Frequency-Doubling Calibrated Measurement of Ultra-Wideband Optoelectronic Frequency Responses

A frequency-doubling calibration technique for accurate characterization of photodetector (PD) frequency response based on second-harmonic detection is proposed and experimentally demonstrated. A carrier-suppressed optical double-sideband (CS-ODSB) signal, generated by biasing a Mach-Zehnder modulator (MZM) at its minimum transmission point, is employed as the probe signal. During the calibration process, the probe signal is injected into a calibrated PD with a known response. By measuring the second-harmonic component present in the generated photocurrent, a calibrated electrical-to-electrical (E-E) response is achieved. In the measurement process, the calibrated PD is replaced by the PD under test. The same detection procedure is repeated to achieve another joint E-E response. By comparing these two measured E-E responses, the frequency response of the PD under test can be precisely extracted. Profiting from the same second-harmonic detection in calibration and measurement processes, the frequency measurement range is doubled relative to the intrinsic bandwidth of the MZM and the measurement is immune to the nonlinearity of the MZM. In the experiment, by using an MZM with a 6-dB bandwidth larger than 65 GHz, commercial PDs undergo precise characterization across the frequency band spanning 20 MHz to 110 GHz, achieving a frequency resolution of 10 MHz and a dynamic range of 78.36 dB@110 GHz. Additionally, the influence of the probe signal on the dynamic range is investigated.