Ultrahigh-Resolution Optical Vector Analysis for Arbitrary Responses Using Low-Frequency Detection

We propose and experimentally demonstrate an ultrahigh-resolution optical vector analyzer (OVA) for arbitrary responses measurement by using microwave photonic frequency down-conversion and fixed low-frequency detection. It avoids high-speed photodetection and ultrawideband electrical phase-magnitude detection, which are the key challenges to the previous OVAs based on microwave photonics (MWPs). Additionally, the proposed OVA has the capability of measuring arbitrary spectral responses. A high-resolution optical frequency-swept signal generated by using MWP-based techniques is separated into two parts. One portion propagates through an optical device under test (DUT) and then directs to a photodetector (PD), which carries out the spectral responses. In the other branch, the optical signal is accurately shifted by a small frequency and then sent to the PD. By square-law detection, the spectral responses of the optical DUT are thus transferred to a low-frequency photocurrent with a frequency equaling to the shifted frequency. Thus, the spectral responses can be obtained by detecting the photocurrent using a low-frequency electrical phase-magnitude detector working at a fixed frequency. In the experiment, a measurement system is established, by which the magnitude and phase responses of optical devices, such as an optical Hilbert transformer and an optical bandpass filter, are successfully measured. The measurement results are verified by the previous MWP-based OVAs and a commercial instrument.