WO2002031550A2

WO2002031550A2 - Waveguide having a light drain

Info

Publication number: WO2002031550A2
Application number: PCT/US2001/028087
Authority: WO
Inventors: Chi Wu
Original assignee: Lightcross, Inc
Priority date: 2000-10-10
Filing date: 2001-09-07
Publication date: 2002-04-18
Also published as: AU2001287139A1; WO2002031550A3

Abstract

A waveguide is disclosed. The waveguide (40) includes a light barrier (42) with a surface between sides. A light signal carrying region is positioned adjacent to the surface (58) of the light barrier. A drain (59) is positioned adjacent to at least one side of the light barrier. The drain is configured to drain light that exits the light signals carrying region from the waveguide.

Description

WAVEGUIDE HAVING A LIGHT DRAIN

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Patent Application Serial Number 09/686,733, filed on October 10, 2000, entitled Waveguide Having a Light Drain and incorporated here in its entirety.

BACKGROUND

1. Field of the Invention The invention relates to one or more optical networking components. In particular, the invention relates to waveguides.

2. Background of the Invention

Many components for optical networking include a plurality of waveguides formed on a substrate. The components often include a light barrier or cladding layer positioned over the substrate. A layer of a light transmitting medium positioned adjacent to the light barrier or cladding layer is formed into a plurality of ridges. The ridges serve as the waveguides where light signals are carried. The light barrier or cladding layer is continuous across the substrate and accordingy extends under each of the waveguides.

The light barrier or cladding layer prevents light from the waveguides from leaking into the substrate. However, when light escapes from one of the waveguides, the light barrier or cladding layer traps the light within the light transmitting medium. The trapped light can enter into other waveguides on the component. Light entering one waveguide from another waveguide is the sourpe of cross talk on the component. There is a need for optical components having reduced cross talk.

SUMMARY OF THE INVENTION The invention relates to a waveguide. The waveguide includes a light barrier having a surface between two sides and a first light transmitting medium having a light signal carrying region adjacent to the surface of the light barrier. A second light transmitting medium is positioned adjacent to at least one side of the light barrier.

In some instances, the second light transmitting medium is positioned to receive light that exits the light signal carrying region. Another embodiment of the waveguide includes a light barrier with a surface between sides. A light signal carrying region is positioned adjacent to the surface of the light barrier. A drain is positioned adjacent to at least one side of the light barrier. The drain is configured to drain light that exits the light signals carrying region from the waveguide. Another embodiment of the invention relates to an optical component having a plurality of waveguides. The optical component includes two or more light barriers having a surface between two sides. A first light transmitting medium forms a light signal carrying region adjacent to the surface of each light barrier. A second light transmitting medium is positioned between the sides of the light barriers.

Another embodiment of the component includes a plurality of light barriers that each has a surface between sides. A light signal carrying region is adjacent to the surface of each light barrier. A drain is positioned between the sides of adjacent light barriers. The drain is configured to drain light that escapes from one or more of the light signal carrying regions from the component.

The invention also relates to a method of forming a waveguide. The method includes forming a light barrier in a light transmitting medium such that the light transmitting medium defines sides of the light barrier. The method also includes forming a light signal carrying region adjacent to a surface of the light barrier, the surface of the light barrier being positioned between the sides defined by the light transmitting medium.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 A illustrates a waveguide formed over a light barrier. The waveguide includes a light signal carrying region where light is constrained. A drain is positioned adjacent to sides of the light barrier for draining light that escapes from the light signal carrying region. Figure IB is a top view of a waveguide.

Figure 1C is a cross section of a waveguide having a triangular light barrier.

Figure ID is a cross section of a waveguide having a semi-circular light barrier.

Figure 2A through Figure 2D illustrate a method for fabricating the component illustrated in Figure 1A.

