High-efficiency vertical fibre-to-polymer waveguide coupling scheme for scalable polymer photonic circuits.

Polymer photonic circuits offer a versatile platform for various applications, including communication, sensing and optical signal processing. Though polymers offer broadband, linear and nonlinear optical properties, the coupling between an optical fibre and a polymer waveguide has been a challenge. In this work, we propose and demonstrate a wafer-scale vertical coupling scheme for polymer waveguides. The scheme uses a silicon nitride grating coupler with an inverse taper to couple between an optical fibre and a SU8 polymer waveguide. We demonstrate a maximum coupling efficiency of -3.55 dB in the C-band and -2.92 dB in the L-band with a 3-dB bandwidth of 74 and 80 nm, respectively. A detailed design and simulation, fabrication, and characterisation results are presented. The scheme demonstrates a scalable and efficient surface grating approach for polymer photonic integrated circuits.

On-chip silicon photonics based grating assisted vibration sensor.

We present a compact, highly sensitive and scalable on-chip photonic vibration measurement scheme for vibration sensing. The scheme uses a silicon photonic diffraction-grating based sensor integrated underneath a silicon cantilever. We demonstrate a static and dynamic measurement sensitivity (ΔT/Δgap) of 0.6 % change in intensity per nm displacement. The electrostatically driven dynamic response measurement of the grating based sensor shows an excellent agreement with commercial Laser Doppler Vibrometer (LDV) measurement. We demonstrate the thermo-mechanical noise measurement on the cantilever in ambience, which is verified using LDV. A minimum displacement of 1.9 pm is measured with a displacement sensitivity of 10 µW/nm for a measurement bandwidth of 16 Hz. The demonstrated sensitivity is 2 orders of magnitude better than that obtained from measurements of static displacement. We also present a detailed 2D-FDTD simulation and optimization of the grating-based sensor to achieve maximum displacement sensitivity.

High efficiency DBR assisted grating chirp generators for silicon nitride fiber-chip coupling.

Silicon Nitride (SiN) is emerging as a promising material for a variety of integrated photonic applications. Given its low index contrast however, a key challenge remains to design efficient couplers for the numerous platforms in SiN photonics portfolio. Using a combination of bottom reflector and a chirp generating algorithm, we propose and demonstrate high efficiency, grating couplers on two distinct SiN platforms. For a partially etched grating on 500 nm thick SiN, a calculated peak efficiency of -0.5 dB/coupler is predicted, while for a fully etched grating on 400 nm thick SiN, an efficiency of -0.4 dB/coupler is predicted. Experimentally measured coupling efficiencies are observed to be -1.17 and -1.24 dB/coupler for the partial and fully etched grating couplers respectively in the C-L band region. Furthermore, through numerical simulations, it is shown that the chirping algorithm can be implemented in eight additional combinations comprising SiN film thickness between 300-700 nm as well as alternate claddings, to achieve a per coupler loss between -0.33 to -0.65 dB.

Compact broadband low-loss taper for coupling to a silicon nitride photonic wire.

We demonstrate an ultra-compact waveguide taper on a silicon nitride platform. The proposed taper provides a coupling efficiency of 95% at a length of 19.5 µm in comparison to the standard linear taper of length 50 µm, which connects a 10 µm wide waveguide to a 1 µm wide photonic wire. The taper has a spectral response >75% spanning over 800 nm and resilience to fabrication variations; ±200 nm change in taper and end waveguide width varies transmission by <5%. We experimentally demonstrate taper insertion loss of <0.1 dB/transition for a taper as short as 19.5 µm, and reduce the footprint of the photonic device by 50.8% compared to the standard adiabatic taper. To the best of our knowledge, the proposed taper is the shortest waveguide taper ever reported in silicon nitride.

Efficient and tunable strip-to-slot fundamental mode coupling.

We present a novel photonic wire-to-slot waveguide coupler in SOI. The phase matching between a wire and slot mode is achieved using a mode transformer. The architecture consists of a balanced 50/50 power splitter and a tunable phase matched taper combiner forming a slot waveguide. We show a theoretical wire-to-slot coupling efficiency of 99 % is achievable and experimentally, we demonstrate a coupling efficiency of 99 % in the 1550 nm band. Based on the coupling scheme, we also show excitation of a slot mode in a slotted ring resonator and verified the excitation through the thermo-optic response of the rings. We show a nearly athermal behaviour of a PMMA filled slot ring with a thermo-optic response of 12.8 pm/°C compare to 43.5 pm/°C for an air clad slot waveguide.