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In this study, a platform guiding single-moded light at wavelengths of 480 nm, 520 nm and 633 nm (blue, green and red) is proposed and designed with several components being fabricated and characterised for the specific wavelength of 633 nm. A waveguide with propagation losses of 3.6 dB/cm is obtained, with a high confinement factor of 90.5% and a tight bending radius of 60 $\mu$m with 0.2 dB losses per bend, offering a good trade-off between losses, confinement factor and compactness. Also, a 1 × 2 MMI is demonstrated, with a footprint of 5 × 161 $\mu$m2, and losses of 0.2 dB. Finally, a silicon nitride single-layer grating coupler has been validated to allow the fibre-to-chip coupling, with losses smaller than 11.7 dB. A comparison of the proposed platform with other state-of-the-art stoichiometric silicon nitride technologies performing in the range of the spectrum of 630–660 nm is shown. The present platform demonstrates losses in the order of the state-of-the-art single-mode waveguides, but with an enhancement of the confinement factor from 61% to 90.5%, which allows to decrease the bending radius by 20 $\mu$m or more compared with other state-of-the-art technologies.

By comparing optical signals travelling through a sensing and a reference arm, interferometric photonic sensors achieve remarkable sensitivities and detection limits using simple single-wavelength laser sources. Sensors based on bimodal waveguides can, in principle, provide the same advantages without requiring a reference arm, by comparing the propagation of two modes travelling through a single sensing waveguide. However, typical implementations of bimodal sensors face two challenges: (i) the abrupt mode excitation and recombination at the sensor input and output is inefficient, unbalanced in power and produces spurious reflections that can mask small sensing signals, (ii) the sinusoidal nature of the output signal can lead to ambiguities in the readout. Here we present a spiralled bimodal refractive index sensor with full mode conversion, multiplexing and demultiplexing and a coherent phase detection, providing an unambiguous linear phase readout with a compact and robust layout. Our sensors have been designed for a 1550 nm central wavelength, fabricated on a silicon nitride platform and validated with bulk sensing experiments, achieving a limit of detection of $\mathbf {1.67\cdot 10^{-7}\,\text{RIU}}$.