Figure 3 A through Figure 3D illustrate another embodiment of a method for fabricating the component illustrated in Figure 1A. Figure 4A through Figure 4C illustrate a method of fabricating a component having a second light transmitting medium adjacent to sides of a light barrier formed in a substrate. The second light transmitting medium and the substrate can optionally be constructed from different materials.

DETAILED DESCRIPTION

The invention also relates to waveguides for carrying of light signals. The waveguides include a light barrier having a surface between two sides. A light signal carrying region is formed adjacent to the surface of the light barrier. A drain is positioned adjacent to at least one side of the light barrier. The drain serves to drain at least a portion of the light signals that exit the lights signal carrying region from the waveguide. Because the light signals are drained, they are prevented from entering into other waveguides that are formed on a component. Accordingly, the drains can reduce cross talk between adjacent waveguides. Figure 1 A illustrates a cross section of a component 10 having a plurality of waveguides 40. The component 10 includes light barriers 42 formed over a substrate 44. Each light barrier 42 includes a barrier surface 56 between two sides 58. A first light transmitting medium 46A is positioned adjacent to the barrier surface 56 and a second light transmitting medium 46B is positioned adjacent to at least one side 58 of the light barrier. Additionally, the second light transmitting medium is positioned between adjacent light barriers. The location of the second light transmitting medium 46B is shown as dashed lines because the second light transmitting medium 46B can optionally be constructed of the same material as the first light transmitting medium 46A and/or the substrate 44.

The first light transmitting medium 46A is formed into a plurality of ridges 54 that each define a portion of a light signal carrying region 62 configured to constrain light signals. The light barriers 42 contain a material that causes light from the light signal carrying region 62 to be reflected back into the light signal carrying region 62. Accordingly, each light barrier 42 serves to constrain light signals to the light signal carrying region. The profile of a light signal constrained in the light signal carrying region 62 is shown by the dashed lines. The second light transmitting medium is positioned to receive at least a portion of the light that escapes from the light signal carrying region. The light that is received by the second light transmitting medium can enter the substrate 44 as shown by the arrow labeled A. Accordingly, the second light transmitting medium 46 serves as a drain 59 that drains light that exits the light signal carrying regions from a waveguide and/or from the component 10. Because the light is drained from the component 10, the light represented by the arrows labeled A does not enter adjacent waveguides 40 and accordingly does not result in cross talk.

In some instances, the bottom or other sides of the component 10 can include an anti-reflective coating. The anti-reflective coating reduces reflection of a light signal off a bottom of the component and accordingly increases the portion of the light transmitted through the bottom of the component. The light transmitting through the bottom of the component is not available to enter other waveguides. As a result, the anti-reflective coating can further reduce the cross talk associated with the component 10. Previous waveguides were formed over a continuous light barrier. For instance, a plurality of waveguides were formed over a single continuous light barrier. When light escaped from one of these waveguides 40, the light was not able to pass the continuous light reflecting layer and accordingly was not drained. Hence, the escaped light was often reflected into another waveguide 40 yielding cross talk between waveguides 40. As described above, the second light transmitting medium 46B positioned between the light reflecting layers reduces this cross talk. Accordingly, the invention also relates to a waveguide 40 having reduced cross talk.

Additionally, the second light transmitting medium 46B positioned between the light barriers 42 can allow more thermal energy to flow across a component 10 than could be achieved with a continuous light barrier 42. For instance, the light barrier can be silica while the second light transmitting medium is silicon. The thermal conductivity of silicon is approximately ten times the thermal conductivity of silica. Accordingly, thermal energy more readily flows through a silicon drain than it would flow through a silica light barrier that extends under more than one waveguide. Hence, positioning the second light transmitting medium 46B adjacent to the sides 58 of the light barrier 42 can provide a component 10 with a reduced thermal sensitivity.

Figure 1A shows a cross section of two waveguides 40 formed on a component 10, however, a component 10 can include one or more waveguides 40 constructed according to Figure 1A.

