BACKGROUND

[0001 J A back reflection may occur when light from an optical source propagates along a waveguide and is reflected back toward the optical source at the interface of the end of the waveguide and another medium, such as air. Back- reflected optical signals in optica! waveguides may cause deleterious effects, such as destabilization of a source laser or splitting of resonances in optical ring resonators.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] The accompanying drawings illustrate various examples of the principles described below, The examples and drawings are illustrative rather than limiting.

[0003] FIG. 1 depicts a b!ock diagram of an example back- reflection reduction device.

[00043 FIGS. 2A-2D depict side views of various end facets of a waveguid used in a back-reflection reduction device.

[0005 FIG. 3A depicts a to view of an example back-reflection reduction device. F!G. 38 depicts a side vie of the example back-reflection reduction device.

[0006] 4 depicts a flow diagram illustrating an example process of reducing back reflections in a waveguide.

[00073 FIG. 5 depicts a flow diagram illustrating an example process of reducing back reflections in a waveguide. DETAILED DESCRIPTION

[0008] Back-reflection reduction devices are presented below that aid in the reduction of back reflections occurring at a termination of a waveguide. A back- reflection reduction device is applicable to planar integrated optical waveguides that use a grating coupler to couple light off the integrated waveguide or into the integrated waveguide. Implementations of the device may be based upon an angled waveguide end facet that couples reflected light into different spatial modes from the spatiai modes of the light that originally propagated along the waveguide to encounter the end facet.

[00093 FIG. 1 depicts a block diagram of an example back-reflection reduction device 100. The back-reflection reduction device 100 may include a mode converter 104, a second waveguide 106, and a coupler 108, where the second waveguide 106 is designed to have an end facet to reduce back reflections,

[0010] The mode converter 104 couples a first waveguide to a second waveguide 106 and converts one or more spatiai modes propagating in the first waveguide to one or more spatial modes whose propagation is supported by the second waveguide 106. The first waveguide may be an optical waveguide that is part of a planar integrated photonic circuit which integrates two or more optical functions and may be fabricated on a substrate, such as silicon. The first waveguide may also be an optical fiber or other type of waveguide. In some implementations, the first waveguide may be a single mode waveguide that supports the propagation of a single spatial mode. Alternatively, the first waveguide may be a multimode waveguide that supports the propagation of multiple spatial modes.

[0011] Examples of mode converters 104 include tapered waveguides, such as adiabaticaSSy tapered waveguides, and a lens thai focuses a light beam after expansion in a medium outside the waveguide, such as air. In some implementations, the mode converter 104 may be an adiabatica!iy tapered waveguide. For example, the adiabatical!y tapered waveguide may convert a single spatia! mode of a first waveguide, which may be a single mode waveguide, to the fundamental mode of the second waveguide 106, which may be a multimode waveguide. As another example, the adiabaticalSy tapered waveguide may convert a first set of spatial modes of the first waveguide, which may be a muStimode waveguide, to a second set of spatial modes of the second waveguide 106, which may be another multimode waveguide, and the second waveguide 106 may or may not be the same type of multimode waveguide as the first waveguide.

|0012| The second waveguide 106 is an optica! waveguide that may be a multimode waveguide. Similar to the first waveguide, the second waveguide 106 may be part of an integrated photonic circuit, an optical fiber, or other type of waveguide. The end facet of the second waveguide 106 is designed to reduce back-reflections to the first waveguide. A smooth or polished end facet that is perpendicular to the direction of propagation of the light in the second waveguide 106 will cause the Sight to be reflected within the second waveguide 106 due to Fresnel reflections. For example, if Sight is propagating in the fundamental mode in the second waveguide 106, the reflected light will remain in the fundamental mode, traverse the mode converter 104 in the backward direction and couple back into the first waveguide. The reflected light may continue propagating backward, resulting in unwanted effects, such as destabilizing a laser source that initially generated the Sight. Types of end facets that may be used to reduce the back-reflected light are described below with reference to FSGS. 2A-2D.

[0013 The coupSer 108 couples Sight out of the second waveguide 106. For example, if the second waveguide 106 were part of a planar integrated photonic circuit, the coupler 108 may couple light out from the second waveguide 106 and into, for example, an optical fiber for transmission to another location. As an example, the coupler 108 may be a grating coupler that is made of grooves etched in the second waveguide 108. Grating couplers may be advantageous with planar integrated photonic circuits because they may facilitate wafer-scale testing, eliminate polishing steps that may be expensive, reduce alignment tolerances, and reduce the cost of manufacturing because the grating coupler can couple Sight to the core of a cleaved single mode fiber, rather than coupling to more expensive tapered Sensed fibers, i0014| ^!f1 general, a coupler 108, such as a grating coupler, is not perfectly efficient. As a result, light propagating along the second waveguide 108 is not entirely coupled out by the coupler 108. Light that is not coupled out by the coupler 108 continues to propagate along the second waveguide until it hits the end facet of the second waveguide 106. OOI 53 The end facet of the second waveguide 106 may be designed to reduce back reflections into the same spatial modes that originally propagated along the second waveguide 106 toward the end facet. FIGS. 2A-2D depict side views of example portions of the second waveguide 106 having various end facets to reduce back reflections to the first waveguide.

