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We propose a spot-size converter (SSC) by exploiting the high-efficiency mode conversion capability that a long-period grating (LPG) possesses. Our proof-of-concept LPG-based SSC, fabricated with optical polymer material on silicon nitride (Si 3 N 4 ) platform, has a length of ∼700 µm, can improve the coupling efficiency between the LP x 01 mode of an ultra-high numerical aperture fiber and the E x 11 mode of the Si 3 N 4 waveguide from ∼24% to ∼33% at 1548 nm wavelength, corresponding to 1.4-dB reduction in insertion loss. Therein, the LPG achieves a maximum mode conversion efficiency of 90% between the two E x 11 modes of the polymer cladding and the Si 3 N 4 core. Our proposed SSC provides a new path to achieve low-loss fiber-to-chip coupling.

A compact and highly sensitive liquid refractive index (RI) sensor using Vernier effect achieved with embedded doublering resonator is proposed and demonstrated. Unlike these conventional sensors using cascaded double-ring resonators to achieve Vernier effect, in our proposed embedded double-ring sensor (EDRS) the sensing ring is embedded on the reference ring, which makes the sensor more compact while achieving high sensitivity. Our typical fabricated EDRS with polymer material, which has a footprint of 1500 µm × 850 µm, achieves the RI sensitivity as high as 7390 nm/RIU in aqueous medium, which is in excellent agreement with the theoretical one.

In this paper, a compact and broadband mode converter, which can achieve efficient LP01-LP11a mode conversion, is demonstrated with cascaded asymmetric Y-junction in polymer waveguide. Our typical fabricated converter has a compact footprint of 1.5 mm × 14 µm and shows a mode conversion efficiency larger than ~ 98%, a crosstalk less than ~ −17.5 dB, and an insertion loss lower than ~ 5.8 dB for both x and y polarizations over the C + L band. Our proposed mode converter could find applications in broadband mode-division multiplexing transmission systems.