The substrate 44, the first light transmitting medium 46A and the second light transmitting medium 46B can be formed of the same or different materials. For instance, the substrate 44, the first light transmitting medium 46A and the second light transmitting medium 46B can all be silicon while the light barrier 42 is air or silica. Alternatively, the substrate 44 and the second light transmitting medium 46B can be silicon and the first light transmitting medium 46A can be silica, while the light barrier 42 is air. Further, the substrate can be silicon, the first light transmitting medium can be GaAs, InP, SiGe, LiNbO₃ or silicon and the second light transmitting medium can be GaAs or InP, or SiGe, or LiNbO₃. Additionally, the first light transmitting medium can be Silicon, GaAs or InP and the light barrier could be SiO₂, SiN_x, SiON_x, air or another gas. Materials such as GaAs and/or InP allow the component to be used for high speed applications. Other suitable materials for first light transmitting medium 46A, the second light transmitting medium 46B and/or the substrate 44, include, but are not limited to, compounds of GaAs such as AlGaAs; compounds of InP such as InGaAsP,

InAlAsP; compounds of Silicon such as SiGe, SiC, SiGeC, SiN, SiGaN; SiON_x, SiN_x, low dielectric constants material, such as SiCOH; polymers; air or other gasses.

The light barrier 42 can have reflective properties such as a metal layer or a metal coating. Alternatively, the light barrier 42 can be a material that transmits light but causes more light reflection at the interface of the light barrier 42 and the first light transmitting medium 46A than results at the interface of the first light transmitting medium 46A and the second light transmitting medium 46. The reflection caused by the light barrier can result from the light barrier 42 having a lower index of refraction than the first light transmitting medium 46 A. For instance, the light barrier 42 can be silica or air when the first light transmitting medium 46A is silicon.

In some instances, the second light transmitting medium 46B has an index of refraction that is greater than or equal to the index of refraction of the first light transmitting medium 46A. For instance, the first light transmitting medium 46A can be silica and the second light transmitting medium 46B can be silicon. The increase in the index of refraction reduces the reflection that occurs at the interface of the first light transmitting medium 46A and the second light transmitting medium 46B. The reduced reflection increases the amount of light that is drained and can accordingly reduce the amount of cross talk. The periphery of the light barriers 42 can trace the periphery of the ridge

54. For instance, Figure IB is a topview of a component 10 with a waveguide 40. The position of the light barrier 42 relative to the ridge 54 is shown in dashed lines. The distance between the periphery of the light barrier 42 and the periphery of the ridge 54 is substantially constant. In some instances, the distance between the periphery of the light barrier and the periphery of the ridge varies along the length of the waveguide. For instance, the periphery of the light barrier can narrow at one or more locations to drain the light signal at the one or more locations. The narrow region can serve to attenuate the light signal at the one or more locations. The light barrier 42 can be sized such that at least a portion of the light that escapes the light signal carrying region 62 can pass along the side of the light barrier 42 and into the substrate 44. As shown in Figure 1A and Figure IB, the light barrier 42 and waveguide 40 can be constructed such that the periphery of the light barrier 42 extends beyond the periphery of the ridge 54 of the waveguide 40. In some instances, the light barrier 42 and waveguide 40 are constructed such that the periphery of the light barrier 42 is substantially the same size as the ridge 54 of the waveguide 40. In other instances, the light barrier 42 and waveguide 40 are constructed such that the periphery of the light barrier 42 is smaller than the ridge 54. Having the periphery of the light barrier 42 smaller than the periphery of the ridge 54 causes the light signal to be drained from the light signal carrying region 62. Accordingly, the light barrier can be sized such that a light signal attenuator is formed in the waveguide. The size of the light barrier periphery can be controlled to achieve a particular amount of light signal drain.

The width of the light barrier can be larger than 150 % of the width of the waveguide 40, larger than 250 % of the width of the waveguide 40, larger than 300 % of the width of the waveguide 40, larger than 400 % of the width of the waveguide 40 or larger than 500 % of the width of the waveguide 40.

