as follows: . 26, sin") J-sin”(( Jsint180-24 +6) 2 2 : (25)

For the purposes of description, equation (25) is also referred to as constraint 2.

Figure 6 is a graph for constraints 1 and 2 in an example embodiment. The graph "is plotted using the following equations. ’ 90~ sin" ("i Jsin@180-24, +6) |~tan| 1 ___|_29 (26) n, i L/ oo h, tan, 26, = sin”, J-sin”((% Jsin(180-26, +6, ) 27) n, n, :

The area 602 confined by the two constraints in the 6,-0, space is available for designing the light coupling device in the example embodiment. it will be appreciated that the equations (26) and (27) and therefore, the area 602 are wavelength dependent because both refractive indices n, and n; in the equations (26) and (27) have wavelength dispersion. } : oo Apart from identifying the two constraints for the design, size effects of the first and second reflective structures e.g. 304, 306 (Figure 3) and 402, 404 (Figure 4) . are also described. It has been recognised that reflected light by the first reflective structures e.g..304, 306 (Figure 3) may have diverging angles due to a diffraction effect of the reflective structures’ edges or size.

oo Figure 7(a) is a graph showing beam width and beam distance results as a base size of the first reflective ‘structure varies in an example embodiment. Using 6, = 0, 6, =60, ne=1.5464( 1 =532nm) where A is the wavelength of a light source used, n=1.6024( A =532nm), and ng.=1.0(air) where ng, is the refractive index of a first reflective structure, the relationship between the beam distance from the reflective structure and the reflected light beam width (1/e) for different L, is calculated by numerical simulation. Results are shown in Figure 7(a). Plot 702 shows the results for L, "at about 1 um, plot 704 shows the results for L, at about 2 um, plot 706 shows the

I0 results for L, at about 3 pm and plot 708 shows the results for L, at about 4 um. }

Using the above values and with a waveguide thickness of 50 um, the reflected light at a first reflective structure e.g. 304, 306 (Figure 3) reaches a second reflective structure: e.g. 402, 404 (Figure 4) when the distance between two reflected points is about 90 ym. In addition, a repetition period of the first reflective structure is 3xLy, ie. WwW, equals to 2xL,.

By comparing the beam width at 90 um distance (see numeral 710) in Figure 7, it is recognised that a smaller L, can advantageously lead to a bigger beam width. This is because each reflected light from a smaller first reflective structure desirably overlaps more, and the intensity of incident light reflected to a second reflective structure is thus more uniform. If intensity is more uniform, it can be beneficial/ladvantageous for manufacturing as an alignment process of the two layers of first and second reflective structures can be omitted. : : | Co

Figure 7(b) is a graph showing reflection efficiency as a base size of the first reflective structure varies in an example embodiment. The parameters used are substantially identical to the conditions for obtaining Figure 7(a). It can be observed that reflection efficiency increases as the base size L, increases. For example, at numeral 712, the efficiency is about 26% for L, at about 1 um while at numeral 714, the efficiency is about 33% for L, at about 4 pm.

oo 20

In view of the trends observed from Figures 7(a) and (b), the size of the first - reflective structures e.g. 304, 306 (Figure 3), in terms of L;, is preferably in a range of ‘about 2 Hm to about 20 pm, more preferably in a range of about 4 ym to about 10 pm. - Figure 8(a) is a graph showing reflection efficiency and relative shift results as the base size of the second reflective structure varies in an example embodiment.

Relative shift refers to a ratio of a shifted distance to the base size of the second reflective structure. For example, given a base size of 6 um for the second reflective structure and the second reflective structure is shifted 3 ym from its initial position, the relative shift is 0.5. In the example embodiment, the first reflective structure is stationary.

