WO2009051562A1

WO2009051562A1 - An optical coupling device and a method of optically coupling light

Info

Publication number: WO2009051562A1
Application number: PCT/SG2007/000355
Authority: WO
Inventors: Jing Zhang; Sik Pong Bryan Lee; Venkata Ramana Pamidighantam; Jayakrishnan Chandrappan; Hon-Shing John Lau; Dim-Lee Kwong
Original assignee: Agency For Science, Technology And Research
Priority date: 2007-10-18
Filing date: 2007-10-18
Publication date: 2009-04-23

Abstract

A coupling device for optically coupling a light source to a single mode fiber or waveguide, a method of optically coupling a light source to a single mode fiber or waveguide and an optical assembly are provided. The coupling device comprises a multimode input plane for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source; a single mode output plane for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide; and a tapered body structure extending between the input plane and the output plane, the tapered body structure being shaped such that multiple modes stimulated at the input plane are trapped and converged to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide.

Description

An Optical Coupling Device And A Method Of Optically Coupling Light

FIELD OF INVENTION

The invention relates broadly to a coupling device for optically coupling a light source to a single mode fiber or waveguide, to a method of optically coupling a light source to a single mode fiber or waveguide and to an optical assembly.

BACKGROUND

Optical coupling is practically important both in design and placement for typical photonic/optoelectronic/waveguide devices. Light sources such as edge-emitting semiconductor lasers that provide an optical source for transmission on optical fibers typically have large-angle asymmetric patterns. However, a single mode optical fiber typically has small receiving angles and a small mode field diameter (MFD). Thus, using butt coupling between a laser and a single mode fiber typically captures less than about 10% of the laser power. Furthermore, butt coupling typically has a low tolerance for misalignment (e.g. tolerance about 1μm) between the laser and the fiber.

To achieve better coupling efficiency, a typical lens may be used to convert the light source from the laser to a smaller angle distribution. However, when the lens is used for improving coupling efficiency, the coupled laser power to the fiber typically decreases significantly, even if there is only a misalignment by some μm. Therefore, for such arrangements, accuracy of the laser, lens and fiber placement are typically important, i.e., the placement has low alignment tolerances.

For achieving a better tolerance with lower coupling losses, various lenses have been investigated for coupling applications. Such lenses include a bi-focus lens, a diffractive lens and a defocus lens. US 2005/0259918 describe a laser and single mode fiber coupling system using bifocal lens. US 2006/0045423 describe a laser and single mode fiber coupling system using refractive lens. In order to achieve large tolerances, the bi-focal lens or diffractive lens typically generate a beam spot that is significantly larger than the size of the fiber core. However, as a consequence of the larger beam spot, coupling efficiency decreases significantly.

A taper coupler is another device typically used to couple light between two devices. The taper coupler is used to match a laser spot size and to maintain a single mode between a single mode laser and a single mode fiber. The taper coupler typically provides mode matching via an adiabatic modal transformation thus resulting in a small coupling loss. Oguro etal. in IEEE ECTC 2002, pp.305-310, 2002 describe a taper coupler which converts modes from a laser diode to a waveguide. The coupler is a mode matched device with a small input aperture matching the laser diode optical output dimensions of about 1um vertical x 2um horizontal and the coupler has an output matching a single mode fiber having dimensions of about 9um x 9um. One problem that may arise is the taper coupler has a low alignment tolerance with the laser diode because the input of the taper coupler supports only a single mode and the input is to match with a single mode from the laser at the laser-coupler interface. In other words, the taper coupler has a small input region that typically results in a small alignment tolerance between the laser and the taper coupler structure.

In order to achieve higher accuracy alignments, Mitomi et. al. in IEEE Journal of Quantum Electronics, vol. 30, No.8, 1787-1793, 1994 describe taper coupler structures fabricated on the same substrate with laser diodes. However, this typically increases the cost of the fabricated laser diodes.

Using the current coupling methods, active alignment has to be carried out for the optical coupling assembly. However, active alignment typically results in a longer time to operation and higher costs.

Hence, there exists a need for a coupling device for optically coupling a light source to a single mode fiber or waveguide, a method of optically coupling a light source to a single mode fiber or waveguide and an optical assembly which seek to address at least one of the above problems. SUMMARY

In accordance with a first aspect of the present invention, there is provided a coupling device for optically coupling a light source to a single mode fiber or waveguide, the coupling device comprising a multimode input plane for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source; a single mode output plane for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide; and a tapered body structure extending between the input plane and the output plane, the tapered body structure being shaped such that multiple modes stimulated at the input plane are trapped and converged to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide.

