A waveguide mode expander couples a smaller optical mode in a semiconductor waveguide to a larger optical mode in an optical fiber. The waveguide mode expander comprises a shoulder and a ridge. In some embodiments, the ridge of the waveguide mode expander has a plurality of stages, the plurality of stages having different widths at a given cross section.

An integrated optical coupler device comprising, on a substrate surface: an integrated optical coupling grating extending in lateral directions parallel to the substrate surface and which, by diffraction at its grating structures, either converts electromagnetic waves, guided parallel to the substrate surface, of at least two waveguide modes of integrated optical waveguides into fiber modes propagating perpendicularly to the substrate surface, or converts electromagnetic waves, propagating perpendicularly to the substrate surface, of a fiber mode into electromagnetic waves, propagating parallel to the substrate surface, of at least two waveguide modes, and a first conductor pair, connected to the coupling grating and formed by a first and a second integrated optical waveguide, through which, in mutually opposite first and second directions parallel to the substrate surface, electromagnetic waves of at least two waveguide modes can be conducted to the coupling grating or can be conducted away from the coupling grating.

A compact polarization beam rotator includes a converter waveguide comprising a first segment and a second segment both in corresponding taper rib shapes sharing a first middle plane and configured to receive an input optical signal with TM polarization mode from an input plane and convert the TM polarization mode to TE1 polarization mode comprising a first arm mode and a second arm mode at a second middle plane. The polarization beam rotator additionally includes a splitter waveguide coupled to the second middle plane for separating the first arm mode and the second arm mode at a third plane respectively coupled to a first branch waveguide to deliver the first arm mode in phase and a second branch waveguide to reverse the second arm mode phase by 180, and a 21 MMI coupler waveguide to combine both arm modes in phase to an output optical signal with TE polarization mode.

Various polarization rotator splitter (PRS) configurations are disclosed. In an example embodiment, a system includes a PRS that includes a silicon nitride (SiN) rib waveguide core that includes a rib and a ridge that extends vertically above the rib, the SiN rib waveguide core having a total height h.sub.SiN from a bottom of the rib to a top of the ridge, a rib height h.sub.rib from the bottom of the rib to a top of the rib, a rib width w.sub.rib, and a top width w.sub.SiN of the ridge. The rib width w.sub.rib varies along at least a portion of a length of the SiN rib waveguide core.

A system may include a polarization rotator combiner. The polarization rotator combiner may include a first stage, a second stage, and a third stage. The first stage may receive a first component of light with a TE00 polarization and a second component of light with the TE00 polarization. The first stage may draw optical paths of the first and second components together. The second stage may receive the first component and the second component from the first stage. The second stage may convert the polarization of the second component from the TE00 polarization to a TE01 polarization. The third stage may receive the first component and the second component from the second stage. The third stage may convert polarization of the second component from the TE01 polarization to a TM00 polarization. The third stage may output the first component and output the second component.

A method of fabricating a waveguide mode expander includes providing a substrate including a waveguide, bonding a chiplet including multiple optical material layers in a mounting region adjacent an output end of the waveguide, and selectively removing portions of the chiplet to form tapered stages that successively increase in number and lateral size from a proximal end to a distal end of the chiplet. The first optical material layer supports an input mode substantially the same size as a mode exiting the waveguide. One or more of the overlying layers, when combined with the first layer, support a larger, output optical mode size. Each tapered stage of the mode expander is formed of a portion of a respective layer of the chiplet. The first layer and the tapered stages form a waveguide mode expander that expands an optical mode of light traversing the chiplet.

A polarization mode converter includes a rectangular waveguide, a first tapered waveguide, and a second tapered waveguide. A height of the rectangular waveguide is a first height (H1). A side of the first tapered waveguide is coupled to the rectangular waveguide. A width of the first tapered waveguide changes gradually. A height of the first tapered waveguide is a second height (H2), and H2 is less than H1. The second tapered waveguide is detached from the rectangular waveguide and the first tapered waveguide. A width of the second tapered waveguide changes gradually. A height of the second tapered waveguide is H1. The first tapered waveguide is located between the rectangular waveguide and the second tapered waveguide.

A method and mode conversion waveguide system for converting a mode of a light is provided. The light is sent through a single mode waveguide, wherein the light has a first mode while traveling through single mode waveguide. The light is sent from the single mode waveguide into a multimode interference region having connected to the single mode waveguide. The light is reflected with a cavity within the multimode interference region in a manner that causes the light to propagate away from the single mode waveguide. The light is output from multimode interference region, wherein the light has a second mode.

An optical assembly for realizing horizontal and vertical spot-size conversion to couple light from a thin waveguide to a thick waveguide is disclosed. The assembly comprises at least one first thin waveguide with a first section having a first optical mode field and a horizontal spot-size expansion section providing spot-size conversion for a first horizontal dimension of said first optical mode field of a light beam propagating in said first waveguide, and at least one second thick waveguide with a second section having a second optical mode field and a horizontal spot-size reduction section providing spot-size conversion for a second horizontal dimension of said second optical mode field of a light beam propagating in said second waveguide. The expanded end of said first waveguide is aligned and rotated to interface with the reduced end of said second waveguide, so that the mode fields in said first and second waveguides are rotated 90 degrees with respect to each other, whereby the spot size of a light beam so coupled between the first and second waveguides is expanded or shrunk in both transverse dimensions, depending on the direction of the light beam.

Structures for a waveguide core and methods of fabricating such structures. The structure comprises a waveguide core including a section having a first trapezoidal portion and a second trapezoidal portion stacked with the first trapezoidal portion. The first trapezoidal portion has a first trapezoidal shape, and the second trapezoidal portion has a second trapezoidal shape different from the first trapezoidal shape.