Using 6,= 0, 6,=60, n,=1.5464(1=532nm) where A is the wavelength of a light source used, n,=1.6024( 4 =532nm), ng;=1.0(air) where Ng, is the refractive index of the first reflective structure, ng,=1.0(air) where ng, is the refractive index of the second reflective structure, L+=1 um, and 6,=6,’=10, and the waveguide thickness is taken as about 50 um, the relationship between the reflection efficiency of the second reflective structure and the relative shift for different sizes of L; is calculated by a numerical simulation. - Results are shown in Figure 8(a). Plot 802 shows the results for L, at about 5um, plot 804 shows the results for L, at about 4um, plot 806 shows the results for L, at about

Bum, plot 808 shows the results for L, at about 3um, plot 810 shows the results for L, at about 2um and plot 812 shows the results for L, at about 1um. ‘Using the above values, it is recognised that the reflection efficiency of the second reflective structure fluctuates when the size, L,, becomes larger. The average efficiency for L, at about 4 pm, about 5 ym and about 6 um (see numerals 804, 802, 806 respectively) shows similar values with periodicity in relation to relative shift. The efficiency for L, at about 1um, about 2um and about 3um (see numerals 812, 810, 808 respectively) appear constant at each relative shift. This may indicate that there is less ‘reflection effect’ when the L; is less than about 3 um.

Figure 8(b) is a graph showing coupling efficiency results as the base size of a second reflective structure varies in an example embodiment. The parameters used are substantially identical to the conditions for obtaining Figure 8(a). Plot 814 shows the

: | Co 21 coupling efficiency results for L, at about 1 um as the base size of the second reflective structure L, varies, plot 816 shows the resuits for L, at about 2 um as L, varies, plot 818 * . shows the results for L, at about 3 pm as L, varies, plot 820 shows the results for L, at about 4 um as L, varies. It can be observed that the coupling efficiency shows an increasing trend as L; increases.

In view of the trends observed from Figures 8(a) and (b),, the size of the second reflective structures e.g. 402, 404 (Figure 4), in terms of L,, is preferably from about 4

Hm to about 200pum, more preferably about 10 pm to about 50um. Preferably, during 0 implementation, the L, size is larger than the L, size selected.

Thus, based on the above size analysis, by combining the first and second reflective structures having the sizes investigated, the light from a light source with a large area can advantageously be coupled into a planar optical waveguide effectively with reduced or no alignment constraint.

In an example implementation, using 8, = 0, 8,=6,’, n;=1.5464( 2 =532nm), n;=1.6024( A =532nm), nr,=1.0(air), and ng,=1.0(air), the available conditions for light coupling can be determined. - : oo . Figure 9 is a graph showing constraints 1 and 2 of equations (21) and (25) plotted using the above values in an example embodiment. Plot 902 shows the results for constraint 1 (compare equation (21)) and plot 904 shows the results for constraint 2 (compare equation (25)). The area 906 is shows the range of available 6,and 8, values available in the above environment. :

Thus; for different 6,and 8, , there is an available range to satisfy the above values of reflective indices. The light coupling efficiency is calculated from,

A+B C+D E+F (28) that is, a product of equations (7) and (11). This assumes that there is no reflection loss at the interface between the first medium having n, and the second medium having n,. It is assumed that all reflected light by a second reflective structure is not interrupted by an adjacent reflective structure in the same medium. Thus, H=0 and therefore, G/(G+H)=1 + and can be omitted from equation (28). : | :

Figure 10 is a graph showing coupling efficiency for different 6,and 6, values in an example embodiment. The parameters used are substantially identical to the conditions for obtaining Figure 9. In the example embodiment, to obtain the coupling efficiency, equations (3) and (6) are considered separately to obtain the product in equation (7). It is assumed that W/Li=2 and L,/L,=2. This is an example that all light enters into the second medium (containing the second reflective structures), and is a different scenario from the drawing of Figure 3 in which some light hits or is interrupted : by an adjacent reflective structure. Thus, Figures 9 and 10 simply depend on angles only. From Figure 10, at 6,=60 and 6, =10 degrees, the calculated coupling efficiency is about 12.2% (see numeral 1002). :