Additionally or alternatively, the width of the light barrier can be smaller than 1000 % of the width of the waveguide 40, smaller than 800 % of the width of the waveguide 40, smaller than 600 % of the width of the waveguide 40, smaller than 500 % of the width of the waveguide 40, smaller than 400 % of the width of the waveguide 40, smaller than 300 % of the width of the waveguide 40 or smaller than 150 % of the width of the waveguide 40. When the waveguide 40 includes a ridge, a suitable measure of the width of the waveugide is the width of the ridge base 60.

The light barrier need not have a rectangular shape as shown above. For instance, the light barrier can have a triangular shape as shown in Figure IC or a curved shape as shown in Figure ID.

Waveguides according to Figure 1 A through Figure ID can be used in conjunction with any optical component that employs waveguides such as multiplexer and/or a demultiplexer, switches, filters, tunable filters, modulators, gain equalizers, fibers dispersion compensators, attenuators, star couplers and arrayed waveguide grating demultiplexers. As an example, the waveguide can be used in conjunction with the switch described in U.S. Patent number 5,581,643. Figures 2A - Figure 2D illustrate a method for fabricating a waveguide 40 such as the waveguide of Figure 1A. Figure 2A shows a plurality of masks 66 formed on a substrate 68. Suitable substrates 68 include, but are not limited to, silicon substrates. The masks 66 are formed such that regions where a light barrier 42 is to be formed remain exposed. An ion implant such as an O₂ ion implant is performed. After annealing, a light barrier 42 is formed between the masks 66 as shown in Figure 2B. For instance, when the ion implant is an O₂ ion implant and the substrate is a silicon substrate 68, the annealing forms silica light barriers 42. The masks 66 are removed and a light transmitting medium 70 such as silicon is grown or deposited on the substrate 68 as shown in Figure 2C. The interface of the light transmitting medium 70 and the substrate 68 is shown as a dashed line because the light transmitting medium 70 and the substrate 68 can be constructed from the same material.

A mask 66 is formed on the light transmitting medium 70 so as to protect regions where ridges will be formed. An etch is performed and the masks 66 are removed to provide the component 10 shown in Figure 2D. In some instances, the substrate 68 can be etched so as to form the waveguides 40 and there is no need to grow or deposit the light transmitting medium 70 as shown in Figure 2C. When the substrate 68 and the light transmitting medium 70 are silicon and the light barriers 42 are silica, the component 10 illustrated in Figure 2D is the component 10 shown in Figure 1A with silica serving as the light barrier 42 and silicon servings as the first light transmitting medium 46A, the second light transmitting medium 46B and the substrate 44.

Figure 3A through Figure 3D shows a method for forming the component 10 illustrated in Figure 1 A with air serving as the light barrier 42. A mask 66 and etch is performed on a substrate 68 such as silicon to provide grooves 71 in a light transmitting medium as illustrated in Figure 3 A. The light barriers 42 will be formed in the grooves 71. Air can be left in the grooves 71 or another light barrier 42 material can be grown or deposited in the grooves 71. Chemical mechanical polishing (CMP) process can optionally be used to smooth the surface of the grooves. Wafer bonding techniques can then be applied to bond a light transmitting medium 70 to the component. For instance, a silicon on insulator wafer 72 can be bonded to the component 10 as shown in Figure 3B. A silicon on insulator wafer typically includes a layer of silica 73 A positioned between a first layer of silicon 73B and a second layer of silicon 73C. The silica layer 73 A and the second silicon layer 73C can be removed to provide the component shown in Figure 3C. The first silicon layer 73B serves as the light transmitting medium. In . some instances, a portion of the first silicon layer 73B can also be removed to provide the light transmitting medium 70 with the desired thickness. Suitable techniques for removing the silica layer 73A, the first silicon layer 73B and/or the second silicon layer 73 C include, but are not limited to, etching, buffing, polishing, lapping, detachment through H implantation and subsequent annealing. Silica remains as the light transmitting medium 14.

