In an example, a photonic system includes a Si PIC with a Si substrate, a SiO.sub.2 box formed on the Si substrate, a first layer, and a second layer. The first layer is formed above the SiO.sub.2 box and includes a SiN waveguide with a coupler portion at a first end and a tapered end opposite the first end. The second layer is formed above the SiO.sub.2 box and vertically displaced above or below the first layer. The second layer includes a Si waveguide with a tapered end aligned in two orthogonal directions with the coupler portion of the SiN waveguide such that the tapered end of the Si waveguide overlaps in the two orthogonal directions and is parallel to the coupler portion of the SiN waveguide. The tapered end of the SiN waveguide is configured to be adiabatically coupled to a coupler portion of an interposer waveguide.

According to an example aspect, there is provided an apparatus comprising a first optical converter coupled to a fibre interface and to two waveguides, a dual rail encoder configured to encode dual rail form light from the two waveguides with payload information, and wherein the dual rail encoder is coupled to the first optical converter or to a second optical converter disposed between the dual rail encoder and the fibre interface, and wherein the first optical converter or the second optical converter is coupled so as to provide polarization encoded light into the fibre interface.

Various examples are provided for tunable radio frequency (RF) filtering. In one example, a RF multiband filter includes a Lyot filter including a tunable birefringence loop including a circulator and a polarization controller (PC) and a phase modulator (PM). The Lyot filter can generate an optical comb based at least in part upon a received optical signal and a polarization rotation angle of the polarization controller. The phase modulator (PM) can generate a modulated tap signal by modulating the optical comb by a RF input signal.

In an example, a coupled system includes a first waveguide, at least one second waveguide, and an interposer. The first waveguide has a silicon (Si) core having first refractive index n1 and a tapered end. The at least one second waveguide each has a silicon nitride (SiN) core having a second refractive index n2. The interposer includes a third waveguide having a third refractive index n3 and a coupler portion, where n1>n2>n3. The tapered end of the first waveguide is adiabatically coupled to a coupler portion of one of the at least one second waveguide. A tapered end of one of the at least one second waveguide is adiabatically coupled to the coupler portion of the third waveguide of the interposer. The third waveguide of the interposer has an optical mode size that is similar to the mode size of a standard single mode optical fiber.

A compact polarization beam splitter is formed by cascading two stages of three restricted MMIs. Each MMI is configured to set ultra compact width and length for a rectangular waveguide body to limit no more than 4 modes therein working as a polarization beam splitter in a 50 nm wavelength window around 1300 nm. Each MMI is further configured to couple an input at a first end and a TE bar output and a TM cross output at a second end of the rectangular waveguide body. The locations of the input/output waveguide ports are designated to be a distance of of the width away from a middle line from the first end to the second end. Two second-stage MMIs have their inputs coupled to the TE bar output and the TM cross output of the first-stage MMI and provide a second-stage TE bar output and a second-stage TM cross output, respectively.

In an example, a photonic system includes a Si PIC with a Si substrate, a SiO.sub.2 box formed on the Si substrate, a first layer, and a second layer. The first layer is formed above the SiO.sub.2 box and includes a SiN waveguide with a coupler portion at a first end and a tapered end opposite the first end. The second layer is formed above the SiO.sub.2 box and vertically displaced above or below the first layer. The second layer includes a Si waveguide with a tapered end aligned in two orthogonal directions with the coupler portion of the SiN waveguide such that the tapered end of the Si waveguide overlaps in the two orthogonal directions and is parallel to the coupler portion of the SiN waveguide. The tapered end of the SiN waveguide is configured to be adiabatically coupled to a coupler portion of an interposer waveguide.

A compact polarization beam splitter is formed by cascading two stages of three restricted MMIs. Each MIMI is configured to set ultra compact width and length for a rectangular waveguide body to limit no more than 4 modes therein working as a polarization beam splitter in a 50 nm wavelength window around 1300 nm. Each MMI is further configured to couple an input at a first end and a TE bar output and a TM cross output at a second end of the rectangular waveguide body. The locations of the input/output waveguide ports are designated to be a distance of of the width away from a middle line from the first end to the second end. Two second-stage MMIs have their inputs coupled to the TE bar output and the TM cross output of the first-stage MMI and provide a second-stage TE bar output and a second-stage TM cross output, respectively.

A planar optical waveguide device includes: a substrate; and an optical waveguide that includes a core and a cladding. The core forms a preceding-stage mode conversion section and a subsequent-stage mode conversion section, the preceding-stage mode conversion section being configured to convert a mode of input light, the subsequent-stage mode conversion section being configured to convert a mode of light output from the preceding-stage mode conversion section. Sectional shapes of the first core portion and the second core portion are not congruent with each other at an input end of the preceding-stage mode conversion section, the sectional shape or size of at least one core is continuously changed along a light waveguide direction, and sectional shapes of the first core portion and the second core portion are congruent with each other at an output end of the preceding-stage mode conversion section.

A compact polarization beam splitter is formed by cascading two stages of three restricted MMIs. Each MIMI is configured to set ultra compact width and length for a rectangular waveguide body to limit no more than 4 modes therein working as a polarization beam splitter in a 50 nm wavelength window around 1300 nm. Each MMI is further configured to couple an input at a first end and a TE bar output and a TM cross output at a second end of the rectangular waveguide body. The locations of the input/output waveguide ports are designated to be a distance of of the width away from a middle line from the first end to the second end. Two second-stage MMIs have their inputs coupled to the TE bar output and the TM cross output of the first-stage MMI and provide a second-stage TE bar output and a second-stage TM cross output, respectively.

Provided is a waveguide mode converter (30) that converts a waveguide mode and that is placed in a transition area (connection section) (43) of a rib-type waveguide (50) and a channel-type waveguide (51). The rib-type waveguide (50) has a tapered section (23b). The tapered section (23b) constitutes a core layer (23) that extends on both sides of a rib (23a) and has a width (Wt) that changes gradually in a direction that is vertical to the waveguide direction.