Figure 11 is a schematic flowchart 1100 illustrating a method of coupling light into or out of a planar waveguide structure in an example embodiment. At step 1102, ‘an array of first light reflective structures formed in a first medium of the planar waveguide structure is used to reflect light into a second medium of the planar ‘waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure. At step 1104, an array of second light reflective structures ‘formed in the second “medium is used to propagate light via internal reflection in the second medium, each second light reflective structure having a second surface with a second angle 6, relative to the longitudinal plane of the planar waveguide structure. At step 1106, the ~ second angle &, and the first angle 0, are inter-related. : Figure 12 is a schematic flowchart 1200 illustrating a method of forming a planar waveguide structure in an example embodiment. At step 1202, an array of first light reflective structures is formed in a first medium of the planar waveguide “structure, each first light reflective structure having a first surface with a first angle 6,

oo : 23 relative to a longitudinal plane of the planar waveguide structure for reflecting light into a second medium of the planar waveguide structure. At step 1204, an array of ~ second light reflective structures is formed in the second medium, each second light reflective structure having a second surface with a second angle 6, relative to the longitudinal plane of the planar waveguide structure. At step 1206, the second angle ¢, and the first angle 6, are inter-related for propagating light via internal reflection in the second medium. )

Based on the above described example embodiments, two sets of reflectors : 10 . each comprising an array of light reflective structures fabricated by for example, but not limited to, stamping, molding or embossing can be provided. The angles for the reflective structures preferably satisfy a region defined by two derived constraints to satisfy a wave propagation condition and to achieve an optimum reflection efficiency.

It has been recognized that a light source or a photodetector placed on a waveguide directly for light propagation by reflection can improve the light coupling efficiency. ~ Thus, a method and structure for coupling light from a light emitter into an optical ‘waveguide and for coupling light from the waveguide to a light receiver can be provided.

Using the above described example embodiments, two layers comprising a specific shaped microprism array and a waveguide having a textured surface structure can be produced by e.g. stamping, injection molding, hot embossing and

UV embossing techniques. Preferably, a thicker multimode waveguide can be constructed with thickness in the range of about 50 ym to about 200 pm.

Furthermore, as alignment of the two layers is reduced or not a constraint, the layer “of irregular waveguide can advantageously be simply picked and placed on the first layer of specific shaped microprism array. Using the above described example embodiments, incident light can be designed such that it is reflected twice using two reflectors to effectively couple light into a waveguide. The two reflectors are a combination of the specific shape microprism array and the irregular waveguide.

Figure 13 is a schematic flow diagram illustrating fabrication of a planar waveguide structure in an example embodiment. At step (a), a silicon wafer 1302 with v-grooves formed thereon is provided. A polydimethylsiloxiane (PDMS) mold structure 1304 is formed using the silicon wafer 1302. At step (b), using the mold structure 1304 (shown flipped horizontally) with the silicon wafer 1302 removed, clad pre-polymer 1306 is poured onto the mold structure 1304. At step (c), by removing the mold structure 1304, a medium comprising first reflective structures e.g. 1308 is formed (shown flipped horizontal). At step (d), a thin cladding layer 1310 is covered over the first reflective structures e.g. 1308 using a lamination technique. At step (e), a stamper structure 1312 is formed. The stamper structure 1312 comprises protrusion structures e.g. 1314. At step (f), a waveguide core material layer 1316 is spun onto the laminated cladding layer 1310 of step (d). At step (g), the stamper structure 1312 “10 is aligned and stamped onto the waveguide core material layer 1316 (see arrow 1318). The protrusions e.g. 1314 are transferred to the waveguide core material layer 1316 and the stamper structure 1312 is removed. At step (h), the planar waveguide structure 1320 is formed.

In the following description, a fabrication example for forming the planar waveguide structure of Figure 13 is provided. For forming a first reflective structure, a master structure is prepared. One example for making a first reflective structure is using a (100) Si wafer. A SiO; layer is formed by a thermal oxidation process at about 1100 °C. Alternatively, an oxidized Si wafer is available commercially. The SiO, surface is cleaned by using a standard cleaning process such as, but not limited to,

CAROS, SC-1, and Os. A positive photoresist, which has dry etching resistivity against fluorinated etching gas, is spun on the cleaned SiO, surface by a spin-coating method.

The photoresist is then heated at about 100 °C for a pre-baking process. The pre-baked photoresist is then exposed by UV light through a photomask, which is designed in order to make desired line-and-space patterns, by using a mask-aligner. The exposed photoresist is then developed by a photoresist developer to dissolve the exposed area. . The developed photoresist is then heated at about 150 °C for a post-baking process.

