ENHANCED COUPLING STRENGTH GRATINGS

• US 20150063753A1
• Filed: 09/05/2014
• Published: 03/05/2015
• Est. Priority Date: 09/05/2013
• Status: Active Grant

• Current Assignee: Oracle International Corporation (Oracle Corporation), Southern Methodist University
• Original Assignee: Oracle International Corporation (Oracle Corporation), Southern Methodist University
• Inventors: Evans, Gary A., Butler, Jerome K., Kirk, Jay B., Yao, Jin, Li, Guoliang, Zheng, Xuezhe, Krishnamoorthy, Ashok V., He, Ruo-Hua
• Granted Patent: US 10,371,898 B2
• Time in Patent Office: Days
• US Class Current: 385/37
• CPC Class Codes: G02B 2006/12107 Grating
• CPC Class Codes: G02B 2006/12176 Etching
• CPC Class Codes: G02B 6/036 core or cladding comprising multiple layers multicore optical fibres G02B6/02042; microstructured properties G02B6/02295; omniguide fibres G02B6/023
• CPC Class Codes: G02B 6/1223 high refractive index type, i.e. high-contrast waveguides
• CPC Class Codes: G02B 6/132 by deposition of thin films
• CPC Class Codes: G02B 6/136 by etching
• CPC Class Codes: G02B 6/34 utilising prism or grating G02B6/293 takes precedence
• CPC Class Codes: H01S 3/0635 provided with a periodic structure, e.g. using distributed feed-back, grating couplers controlling, e.g. modulating distributed feed-back lasers H01S3/102
• CPC Class Codes: H01S 5/12 the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers comprising a photonic bandgap structure H01S5/11; surface-emitting lasers H01S5/18
• CPC Class Codes: H01S 5/1231 Grating growth or overgrowth details
• CPC Class Codes: H01S 5/125 Distributed Bragg reflector [DBR] lasers
• CPC Class Codes: H01S 5/187 using Bragg reflection

Abstract

The present invention includes an optical waveguide with a grating and a method of making the same for increasing the effectiveness of the grating. In one example, the grating is at least partially covered by a liner layer disposed on at least a portion of a grating; and a cover layer disposed on the liner layer, wherein a first material selected for the core and ridges and a second material selected for the liner layer are selected to provide a difference in the index of refraction between the first and second material that is sufficient to provide a contrast therebetween.

Claims

1. An enhanced coupling strength grating, comprising:

a substrate (12);

a core (14) disposed on the substrate (12);

one or more ridges (16) and one or more grooves (17) formed on the core (14), wherein the one or more grooves (17) are adjacent to, or between the one or more ridges (16), wherein the ridges (16) and grooves (17) form a grating (19);

a liner layer (18) disposed on at least a portion of a grating cycle; and

a cover layer (20) disposed on the liner layer (18), wherein a first material selected for the core (14) and ridges (16) and a second material selected for the liner layer (18) are selected to provide a difference in the index of refraction between the first and second material that is sufficient to provide a contrast therebetween.

2. The grating of claim 1, wherein the liner layer (18) is disposed on at least one of:

the bottom (28) of the groove (17);

one or more sidewalls (22, 24) of the ridges (16);

on the top (26) of the one or more ridges (16);

two or more liner layers (18) in the groove (17);

or

on the sides of the ridges (16) that do not have a top (26).

3. The grating of claim 1, wherein the liner layer (18) selected from one or more of the following optional configurations:

(a) the liner layer (18) is not contiguous;

(b) the liner layer (18) is disposed on a first sidewall (22), a second sidewall (24) or both the first and second sidewalls (22, 24) of the ridges (16);

(c) the liner layer (18) is defined further as one or more liner layers (18) that are contiguous and that follow the contour of the ridges (16) and the grooves (17);

(d) the liner layer (18) is not contiguous, wherein the liner layer (18) is defined further as being substantially parallel to a bottom of the one or more grooves (17), and the non-contiguous layers are separated by one or more cover layers (30);

(e) the liner layer (18) is defined further as two or more liner layers (18) that are contiguous and that follow the contour of the ridges (16), and each of the two or more liner layers (18) are separated by one or more cover layers (20);

(f) the liner layer (18) is disposed on one or more tops (26) of the ridges (16), one or more grooves (17) between the ridges (16), or both the top (26) of the ridges (16), and the grooves (17) between the ridges (16);

(g) the liner layer (18) is disposed on a first sidewall (22), one or more tops (26) of the ridges (16), and one or more grooves (17) between the ridges (16), to provide an effective blazed grating;

(h) the liner layer (18) is disposed on a first sidewall (22) and one or more tops (26) of the ridges (16);

or

(i) the liner layer (18) is disposed on one or more first sidewall (22) or second sidewall (24) of one or more waveguiding structures for grating coupling (inward or outward).