The input plane may have a numerical aperture of about 0.3 or larger.

The tapered body structure may be symmetrically tapered in a lateral direction from the input plane to the output plane.

The tapered body structure may be linearly tapered in a lateral direction from the input plane to the output plane.

The tapered body structure may be non-linearly tapered in a lateral direction from the input plane to the output plane.

The tapered body structure may comprise two or more pseudo-vertical tapers in a vertical direction.

The tapered body structure may be linearly tapered in a vertical direction.

The tapered body structure may be non-linearly tapered in a vertical direction.

The multimode input plane may be disposed at an angle with respect to an optical axis of the light source. The angle may be about 0.2 degrees.

In accordance with a second aspect of the present invention, there is provided a method of optically coupling a light source to a single mode fiber or waveguide, the method comprising providing a multimode input plane for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source; providing a single mode output plane for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide; and stimulating multiple modes at the input plane; and trapping and converging the multiple modes to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide, using a tapered body structure extending between the input plane and the output plane.

The input plane may have a numerical aperture of about 0.3 or larger.

The method may further comprise symmetrically tapering the tapered body structure in a lateral direction from the input plane to the output plane.

The method may further comprise tapering the tapered body structure linearly in a lateral direction from the input plane to the output plane.

The method may further comprise tapering the tapered body structure non- linearly in a lateral direction from the input plane to the output plane.

The method may further comprise tapering the tapered body structure linearly in a vertical direction.

The method may further comprise tapering the tapered body structure non- linearly in a vertical direction. The method may further comprise disposing the multimode input plane at an angle with respect to an optical axis of the light source.

The angle may be about 0.2 degrees.

In accordance with a third aspect of the present invention, there is provided an optical assembly, the optical assembly comprising a substrate; a coupling device fabricated on the substrate; a light source mounted on the substrate and optically coupled to the coupling device; and a single mode fiber or waveguide mounted on the substrate and optically coupled to the coupling device; wherein the coupling device comprises a multimode input plane for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source; a single mode output plane for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide; and a tapered body structure extending between the input plane and the output plane, the tapered body structure being shaped such that multiple modes stimulated at the input plane are trapped and converged to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

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(a) is a schematic top view diagram illustrating an optical assembly in an example embodiment.

Figure 1 (b) is a schematic side view diagram illustrating the optical assembly.

Figure 2 is a graph of coupling efficiency vs offset in the x direction between a taper coupler and a laser diode of the optical assembly. Figure 3 is a graph of coupling efficiency vs offset in the y direction between the taper coupler and the laser diode of the optical assembly.

Figure 4 is a schematic top view diagram illustrating an optical assembly in another example embodiment.

Figure 5 is a graph of coupling efficiency vs tilting angle θ between the taper coupler and the laser diode of the optical assembly.

Figure 6 is a schematic diagram illustrating light propagation in the lateral direction from a laser to a taper coupler and to a fiber.

Figure 7(a) is a schematic top view diagram illustrating an optical assembly in another example embodiment.

Figure 7(b) is a schematic side view diagram illustrating the optical assembly.

Figure 8(a) is a schematic top view diagram illustrating an optical assembly in another example embodiment.

Figure 8(b) is a schematic side view diagram illustrating the optical assembly.

Figure 9(a) is a schematic top view diagram illustrating an optical assembly in yet another example embodiment.

Figure 9(b) is a schematic side view diagram illustrating the optical assembly.

Figure 10 is a schematic flowchart illustrating a method of optically coupling a light source to a single mode fiber or waveguide.

Figure 11 (a) is a schematic top view diagram illustrating an optical assembly in another example embodiment.

Figure 11(b) is a schematic side view diagram illustrating the optical assembly. Figure 12(a) is a schematic top view diagram illustrating an optical assembly in another example embodiment.

Figure 12(b) is a schematic side view diagram illustrating the optical assembly.

Figure 13(a) is a schematic top view diagram illustrating an optical assembly in another example embodiment.

Figure 13(b) is a schematic side view diagram illustrating the optical assembly.

