High-speed electro-optic Phase Modulators (PMs) are critical devices in microwave photonics and optical communication. Optical spectrum analysis is widely used to measure the modulation depth of PMs thanks to simple structure and wide measurement frequency range. To further obtain the accurate frequency response especially the half-wave voltage of PMs, the microwave driving power input to the electrical port of PMs must be measured accurately. However, the source adapter network such as coaxial cables and connectors for connecting the microwave source and the PM under test will result in degraded response, which should be de-embedded. In this study, an improved optical spectrum analysis method is proposed to measure the frequency response of PMs based on microwave fixture de-embedding. In the proposed scheme, the microwave power transmitted from the microwave source to the PM is investigated by using signal flow graph to track the accurate driving power input to the PM, including transmission attenuation and impedance mismatch of the source adapter network and the PM. The accurate frequency response including modulation depth, half-wave voltage and relative response of the PM are obtained. In a proof-of-concept experiment, a commercial PM has been accurately measured in the frequency range of 1–30 GHz to demonstrate the feasibility of the proposed method. Our method fully calibrates the microwave power loss and the port impedance mismatch, which is applicable for those PMs even without good impedance match. Moreover, our method features simple structure and broadband measurement thanks to the optical spectrum analysis scheme.
An improved self-reference photonic sampling method is proposed to measure the frequency response of photodiode (PD) chips. In the proposed scheme, the uneven response of the Mode-Locked Laser Source (MLLS) is eliminated by using the half-frequency photonic sampling measurements. The microwave de-embedding under short-open-load-device termination is used to realize on-chip de-embedding of the adapter network connected to the receiver of the microwave network analyzer in terms of the transmission loss and the impedance mismatch. The proposed on-chip measurement method is free of any extra electro-optical transducer standard, and an accurate measurement can still be realized without an impedance match.
An approach to measuring high-frequency responses of electro-absorption modulated lasers (EMLs) is proposed based on fixed low-frequency pilot analysis. The fixed low-frequency pilot is inserted into the microwave driving signal loaded on the EML through amplitude modulation. Then, the high-frequency response of EML can be obtained by extracting and analyzing the pilot (kHz level) after photodetection, thereby realizing the low-frequency detection for EML measurement. Moreover, the method is independent of the responsivity fluctuation of the photodetector due to the fixed frequency analysis and enables the self-calibrated frequency response measurement of high-speed EML.
A slowly-varying-envelope photonic sampling method is proposed for hyperfine and ultra-wideband frequency response measurement of high-speed photodetectors (PDs). The measuring frequency range of PD is firstly divided into several segments by the repetition frequency fr of a mode-locked laser diode (MLLD), and the hyperfine frequency response measurement of the PD in every segment is then achieved by applying a slowly-varying-envelope microwave modulation sweeping up to fr/2, which is also independent of the uneven responses of the MLLD and the Mach-Zehnder modulator (MZM). Finally, through carefully choosing the joint-frequency of photonic sampling, the ultra-wideband frequency response of the PD can be obtained with the help of seamlessly stitching different segments. Most importantly, the frequency response of the PD at any frequency can be measured by subtly changing the frequency of photonic sampling from DC to fr/2. Moreover, the measuring frequency range of the PD can be extended by 2(n+1)-fold relative to the modulation frequency range of the MZM, where the microwave frequency swept up to fr/2 enables the measuring frequency range up to (n+1)fr.
A frequency-shifted dual-carrier method is proposed for microwave characterization of Mach-Zehnder modulators based on low frequency detection. The proposed method utilizes the heterodyne products between the beats of two modulated sidebands, and achieves calibration-free microwave measurement of Mach-Zehnder modulators with the help of electrical spectrum analysis. Our method features low-frequency detection with only one microwave source and avoids the responsivity correction introduced by the photodetector. In the experiment, the frequency response of a Mach-Zehnder electro-optic intensity modulator is measured by using the proposed method, where the measurement results fit in with those obtained by using the conventional optical spectrum analysis method.
A self-calibrated method is proposed for electrical spectrum measurement of optical frequency comb (OFC) based on segmental electro-optic up-conversion. In the method, every N comb teeth of OFC are divided into one segment in the frequency domain, and M segments are investigated with the measuring frequency range of M×N×fr (fr is the repetition frequency of the OFC). Through symmetric frequency modulation, intra-segment measurement and seamless stitching between different segments are performed. Finally, only a low-frequency microwave source is required to achieve the electrical spectrum measurement of OFC within ultra-wideband frequency range, and the measuring frequency range can be 2M-fold expanded with respect to the modulation frequency rang. Meanwhile, the frequency responses of MachZehnder modulator and photodetector are fully cancelled out, realizing the self-calibrated electrical spectrum measurement of OFC within ultra-wideband frequency range.
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