4. The grating of claim 1, wherein the liner layer (18) is disposed on a high index contrast Si/SiO₂ waveguide to further enhance the performance of the grating.

5. The grating of claim 1, wherein the cover layer (20) is selected to provide a similar index contrast between the cover layer (20) and the liner layer (18) as the contrast provided between the core (14) and the liner layer (18).

6. The grating of claim 1, wherein the liner layer (18) is selected from at least one of SiO, SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O_4-5, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe, polymers, and silicon nitride.

7. The grating of claim 1, wherein the liner layer (18) comprises a variable thickness to provide at least one of varying the strength of the coupling, an effective variable duty cycle, an effective variable grating depth, a Gaussian profile in a radiating coupler grating, or a near-Gaussian profile in a radiating coupler grating.

8. The grating of claim 1, wherein the liner layer (18) is selected to provide at least one of an optical loss or an optical gain.

9. The grating of claim 1, wherein the optical waveguide is at least one of a distributed Bragg reflectors (DBRs) or a distributed Bragg deflectors (DBDs).

10. The grating of claim 1, wherein the optical waveguide is defined further as comprising at least two ECS gratings to make an edge-emitting DBR laser;

one ECS grating and one regular DBR grating to make an edge-emitting DBR laser;

two ECS gratings with a straight ECS outcoupler grating to make a surface-emitting laser;

ECS grating and one regular DBR grating with a straight ECS outcoupler grating to make a surface-emitting laser;

two ECS gratings with a “

fan-out”

ECS outcoupler grating to make a surface-emitting laser;

one ECS grating and one regular DBR grating with a “

fan-out”

ECS outcoupler grating to make a surface-emitting laser;

two ECS gratings with a standard grating outcoupler grating to make a surface-emitting laser;

one ECS grating and one regular DBR grating with a standard grating outcoupler grating to make a surface-emitting laser;

one or more ECS grating output couplers with low back reflection on both ends to make a surface-normal coupled semiconductor optical amplifier (SOA) or optical gain block;

or one or more ECS gratings or regular DBR gratings configures as a mirror with high reflectivity and another ECS grating as an output coupler to make a surface-emitting reflective semiconductor optical amplifier (RSOA) or an optical gain block;

a hybrid external cavity laser and tunable laser using SOA or RSOA with ECS grating output couplers integrated with a waveguide or free space wavelength control optics;

or an enhanced grating for high density and low loss integration of III/V laser sources for silicon photonic interconnects.

11. The grating of claim 1, wherein the optical waveguide provides lateral optical confinement with a mesa structure and the enhanced grating is on at least one of:

(1) the top surface of the mesa structure;

(2) one or more lateral surfaces of the mesa structure;

or (3) on the surfaces adjacent the mesa structure.

12. The grating of claim 1, wherein an enhanced grating for grating-assisted directional couplers;

enhanced grating for multiply resonant distributed feedback lasers;

or an enhanced grating for multiplying resonant distributed Bragg reflector lasers;

an enhanced grating in optical fibers for sampling or detecting light in optical fibers by grating outcouplers;

an enhanced gratings in optical fibers for (1) sampling or detecting light in optical fibers by grating outcouplers operating near the second order Bragg condition;

(2) sampling or detecting light in optical fibers by grating outcouplers operating as distributed Bragg deflectors;

(3) to couple light into optical fibers;

a curved, enhanced gratings to make unstable resonator semiconductor lasers;

an enhanced grating to reduce the etch depth for the placement of distributed Bragg reflector gratings in semiconductor lasers, which results in simplified processing for DBR lasers;

an enhanced grating to reduce the etch depth for the placement of distributed Bragg reflector gratings in photonic devices, which results in simplified processing for photonic devices; and

enhanced grating to reduce the etch depth for the placement of gratings in photonic devices, which results in simplified processing for photonic devices;

or an enhanced grating to reduce the etch depth for the placement of coupling gratings in photonic devices, which results in simplified processing for photonic devices.