Figure 14(a) is a schematic top view diagram illustrating an optical assembly in another example embodiment.

Figure 14(b) is a schematic side view diagram illustrating the optical assembly.

Figure 15 shows a schematic layout and pictures of a sample optical assembly comprising a tapered coupler structure.

Figure 16(a) is a schematic top view diagram illustrating a sample optical assembly.

Figure 16(b) is a graph of insertion loss (dB) against horizontal misalignment (um) for the optical assembly of Figure 16(a).

Figure 16(c) is a graph of insertion loss (dB) against vertical misalignment (um) for the optical assembly of Figure 16(a).

DETAILED DESCRIPTION

In the example embodiments described below, a taper coupler may be used for light coupling between a laser diode and a single mode fiber. The taper coupler has a large input aperture to enhance placement tolerances for die placement and an output aperture matched to a single mode fiber or a waveguide. The large input dimensions result in multiple modes that are excited in the coupler and the different modes converge to give a single mode output. Thus, the coupler can act as a coupling element that eases assembly processes and simplifies the packaging. In the example embodiments, the coupler has a symmetrical taper in a lateral direction. In a vertical direction, the taper can be a linear taper or have pseudo-vertical taper shapes.

Figure 1(a) is^" a schematic top view diagram illustrating an optical assembly 100 in an example embodiment. Figure 1(b) is a schematic side view diagram illustrating the optical assembly 100. The optical assembly 100 comprises a taper coupler 102 optically coupling a laser diode 104 to a single mode fiber 106. The taper coupler 102 comprises a linear lateral taper (see profile numeral 110) and a two-layer 112, 114 pseudo-vertical taper structure. The laser diode 104, e.g. a Distributed Feedback (DFB) laser diode, has a relatively small beam spot of about 1um(V) x 2um(H) at its output 116 with wide divergence angles of 20 degrees horizontally and 40 degree vertically. The taper coupler 102 has an input 118 having a size larger than that of the laser active waveguide i.e. see laser output 116. Furthermore, the taper coupler 102 has relatively large numerical apertures of about 0.3 or larger in example embodiments. The numerical aperture is set to enable the taper coupler 102 to collect light from the laser diode 104. As an example, for a laser diode having a divergence angle of 35 degrees, in order to collect the light, the numerical aperture is calculated to be NA=sin(17.5degrees)=0.3. Thus, by using laser diodes with divergence angles of 35 degrees or larger, the numerical aperture of the taper coupler 102 can be 0.3 or larger. Therefore, in the example embodiment, the light sources from the single mode laser diode 104 enter the taper coupler 102 with a large divergence angle and stimulate multiple modes in the taper coupler 102. In the coupler 102, due to the taper structure, the light converges and, although some light may leak out of the coupler 102, a significant amount of the light is trapped in the coupler 102 and is coupled to a fundamental mode at an output 120. At the output 120 of the taper coupler 102, the light matches a core size 122 and the fundamental mode with the single mode fiber 106.

In the example embodiment, the dimensions of the input 118 opening are significantly larger than the size of the mode from the laser diode 104. The dimensions and taper angles of the lower layer 112 and the upper layer 114 of the pseudo-vertical taper structure affects the light coupling between the two layers 112, 114 and the light coupling into the fiber 106. Hence, the dimensions and taper angles are optimized to achieve a relatively high light coupling efficiency from the input 118 to the output 120. The optimisation may be achieved by varying design parameters such as refractive index differences between polymer core materials and silicon dioxide clad materials, waveguide cross-section dimensions, taper lengths for the laser diode operating wavelength etc. Multiple modes in the coupler 102 may also be suppressed by providing a tilt angle between the optical axis of the laser diode 104 and the coupler 102. In the example embodiment, the dimensions of the taper coupler is about 20μm x 20μm at the input 118 opening and about 10μm x 10μm at the output 120 opening while the length of the lower layer 112 is about 1450 μm and the length of the upper layer 114 is about 1700 μm.

In the example embodiment, the taper coupler 102 comprises a core having high refractive indexes e.g.1.51. After fabrication of the pseudo-vertical taper trench (see e.g.