The exposed SiO, layer is then removed by the dry etching process by using fluorinated etching gas, such as CF,, and/or CHF. After this process, the Si surface is partially exposed with the designed pattern by the photomask. The etching gas is then changed "to O; for an ashing process to remove the photoresist. The wafer is then cleaned by the

CAROS process in order to remove the photoresist completely. The cleaned wafer is the treated by 1% HF solution for about 2 minutes to remove the thin oxidized layer, which is

25 | oo formed through the ashing process and CAROS cleaning process, on the exposed Si surface. The wafer is then etched by 13% KOH solution at about 85 °C for about 40 min to form the v-shape groove. The wafer with v-groove is then etched by 1% HF solution for about 20 minutes in order to dissolve the patterned SiO, layer. : ~ The master structure prepared by the above process is then transferred to polydimethylsiloxiane (PDMS) for preparing a mold structure. The surface of the Si wafer is then treated by dimethyloctadecylchlorosilane solution in order to enhance the mold release characteristics. The PDMS pre-polymer and the curing agent, such as Sylgard 184 (Dow Corning, Co., MI), are mixed with a ratio of 10 to 1. The well stirred mixture is then placed under a vacuum condition for the deairing for about 15 minutes. The pre- polymer mixture is then poured onto the Si wafer and cured for about 1 hour at about 65 °C followed by a further curing process for about 15 minutes at about 135 °C. The PDMS mold is then taken out from the Si master structure. : The master structure prepared by the above process is then replicated by the

PDMS mold prepared using the above method. The under clad pre-polymer, such as UV curable epoxy, is then poured onto the PDMS mold and cured by UV irradiation for about ~ 410 min. The PDMS mold is then removed after the curing process. The reflective structure is then covered by a thin cladding layer, between about 10 and about 20 micro- meter thickness, by using a lamination technique.

For forming a second reflective structure, a stamper structure is prepared. One example for making a second reflective structure is using a grayscale photomask.

Photoresist is spun on a quartz substrate by a spin-coating method. The thickness of the photoresist is designed in order to obtain a protrusion structure with desired depth and angle by considering the etching selectivity to the quartz. The photomask is designed by considering the photoresist sensitivity, such as the relationship between the UV dosage and residual photoresist thickness. The photoresist is then 30. heated for a pre-baking process. The pre-baked photoresist is then exposed by UV light through the grayscale photomask by using a mask-aligner. The exposed photoresist is then developed by the photoresist developer to dissolve the exposed area. The developed photoresist is then heated for a post-baking process. The quartz substrate is then etched by using fluorinated etching gas, such as CF, and/or © CHF: | oo

For the waveguide fabrication process, the waveguide core material is spun _. on the laminated cladding layer. The waveguide core material is then heated at about 100 °C for a pre-baking process. The stamper structure is then aligned to the wafer and the protrusion structure is transferred. The pre-baked waveguide core material is then exposed by UV light through the photomask by using the mask-aligner. The exposed waveguide core material is then developed by the developer to dissolve unexposed areas. The developed waveguide core structure is then heated at about 150 °C for a post-baking process.

Figure 14 is a schematic flowchart 1400 illustrating a method of coupling light out of a planar waveguide structure in an example embodiment. At step 1402, an array of first light reflective structures formed in a first medium of the planar - waveguide structure is used to reflect light propagating in the planar waveguide = structure out of the first medium into a second medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 0, relative to a longitudinal plane of the planar waveguide structure. At step 1404, an array of second light reflective structures formed in the second medium is used to reflect light out of the second medium, each second light reflective structure having a second surface with a second angle 6, relative to the longitudinal plane of the planar waveguide structure.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific ~ embodiments ‘without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive. :

Claims

1. ~ Aplanar waveguide structure, the structure comprising, - an array of first light reflective structures formed in a first medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure _ for reflecting light into a second medium of the planar waveguide structure; + an array of second light reflective structures formed in the second medium, each second light reflective structure having a second surface with a second angle

10 . 6, relative to the longitudinal plane of the planar waveguide structure; wherein the second angle 8, and the first angle 6, are inter-related for propagating light via internal reflection in the second medium.

: 2. The structure as claimed in claim 1, wherein the array of the second light reflective structures is formed as a textured surface of the second medium.

