Wireless Microwave-to-Optical Conversion on Thin-Film Lithium Niobate

Microwave-to-optical converters (MOCs) are critical components for building a bridge between photonic and radio-frequency (RF) engineering. High bandwidth and sensitivity, immunity to noise, and compactness are some of the requirements of MOCs. Integrated MOCs that incorporate RF antennas and optical waveguides on the same chip can address these requirements. In this work, integrated MOCs are demonstrated on thin-film lithium niobate (TFLN). Two antenna types (patch and bow-tie) are designed at a resonant frequency of 28 GHz on silicon substrates. The antennas are seamlessly integrated with single-mode TFLN optical waveguides, operating at a nominal wavelength of 1550 nm. Experimental results demonstrate that the TFLN MOCs employing patch and bow-tie antennas yield figures of merit (FOM) of 0.7 W
$^{-\frac{1}{2}}$
and 0.25 W
$^{-\frac{1}{2}}$
, respectively, corresponding to field enhancement (FE) factors of 590 and 487. The MOC based on bow-tie antennas shows a wider 3-dB bandwidth of 5 GHz, which is 2 GHz broader than its patch antenna counterpart. Furthermore, We predict the FOM can be improved to a value of 1.46 W
$^{-\frac{1}{2}}$
for MOCs fabricated on quartz substrates.