112), the optical assembly 100 silicon wafer is coated with an oxide layer. The oxide layer (SiO2) acts as a cladding for the taper coupler 102. The cladding of the taper coupler 102 is a lower refractive-index material, e.g. 1.46. Therefore, more than one mode of wavefronts are stimulated once the light with a large divergence angle from the laser diode 104 enters the taper coupler 102. The output 120 of the taper coupler 102 has a core size whose fundamental mode matches with the mode in the single mode fiber 106. In the example embodiment, the output 120 of the taper coupler 102 has a square-shaped cross-section.

Figure 2 is a graph of coupling efficiency vs offset in the x direction between the taper coupler 102 (Figure 1(a)) and the laser diode 104 (Figure 1(a)). For a coupling variation of less than 2dB (see e.q. 202), the lateral tolerance of the laser104 (Figure 1) placement can be about +/-5μm.

Figure 3 is a graph of coupling efficiency vs offset in the y direction between the taper coupler 102 (Figure 1(a)) and the laser diode 104 (Figure 1(a)). For a coupling variation of less than 3dB (see e.g. 302), the vertical tolerance is in the range of about - 3 to 6μm. Thus, as shown in Figures 2 and 3, by using the taper coupler 102, a 3dB coupling efficiency is achievable and advantageously, with better alignment tolerances, as compared to the prior art.

It is noted that the results shown in Figures 2 and 3 are based on an assumption that there is no tilting at an active waveguide 126 (Figure 1) of the laser diode 104 (Figure 1) relative to the optical axis of the taper coupler 102 (Figure 1) and fiber 106 (Figure 1).

Figure 4 is a schematic top view diagram illustrating an optical assembly 400 in another example embodiment. The optical assembly 400 comprises a laser diode 402 coupled to a taper coupler 404 and the taper coupler 404 is coupled to an optical fiber 406. A lateral tilt θ is provided between the laser diode 402 and the taper coupler 404. The laser diode 402, the taper coupler 404 and the fiber 406 are substantially the same as the laser diode 104, the taper coupler 102 and the fiber 106 respectively described with reference to Figure 1.

Figure 5 is a graph of coupling efficiency vs tilting angle θ between the taper coupler 404 (Figure 4) and the laser diode 402 (Figure 4). With a lateral tilting angle θ, the coupling efficiency between the laser diode 402 to the coupler 404 and to the fiber 406 may be improved. It is shown that the best tilting angle is about 0.2 degrees with a 1 degree tolerance overall that results in a coupling variation of less than 1dB. The tilting may be carried out in either the clockwise or the anti-clockwise direction.

Figure 6 is a schematic diagram illustrating light propagation in the lateral direction from a laser 602 to a taper coupler 604 and to a fiber 606, based on the taper structure described above.

Variations of the above example embodiment may be implemented to achieve coupling for a laser diode and a fiber with larger allowable tolerances, as compared to the prior art.

Figure 7(a) is a schematic top view diagram illustrating an optical assembly 700 in another example embodiment. Figure 7(b) is a schematic side view diagram illustrating the optical assembly 700. The optical assembly 700 comprises a taper coupler 702 coupling a laser diode 704 to an optical fiber 706. As illustrated, the taper coupler 702 is linearly tapered in the lateral direction (see e.g. 708) and is linearly tapered in a vertical direction (see e.g. 710).

Figure 8(a) is a schematic top view diagram illustrating an optical assembly 800 in another example embodiment. Figure 8(b) is a schematic side view diagram illustrating the optical assembly 800. The optical assembly 800 comprises a taper coupler 802 coupling a laser diode 804 to an optical fiber 806. As illustrated, the taper coupler 802 has a non linear taper in the lateral direction (see e.g. 808) and a non linear taper in the vertical direction (see e.g. 810).

Figure 9(a) is a schematic top view diagram illustrating an optical assembly 900 in yet another example embodiment. Figure 9(b) is a schematic side view diagram illustrating the optical assembly 900. The optical assembly 900 comprises a taper coupler 902 coupling a laser diode 904 to an optical fiber 906. As illustrated, the taper coupler 902 has a linear lateral taper (see e.g. profile numeral 908) and three layers of pseudo-vertical tapers (see 910, 912, 914).

Apart from laser to fiber coupling, the above described example embodiments can alternatively be used for laser to waveguide coupling.

Table 1 below tabulates the comparison of typical coupling methods with the taper coupler described in the example embodiments.

Table 1

From Table 1, it is observed that described example embodiments can provide the larger allowable tolerances, as compared to the prior art, while maintaining good coupling performance.