3. The structure as claimed in claim 2, wherein the textured surface is disposed at an interface between the second medium and a third medium of the planar waveguide structure.

-

4. The structure as claimed in any one of claims 2 to 3, wherein the textured surface comprises a micromirror array. :

5. The structure as claimed in any one of claims 1 to 4, wherein the array of first reflective structures is disposed at an interface between the first medium and the second medium.

6. The structure as claimed in any one of claims 1 to 5, wherein a filler ~ medium of each first reflective structure has a lower refractive index than the first . medium.

7. ~The structure as claimed in claim 6, wherein the filler medium of each + first reflective structure comprises air.

o 8. The structure as claimed in any one of claims 1 to 7, wherein a filler medium of each second reflective structure has a lower refractive index than the second medium.

C9. The structure as claimed in claim 8, wherein the filler medium of each second reflective structure comprises air. : : (J | | .

10. The structure as claimed in any one of claims 1 to 9, wherein 8, is based : on a first _ constraint : 90-sin™ (", Jsin6180-26, +0) tan — > 20, hn, L,/ _1 h, tan, ‘where n, is the refractive index of the first medium, n, is the refractive index of the second medium, 6,is a light incident angle for light incident on the first surface, L, is a length of a base of each second light reflective structure and h; is a height of each second light reflective structure.

11. The structure as claimed in any one of claims 1 to 10, wherein 0, is based on a second constraint 26, > sin” VA J-sin(( JsinG180-25, +6) 2 / where ny is the refractive index of the first medium, n, is the refractive index of the second medium, 6,is a light incident angle for light incident on the first surface, L, is a i length of a base of each second light reflective structure and h; is a height of each second light reflective structure.

12. The structure as claimed in any one of claims 1 to 11, wherein each first light reflective structure is formed with a base length of between about 2 gm to about 20 ym.

- 29 Co : 13. The structure as claimed in any one of claims 1 to 12, wherein each second light reflective structure is formed with a base length between a range of about 4 um to about 200pm. :

14. The structure as claimed in ‘any one of claims 1 to 13, wherein the second medium is formed as a waveguide with a thickness in a range of about 50um to about 200m. - 15. A method of coupling light into or out of a planar waveguide structure, the method comprising, Co using an array of first light reflective structures formed in a first medium of the planar waveguide structure to reflect light into a second medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure; using an array of second light reflective structures formed in the second ‘medium to propagate light via internal reflection in the second medium, each second light reflective structure having a second surface with a second angle 6, relative to the longitudinal plane of the planar waveguide structure; and : wherein the second angle 6, and the first angle 6, are inter-related.

16. The method as claimed in claim 15, wherein the array of the second light reflective structures is formed as a textured surface of the second medium.

17. The method as claimed in claim 16, wherein the textured surface is disposed at an interface between the second medium and a third medium of the planar waveguide structure.

18. The method as claimed in any one of claims 16 or 17, wherein the textured surface comprises a micromirror array.

19. The method as claimed in any one of claims 15 to 18, wherein the array of first reflective structures is disposed at an interface between the first medium and the second medium.

20. The method as claimed in any one of claims 15 to 19, wherein a filler medium of each first reflective structure has a lower refractive index than the first medium.

21. The method as claimed in claim 20, wherein the filler medium of each first reflective structure comprises air. | :

22. The method as claimed in any one of claims 15 to 21, wherein a filler medium of each second reflective structure has a lower refractive index than the : - second medium. : : 23. The method as claimed in claim 22, wherein the filler medium of each second reflective structure comprises air. oo ;

24. The method as claimed in any one of claims 15 to 23, wherein 8, is based on a first constraint 90 —sin™' (" Jsin(180-26,+6) tan — > 26, mn L/ _1 h, /tand, where n, is the refractive index of the first medium, n, is the refractive index of the second medium, 4, is a light incident angle for light incident on the first surface, L; is a length of a base of each second light reflective structure and h; is a height of each second light reflective structure. -

‘25. The method as claimed in any one of claims 15 to 24, wherein 6, is based on a second constraint 26, sin”, J-sin”(( Jsin(180-26, +6) : 2 2

Co 31 ~~ where n, is the refractive index of the first medium, n; is the refractive index of the second medium, 6.is a light incident angle for light incident on the first surface, L. is a oo length of a base of each second light reflective structure and h;, is a height of each : second light reflective structure. :

26. The method as claimed in any one of claims 15 to 25, wherein each first light reflective structure is formed with a base length of between about 2 um to about 20 pm.

