High-Order Electrical Spectrum Method for Measuring the Chirp of a Silicon Mach–Zehnder Modulator

The chirp parameter of a silicon optical modulator (SOM) is difficult to measure precisely because the plasma dispersion effect is due to electro-refraction and electro-absorption occurring simultaneously. A novel method for measuring the chirp parameter of a SOM is presented, which is adopted from the beat-frequency method. Firstly, based on the model of the differential driver silicon Mach-Zehnder modulator(DDSMZM), the chirp parameter of the DDSMZM is presented by mathematical derivations, and we prove that achieving the multi-order coefficients of the radio frequency phase shift is a key point for characterizing the chirp accurately. In the proposed method, a wavelength tunable laser, as a local source, is applied to mix with the chirped modulated optical signal of the DDSMZM. In this way, the multi-order electric signals of beat frequency are mapped from high-frequency to low-frequency. Meanwhile, a balanced detector with a low-speed trans-impedance amplifier(TIA) is employed for realizing photo-electric conversion, which can improve the signal-to-noise ratio of the measurement. Finally, we have examined the chirp parameter of the DDSMZM in the case of low-frequency and high-frequency modulation signal condition and the results show the beat-frequency method has a higher frequency resolution and measurement accuracy for measuring the chirp parameter of the DDSMZM compared with the optical spectrum analysis(OSA) method and the optical heterodyne method.