Development and characterization of LiNbO 3 electro-optic phase modulator at 220 GHz for millimeter-wave imaging system

Abstract

Lithium niobate (LN) modulators have been for years the devices of choice for the telecommunication market due to their high modulation rate and their robustness. Matching RF and optical velocities has been the most critical factor that limited this technology to 40 Gb/s since the early 2000's. However, recent interest in millimeter-wave (mmW) imaging, which requires good resolution images for object recognition, has led to significant progress in modulator's design and fabrication techniques [1]. 100 GHz modulation bandwidth has been reported [2]. For passive mmW imaging purpose at 77 GHz and beyond, we have developed an electro-optic phase modulator that can work up to 220 GHz. Modulator design and fabrication techniques are presented, supported by experimental measurements of optical response. We demonstrate optical upconversion up to 220 GHz by achieving RF and optical index matching combined with substrate modes elimination and low dielectric and conduction losses. The RF index is matched to the optical group velocity at 2.19 through CPW ridged structure and silicon dioxide layer deposition. Accurate index matching is obtained by controlling the thickness and the topology of the silicon dioxide layer, whereas substrate modes are reduced by thinning the LN substrate down to 50 μm. A fiber-modulator-fiber optical insertion loss of 4.5 dB also ensures good optical upconversion efficiency. We have applied the phase modulator to our mmW imaging system and obtained high quality mmW images in W-band.