As an alternative to a silicon on insulator wafer, a silica wafer can be bonded to the substrate. The support layer that generally accompanies the silica layer can be removed so the silica serves as the light transmitting medium 70. The component of Figure 3C can also be formed by bonding a wafer 74 such as a silicon wafer or a silica wafer to the component 10 of Figure 3 A. The wafer 74 serves as the light transmitting medium 70. A portion of the wafer 74 can be removed to provide the light transmitting medium 70 with the desired thickness. The light transmitting medium 70 is masked so as to protect the regions of the light transmitting medium 70 where the ridge will be formed. The masked component 10 is etched and the masks removed to provide the component 10 illustrated in Figure 3D. When the substrate 68 and the light transmitting medium 70 are silicon and the light barriers 42 are air, the component 10 illustrated in Figure 3D is the component 10 shown in Figure 1A with air serving as the light barrier 42 and silicon servings as the first light transmitting medium 46A, the second light transmitting medium 46B and the substrate 44. When the substrate 68 is silicon, the light transmitting medium 70 is silica and the light barriers 42 are air, the component 10 illustrated in Figure 3D is the component 10 shown in Figure 1 A with air serving as the light barrier 42, silica serving as the first light transmitting medium 46A and silicon serving as the second light transmitting medium 46B and the substrate 44. Figure 4A through Figure 4C show a method of fo-rming the optical component with a second light transmitting medium 46B that is different from the substrate 44. Figure 4A shows the substrate 68 of Figure 3 A. A layer of material 76 can be grown or deposited on the substrate as shown in Figure 4B. Suitable layers of material 76 include, but are not limited to, silica. As discussed above, the light transmitting medium 70 is bonded to the layer of material with air remaining as the light barriers 42. The ridges 54 are formed in the light transmitting medium to provide the component shown in Figure 4C. The layer of material 76 adjacent to the side of the light barrier 42 serves as the second light transmitting medium of Figure IB. When the substrate 68 is silicon and the layer of material 76 and the light transmitting medium 70 are silica, the component 10 illustrated in Figure 4C is the component 10 shown in Figure 1 A with air serving as the light barrier 42, silicon servings the substrate 44 and silica serving as the first light transmitting medium 46A and the second light transmitting medium 46B.

Although the substrates illustrated above are shown as being constructed of a single material, the substrates can be constructed from two or more layers of material.

Although the optical component 10 is disclosed in the context of optical components having ridge waveguides, the principles of the present invention can be applied to optical components having other waveguide types. Suitable waveguide types include, but are not limited to, buried channel waveguides and strip waveguide.

Other embodiments, combinations and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. For instance, the principles of the invention can be applied to wavelength add/drop multiplexers, wavelength cross connect switches and N x N wavelength routing applications. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.

Claims

1. An optical component for carrying light signals, comprising: a light barrier having a surface between two sides; a first light transmitting medium forming a light signal carrying region adjacent to the surface of the light barrier; and a second light transmitting medium adjacent to at least one side of the light barrier.

2. The component of claim 1, wherein the first light transmitting medium and the second light transmitting medium are continuous and formed of the same material.

3. The component of claim 1, wherein the first light transmitting medium is formed into a ridge and a perimeter of the light barrier extends beyond a perimeter of the ridge.

4. The component of claim 1, wherein the light barrier is positioned adjacent to a substrate.

5. The component of claim 4, wherein the substrate, the first light transmitting medium and the second light transmitting medium are each formed of the same material.

6. The component of claim 5, wherein the substrate, the first light transmitting medium and the second light transmitting medium are each formed of silicon.

7. The component of claim 1 , wherein the light barrier includes silica.

8. The component of claim 1, wherein the light barrier includes air.

9. The component of claim 1, wherein the second light transmitting medium has an index of refraction that is greater than or equal to the index of refraction of the first light transmitting medium.

