Wideband Optical Vector Analysis Based on Microwave Photonic Frequency Downconversion

A high-resolution optical vector analyzer (OVA) with wide measurement bandwidth based on microwave photonic frequency downconversion is proposed and experimentally demonstrated. In the proposed OVA, two pairs of frequency-swept dual-tone signals with different frequency intervals are used as the probe light. After passing through a device under test (DUT), the probe light is transmitted into a low-speed photodetector to perform frequency downconversion. Then, an intermediate frequency (IF) photocurrent with a fixed frequency is generated, which carries the DUT response. Hence, a low-speed electrical phase-magnitude detector (EPMD) can be employed to extract the magnitude and group delay responses of the DUT. Compared with the OVAs using the high-speed PD and ultrawide EPMD, the proposed approach is low-cost and has a high signal-to-noise ratio. In an experiment, the probe light is generated by the 2nd-order nonlinearity of the electro-optic modulator. Therefore, compared with conventional OVAs based on optical single-sideband (OSSB) or optical double-sideband (ODSB) modulation, the proposed OVA can achieve wide measurement bandwidth, which is four times (or two times) of that of the OSSB-based OVA (or the ODSB-based OVA). A frequency resolution of 6 MHz and a measurement bandwidth of 120 GHz are achieved.