Investigation For The 60GHz LiNbO₃ Single-Sideband Modulator Technology

Optical external modulators are essential components for the transmission of analog microwave/millimeter-wave and high-speed digital signals over optical fibers. In the radio-over-fiber (ROF) systems, the fiber chromatic dispersion will cause severe RF power penalty due to the optical double-sideband (DSB) modulation scheme. In order to overcome the power penalty, some optical single-sideband (SSB) modulation techniques have been implemented. However, all above schemes are complicated to implement and require a phase shift device.We designed a LiNbO3 traveling-wave modulator with the period phase reversal electrode to implement SSB modulation. The optical structure of the modulator is a Mach-Zehnder interferometer and the electrode which consists of a feeder and two modulating period phase reversal electrodes that are used to overcome the velocity mismatch between microwaves and optical signals. Due to the velocity mismatch, we can control the optical phase shift of the each arm in the modulator by changing the structure of the period phase reversal electrode. To shift the reversal position along the traveling-wave of the electrode on one arm, the phase shift difference of the two arms is 90-degree. Thus, the SSB optical signal is reached after interference at output of the modulator.We describe the fundamental theory and types of the electro-optical modulators in detail, and make an introduction to the means used to analyze the modulators, especially, to the Schwartz-Christoffel (SC) conformal mapping (CM) technique. Then a quasi-static analysis using the conventional conformal mapping approaches based on elliptic integrals has been used in evaluation of the microwave characteristics of the coplanar waveguide modulator and asymmetrical strip line modulator, the effective index, the characteristic impedance and the half wave voltage.We theoretically analyze the principle of the SSB modulator and the time-domain and frequency-domain response of the period phase reversal electrode. The simulation results show that the SSB modulation can be implemented well and an optical sideband suppression ratio is over 40dB in 60GHz ROF system that could successfully reduce the power penalty caused by the chromatic dispersive. Comparing to conventional EO SSB modulators, this scheme does not require a 90-degree RF hybrid circuit or tuning of a phase of RF modulation signals and it can be operated under a high microwave frequency as long as setting an appropriate period.