High-performance and compact integrated photonics platform based on silicon rich nitride-lithium niobate on insulator APL Photonics 6, 116102 (2021);

Silicon nitride (SiN) emerges as an important platform for ultralow loss photonic integrations with complementary metal‐oxide‐semiconductor compatibility. However, active devices, such as modulators, are difficult to realize on pure SiN due to the lack of any electro‐optic (EO) properties of the material. Here, an SiN and lithium niobate (LN) heterogenous integration platform supporting high‐performance EO modulators on SiN waveguide circuits is introduced. An efficient evanescent coupling structure is realized for low‐loss light transitions between the SiN waveguide and the LN ridge waveguide with a measured mode transition loss of only 0.4 dB. Based on this heterogeneous platform, an EO Mach–Zender interference modulator on SiN is built with unprecedented loss, efficiency, and bandwidth performances. A half‐wave voltage of 4.3 V with a modulation bandwidth of 37 GHz and an overall insertion loss of 1 dB is measured for a 7‐mm long device. Data transmission up to 128 Gb s−1 with a bit‐error‐rate of <2.4 × 10‐4 is also demonstrated.

High-performance silicon and thin-film lithium niobate hybrid electro-optic modulators are demonstrated. In order to break the voltage–bandwidth limit in a normal traveling-wave modulator, a periodic capacitively loaded traveling-wave electrode is employed in this hybrid platform. The silicon substrate is undercut-etched to achieve index matching of the optical wave and microwave. A hybrid waveguide with a lithium niobate thin film bonded on a silicon wire is employed. Lithium niobate etching is not required for making the hybrid optical waveguides. We realize an intensity modulator of 12.5 mm long modulation section, which exhibits a low half-wave voltage of 1.7 V and a large 3 dB modulation bandwidth of >70 GHz. Data transmissions with various modulation formats beyond 100 Gbit/s are successfully achieved with dynamic extinction ratios of >8 dB. Combining the advantages of the silicon and thin-film lithium niobate platforms, a compact dual polarization coherent modulator is also experimentally demonstrated, on which 96 Gbaud 16-level quadrature amplitude modulation signals in both polarizations are successfully transmitted.

We experimentally demonstrate an integrated visible light modulator at 532 nm on the thin-film lithium niobate platform. The waveguides on such platform feature a propagation loss of 2.2 dB/mm while a grating for fiber interface has a coupling loss of 5 dB. Our fabricated modulator demonstrates a low voltage-length product of 1.1 V·cm and a large electro-optic bandwidth with a roll-off of -1.59 dB at 25 GHz for a length of 3.3 mm. This device offers a compact and large bandwidth solution to the challenge of integrated visible wavelength modulation in lithium niobate and paves the way for future small-form-factor integrated systems at visible wavelengths.

A grating coupler with a high coupling efficiency and low back reflections is designed and demonstrated on the thin film lithium niobate platform, which facilitates an efficient interface between a lithium niobate ridge waveguide and a standard single mode fiber. The excellent performances of the present grating coupler are enabled by a cavity-assisted grating structure and a top metal mirror, even though a uniform grating is adopted as the diffractive element. Experimentally, a coupling loss of -0.89 dB at 1552 nm is demonstrated with a 1-dB bandwidth of 45 nm. Low back reflections of &lt; -16.5 dB to the waveguide and &lt; -13.7 dB to the fiber are also achieved from 1536 nm to 1579 nm.