Objective Optoelectronic integrated chips are continuously evolving towards ultra-wideband, multifunctionality, and high density. Chip characterization spans the design, fabrication, and packaging processes. Particularly, on-wafer and in-line testing technologies can significantly enhance measurement efficiency, thereby aiding in yield improvement. In the past decades, numerous methods have been proposed for measuring the frequency response of optoelectronic integrated chips, categorized into optical spectrum and electrical spectrum methods. The optical spectrum analysis method involves measuring the power ratio of modulation sidebands relative to the optical carrier using an optical spectrum analyzer (OSA). This method is direct and effective for high-frequency and ultra-wideband operations. However, commercially available grating-based OSAs restrict the best resolution to 1.25 GHz (0.01 nm @ 1550 nm). Additionally, OSA-based methods are applicable primarily to electro-optical modulators (EOMs). Currently, the electro-optic frequency sweep (EOFS) scheme, a prevalent electrical spectrum analysis method, is widely adopted for measuring both EOMs and photodetectors (PDs) with the aid of optical/electrical (O/E) or electrical/optical (E/O) transducer standards. To streamline the O/E and E/O calibration procedures, an improved EOFS method based on electro-absorption modulators (EAMs) is proposed. This method assumes that the frequency responses of the EAM used as an EOM and PD are identical. To further streamline the calibration process, we have proposed a self-calibration method for measuring the EOM and PD based on two-tone modulation. This method allows for obtaining the frequency responses of the EOM and PD by analyzing the sum- and difference-frequency components of the two-tone mixing signals. Recently, we have presented a cascaded modulation mixing method to achieve damage-free and self-calibrated frequency response measurement of an integrated silicon photonic transceiver. However, it is important to note that a packaged EOM or PD with a good impedance match is required for this method. Therefore, methods capable of characterizing wafer-level optoelectronic chips, even without a good impedance match, and simultaneously free of extra E/O or O/E calibration, are of great interest. Methods As illustrated in Fig. 2, an optical pulse train from an optical frequency comb (OFC) with the repetition frequency f(r) is directed into the EOM to sample the frequency-sweep microwave signal f(n)=nf(r)+Delta f. The optical sampling signal is then detected by the PD. In the case of EOM chip measurement, the frequency-sweep microwave signal is down-converted to the same low-frequency component at Delta f, which combines the frequency responses of OFC, EOM, and adaptor network A (AN-A). For PD chip measurement, the fixed microwave signal Delta f is up-converted to the high-frequency component at f(n), incorporating the frequency responses of OFC, PD, and adaptor network B (AN-B). Subsequently, the uneven comb intensity response of the OFC can be obtained based on the time-frequency transformation theory of the hyperbolic secant pulses. Furthermore, microwave de-embedding with short-open-load-thru (SOLT) and open-short-load (OSL) terminations is implemented to accurately characterize the degradation factor of AN-A and AN-B in terms of transmission attenuation and impedance mismatch. Finally, the intrinsic frequency responses of EOM and PD chips are respectively extracted after de-embedding the frequency responses of AN-A and AN-B. Additionally, the measured results are compared to the EOFS method to verify consistency and accuracy. Results and Discussions The frequency response of the EOM chip within the frequency range of 222.42 GHz to 40.036 GHz is determined by detecting the down-converted fixed low-frequency signal at 202.485 MHz. Similarly, the frequency response of the PD chip across the same frequency range is obtained by up-converting a fixed microwave signal at 202.485 MHz to higher frequencies. Utilizing the time-frequency transformation theory, the uneven comb intensity response of the OFC, characterized by a pulse width of 5.16 ps, is calculated and shows a degradation of approximately 1.87 dB at 40.036 GHz. The frequency responses of AN-A and AN-B are extracted through microwave de-embedding, as depicted in Fig. 8. AN-B exhibits a more irregular response compared to AN-A, attributed to higher resistance in the PD chip. Analysis using the Smith chart reveals that the EOM chip does not achieve a perfect 50 Omega match across the entire modulation frequency range, while the PD chip exhibits significant deviation from a 50 Omega match. Reflection coefficients further confirm the robustness of the proposed method against impedance mismatches. Finally, comparison with results obtained using the EOFS method demonstrates good agreement, validating the feasibility and accuracy of the proposed method. Conclusions We propose an on-wafer and in-line measurement method for optoelectronic chips based on photonic sampling using an OFC as the optical source. The method eliminates the need for additional E/O or O/E calibration and proves resilient against impedance mismatches. It enables high-frequency measurement of electro-optic modulator chips through low-frequency photodetection and wideband measurement of photodetector chips via narrowband electro-optic modulation. These capabilities make the approach promising for in-line testing of wafer-level optoelectronic chips.
