Frequency response measurement of high-speed photodetectors using the spectrum power method in a delay self-heterodyne system

An improved delay self-heterodyne system consisting of a distributed Bragg reflector (DBR) tunable laser and an optical fiber ring interferometer is used to measure the frequency responses of high-speed photodetectors. By adjusting the amplitude of the phase-tuning-voltage which is applied across the phase section of the DBR laser, the beat signals with frequencies from zero to several tens of gigahertz will be generated at the photodetectors. The peak power method (PPM) and spectrum power method (SPM) were used to measure the frequency response of photodetectors. The effects of the phase-tuning-voltage over different ranges on the measurement accuracy were experimentally investigated. Theoretical analysis indicates that the variations of optical linewidth and lineshape are the main reasons for producing a large error in the measurement by PPM, and that the fluctuation of optical power did not generate errors in the frequency response measurements by both PPM and SPM. The experimental results show that when a low phase-tuning-voltage range is used in measurement by SPM, the error of the measuring frequency response of the photodetector can be sharply reduced.