Frequency chirp modulation by electro-optic modulator integrated with microwave rat-race circuit and measurement of chirp parameter by using optical fiber dispersion

The rat-race (RR) circuit, which operates as a 180-degree hybrid, with an asymmetric power split ratio was integrated with the dual electrode type electro-optic modulator on the same substrate. Thereby, frequency chirp modulation with a compact configuration was realized. The RR circuit was designed with 4.5:1 power split ratio on a z-cut LiNbO₃ substrate. The operation of the modulator was confirmed by observing the output light spectra. Moreover, the chirp parameter was directly measured by using the chromic dispersion of the optical fiber, where the power penalty of detected modulation signals transmitted through optical fiber was observed as a function of the fiber distance. The chirp parameter of the modulated light at 10 GHz was confirmed to be 0.47 by the measurement.

I. Introduction

The frequency chirp in optical intensity modulation is caused by the parasitic phase shift induced with the intensity variation. It is known that the frequency chirp of optical signals degrades performance in high speed and long distance fiber transmission [1]. On the other hand, if the frequency chirp can be controlled in electro-optic (EO) modulators, a new application such as the flat optical comb signal generation will be offered [2]. Optical comb signals generated by EO modulators are expected to be used in multi-wavelength laser source, ultra-short pulse generation and optical frequency reference. The pre-chirp modulation for compensating the effect of optical fiber dispersion is also realized by such frequency chirp modulation.