13. The grating of claim 1, wherein the grating (19) comprise a period that is equal to about the wavelength of the light propagating in the optical waveguide to produce an outcoupling in about 10 to 50 grating cycles.

14. The grating of claim 1, wherein the grating (19) comprise a period that is equal to about one half the wavelength of the light propagating in the optical waveguide, and in plane reflectivity of up to about 100% occurs in about 5 to 50 grating cycles for light in a typical III-V waveguide.

15. The grating of claim 1, wherein the cover layer (20) is defined further as an amorphous or crystalline cover layer selected from at least one of Si, GaAs, AlGaAs, InP, InGaAsP, GaN, AlGaN, InGaAsPSb, GaP, spin polymers, other column IV, column III-V, or column II-VI semiconductors.

16. The grating of claim 1, wherein the cover layer (20) is defined further as an amorphous or crystalline high index layer or an amorphous or crystalline low index cover layer and the cover layer (20) is deposited or formed by at least one of sputtering, vapor phase deposition, plasma enhanced chemical vapor deposition, vapor phase epitaxy, molecular beam deposition, molecular beam epitaxy, spin-on, or atomic layer deposition or epitaxial growth over dielectrics through openings in the dielectric to exposed epitaxial material.

17. The grating of claim 1, wherein the cover layer (20) is defined further as an amorphous low index cover layer selected from at least one of silicon nitride, polymer, SiO, SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O_4-5, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe, and other dielectrics.

18. The grating of claim 1, wherein the cover layer (20) is defined further as an amorphous low index cover layer deposited by at least one of sputtering, vapor phase deposition, plasma enhanced chemical vapor deposition, vapor phase epitaxy, molecular beam deposition, molecular beam epitaxy, atomic layer deposition, or by a spin-on processes.

19. The grating of claim 1, wherein the cover layer (20) converts a grating from a grating region that does not support a bound-mode to a grating region that does support a bound-mode.

20. The grating of claim 1, wherein the optical waveguide is defined further as comprising a non-grating transition waveguide, wherein the non-grating transition waveguide comprises a high index cover layer or a low index cover layer that converts high loss discontinuity between the waveguide and the transition waveguide to a low loss discontinuity, and may optionally further comprise a second contrasting cover layer.

21. The grating of claim 1, wherein the cover layer (20) when applied over a liner layer converts a grating from a grating region that does not support a bound-mode to a grating region that does support a bound-mode.

22. The grating of claim 1, wherein the ridges (16) of the grating (19) extend above the core layer (14).

23. The grating of claim 1, wherein the thickness of each of the core layer (14), grating (19) liner layer (18), and cover layer (20) are varied to optimize the ratio of upward coupled radiation to downward coupled radiation or in the upwards or downwards direction.

24. The grating of claim 1, wherein a period is selected that couples radiation at an angle sufficiently tilted from a surface-normal to reduce or eliminate second-order in-plane Bragg reflection.

25. The grating of claim 1, wherein the optical waveguide further comprises one or more additional grooves (17) or ridges (16) each with enhanced coupling strength gratings to provide a partially reflecting mirror that reduces or cancels a second-order in-plane Bragg reflection by destructive interference.

26. The grating of claim 1, wherein the optical waveguide further comprises one or more additional grooves (17) or ridges (16) to provide a partially reflecting mirror that reduces or cancels a second-order in-plane Bragg reflection by destructive interference.

27. The grating of claim 1, wherein the optical waveguide further comprises one or more additional grooves (17) or ridges (16) that are not covered by at least one of the liner layer (18) or cover layer (20) to provide a partially reflecting mirror that reduces or cancels a second-order in-plane Bragg reflection by destructive interference.

28. The grating of claim 1, wherein the optical waveguide further comprises one or more additional grating ridges that are not covered by at least one of a liner layer or a cover layer.

29. The grating of claim 1, wherein the index of refraction of the liner layer (18) is the range of ˜

1.3 to ˜

1.7, 1.7 to ˜

2.2, ˜

2.2 to ˜

3, or ˜

3 to ˜

3.8.