In the above described example embodiments, the taper coupler has relatively larger dimensions at an input that is coupled to a single mode laser diode. The taper coupler has large numerical apertures at the input to collect the light with large divergence angles from the laser. The taper coupler matches the dimensions and a fundamental mode with a single mode fiber at its output. The taper coupler can have both lateral and vertical tapers. The lateral taper may be linear or nonlinear. The vertical taper may be a linear, nonlinear or pseudo-vertical taper. At the input of the taper coupler, multiple modes are stimulated on entrance, and during the light propagation in the taper coupler, a significant amount of modes are trapped inside the taper coupler and coupled to the fundamental mode at the output. A slight tilting angle between the laser diode and the taper coupler may lead to a better coupling performance in the described example embodiments. Using the described example embodiments, tolerance margins of the taper coupler may be improved over the prior art. Simulation and test data show that for device assembly while maintaining coupling efficiency, the described example embodiments can achieve a +/-5μm misalignment tolerance in the x- and y- directions and a +/-0.5 degree tilting angle tolerance.

Figure 10 is a schematic flowchart 1000 illustrating a method of optically coupling a light source to a single mode fiber or waveguide. At step 1002, a multimode input plane is provided for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source. At step 1004, a single mode output plane is provided for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide. At step 1006, multiple modes are stimulated at the input plane. At step 1008, the multiple modes are trapped and converged to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide, using a tapered body structure extending between the input plane and the output plane.

Figure 11 (a) is a schematic top view diagram illustrating an optical assembly 1100 in another example embodiment. Figure 11(b) is a schematic side view diagram illustrating the optical assembly 1100. The optical assembly 1100 functions substantially the same as the optical assembly 100 with reference to Figure 1. The laser diode 1102 is assembled using flip-chip technology that can improve the vertical coupling of the taper coupler 1104.

Figure 12(a) is a schematic top view diagram illustrating an optical assembly 1200 in another example embodiment. Figure 12(b) is a schematic side view diagram illustrating the optical assembly 1200. The optical assembly 1200 functions substantially the same as the optical assembly 700 with reference to Figure 7. The laser diode 1202 is assembled using flip-chip technology that can improve the vertical coupling of the taper coupler 1204.

Figure 13(a) is a schematic top view diagram illustrating an optical assembly

1300 in another example embodiment. Figure 13(b) is a schematic side view diagram illustrating the optical assembly 1300. The optical assembly 1300 functions substantially the same as the optical assembly 800 with reference to Figure 8. The laser diode 1302 is assembled using flip-chip technology that can improve the vertical coupling of the taper coupler 1304.

Figure 14(a) is a schematic top view diagram illustrating an optical assembly 1400 in another example embodiment. Figure 14(b) is a schematic side view diagram illustrating the optical assembly 1400. The optical assembly 1400 functions substantially the same as the optical assembly 900 with reference to Figure 9. The laser diode 1402 is assembled using flip-chip technology that can improve the vertical coupling of the taper coupler 1404.

Figure 15 shows a schematic layout and pictures of a sample optical assembly comprising a tapered coupler structure. The schematic layout 1502 illustrates the design of the sample optical assembly. The pictures e.g. 1504, 1506 show the fabrication result of the sample optical assembly.

Figure 16(a) is a schematic top view diagram illustrating a sample optical assembly 1600. The optical assembly 1600 comprises a taper coupler 1602 coupling a single mode fiber light source 1604 to a single mode fiber output 1606. The light source

1604 may be shifted horizontally (see numeral 1608) with respect to the taper coupler 1602 to impart horizontal misalignment to the optical assembly 1600.

Figure 16(b) is a graph 1610 of insertion loss (dB) against horizontal misalignment (um) for the optical assembly 1600. Figure 16(c) is a graph 1612 of insertion loss (dB) against vertical misalignment (um) for the optical assembly 1600. From the graphs 1610 and 1612, it can be observed that the optical assembly 1600 comprising the taper coupler 1602 can achieve a coupling loss of up to about 3dB for misalignments up to about +/-5um both in the horizontal and vertical directions. Therefore, the taper coupler 1602 can allow larger alignment tolerances for the optical assembly 1600. This corresponds to the tabulated results of Table 1.

