Integration of Optical Waveguide Array and Multilayer Substrate Integrated Waveguide for Electrooptical Modulator

Integration of an optical waveguide array using four waveguides in a Mach-Zehnder (MZ) structure and a two-layer low-loss substrate integrated waveguide (SIW) structure is presented to propose a new class of electrooptical phase modulators. In the proposed SIW modulator, synthesized rectangular waveguide in planar form is used as “electrodes” to guide millimeter-wave signals instead of conventional coplanar waveguide (CPW) for an electrooptical modulator, thus providing low-loss non-TEM mode propagation. In this study, two layers of a lithium niobate (LiNbO₃) substrate for creating a high field interaction between the millimeter-wave and optical signals are considered and studied. Fabrication of the proposed structure by titanium diffusion in an LiNbO₃ substrate as the optical waveguides and laser micromachining of the SIW via-holes has been done. The validation of our simulated results is also performed by the measurement of CPW-to-SIW transitions in the LiNbO₃ substrate. Optical loss of 0.8 dB/cm at the wavelength of 1550 nm is obtained for an MZ structure using an optical waveguide array consisting of four waveguiding channels. In addition, it is shown that 5-GHz bandwidth over a 60-GHz operating frequency range can easily be obtained for the proposed modulator.

Electrooptical modulators are key components of transmitters in optical communication systems. The electrode structures of electrooptical modulators for applications over millimeter-wave frequency ranges are usually in the form of conventional coplanar waveguides (CPWs). Such “electrical” structures present a number of unfavorable factors in the design and applications of millimeter-wave electrooptical modulators, namely, high loss and high crosstalk. Those problems are responsible for limited bandwidth and packaging difficulty. On the other hand, the state-of-the-art millimeter-wave devices including conventional lithium niobate (LiNbO₃) modulators are usually designed as low-pass devices with TEM-mode CPW electrodes. Unfortunately, modulation depth or bandwidth of these conventional modulators is significantly decreased at high frequencies because of optical/millimeter-wave velocity mismatch and high millimeter-wave loss [1], [2] for which the TEM mode operation does not present particular advantages.