30. The grating of claim 1, wherein the cover layer (20) is at least one of amorphous or crystalline silicon and is defined further as a high index cover layer that is compatible with silicon processing.

31. The grating of claim 1, wherein the grating period (19) is adapted for use with wavelengths in the range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to 1.1, and greater than 1.1.

32. The grating of claim 1, wherein the selection of the materials for the ridges (16) is adapted for use with wavelengths in the range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to 1.1, and greater than 1.1.

33. The grating of claim 1, wherein the core (14) and the ridges (16) are unitary.

34. The grating of claim 1, wherein the grating forms at least a portion of an optical waveguide.

35. A method of making a grating comprising:

depositing on a first portion of substrate (12), a core (14) on the substrate (12), a superstrate (42) on the core (14), and a photoresist (44) on the superstrate (42);

etching the superstrate (42) to the core (14);

etching through a grating mask (48) formed on the core (14) to form the grating (48);

removing the grating mask depositing a liner layer (50) on the grating 48, wherein a first material is selected for the core (14) and ridges (16) and a second material selected for the liner layer (50), wherein the first and second materials are selected to provide a difference in the index of refraction sufficient to provide a contrast therebetween; and

depositing a cover layer (52) on the liner layer (50).

36. The method of claim 35, wherein the grating (48) comprises at least a portion of a waveguide (40).

37. The method of claim 35, wherein the grating (48) is defined further as comprising on at least one of:

a bottom (28) of a groove (17);

one or more sidewalls (22, 24) of one or more ridges (16);

a top (26) on the one or more ridges (16);

two or more liner layers (18) in the groove (17);

or

one or more sidewalls (22, 24) on the one or more ridges (16) that do not have a top (26).

38. The method of claim 37, wherein the liner layer (18) selected from one or more of the following optional configurations:

(a) the liner layer (18) is not contiguous;

(b) the liner layer (18) is disposed on a first sidewall (22), a second sidewall (24) or both the first and second sidewalls (22, 24) of the ridges (16);

(c) the liner layer (18) is defined further as one or more liner layers (18) that are contiguous and that follow the contour of the ridges (16) and the grooves (17);

(d) the liner layer (18) is not contiguous, wherein the liner layer (18) is defined further as being substantially parallel to a bottom of the one or more grooves (17), and the non-contiguous layers are separated by one or more cover layers (30);

(e) the liner layer (18) is defined further as two or more liner layers (18) that are contiguous and that follow the contour of the ridges (16), and each of the two or more liner layers (18) are separated by one or more cover layers (20);

(f) the liner layer (18) is disposed on one or more tops (26) of the ridges (16), one or more grooves (17) between the ridges (16), or both the top (26) of the ridges (16), and the grooves (17) between the ridges (16);

(g) the liner layer (18) is disposed on a first sidewall (22), one or more tops (26) of the ridges (16), and one or more grooves (17) between the ridges (16), to provide an effective blazed grating;

(h) the liner layer (18) is disposed on a first sidewall (22) and one or more tops (26) of the ridges (16);

or

(i) the liner layer (18) is disposed on one or more first sidewall (22) or second sidewall (24) of one or more waveguiding structures for grating coupling (inward or outward).

39. The method of claim 35, wherein the liner layer (50) is disposed on a high index contrast Si/SiO₂ waveguide to further enhance the performance of the grating.

40. The method of claim 35, wherein the cover layer (20) is selected to provide a similar index contrast between the cover layer (20) and the liner layer (18) as the contrast provided between the core (14) and the liner layer (18).

41. The method of claim 35, wherein the liner layer (50) is selected from at least one of SiO, SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O_4-5, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe, polymers, and silicon nitride.

42. The method of claim 35, wherein the liner layer (50) comprises a variable thickness to provide at least one of varying the strength of the coupling, an effective variable duty cycle, an effective variable grating depth, a Gaussian profile in a radiating coupler grating, or a near-Gaussian profile in a radiating coupler grating.

43. The method of claim 36, wherein the optical waveguide is at least one of a distributed Bragg reflectors (DBRs) or a distributed Bragg deflectors (DBDs).