10. An optical component for carrying light signals, comprising: two or more light barriers having a surface between two sides; a first light transmitting medium forming a light signal carrying region adjacent to the surface of each light barrier; and a second light transmitting medium between the sides of the light barriers.

11. The component of claim 10, wherein the first light transmitting medium and the second light transmitting medium are continuous and formed of the same material.

12. The component of claim 10, wherein the first light transmitting medium is formed into a ridge and a perimeter of the light barrier extends beyond a perimeter of the ridge.

13. The component of claim 10, wherein the light barrier is positioned adjacent to a substrate.

14. The component of claim 13, wherein the substrate, the first light transmitting medium and the second light transmitting medium are each formed of the same material.

15. The component of claim 14, wherein the substrate, the first light transmitting medium and the second light transmitting medium are each formed of silicon.

16. The component of claim 15, wherein each light barrier includes silica.

17. The component of claim 10, wherein each light barrier includes air.

18. A method of forming an optical component for carrying light signals, comprising: forming a light barrier in a light transmitting medium such that the light transmitting medium defines sides of the light barrier; and forming a light signal carrying region adjacent to a surface of the light barrier, the surface of the light barrier being positioned between the sides defined by the light fransmitting medium.

19. The method of claim 18, wherein forming a light signal carrying region adjacent to the surface of the light barrier includes forming a second light transmitting into a ridge positioned over the light barrier.

20. The method of claim 18 wherein forming a light barrier in a light transmitting medium such that the light transmitting medium defines sides of the light barrier includes etching a groove sized to serve as the light barrier in the light transmitting medium; and forming a light signal carrying region adjacent to a surface of the light barrier includes positioning a second light transmitting medium over the groove.

21. The method of claim 20, wherein wafer bonding techniques are used for positioning the second light transmitting medium over the groove.

22. The method of claim 20, wherein forming a light signal carrying region adjacent to a surface of the light barrier includes etching a ridge in the second light transmitting medium.

23. The method of claim 18 wherein forming a light barrier in a light transmitting medium such that the light transmitting medium defines sides of the light barrier includes implanting ions in the first light transmitting medium where the light barrier is to be formed; and performing an anneal so as to react the ions with the light fransmitting medium.

24. The method of claim 23, wherein forming a light signal carrying region adj acent to a surface of the light barrier includes positioning a second light fransmitting medium over the first light transmitting medium; and etching the second light transmitting medium.

25. A waveguide, comprising: a light barrier with a surface between sides; a light signal carrying region positioned adjacent to the surface of the light barrier; and a drain adjacent to at least one side of the light barrier for draining light that exits the light signals carrying region away from the light signal carrying region.

26. The waveguide of claim 25, wherein the light signal carrying region is formed in a first light fransmitting medium.

27. The waveguide of claim 25, wherein the drain includes a second light transmitting medium positioned adjacent to at least one side of the light barrier.

28. The waveguide of claim 25, wherein the first light transmitting medium and the second light transmitting medium are the same material.

29. The component of claim 25, wherein the first light transmitting medium is formed into a ridge and a perimeter of the light barrier extends beyond a perimeter of the ridge.

30. The component of claim 25, wherein the first light fransmitting medium and the second light transmitting medium are each formed of the same material.

31. The component of claim 30, wherein the first light transmitting medium and the second light fransmitting medium are each formed of silicon.

32. The component of claim 25, wherein the first light transmitting medium is formed into a ridge having a base and a width of the light barrier is greater than a width of the base of the ridge.

33. The component of claim 25 , wherein the light signal carrying region is the light signal carrying region of a waveguide and a width of the light barrier is greater than 150 % of a width of the waveguide.

34. The component of claim 25, wherein the light signal carrying region is the light signal carrying region of a waveguide and a width of the light barrier is greater than 250 % of a width of the waveguide.

35. A component having a plurality of waveguides, comprising: a plurality of light barriers that each have a surface between sides; a light signal carrying region adjacent to the surface of each light barrier; and a drain between the sides of adjacent light barriers.