[00163 FIG. 2A depicts an example second waveguide 206 having an end facet 210 that is flat and angled with respect to the optica! axis 21 1 of the second waveguide 206. Light that is not coupied out of the second waveguide 206 by the coupler 108 propagates to and hits th angled end facet 210 and is subsequently reflected at an angle from the incident direction to propagate in the backward direction. Thus, the reflected Sight does not propagate backward in the same spatial modes as upon arrival at the angled end facet 210, rather the light propagates in different spatial modes back toward the mode converter 104. However, because the light is no longer propagating in the same spatial modes as it was after it traversed the mode converter 104 the first time, the mode converter 104 wiil not convert the back-reflected light into the original set of spatial modes that originally propagated along the first waveguide.

[00173 F!G. 2B depicts a side view of an example second waveguide 207 that is a planar integrated optical waveguide fabricated on a substrate 220. The second waveguide 207 may have an end facet 202 that is flat and angled in a direction out of a plane defined by a surface plane of the planar integrated optical waveguide. The angle of the end facet 202 performs the same function as the angie of the end facet 202 when it is in the plane of the surface plane of the second waveguide 208 as described above with reference to FIG. 2A.

{0018| F!G. 2C depicts an exarnpie second waveguide 206 with an end facet 210 that has an anti-reflection coating 230. The anti- reflection coating 230 is designed to be effective at the wavelengths of the light propagating in the second waveguide 208. The a ti -reflection coating 230 may be applied to the end facet 210 of the second waveguide 206 for any configuration of the end facet 210, for example, for any angle in any plane. Although anti-reflection coatings are not perfect, the anti-reflection coating 230 serves to reduce the amount of light reflected in the backward direction along the second waveguide 206, and thus, further reduces any light that couples back into the first waveguide. 0019| FIG. 2D depicts an example second waveguide 206 with an end facet that has a rough surface 240. The rough surface 240 spoils the reflection of the light propagating along the second waveguide 206 by reflecting different portions of the light in different directions and into different spatial modes of the second waveguide 208. Alternatively, the end facet of the second waveguide 206 may have a curved surface, such as a parabolic surface, where the curvature of the surface may be in any plane or combination of planes. The effect is to reduce the amount of light that propagates back into the first waveguide (not shown in FIG. 2D) at the original spatial modes in which the light propagated along the first waveguide.

[0020] FIG. 3A depicts a top view 300A of an example back-reflection reduction device for planar integrated optica! waveguides, and FIG. 3B depicts a side view 300B of the example back-reflection reduction device for planar integrated optica! waveguides.

|0021| The back-reflection reduction device shown in 300A and 3008 is a planar integrated optical device that has a first waveguide 302, a mode converter 304, and a second waveguide 306, In some implementations, the first waveguide 302 may be a single mode integrated optical waveguide along which light propagates; the mode converter 304 may be an adiabaticaliy tapered integrated optical waveguide coupling the single mode waveguide to a multimode waveguide to convert the light in the single mode waveguide to a fundamenia! mode of the multimode waveguide; and the second waveguide 306 may be the multimode integrated optical waveguide along which the converted light propagates in the fundamental mode. Further, the back-reflection reduction device has a grating coupler 308 to couple the tight in the fundamental mode out of the multimode waveguide 308.

{00223 ^e multimode waveguide has an end facet 310 away from the adiabaticaliy tapered waveguide to reduce back reflection of light in the fundamental mode. Thus, light not coupled out of the second waveguide 306 by the grating coupler 308 hits the end facet 310 and is reflected into a different set of spatial modes from the fundamental mode. The light in the different set of spatial modes then propagates in the backward direction along the second waveguide 306. The mode converter 304 converts the backward propagating light into higher order modes that are not supported by th first waveguide 302. As a result, the Sight reflected from the end facet 310 may couple to unsupported cladding modes and dissipate after a short propagation distance.

{0023] In some implementations, the end facet 310 may be angled in a direction out of a plane defined by the second waveguide 306, have an anti-reflection coating at wavelengths of the light, and/or have a rough surface.

[0024] FIG. 4 depicts a flow diagram illustrating an example process 400 of reducing back-reflections in a waveguide. The process begins at block 405 where a mode converter may be used to convert light in a first set of spatial modes propagating in a first waveguide to a second set of spatial modes for propagating in a second waveguide. For example, in some implementations, the first waveguide may be a single mode waveguide, and the second waveguide may be a multimode waveguide, where the second set of spatial modes may include the fundamental mode. [00253 At block 410, a portion of the Iight in the second set of spatial modes may be coupled out of the second waveguide. For example, a grating coupler may be used to couple the portion of the light in the second set of spatial modes out of the second waveguide,

[0026] At block 415, the light in the second set of spatial modes not coupled out of the second waveguide may be reflected into a third set of spatial modes different from the second set of spatial modes. For example, light in the fundamental mode of the second waveguide may be reflected into higher order modes of the second waveguide.

{00273 FSG. 5 depicts a flow diagram illustrating an example process 500 of reducing reflections in a waveguide. The process begins at block 505, which may be similar to block 405 described with respect to the process 400 of FIG. 4. Blocks 510 and 515 may also be similar to blocks 410 and 415, respectively, of FIG. 4.

[0028] At block 520, the third set of spatial modes may be converted into a fourth set of spatial modes using the mode converter, where the fourth set of spatial modes is different from the first set of spatial modes. For example, the fourth set of spatial modes may be higher order spatial modes that are not supported by the first waveguide. Thus, light may be converted into the higher order spatiai modes of the first waveguide, and if the first waveguide is a single mode waveguide, the light does not propagate ver far in the first waveguide.

[0029] ot all of the steps, or features presented above are used in each implementation of the presented techniques.

[0030] As used in the specification and claims herein, the singula forms "a," "an," and the" include plural referents unless the context clearly dictates otherwise.