44. The method of claim 36, wherein the optical waveguide is defined further as comprising at least two ECS gratings to make an edge-emitting DBR laser;

one ECS grating and one regular DBR grating to make an edge-emitting DBR laser;

two ECS gratings with a straight ECS outcoupler grating to make a surface-emitting laser;

ECS grating and one regular DBR grating with a straight ECS outcoupler grating to make a surface-emitting laser;

two ECS gratings with a “

fan-out”

ECS outcoupler grating to make a surface-emitting laser;

one ECS grating and one regular DBR grating with a “

fan-out”

ECS outcoupler grating to make a surface-emitting laser;

two ECS gratings with a standard grating outcoupler grating to make a surface-emitting laser;

one ECS grating and one regular DBR grating with a standard grating outcoupler grating to make a surface-emitting laser;

one or more ECS grating output couplers with low back reflection on both ends to make a surface-normal coupled semiconductor optical amplifier (SOA) or optical gain block;

or one or more ECS gratings or regular DBR gratings configures as a mirror with high reflectivity and another ECS grating as an output coupler to make a surface-emitting reflective semiconductor optical amplifier (RSOA) or an optical gain block;

a hybrid external cavity laser and tunable laser using SOA or RSOA with ECS grating output couplers integrated with a waveguide or free space wavelength control optics;

or an enhanced grating for high density and low loss integration of III/V laser sources for silicon photonic interconnects.

45. The method of claim 35, wherein the optical waveguide is formed into at least one of an enhanced grating on wherein the optical waveguide provides lateral optical confinement with a mesa structure and the enhanced grating is on at least one of:

the top surface of the mesa structure;

one or more lateral surfaces of the mesa structure;

or on the surfaces adjacent the mesa structure;

an enhanced grating for grating-assisted directional couplers;

an enhanced grating for multiply resonant distributed feedback lasers;

or an enhanced grating for multiplying resonant distributed Bragg reflector lasers;

an enhanced grating in optical fibers for sampling or detecting light in optical fibers by grating outcouplers;

an enhanced gratings in optical fibers for (1) sampling or detecting light in optical fibers by grating outcouplers operating near the second order Bragg condition;

(2) sampling or detecting light in optical fibers by grating outcouplers operating as distributed Bragg deflectors;

or (3) to couple light into optical fibers;

a curved, enhanced gratings to make unstable resonator semiconductor lasers;

an enhanced grating to reduce the etch depth for the placement of distributed Bragg reflector gratings in semiconductor lasers, which results in simplified processing for DBR lasers;

an enhanced grating to reduce the etch depth for the placement of distributed Bragg reflector gratings in photonic devices, which results in simplified processing for photonic devices; and

enhanced grating to reduce the etch depth for the placement of gratings in photonic devices, which results in simplified processing for photonic devices;

or an enhanced grating to reduce the etch depth for the placement of coupling gratings in photonic devices, which results in simplified processing for photonic devices.

46. The method of claim 35, wherein the grating (19) comprise a period that is equal to about the wavelength of the light propagating in the optical waveguide to produce an outcoupling in about 10 to 50 grating cycles.

47. The method of claim 35, wherein the grating (19) comprise a period that is equal to about one half the wavelength of the light propagating in the optical waveguide, and up to about 100% in-plane reflectivity occurs in about 5 to 50 grating cycles for light in a typical III-V waveguide.

48. The method of claim 35, wherein the cover layer (52) is defined further as an amorphous or crystalline cover layer selected from at least one of Si, GaAs, AlGaAs, InP, InGaAsP, GaN, AlGaN, InGaAsPSb, GaP, other column IV, column III-V, column II-VI semiconductors, SiO, SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O_4-5, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe, spin-on polymers, and silicon nitride.

49. The method of claim 35, wherein the cover layer (52) is defined further as an amorphous or crystalline high index layer or an amorphous or crystalline low index cover layer and the cover layer (52) is deposited or formed by at least one of sputtering, vapor phase deposition, plasma enhanced chemical vapor deposition, vapor phase epitaxy, molecular beam deposition, molecular beam epitaxy, spin-on process, or atomic layer deposition or epitaxial growth over the liner layer through openings in the liner layer to exposed epitaxial material.

50. The method of claim 35, wherein the cover layer (52) is defined further as an amorphous low index cover layer deposited by at least one of sputtering, vapor phase deposition, plasma enhanced chemical vapor deposition, vapor phase epitaxy, molecular beam deposition, molecular beam epitaxy, atomic layer deposition, or by a spin-on processes.

