Frequency Response Measurement Method of Photodetector Based on Multiple Harmonics

This letter proposes a simple method for measuring frequency responses of photodetectors (PDs) based on the multiple harmonic responses of a direct-modulated laser (DML). This method provides numerous harmonics as carriers for subsequent Mach–Zehnder modulator (MZM) modulation based on the nonlinear effect of the direct modulation laser. The modulated signal is injected into the photodetector under test, and the frequency response of the photodetector detector is obtained by sampling the MZM modulation component around multiple harmonics on the electronics spectrum analyzer (ESA). By measuring the relative amplitude of the modulation components around different harmonics, measurement error caused by harmonic amplitude differences can be eliminated. This method greatly expands the measurement range of the frequency response from DC to 2nfm. Moreover, since the modulator modulation frequency range occupies only
$\frac {1}{2{\mathrm {n}}}$
of the entire measurement range, the uneven frequency response caused by the modulator is reduced, and extra calibration is avoided. According to this method, we use a 35GHz Mach-Zehnder modulator to measure the frequency response of photodetectors with the bandwidth of 35GHz and 50GHz, respectively. We also compared our method with those conventional methods, and the results show that the method which has a wide frequency response can accurately measure the frequency response of the PD under the condition of the method.