The taper coupler in the described example embodiments can have large assembly tolerances compared with existing methods. The relatively large allowable tolerance in the described example embodiments may make passive assembly of photonic devices feasible. Considering the coupling efficiency as well as the allowable W

15 tolerances, the described example embodiments may also be applied to photonics packaging. With higher allowable tolerances and lower coupling loss, the described example embodiments may result in cheaper passive assembly and easier assembly for photonic packaging as compared to using typical lens coupling methods.

It will be appreciated by a person skilled in the art that the taper coupler of the described example embodiments is not limited to coupling to a single mode laser. The taper coupler can work with multi mode lasers, which are typically Vertical-Cavity

Surface-Emitting Lasers (VCSELs) that are surface emitting in nature and are suitable for non-optical bench applications.

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. A coupling device for optically coupling a light source to a single mode fiber or waveguide, the coupling device comprising a multimode input plane for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source; a single mode output plane for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide; and a tapered body structure extending between the input plane and the output plane, the tapered body structure being shaped such that multiple modes stimulated at the input plane are trapped and converged to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide.

2. The coupling device as claimed in claim 1, wherein the input plane has a numerical aperture of about 0.3 or larger.

3. The coupling device as claimed in claims 1 or 2, wherein the tapered body structure is symmetrically tapered in a lateral direction from the input plane to the output plane.

4. The coupling device as claimed in any one of claims 1 to 3, wherein the tapered body structure is linearly tapered in a lateral direction from the input plane to the output plane.

5. The coupling device as claimed in any one of claims 1 to 3, wherein the tapered body structure is non-linearly tapered in a lateral direction from the input plane to the output plane.

6. The coupling device as claimed in any one of claims 1 to 5, wherein the tapered body structure comprises two or more pseudo-vertical tapers in a vertical direction. W

17

7. The coupling device as claimed in any one of claims 1 to 6, wherein the tapered body structure is linearly tapered in a vertical direction.

8. The coupling device as claimed in any one of claims 1 to 6, wherein the tapered body structure is non-linearly tapered in a vertical direction.

9. The coupling device as claimed in any one of claims 1 to 8, wherein the multimode input plane is disposed at an angle with respect to an optical axis of the light source.

10. The coupling device as claimed in claim 9, wherein the angle is about 0.2 degrees.

11. A method of optically coupling a light source to a single mode fiber or waveguide, the method comprising providing a multimode input plane for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source; providing a single mode output plane for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide; and stimulating multiple modes at the input plane; and trapping and converging the multiple modes to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide, using a tapered body structure extending between the input plane and the output plane.

12. The method as claimed in claim 11 , wherein the input plane has a numerical aperture of about 0,3 or larger.

13. The method as claimed in claims 11 or 12, further comprising symmetrically tapering the tapered body structure in a lateral direction from the input plane to the output plane.

14. The method as claimed in any one of claims 11 to 13, further comprising tapering the tapered body structure linearly in a lateral direction from the input plane to the output plane.

15. The method as claimed in any one of claims 11 to 13, further comprising tapering the tapered body structure non-linearly in a lateral direction from the input plane to the output plane.

16. The method as claimed in any one of claims 11 to 15, wherein the tapered body structure comprises two or more pseudo-vertical tapers in a vertical direction.

17. The method as claimed in any one of claims 11 to 16, further comprising tapering the tapered body structure linearly in a vertical direction.

18. The method as claimed in any one of claims 11 to 16, further comprising tapering the tapered body structure non-linearly in a vertical direction.

19. The method as claimed in any one of claims 11 to 18, further comprising disposing the multimode input plane at an angle with respect to an optical axis of the light source.

20. The method as claimed in claim 19, wherein the angle is about 0.2 degrees.

21. An optical assembly, the optical assembly comprising a substrate; a coupling device fabricated on the substrate; a light source mounted on the substrate and optically coupled to the coupling device; and a single mode fiber or waveguide mounted on the substrate and optically coupled to the coupling device; wherein the coupling device comprises a multimode input plane for coupling to the light source, the input plane having dimensions that are larger than a spot size of the light source; a single mode output plane for coupling to the single mode fiber or waveguide, the output plane being mode-matched to the single mode fiber or waveguide; and a tapered body structure extending between the input plane and the output plane, the tapered body structure being shaped such that multiple modes stimulated at the input plane are trapped and converged to a fundamental mode at the output plane, for coupling to the single mode fiber or waveguide.