51. The method of claim 35, wherein the cover layer (52) converts a grating from a grating region that does not support a bound-mode to a grating region that does support a bound-mode.

52. The method of claim 36, wherein the optical waveguide is defined further as comprising a non-grating transition waveguide, wherein the non-grating transition waveguide comprises:

a. a high index cover layer or a low index cover layer that converts a high loss discontinuity between the waveguide and the transition waveguide to a low loss discontinuity,

b. a high index cover layer or a low index cover layer that converts a high loss discontinuity between the waveguide and the transition waveguide to a low loss discontinuity with a second contrasting cover layer;

c. a tapered waveguide;

or

d. an inverse-tapered waveguide.

53. The method of claim 37, wherein the ridges (16) of the grating (19) extend above the core layer (14).

54. The method of claim 37, wherein the thickness of each of the core layer (14), grating (19) liner layer (18), and cover layer (20) are varied to optimize the ratio of upward coupled radiation to downward coupled radiation or in the upwards or downwards direction.

55. The method of claim 35, wherein a period is selected that couples radiation at an angle sufficiently tilted from a surface-normal to reduce or eliminate second-order in-plane Bragg reflection.

56. The method of claim 37, wherein the optical waveguide further comprises one or more additional grooves (17) or ridges (16) each with enhanced coupling strength gratings to provide a partially reflecting mirror that reduces or cancels a second-order in-plane Bragg reflection by destructive interference.

57. The method of claim 37, wherein the optical waveguide further comprises one or more additional grooves (17) or ridges (16) to provide a partially reflecting mirror that reduces or cancels a second-order in-plane Bragg reflection by destructive interference.

58. The method of claim 37, wherein the optical waveguide further comprises one or more additional grooves (17) or ridges (16) that are not covered by at least one of the liner layer (18) or cover layer (20) to provide a partially reflecting mirror that reduces or cancels a second-order in-plane Bragg reflection by destructive interference.

59. The method of claim 35, wherein the optical waveguide further comprises one or more additional grating ridges that are not covered by a cover layer.

60. The method of claim 35, wherein the index of refraction of the liner layer (50) is the range of ˜

1.3 to ˜

1.7, 1.7 to ˜

2.2, ˜

2.2 to ˜

3, or ˜

3 to ˜

3.8.

61. The method of claim 35, wherein the cover layer (52) is at least one of amorphous or crystalline silicon and is defined further as a high index cover layer that is compatible with silicon processing.

62. The method of claim 37, wherein the grating period (16) is adapted for use with wavelengths in the range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to 1.1, and greater than 1.1.

63. The method of claim 37, wherein the selection of the materials for the ridges (16) is adapted for use with wavelengths in the range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to 1.1, and greater than 1.1.

64. The method of claim 37, wherein the core (14) and the ridges (16) are unitary.

65. An optical waveguide comprising:

one or more cladding layers deposited on one or more core layers, wherein the cladding layers comprise a refractive index that is lower than the refractive index of the core layers, wherein the optical waveguide is further defined as comprising a grating etched into at least one of a cladding layer, the core, or the cladding and core layers.

66. The optical waveguide of claim 65, wherein the refractive index of the cladding layers and core layers are ˜

1 to 2.

67. The optical waveguide of claim 65, wherein the optical waveguide is further defined as comprising a grating that has a liner layer disposed thereon and the liner layer has a cover layer, wherein the cover layer has at least one of a lower-index of refraction than the liner layer or a higher index of refraction than the liner layer.

68. The optical waveguide of claim 65, wherein the optical waveguide is further defined as comprising a grating that has a cover layer disposed thereon and the index of refraction of the cover layer is ˜

1 to ˜

2, or ˜

2 to 4.

69. The optical waveguide of claim 65, wherein the optical waveguide is further defined as comprising a grating with a period that is equal to about the wavelength of the light propagating in the waveguide to produce a coupling in 5 to 50 grating cycles.

70. The optical waveguide of claim 65, wherein the optical waveguide is further defined as comprising a grating with a period that is equal to about the wavelength of the light propagating in the waveguide to produce a coupling in 5 to 50 microns for light in a typical III-V waveguide at a free space wavelength of about 1.5 micron.