27. The method as claimed in any one of claims 15 to 26, wherein each second light reflective structure is formed with a base length between a range of ~ abdut 4 ym to about 200m. : Co

28. The method as claimed in any one of claims 15 to 27, wherein the | second medium is formed as a waveguide with a thickness in a range of about 50um "to about 200um.

29. A method of forming a planar waveguide structure, the method comprising, forming an array of first light reflective structures in a first medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure for reflecting light into a second medium of the planar waveguide structure; forming an array of second light reflective structures in the second medium, © each second light reflective structure having a second surface with a second angle : g, relative to the longitudinal plane of the planar waveguide structure; oo wherein the second angle 6, and the first angle 6, are inter-related for propagating light via internal reflection in the second médium.

30. The method as claimed in claim 29, comprising forming the array of the second light reflective structures as a textured surface of the second medium.

31. The method as claimed in claim 30, further comprising forming the textured surface at an interface between the second medium and a third medium of the planar waveguide structure.

32. The method as claimed in any one of claims 30 or 31, further comprising forming the textured surface comprising a micromirror array. . + 33. The method as claimed in any one of claims 29 to 32, comprising forming the array of first reflective structures at an interface between the first medium and the second medium. oo :

34.. The method as claimed in any one of claims 29 to 33, wherein a filler medium of each first reflective structure has a lower refractive index than the first medium. : 35. The method as claimed in claim 34, wherein the filler medium of each first reflective structure comprises air. oo ] 36. The method as claimed in any one of claims 29 to 35, wherein a filler medium of each second reflective structure has a lower refractive index than the second medium.

37. The method as claimed in claim 36, wherein the filler medium of each second reflective structure comprises air. oo

38. The method as claimed in any one of claims 29 to 37, wherein 0, is - based on a first constraint : 90 sin" (i Jsin(iso- 26,+6,) |—tan™ rr > 26, oo mn ' L,/ _1 : h, /tanf, where n, is the refractive index of the first medium, n, is the refractive index of the second medium, 6,is a light incident angle for light incident on the first surface, L; is a

33 | . length of a base of each second light reflective structure and h;, is a height of each second light reflective structure. :

39. The method as claimed in any one of claims 29 to 38, wherein 6, is 5° based on a second constraint 26, > sin”(", J-sin((, Jsin(180-26 +6) 2 2 where n, is the refractive index of the first medium, n; is the refractive index of the second medium, 8, is a light incident angle for light incident on the first surface, L, is a length of a base of each second light reflective structure and h, is a height of each second light reflective structure. : oo

40. The method as claimed in any one of claims 29 to 39, further comprising forming the first light reflective structure with a base length of between about 2 ym to about 20 pm.

41. The method as claimed in any one of claims 29 to 40, further comprising forming the second light reflective structure with a base length between a range of about 4 um to about 200um.

42. The method as claimed in any one of claims 29 to 41, further comprising forming the second medium as a waveguide with a thickness in a range of about 50um to about 200um.

43. A planar waveguide structure, the structure comprising, an array of first light reflective structures formed in a first medium of the = planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, relative to a longitudinal plane of the planar waveguide structure for reflecting light propagating in the planar waveguide structure out of the first medium into a second medium of the planar waveguide structure; an array of second light reflective structures formed in the second medium,

30 . each second light reflective structure having a second surface with a second angle

9, relative to the longitudinal plane of the planar waveguide structure for reflecting light out of the second medium.

44. A method of coupling light out of a planar waveguide structure, the method comprising, using an array of first light reflective structures formed in a first medium of the planar waveguide structure to reflect light propagating in the planar waveguide structure out of the first medium into a second medium of the planar waveguide structure, each first light reflective structure having a first surface with a first angle 6, - relative to a longitudinal plane of the planar waveguide structure; : using an array of second light reflective structures formed in the second medium to reflect light out of the second medium, each second light reflective structure having a second surface with a second angle 9, relative to the longitudinal plane of the planar waveguide structure.