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Photodetectors (PDs) are optoelectronic (O/E) devices to achieve optical-to-electrical conversion, which are essential and of great importance in optical communication, optoelectronic oscillator, etc. Measuring O/E frequency responses, including magnitude response and phase response, is a fundamental measurement processing in their development and application. Microwave photonics (MWP) is a promising solution to achieve ultrahigh-resolution characterization. However, the frequency measurement range is restricted by the relatively small working bandwidth of modulators. To enlarge the measurement range, an approach to measure magnitude response of O/E devices is proposed and experimentally presented. In the approach, two optical double-sideband (ODSB) signals with the carrier suppression are generated. One ODSB signal filtered out +1st-order sideband is used as the frequency-shifted carrier. By coupling the frequency-shifted carrier and the other ODSB signal, an asymmetrical ODSB signal is thus achieved and served as a probe signal. After square-law detection of a PD under test, a photocurrent is produced. Detecting the frequency downconversion component in the produced photocurrent, the magnitude response in the low-frequency regime is obtained. Similarly, the magnitude response in the high-frequency regime is observed via extracting the magnitude information of the frequency up-conversion component. Thanks to the MWP-based frequency conversion, the measurement range is doubled, and the nonlinear error is suppressed. Furthermore, an ultrahigh-frequency resolution up to Hz or even sub-Hz is theoretically achievable. In an experiment, a 20-GHz commercial PD is accurately measured using a 200-kHz resolution. A measurement range as large as 67 GHz is enabled by 33.5 GHz RF frequency sweeping.
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