Compensation of third-order intermodulation distortion of electro-optic modulator by using frequency chirp modulation

A new electro-optic modulator for compensating the nonlinear distortion is proposed. The third-order intermodulation distortion (IMD3) in receiver output is optically suppressed by the frequency-chirp modulation in the dual-parallel Mach-Zehnder modulator (DPMZM) structure. All of optical branches of the DPMZM are symmetric. The performance was analyzed by numerical calculations. First, we made an experiment to confirm the performance using a DPMZM with a microwave hybrid coupler for applying 10GHz two-tone signals to each electrode of the DPMZM. The chirp parameters of two Mach-Zehnder modulators (MZMs) of the DPMZM were set to opposite signs to each other, +1 and -1. It is seen from the experiment that the intensity ratio of IMD3 to signal in the receiver output was less than -58dB at modulation indices below 0.55 (0.175 €) rad and that the minimum of the ratio was less than -80dB at around 0.5 (0.16π) rad. In contrast, for a conventional single-MZM the ratio was less than -27dB below 0.55rad modulation indices. These experimental results agreed well with the calculation ones. Next, we designed and fabricated the modulator of the single-chip structure and the single-input operation for smallsizing and simple operation. The microwave rat-race circuit (RR), which operates as a 180° hybrid coupler, was designed on a Lithium Niobate (LN) substrate at 10GHz and was integrated with modulation electrodes of the DPMZM. The RR and modulation electrodes were formed with a gold thin film pattern at a time. The IMD3 suppression operation was confirmed for the fabricated modulator as well.